TRANSDUCERS

TRANSDUCERS
prepared by g.bancale
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Why we’re here today?
• To learn a technical subject ( the content) in
English (the language)
• Because there is no other way to learn a
language than listening and talking
• Because 90% of technical literature is in English
• Because we’d like you to be competitive with
European students of your age
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Some friendly reminders
• From now till the end of this presentation we
will talk in English only!
• I will speak in plain English and slowly but I
won’t speak in Italian/English and neither in
English/Italian!
• Questions are most welcome … if they are
simple!
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What is a transducer ?
(a fairly good definition)
A transducer is a device which transforms a non-electrical
physical quantity (i.e. temperature, sound or light) into an
electrical signal (i.e. voltage, current, capacity…)
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Where are they used and what for?
• Antenna: is the most basic transducer and can be made from a simple
piece of wire. It converts electromagnetic energy into electricity when it
receives signals and does the opposite when it transmits
• Strain gauges: have a long thin wire attached to a foil backing which is
glued to an object. When the object changes shape, the strain gauge also
changes shape and its resistance changes. The amount of stress or strain
in the object is calculated from this change in resistance
• Accelerometer: which converts the change in position of mass into an
electrical signal. Accelerometers measure the force of acceleration and
deceleration. They are used in car airbags, stability control, hard drives, and
many electronic gadgets.
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…more
• Geiger counter: detects radiation levels by using a transducer called a
Geiger-Muller tube
• Microphone and Speaker. Microphones convert sound pressure waves
into electrical current, while speaker convert electrical current into sound
pressure waves.
• And many many others
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non-electrical physical quantity
electrical signal
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What is its structure
A transducer is made of three blocks:
o Input I/F
o Sensor
o Output I/F
Transducer
Input
I/F
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sensor
Output
I/F
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Where does it fit in the DAQ
Transducer
Conditioning +
filtering
Sampling
A/D
Control logic
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P
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Important parameters of a transducer
• Static response: how does it respond to slowly
variable signals, is it precise and accurate
• Dynamic response: how does it respond to
quickly variable signals (bandwidth of control
system, tr, ts !!!)
• Environmental factors: how these factors are
affecting transducer performance
• Reliability: MTBF
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Precision
Accuracy
Precise
Not precise
Precise
Not precise
not accurate
accurate
+ accurate
not accurate
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TRANSDUCERS
• Temperature transducers
▫ Thermocouples
▫ Resistance - Temperature Detectors (RTD)
▫ Thermistors
• Resistive position transducers
• Displacement transducers
• Strain gauge
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TRANSDUCERS
• Temperature transducers
▫ Thermocouples
▫ Resistance - Temperature Detectors (RTD)
▫ Thermistors
• Resistive position transducers
• Displacement transducers
• Strain gauge
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What is a Thermocouple
It’s a temperature sensitive device which works
thanks to the Seebeck effect:
“a voltage is generated in a circuit containing
two different metals by keeping the junctions
between them at different temperatures”
Estonian physician Thomas Seebeck (1770–1831)
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Pros and Cons in temperature
measuring using Thermocouples
• Pros
▫ They are inexpensive.
▫ They are rugged and reliable.
▫ They can be used over a wide temperature range.
• Cons
▫
▫
▫
▫
low output voltage
low sensitivity
non-linearity
electrical connections.
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How does a thermocouple look like ?
Here it is!
please note the two wires (of two different metals) joined in the
junction.
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How does a thermocouple work ?
High impedance voltmeter !
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In normal operation, cold junction is
placed in an ice bath
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What types of thermocouples can we
have ?
temp. range (°C)
•
•
•
•
•
Type K
Type J
Type E
Type N
Type T
: Chromel - Alumel
-270 / 1370
: Iron-Constantan
-210 / 1050
: Chromel -Constantan -270 / 790
: Nicros -Nisil
-260 / 1300
: Copper-Constantan -270 / 400
It is important to note that thermocouples measure the temperature
difference between two points, not absolute temperature
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More features:
• Type K
‘General Purpose' and low cost thermocouple, very popular
• Type J
Limited range (-40 to +750°C), less popular than type
K.
