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A CONTROL ENGINEER usually begins the design of a sampled-data feedback control system with two sets of data. These are the fixed components that the system must include and the specifications which the final system must meet. The fixed components, which will be referred to as the plant, are inherent in the process to be controlled and in many cases are not subject to change. The commonest specifications are generally in terms of time-response and may include rise time, peak overshoot, and steady-state position errors. In general, the plant will not, of itself, satisfy the specifications; hence, the designer's problem is to select such additional components as will produce the desired system. These components, called the compensator, may then give the final system the form shown in Fig. 1, where Gp(s) represents the transfer function of the plant and Gc(s) is the compensator. Determination of this compensator is the central problem in feedback control system design.