NEETS Module 15 — Principles of Synchros, Servos, and Gyros
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decreased, the error signal would increase in amplitude and cause the motor to speed up. In the same way, if
the antenna were to speed up, the tach output would increase, decreasing the error signal and the motor would slow
down. Without the velocity loop to compensate for changing conditions, the load could not respond in the desired
The system shown in figure 2-7 is a simplified version of a velocity loop. In practice, the reaction of
the motor to error voltage and the output of the tach would not be equal (10 rpm per volt and 1 volt per 10 rpm).
This would be compensated for by gearing between the motor and load and between the load and tach, or by using a
summation network in which the resistors (R2 and R3) are riot equal. This use of unequal
resistors is called a SCALING FACTOR and compensates for tach outputs and required motor inputs. This is just
another way of saying that the individual components of the velocity loop must be made to work together so that
each can respond in a manner that produces the desired system result.
Q-9. What are two major
differences between velocity servos and position servos?
Q-10. In a typical velocity servo
block diagram what device is placed in the feedback loop that is not present in the position servo?
What is the advantage of using a closed-servo loop to control load velocity?
The acceleration servo is similar to the two loops we just discussed except that the acceleration of the load is
sensed, rather than the position or velocity. In this loop, the tachometer of the velocity loop is replaced by an
accelerometer (a device that generates a signal in response to an acceleration) as the feedback device.
We have not provided an illustration of the acceleration servo because of the complexity of its applications as
well as its components. This type of servo is widely used in the rocket and missile fields, and is used whenever
acceleration control is required.
Servo characteristics vary primarily with the job the servo is designed to do. There are almost as many types
of servos as there are jobs for servos. All servos usually have the common purpose of controlling output in a way
ordered by the input. Ideally, motion and output shaft position should duplicate the track of the input shaft.
However, this ideal performance is never achieved. We will discuss the major reasons for this, and show some
methods used in the attempt to approach the ideal.
Because a servo compares an input signal with a
feedback response, there will always be a TIME LAG between the input signal and the actual movement of the load.
Also, the weight of the load may introduce an additional time lag. The time lag of the servo can be decreased by
increasing the gain of the servo amplifier. If the gain is set too high, however, the servo output will tend to
oscillate and be unstable. From this you can see that the gain of a servo is limited by stability considerations.
Servo sensitivity must be considered along with stability to reach a "happy medium."
To reduce time lag, the gain of the servo amplifier could be increased. Increasing the gain of the servo
amplifier will decrease the lag time and cause the load to move faster. However, there is a serious drawback
because the load is moving faster, its inertia will likely cause it to go past the desired position
(overshoot). When the load attempts to drive back to the desired position, the high gain of the
amplifier may cause it to overshoot in the opposite direction. Therefore, the system must be stabilized to
minimize or eliminate the problem of overshoot. This is done through DAMPING. Damping can be done by either
introducing a voltage in opposition to the signal voltage or placing a physical restraint on the servo output. The
actual function of this anti-hunting is to reduce the amplitude and duration of the oscillations that may exist in
the system. Every system has one or more natural oscillating frequencies that depend on the weight of the load,
designed speed, and other characteristics.
The degree of damping is determined by the purpose and the use of the system. If the system is
OVERDAMPED, it will not be bothered by oscillations. However, the large amount of restraint placed on the servo
presents an additional problem. This is an excessive time requirement for the system to reach synchronization.
Figure 2-8 is a graphic representation showing the time relationship with regard to degree of damping.
Figure 2-8.—Degree of damping.
An UNDERDAMPED servo system has other traits. The favorable one is its instantaneous response to an error
signal. The unfavorable trait is an erratic operation around the point of synchronization because of the low
amount of restraining force placed on the servo. Somewhere between overdamped and underdamped, there is a
combination of desirable accuracy, smoothness, and moderately short synchronizing time.
The simplest form
of damping is FRICTION damping. Friction damping is the application of friction to the output shaft or load that
is proportional to the output velocity. The amount of friction applied to the system is critical, and will
materially affect the results of the system. Friction absorbs power from the motor and converts that power to
A pure friction damper would absorb an excessive amount of power from the system. However, two
available systems have some of the characteristics of a friction damper, but with somewhat less power loss. These
are the friction clutch and the magnetic clutch.
Q-12. If a position servo system tends to
oscillate whenever a new position is selected, is the system overdamped or underdamped?
Q-13. If a position servo system does not respond to small changes of the input, is the system
overdamped or underdamped?
