Frequently Asked Questions

Transcription

Pressure • Load • Torque • Acceleration • Displacement • Instrumentation
Frequently Asked Questions
H o n e yw e l l S e n s o t e c
(800) 867-3892
2080 Arlingate Lane
Columbus, Ohio 43228
USA
Tel: 614-850-6000
Fax: 614-850-1111
www.honeywell.com/sensotec
www.sensotec.com
Table of Contents
T AB L E O F C O N T E N T S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
AC C E L E R O M E T E R S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
HOW DOES A PIEZO-ELECTRIC ACCELEROMETER WORK? ......................................................... 6
WHAT ARE THE DIFFERENT TYPES OF ACCELEROMETER? ........................................................ 6
WHAT IS A SINGLE ENDED COMPRESSION ACCELEROMETER? ................................................... 7
WHAT IS AN ISOLATED COMPRESSION ACCELEROMETER? ........................................................ 7
WHAT IS A SHEAR TYPE ACCELEROMETER? ......................................................................... 8
WHAT IS A PIEZO-RESISTIVE ACCELEROMETER? ................................................................... 8
WHAT IS A STRAIN GAGE BASED ACCELEROMETER? ............................................................... 9
WHAT IS THE USEABLE FREQUENCY RANGE? ....................................................................... 9
WHAT IS AN IEPE ACCELEROMETER?...............................................................................10
WHAT IS AN ICP ACCELEROMETER? ................................................................................11
WHAT IS A CHARGE OUTPUT ACCELEROMETER?...................................................................11
WHAT IS THE NATURAL FREQUENCY OF AN ACCELEROMETER? .................................................11
WHAT IS THE MOUNTED NATURAL FREQUENCY? ...................................................................12
WHAT IS BASE STRAIN SENSITIVITY? ................................................................................13
WHAT IS CROSS SENSITIVITY OR TRANSVERSE SENSITIVITY? .................................................. 13
WHAT IS DYNAMIC RANGE? ............................................................................................14
WHAT IS AMPLITUDE LINEARITY? .....................................................................................14
WHAT ARE THE DIFFERENCES BETWEEN QUARTZ CRYSTAL BASED AND CERAMIC CRYSTAL BASED
ACCELEROMETERS?.....................................................................................................15
HOW DO I MOUNT AN ACCELEROMETER? ...........................................................................15
WHAT IS A QUICKFIT MOUNT? .........................................................................................17
WHEN SHOULD I USE A VELOCITY OUTPUT ACCELEROMETER? .................................................17
WHAT SIGNAL CONDITIONING DO I NEED FOR MY ACCELEROMETER? .........................................18
WHAT ARE GROUND ISOLATED ACCELEROMETERS? ..............................................................19
WHAT IS AN ISOLATED STUD?.........................................................................................22
HOW DO I INSTALL A CHARGE AMPLIFIER? .........................................................................22
WHAT IS THE TRIBO-ELECTRIC EFFECT?............................................................................23
HOW DO I CHOOSE THE SENSITIVITY OF AN ACCELEROMETER? ................................................24
WHAT IS THE OUTPUT OF AN IEPE ACCELEROMETER? ..........................................................24
WHAT IS AN FFT? ......................................................................................................24
WHAT IS CONDITION MONITORING? ..................................................................................25
WHAT FREQUENCY RESPONSE DO I WANT FROM MY ACCELEROMETER? ......................................25
WHAT TYPE OF ACCELEROMETER BEST SUITS MY APPLICATION? ..............................................28
C AL I B R AT I O N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 9
WHY SHOULD I CALIBRATE?...........................................................................................30
WHAT IS NIST? .........................................................................................................30
WHAT IS A NIST TRACEABLE CALIBRATION? ......................................................................30
CAN A LOAD CELL BE MADE TRACEABLE TO NIST? ...............................................................31
CAN AN ORGANIZATION BE NIST TRACEABLE? ....................................................................31
WHAT IS A2LA OR NVLAP ACCREDITATION? .....................................................................32
WHAT IS UNCERTAINTY? ...............................................................................................32
WHEN IS UNCERTAINTY IMPORTANT? ................................................................................32
HOW IS UNCERTAINTY MEASURED? ..................................................................................33
C AL I B R AT I O N C L AS S L O AD C E L L S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4
WHAT DOES A LOAD CELL CALIBRATION CONSIST OF? ...........................................................35
WHAT LOAD CELL CALIBRATION STANDARD SHOULD I ADOPT? .................................................35
WHAT ARE IMPORTANT PARAMETERS FOR A CALIBRATION CLASS LOAD CELL? ..............................35
WHAT ARE THE CHARACTERISTICS OF A LOW UNCERTAINTY CALIBRATION CLASS LOAD CELL? ..........36
WHY DOES A CALIBRATION CLASS LOAD CELL HAVE A BASE PLATE AND A CALIBRATION ADAPTER? ....36
WHAT IS THE UNCERTAINTY OF A SENSOTEC LOAD CELL? ......................................................37
WHAT UNCERTAINTY SHOULD MY CALIBRATION REFERENCE LOAD CELL HAVE? .............................37
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Honeywell Sensotec | www.sensotec.com | (800) 867-3892
Copyright 2003 Honeywell International Inc.
DOES A LOAD CELL HAVE TO BE CALIBRATED WITH ITS DISPLAY? .............................................38
WHAT DOES THE ASTM E74 STANDARD SPECIFICALLY SAY ABOUT CALIBRATION CLASS LOAD CELLS?
.............................................................................................................................38
G E N E R AL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 0
HOW DO I KNOW WHAT ACCURACY CLASS TO USE FOR MY SENSOR? .........................................41
WHAT IS SENSITIVITY? ................................................................................................41
WHAT IS NON-LINEARITY? ............................................................................................43
WHAT IS HYSTERESIS? ................................................................................................46
WHAT IS STATIC ERROR BAND? .....................................................................................46
WHAT IS CALIBRATION FACTOR? ....................................................................................47
WHEN SHOULD I FIT A CONNECTOR OR INTEGRAL CABLE? ....................................................48
WHAT IS THE DIFFERENCE BETWEEN SUBMERSIBLE AND WATERPROOF? ...................................50
WHAT ARE NEMA AND IP DEFINITIONS FOR ENVIRONMENTAL PROTECTION?..............................51
L O AD C E L L S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3
WHAT IS OVERLOAD PROTECTION ON A LOAD CELL? ............................................................54
WHAT IS A COMPRESSION ONLY LOAD CELL?.....................................................................55
WHAT IS A TENSION ONLY LOAD CELL? ............................................................................56
WHAT IS A TENSION AND COMPRESSION ONLY LOAD CELL? ...................................................58
WHAT IS A ROD END BEARING? ......................................................................................58
WHAT IS A LOAD BUTTON? ............................................................................................59
WHAT IS LOAD CELL SYMMETRY? ...................................................................................60
WHAT IS ZERO BALANCE FOR A LOAD CELL?.......................................................................61
WHAT IS ZERO BALANCE TEMPERATURE EFFECT? ................................................................62
WHAT IS OUTPUT SPAN TEMPERATURE EFFECT? .................................................................63
WHEN SHOULD I HAVE ZERO AND SPAN ADJUSTMENTS ON MY LOAD CELL?.................................65
WHAT ARE THE ADVANTAGES AND DISADVANTAGES OF HAVING A SENSOR INTERNAL AMPLIFIER ON A
LOAD CELL? ..............................................................................................................66
WHY IS THE OUTPUT OF MY PRESSURE DETECTOR QUOTED IN MV/V? ........................................69
HOW DO I KNOW WHAT ACCURACY CLASS TO USE FOR MY SENSOR? .........................................70
HOW DOES TEMPERATURE AFFECT A LOAD CELL? ...............................................................71
HOW DO YOU COMPENSATE FOR TEMPERATURE IN A LOAD CELL? .............................................71
WHAT IS THE TEMPERATURE COMPENSATION RANGE? ...........................................................71
WHAT IS THE TEMPERATURE OPERATING RANGE? ................................................................71
HOW DO I PICK THE RIGHT FULL SCALE OUTPUT FOR A LOAD CELL? ..........................................72
WHAT IS THE EFFECT OF DYNAMIC LOADS ON A LOAD CELL?...................................................72
HOW DOES A LOAD CELL WORK? .....................................................................................73
WHAT LOAD RANGE SHOULD I CHOSE FOR A LOAD CELL? .......................................................73
WHAT THINGS DO I NEED TO CONSIDER WHEN MOUNTING A LOAD CELL? ...................................74
PRESSURE SENSORS.................................................................................................76
HOW DOES A BONDED FOIL STRAIN GAGE-BASED PRESSURE SENSOR WORK? ...........................77
HOW DOES A SILICON-BASED PRESSURE SENSOR WORK? ....................................................78
WHAT ARE ADVANTAGES BETWEEN BONDED FOIL STRAIN GAGE-BASED AND SILICON-BASED
PRESSURE SENSORS? .................................................................................................79
WHAT IS A GAGE PRESSURE SENSOR? .............................................................................79
WHAT IS A TRUE GAGE PRESSURE SENSOR? .....................................................................80
WHAT IS AN ABSOLUTE PRESSURE SENSOR? .....................................................................81
WHEN SHOULD I USE AN ABSOLUTE PRESSURE SENSOR RATHER THAN A GAGE PRESSURE SENSOR? .. 82
WHAT IS A DIFFERENTIAL PRESSURE SENSOR? ..................................................................83
WHAT IS A VACUUM PRESSURE SENSOR? .........................................................................83
WHAT IS A BAROMETRIC PRESSURE SENSOR? ....................................................................84
WHICH PRESSURE REFERENCE SHOULD I USE FOR MY APPLICATION? .......................................85
WHAT IS OVERLOAD PROTECTION ON A PRESSURE SENSOR? .................................................86
WHAT CONSIDERATIONS SHOULD I MAKE WHEN MOUNTING A PRESSURE SENSOR? ........................87
HOW CAN I PROTECT AGAINST WATER HAMMER? .................................................................88
WHAT IS ZERO BALANCE FOR A PRESSURE TRANSDUCER? ......................................................89
WHAT IS ZERO BALANCE TEMPERATURE EFFECT? ................................................................90
WHAT IS OUTPUT SPAN TEMPERATURE EFFECT? .................................................................92
WHEN SHOULD I HAVE ZERO AND SPAN ADJUSTMENTS ON MY PRESSURE DETECTOR? ...................94
WHAT ARE THE ADVANTAGES AND DISADVANTAGES OF HAVING A SENSOR INTERNAL AMPLIFIER ON A
PRESSURE DETECTOR?.................................................................................................95
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WHY
IS THE OUTPUT OF MY PRESSURE DETECTOR QUOTED IN MV/V? ........................................97
TORQUE SENSORS .................................................................................................. 100
WHAT IS TORQUE? .................................................................................................... 101
WHAT IS REACTION TORQUE? ....................................................................................... 101
WHAT IS ROTARY TORQUE? ......................................................................................... 102
HOW DO YOU MEASURE ROTARY TORQUE? ....................................................................... 102
HOW DO ROTARY TORQUE SENSORS WORK? ..................................................................... 103
WHEN SHOULD I CHOOSE AN IN-LINE SENSOR? ................................................................. 105
WHEN SHOULD I CHOOSE A CLAMP ON COLLAR? ................................................................ 105
WHEN SHOULD I STRAIN GAGE MY SHAFT AS MY TORQUE TRANSDUCER? .................................. 106
WHAT ARE THE MAJOR DIFFERENCES BETWEEN THE VARIOUS FORMS OF TORQUE MEASUREMENT? .. 107
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ACCELEROMETERS
5
HOW
D O E S A P I E Z O - E L E C T R I C AC C E L E R O M E T E R W O R K ?
Piezo-electric crystals are man-made or naturally occurring crystals that
produce a charge output when they are compressed, flexed or subjected to
shear forces. The word piezo is a corruption of the Greek word for squeeze. In
a piezo-electric accelerometer a mass is attached to a piezo-electric crystal,
which is in turn mounted to the case of the accelerometer. When the body of
the accelerometer is subjected to vibration the mass mounted on the crystal
wants to stay still in space due to inertia and so compresses and stretches the
piezo electric crystal. This force causes a charge to be generated and due to
Newton law F=ma this force is in turn proportional to acceleration. The charge
output is either is converted to a low impedance voltage output by the use of
integral electronics (example: in an IEPE accelerometer) or made available as a
charge output (Pico-coulombs /g) in a charge output piezo-electric
accelerometer.
W H AT
AR E T H E D I F F E R E N T T Y P E S O F AC C E L E R O M E T E R ?
There are many different type of accelerometers and each has unique
characteristics, advantages and disadvantages. The different types include:
Different technologies
Piezo-electric accelerometers
Piezo-resistive accelerometers
Strain gage based accelerometers
Different output accelerometers
Charge output
IEPE output (2-wire voltage)
Voltage output (3 wire)
4-20mA output
Velocity output accelerometers
Different designs of accelerometer
Shear type design
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Single ended compression design
Isolated compression
Inverted compression
Flexural design
W H AT
I S A S I N G L E E N D E D C O M P R E S S I O N AC C E L E R O M E T E R ?
