Comments about frequency response plots of Keller and Anderson and Rattan

It is noticed that the method of comparison of frequency responses by using the Nyquist plots of the closed-loop system spectral transfer functions, applied in Keller-Anderson (ibid. vol.37, no.2, p.214-23 (1992)) and Rattan (ibid. vol.AC-29, p.282-5 (1984)), is not appropriate. Namely not only the shape of these plots but also the frequency distribution on them must be taken into account. It is shown that significantly more legible is utilization of the Bode plots of the control index presented by Findeison (1978) which has a good physical insight.<
>