Friction Clutch Damping
The friction clutch damper uses a friction clutch to
couple a weighted flywheel to the output drive shaft of the servo motor. As the servo motor rotates, the clutch
couples some of this motion to the flywheel. As the flywheel overcomes inertia and gains speed, it approaches the
motor speed. The flywheel, in turning, absorbs energy (power) from the servo motor. The amount of energy stored in
the flywheel is determined by its speed (velocity). Because of inertia, the flywheel resists any attempt to change
As the correspondence point of the system is approached, the error signal is reduced and
the motor begins to slow down. In an attempt to keep the output shaft turning at the same speed, the flywheel
releases some of its energy into the shaft. This causes the first overshoot to be large. When the servo system
drives past the point of correspondence, a new error signal is developed. The new error signal is of opposite
polarity and causes the servo system motor to drive in the opposite direction. Once again the flywheel resists the
motor movement and absorbs energy from the system. This causes a large reduction in the second overshoot and all
subsequent overshoots of the system. The overall effect is to dampen the oscillations about the point of
correspondence and reduce the synchronizing time.
The motor rotation is transmitted to the flywheel
through the friction clutch. The inertia of the flywheel acts as an additional load on the motor. The friction
clutch is designed to slip with a rapid change of direction or speed. This slipping effectively disconnects the
flywheel instantaneously, and thus governs the amount of power the flywheel draws from the motor.
Another type of damper is the MAGNETIC CLUTCH. This type is similar
in function to the friction- clutch damper. The main difference between the two is the method used to couple the
flywheel to the shaft of the servo motor. There are two distinct types of magnetic clutch dampers. The first uses
a magnetic field to draw two friction clutch plates together to produce damping. The action is similar to the
friction clutch we just described.
The second version of the magnetic clutch uses the action of a
magnetic field generated by two sets of coils, or one set of coils and the induced eddy currents, which result
from rotation of the single set of coils near a conducting surface (the flywheel).
Coupling in this type
of clutch is made by the interaction of two magnetic fields without a physical contact between the two. The
two-coil or eddy-current type of magnetic clutch offers smoother operation than a pure friction clutch and has no
problem of wear because of friction.
In summary, a smooth, efficient operating servo system can only be achieved by a system of compromises.
As you recall, earlier we increased the gain of the amplifier to reduce time lag. This had the drawback of
increasing hunting or oscillations about the point of correspondence. We overcame this difficulty through friction
damping. This solved the problem of hunting and smoothed out servo operation but acted as part of the servo load.
It caused a large first overshoot and increased the time lag. Some form of damping that can be used with high
amplification to obtain smooth servo operation and minimum time lag is needed. The answer lies with the use of
Q-14. Why is damping needed in a practical servo system?
Error-rate damping is a method of damping that
"anticipates" the amount of overshoot. This form of damping corrects the overshoot by introducing a voltage in the
error detector that is proportional to the rate of change of the error signal.
This "correction" voltage
is combined with the error signal in the proper ratio to obtain the desired servo operation with reduced
overshooting and minimum time lag.
The advantages of error-rate damping are as follows:
Maximum damping occurs when a maximum rate of change of error signal is present. This normally would occur as the
servo load reverses direction.
2. Since a CHANGE in the signal causes damping, there is a minimum
amount of damping when no signal, or a signal of constant strength, is present.
Error-rate voltages are
generated by either electromechanical devices or electrical networks in the equipment. One electromechanical
device widely used to generate an error-rate voltage is the tachometer generator. As you learned previously, its
output voltage is proportional to the output velocity of the servo. Hence, the output voltage of the tachometer
can be used to anticipate sudden movement changes of the load.
The compensating electrical network used
for error-rate damping consists of a combination of resistors and capacitors forming an RC, differentiating or
integrating network. You should recall that a differentiating circuit produces an output voltage that is
proportional to the rate of change of the input voltage and that an integrating circuit produces an output
proportional to the integral of the input signal.
Figure 2-9 shows a basic RC INTEGRATOR. It can be recognized by the output voltage being taken across the
capacitor. R 1 is added in this circuit to develop the transient error signal (small variation in the signal from
the error detector). The RC integrator is sometimes referred to as an INTEGRAL CONTROL CIRCUIT and will be used to
explain electrical error-rate damping.
Figure 2-9.—Error rate stabilization network using an RC integrator.
The network consists of a capacitor and two resistors connected in series with the servo amplifier. The
components of this circuit are designed to work with a constant or very slowly changing error signal.