A single ended compression accelerometer is where the crystal is mounted to
the base of the accelerometer and the mass is mounted to the crystal by a
setscrew, bolt or fastener.
A single ended compression accelerometer
W H AT
I S AN I S O L AT E D C O M P R E S S I O N AC C E L E R O M E T E R ?
Single ended compression accelerometers can be susceptible to base strain
and so to alleviate this problem the crystal is isolated from the base by
mounting on an isolation washer or by reducing the mounting area by which the
crystal is mounted to the base.
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Isolated compression accelerometers
W H AT
I S A S H E AR T Y P E AC C E L E R O M E T E R ?
A shear type accelerometer is where the seismic mass is attached to the crystal
so that it exerts a shear load on the crystal rather than a compressive load.
Shear type accelerometers are designed for applications that are likely to
encounter significant base distortion from thermal transients or where they are
mounted onto flexible structures.
Shear type piezo-electric accelerometer
W H AT
I S A P I E Z O - R E S I S T I V E AC C E L E R O M E T E R ?
A piezo-resistive accelerometer is an accelerometer that uses a piezo-resistive
substrate in place of the piezo electric crystal and the force exerted by the
seismic mass changes the resistance of the etched bridge network and a
whetstone bridge network detects this. Piezo-resistive accelerometers have the
advantage over piezo-electric accelerometers in that they can measure
accelerations down to zero Hertz.
8
W H AT
I S A S T R AI N G AG E B AS E D AC C E L E R O M E T E R ?
A strain gauged based accelerometer is based on detecting the deflection of a
seismic mass by using a silicon or foil strain gauged element. A whetstone
bridge network detects the deflection. The deflection is directly proportional to
the acceleration applied to the sensor. Like the piezo-resistive accelerometer it
has a frequency response down to zero Hz.
W H AT
I S T H E U S E AB L E F R E Q U E N C Y R AN G E ?
For an accelerometer to be useful the output needs to be directly proportional
to the acceleration that it is measuring. This fixed ratio of output to input is only
true for a range of frequencies as described by the frequency response curve.
9
Typical Piezo-electric frequency response curve.
The usable frequency response is the flat area of the frequency response curve
and extends to approximately 1/3 to ½ of the natural frequency. The definition
of flat also needs to be qualified and is done so by quoting the roll off of the
curve in either percentage terms (typically 5% or 10%) or in dB terms (typically
+/- 3db)
W H AT
I S AN
IEPE
AC C E L E R O M E T E R ?
IEPE stands for Integrated Electronics Piezo Electric and defines a class of
accelerometer that has built in electronics. Specifically it defines a class of
accelerometer that has low impedance output electronics that works on a two
wire constant current supply with an voltage output on a DC voltage bias. IEPE
two wire accelerometers are easy to install, have a wide frequency response,
can run over long cable lengths and are relatively cheap to purchase. The IEPE
technology has generally replaced most 3 wire accelerometers and is broadly
used for most applications except for specialist applications such as zero Hz
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accelerometers, high temperature applications or 4-20mA accelerometers used
in the process industries.
W H AT
I S AN
ICP
ICP is the trademarked PCB name for IEPE accelerometers. It stands for
‘Integrated circuit-piezo electric’
W H AT
I S A C H AR G E O U T P U T AC C E L E R O M E T E R ?
All piezo-electric accelerometers work by measuring the charge generated by a
crystal that is being compressed or shear loaded by a mass influenced by
acceleration. In most applications this high impedance charge output is
converted to a low impedance voltage output by the use of integral electronics.
However in some applications integral electronics are not appropriate such as
high temperature or high radiation applications. Charge output accelerometers
are self-generating and would typically have amplifying electronics mounted
several feet away from the local heat or local radiation source.
W H AT
I S T H E N AT U R AL F R E Q U E N C Y O F AN AC C E L E R O M E T E R ?
The natural frequency of an accelerometer is the frequency where the ratio of
output is at it highest. The natural frequency of an accelerometer is defined by
the equation
From a frequency roughly 1/3 to ½ of the natural frequency the ratio of output
to input becomes non-linear and therefore makes measurements from this
region difficult to interpret. Therefore the higher the natural frequency of an
accelerometer the higher frequencies where the output to input is linear and the
higher the frequencies that can be measured.
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It can be seen from the formula for natural frequency that to increase the
natural frequency the mass needs to be as small as possible and the stiffness
needs to be as high as possible. A small mass usually means a lower sensitivity
and this is true of most high frequency accelerometers.
W H AT
I S T H E M O U N T E D N AT U R AL F R E Q U E N C Y ?
An accelerometer has a different natural frequency when it is in free space to
that when it is mounted. The only frequency that is of interest to the user is of
course the mounted natural frequency and is often the one quoted in the
specifications. The mounted natural frequency is of course dependent on the
stiffness of the mounting structure to which it is attached and is therefore
quoted as the natural frequency of the accelerometer as installed according to
manufacturers instructions. Gluing, magnetically mounting or loose bolting
down to a surface will significantly reduce the mounted natural frequency.
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W H AT
I S B AS E S T R AI N S E N S I T I V I T Y ?
Base strain sensitivity is the erroneous signal that is generated by an
accelerometer when the base is subjected to bending, torque or distortion
either by mechanical movement or thermal stressing. The relative movement of
the base of the accelerometer squeezes the crystal in an accelerometer and the
seismic mass mounted on the crystal. Base strain is where the base distorts the
mass while acceleration causes the seismic mass to distort the crystal. These
two forces on the crystal are indistinguishable and so reduction of the base
strain is vital for good signals only to be generated. The more indirectly that a
crystal is mounted to the base under strain the less sensitive the accelerometer
is to base strain. Single ended compression sensors are the most prone to base
strain sensitivity and shear type accelerometers the least. Isolated compression
accelerometers are a good compromise between have good base strain
immunity and the disadvantages that shear type accelerometers bring in terms
of sensitivity and robustness.
Base Strain Sensitivity
W H AT
I S C R O S S S E N S I T I V I T Y O R T R AN S V E R S E S E N S I T I V I T Y ?
An accelerometer produces a charge output when the crystal is compressed.
That same crystal also produces a charge, albeit a much smaller one, when a
shear load is exerted on the crystal. The accelerometer therefore produces a
charge when it is vibrated in the axis 90 degrees to the main axis of
measurement, which is indistinguishable from acceleration in the main axis.
Conversely shear type accelerometers produce an erroneous signal when they
experience cross axis acceleration only this time it loads the crystal in
compressive mode.
13
Cross axis sensitivity
The sensitivity of the accelerometer to a transverse vibration is known as the
transverse sensitivity and is typically less than 5% of the sensitivity to an ‘on
axis’ acceleration.
W H AT
I S D Y N AM I C R AN G E ?
The dynamic range of an accelerometer is the range between the smallest
acceleration detectable by the accelerometer to the largest. A piezo-electric
accelerometer produces a charge proportional the force applied to the crystal,
which due to the seismic mass on the crystal is proportional to acceleration
applied. The piezo electric effect can be detected for very small forces or
accelerations all the way through to very large accelerations. In most cases the
smallest acceleration is dictated by the amplifying electronics noise floor and
for high g levels to the voltage rail used by the power supply. The design of the
accelerometer will also play a part in what shock g levels an accelerometer can
withstand before the crystal is irreparably damaged or the structure holding the
crystal is distorted. Compression accelerometers are the most shock resistant
design of accelerometer.
Accelerometers with integral electronics have a maximum output voltage
determined by the circuit design and the input voltage. The maximum output for
an IEPE accelerometer is typically 4-8 volt s. An accelerometer with a sensitivity
of 100mV/g with electronics that has a maximum output of 5V will obviously
have a dynamic range of +/- 50g while an accelerometer of sensitivity of
10mV/g will have a dynamic range of +/- 500g
W H AT
I S AM P L I T U D E L I N E AR I T Y ?
The amplitude linearity of an accelerometer is the degree of accuracy that an
accelerometer reports the output in voltage terms as it moves from being
excited at the smallest detectable acceleration levels to the highest. This
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accuracy is qualified by the linearity. Typically the amplitude linearity is 1%.
The dynamic range describes the minimum to maximum accelerations that can
be detected. The output of an IEPE accelerometer can typically go from 100
micro g to 500g. This dynamic range is dependent on the electronics used with
the accelerometer either internal or external, as is the output linearity over the
dynamic range.
W H AT AR E T H E D I F F E R E N C E S B E T W E E N Q U AR T Z C R Y S T AL B AS E D AN D C E R AM I C C R Y S T AL
B AS E D AC C E L E R O M E T E R S ?
Ceramic Crystals
Quartz Crystals
Man made piezo electric crystals
Higher output sensitivity
Less expensive
Higher pyro-electric effect at
elevated temperatures
Higher crystal decay rates at
Lower temperature of operation
Natural piezo electric crystals
Lower output sensitivity
More expensive
Lower pyro-electric effect at
No crystal decay rates with time or
temperature
Higher temperature operation
HOW
DO
I
M O U N T AN AC C E L E R O M E T E R ?
The mounting of an accelerometer affects its frequency response. The mounted
natural frequency is dependent directly on the stiffness of the mounting. The
higher the stiffness the more the mounted natural frequency approaches its
maximum. The least stiff mounting of an accelerometer is magnetic mounting
and the highest stiffness is using a high tensile setscrew tightened to the
correct torque mounted on a hard flat surface. Other mounting methods come in
between these two extremes.
15
It is important to ensure that the site chosen for the accelerometer is ground
flat for at least an area larger than the base of the accelerometer. A slight
smear of Silicone grease will ensure a stiff bond between accelerometer and
structure.
Surface preparation and set screw installation
When using a mounting stud it is important to ensure that the stud does not
bottom out in either the base of the accelerometer or the drilled location hole.
High tensile strength set screws that have a shoulder will prevent this
eventuality from happening.
16
Incorrectly mounted accelerometer
W H AT
IS A QUICKFIT MOUNT?
A quickfit mount is used in installations where the accelerometer will be
removed between monitoring the acceleration or velocity vibration yet will be
repeatedly place back in the same location. Such installations include machine
health monitoring using data collectors.
Quickfit mounting for an accelerometer
WHEN
SHOULD
I
U S E A V E L O C I T Y O U T P U T AC C E L E R O M E T E R ?
Velocity output accelerometers are usually used in condition monitoring
applications where velocity is a much better parameter for judging the health of
a machine. Doubling of velocity vibration equates to a doubling of the
deterioration of the health of the machine. Velocity can also be used in lower
frequency applications where the acceleration amplitude of vibration is too
small to measure and the velocity vibration maybe of a higher and more
17
meaningful value. Velocity vibration accelerometers are only really effective if
the frequency of vibration is higher than 2Hz but more ideally 5 Hz.
W H AT
S I G N AL C O N D I T I O N I N G D O
I
N E E D F O R M Y AC C E L E R O M E T E R ?
All internally amplified accelerometers need a power supply be it a constant
current IEPE supply, a 4-20mA loop, a 10V bridge excitation or a bipolar +/15V supply for a three wire accelerometer. The output of the accelerometer is
now conditioned to an AC voltage whose amplitude is proportional to the
amplitude of vibration with a frequency the same as the frequency of the
vibration. An AC voltage signal needs further signal conditioning to retrieve any
useful data.
This signal conditioning takes three main forms:
a) Overall voltage levels in either RMS or peak to peak
b) Spectral content analysis
c) Snap shot time domain analysis
Overall acceleration levels in RMS terms
Overall acceleration levels in Peak-Peak terms
18
Breaking the acceleration signal into its frequency components
Viewing the acceleration signal on a storage scope or transient recorder
W H AT
AR E G R O U N D I S O L AT E D AC C E L E R O M E T E R S ?
Ground loops can be a significant problem to all type of sensors where the
signal is un-amplified or the signal levels are low. Ground loops occur when
different parts of the structure lab or building have different electrical grounds.
These grounds may only differ by a few millivolts or less. When areas with
different grounds are connected by sensor cables then unless measures are
taken to prevent it a ground loop are set up in the cable that can be significant
when compared to low level voltage signals that come from the sensor.
19
Ground loops are often very difficult to detect so it is prudent to take
precautions to prevent their effects.
There are a number of ways that ground loops can be prevented. The first is to
hard wire different parts of the structure to ensure that each area has exactly
the same ground.
Preventing ground loops by ensuring all parts of structure have same ground
Ensuring different parts of a plant have the same ground may not be so easy
particularly when long distances are involved or structures carry noise
generating machinery. In these cases it may be better not to eliminate ground
loops but to prevent their effects influencing the sensor output. This can be
achieved by mounting the accelerometer on an electrically isolated mounting
stud. In this way the accelerometer sits on a locally constructed instrument
20
ground and ensures that now ground loop exists between this and the
measuring instrument.
Isolated mounting bases eliminate problems with ground loops
The same effect as mounting the accelerometer on an electrically isolated
mounting base can be achieved by isolating the accelerometer internals from
the outer case of the accelerometer. This is done by the manufacturer.
