| Abstract: | Phase Locked servo systems have received considerable interest in the literature because of their efficiency in achieving high accurate drive with long-term stability while permitting speed control with high degree of precision. When the PLL operates around a central frequency from which it deviates very little, it is often designed using its continuous linear model. Whereas, in servo systems, where the speed has to vary in very large limits, the analysis of this condition is much more difficult, due to the non linear nature of the system. This fundamental problem has not been solved correctly. It has been discussed by some authors but the results are mostly simulations or approximations for a special case. The main contribution of this paper is to carry out a modeling for the system with emphasis on its non linear nature and which represents better the physical reality. For this purpose, we propose two sampled linear models: the Dirac Sampled Model (DSM) and the Zero Order Hold Model (ZOHM). The first one is useful for the local studies of stability, the second permits to consider important transitions of speed. The simulated and experimental results have shown good agreement with DSM and ZOHM models.
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