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THERE is an important class of feedback control systems known as sampled-data systems or sampling servomechanisms in which the data at one or more points consist of trains of pulses or sequences of numbers. Such systems may have a variety of forms, a common example of which is shown in Figure 1. In the case illustrated, the sampling is performed on the control error by a so-called sampler which is indicated as a mechanical switch which closes momentarily every T seconds. The data at the output of such a switch consist of a train of equally spaced pulses of short duration whose envelope is the control error function. In some practical systems, the separation between successive pulses is controlled by some characteristic of the input signal and consequently is not constant. Such systems will not be considered in this paper.