Use of modified relay techniques for the design of model-based controllersfor chemical processes

In this paper the possibility of using relay techniques to build parametric models for the design of advanced controllers is analyzed. A modified rela y technique (ATV(+) method), which requires more experimental runs and can lead to the design of high-order controllers, is compared with the "two-cha nnel relay'' (TCR) technique, which requires simpler experimental tests and leads to PI/PID control. The comparison is carried out for a large number of typical process dynamics by considering ease of application, duration of experimental tests, and achievable performance. For a quantitative evaluat ion of these features, several indexes are defined and simulations of time responses are analyzed. As a synthetic conclusion, performance can be large ly improved by adopting the ATV(+) method, with model-based controllers, at the expense of a longer duration of experiments. The time required for TCR depends on initial knowledge of the process. This paper gives more details to quantify the tradeoff between performance and times. The presented resu lts can be used as a guideline for the process control engineer to help in the choice of the more suitable technique, according to the real needs of t he plant.