6.5 Summary

The closed-loop Ziegler-Nichols method (Table 6-1) was shown to lead to oscillatory closed-loop behavior, which is one of the major disadvantages to the approach. A major advantage to the approach is that a process model is not needed. Tyreus-Luyben parameters (Table 6-2) were shown to be less oscillatory and are generally recommended over the Ziegler-Nichols parameters.

The open-loop Ziegler-Nichols (Table 6-3) and the Cohen-Coon (Table 6-4) methods also tend to lead to oscillatory closed-loop behavior; we have covered these approaches primarily for historical reasons.

In the direct synthesis design procedure, a desired closed-loop response (or closed-loop transfer function) is specified. A feedback controller is then synthesized to obtain that response. We have seen that a PID-type controller often results. Some problems can arise if the process has a time delay or inverse response. In these cases, the desired closed-loop response must also have a time delay or inverse response. We develop a more transparent method for designing controllers for first-order + time-delay processes in Chapters 8 and 9.