Mounting the accelerometer on an isolating base or internally isolating the
accelerometer does reduce the stiffness of the accelerometer and therefore
reduces the mounted natural frequency. It is for this reason that not all
accelerometers come automatically with internal isolation.
Internally isolated accelerometers can prevent ground loops but have a reduced
frequency response as a result
21
W H AT
I S AN I S O L AT E D S T U D ?
An accelerometer isolated stud is used in application where the possibility for
ground loops exists which can corrupt the output of the sensor. Isolated studs
do reduce the frequency response of the accelerometer somewhat so caution
should be taken if high frequency data needs to be measured.
HOW
DO
I
I N S T AL L A C H AR G E AM P L I F I E R ?
Charge output accelerometers are used in applications where:
High temperatures environments are encountered
High radiation environments are encountered
Very high frequency accelerometers are used where no room exists for internal
electronics
Charge output accelerometers are self-generating and so no excitation is
required but a local charge amplifier is used to convert the charge output to a
voltage. The charge output accelerometers do however have high output
impedance. This high output impedance makes them susceptible to noise, cable
movement (tribo-electric effect) and low insulation resistance. To minimize
these effects it is important to have; a charge amplifier-impedance converter
mounted as close to the accelerometer as possible, prevent cable movement,
use low noise co-axial cable and ensure all surfaces are kept clean and dry.
22
Installing a charge output accelerometer and clamping low noise co-axial cable
Charge amplifier is located as close to accelerometer as possible but away from
the hostile environment
W H AT
IS THE TRIBO-ELECTRIC EFFECT?
Tribo-electric effect is when a spurious signal is generated by a charge output
accelerometer by the movement of the co-axial cable. To prevent the triboelectric effect the low noise cable needs to be clamped down as close to the
accelerometer as possible. See How do I install a charge amplifier?
Tribo-electric effect
23
HOW
DO
I
C H O O S E T H E S E N S I T I V I T Y O F AN AC C E L E R O M E T E R ?
Accelerometers with integral electronics have a maximum output voltage
determined by the circuit design and the input voltage. The maximum output for
an IEPE accelerometer is typically 4-8 volt s. An accelerometer with a sensitivity
of 100mV/g with electronics that has a maximum output of 5V will obviously
have a dynamic range of +/- 50g while an accelerometer of sensitivity of
10mV/g will have a dynamic range of +/- 500g
If the maximum g levels likely to be experienced is known then dividing this
number by 5 volts will give the maximum sensitivity that should be used to get
this dynamic range
Example; Vibration expected to be seen is 300g. Sensitivity will be 5 divided by
200, which equals 16.6 mV/g. The nearest sensitivity would be a 10mV/g
accelerometer.
W H AT
I S T H E O U T P U T O F AN
IEPE
An IEPE accelerometer is a two-wire sensor that requires a constant current
supply and outputs an AC voltage output on a DC voltage bias. The DC bias is
often removed by the use of a decoupling capacitor.
W H AT
I S AN
FFT?
An FFT is short for Fast Fourier Transform and is an algorithm that is used to
obtain frequency content data from time domain signal. Spectral analysis,
frequency analysis are terms also used to describe obtaining frequency content
data from time domain signals.
24
W H AT
IS CONDITION MONITORING?
Condition monitoring is where the health of a rotating machine is monitored
using vibration levels. As the health of a machine (example becomes
unbalanced, fan blades corrode, bearing surfaces degrade) deteriorates so the
amplitude of the vibration the machine generates increases. By monitoring the
vibration levels over a long period of time this gradual deterioration of the
health of the machine can be assessed until the vibration levels get to a point
where the machine needs to be taken out of service and overhauled. Analysis
of the frequency content of the machine vibration signal will indicate not only
that the health of the machine has deteriorated but also root causes can be
attributed to the problem.
Example: An 8 bladed pump running at 6000 rpm (100Hz) will produce a
vibration signal with 100 Hz frequency if it becomes unbalanced, 200 Hz if it
becomes misaligned 800 Hz if the blades become corroded and 43-47 Hz if the
bearings start to go into oil whirl.
W H AT
FREQUENCY RESPONSE DO
I
W AN T F R O M M Y AC C E L E R O M E T E R ?
The frequency response of the accelerometer needed for testing depends on
what frequencies of vibration are required to be measured. An accelerometer
should have a high enough natural frequency as to capture all the frequencies
required to be measured.
25
Natural frequency sufficiently high to capture all frequencies in signal
Problems start to arise however when the vibration content of the acceleration
to be measured gets close to the natural frequency of the accelerometer.
Frequencies to be measured approach the natural frequency of the
accelerometer
In these instances distortion of the acceleration by the high gains seen near the
natural frequency can give a false picture of the reported acceleration
amplitudes at high frequencies.
26
Acceleration signal is misrepresented by non-unity gain of the higher
frequencies
To overcome this problem one of two things needs to happen:
a higher frequency accelerometer needs to be used
A higher natural frequency accelerometer solves the problem of measuring high
frequency accelerations
If the higher frequencies are not required to be measured then using a low pass
filter should filter them out.
27
A low pass filter removes high frequency components of the measured signal
W H AT
T Y P E O F AC C E L E R O M E T E R B E S T S U I T S M Y AP P L I C AT I O N ?
Accelerometer Type
Single ended
compression
Isolated base
compression
Shear
Charge output
Piezo-resistive
Strain Gage based
Advantages
Robust
Highest natural
frequency
High shock resistance
Robust
High natural frequency
Disadvantages
Poor base strain
characteristics
Best base strain
performance
Best temperature
transients immunity
Smallest size
High temperature
operation
Suitable for radiation
environments
Small size
Measures down to zero
Hz
Measures down to zero
Hz
High shock resistance
Less robust
Lower shock
resistance
Better base strain
performance
Requires local charge
amplifier
Susceptible to triboelectric effect
Limited high frequency
response
Limited high frequency
response
28
CALIBRATION
29
WHY
SHOULD
I
C AL I B R AT E ?
Any measurement is subject to degradation due to use, abuse, drift or ageing.
To understand this degradation calibration at regular intervals needs to be
carried out to characterize the instrument after degradation, to restore the
instrument to an ‘as new’ condition as regards its measurement performance
and to reference the measurement to National Standards.
ISO9000 and many other standards specify the maximum period between recalibration as once every two years and more frequently if the instrument
degradation is significant during that period. (Typically 1% degradation) Many
users adopt an annual calibration as the standard interval between calibrations.
Sensotec Note
Many customers adopt the annual calibration but very few do a comparison
between the current calibration and the previous calibration to ascertain the
degree of degradation and determine a suitable re-calibration time period.
W H AT
IS
NIST?
NIST is the National Institute of Standards and Technology, which is the US
federal government agency responsible for the maintenance of National
Standards.
W H AT
IS A
NIST
T R AC E AB L E C AL I B R AT I O N ?
A NIST traceable calibration means that the calibration can be traced by an
unbroken chain of documented steps, comparisons and stated uncertainties
right back to the national standards.
30
C AN
A L O AD C E L L B E M AD E T R AC E AB L E T O
NIST?
No. Only the results produced by that load cell are traceable to NIST provided
that the conditions under which the results are obtained are clearly understood
and under control. For example if a load cell and signal conditioning are
certified by NIST the readings taken by the load cell are not NIST traceable
unless the way the measurements are taken is clearly understood and the
conditions under which they are taken is under control.
C AN
AN O R G AN I Z AT I O N B E
NIST
T R AC E AB L E ?
No an organization cannot be NIST traceable. Only the results of the
organization can be traceable
31
W H AT
IS
A2LA
OR
NVLAP
AC C R E D I T AT I O N ?
In order to reduce the uncertainty of a load cells measurements or results the
calibration needs to be carried out by a competent person using appropriate
calibration equipment and adopting good calibration practices
A2LA and NVLAP are two certifying bodies that audit companies against
ISO17025, which is a standard that ensures competent people, carry out good
calibration practices using good calibration equipment
W H AT
I S U N C E R T AI N T Y ?
Uncertainty is a tolerance band around any measurement result that indicates
the range of results that would be reported if the test were carried out on
infinitely accurate equipment using standards held at NIST or any other
internationally recognized standards.
Uncertainty is used to qualify measurements and their absolute accuracy and is
expressed as a tolerance foe example:
422 lbf
+/- 0.28 lbs. (+/- 0.28 is the uncertainty)
That means that if NIST standards were on infinitely accurate equipment that
the results that would be reported would lie somewhere in the range of 400.28
lbf and 399.72 lbf.
Uncertainty estimations are developed by very detailed mathematical analysis
and by observations
WHEN
I S U N C E R T AI N T Y I M P O R T AN T ?
Uncertainty is important when carrying out critical absolute measurements and
is not important when making relative measurements. When measuring breaking
forces for seat belts it might well be important to know if it breaks at 740 lbs or
760lbs. If however measurements are being made on the force required to
press fit gearbox bearings on an automotive production line the absolute value
of the load may not be as important as to ensure that the same load is applied
every time.
When absolute measurements are stated they should always be qualified by
adding the uncertainty example 28 lbf +/- 0.16 lbf or somewhere a qualification
statement should be stated such as “all readings have an uncertainty of 0.07%.
Uncertainty is very important when calibrating load cells, as calibration is the
time that a cell is checked on how it measures absolute loads and checked
against national standards or loads that are traceable to national standards.
Obviously a load cell that has an uncertainty of +/- 0.15lbf is better than a load
cell that has an uncertainty of +/- 1.2 lbf. This might be because it is a better
32
load cell or alternatively the load cell with the better uncertainty might just have
been calibrated to a higher standard.
HOW
I S U N C E R T AI N T Y M E AS U R E D ?
Uncertainty is not measured but is estimated. Uncertainty estimations are
developed by very detailed mathematical analysis and by observations.
Uncertainty and its determination is a science all of its own and is often very
difficult, complicated and time consuming. Many calibration labs employ people
full time just to determine uncertainty of measurement results obtained within
their facility.
NIST recognizes two main methods of obtaining uncertainty.
Method A
Determined by analysis, measurements and observations of results. All the
contributing elements that give rise to any error in measurement are measured
and the standard deviation obtained. The measurement results of all the
contributing components are then added together using the square root of the
sum of the squares and an overall standard deviation determined. The overall
uncertainty is then stated as either 2 times the standard deviation or 3 times
the standard deviation.
Method B
Determination of the uncertainty by a theoretical and mathematical analysis
rather than by observation.
Sensotec Note
All uncertainty measurements carried out at Sensotec are carried out using
Method A
33
CALIBRATION CLASS LOAD CELLS
34
W H AT
D O E S A L O AD C E L L C AL I B R AT I O N C O N S I S T O F ?
There is no specific standard for load cell calibration except ASTM E74, which
is targeted toward the force testing machines. A number of large companies
and organizations such as Boeing and the USAF also have standards for load
cell calibration. Standards such as ISO 17025 and Z54 lay out guidelines for
calibration but do not specifically address load cells. As a result each
manufacturer has their own standard of load cell calibration. However a typical
calibration will indicate sensitivity while more comprehensive calibration might
indicate linearity, error, best fit straight line hysteresis. A more elaborate
calibration might calibrate the load cell in tension and compression and might
use more data points. All calibrations should be traceable to NIST standards.
W H AT
L O AD C E L L C AL I B R AT I O N S T AN D AR D S H O U L D
I
AD O P T ?
Calibration costs time and money so it is important to adopt a standard that is
comprehensive enough to cover the needs of the application but not so
comprehensive that time and money is spent needlessly. The calibration should
be comprehensive enough to ensure that the uncertainty is four times better
than the system that is to be calibrated.
If measuring the uncertainty is too cumbersome, too complex or insufficient
time can be invested in the project then at least an appreciation of the
uncertainty should be attempted.
For example:
If the test rig on which the load cell is being used can only consistently
reproduce loads to an accuracy of 5lbs then a load cell calibration that ensures
better than 1.25 lbs is likely to be sufficient.
W H AT
AR E I M P O R T AN T P AR AM E T E R S F O R A C AL I B R AT I O N C L AS S L O AD C E L L ?
Any load cell can be a calibration class load cell provided
a) It’s uncertainty is known
b) It is used to calibrate load cells whose uncertainty only needs to be 4 x more
(the generally accepted ratio) than this calibration standard
c) It is only used over part of its measurement range (say 20% to 100%) which
is dictated by the uncertainty of the cell
For example
A 1000 lb load cell with an uncertainty of 0.5 lbs (which means that if it
measures a load and it indicates 760 lbs it could actually be anywhere between
759.5 lbs to 760.5 lbs.) Is used as a reference standard to calibrate other load
cells provided that these cells under test are not required to report loads with a
greater uncertainty of +/- 2 lbs. In addition this reference cell would not be able
to carry out calibrations with loads less than 200 lbs. This lower limit is
35
determined by multiplying the uncertainty by 400 for class A load cells and
2000 for class AA load cells.
W H AT
AR E T H E C H AR AC T E R I S T I C S O F A L O W U N C E R T AI N T Y C AL I B R AT I O N C L AS S L O AD C E L L ?
