Ubiquitous Fractional Order Controls?

There is an increasing interest in dynamic systems and controls of noninteger orders or fractional orders. Clearly, for closed-loop control systems, there are four situations. They are 1) IO (integer order) plant with IO controller; 2) IO plant with FO (fractional order) controller; 3) FO plant with IO controller and 4) FO plant with FO controller. However, from engineering point of view, doing something better is the major concern. This review article will first show two examples that the best fractional order controller outperforms the best integer order controller. Then, we try to argue why consider fractional order control even when integer (high) order control works comparatively well. We will also address issues in fractional order PID controller tuning. Using several real world examples, we further argue that, fractional order control is ubiquitous when the dynamic system is of distributed parameter nature.