Phase-locked Loop Servo Systems

Phase-locked Loop Servo Systems

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Phase-locked Loop Servo Systems is a closed-loop control system that is used widely in communication systems for frequency demodulation and bit synchronization. Phase-locked loop servos are also use for velocity control, especially when a high deg...

Phase-locked Loop Servo Systems is a closed-loop control system that is used widely in communication systems for frequency demodulation and bit synchronization. Phase-locked loop servos are also use for velocity control, especially when a high degree of speed regulation and accuracy is desired.

A phase-locked loop in its basic form is represented by the block diagram of Fig. 37. The input signal and the feedback signal are sinusoidal and upon locking both signals will have the same frequency and a constant phase difference. Any deviation from the desired phase difference is detected by the phase detector and is transmitted to the vo;tage-controlled oscillator (VCO) to correct the error. Since the phase difference is constant, the frequencies of the input and the feedback signals are the same. This principle can be applied to servo systems for which the control objective is speed regulation.

In the case of a phase-locked loop servo for speed control, the VCO is replaced by an amplifier-motor-encodercombination. The output of the encoder is a sinusoidal signal with a frequency proportional to the angular velocity of the motor. Fig. 38 shows the block diagram of a phase-locked loop servo.

The main difference between a phase-locked servo and the conventional servo systemis that the reference input for a phase-locked servo is a periodic signal from the encoder is a pulse train whose frequency is proportional to the speed of the motor. The phase detector compares the frequencies or phase of the reference and feedback signals, and generates an error voltage proportional to the difference. The error voltage is than filtered and sent to the motor, the motor speed changes to reduce the difference between the frequencies or phases of the two signals. Thus the motor speed can be syncronized to the reference frequency.

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