• Type E
High output (68 mV/°C)  well suited to low
temperature (cryogenic) use
• Type N
High stability and resistance to high temperature
oxidation, designed as an 'improved' type K, it’s
becoming more popular.
• Type T
They are used for moist or sub-zero temperature
monitoring applications because of superior corrosion
resistance
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Alloys used:
• Constantan:
55% Copper and 45% Nickel
• Cromel:
90% Ni + 10% Cr
• Alumel: 95% Ni + 2% Mn + 2% Al + 1% Silicon
• Nicrosil: 14.4%Cr +1.4 Silicon + 0.1% Mn + Ni
• Nisil:
same as Nicrosil but different %
How much are thermocouples ?
• Type K Thermocouple (Exposed wire, fiberglass insulated) Tip
Diameter: 1.5 mm Tip Temperature: -60 to +350 °C Price $9.90
• Type K Thermocouple (Insertion Probe) Tip Diameter: 3.3 mm Tip
Temperature: -50 to +250 °C Price $39.60
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Magnitude of thermal EMF
The temperature is usually expressed as a polynomial function of
the measured voltage. Sometimes it is possible to get a decent linear
approximation over a limited temperature range.
2
1
2
2
E  c(T1  T2 )  k (T  T )
where
c and k = constants of the thermocouple materials
T1 = the temperature of the ‘hot’ junction
T2 = the temperature of the ‘cold’ or ‘reference’ junction
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Measurement circuit: issues
We would like DVM to read only V1, but the voltmeter created two more metallic junctions: J2 and
J3  voltmeter reading V will be proportional to the temperature difference between J1 and J2 
we cannot find the temperature at J1 unless we first find the temperature of J2.
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The circuit will provide accurate readings, but it is desirable to eliminate the ice bath
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One way to do this is to replace the ice bath with another isothermal block
Cu
Fe
C
Cu
C
directly measures the
temperature of the
isothermal block (the
reference junction) and
use that information
to compute the
unknown temperature
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Last but not least: reference junction compensation circuit
The AD580 is a threeterminal, low cost,
temperature
compensated,
voltage reference which
provides a fixed 2.5 V
output for inputs between
4.5 V and 30 V.
AD590 is an integrated-circuit temperature
transducer which produces an output current
proportional to absolute temperature.
The device acts as a high impedance constant
current regulator, passing 1µA/oK for supply
voltages between +4V and +30V
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Thermocouple - applications
• Thermocouples are most suitable for measuring
over a large temperature range, up to 1800 K.
• They are less suitable for applications where
smaller temperature differences need to be
measured with high accuracy, for example the
range 0–100 °C with 0.1 °C accuracy. For such
applications, Thermistors and RTD’s are more
suitable.
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TRANSDUCERS
• Temperature transducers
▫ Thermocouples
▫ Resistance - Temperature Detectors (RTD)
▫ Thermistors
• Resistive position transducers
• Displacement transducers
• Strain gauge
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Resistance temperature detectors
(RTDs)
RTDs are temperature sensors that exploit the
predictable change in electrical resistance of
some materials with changing temperature.
Temperature
Metal Resistance
The resistance ideally changes linearly with
temp.
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Principle of operation
RTDs are manufactured from metals whose
resistance increases with temperature.
Within a limited temperature range, this
resistivity increases linearly with temperature:
where:
Rt = resistance at temperature t
R0= resistance at a standard temperature t0
 = temperature coefficient of resistance (°C-1)
PRTs ( platinum RTDs )
• They are the most popular RTD type
• Nearly linear over a wide range of temperatures
• Small enough to have response times of a
fraction of a second
• They are among the most precise temperature
sensors available with resolution and
measurement uncertainties of ±0.1 °C or better
Why Platinum
• Chemical stability
• Availability in a pure form
• Highly reproducible electrical properties
• Platinum probes will read 100Ω at 0°C and at 100°C the
DIN grade (i.e. pure platinum intentionally contaminated with other platinum group
metals)
platinum RTD will read 138.5Ω
• Only platinum RTDs have an international standard ( =
0.00385 Ω/Ω/°C )
How does an RTD look like ?
