 While PID controllers are applicable to many control problems, and often perform satisfactorily without any improvements or only coarse tuning, they can perform poorly in some applications, and do not in general provide "optimal" control.The fundamental difficulty with PID control is that it is a feedback control system, with "constant" parameters, and no direct knowledge of the process, and thus overall performance is reactive and a compromise.While PID control is the best controller in an observer without a model of the process, better performance can be obtained by overtly modeling the actor of the process without resorting to an observer.