A low uncertainty calibration class load cell has the following attributes:
a) The cell has a high repeatability (or a low repeatability error)
b) The linearity curve is well known. (It does not have to be highly linear. It just
has to be well known either as a polynomial curve or a look up table)
c) The cell should have a low hysteresis
d) The cell should have very low creep
e) The cell should have low drift
f) The cell should be calibrated with a low uncertainty i.e. with good calibration
practices on good calibration equipment.
g) The read out should be highly accurate and have a good uncertainty
h) The fixturing used should be carefully designed that ensures accurate
calibration with high repeatability
WHY
DOES A
AD AP T E R ?
C AL I B R AT I O N C L AS S
L O AD C E L L H AV E A B AS E P L AT E AN D A C AL I B R AT I O N
In order to get high repeatability (low repeatability error) and low creep from a
calibration class load cell Then the following conditions need to be met:
a) The cell sensing element must be mounted on a flat surface
b) The load cell sensing element must be rigidly fixed to the structure.
Rigidly fixed usually means bolted down
c) The load cell element should be torqued down evenly to avoid distortion
d) Any threads used in the loading path should be pre-tensioned to avoid
thread creep during the load cycle
e) All compressive forces should be applied absolutely perpendicular to the
sensing element and any side loading should be avoided.
f) All tensile forces should be applied to the load cell absolutely
perpendicular to the sensing element.
The calibration adapter and base plate help achieve all of these goals as shown
in the diagram.
36
W H AT
I S T H E U N C E R T AI N T Y O F A
SENSOTEC
L O AD C E L L ?
The uncertainty of a Sensotec load cell is determined by a number of factors
a) The type of load cell
b) The calibration procedure used in its calibration
c) The range of the load cell
d) The calibration test stand that was used to do the calibration
e) If the low uncertainty option was specified at time of manufacture
f) If the load cell includes pull plate and calibration adaptor.
An 10,000 lb imperial class calibration load cell with pull plate and calibration
adapter and SC2000 calibration class signal conditioning may have an
uncertainty of 0.75 lbf or 0.0075%
A 100 lbf model 41 would have an uncertainty of 0.05 lbf or 0.05%
W H AT
U N C E R T AI N T Y S H O U L D M Y C AL I B R AT I O N R E F E R E N C E L O AD C E L L H AV E ?
The simple answer is that a calibration reference load cell should have an
uncertainty that is 4 times better that the load cell it is going to be used in
calibrating. The more complete answer is however that it is not strictly the load
cell that has the uncertainty but the results obtained by that load cell. In order
to get results from a calibration reference load cell the load cell needs a
display, it needs signal conditioning, it needs a calibration stand or means of
loading and it needs a calibration procedure. If you look at the contributing
37
uncertainties in the results obtained from a calibration load cell the largest
contributors to error in descending order are likely to be:
• Fixturing
• Calibration rig or means of loading
• Reference load cell display and signal conditioning
• Reference calibration load cell calibration
• Reference Load cell creep
• Reference Load cell hysteresis
• Reference Load cell linearity
If a low uncertainty calibration result is desired then it makes sense to reduce
individual contributors to the overall uncertainty budget. If the fixturing,
calibration rig, display and signal conditioning are poor it would seem pointless
to spend time and money employing a low (good) uncertainty load cell. If the
fixturing, calibration rig, display and signal conditioning are good then it would
be worthwhile spending time and money getting a low uncertainty calibration
carried out on a low creep, low hysteresis, high linearity load cell.
.
DOES
A L O AD C E L L H AV E T O B E C AL I B R AT E D W I T H I T S D I S P L AY ?
Strictly speaking the answer is no. However when using the load cell the
uncertainty of the signal conditioning cabling and the display unit need to be
determined and added to that of the load cell. In addition the signal
conditioning/display unit needs to be within its annual calibration. It is for these
reasons that the signal conditioning/display unit is often included in the
calibration of the load cells as it a) determines the uncertainty of the load cell
and signal conditioning unit combined and b) ensures that the signal
conditioning unit gets its annual or biannual calibration.
W H AT D O E S T H E
CELLS?
ASTM E74
S T AN D AR D S P E C I F I C AL L Y S AY AB O U T C AL I B R AT I O N C L AS S L O AD
a) It outlines a procedure on how load cells should be calibrated and the
terminology used
b) Load cells should be tested by deadweight testing machines and
hydraulic test machines and specifies the uncertainty of these weights
and how local gravity needs to be determined.
c) It defines how the linearity curve should be defined as a polynomial curve
with a 2 n d order fit but that up to 5 t h order can be used.
d) It states that a calibration should be carried out at 10%, 20%, 30%, 40%
50%, 60%, 70%, 80%, 90%, 100% in ascending loading only. It also
states that if increments in deadweights cannot be obtained to carry out
these percentages that alternatives can be used but need to be specified
in the calibration certificate
e) A calibrated load cell cannot be used below 10% unless weights were
applied below 10% and calibration results obtained.
38
f) A load cell can never be used below 2000 times the uncertainty of the
cell for a class AA load cell and 400x the uncertainty
g) A load cell should never be used below 2% of its full scale output.
h) It specifies good calibration practices to adopt like temperature control
and application of the weights etc.
i) It specifies that after the cell is taken through one 19 point calibration
cycle that the cell should be rotated by 120 degrees and a 19 point
calibration carried out again and then rotated again through 120 degrees
for a third 19 point calibration carried out.
j) The uncertainty of the cell is reported as 2.4 x the standard deviation of
the results obtained during the calibration process.
k) The uncertainty for a class A standard load cell should never exceed
0.25% and 0.05% for a class AA load cell
l) The temperature error over the stated temperature range should not
exceed 0.01% for Class AA load cells and 0.05% for Class A load cells
m) Load cells need to be re-calibrated 1 year after manufacture and then
every 2 years provided the calibration has not changed by more than
0.1%. If the calibration has changed then the cells need to be recalibrated more frequently until a new time interval is established.
n) It specifies a format for the report or calibration certificate
ASTM E74 does not specify the required accuracy of a load cell nor does it
specify linearity or hysteresis
39
GENERAL
40
HOW
DO
I
K N O W W H AT AC C U R AC Y C L AS S T O U S E F O R M Y S E N S O R ?
It depends on the application. How much does error in your project matter? If
you’re filling a tub with water then it may not matter at all. If you’re filling of a
tub with chemicals, the consequence of error could be severe.
If the consequences of error are significant, you should identify all sources of
error and their contribution to total error.
You can then “snap” pressure sensors with different levels of accuracy into your
evaluation and see the impact of sensor error on each.
If you have other error factors that are much larger than the transducer then
upgrading the transducer accuracy may not matter.
Remember to translate the accuracy into hard numbers to get a better
perspective. Is a 1000 PSI pressure transducer with .1% accuracy good
enough? The real question is whether an accuracy of +/- 1 PSI is accurate
enough. Is detecting between 999 and 1001 PSI at a true pressure of 1000 PSI
acceptable? It depends on the application.
You can always opt for the highest accuracy, but it can be more beneficial to
analyze accuracy in the context of the application’s needs, transducer costs
and transducer lead times.
W H AT
IS
SENSITIVITY?
The ratio of change between a transducer’s output and input is known as its
sensitivity. For example, a transducer that produces 1 mV for every 100 psi
has a sensitivity value of .01 mV/psi.
Under ideal conditions, a transducer’s sensitivity value does not change
between zero and full scale. A transducer that produces 1 mV for every 100 psi
would then, under ideal conditions, also produce 2 mV for an applied pressure
of 200 psi, 3 mV for an applied pressure of 300 psi, and so.
A transducer’s ideal sensitivity can therefore be mapped as a straight line, and
the transducer’s sensitivity value, expressed as the ratio of output to input,
then equates to the slope of that line
41
Ideal Sensitivity is Represented as a Straight Line
Notice also that under ideal conditions, there is zero output when there is zero
input.
However, the actual sensitivity of a transducer fluctuates slightly between zero
balance and full scale. Some reasons for this might be due to manufacturing
and materials imperfections, electrical interference, and even the age of the
transducer. In addition, a transducer usually produces some amount of output
even at zero balance. Thus, true sensitivity actually equates to a non-linear
function with a zero offset.
True Sensitivity is Represented as a Curve
Because true sensitivity is non-linear, the true sensitivity value of a transducer
(the ratio of output to input) will not always be the same at any point between
zero balance and full scale. In order for a sensitivity value to be constant, the
sensitivity must be expressed linearly. Most manufacturers use a best fit
straight line to represent sensitivity.
42
Example of a Best Fit Straight Line
The sensitivity value can then be expressed as the slope of the best fit straight
line, which becomes the value quoted on the transducer’s calibration certificate.
Sensitivity as the Slope of the Best Fit Straight Line
Sensotec Note
In some cases, Sensotec uses the slope of a best fit straight line as a
transducer’s quoted sensitivity on its calibration certificate. In other cases,
Sensotec uses the slope of a terminal point straight line. (See What is NonLinearity?)
W H AT
IS
N O N - L I N E AR I T Y ?
In its broadest sense, non-linearity refers simply to a departure from something
that is linear.
43
In the world of transducers, non-linearity is the maximum deviation in output
between a transducer’s sensitivity curve and a linear representation of its true
sensitivity curve drawn between nominal zero and full scale. Non-linearity is
measured on increasing input only, and is expressed as a percent of full scale
output. An example of non-linearity for a transducer is ±0.15% F.S.
Determining non-linearity for a transducer raises a question of how to create
the linear representation of a transducer’s true sensitivity. Often a best fit
straight line, which is based on the least squares method, is employed.
Best Fit Straight Line Compared to Ideal Sensitivity
When a best fit straight line is used, transducer non-linearity is simply the
greatest deviation between the transducer’s sensitivity curve and the best fit
straight line obtained mathematically using the least squares fit method
Non-Linearity Based on Best Fit Straight Line
44
In other cases, a terminal point straight line is used to determine transducer
non-linearity. The terminal point straight line is drawn between nominal zero
and output at full scale.
Terminal Point Straight Line
Terminal point straight line is often a more practical best straight line, as it is
easy to understand and implement. The user simply takes the output at zero
and the output at full scale and assumes a straight line relationship. Using a
terminal point straight line results in a greater (worse) value for non-linearity
than using a best fit straight line obtained mathematically.
Non-Linearity Based on Terminal Point Straight Line
Sensotec Note
Sensotec uses the terms linearity and non-linearity interchangeably. Sensotec
uses the terminal point straight line method and least squares fit best straight
line to determine its transducer’s non-linearity. The datasheets indicate the
method used when quoting specifications.
45
W H AT
IS
HYSTERESIS?
Hysteresis refers to the behavior of a transducer to produce different output
values for a common input point depending on whether applied input is
increasing or decreasing.
Hysteresis is due to the behavioral patterns of metal crystals, which expand
and contract differently. As applied pressure on a transducer increases, the
non-linear representation of the transducer’s output traces its true sensitivity
curve. But as applied pressure on a transducer decreases, the non-linear
representation of transducer output results in a different sensitivity curve.
Hysteresis is then the greatest difference between output readings for a
common input point, one reading obtained while increasing from zero input, and
the other while decreasing from full scale output. The deviation is expressed as
a percent of full scale. An example of hysteresis for a transducer is ±0.10%
F.S.
Hysteresis as the Deviation between Increasing and Decreasing Values
W H AT
IS
S T AT I C E R R O R B AN D ?
Static error band is a performance specification that takes into account the
effects of transducer non-linearity and hysteresis.
The static error band is an error envelope that is determined by drawing two
lines parallel to the best fit straight line (going through normalized zero point)
with a width that is determined by the hysteresis curve. An example of static
error band for a transducer is ±0.04% F.S.
Notice that a best fit straight line, rather than a terminal point straight line, is
used to calculate transducer error band. The best fit straight line must take
into account both curves. Once the best fit straight line has been determined,
two lines, which are both parallel to the best fit straight line, are then drawn
through the points of maximum deviation. The entire region between these
outer lines is known as the static error band.
46
Static Error Band and the Best Fit Straight Line
W H AT
IS
C AL I B R AT I O N F AC T O R ?
Calibration is the process of standardizing an instrument by determining its
deviation from a desired standard. It is through the calibration process that one
obtains the proper correction factors for the transducers deviation. Calibration
is essentially the comparison of transducer outputs when compared to a
reference standard.
Every transducer is shipped with a sensitivity on its calibration certificate so
that the electronic equipment associated with the transducer can be set up
correctly.
Sensotec Note
Sensotec expresses the sensitivity of the transducer by stating a calibration
factor rather than a sensitivity.
The calibration factor for a transducer is the transducer’s output value at full
scale when the output has been normalized (i.e. zeroed). The line drawn
through normalized zero and a transducer’s calibration factor equates to the
best fit straight line of the transducer output. Thus, the transducer’s calibration
factor in effect establishes the transducer’s sensitivity.
47
Calibration Factor Quoted on the Calibration Certificate
WHEN SHOULD I FIT
A
CONNECTOR
OR
I N T E G R AL C AB L E ?