• Usually they are provided encapsulated in
probes
• They have an external indicator, controller or
transmitter, or enclosed inside other devices
where they measure temperature as a part of the
device's function (i.e. temperature controller,
precision thermostat... )
…more
• price $ 65.00
• Temperature Range: -200 to 260°C
• High-Accuracy
• Platinum Elements
•3-Wire Construction Standard, 2 and
4-Wire Constructions Available
• Wall mount transducer
• Temperature, Temperature/Humidity and
Barometric Pressure Transmitter Models
• Low-Cost Miniature Design
Stylish Design Blends in well with Your
Office, Computer Room or Laboratory Décor
• $46.00 Wall mount RTD sensor
Pros and Cons in temperature
measuring using RTDs
Pros
• stable output for long period of time
• ease of recalibration
• accurate readings over relatively narrow
temperature ranges
…more
Cons (compared to thermocouples)
• Smaller overall temperature range
• Higher initial cost
• Less rugged in high vibration environments
• Active devices requiring an electrical current to
produce a voltage drop across the sensor that
can be then measured by a calibrated read-out
device
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Nickel
Tungsten
Copper
Platinum
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Construction of a PRTs
• The coiled element sensor, made by inserting the helical sensing wires into a
packed powder-filled insulating mandrel, provides a strain-free sensing element.
• All work is done manually under a microscope.
• Strain-free elements required for industrial measurements below –200°C.
•They also insure superior interchangeability and stability to the highest temp.
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…more
•The thin film sensing element is made by depositing a thin layer of platinum
in a resistance pattern on a ceramic substrate.
•A glassy layer is applied for seal and protection.
Specifications
•
•
•
•
•
Wiring configuration (2, 3, or 4-wire)
Self-heating
Stability
Repeatability
Response time
A. Wiring Configuration
Serious lead-wire resistance errors for
2/wire RTD especially in a 100Ω sensor.
•
•
•
If lead wire resistance remains
constant, it can be offset and not affect
the temperature measurement.
If ambient temperature conditions
change, the wire resistance will also
change, introducing errors. If the wire
is very long, this source of error could
be significant.
Two-wire RTDs are typically used only
with very short lead wires, or with a
1000Ω element.
B. Wiring Configuration
(L1+L2+Rt)xR2=R1x(L2+L3+R3)
In a 3-wire RTD, there are 3 leads coming from But: L1=L3 therefore:
the RTD instead of 2. L1 and L3 carry the
(L3+L2+Rt)xR2=R1x(L3+L2+R3)
measuring current, while L2 acts only as a
potential lead. Ideally, the resistances of L1 and L3
are equal and therefore canceled. The resistance in
R3 is equal to the resistance of the sensor Rt at a
given temperature—usually the beginning of the
temperature range. At this point, V out = zero. As
the temperature of the sensor increases, the
resistance of the sensor increases, causing the
resistance to be out of balance and indicated at V
out. Resistances L1 and L3 in leads up to tens of
feet long usually match well enough for 100 ohm
three-wire RTDs.
Self-Heating
• To measure resistance, it is necessary to pass a current
through the RTD
• The resultant voltage drop across the resistor heats the
device ( Joule heating )
• The sensor's indicated temperature is therefore slightly
higher than the actual temperature
• The amount of self-heating also depends heavily on the
medium in which the RTD is immersed. An RTD can
self-heat up to 100x higher in still air than in moving
water, so self-heating specifications are just a
conservative guide.
Stability
• Sensor’s ability to maintain a consistent output when a
constant input is applied.
• Unintended physical or thermal shocks can cause small,
one-time shifts. The material that the platinum is
adhered to, when wound on a mandrel or deposited on a
substrate, can expand and contract differentially to cause
strain incorporated in normal performance but not cause
shifts.
• The stability of platinum RTDs is exceptional, with most
experiencing drift rates < 0.05°C over five years.
