Sampled-Data Control Systems
Constantine H. Houpis, Stuart N. Sheldon · 2013
The methods presented in the previous chapters deal with the fundamental properties of continuous systems [1-5]. However, digital computers are available not only for the design of control systems but also to perform the control function. Digital computers and microprocessors are used in many control systems. The size and cost advantages associated with the microcomputer make its use as a controller economical and practical. Using such a digital processor with a plant that operates in the continuous-time domain requires that its input signal be discrete, which requires the sampling of the signals used by the controller. Such sampling may be an inherent characteristic of the system. For example, a radar or a missile tracking system supplies information on an airplane’s or an incoming missile position and motion to a digital processor at discrete periods of time. This information is therefore available as a succession of data points. When there is no inherent sampling, an analog-to-digital (A/D) converter must be incorporated in a digital or sampled-data (S-D) control system. The sampling process can be performed at a constant rate or at a variable rate, or it may be random. The discussion in this chapter is based on a constant-rate sampling. The output of the controller must then be converted from discrete form into an analog signal by a digital-to-analog (D/A) converter. A functional block diagram of such a system is shown in Figure 18.1, in which the signals e* and u* are in discrete form, u is piecewise continuous, and the remaining signals are continuous.