Feedback controllers time-delay compensation

The Smith predictor is a model-based control strategy that involves a more complicated block diagram than that for a conventional feedback controller, although a PID controller is still central to the control strategy (see Fig. 8-37). The key concept is based on better coordination of the timing of manipulated variable action. The loop configuration takes into account the facd that the current controlled variable measurement is not a result of the current manipulated variable action, but the value taken 0 time units earlier. Time-delay compensation can yield excellent performance however, if the process model parameters change (especially the time delay), the Smith predictor performance will deteriorate and is not recommended unless other precautions are taken. 

Apply classical controller analysis to cascade control, feedforward control, feedforward-feedback control, ratio control, and the Smith predictor for time delay compensation. 

In this section we present an advanced control technique, time-delay compensation, which deals with a problematic area in process control—namely, the occurrence of significant time delays. Time delays commonly occur in the process industries because of the presence of distance velocity lags, recycle loops, and the analysis time associated with composition measurement. As discussed in Chapters 12 and 14, the presence of time delays in a process hmits the performance of a conventional feedback control system. From a frequency response perspective, a time delay adds phase lag to the feedback loop, which adversely affects closed-loop stabihty. Consequently, the controller gain must be reduced below the value that could be used if no time delay were present, and the response of the closed-loop system will be sluggish compared to that of the control loop with no time delay. 

The previous discussion of time-delay compensation assumed that measurements of the controlled variable were available. In some control applications, the process variable that is to be controlled cannot be conveniently measured on-line. For example, product composition measurement may require that a sample be sent to the plant analytical laboratory from time to time. In this situation, measurements of the controlled variable may not be available frequently enough or quickly enough to be used for feedback control. 

The IMC block diagram in Fig. 12.5 can be expanded to include a block A in the feedback path as well as a disturbance transfer function G. The block A can be used to predict the effect of the disturbance on the error signal to the controller, and it can also provide time-delay compensation. This two-degree-of-freedom controller (see Chapter 12) is known as an analytical predictor (Doss and Moore, 1982 Wellons and Edgar, 1987). 

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