Using a connector makes it easy to disconnect (or reconnect) a sensor’s
cabling which also makes it easier to remove or replace the sensor too. Thus,
using connectors is an excellent choice for temporary sensor applications.
However, in addition to sometimes being more expensive than integral cable
options, connectors introduce a point of vulnerability for potential water,
moisture, and mechanical damage in the connection itself.
Connector version of sensor
Because of size constraints, it is often impractical or impossible to fit
connectors to small sensors; thus, integral cable technology is often the only
option for small sensors. As can be imagined, integral cables are often used
for more permanent installations, where connecting and disconnecting a
sensor’s cabling is not planned. Because the cable is integrated with the
sensor, the points of vulnerability with respect to water, moisture, and
mechanical damage that can occur with connector technology, are eliminated;
on the other hand, integral cables require strain relief protection to prevent the
cable from getting sheared off or ripped out. If an integral cable is ever
damaged, the entire sensor must be repaired or replaced.
Miniature sensor with integral cable
48
While it is possible to fit a connector to a submersible sensor (using a
submersible connector), using an integrated cable is much more cost effective.
Because a submersible sensor (Fig 24c) is typically part of a permanent
installation, an integrated cable becomes a much more practical choice. In
some cases, though, an application might not require true submersibility, but
only a degree of water protection; thus one must be able to identify the true
need.
Submersible Sensor
The following table summarizes the advantages and concerns for connectors
and integral cables:
Connector - Cable Comparison
Connector
Integral Cable
Easy to disconnect
cabling from the
sensor
Ideal for permanent
installations
Easy to replace the
sensor
Often the only option
on small sensors
Ideal for temporary
installations
Cable must be
protected to avoid
damage
More expensive
Strain relief often
required
Submersible
Submersible connectors
are very expensive
Integral cable is a more
cost effective solution
More permanent
installation (by
definition)
Ensure of definition
b e t we e n wa t e r p r o o f a n d
submersible
Point of vulnerability
S u b j e c t t o wa t e r ,
moisture, mechanical
damage
Cable damage means
replacing or repairing
sensor
Connector - Cable Comparison Table
49
W H AT
IS THE
DIFFERENCE BETWEEN SUBMERSIBLE
AN D
W AT E R P R O O F ?
There are many categories of environmental protection against water. For
example, a product might be designed for water protection in various forms,
such as protection from dripping; spraying; splashing; jet spray; immersion;
submersion; and so on. A product with an environmental protection rating
against dripping water might not have sufficient protection against splashing
water or jet spray. Similarly, a product protected against spraying or splashing
might not have sufficient protection against submersion.
Three Environmental Protections for Enclosures
Waterproof is a general term with respect to environmental protection. A
waterproof sensor might actually be rated only against a specific type of water
ingress, such as splashing, dripping, spraying, and so on. A sensor that is
rated against submersion, on the other hand, is probably also protected against
all other forms of water ingress.
Submersible sensors, which are rated by depth of submersion, enjoy the
highest environmental protection against water.
50
W H AT
AR E
NEMA
AN D
IP D E F I N I T I O N S
FOR
E N V I R O N M E N T AL P R O T E C T I O N ?
The National Electrical Manufacturers Association (NEMA) has been developing
standards for the electrical manufacturing industry for more than 70 years.
NEMA’s environmental protection standards, which are used in America, are
expressed numerically as follows:
NEMA Number Definitions: NEMA-#
#
Meaning
1
General Purpose (Indoor)
2
Water Drip Proof (Indoor)
3R
Dust Tight, Rain Tight, and Ice Resistant (Outdoor)
4
Water Tight and Dust Tight (Indoor/Outdoor)
Water Tight, Dust Tight, and Corrosion Resistant
(Indoor/Outdoor)
Indoor Hazardous Locations (Not Applicable to EMS
Equipment)
Industrial Use - Dust Tight and Drip Tight (Indoor)
4X
9
12
13
Oil Tight and Dust Tight (Indoor)
NEMA Number Definitions Table
Thus, an enclosure that is rated as NEMA-4 is both water tight and dust tight,
whether indoors and outdoors.
Europe uses a different system (IP) to express environmental protection for
enclosures. Protection categories are expressed by two numbers. Each
number defines the protection level. The first number refers to a particles
protection; the second number refers to water protection.
51
IP Number Definitions: IP##
1st #
0
1
2
3
4
5
6
Meaning
No Special
Protection
Protected Against
Solid Objects > 50
mm in Diameter
Protected Against
Solid Objects <12
mm in Diameter
Protected Against
Solid Objects <2.5
mm in Diameter
Protected Against
Solid Objects <1
mm in Diameter
2nd #
Meaning
0
No Special Protection
1
Protected Against
Dripping Water
2
Protected Against
Dripping Water When
Tilted Up to 15D C
From Normal Position
3
Protected Against
Spraying Water
4
Protected Against
Splashing Water
Protected Against
Water Jet Spray
Protected Against
Dust Tight
6
Heavy Jet Spray
Protected Against the
7
Effects of Immersion
Protected Against
8
Submersion
IP Number Definitions Table
Dust Protected
5
For example, IP54 is considered both dust protected and splash protected.
Note that there is a difference between the effects of immersion and
submersion. Immersion protection means that an ingress of water will not
cause harmful effects when the enclosure is temporarily immersed in one meter
of water for standard conditions of pressure and time. Submersion, on the
other hand, means that an ingress of water will not cause harmful effects when
the enclosure is continuously immersed in water under more severe conditions,
which are agreed upon by the manufacturer and user.
52
LOAD CELLS
53
W H AT
IS
O V E R L O AD P R O T E C T I O N
ON A
L O AD C E L L ?
Overload protection in a general sense refers to how a system or device is
protected from damage that can result from an input that exceeds a designed
limit. For example, overload protection in an electrical application might
involve using a fuse or circuit breaker to protect the system from a current
overload.
Overload protection on a load cell refers specifically to the means used to
prevent the cell’s diaphragm from deflecting beyond its designed elastic limit.
Without overload protection, a load cell’s diaphragm experiences irreversible
damage under too much applied input.
To achieve overload protection in a load cell, a mechanical stop is inserted to
prevent the diaphragm from deflecting beyond its elastic limit The mechanical
stop bottoms out when excessive load is applied.
Overload Protection on a load cell (in this case mounted between base plate
and load cell)
Load cells that do not have mechanical overload protection enjoy an overload
capacity by default typically 50%. This means that a 100 lbf load cell without
mechanical overload protection can sustain a 150 lbf load without incurring
damage.
Notice that when a load cell’s maximum designed input limit is exceeded, the
load cell’s output does not increase further (the output becomes asymtopic)
Mechanical overload protection lends itself more easily to compression load
cells than to tension load cells. Because of the internal mechanical design of
54
tension load cells, it is difficult to insert a mechanical stop that provides
sufficient overload protection.
Sensotec Note:
Sensotec sensors operate in an elastic range of .002 - .003 inch.
Because of the typical design of most load cells, Sensotec can fit overload
stops on most compression load cells, but on only a few tension load cells.
There are also some load cells that, because of their unique shape, are unable
to have overload stops in either compression or tension.
At Sensotec, every load cell has a slightly different deflection. Because
tolerances on a load cell’s internal dimensions are not tight enough to permit a
generic overload protection stop, any protection stop that is inserted must be
custom made, custom fitted, and then custom tested. This process increases
the cost significantly.
W H AT
IS A
C O M P R E S S I O N O N L Y L O AD C E L L ?
A compression only load cell is a load cell that has been designed specifically
to measure only compression.
Most load cells work in both compression and tension to some degree.
However, some load cells, by virtue of their physical construction, are better
suited to either compression or tension.
Sometimes it is preferable to use a load cell that measures both compression
and tension without enabling its tension measuring capability. Rather than
design a load cell that measures only compression, a load cell capable of
measuring compression and tension can be shipped with calibration simply
carried out only for compression. This type of load cell might be considered a
compression-only load cell, although it is technically a compression and tension
load cell being used only for compression.
Compression-only load cells are usually fitted with a load button to minimize
side loading.
55
The physical design of the load cell is typically better suited for either
compression or tension. Compression-only load cells tend to have a high
diameter to height ratio. Tension-only load cells, on the other hand, tend to
have a high height to diameter ratio.
When a compression and tension load cell is used only for compression,
calibration is provided only for compression loading. It is more expensive to
provide calibration in both directions for a load cell. It is less expensive, on the
other hand, to provide calibration in just one direction (tension only or
compression only).
It is also important to consider which physical attachments will come into play
with a load cell’s application. Some load cells will be better suited for specific
attachments.
W H AT
IS A
T E N S I O N O N L Y L O AD C E L L ?
A tension only load cell is a load cell that has been designed specifically to
measure only compression.
Most load cells work in both compression and tension to some degree.
However, some load cells, by virtue of their physical construction, are better
suited to either compression or tension.
56
Physical design of a tension load cell
Sometimes it is preferable to use a load cell that measures both compression
and tension without enabling its compression measuring capability. Rather than
design a load cell that measures only tension, a load cell capable of measuring
compression and tension can be shipped with calibration simply carried out only
for tension. This type of load cell might be considered a tension-only load cell,
although it is technically a compression and tension load cell being used only
for tension.
Tension Only Application and load cell fitted with rod end bearings
Tension-only load cells tend to be long in shape, and are often fitted with rodend bearings, which minimize side loading.
57
W H AT
IS A
TENSION
AN D
C O M P R E S S I O N O N L Y L O AD C E L L ?
A tension and compression load cell is a load cell that has been designed
specifically to measure both tension and compression. Most load cells work in
both compression and tension to some degree. However, some load cells, by
virtue of their physical construction, are better suited to either compression or
tension. Thus, a tension and compression load cell is one that has been
physically designed in such a way to be able to measure both compression and
tension effectively.
Note that a tension and compression load cell should come with two calibration
factors: one for compression measurement, and one for tension measurement.
Sensotec Note:
Load cells are supplied with tension-only calibration unless the
tension/compression calibration option is ordered.
W H AT
IS A
R O D E N D B E AR I N G ?
Rod end bearings are self-aligning spherical bearings used in tension
applications to prevent side loading. Using male or female attachments, rod
end bearings attach to a static rod. The bearing itself swivels in order to
accommodate the rod’s varying misalignment. Ratings exist for both load
capacity and lubrication requirements. Some rod end bearings can actually be
created with integrated threaded studs in the bearing. Common applications for
58
rod end bearings include linkages, shift control rods, and use with tension load
cells.
Male Rod End Bearing
W H AT
IS A
Female Rod End Bearing
L O AD B U T T O N ?
A load button is a physical feature of a compression load cell. A load button is
a domed shaped loading point that is welded or threaded onto the side of the
load cell that has been designated to receive the applied load. Load buttons
are used to measure compression loading correctly by eliminating the potential
for side loading. A load button is rounded so that the load being measured
always rests on the highest point of the button. A load button ensures that the
entire applied load issues a force in a direction that is perpendicular to the load
cell.
Load Button Ensures a Force Perpendicular to the Load Cell
59
W H AT
IS
L O AD C E L L S Y M M E T R Y ?
Load cell symmetry refers to whether a load cell exhibits the same sensitivity
for both compression and tension usage.
All load cells have a non-symmetry of a greater or lesser extent. The linear
representation for compression in a load cell is not, by default, symmetrical to
the linear representation for tension. Thus, when both sensitivities are
mapped, the sensitivity curves for compression and tension are different and
have a different slope.
Most tension and compression load cells are used in only one direction for an
application so the question of symmetry is not an issue. On some occasions,
though, applications might call for true symmetry in a load cell. An example
might be an application, which measures both compression and tension in a
hydraulic line. In this case, working with a single calibration factor for both
directions might be preferred to having to balance two calibration factors. This
convenience can be achieved if the load cell exhibits near true symmetry.
It is possible to achieve near true symmetry in a load cell. The process,
however, is expensive.
Sensotec Note:
Symmetry can be calculated and minimized for customers where this
phenomenon is an issue. Calibration class load cells have very low symmetry
characteristics.
60
W H AT
I S Z E R O B AL AN C E F O R A
L O AD
CELL?
Zero Balance is the load cell output when no load and proper supply voltage are
applied. Zero balance is expressed as a percentage of full scale. Ideally, the
output would be exactly zero, but it is not due to several factors.
The electrical and mechanical components of transducers have inherent
stresses and offsets during the assembly process. These offset the zero
balance.
Zero offset present during manufacture
We add resistance to ensure satisfactory zero balance for new transducers.
Zero balance resistor added to bridge network
Operating environment and transducer history can change the zero balance.
Factors include temperature, transducer age, and transducer overload.
If customers notice a change in their zero balance, they may want to adjust
their balance using either external instrumentation or local zero adjustments.
61
Annual testing and certification can ensure precise output throughout the life of
the transducer.
W H AT
I S Z E R O B AL AN C E T E M P E R AT U R E E F F E C T ?
Zero balance is the load cell output at no load. This balance changes as
temperature changes
We can minimize the zero balance change due to temperature by inserting
compensating resistors. When a temperature changes drive the transducer
output higher, the changes are also driving the compensation resistor to lower
output.
