3-2 PID Modes, Structure, and Form

Basic feedback control is performed by a controller that has proportional, integral, and derivative modes. Except for temperature loops, the derivative mode is usually turned off by setting the rate time (derivative time) setting to zero. Users make a distinction here and call a controller with no derivative action a PI controller, but the open literature often does not do this. In many papers, the performance of PI controllers (labeled as PID controllers) is frequently compared to advanced control algorithms. This section will discuss how heavily performance comparisons depend on PID structure. Equations 2-2a and 2-2b in chapter 2 showed how strongly performance depends on tuning settings. The authors of the technical literature choose structures and methods to prove the value of their new tuning methods or algorithms. In cases where derivative control is useful and noise and interaction are negligible, an aggressively tuned PID controller offers the best rejection of unmeasured disturbances at the input to a process ^{[3-1] [3-2]}. Often overlooked are the special techniques that can be readily added to the PID controller, such as batch preload and dead-time compensation via external reset (mentioned in this section) and the optimal switching of PID output to its final resting value (detailed in section 3-5).