Comment on ´biased model reduction by simplified Routh approximation method´

For the original article see ibid., vol.23, p.1045-7 (1987). Sastry and Krishnamurthy recently suggested a ´simplified´ Routh approximation method (SRAM) for model order reduction of linear systems. They claim that one of the major advantages of the technique over the much used Routh approximation method is having only to use the so-called alpha -parameters to obtain the reduced model instead of both the alpha - and beta -parameters. They also claim that the models produced by this simplified approach exhibit the desirable properties of the Routh method, i.e. stability preservation and time moment/Markov parameter retention. While the latter claim cannot be doubted, the former is based on experimental evidence only, and not proved mathematically; consequently it must be viewed with suspicion. The commenter shows that stability is not preserved in general when using this simplified approach to obtain reduced model of orders higher than 4. It is suggested that great care be applied, if this method is used to check the stability of the reduced models.