62
The zero balance offset is described as a percentage of full scale change per
degree Fahrenheit (% F.S. / F) A 10000 lb sensor with .002% F.S. / F will have
a zero shift of 0.2 lbs for every degree change from the calibration reference
temperature.
W H AT
IS
O U T P U T S P AN T E M P E R AT U R E
EFFECT?
Output Span refers to the output between zero and full scale. The output span
is the output value of the sensor at full load (Calibration factor), expressed as
mV/V. A sensor with a calibration factor of 3 mV/V will have an output of 30 mV
at full load if it is being supplied with 10V power.
63
Output span varies with temperature, and we insert span compensating
resistors to minimize this variation.
800 lb load
10V
75 F
Given output
for a given load
at reference
temperature
64
The temperature effect on span is measured as a percentage change in rated
output per degree of temperature change. A 10000 lb load cell with .002 Rdg/F
will exhibit 0.2 lb span shift for each degree of temperature change.
Customers can adjust the zero and span of their outputs by adjusting the zero
and span adjustments on their transducer (If they have internal amplification),
or on the instrumentation that reads the transducers.
WHEN SHOULD I
H AV E
ZERO
AN D
S P AN
AD J U S T M E N T S O N M Y L O AD C E L L ?
Zero and span may shift due to temperature, repeated loading, or sensor aging.
The preferred method to adjust zero and span is through the use of external
instrumentation. This allows users to track the changes they’ve made and revert
back to previous values if needed.
65
Customers may purchase amplified output load cells with zero/span
adjustments directly on the unit. This also allows for zero and span adjustment,
but does not allow the user to track changes or revert to previous values. Once
adjusted, the sensor is no longer calibrated as it was from the factory.
Customers will also want to consider the operating environment of the
transducer when selecting whether or not to use sensors with zero/span
adjustments. If the sensor were going to be inaccessible, then it would be
better to not have adjustments directly on the sensor.
A significant advantage of having the adjustment screws is that users can get
calibrated, precise output by use of annual testing and certification, even as the
transducer wears or ages in service.
W H AT AR E T H E AD V AN T AG E S AN D D I S AD V AN T AG E S O F H AV I N G A S E N S O R I N T E R N AL
AM P L I F I E R O N A L O AD C E L L ?
Systems without internal amplification must receive supply voltage from an
external source, and must send small signals (i.e. 30mV) back to the amplifying
source.
A good sensor output may become distorted by the electrical noise. These
errors can be large and give signal to noise ratios of less than 20.
66
Internal amplification is a good way of reducing the effects of noise. The
internal amplifiers are housed in the same unit as the sensor. This ensures the
signal amplification is accomplished inside the transducer.
This makes the system less vulnerable to electrical noise and creates a higher
signal to noise ratio. The larger output also allows A/D converters to create a
higher resolution output. Because the internal amplifiers are so close to the
sensor, line drops in excitation are eliminated.
67
The amplifier outputs are low impedance, and internal amplifiers, although they
have some ripple associated with them don’t contribute noticeably to system
inaccuracy and signal to noise ratios of 10,000 are not uncommon.
Internal amplifiers may not be feasible under certain conditions. Specifically,
the circuitry in the amp cannot be subjected to extreme temperatures. If sensor
placed in a location inaccessible to users (hazardous environment, small space,
long distance), zero and span adjustments may not be able to be tweaked when
needed. Internal amplifiers increase the overall size of the unit, which may be
concern in some applications.
68
WHY
IS THE OUTPUT OF MY PRESSURE DETECTOR QUOTED IN MV/V?
mV/V output allows you to eliminate much of the error due to power supply
voltage change.
A mV/V output implies that different levels of excitation may be provided to the
transducer. The full-scale output of the transducer varies directly with the
excitation. A sensor with a calibration factor of 3 mV/V will exhibit 30 mV at full
pressure if it is being supplied with 10V power, but only 15 mV at full pressure
if it is being supplied with 5 V.
Output varies with supply voltage. If we don’t know how much the change in
supply voltage affected our output, then we cannot possibly know how much our
change in output was due to an actual change in pressure
Many users monitor transducer output AND power supply excitation. Changes in
output are compared to the supply voltage to discount effects from voltage
69
shifts. Using the mV/V relationship, users can tell how much of their output
change was due to an actual change in pressure
This approach is known as a ratio metric approach because it relies on the ratio
of voltage output to the Calibration Factor (mV/V) to determine pressure.
For example, if we have a 3 mV/V, 100 lb load cell:
Supply Voltage
Load
Output
10 Volts
100 lbs
30 mV
5 volts
100 lbs
15 mV
HOW
DO
I
K N O W W H AT AC C U R AC Y C L AS S T O U S E F O R M Y S E N S O R ?
It depends on the application. How much does error in your project matter? If
you’re filling a tub with water then it may not matter at all. If you’re filling of a
tub with chemicals, the consequence of error could be severe.
If the consequences of error are significant, you should identify all sources of
error and their contribution to total error.
You can then “snap” load cells with different levels of accuracy into your
evaluation and see the impact of sensor error on each.
If you have other error factors that are much larger than the transducer then
upgrading the transducer accuracy may not matter.
Remember to translate the accuracy into hard numbers to get a better
perspective. Is a 1000 lb load cell with 0.1% accuracy good enough? The real
question is whether an accuracy of +/- 1 lb is accurate enough. Is detecting
70
between 999 and 1001 lbs at a true load of 1000 lbs acceptable? It depends on
the application.
You can always opt for the highest accuracy, but it can be more beneficial to
analyze accuracy in the context of the application’s needs, transducer costs
and transducer lead times.
H O W D O E S T E M P E R AT U R E
AF F E C T A
L O AD
CELL?
Temperature causes changes in zero and span shift for load cells.
HOW
D O Y O U C O M P E N S AT E F O R T E M P E R AT U R E I N A
L O AD
CELL?
The characteristics of all transducers vary with temperature. We install
additional components that act opposite of the transducer’s inherent
temperature characteristics. If the transducer’s components increase output
with temperature, the compensation components decrease output, and viceversa.
To select the proper compensation components, we conduct a thorough test.
We vary the pressure and temperature while measuring the output. We use this
data to select components that best compensate for temperature in each
specific load cell.
W H AT
I S T H E T E M P E R AT U R E C O M P E N S AT I O N R AN G E ?
The temperature compensation range is the temperature range in which our
sensor can be operated up to full scale while still maintaining our accuracy
specifications. This is different than the temperature operating range, which is
the temperature range in which the sensor may be operated safely but the
accuracy specifications may not be met.
W H AT
I S T H E T E M P E R AT U R E O P E R AT I N G R AN G E ?
The temperature operating range is different than the temperature
compensation range, and it is critical to understand the difference between the
two. The temperature operation range is the range of temperatures in which our
sensor can be safely operated up to full scale (accuracy specifications MAY
NOT be met). The temperature compensation range is the range in which our
sensor can be operated up to full scale while still meeting our accuracy
specifications.
71
HOW
DO
I
P I C K T H E R I G H T F U L L S C AL E O U T P U T F O R A L O AD C E L L ?
Several factors should be considered when selecting the right full scale output
for a load cell.
What is the maximum transient load your system will see? Transients may
degrade sensor performance if they are too extreme. To evaluate if your
transient is acceptable, compare it to the safe overload rating. The safe
overload rating of the transducer is the maximum load that can be applied
without permanently degrading the sensor’s characteristics.
How often will the load in your system cycle? Transducers, like all mechanical
components, wear over the course of many cycles. This wear can change your
transducer characteristics. One way to extend your transducer’s life over many
cycles is to pick a larger range load cell.
Dynamic loading can be a very significant factor when considering the load
range to choose for your application. See Dynamic loading of load cells.
W H AT
IS THE EFFECT OF
D Y N AM I C
L O AD S O N A L O AD C E L L ?
Dynamic loads on a load cell can have a dramatic effect on your load cell and
in many cases destroy your load cell without you being aware of it. Dynamic
loads are fast acting loads that require high frequency signal conditioning to
detect it because often they happen so fast. If the system is designed correctly
unknown dynamic loads are not a problem. Dynamic loads often occur during
installation when the cell is at its most venerable. Consider a 100lb load cell
sitting on the floor. If a 3/8inch ball bearing weighing less than 2 oz’s is
dropped from a height of 18 inches the load cell experiences a 161 lb load. This
innocuous effect permanently damages the load cell. Any signal conditioning
attached to the load cell would have to have an update rate of 300 updates per
second to detect this high-speed event. Thus a dropped wrench on the load cell
or the dropping of the load cell on the floor from just a few inches can damage
the cell. Damage is dramatically reduced when the forces applied on the load
cell are not generated by hard incompressible surfaces. If the floor is wood or
hammer blow was inflicted by a brass head instead of steel, then forces
generated and damage done is considerably less.
72
HOW
D O E S A L O AD C E L L W O R K ?
At the heart of a load cell is a strain gage. A strain gage is a device that
changes resistance when it is deformed or stressed. The precise positioning of
the gage, the mounting procedure, and the materials used all have a
measurable effect on overall performance of the load cell.
A strain gage is then cemented to the surface of a beam, diaphragm or column
within a load cell. As the surface to which the gage is attached becomes
strained, the fine wires of the strain gage wires expand or compress changing
their resistance proportional to the applied load. In most strain gages four
gages (or sometimes eight gages are used in the making of a load cell.
Multiple strain gages are connected to create the four legs of a Whetstonebridge configuration. When an input voltage is applied to the bridge, the output
becomes a voltage proportional to the load on the cell. The more load that is
applied to the cell the more the bridge becomes unbalance and the larger the
output. This output can be amplified and processed by signal conditioning or
data acquisition equipment. In order to increase sensitivity of the whetstone
bridge all the arms are active and the four strain gages are arranged so that
two arms of the bridge is in compression while the other two arms are in
tension.
W H AT
L O AD R AN G E S H O U L D
I
C H O S E F O R A L O AD C E L L ?
The load range for load cell should obviously be a little more than the maximum
load that the cell will encounter during normal use. Built into all Sensotec load
cells is a 50% overload capability. That means that if a cell is rated to 100lbs
that it can endure 150 lbs before permanent damage is done to the cell. The
73
load cell will not hold specification between 100 and 150lbs. Above the
overload range is a loading region where the cell becomes progressively
damaged. The further into this region, the more damage that is done, until the
cell eventually breaks. Cells that have been loaded beyond 150% are not
always damaged so that they cannot be used but they will permanently have an
electrical offset due to the ‘set’ that the load cell has now taken. This offset can
be easily zeroed out by the load cell electronics. However because of this
permanent damage the cell will have a lower future overload capability
dependent on how much progressive damage was sustained. It is also important
to take into consideration the dynamic loading on a load cell when specifying
the loading range of a load cell. Dynamic loading can be catastrophic to a load
cell. In this demonstration a 3/8 inch ball bearing weighing 2 ozs is dropped
from 1ft onto a 100 load cell. The load cell experiences 140 lbs of load. If the
ball bearing were to have been dropped from any higher the cell would have
suffered irreparable damage.
W H AT
THINGS DO
I
NEED TO CONSIDER WHEN MOUNTING A
L O AD C E L L ?
A load cell will only perform to its specification if it is mounted correctly. Many
times the load cell is blamed for poor measurement performance when in fact
the mounting arrangement is the source of the inaccuracy. In order of for the
load cell to perform correctly the strain gauged element must be uniformly
stressed when under load. In order for this to happen the following conditions
should be met:
Flat surface
The mounting surface for the load cell should be flat, preferably a ground
surface. This ensures that the load cell is strained evenly and that high spots
do not induce an uneven loading on the load cell and therefore uneven stress
levels.
Hard, rigid surface
The mounting surface for the load cell should be hard and rigid, This ensures
that the surface does not distort or bend and twist under loading conditions.
The loading stresses on the surface can be very severe particularly when
miniature load cells are being used. Loads of 40,000 lbs per square inch are
not uncommon. Once again if the surface is hard and rigid it will not distort and
this ensures that the load cell is strained evenly and therefore experiences
even stress levels.
Level surface
The surface should be level so that all the load is applied parallel to the main
axis of the load cell. This ensures no cosine error is induced but also ensures
that side loading does not create problems for the load cell, which will only
have some degree of immunity.
Loading applied parallel to the main axis.
To ensure that the load cell only sees loads that are parallel to the main axis.
In tension load cells rod end bearings can ensure that the load is always
74
applied parallel to the main axis and in compression load cells load buttons
fulfill a similar function.
75
PRESSURE SENSORS
76
HOW DOES
A
B O N D E D F O I L S T R AI N G AG E - B AS E D P R E S S U R E S E N S O R W O R K ?
A bonded foil strain gage-based pressure sensor measures an applied pressure in
one of two ways. In some models, such as miniature pressure sensors, foil strain
gages are bonded to a diaphragm
Gauged diaphragm on a miniature pressure sensor
But in other models, foil strain gages are bonded to an element that is
mechanically connected to a diaphragm, such as by a bolt the strain gages are
strained as applied pressure is transmitted from the diaphragm to the gauged
element.
