INDUSTRIAL SERVO CONTROL SYSTEMS Fundamentals and

Transcription

3
Components of Servos
3.1
HYDRAULIC/ELECTRIC CIRCUIT EQUATIONS
From a basic hardware point of view it is important to understand the
steady-state equations of a hydraulic and electric actuator.
The majority of hydraulic positioning or machine feed drives use a
rotary actuator with a servo valve attached to the motor to minimize the
amount of trapped oil under compression. By minimizing the volume of oil
that is trapped between the servo motor and servo valve, the hydraulic
resonance will be increased and the drive made more stable. The hydraulic
flow equations for a fixed displacement servo motor are shown in Figure 1.
The steady-state direct current (DC) motor equations are also shown
in Figure 1. The voltage and torque equations include the motor back emf
constant (Ke) and the torque constant (KT). These motor constants are used
throughout the basic and advanced portions of this book. Therefore it is
important to know where to find them and how they are used in the
forthcoming servo calculations.
3.2
ACTUATORS—ELECTRIC
It is important from the hardware point of view to be familiar with
manufacturers’ motor specifications to find the design parameters such as
Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved
Fig. 1
Hydraulic and electric motor circuits.
the voltage constant and torque constants that are required for servo drive
calculations. Sample electric drive manufacturer specifications are included
for DC and alternating current (AC) servo drives. DC motor specifications
for Gettys motors are shown in Figure 2. AC (brushless DC) Allen-Bradley
motor specifications are shown in Figure 3. The motor voltage and torque
constants are included in these specifications. For all drive calculations the
torque constants for a hot motor (408C) should be used wherever possible.
Fig. 2 Gettys DC motor data. (Courtesy of Gettys Corp.)
Fig. 3
Allen-Bradley AC motor data. (Courtesy of Rockwell Automation/Allen-Bradley.)
In addition to the motor specifications, it is important to be familiar
with the speed/torque characteristics of a given drive motor. These
characteristics show how much torque can be developed at a given speed
to stay within the rating of the drive. It should be noted that the speed
torque ratings are for the motor/amplifier combination. The speed torque
characteristics for two Gettys DC motors are shown in Figure 4. As the
operating point moves to the right (higher torque) the motor will develop
increasing sparking at the brushes and thus the drive must be derated. These
other operating zones can be used intermittently for drive forcing and
acceleration. If the required torque moves further to the right, beyond the
operating zones, the motor will be damaged. To prevent this from
happening, all commercial servo amplifiers use ‘‘current limit.’’
Speed/torque characteristics for two Allen-Bradley brushless DC
motors are shown in Figure 5. These AC motors do not have the
commutation limits of the DC motors and therefore can be operated at
higher speeds for rated torque conditions. While the DC drive (amplifier and
motor) can use 450% rated torque intermittently, for drive forcing, the AC
drive package can only operate at about 200% rated torque for drive
forcing. As the state of the art in drive amplifiers advances, this 200% limit
for the AC (brushless DC) drive should increase.
3.3
ACTUATORS—HYDRAULIC
Most industrial hydraulic servos are of the servo valve classification.
Hydraulic servo pump designs usually have a large amount of trapped oil,
which results in a low hydraulic resonance and stability problems. Hydraulic
servo valve designs using a piston actuator can also have large amounts of
trapped oil volume for the longer stroke designs. Therefore most industrial
hydraulic positioning or feed drives use rotary actuators. Like electric
motors, the hydraulic servo motors have manufacturer specifications. These
motors also have a torque constant with units of so many inch-pounds per
100 psi pressure. Hydraulic rotary actuators are generally of the piston type
motor or the roll-vane type of motor. A typical specification sheet for a
Hartman roll-vane motor is shown in Figure 6.
The advantage of the hydraulic servo is the large amount of torque
that can be produced. They also have a disadvantage of oil contamination
requiring oil filtration of 10 mm. The oil viscosity can change with operating
temperature, which can affect the stability of the servo. Oil temperature
should be held below 1308F. If the operating temperature exceeds 1408F for
Fig. 4
Gettys DC motor speed/torque data. (Courtesy of Gettys Corp.)
Fig. 5 Allen-Bradley AC motor speed/torque data. (Courtesy of Rockwell
Automation/Allen-Bradley.)
