Velocity Servo Loop
The VELOCITY SERVO is based on the same principle of error-signal generation as the position servo, but there are some operational differences.
Two major differences are as follows:
This type of servo is used in applications where the load is required to be driven at a constant speed. This speed is governed by the level of the error signal present. Radar antennas, star-tracking telescopes, machine cutting tools, and other devices requiring variable speed regulation are all examples of the types of load this servo may be used to drive.
Figure 2-7 is a block diagram of a velocity servo. It is similar to the block diagram of the position servo loop except that the velocity servo loop contains a TACHOMETER in the feedback line. The tachometer (tach) is a small generator that generates a voltage proportional to its shaft speed.
Figure 2-7. - Block diagram of a velocity servo.
In this application, the tach is used as a feedback device and is designed to produce 1 volt of feedback for each 10 rpm.
Let's assume that the motor is designed to turn 10 rpm for each volt of error signal. Figure 2-7 shows the tach mechanically connected to the load. With this arrangement, the shaft of the tach rotates as the load rotates, and the tach can be said to "sense" the speed of rotation of the load. For purposes of explanation, we win assume that the load is an antenna that we want to rotate at 30 rpm.
Initially, the wiper arm of R1 is set at the 0-volt point (mid-position). This applies 0 volts to the left side of R2. Since the motor is not turning, the load is not being driven, and the tach output is 0 volts. This applies 0 volts to the left side of R3. Under these conditions, 0 volts is felt at the sum point and the motor is not driven. The voltage at the sum point is the error signal. When the wiper arm of R1 is moved to the -9 volt point, an error signal appears at the sum point. At the first instant, the error signal (at the sum point) is -4.5 volts. This is because, at the first instant, the load and tach have not started to move. With the tach output at 0 volts, and the wiper of R1 at -9 volts, -4.5 volts is present at the sum point. This voltage will cause the motor to start to rotate the load.
After a period of time, the load (and tach) are rotating at 10 rpm. This causes the tach to have an output of +1 volt. With +1 volt from the tach applied to the bottom of R3, and -9 volts (from R1 wiper) applied to the top of R2, the voltage at the sum point (error signal) is -4 volts. Since the motor will turn 10 rpm for each volt of error signal, the motor continues to speed up. When the load reaches 30 rpm, the tach output is +3 volts. With this +3 volts at the bottom of R3 and the -9 volts at the top of R 2, the error signal at the sum point is -3 volts. This -3 volts is the voltage required to drive the motor at 30 rpm, and places the system in balance. This satisfies the two conditions of the velocity servo. (1) The velocity of the output is sensed (by the tach), and (2) an error signal (-3 volts) is still present and the load continues to move when the velocity loop is at correspondence (30 rpm).
You may ask why the velocity loop and feedback are necessary. If this motor turns 10 rpm for each 1 volt error signal, why not simply feed -3 volts into this amplifier from the wiper of R1 and not have a tach or summing network?
The answer is that the velocity loop will regulate the speed of the load for changing conditions. If the load in figure 2-7 were a rotating antenna on a ship, the antenna would tend to slow down as the wind opposed its movement and speed up as the wind aided its movement. Whenever the antenna slowed down, the output of the tach would decrease (since the tach is connected to the load). If the tach output 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 manner.
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?
The Acceleration Servo
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 antihunting 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 heat.
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?