Gauged element and mechanical transmitter
77
HOW DOES
A
S I L I C O N - B AS E D P R E S S U R E S E N S O R W O R K ?
A silicon-based pressure sensor measures an applied pressure by detecting the
effect of the pressure against a silicon substrate. Strain gages in a silicon-based
pressure sensor are not bonded to, but etched onto a silicon substrate.
Notice that the silicon substrate is not connected to the diaphragm via any
mechanical transmitter; rather, a transmission fluid (oil) is used to transmit the
pressure from the diaphragm to the silicon substrate.
Silicon based pressure sensor
As an applied pressure pushes against the diaphragm, the transmission fluid
presses against the silicon substrate. As the silicon substrate experiences
deformation, the strain gages, which are etched onto the silicon substrate, register
this change.
As with bonded foil strain gage-based pressure sensors, strain gages in a siliconbased pressure sensor are arranged in a whetstone bridge circuit formation. The
bridge circuit detects any change in electrical resistance, which occurs once a
strain gage has been bent due to deformation of the silicon substrate, as a
deflection from the initial zero voltage reading. The deflection means the applied
pressure to the sensor has changed.
Sensotec Note:
Sensotec uses bonded foil strain gages, bonded semiconductor strain gages,
gauged silicon substrates, and sputtered technologies in its pressure sensors
depending on which is most suitable for the application.
78
W H AT AR E A D V AN T AG E S B E T W E E N B O N D E D F O I L S T R AI N G AG E - B AS E D
PRESSURE SENSORS?
AN D
S I L I C O N - B AS E D
Bonded foil strain gauge based pressure sensors are excellent choices for high
pressure and high temperature applications. Strain gage based sensors are also
very flexible in the number of designs they can be used for. This makes them ideal
for short run specials or unique, application specific solutions. Silicon-based
pressure sensors offer a higher frequency response with a higher overload
capability. Silicon-based pressure sensors are often used in low pressure
applications due to their higher sensitivity.
Sensotec Note
Sensotec-specific pressure sensor information is summarized in the following
chart.
Silicon vs. bonded foil sensor comparison chart
W H AT
IS A
G AG E P R E S S U R E S E N S O R ?
A gage pressure sensor is a bonded foil strain gage-based pressure sensor
designed to measure applied pressure referenced to sealed atmospheric
pressure. The atmospheric pressure in a gage pressure sensor, which is sealed
to prevent moisture and other air particles from entering the sensor, always
reflects the atmospheric pressure on the day of manufacture, as opposed to the
current or other date.
79
Gage Pressure Sensor
A gage pressure sensor operates in the same way as a bonded foil strain gagebased pressure sensor. As pressure is applied to a diaphragm, pressure is
transmitted to a gauged element via a mechanical connection, such as a bolt. As
the gauged element is deflected, strain gages, which are bonded to the element,
reflect this as a change in resistance. The bridge circuit identifies the resulting
change in electrical resistance as a change in applied pressure to the sensor.
(Consult the Pressure Reference Chart to compare how the various pressure
sensors reference pressure.)
W H AT
IS A
T R U E G AG E P R E S S U R E S E N S O R ?
A True gage pressure sensor is a bonded foil strain gage-based pressure sensor
designed to measure applied pressure referenced to current atmospheric pressure.
A true gage pressure sensor employs a dual diaphragm. One side of the diaphragm
has the pressure to be measured applied while to second diaphragm, required to
prevent moisture and other air particles from entering the sensor is vented to
reference the current atmospheric pressure.
80
True Gage Pressure Sensor
A true gage pressure sensor operates in the same way as a bonded foil strain
gage-based pressure sensor. As pressure is applied to a diaphragm, pressure
is transmitted to a gauged element via a mechanical connection, such as a bolt.
As the gauged element is deflected, strain gages, which are bonded to the
element, reflect this as a change in resistance. The bridge circuit identifies the
resulting change in electrical resistance as a change in applied pressure to the
sensor.
Sensotec Note
Sensotec is one of only a few companies offering true gage pressure
transducers. It also unique in offering a second diaphragm that protects the
sensor from corrosion and damage by venting to air with humidity and dirt.
W H AT
I S AN
ABSOLUTE PRESSURE SENSOR?
An absolute pressure sensor is a bonded foil strain age-based pressure sensor
designed to measure applied pressure referenced to sealed vacuum, or absolute
zero pressure.
81
Absolute Pressure Sensor
An absolute pressure sensor operates in the same way as a bonded foil strain
gage-based pressure sensor. As pressure is applied to a diaphragm, pressure is
transmitted to a gauged element via a mechanical connection, such as a bolt. As
the gauged element is deflected, strain gages, which are bonded to the element,
reflect this as a change in resistance. The bridge circuit identifies the resulting
change in electrical resistance as a change in applied pressure to the sensor.
WHEN SHOULD
SENSOR?
I
U S E AN AB S O L U T E P R E S S U R E S E N S O R R AT H E R T H AN A G AG E P R E S S U R E
The primary consideration concerns whether the atmosphere plays a role in the
application for which the pressure sensor is being used. If any part of the
application is exposed to the atmosphere, such as an automotive emission
system or in level measurement in an open tank, a gage pressure sensor should
be used. But if the atmosphere has no effect on the application, either type of
pressure sensor can be used. However, when measuring extremely high
pressures, such as those found in hydraulic applications, an absolute pressure
sensor is typically used. (Consult the Pressure Reference Chart to compare how
the various pressure sensors reference pressure.)
82
W H AT
IS A
D I F F E R E N T I AL P R E S S U R E S E N S O R ?
A differential pressure sensor designed to detect a change in pressure as the
difference between two applied pressures. A differential pressure sensor
employs a dual diaphragm with each diaphragm linked to a common central
gauged element (foil based sensors) or silicon substrate (silicon based
sensors). If both applied pressures are equal, both diaphragms are balanced,
and the reading shows zero; but if either diaphragm see a different pressure
from the other this differential pressure is transmitted to the gauged element.
As the gauged element or silicon substrate is deflected so the gages register a
change in resistance. The bridge circuit identifies the resulting change in
electrical resistance as a change in applied pressure to the sensor.
Differential Pressure Sensor
Differential pressure sensors are commonly used to measure the rate of flow in a
pipe. By monitoring the pressure drop on either side of an orifice plate, the mass
flow rate of the fluid in the pipe can be determined.
W H AT
IS A
V AC U U M P R E S S U R E S E N S O R ?
Vacuum can be measured in two ways. By a) a Gage Pressure Sensor measuring
pressure below atmospheric pressure (i.e. referenced to atmospheric pressure) or
b) by an Absolute Pressure Sensor measuring pressure greater than absolute zero
but less than atmospheric pressure. For user convenience the Gage Pressure
Sensor designed for vacuum applications is usually scaled to report a decrease in
pressure below atmospheric pressure as an increase in positive voltage. Thus, at
83
current atmospheric pressure, a vacuum pressure sensor actually reports 0 psi; but
at absolute zero pressure, a vacuum pressure sensor reports the value of the
current atmospheric pressure as psi of vacuum.
Pressure Reference for Vacuum Pressure Sensor
Because vacuum pressure sensors are measuring small pressure changes (max of
15 psi) the output can be scaled inches of water, inches of mercury or in psi.
Sensotec Note:
Sensotec uses the term vacuum gages to define gage pressure sensors scaled in
psi of vacuum or inches of water of vacuum. Absolute gages that are scaled to 15
psi are just considered absolute gages.
W H AT
IS A
B AR O M E T R I C P R E S S U R E S E N S O R ?
A barometric pressure sensor is an absolute pressure sensor that is used to
measure barometric pressure. The full scale output can be calibrated in a number
of different ways but is typically 16 to 32 or 0-30 inches of mercury.
84
Pressure Reference for Barometric Pressure Sensor
WHICH PRESSURE REFERENCE
SHOULD
I
U S E F O R M Y AP P L I C AT I O N ?
It really depends on what the application is. However the best way to answer to this
question is to show all of the various options so that an informed decision can be
made. The Pressure Reference Chart below (See Figure 8a) shows the different
points of reference against which pressure is measured by the various pressure
sensors.
Absolute pressure sensors reference pressure above absolute zero pressure; thus,
absolute pressure sensors are capable of measuring pressure at any level.
Gage pressure sensors reference pressure above a sealed atmospheric pressure;
the atmospheric pressure used is the atmospheric pressure that existed on the day
the sensor was sealed.
85
Pressure Reference Chart
True gage pressure sensors reference pressure above the current atmospheric
pressure; because air can enter and exit through a vent in the sensor, the
atmospheric pressure referenced is today’s atmospheric pressure.
Vacuum pressure sensors reference pressure of a vacuum below atmospheric
pressure; vacuum pressure sensors are scaled to report decreasing negative
pressure as increasing positive voltage.
W H AT
IS
O V E R L O AD P R O T E C T I O N
ON A
PRESSURE SENSOR?
Overload protection on a pressure sensor is similar to overload protection on a
load cell. A mechanical stop is inserted to prevent the sensor’s diaphragm from
deflecting beyond its elastic limit.
Overload Protection as a Mechanical Stop
86
Mechanical overload protection stops are used less often in pressure sensors
than in load cells. Bonded foil strain gage based pressure sensors can have
mechanical overload protection easily fitted; silicon-based pressure sensors,
because of their minute size and unique deflection/deformation process, have
mechanical overload protection fitted less often.
Notice that although silicon-based pressure sensors rarely have mechanical
overload protection that comes with a mechanical stop, they exhibit a higher
overload capacity than non-mechanically protected foil strain gage based
pressure sensors. Silicon-based pressure sensors can sustain a 400%
overload capacity. Bonded foil strain gage based pressure sensors without
mechanical stops can sustain only a 50% (typical) overload capacity.
Thus, a 100 psi silicon-based pressure sensor can sustain a 400 psi applied
pressure without incurring damage. A 50,000 psi bonded foil strain gage
pressure sensor, on the other hand, without a physical overload protection stop,
can sustain a 75,000 psi applied pressure before experiencing irreversible
damage.
W H AT
C O N S I D E R AT I O N S S H O U L D
I
M AK E W H E N M O U N T I N G A P R E S S U R E S E N S O R ?
A Sensotec pressure sensor can be mounted to a pressure line or vessel in a
number of different ways: Male thread, female thread, tapered thread, ‘clean in
place’ connection, flush mount or through hole. The connection will depend on
the application, the fluid of which the pressure is being measured and the
pressure rating of that fluid. Selection needs to be made carefully particularly if
the pressures are high.
Male or female threads are used extensively while tapered threads are used
when the thread is relied upon to perform the sealing.
Clean in place connections ensure that when the fluid is not present in the
system that no crevices exist in the system where fluid can be trapped and
become contaminated when the next lot of fluid is passed through the system.
Typical applications include the food and beverage industry and the medical
industry.
87
Flush mount connections are made when bleed off paths are not practical or
influence the measurement.
HOW
C AN
I
P R O T E C T AG AI N S T W AT E R H AM M E R ?
Water hammer is one of the most common reasons for failure in a pressure
sensor. Water hammer is the phenomenon where a fast moving fluid is suddenly
stopped by the closing of a valve. The fluid has momentum that is suddenly
arrested causes the incompressible fluid to minutely stretch the vessel in which
it is constrained. A large bang is heard large pressure spike is generated. Any
weak part of the system is subject to distortion. The pressure sensor sees this
pressure spike and can easily be and is very often damaged by it. The effects
88
of ‘water hammer’ can be dramatically reduced with the insertion of a snubber
in the pressure line just before the pressure sensor. The snubber is usually a
mesh filter or sintered material that allows pressurized fluid through but does
not allow large volumes of fluid through and therefore pressure spikes though in
the event of water hammer.
W H AT
I S Z E R O B AL AN C E F O R A P R E S S U R E T R AN S D U C E R ?
Zero Balance is the pressure sensor output when no pressure and proper
supply voltage are applied. Zero balance is expressed as a percentage of full
scale. For Gage pressure sensors, zero balance is measured at atmospheric
pressure (0 PSIG). For absolute pressure sensors, it is measured at full vacuum
(0 PSIA). Ideally, the output would be exactly zero, but it is not due to several
factors.
The electrical and mechanical components of transducers have inherent
stresses and offsets during the assembly process. These offset the zero
balance.
89
Zero offset present during manufacture
We add resistance to ensure satisfactory zero balance for new transducers.
Zero balance resistor added to bridge network
Operating environment and transducer history can change the zero balance.
Factors include temperature, transducer age, and transducer overload.
If customers notice a change in their zero balance, they may want to adjust
their balance using either external instrumentation or local zero adjustments.
Annual testing and certification can ensure precise output throughout the life of
the transducer.
W H AT
I S Z E R O B AL AN C E T E M P E R AT U R E E F F E C T ?
Zero balance is the pressure sensor output at 0 pressure. This balance changes
as temperature changes
90
We can minimize the zero balance change due to temperature by inserting
compensating resistors. When a temperature changes drive the transducer
output higher, the changes are also driving the compensation resistor to lower
output.
