SERVO AMPLIFIERS

2-26 diodes are cut off, and zero potential is felt across the output terminals. The circuit will remain in this condition until an ac error signal is applied. As we make this circuit work, you will notice that CR₁ will conduct when the input signal is in phase with the reference voltage and then only on the positive half- cycle. CR₂ will remain in cutoff unless the phase relationship between the ac error signal and the reference voltage changes by 180º. At this time CR₁ will cut off. This change could be brought about by the error detector in the servo system sensing a change in the direction of the load. Effectively, we have a one-diode circuit for one direction of rotation. Assume that an ac error signal is applied to T₂, making the anode of CR₁ positive and the anode of CR 2 negative. At the same time, the reference voltage on the anodes of CR₁ and CR₂ is on its positive half-cycle. Under these conditions, CR₁ will conduct and CR₂ will be cut off. A positive voltage will be developed across Ri and felt on the output terminals. During the negative half-cycle, a negative voltage will be felt on the anodes of CR₁, and CR₂ and will cut them off. The output of the circuit for one complete cycle of the reference signal will be a filtered, pulsating, dc voltage. As long as the input and reference signals are in phase, the circuit acts as a half-wave rectifier and a filter network. As we mentioned earlier, this circuit will also respond to a 180º phase reversal between the input and reference signals. For instance, when the error signal applied to T₂ is 180º out of phase with the reference signal, CR₂ conducts and CR 1 cuts off, causing the output voltage to change polarity. You may encounter variations of the diode phase detector; however, they all depend on the same basic principle of operation. To quickly summarize, the demodulator converted the ac input signal to a dc error signal. The polarity of the dc error signal was determined by the phase relationship between the ac error input signal and the reference signal. The amplitude of the dc error signal was directly proportional to the magnitude of the ac input signal. Q-23. What is the purpose of a demodulator in a servo system? SERVO AMPLIFIERS The servo amplifiers previously discussed were used in servo systems to amplify either the ac or dc error signal to a sufficient amplitude to drive the servo motor. These amplifiers are the same amplifiers in principle as covered in NEETS Module 8, Introduction to Amplifiers. The basic amplifier chosen for use in the servo system must have the following characteristics: 1. Flat gain versus frequency response over the broad band of frequencies of interest. 2. Minimum phase shift with a change in input signal (zero phase shift is desired, but a small amount of phase-shift is acceptable, if constant). 3. A low output impedance. 4. A low noise level. Up to this point in our discussion of servos, the amplifiers have been directly connected to the motor that drove the load. Servo amplifiers are also used within the system itself to amplify the error signal. For example, the signal from the demodulator or filter network may require additional amplification to maintain signal strength. In cases where the amplifier is used to feed large drive motors, to move large loads, the basic electronic amplifier that was presented earlier in this training series is not adequate to do the job. This type of work is done by large power amplifying devices such as the amplidyne generator (NEETS, Module 5, Introduction to Generators and Motors) and the MAGNETIC AMPLIFIER, which we will discuss later in this chapter.