Repeatability
• Sensor’s ability to give the same output or
reading under repeated identical conditions
• Absolute accuracy is not necessary in most
applications. The focus should be on the stability
and repeatability of the sensor (i.e. if an RTD in a
100.00°C bath consistently reads 100.06°C, the
electronics can easily compensate for this error)
Response Time
• Sensor's ability to react to a change in temperature, and
depends on the sensor's thermal mass and heat transfer
from the material being tested
• Surface RTDs respond quickly to surface temperature
change
• RTD specifications will list the sensor's time constant,
which is the time it takes for an RTD to respond to a step
change in temperature and come to 63% of its final
equilibrium value.
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TRANSDUCERS
• Temperature transducers
▫ Thermocouples
▫ Resistance - Temperature Detectors (RTD)
▫ Thermistors
• Resistive position transducers
• Displacement transducers
• Strain gauge
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Milestones of Thermistors
THERMal resISTORS
• 1833: negative temperature coefficient of silver
sulphide was first observed by M. Faraday
• Before 2003 only ceramic materials (a mix of
different metal oxides) were used for production
of Thermistors
• After 2003 AdSem started manufacturing of Si
and Ge high temperature NTC Thermistors with
better performance than any ceramic NTC
Thermistors
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Types
• NTC - Negative Temperature Coefficient, used
mostly in temperature sensing
Temperature
semiconductor resistance
• PTC - Positive Temperature Coefficient, used
mostly in electric current control
• Symbol
Key features
• the change in electrical resistance when subjected to a
corresponding change in body temperature is
▫ Predictable
▫ Precise
▫ Stable
• extremely high temperature coefficient of resistance
• typical temperature range of -100° to over +600° F
• Thermistors are generally accepted to be the most advantageous
sensor for many applications including temperature measurement
and control.
THERMISTOR APPLICATIONS
And…
And…
And…
And
How they look like
How they look like
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Thermistor
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Semiconductor Temp. Sensors
• The ordinary semiconductor diode may be used as a
temperature sensor
• The forward biased voltage across a diode has a
temperature coefficient of about 2.3mV/°C and is
reasonably linear
• The bias current should be held as constant as possible
using constant current source, or a resistor from a stable
voltage source
…more
• If two identical transistors are operated at different but
constant collector current densities, then the difference
in their base-emitter voltages is proportional to the
absolute temperature of the transistors
• The transistor sensor is used in diode mode by
connecting the base and collector together.
• VBE changes by approx. -2.2 mV/C°
• The result is a sensor




very low power
sensitive
linear
but electrically and mechanically delicate
IC temperature sensors
• The AD590 and the LM35 have traditionally
been the most popular devices, but over the last
few years better alternatives have become
available.
• They provide a current or voltage output signal
with relatively low output impedance
• Require an excitation power source and are
essentially linear
AD590:Product Description
•The AD590 is a two-terminal integrated circuit temperature transducer
• produces an output current proportional to absolute temperature(1µA/K
which means 298.2 µA  298.2K (25°C)
•The AD590 should be used in any temperature sensing application below
150°C
• low cost
• Linearization circuitry, precision voltage amplifiers, resistance measuring
circuitry and cold junction compensation are not needed in applying the
AD590
•The AD590 is particularly useful in remote sensing applications. The
device is insensitive to voltage drops over long lines due to its high
impedance current output.
LM 35:Product Description
• Calibrated directly in ° Celsius +10.0 mV/°C
• Rated for full -55° to +150°C range
• Suitable for remote applications
• Low cost due to wafer-level trimming
• Operates from 4 to 30 volts
• Low self-heating, 0.08°C in still air
• Nonlinearity only ±¼°C typical
• Low impedance output, 0.1 Ohm for 1 mA load
Price: Qty 1K+ 
$0.73
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TRANSDUCER
• Temperature transducers
▫ Thermocouples
▫ Resistance-Temperature Detectors (RTD)
▫ Thermistors
• Resistive position transducers
• Displacement transducers
• Strain gauge
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Resistive position transducers
Distance
Electrical signal
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Principle of operation of an RPT
• The principle of the resistive position transducer is that
the measured quantity (i.e. the position of an object, or
the distance it has moved) causes a resistance change
in the sensing element.
