PWM motor control: Model and servo analysis

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Title: PWM motor control: Model and servo analysis
Author: Sandor, Rick J.
Abstract: In recent years, the performance requirements of high power servo motor systems utilizing pulse width modulated (PWM) switching amplifiers have steadily increased. These PWM motor amplifiers perform an important function in the d.c. servo system by boosting the low level command signal to the high voltage and current levels required by the motor. Ideally, this power gain is to be constant over all input frequencies but, in reality, gain is frequency dependent which affects system dynamics. The amplifier gain and phase versus frequency relationships an*i amplifier noise and d.c. offsets which may affect system response must be known to the servo designer to properly design the motor control system. The switching effects of the PWM amplifier may result in making the overall system unstable if the system bandwidth is kept high with respect to the PWM switching frequency. Since the standard servo design techniques utilize linear system modeling, analysis, and compensation, it would be very advantageous to the design engineer to have a linear model which best approximates the true nonlinear PWM amplifier. This work will look at the output response of the PWM amplifier with respect to stability and output ripple. A linear model will be developed which simulates these stability and ripple effects in a position control servo system and which is valid as system bandwidth reaches one-third the PWM switching frequency. This work extends the application of the "Principle of Equivalent Areas" [141 to the bipolar PWM amplifier. It is then combined with a detailed analysis of the PWM waveform by Double Fourier Transform to yield the unique PWM switching effects in a position control servo system. Theoretical results of the newly derived sampling plus harmonic linear model are verified by computer simulation.
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Date: 1984-04

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