An enhanced automatic tuning procedure developed for process control of PI and PID controllers addresses several potential problems present in current standard autotuners. The proposed enhanced autotuner uses a novel technique based on relay feedback to estimate the process frequency response at two specified phase lags on the Nyquist curve automatically. An iterative procedure then uses these two points to obtain a transfer--function model of the process. Based on this model and a controller-selection scheme, an appropriate controller (PI or PID) is applied to the process automatically. The controller is tuned so that the Nyquist curve of the compensated system is appropriately shaped to satisfy a combined gain and phase-margin type of specGcation. The effectiveness of this enhanced autotuner is demonstrated both in simulations and in real-time experiments for level control of a coupled-tanks system. frequency can be identified. When the critical point, that is, the ultimate gain and frequency, is known, it is straightforward to apply the classic Ziegler-Nichols tuning rules (1943)

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In process control, the Smith-predictor controller introduced by Smith (1957) is a useful technique for deadtime compensation. One main problem with this scheme, however, is that a model of the process is required for output prediction. As in the case of other model-based controllers, it therefore suffers from a sensitivity problem. This has posed a big obstacle to its real-time applications, since in practice, no mathematical model will be a perfect representation of the real process. In realistic situations, there is no guarantee that a Smith-predictor controller will give a better performance over a single-loop controller for the same process even when it has a significant deadtime (Palmor, 1980;Yamanaka and Shimemura, 1987). Therefore, often the use by engineers of a single-loop PID control though deadtime compensation is highly desirable to enhance performance. This has been a persistent problem associated with the practical implementation of the Smith-predictor controller, and thus far there has been no satisfactory solution to this problem.This article gives a different perspective to the analysis and design of Smith-predictor controllers. An equivalent representation is first presented. It shows that the Smith system contains an inherent phase-compensated element in the feedback loop and, further, the single-loop controller can be viewed as a special mismatched Smith system. Under perturbed conditions, all the uncertainty is concentrated in this element so that its properties are indicative of the achievable closed-loop performance. This provides a unified framework for analysis and design of Smith systems and single-loop controllers, and it clearly shows when we should use the Smith controller and what benefits the controller offers over the single-loop controller. For this assessment a suitable performance measure is formulated based on the frequency response characteristics of the feedback element which depends on the process model used for the output prediction. Process model selection may then be posed as an optimization problem whose solution corresponds to the most desirable performance measure. As a result, we observe that in contrast to intuition a mismatched Smith system may give a better performance over the perfectly matched one. Examples are included for illustration.The Otto Smith predictor scheme is reviewed. An equivalent representation is given together with an account of the significant features. A general measure for the assessment of the achievable closed-loop performance is formulated. Model identification is considered, the controller design is discussed, and the effects of reduced-order modeling on the closed-loop performance of the Smith-predictor are investigated with several examples. Smith-Predictor Controller-A ReviewThe Smith-predictor controller was proposed by Smith (1957) for deadtime compensation and is shown in Figure 1

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