• One type of displacement transducer uses a resistance
element with a sliding contact linked to the object being
monitored.
• Thus the resistance between the slider and one end of
the resistance element depends on the position of the
object.
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Resistive position transducers
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Eout
R2

E
R1  R2
…more
• The output voltage depends on the wiper
position and therefore is a function of the shaft
position.
• The output voltage Eout is a fraction of E,
depending on the position of the wiper.
• The element is considered perfectly linear if
the resistance of the transducer is distributed
uniformly along the length of travel of wiper.
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TRANSDUCER
• Temperature transducers
▫ Thermocouples
▫ Resistance-Temperature Detectors (RTD)
▫ Thermistors
• Resistive position transducers
• Displacement transducers
• Strain gauge
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Displacement transducers
• Capacitive transducer
• Inductive transducer
• Variable inductance transducer
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Capacitive transducers
• The capacitance of a parallel-plate capacitor is
given by
 o A
C
d
• ε = dielectric constant
εo = 8.854 x 1o-12, in farad per meter
A = the area of the plate, in square meter
d = the plate spacing in meters
• Since C is inversely proportional to d, any
change in d will cause a change in C.
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Capacitive transducers – physical
design
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Inductive transducers (IT)
• Inductive Transducers may be either
▫ Self - generating type transducers
▫ Passive type transducers.
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Self-Generating IT
• Self-Generating IT utilises the basic electrical
generator principle that: when there is relative
motion between conductor and magnetic field, a
voltage is induced in the conductor.
• An example of this is Tachometer that directly
converts speeds or velocity into an electrical
signal.
Tachometers
• Examples of a Common Tachometer
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IT– tachometer with a PM stator
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IT– tachometer with a PM rotor
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Passive type IT (LVDT)
• Passive inductive transducers require an
external source of power.
• The Differential transformer is a passive
inductive transformer, well known as Linear
Variable Differential Transformer (LVDT).
• It consists basically of a primary winding and two
secondary windings, wound over a hollow tube
and positioned so that the primary is between
two of its secondaries.
Linear Variable Differential Transformer
(LVDT)
• Some examples of LVDTs.
Linear Variable Differential Transformer
• An example of LVDT electrical wiring.
Linear Variable Differential Transformer
• An iron core slides within the tube and therefore
affects the magnetic coupling between the
primary and two secondaries.
• When the core is in the centre , the voltage
induced in the two secondaries is equal.
• When the core is moved in one direction of
centre, the voltage induced in one winding is
increased and that in the other is decreased.
Movement in the opposite direction reverses this
effects.
Linear Variable Differential Transformer
• In next figure, the winding
is connected ‘series opposing’
-that is the polarities of V1
and V2 oppose each other
• Consequently, when the core
is in the center so that V1=V2,
there is no voltage output,
Vo = 0V.
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Linear Variable Differential Transformer
• When the core is moved in one direction from
the center, the voltage induced in one winding is
increased and that in the others is decreased.
• Movement in the opposite direction reverse the
effect.
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Variable Inductance Transducers –
operation
Core at the center
V1 = V2
Vo = 0
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Variable Inductance Transducers –
operation
Core moves towards S1
V1 > V2
Vo increase
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Variable Inductance Transducers –
operation
Core moves towards S2
V2 > V1
Vo decrease
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Variable Inductance Transducers –
operation
Thus, the amplitude of Vo is
a function of distance the
core has moved. If the core is
attached to a moving object,
the LVDT output voltage can
be a measure of the position
of the object.
The farther the core moves
from the centre, the greater
the difference in value
between V1 and V2, and
consequently the greater the
value of Vo.
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Advantages of LVDT
• It produces a high output voltages for small
changes in core position.
• Low cost
• Solid and robust - capable of working in a wide
variety of environments.