Initially, all of the error voltage is divided between R 1 and R2. But the longer the error voltage is applied,
the more C1 charges, and the greater the voltage at the input of the amplifier. Because of the RC time of the
circuit, it takes time for the capacitor to charge to the value of the error input signal. Because of the long
charge time of C1, the circuit can not respond instantaneously to a rapid change in error signal.
What this means is that all error signals will be integrated (or smoothed out). The load will not
respond as quickly. The inertia of the load will be reduced, and the system will be damped.
capacitor, by not responding instantaneously to the error signal, causes the damping action. This action is used
to stabilize the servo system at the new velocity. By tailoring the stabilization network (through the proper
selection of the RC components) to the system's performance requirements and the type of load to be driven,
undesirable load or performance characteristics can be minimized.
The various compensating networks that
you will encounter will depend on the design of the individual servo system and will be covered in the associated
system's technical manual.
In summary, the key to understanding compensating networks is to realize that
components are chosen so the capacitor does not have time to charge and discharge in response to large, rapid
Q-15. Error-rate damping is effective because the circuitry has the capability
the amount of overshoot before it happens.
frequency response of a servo is the range of frequencies to which the system is able to respond in moving the
load. It is a characteristic of the system, chosen by the designers so the system will be able to respond to
whatever frequencies are expected to be present in the input signal for the particular application.
Oscillating Input Signal
At first, we considered the input order to a servo as being suddenly put
at a fixed desired value. Later, we studied the case where the order slowly increased to the desired value.
Actually, the input order to a servo in a given application may accelerate, start, stop, or oscillate about a
fixed point. We will now consider the actions of a servo while the order oscillates. When the order is constant,
oscillations of the load are undesirable. When the order oscillates, the load must oscillate in a similar manner.
Let's assume that an oscillating input signal (order) is applied to a servo. The load may behave in several
ways. Ideally, it would respond in perfect sync with the order. Actually, the amplitude and phase of the load are
different from those of the order, figure 2-10. As we noted above, the frequency response of the system is
normally designed so the load is able to respond to the order.
Figure 2-10.—Frequency response.
A servo may follow the order in amplitude and differ in phase; it may follow the order in phase, and differ in
amplitude; or it may differ in both phase and amplitude.
Bandpass Frequencies in a Servo system
Servos are plagued by noise signals that ride through the system on desired electrical signals. These noise
signals cause roughness in the servo system and must be eliminated to obtain smooth servo operation.
examining the different signals in a servo system, we can determine which frequencies are related
movement of the load and which ones are from noise sources, such as static, motors, harmonics, and mechanical
Filters in the signal circuit can be used to shunt some of the unwanted frequencies away from the amplifier, and
allow only those frequencies that represent load movement to enter the amplifier. This can also be accomplished by
designing the BANDWIDTH of the servo amplifier to accept only the range of frequencies that represents valid servo
signals and to reject all others. This smooths servo response, but has the drawback of reducing amplifier gain.
Reduced amplifier bandwidth is another compromise in achieving optimum servo operation.
In a properly designed servo system that has an oscillating input (order), what should be the response of the
Q-17. What is the advantage of designing a limited bandwidth into a servo amplifier?
SERVO COMPONENTS AND CIRCUITS
In this section we will discuss the circuits and components that make up a typical servo system. We cannot
cover all possible servo applications here because of the vast number of servo system configurations. The circuits
and components discussed in the following pages are the most commonly used and represent a broad view of the
systems used in the Navy today. We have not attempted to put the units into any rigid classification system. We
will mention some of the more common terms used by manufacturers and the Navy to classify the devices to
familiarize you with the wide variety of nomenclatures.
We will be covering much of the electronic
application without discussing the theory of the units. You may want to review some of the applicable NEETS
modules or other sources before or during this discussion. You will find that much of the material necessary to
understand these subjects is contained in the basic theory of electricity and electronics.
A position sensor is a device that changes a mechanical position into a voltage
that represents that position. The output of a position sensor can be either ac or dc voltage. There are many
different kinds of position sensors. In the last chapter you learned about the CX, a synchro device that
represents the position of its rotor by a voltage on its stators. You saw a CX used as a position sensor in a
servo system earlier in this chapter. Other devices can be used as position sensors. The potentiometer is one of
Potentiometer position sensors are generally used
only where the input and output of the servo mechanism have limited motion They are characterized by high accuracy
and small size, and may have either a dc or an ac output voltage. Their disadvantages include limited motion and a
life problem resulting from the wear of the brush on the potentiometer wire. Also the voltage output of the
potentiometer changes in discrete steps as the brush moves from wire to wire. A further disadvantage of some
potentiometers is the high drive torque required to rotate the wiper contact.
