2-18
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
Electrical 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.