91
The zero balance offset is described as a percentage of full scale change per
degree Fahrenheit (% F.S. / F) A 10000 PSI sensor with .002% F.S. / F will
have a zero shift of .2 PSI for every degree change from the calibration
reference temperature.
W H AT
IS
O U T P U T S P AN T E M P E R AT U R E
EFFECT?
Output Span refers to the output between zero and full scale. The output span
is the output value of the sensor at full pressure (Calibration factor), expressed
as mV/V. A sensor with a calibration factor of 3 mV/V will exhibit 30 mV at full
pressure if it is being supplied with 10V power.
92
Output span varies with temperature, and we insert span compensating
resistors to minimize this variation.
93
The temperature effect on span is measured as a percentage change in rated
output per degree of temperature change. A 10000 PSI sensor with .002 Rdg/F
will exhibit .2 PSI span shift for each degree of temperature change.
Customers can adjust the zero and span of their outputs by adjusting the zero
and span adjustments on their transducer (If they have internal amplification),
or on the instrumentation that reads the transducers.
WHEN SHOULD I
H AV E
ZERO
AN D
S P AN
AD J U S T M E N T S O N M Y P R E S S U R E D E T E C T O R ?
Zero and span may shift due to temperature, repeated loading, or sensor aging.
The preferred method to adjust zero and span is through the use of external
instrumentation. This allows users to track the changes they’ve made and revert
back to previous values if needed.
Customers may order pressure detectors with zero/span adjustments directly on
the unit. This also allows for zero and span adjustment, but does not allow the
user to track changes or revert to previous values. Once adjusted, the sensor is
no longer calibrated as it was from the factory. Customers will also want to
consider the operating environment of the transducer when selecting whether or
not to use sensors with zero/span adjustments. If the sensor is going to be
inaccessible, then it would be better to not have adjustments directly on the
sensor.
A significant advantage of having the adjustment screws is that users can get
calibrated, precise output by use of annual testing and certification, even as the
transducer wears or ages in service.
94
W H AT AR E T H E AD V AN T AG E S AN D D I S AD V AN T AG E S O F H AV I N G A S E N S O R I N T E R N AL
AM P L I F I E R O N A P R E S S U R E D E T E C T O R ?
Systems without internal amplification must receive supply voltage from an
external source, and must send small signals (i.e. 30mV) back to the amplifying
source
What may start out as clean power can become degraded because of electrical
noise between the excitation and the sensor.
95
A good sensor output may become distorted by the electrical noise as well
The internal amplifiers are housed in the same unit as the sensor. This ensures
the power to the sensor and the signal amplification are accomplished inside
the transducer.
This makes the system less vulnerable to electrical noise and creates a higher
signal to noise ratio. The larger output also allows A/D converters to create a
higher resolution output. Because the internal amplifiers are so close to the
sensor, line drops in excitation are eliminated. The amplifier outputs are low
impedance, and internal amplifiers don’t contribute noticeably to system
inaccuracy.
96
Internal amplifiers may not be feasible under certain conditions. Specifically,
the circuitry in the amp cannot be subjected to extreme temperatures. If sensor
placed in a location inaccessible to users (hazardous environment, small space,
long distance), zero and span adjustments may not be able to be tweaked when
needed. Internal amplifiers increase the overall size of the unit, which may be
concern in some applications.
WHY
IS THE OUTPUT OF MY PRESSURE DETECTOR QUOTED IN MV/V?
mV/V output allows you to eliminate much of the error due to power supply
voltage change. A mV/V output implies that different levels of excitation may be
provided to the transducer. The full scale output of the transducer varies
directly with the excitation. A sensor with a calibration factor of 3 mV/V will
exhibit 30 mV at full pressure if it is being supplied with 10V power, but only 15
mV at full pressure if it is being supplied with 5 V.
97
Output varies with supply voltage. If we don’t know how much the change in
supply voltage affected our output, then we can not possibly know how much
our change in output was due to an actual change in pressure
Many users monitor transducer output AND power supply excitation. Changes in
output are compared to the supply voltage to discount effects from voltage
shifts. Using the mV/V relationship, users can tell how much of their output
change was due to an actual change in pressure
98
This approach is known as a ratio metric approach because it relies on the ratio
of voltage output to the Calibration Factor (mV/V) to determine pressure.
For example, if we have a 3 mV/V, 100 PSI sensor:
Supply Voltage
10 Volts
5 volts
Pressure
100 PSI
100 PSI
Output
30 mV
15 Mv
99
TORQUE SENSORS
100
W H AT
IS TORQUE?
Torque is a measure of the forces that cause an object to rotate. This
measurement is a combination of two concerns: the amount of force applied to
cause an object to turn on an axis; and the distance between the object’s axis
and the point at which the force is applied. Torque is the product of these two
measurements -- a force and a length -- expressed in pound-feet or footpounds. When torque is very small, it is expressed in ounce-inches or inchounces.
Consider a bolt that must be tightened. If 20 pounds of force is applied to the
end of a two-foot wrench, then a torque of 40 foot-pounds has been applied.
Because torque relies on two interrelated measurements -- a force and a length
-- a proportional relationship exists when one measurement increases or
decreases. For example, if one doubles the distance between the axis of
rotation and the point at which an unchanging force is applied, the torque is
also doubled. Similarly, if the distance is halved, the torque is also halved.
Torque is sometimes referred to as a moment, and the distance between an
object’s axis of rotation and the point at which a perpendicular force is applied
to turn the object is referred to as the moment arm. Thus, an application that
requires a torque of 100 foot-pounds also requires a moment of 100 footpounds.
There are two types of torque: reaction torque and rotary torque. (See What is
reaction torque? and What is rotary torque?)
W H AT
I S R E AC T I O N T O R Q U E ?
Reaction torque is the force required to turn an object that is not free to rotate
about an axis. For example, using a screwdriver to drive a screw into very hard
wood or metal requires reaction torque to turn the bit as resistance is
encountered.
101
W H AT
I S R O T AR Y T O R Q U E ?
Rotary torque is the force required to cause something that is free to rotate to
do so (rotate) continuously. For example, a propeller shaft or transmission
shaft requires rotary torque to rotate continuously (360º).
Some other examples of rotational torque include industrial motor drives and
gear reducers. Rotary torque is differentiated from reaction torque. (See What
is reaction torque?)
HOW
D O Y O U M E AS U R E R O T AR Y T O R Q U E ?
Rotary torque is the force required to cause something that is free to rotate to
do so continuously (360º). Rotary torque can be measured by a rotary torque
sensor. A rotary torque sensor, or transducer, measures rotary torque by
converting torque into an electrical signal. Selecting a torque sensor for your
application depends upon a number of considerations, such as long-term
reliability, physical constraints, portability, and budget.
One way to measure rotary torque is to strain gage the shaft.
In this process one or more strain gages are bonded directly to the shaft that
rotates. As the shaft deforms due to applied torque, so does the resistance in
the bonded foil strain gage. A Wheatstone bridge converts the resistance
change into a calibrated output signal. Direct torque sensor measurement is
generally preferred to remote or indirect methods of calculating torque.
102
Another way to measure rotary torque is to mount a transducer in the machine
train as an in-line pre-gauged sensor.
Wireless torque cells consist of a rotary torque transformer connected in-line
with standard industry flanges. The rotary torque transformer is coupled to the
stationary portion of the assembly by wireless transmission. A stationary loop
antenna induces power into an embedded antenna on the rotating torque cell.
The induced power supplies the excitation voltage to the strain gages and
powers the radio transmitter mounted in the torque sensor. The radio
transmitter then modulates the strain signal for transmission to a stationary
antenna. Still another way to measure rotary torque is to use a clamp on collar.
A clamp-on torque cell is a pre-calibrated bending beam mounted between two
collars that clamp onto the shaft. Appropriately spaced knife-edges provide an
accurate, reliable shaft torque measurement without marring or modifying the
shaft. A clamp-on torque cell handles shaft diameters from 3 to 32 inches, and
as much as 100,000 hp.
HOW
D O R O T AR Y T O R Q U E S E N S O R S W O R K ?
There are two types of rotary torque sensors. Some rotary torque sensors,
such as wireless models, are non-contacting.
103
Wireless rotary torque sensors, which are based on radio telemetry, are reliable
and easy to install. Wireless rotary torque sensors are more expensive than
contacting torque sensors; however, because wireless rotary torque sensors
are non-contacting, they do not require support bearings or mechanical
contacting parts. As a result, maintenance is eliminated.
A stationary antenna induces power in a loop antenna on the rotating shaft.
The power from the rotating shaft antenna is conditioned and excites the strain
gages. A shaft-mounted radio transmitter then sends the measurement signal
back to the stationary antenna.
The telemetry antennas need to be somewhat flexible for ease of mechanical
installation. Receivers should also have adjustments for peak coupling of the
antenna for maximum induced power and received signal strength. The radio
antenna gap is normally less than 3/4 in.
Other rotary torque sensors are contacting. Slip rings are often used in
contact-type torque sensors to apply power to and retrieve the signal from
strain gages mounted on the rotating shaft. However, slip rings are susceptible
to wear. Maintaining an oil-free slip ring is not always easy in many industrial
applications. Slip ring brushes, as well as the support bearings that are
internal to these torque sensors, eventually wear out.
In-line rotary torque transformers are ideal for measuring torque when
transducers are mounted in line with the rotating shaft. These consist of a
strain gage torque cell having a calibrated output and inductively coupled to the
stationary windings on the assembly by the rotary transformer. The rotary
transformer couples the strain gages for power and signal return.
The rotary transformer works on the same principle as any conventional
transformer except that either the primary or secondary coils rotate. The rotary
transformer is simple and easy to use, and is usually applied to smaller
104
machine trains. Rotary transformers have some susceptibility to noise and
require bearings for support; thus, maintenance is required. The act of
mounting the in-line transducer also changes system dynamics and can mean
the torque values themselves may change.
WHEN
SHOULD
I
C H O O S E AN I N - L I N E S E N S O R ?
In-line torque cells maintain system dynamics by offering high torsional
stiffness. They are pre-calibrated, and most have an internal calibration
system that supplies a calibrated output signal to adjust instrument span in the
absence of a known static torque.
The wireless telemetry feature also eliminates support bearings and their
maintenance. However, in-line torque systems require cutting the shaft or
lengthening the machine train to accommodate the inserted in-line transducer.
Thus, if cutting or lengthening the shaft is not a feasible option, another
measuring approach must be employed. (See What are the major differences
between the various forms of torque measurement?)
WHEN
SHOULD
I
C H O O S E A C L AM P O N C O L L AR ?
Because a clamp-on torque cell is based on wireless telemetry, it has the
inherent advantages of a non-contact system.
A clamp-on torque cell is immune to oil and dirt. However, unlike in-line
sensors, clamp-on torque cells do not require cutting or lengthening the rotating
shaft. The clamp-on feature also allows the torque measuring system to be
moved to other similar installations easily in less than 30 minutes. Thus, a
clamp-on torque cell is ideal when torque or horsepower monitoring forms part
105
of the final check-out of multiple machines. (See What are the major differences
between the various forms of torque measurement?)
WHEN
SHOULD
I
S T R AI N G AG E M Y S H AF T AS M Y T O R Q U E T R AN S D U C E R ?
When strain gages are bonded to the shaft, the shaft becomes the transducer.
The general guideline for strain gauging a shaft is that the applied torque must
induce at least 150 to 175 micro-strain. The shaft must also be calibrated, a
process that usually involves loading the shaft statically and tabulating the
results. Calibrating a shaft is easy to do in small systems, but as loads and
shaft size increase, it becomes an onerous task. Other concerns associated
with strain gauging a shaft involve selecting a location for the strain gages,
mounting the strain gages onto the shaft, and protecting the strain gages from
damage through the application itself. Any of these tasks can create problems
for users who are inexperienced in such techniques. Therefore, outside
contractors are usually available through the torque sensor suppliers for most
applications and locations. (See What are the major differences between the
various forms of torque measurement?)
106
W H AT AR E T H E M AJ O R D I F F E R E N C E S B E T W E E N T H E V AR I O U S F O R M S O F T O R Q U E
M E AS U R E M E N T ?
The following chart compares the various forms of torque measurement:
Comparison of Torque Measurement Techniques
In-line Sensor
Clamp-on Collar
Strain Gauged
Shaft
Based on wireless
telemetry
Based on wireless
telemetry
Strain gages are
bonded directly to
the shaft
No support bearings
eliminate
maintenance
Immune to oil and
dirt effects
The shaft
becomes the
transducer
Requires cutting the
shaft or lengthening
the machine train to
accommodate the
inserted in-line
transducer
Does not require
cutting or
lengthening the
rotating shaft
Must select a
location for the
strain gages,
mount the strain
gages, and
protect the strain
gages
Pre-calibrated
Can be moved to
other similar
installations easily
in less than 30
minutes
The shaft must be
calibrated
Ideal application
when torque or
horsepower
monitoring forms
part of the final
check-out of
multiple machines
Generally
preferred to
remote or indirect
methods of
calculating torque
107

Frequently Asked Questions

Transcription

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