Optimal Routh approximants through integral squared error minimisation: computer-aided approach

A computer-aided method for obtaining a reduced-order approximant of a given (stable) single-input single-output system based on the minimisation of integral squared error (ISE) pertaining to a unit-step input is presented. Both the numerator and denominator coefficients of the model are treated as free parameters in the process of optimisation. The method has a built-in stability-preserving feature. The problem of formulating the ISE is circumvented by introducing a set of equality constraints.