Low-order modelling via discrete stability equations

The continuous-time stability equation method is extended to reduce the order of discrete stable high-order transfer functions. The operations of the proposed method are carried out entirely in the z domain. Thus, use of the bilinear transformation is avoided, and considerable savings in computation are achieved. Furthermore, all the desired properties of the stability equation method, such as stability preservation, satisfactory approximation at large times, estimation of the reduced order and accurate preservation of the oscillation frequencies are retained.