Design of low-frequency compensators for improvement of plantwide regulatory performance

One of the major weaknesses of classical feedback controllers is their inability to track ramplike disturbances without offset. This hinders their performance in plantwide environments where load-transfer functions with large load time constants transform step load disturbances into ramp-like disturbances. This paper presents a solution to this problem. Two types of low-frequency compensators are introduced: double integral compensators and lag compensators. Tuning rules are developed for these compensators for the case where the base controller is of the PI variety. The effectiveness of low-frequency compensation in improving the performance of PI controllers is demonstrated first through the use of a simple transfer function model and then through a rigorous simulation involving a more realistic plantwide system. The idea of low-frequency compensation has been used in other engineering disciplines for many years for tracking ramping setpoint signals (i.e., in the fields of mechanical and electrical engineering, ''type 2'' servomechanisms are often used when ramps in setpoint signals are anticipated (Chestnut and Mayer, 1951; Nixon, 1954; Franklin et al., 1994), but it has not been used in the process control field for rejecting the effects of frequently occurring ramp-like disturbances.