Quality of control and optimisation

Controller tuning can be defined as an optimisation process that involves a performance criterion related to the form of controller response and to the error between the process variable and the set point. When tuning a controller, some of the questions that may be asked include  

These questions address some of the performance criteria that are used in the tuning of a controller, including overshoot, decay ratio, and error performance. 

2 critically damped fastest response without oscillation  

3 underdamped fast return to set point but with considerable oscillation. 

From these three general extremes, we can see that selection of good control is a trade off between the speed of response and deviation from the set point. A highly tuned controller may become unstable if large disturbances occur, whereas a sluggishly timed controller provides poor performance but is very robust. What is typically required for most process control loops is a compromise between performance and robustness. 

---