A Simultaneous Tuning Method for Cascade Control Systems Based on Direct Use of Plant Data

Abstract

This paper presents a novel method for tuning cascade control systems in which both primary and secondary controllers are tuned simultaneously by directly using plant data without resorting to process models. The required plant data are collected from a one-shot step test that can be conducted under either closed-loop or open-loop conditions. The goal of the proposed design is to obtain the parameters of two proportional-integral-derivative (PID) controllers such that the resulting inner and outer loops behave as closely to appropriately specified reference models as possible. The optimization problems related to the proposed design are derived. On the basis of the rationale behind cascade control, the secondary controller is designed to attenuate disturbance faster. The primary controller is designed to accurately account for the inner loop dynamics without requiring an additional test. Moreover, robustness consideration is included in the proposed tuning method, which enables the designer to explicitly address the trade-off between performance and robustness for inner and outer loops independently. Simulations show that the proposed method exhibits superior control performance compared with the existing (model-based) design methods, confirming the effectiveness of this novel method of PID controller design for cascade control systems. INTRODUCTIONCascade control is one of the most successful structures for enhancing the performance of single-loop control, particularly when disturbances are associated with the manipulated variable. 1 Thus, cascade control is applied extensively in chemical process industries. In the standard cascade-control approach, one feedback loop is nested inside another feedback loop using two controllers. The controller of the inner loop is called the secondary (or slave) controller, whereas the controller of the outer loop is the primary (or master) controller. The rationale behind this configuration is that the fast dynamics of the inner loop enable fast attenuation of disturbance and minimize the possible effects of disturbances before they affect the primary output, which is the controlled variable of interest.Although sophisticated schemes for cascade control have been proposed, 2−4 the basic scheme still comprises two nested loops with two proportional-integral-derivative (PID) controllers. Because this scheme involves tuning two PID controllers, the design of cascade control systems is more complex than the design of single-loop control systems. The usual approach involves first tuning the secondary controller by setting the primary controller on a manual mode. The primary controller is then tuned by considering the action of the secondary controller on the inner loop. Such a tuning procedure is time-consuming because at least two runs of the plant test are typically required. 5,6 However, the sequential tuning procedure has been improved so that only a single experiment is conducted for tuning the two controllers simultaneously. 7−12 In most methods of simultaneous tuning, low-ord...

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