Fig. 6
Hartman hydraulic motor data. (Courtesy of Hartmann Controls Inc.)
a long period of time the oil can start to break down, causing the servo valve
spool to stick, resulting in a stall or runaway condition. Hydraulic pump
noise can also be an annoying condition. From the government Walsh–
Healy act, hydraulic pump noise must be limited to 90 dB on the A scale.
3.4
AMPLIFIERS—ELECTRIC
In Figure 2 for the classification of drives, three classes of amplifiers were
shown: rotary amplifiers, electronic amplifiers, and static amplifiers. Of these
three types of amplifiers the static solid-state amplifier is predominant. The
silicon controlled rectifier (SCR) has been used extensively in industrial
applications. The SCR is the solid-state version of a thyratron. As an
amplifier component the SCR is very rugged and available in very high
current ratings, but by itself it is just a switch that conducts for a part of a half
cycle of the AC power. Therefore, for part of the conduction half cycle no
current flows. The period where no current is flowing is called the off time of
the SCR, which is referred to as the transport lag of the amplifier and is
discussed further in Part II. SCR amplifiers are designed in various circuit
configurations, with the most popular circuit being the three-phase, half-wave
amplifier. For reference, this circuit design for a Gettys DC drive is shown in
Figure 7. Each circuit design for the SCR has an efficiency rating, represented
by the form factor, which is a measure of the amount of ripple current in its
output relative to the average DC value of its output. Thus the form factor is
equal to Irms/Idc. For DC SCR amplifiers it is necessary to derate the drive
rated torque. The rated torque is divided by the form factor. The derating
form factor for various SCR amplifier designs are listed as follows:
SCR amplifiers
Single-phase, full-wave
Single-phase, full-wave/inductor
Three-phase, half-wave
Three-phase, half-wave/
inductor
Pulse width modulation or
brushless DC amplifiers
Derating form factor
1.66
1.2
1.25
1.05
1.0
Another class of DC static amplifiers uses pulse width modulation
(PWM) techniques. These amplifiers use transistors for current control to
the motor. While the PWM amplifier has a high switching rate, allowing for
much higher servo performance, it has much lower current ratings than DCSCR drives. In general, these drives have a maximum rating of 200% rated
current. The form factor of these drives is one, so no derating is required. A
typical block diagram of an Allen-Bradley DC pulse width modulation
amplifier is shown in Figure 8. While these drives have a servo bandwidth of
about 40 Hz, they are limited in velocity because of the mechanical
commutator in the DC motor.
Fig. 7
Gettys DC SCR electric drive diagram. (Courtesy of Gettys Corp.)
Fig. 8 Allen-Bradley DC PWM electric drive diagram. (Courtesy of Rockwell Automation/Allen-Bradley.)
Lastly, the brushless DC drives offer higher speeds and higher
performance. Most industrial versions of this drive use a synchronous AC
motor. A position transducer in the motor measures the armature position
and provides a signal to the amplifier to commutate the three-phase
armature currents. Thus the motors can rotate at higher velocities than the
DC motors. Speeds of 3000–5000 rpm are common. The brushless DC
transistor amplifier also has a high switching rate with a 200% limit of rated
torque. The amplifier and motor can be treated as a DC drive. A typical
block diagram for an Allen-Bradley digital brushless DC drive is shown in
Figure 9.
Fig. 9 Allen-Bradley brushless DC block diagram. (Courtesy of Rockwell
Automation/Allen-Bradley.)
3.5
AMPLIFIERS—HYDRAULIC
Hydraulic servo drive amplifiers take the form of a servo valve. Servo pump
drives are not being considered. For reasons stated in Section 2.2, to
maintain a minimum of trapped oil volume, the servo valve is mounted to the
actuator with a manifold. Oil is supplied to the servo valve from a pump
through a filter, which usually has a rating of 10 mm. Based on maximum
power transfer, the hydraulic pressure at the servo motor through a servo
valve is equal to two-thirds of the pressure at the input to the servo valve. In
general it is assumed there is approximately a 300 psi line loss between the
pump and the servo valve. When selecting a servo valve based on motor
speed and required flow, it is important to allow additional oil flow to
compensate for system leakage. As an index of performance, 30% more oil
flow than required should be used in selecting the flow rating of the servo
valve. The typical servo valve force motor (or torque motor) is shown in
schematic form in Figure 10. The armature and drive arm are suspended on
the flexure tube, which acts as a pivot point and also provides a spring force
tending to keep the armature on center between the poles of the ‘‘C’’ section.
