Comments on two methods for designing a digital equivalent to a continuous control system

Rattan (1984, 1989) suggests two methods for designing a digital control system to match the performance of a given continuous control system as closely as possible. In this note, two main points are made. First, there are a number of errors in the paper by Rattan (1989) that significantly affect its conclusions. Second, neither of the methods is guaranteed to produce a stable closed-Loop system, even if the original closed-loop system is stable. In fact, the two methods will generally lead to unstable closed-loop systems when the number of free parameters in the compensator design process is large.<>