Refined design method for sampled-data control systems: the pseudo-continuous-time (PCT) control system design

A refined design method, based upon known principles, is presented for the design of sampled-data SISO control systems. Based upon the Fourier analysis of a sampled signal and the Padé approximation, a sampled-data system may be approximated by a pseudo-continuous-time (PCT) control system. This approximation of a sampled-data system permits the use of continuous-time domain methods for designing cascade and/or feedback controllers based upon step-response specifications. When the rules governing the use of the Padé approximation and the Tustin transformation are satisfied, the PCT approach is a valuable technique for the design of sampled-data control systems, especially when the sampling time T is much smaller than 1 s. For T < 1 s, the accuracy needed in performing the calculations in the z-domain increases as T decreases. The same degree of accuracy is not required while designing the controller in the s-plane. A design technique based upon the use of the PCT system is presented along with examples. The associated controller implementation problems are discussed.