The force motor has two symmetrical flux paths as shown by the dotted lines.
Assuming an electrical current is applied to coil ‘‘A,’’ magnetic flux
will be produced in air gap ‘‘a,’’ which tends to draw the armature toward
that pole. The armature will move a distance such that the restoring force of
the spring suspension (flexure tube) equals the magnetic pull. Thus, the
greater the current in the coil, the greater the magnetic pull, and the further
the stroke. Since the flexure tube acts as a pivot, the tip of the drive arm
strokes in the opposite direction as the armature. The motion at the tip of
the drive arm (also known as the flapper) is the force motor output. In
summary, the force motor produces a displacement output proportional to
differential current. If the coil currents are not the same, then the pull on one
side will be greater and the armature will stroke an amount proportional to
the difference in the two currents. As the flapper strokes one way, the nozzle
pressures A and B will be unbalanced. This causes the spool to move in a
direction to reestablish a balance of pressure at the nozzles. As the valve
spool moves to a new point of equilibrium, oil will flow through the valve to
the motor. Therefore the servo valve can be considered as a positioning
servo where the spool is being positioned to allow oil flow. Servo valve
spools can be made with overlap (creating a dead zone in output flow),
underlap (which can cause system instability), and critical or zero lap.
For most hydraulic servo drive applications, a very small overlap is
used on the spool. Since the 1970s, advances have been made in hydraulic
servo valve and actuator designs to include feedback transducers such as
linear differential transducers (LDT).
Fig. 10
Pegasus hydraulic servo valve diagram. (Courtesy of Schenck Pegasus.)
3.6
TRANSDUCERS (FEEDBACK)
One of the important, if not the most important, elements of an industrial
servo drive is the measuring device. One of the purposes of the feedback
device is to accurately place a tool or workpiece at some desired location,
prior to some machine operation. For the control system to know where the
tool or workpiece is located, some measuring device must be used to provide
this information in a language the control will understand. Measuring
devices in general can be called transducers, a device used to transform one
form of energy to another form of energy. A simple thermostat is an
example of a transducer, where temperature is registered as a physical
bending in a bimetallic strip. For some specified deflection of the bimetallic
strip as the result of a temperature change, an electric circuit will be
activated to run a furnace in the case of a heating control, or a compressor
in the case of a refrigerator. There are many examples of transducers in the
average home, but the type used with numerical control systems will convert
position information from the motion of a machine drive screw or element
into electrical signals. These electrical signals are the feedback information
Fig. 11
Feedback devices.
to the control system in a language the control can understand. A
classification of feedback devices is shown in Figure 11. There are two basic
types of feedback signals, which also define the control system as to the data
information used within the system. The first type of signal is called analog,
and the control is usually referred to as an analog system. Analog signals
have two characteristic features. They directly represent some physical
quantity and are continually varied. A tachometer is an example of an
analog measuring device where the output voltage is directly proportional to
the velocity of input shaft rotation.
The second type of signal is called digital, and the control system is
usually referred to as a digital system. Physical quantities are represented
through the medium of digits or numbers, which would be discrete electrical
pulses in the digital control system. Digital signals or information can be
further defined as of two types: incremental and absolute. Incremental
signals are merely a train of pulses where each pulse has a specific weighted
value such as 0.01 in per pulse. The absolute digital information will take the
form of a pattern of pulses. The pattern can have the form of a binary
numbering code or some special coding.
There are many commercial versions of transducers available. Perhaps
the best known device is the tachometer, which is a small generator,
sometimes producing AC output voltage but more often a DC output
voltage. Tachometers are used to a large extent with velocity regulators.
With brushless DC drives, the velocity feedback is synthetically generated.
Since these drives have a position transducer as part of the motor (used for
commutating motor currents in the amplifier), the position signal is
differentiated in the amplifier creating a synthetic velocity signal to close a
velocity servo loop.
