Stability of a Servomechanism Operated by PWM Mode

Abstract

In a servomechanism operated by the PWM mode, the output contains the dither corresponding to the carrier pulse. When the loop gain becomes high, the dither component is fed back and modulates the pulse width. Therefore, we must consider not only the ordinary stability problem of the servo loop but also the stability of the dither. First, the stability of the fundamental circuit was analyzed. In this case, it was shown that the various waveshapes took place depending on the ratio of the pulse transmission lag to the pulse period and the loop gain. The ranges of the loop gain were also obtained for various waves. Next, the stability was studied with regard to the servomechanism in which two types of compensating circuits were used. One was a first-order system and the other was a phase lead network. It was shown that these compensating circuits were effective for stabilizing the servomechanism, if they were selected properly. The optimum time constants were obtained for these compensating circuits. Finally, the theoretical results were verified by experiments which were carried out by an electrohydraulic servomechanism.