The VELOCITY SERVO LOOP 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:
1. In this loop the VELOCITY of the output is sensed rather than the position of the load.
2. When the velocity servo loop is at correspondence or null position, an error signal is still present and the load is moving.
The velocity 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 the velocity servo loop may be used to drive.
The illustration below is a block diagram of a velocity servo loop. 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.
Block diagram of a velocity servo loop.
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. The illustration above 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.
Let's assume that the motor is designed to turn 10 rpm for each volt of error signal. The picture above 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 servo 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 servo loop will regulate the speed of the load for changing conditions. If the load in the illustration above 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 the illustration presented above 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.