The velocity regulator may not be the complete control system, but it
is quite often part of a positioning system. Transducers used for positioning
systems have two basic forms as analog devices. Probably the simplest
analog device for positioning systems is the potentiometer. As a feedback
measuring device, the potentiometer can be a single-turn or a multiturn
device. One of the most common forms of feedback measuring devices is the
synchro, an AC electromechanical device providing a mechanical indication
of its shaft position for some electrical input, or providing some electrical
output that is some function of the angular position of its rotor shaft.
Another electromechanical feedback device often used is the resolver.
The device is similar in appearance to the synchro, but the electrical
construction is not the same. Both types are a form of variable transformer,
with more or less decoupling between the rotor and stator windings as the
armature rotates. A linear version of the resolver is known as the
Inductosyn. These devices are discussed later in more detail.
The synchro construction has a single winding on the rotor with three
windings on the stator. The stator windings are wound, so that induced AC
voltages in each of three windings from the rotor winding are 1208 apart. In
application, these units are used as a transmitter or receiver in the
transmission of data in position-type servos. A variation of the usual type
synchro is the differential synchro. This device has a set of three windings on
both the stator and rotor. The differential is used to add another mechanical
input to the data transmission system of servo drives.
The resolver is a precision induction-type device acting like a variable
transformer, with the amount of coupling varying as the sine and cosine of
the angular position of its rotor shaft. In control systems, the resolver is
used for coordinate transformation in analog computer applications, in
position servos as applied to industrial controls, and in other control
applications where rectangular conversions to polar coordinate or vice versa
are desired. The rotor and stator have two windings, which are wound at 90
electrical degrees with respect to each other. As with the synchro, the
resolver can be used as a data transmitter or receiver. The resolver has the
advantage that it can transmit data either side of zero degrees (operate in all
four quadrants) whereas the synchro can only operate in quadrant one. In
addition, the same resolver used to transmit or receive AC signals can be
used as a differential unit.
An electrical drawing of a resolver is shown in Figure 12. The resolver
could be considered as a variable coupling transformer, with the amount of
coupling dependent on the angle of the rotor shaft. Considering voltages V1
and V2 as primary voltages, the secondary voltages would be Vr1 and Vr2.
Each secondary voltage would be a function of both primary voltages V1
and V2, plus the angle of the rotor shaft. Therefore, the two secondary
Fig. 12
Resolver circuit.
The difference between the actual angle ya and the desired angle yd is
the position error. However, the input and output of the resolver windings
are electrical voltages, not angles. The input voltages can be a function of
the desired angle, as from the equations for V1 and V2 . The output rotor
voltage is a function of the difference between the actual angle ya and the
desired angle yd . When ya ¼ yd the voltage Vr1 is zero, which is the same as
saying the machine slide is in position. This fact can be determined from the
preceding equations when yd and ya ¼ 0 :
Vr1 ¼ 0 cos 0 þ Eð0Þ ¼ 0
Vr2 ¼ 0ð0Þ E cos 0 ¼ E
ðnot usedÞ
Note: Only one resolver rotor winding is used.
Thus Vr1 satisfies the condition of zero output when yd ¼ ya and it is
therefore used; Vr2 is not used.
It should be apparent that Vr1 can be made zero (the positioning
system null position) by varying either the rotor shaft ya or varying V1 and
V2 as functions of the desired angle yd .
Most of the resolver devices are used as measuring devices coupled to a
machine drive screw. With most industrial machines, the drive screw will
have some backlash between its angular position and the position of the
element it is driving—a machine slide, for example. For this reason, there
has been difficulty in obtaining accurate positioning when measuring from
the machine drive screw. Some manufacturers have designed their controls
so positioning will always occur from the same direction. In general,
measuring from the machine drive screw is not ideal. A considerable effort
has gone forth to create a linear measuring device that can be used right at
the moving machine slide rather than through a drive screw. These devices
are called linear transducers (e.g., Inductosyn) and take the commercial
forms of linear resolvers. With the linear analog measuring devices, either
the stator or rotor is mounted on the moving part, and the other part is
mounted on the stationary machine element. There is an associated
attenuation with these devices, and amplification is required. The linear
digital measuring devices have taken the form of optical grating devices, and
some are magnetic coupling devices that generate a series of pulses, with
each pulse being equal to some given distance.