A potentiometer is one of
the simplest means of converting mechanical positional information to a proportional voltage. A schematic
representation of a potentiometer is shown in figure 2-11.
Figure 2-11.—A potentiometer.
A potentiometer is a variable voltage divider, with an output voltage that is a percentage of the input
voltage. The amount of output voltage is proportional to the position of the wiper relative to the grounded end.
For example, if the resistance from ground to the wiper is 50% of the total, the output voltage sensed by the load
will be 50% of the total voltage across the potentiometer.
A basic, closed-loop servo system using a
balanced potentiometer as a position sensor is shown in figure 2-12.
Figure 2-12.—Balanced potentiometer used In position sensing.
The command input shaft is mechanically linked to R1, and the load is mechanically linked to R2.
A supply voltage is applied across both potentiometers.
The system is designed so that when the input and
output shafts are in the same angular position, the voltages from the two potentiometers are equal and no error
voltage is felt at the amplifier input. If the input shaft is rotated, moving the wiper contact of R1, an error
voltage is applied to the servo amplifier. This error voltage is the difference between the voltages at the wiper
contents of R1 and R2. The output of the amplifier causes the motor to rotate the load and
the wiper contact of R2. This continues until both voltages are again equal. When the voltages are
equal, the motor stops. In effect, the position of the output shaft has been sensed by the balanced potentiometer.
Q-18. When the input and output wipers of a balanced potentiometer are in the same angular
position, what is the value of the error voltage?
error detectors may be either ac or dc devices, depending upon the requirements of the servo system. An ac device
used as an error detector must compare the two signals and produce an error signal in which the phase and
amplitude will indicate the direction and amount of control, respectively, that are necessary for correspondence.
A dc device differs in that the polarity of the output error signal determines the direction of the necessary
correction. We will discuss in the following paragraphs various devices that are commonly used in servo systems.
Summing networks, as we mentioned earlier, are used as error
detectors in servo applications where the servo output must be proportional to the algebraic sum of two or more
inputs. A typical circuit is shown in figure 2-13.
Figure 2-13.—Summing network as an error detector.
As in the case of potentiometers, the networks may use either ac or dc voltage, with the phase or polarity of
the input voltage determining whether the signals are additive or subtractive. Refer to figure 2-13. If two input
signals E1 and E2 are applied to the network, the network will provide an error voltage
output that is proportional to the algebraic sum of the two signals. The servo motor drives the load and also a
tachometer that supplies feedback voltage to resistor Rf. Resistor Rf nulls the error signal.
In some installations, the servo motor may position the wiper arm of a potentiometer instead of driving a
tachometer to supply the feedback voltage.
The E-transformer is a
type of magnetic unit that is used as an error detector in systems in which the load is not required to move
through large angles.
In the basic E-transformer shown in figure 2-14, an ac voltage is applied to the primary coil (2) located on the
center leg of the laminated, E-shaped core. Two secondary coils (1 and 3) are wound series-opposing on the outer
poles of the core. The magnetic coupling between the primary (coil 2) and the two secondaries varies with the
position of the armature. The armature can be physically moved left or right in the magnetic circuit by mechanical
linkage to the load. This changes the reluctance between either pole and the armature.
Figure 2-14.—Basic-E transformer.
When the armature is located in the center of the E-shaped core, as shown in the figure, equal and opposite
voltages are induced in the secondary coils. The difference between them is zero. Thus, the voltage at the output
terminals is also zero.
But, if the armature is moved, say to the tight, the voltage induced in coil 1
increases, while the voltage induced in coil 3 decreases. The two voltages are then unequal, so that the
difference is no longer zero. A net voltage results at the output terminals. The amplitude of this voltage is
directly proportional to the distance the armature has been moved from its center position. The phase of this
output voltage, relative to the ac on the primary, controls the direction the load moves in correcting the error.
The basic E-transformer will detect movement of the armature in one axis only (either the horizontal or
vertical depending upon the way the unit is mounted). To detect movement in both the horizontal and vertical axes,
a CROSSED-E-TRANSFORMER is used.
If you place two E-transformers at right angles to each other and replace the bar armature with a
dome-shaped one (fig. 2-15), you have the basic configuration of what is known as the crossed-E transformer, or
pickoff. In most applications the dome-shaped armature is attached to a gyro, and the core assembly is fixed to a
gimbal, which is the servo load.
Figure 2-15.—Crossed-E transformer.
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics