Electronic control of commutator-type machines: Motor speed control with a phase-locked loop

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.The speed-control scheme shown in FIG. 19 is similar to servo systems, feedback circuits, and regulated power supplies in one respect. All of these are “automatic mulling” applications where the error between a sampled portion of the output signal and a reference signal is reduced to zero. In order for the error signal to become zero, the output signal is forced to return to the level from which it attempted to deviate. Thus, in a simple regulated power supply, any tendency of the output voltage to fall produces an error signal because the sampled portion of the output voltage would then differ from the reference voltage. Such an error signal would then in crease the conductivity of the series-regulator transistor, which would in turn in crease the output voltage until the error signal was restored to zero (a tendency for the output voltage to rise would invoke the opposite reaction). A regulated dc power supply is shown in FIG. 20. Its operation provides a good analogy for understanding the phase-locked loop.

FIG. 19 Speed-control by means of a phase-locked loop.

The salient features of the regulated power supply, analog servo systems, and many other self-corrective circuits are:

• Negative feedback is used—a sample of the output is returned to the input.

• At the input, the feedback signal is compared with a reference voltage, and the difference between the two generates an error signal.

• The error signal is amplified and deployed to change the output signal in the direction tending to extinguish the error.

The phase-locked loop is also a self-correcting system. It differs in the following ways from the more familiar analog, or “linear,” system just described:

• The reference signal is an ac signal.

• The error signal is an ac voltage or a digital pulse train. (Departure from the reference frequency is the “error.”)

• Regulation involves the stabilization of an output frequency, which can be made to represent the speed of a motor.

• The basic circuit action is to cause the frequency of the sampled output to be come identical to that of the reference frequency.

The basic phase-locked loop is illustrated in FIG. 21A. This arrangement is of ten found in communications systems. Thus, if an FM signal is applied to the phase comparator, the filtered error signal constitutes the audio modulation. The phase locked loop in such an application serves as a frequency discriminator. The circuit action is such that the output frequency of the voltage-controlled oscillator always seeks the instantaneous frequency of the FM signal. The phase-locked loop there fore displays selectivity and can be used in place of conventional tuned circuits. The error signal is stripped of its high-frequency residue by the low-pass filter. Because of its position in the loop, the voltage-controlled oscillator is caused by the error signal to make its generated frequency identical with that of the incoming signal. It can also be said that the action is such that the error signal tends to extinguish itself, as it does with analog servo systems. Now, consider the adaptation of this basic sys tem to the control of motor speed. FIG. 21B is functionally the same as the integrated circuit system in FIG. 19.

FIG. 20 Error-signal nulling action in a regulated dc supply. The reference and error voltages in this case are dc, but the nulling or servo action is analogous to that taking place in a phase-locked loop where the reference and the error signal are ac. Error signal, is the difference between the feedback voltage and the reference voltage. The error signal always tends to extinguish itself. Feedback path; Unregulated supply; Series-regulator transistor; Regulated dc output; output

FIG. 21 B shows that the electronic voltage-controlled oscillator is replaced by a dc shunt motor coupled to an ac tachometer. The tachometer can be a small ac generator, or it can be an optical encoder formed from a slotted wheel in conjunction with a light source and a photoelectric detector. (This could be an LED and a photo- transistor.) For simplicity, the power supply for the motor armature is not shown.

FIG. 21 Simplified block diagram of phase-locked loop systems. A. Basic phase-locked loop. Returned frequency is locked to that of the input signal; B. Phase-locked loop for speed control of a dc shunt motor. Returned frequency is locked to reference

A number of circuits can provide the function of phase comparator. In FIG. 19, a four-quadrant multiplier within the IC is used as the phase comparator. Other phase comparators are exclusive OR circuits, edge-triggered flip-flops, and LC resonant circuits.

The low-pass filter not only removes high frequencies and transients from the error signal, but also governs the dynamic behavior of the overall system. Because this filter is usually of the simple RC variety or is an active filter designed around an op-amp, the phase-gain characteristics of the overall system are fairly easy to manipulate. The usual objectives are to obtain minimal over-shoot together with fast response to a disturbance, such as a suddenly increased motor load. This condition corresponds to critical damping in conventional servo systems.

This speed-control system has very good potentialities, because extremely close speed regulation is possible with its use. It makes feasible such techniques as coordinating motor speed with digital clocks (which is needed in certain computer peripherals) or synchronizing the speed of several motors in a conveyer system. And once the basic operation of this system is grasped, its use can be extended to other types of motors. Alternating-current motors could conceivably be used if a voltage- controlled oscillator is inserted between the low-pass filter and the power amplifier in FIG. 21B.

Although the circuits of FIG. 19 and 21B control the field of a shunt motor, armature current can also be controlled. Of course, a more powerful amplifier would be needed.

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