Some manufacturers have used precision gear racks on the moving
machine element with a rotary measuring device fastened to the stationary
element. This technique obviously is better than measuring off the machine
drive screw. Each of the three methods of measuring—from the machine
drive screw, a precision rack, or a linear transducer—has advantages and
disadvantages in cases of application to a machine. In general, accuracy
The difference between the actual angle ya and the desired angle yd is
the position error. However, the input and output of the resolver windings
are electrical voltages, not angles. The input voltages can be a function of
the desired angle, as from the equations for V1 and V2 . The output rotor
voltage is a function of the difference between the actual angle ya and the
desired angle yd . When ya ¼ yd the voltage Vr1 is zero, which is the same as
saying the machine slide is in position. This fact can be determined from the
preceding equations when yd and ya ¼ 0 :
Vr1 ¼ 0 cos 0 þ Eð0Þ ¼ 0
Vr2 ¼ 0ð0Þ E cos 0 ¼ E
ðnot usedÞ
Note: Only one resolver rotor winding is used.
Thus Vr1 satisfies the condition of zero output when yd ¼ ya and it is
therefore used; Vr2 is not used.
It should be apparent that Vr1 can be made zero (the positioning
system null position) by varying either the rotor shaft ya or varying V1 and
V2 as functions of the desired angle yd .
Most of the resolver devices are used as measuring devices coupled to a
machine drive screw. With most industrial machines, the drive screw will
have some backlash between its angular position and the position of the
element it is driving—a machine slide, for example. For this reason, there
has been difficulty in obtaining accurate positioning when measuring from
the machine drive screw. Some manufacturers have designed their controls
so positioning will always occur from the same direction. In general,
measuring from the machine drive screw is not ideal. A considerable effort
has gone forth to create a linear measuring device that can be used right at
the moving machine slide rather than through a drive screw. These devices
are called linear transducers (e.g., Inductosyn) and take the commercial
forms of linear resolvers. With the linear analog measuring devices, either
the stator or rotor is mounted on the moving part, and the other part is
mounted on the stationary machine element. There is an associated
attenuation with these devices, and amplification is required. The linear
digital measuring devices have taken the form of optical grating devices, and
some are magnetic coupling devices that generate a series of pulses, with
each pulse being equal to some given distance.
Some manufacturers have used precision gear racks on the moving
machine element with a rotary measuring device fastened to the stationary
element. This technique obviously is better than measuring off the machine
drive screw. Each of the three methods of measuring—from the machine
drive screw, a precision rack, or a linear transducer—has advantages and
disadvantages in cases of application to a machine. In general, accuracy
improvements of a factor of 10 can be obtained between measuring from a
machine drive screw and a linear transducer. Measuring at the drive screw
can provide accuracies of one thousandth of an inch when using a good ballbearing screw. Both analog and digital systems are in use, and equally good
results have been obtained with both.
Most process-type control systems and numerical contouring systems
use a single measuring device. It is characteristic of these types of control
systems for the actual position to vary with the varying command position
such as to make the position error of the control system very small. A plot of
a resolver output voltage vs the rotor shaft angle is shown in Figure 14. With
a continuously varying control voltage (V1 and V2 ) the resolver output
voltage seldom gets more than a few degrees away from the resolver null
position.
Digital feedback is used extensively in many industrial servos. The
digital feedback transducers can provide improved accuracy over the analog
resolver or Inductosyn if the resolution of the pulses count per revolution is
high enough (e.g., 500,000 pulses per revolution). For short-travel machine
slides, the absolute digital feedback is practical, but in general most digital
feedback devices used are incremental pulse generators. For maximizing the
accuracy of positioning, the transducer is mounted on the machine slide.
Fig. 14
Resolver output voltage.
Fig. 15
Feedback devices.
Therefore for short-travel machine slides there are many linear digital
incremental feedback devices that can be used. These linear devices are
costly and must be justified. Examples of some of the feedback devices are
shown in Figure 15.

INDUSTRIAL SERVO CONTROL SYSTEMS Fundamentals and

Transcription

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