A New Perspective on the Tuning, Stability, and Benefits of Cascade Control

摘要

It is often said that for a cascade control system to be stable, the outer loop should be tuned for a significantly slower response than the inner loop. However, the minimum ratio of outer to inner loop response time required for stability is subject to some dispute. Guidance given in this regard has ranged from ratios as low as 3:1 to as much as 20:1. This paper explores the minimum response-time ratio required for cascade control systems to be stable, and the effect that process characteristics and tuning methods have on this minimum. It also makes recommendations about the type of tuning methods that should be used to obtain stable, responsive cascade control systems. It has also been recommended that for cascade control to be beneficial, the inner loop in a cascade control system must respond at least five times faster than the outer loop. This paper analyzes a few typical, but distinctly different cascade control applications, and evaluates the benefits of cascade control in each regard. It shows that in practice, cascade control is not always applied for improving control performance - and explains how cascade control is sometimes applied solely to simplify a control strategy.