Smith Predictor-Based Control Schemes for Dead-Time Unstable Cascade Processes

This paper presents two simple and efficient Smith predictor (SP) based control schemes which can be used to control open-loop stable or unstable time-delay cascade processes. The proposed structures have two control loops, a secondary inner loop and a primary outer loop. Similar to previous approaches, the secondary loop uses an internal model control (IMC) structure. Two different schemes are proposed for the outer loop that has an unstable open-loop behavior. Contrary to previous proposed controllers, where a delayed model should be used in the stabilization and tuning procedure by considering some kind of polynomial approximation of the dead time, in the proposed structures, internal stability is naturally achieved through a suitable implementation and tuning of the controller without using any delay approximation. To illustrate this, simulation comparative results with some of the schemes recently presented in the literature are presented, showing the simplicity of the proposed design. Moreover, the simulations show that the proposed schemes allow one to obtain some improvement in disturbance rejections performance.