• No permanent damage to the LVDT if
measurements exceed the designed range.
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TRANSDUCER
• Temperature transducers
▫ Thermocouples
▫ Resistance-Temperature Detectors (RTD)
▫ Thermistors
• Resistive position transducers
• Displacement transducers
• Strain gauge
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Strain gauge
• The Strain Gauge is an example of a passive
transducer that uses electrical resistance
variation in wires to sense the strain produced
by a force on the wire.
• It is a very versatile detector and transducer for
measuring weight, pressure, mechanical force
or displacement.
Strain gauge: how they look like
The construction of a bonded strain gauge shows a
fine wire looped back and forth on a mounting plate,
which is usually cemented to the element that
undergoing stress.
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Strain gauge
From the equation of resistance,
L
R
A
R = resistance
ρ = specific resistance of the conductor material
L = the length of the conductor in meters
A = the area of the conductor in square meters
When a strain produced by a force is applied on
the wires, L increase and A decrease.
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Strain gauge
L
increase
A
decrease
L
From the equation of resistance, R 
A
R
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increase
102
Strain gauge – the gauge factor
R / R
K
L / L
K = the gauge factor
R = the initial resistance in ohms (without strain)
ΔR = the change of initial resistance in ohms
L = the initial length in meters (without strain)
ΔL = the change of initial length in meters
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Strain gauge – the gauge factor
R / R
K
L / L
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Strain gauge – the gauge factor
R / R
K
G
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Stress equation
For many common materials, there is a constant
ratio between stress and strain. Stress is defined
as the internal force per unit area.
F
S
A
S = the stress in kilograms per square meter
F = the force in kilograms
A = the area in square meters
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Young’s modulus
The constant of proportionality between stress
and strain for the curve is known as the modulus
of elasticity of the materials
S
E
G
E = Young’s modulus in kilograms per square
meter
S = the stress in kilograms per square meter
G = the strain (no unit)
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Measuring Circuits
Measuring Circuits
Application & Installation
• The output of a strain gage circuit is a very low-level voltage signal
requiring a sensitivity of 100 microvolts or better.
• The low level of the signal makes it particularly susceptible to
unwanted noise from other electrical devices.
• Capacitive coupling caused by the lead wires' running too close to
AC power cables or ground currents are potential error sources in
strain measurement.
• Other error sources may include magnetically induced voltages
when the lead wires pass through variable magnetic fields, parasitic
(unwanted) contact resistances of lead wires, insulation failure, and
thermocouple effects at the junction of dissimilar metals. The sum of
such interferences can result in significant signal degradation.
Evaluation test on Transducers
Name………………………………………………………….. 5a CE Thursday, 5th March 2009
1. where does a transducer fit in the DAQ (i.e. Data AcQuisition)
2. two important parameters of a transducer are Static and Dynamic response. Can you
explain the difference between these two?
3. explain the principle of operation of a thermocouple
4. are thermocouples suitable for heavy industrial applications? Why?
5. why should you use an ice bath when measuring temperature with a thermocouple?
6. explain the principle of operation of RTD’s
7. are RTD’s and specifically PRT’s precise temperature sensors?
8. how can you minimize lead - wire resistance errors for RTD
9. an ordinary semiconductor diode may be used as a temperature sensor, can you explain
how?
10. LM35 is by far the most popular temperature sensor among students. Can you remember
why?
11. explain the principle of operation of a resistive position transducer
12. LVDT’s (Linear Variable Differential Transformer) are passive type inductive displacement
transducers. Can you explain how they work?
13. what can you measure with a strain gauge?
14. what is the equation that explains the operation of a strain gauge
15. the gauge factor is indicated as K in the following formula. Can you tell the meaning of the
terms which appear in the formula?
K=
ΔR / R
ΔL / L
Please note that:
• all answers must be in English, answers in Italian will not be evaluated
• you are free to use drawings to better explain your answers
• all questions have the same weight, so in order to have a sufficient score you have to
provide correct answers to 9 questions
• you are not allowed to use any book or material with related information, you are allowed to
use English dictionary