DEMODULATORS IN THE SERVO SYSTEM

As you know, servo systems use both ac and dc servo motors depending upon the requirements of the system. Systems that are required to move light loads at constant speed use ac motors. Systems that are required to move heavy loads with a wide speed range use dc motors. When the requirements of the system call for a dc motor or other dc devices, the ac error signal within the servo system must be converted to a dc error signal before being fed to the dc servo amplifier. The conversion is made by the circuit known as a DEMODULATOR.

As with the modulator, the demodulator maintains the same relationships between its input and output signals. Just like the modulator, the demodulator's output amplitude is proportional to its input signal and its output polarity is determined by the phase of the input signal. These relationships, as in the modulator you just studied, are necessary so the "new" error signal will control the servo motor in the desired manner.

Diode Demodulator

One example of a servo demodulator is the DIODE DEMODULATOR, sometimes called a phase detector, shown in figure 2-18. This circuit is used in servo systems because it not only converts ac to dc, but it is also able to distinguish the phase of the ac signal by comparing it to a reference voltage. Do not confuse this circuit with other phase detector circuits, such as those used in radar or communications systems. This demodulator (phase detector) distinguishes signals that are either in phase or 180 out of phase. For this reason this circuit is useful in servo systems where the ac output from the error detector (CT) is either in phase with the reference signal or 180 out of phase. Whatever type of error detector is used in the servo system, the reference voltage to the error detector and to the demodulator must be IN PHASE with each other for the demodulator to do its job.

Figure 2-18. - Diode demodulator.

As shown in figure 2-18, the anodes of the two diodes are supplied with the same reference voltage.

With no ac error input signal applied to T₂ (quiescent state), both diodes will conduct equally on the positive half-cycle of the reference voltage. The voltage drops across R₁1 and R₂ are equal. This results in the two output terminals being at the same potential; therefore, the output voltage is zero for the positive half-cycle. During the negative half-cycle, a negative voltage is felt on the anodes of both diodes, both 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 180. 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 CR₂ 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 180 phase reversal between the input and reference signals. For instance, when the error signal applied to T₂ is 180 out of phase with the reference signal, CR₂ conducts and CR₁ 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:

Flat gain versus Frequency response over the broad band of frequencies of interest. 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). A low output impedance. 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.

AC SERVO MOTORS

Large ac motors are too inefficient for servo use. To move large loads, the ac motor draws excessive amounts of power, and is difficult to cool. Hence, ac servo motors are used primarily to move light loads. Most of the ac servo motors are of the two-phase or split-phase induction type. Fundamentally, these motors are constant-speed devices, although their speeds can be varied within limits by varying the amplitude of the voltage to one of the motors stator windings. When the load becomes heavy, the workhorse dc servo motor is used.

DC SERVO MOTORS

The control characteristics of dc servo motors are superior to those of ac servo motors. The dc servo motor can control heavy loads at variable speeds. Most dc servo motors are either the permanent magnet type, which are used for light loads, or the shunt field type, which are used for heavy loads. The direction and speed of the dc motor's rotation is determined by the armature current. An increase in armature current will increase the motor's speed. A reversal of the motor's armature current will change the motor's direction of rotation More thorough explanations of ac and dc motors are given in NEETS Module 5, Introduction to Generators and Motors.

SYNCHRONIZING CIRCUITS

As we explained in chapter 1, the use of a multi-speed synchro transmission system increases the accuracy of data transmission. The accuracy of the servo system depends in part upon the accuracy of the input fed from the synchro system. For example, a dual-speed synchro system operating in conjunction with a servo system uses two CTs (one coarse and one fine) to define a quantity accurately. This is done by feeding the output of the COARSE CT to the servo amplifier when the system is far out of correspondence and then shifting to the output of the FINE CT when the system is within 2 or 3 degrees of synchronization. A circuit that will perform this job is known as a SYNCHRONIZING NETWORK.

A synchronizing network (also called a crossover or switching network) senses how far the servo load is from the ordered position and then switches either the coarse signal or the fine signal into control. The signal selected by the circuit is the input to the amplifier. The selection is based on the size of the error signals the circuit receives. The coarse signal is the predominant factor in the selection, since it is a measure of the servo's output position throughout its limit of motion. The coarse signal drives the system into approximate synchronization, and then the fine signal is shifted into control.

Semiconductor-Diode Synchronizing Network

The SEMICONDUCTOR-DIODE SYNCHRONIZING NETWORK is fairly common and typical of the type used in servo systems. Let's take a look at a circuit that uses this technique. Figure 2 - 19 is an illustration of the circuit. In the following explanation, we will assume that the system is far out of correspondence (more than 3). At this time, the coarse signal is large in amplitute. With this condition, CR₃ and CR₄, or CR₅ and CR₆, will be forward-biased, depending upon the polarity of the input signal. This will cause current to flow through R₁. The voltage developed across R₁ is felt on one leg of the summing newtork. A large amplitude fine signal CANNOT be present in the summing network, because CR₁ and CR₂ are designed to limit the fine amplitude to a small value. With this condition present at the summing network, the coarse signal maintains control and drives the load toward correspondence.

Figure 2 - 19. - Semiconductor diode synchronizing network.

When the load is within 3 of correspondence, the coarse signal is no longer large enough to forward bias the coarse diode network. The effect of this is to cause a large impedance across the diode network, which then drops most of the coarse signal. Practically no coarse signal voltage is felt across R₁ and one leg of the summing network. On the other hand, the fine signal is also small at this time, since the load is close to correspondence. Small fine signals are unaffected by CR₁ and CR₂. Therefore, the small fine signal is impressed across the summing network. With the fine signal being the only signal felt at the summing network, it takes control and drives the load to the exact point of correspondence. There are various types of synchronizing circuits used in servo systems. Some applications call for electron tubes, relays, and different types of semiconductor diodes. The theory of the specific type you will encounter in servo equipments will be explained in detail in the equipment's technical manual.

Q.24 What is the purpose of a synchronizing network in a servo system?