Transform method of controller design is generally suitable for designing simple analog control circuits for SISO (Single input single output) system). Control of a realistic system may require many sensors and actuators and since all of them are related with a single system, their performances are coupled. Hence, design of multiple decoupled SISO controllers is not feasible in such cases.
Moreover, transform method is difficult to implement for a MIMO system, since for m number of actuators and n number of sensors, one will have mn number of transfer functions and one has to design as many controllers for the system. With the advent of computers and computer based digital control, one can design control systems for the MIMO systems directly in the time domain. The governing equation of motion in such linear dynamic systems is represented as a set of first order differential or difference equations commonly known as state space representation of the system.
