1. Definition

A servo system is designed to perform a certain task: deliver an output signal that is a known function of the input signal. Its performance is therefore judged by comparing the actual output with the desired output. Symbolically, this leads to the diagram in figure 1 , in which the dashed chain is fictitious.

Most often, we want to obtain a physical quantity at the output that is ideally a copy of the input, to within a constant factor α, α having a dimension since the physical natures of the input and output are generally different. By explaining the loop structure of the servo system, we obtain the diagram shown in figure 2 .

If the sensor is chosen...