Pergamon Chemical Engineering Science, Vol. 51, No. 6, pp. 931-945, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0009 2509/96 $15.00 + 0.00 0009-2509(95)00338-X A PARALLEL CONTROL STRATEGY ABSTRACTED FROM THE BARORECEPTOR REFLEX MARTIN POTTMANN, MICHAEL A. HENSON t, BABATUNDE A. OGUNNAIKE* and JAMES S. SCHWABER Advanced Process Control Group, E. I. du Pont de Nemours and Company, Wilmington, DE 19880-0101, U.S.A. (First received 17 February 1995; revised manuscript received 9 August 1995; accepted 20 August 1995) Abstract--A parallel control strategy is developed for process applications by "reverse engineering" the functions of the baroreceptor reflex--the biological control system that regulates arterial blood pressure. The specific control architecture and algorithm employed by the reflex are analyzed from a process control perspective. A parallel control structure for process applications is then developed by reparameterizing the controllers in the biologically derived architecture. The resulting structure allows independent design of the parallel controllers via Hz-optimal control theory. The parallel control technique is applicable to single- input processes for which two types of output measurements are available: (i) a primary measurement of the controlled output whose dynamic response to input changes is unfavorable (e.g. delayed); and (ii) a second- ary measurement of a different output whose dynamic response is more favorable (e.g. undelayed). The parallel control system uses the primary and secondary outputs in a coordinated fashion in order to provide high performance disturbance rejection. Compared to conventional cascade control, the parallel control strategy provides improved stability and robustness characteristics. Two simulation examples demonstrate the superior performance and failure tolerance that can be achieved with the parallel control strategy compared to cascade control and single-input, single-output control techniques. 1. INTRODUCTION 1.1. Background Biological systems provide high-performance, fault- tolerant control of physiological systems that are con- siderably more complex than the most sophisticated chemical plants. Thus, by "reverse engineering" the control functions of these biological systems, it is possible to develop novel and effective control strat- egies for chemical process applications (Doyle et al., 1995; Henson et al., 1994, 1995). One such biological control system is the baroreceptor reflex, which provides short-term regula- tion of arterial blood pressure. This control system coordinates the activity of several lower-level control systems, one of which affects the heart rate. The heart rate is manipulated by two parallel neural controllers which receive two different types of blood pressure measurements: mean pressure and rate-of-change of pressure. The rate-of-change measurement is obtained very quickly, while the mean pressure measurement is available only after a comparatively large delay. The blood pressure measurements are selectively pro- cessed such that the rate-of-change signal is used for dynamic control and the mean pressure signal is em- ployed primarily for steady-state control. In this paper, we demonstrate how a parallel con- trol strategy can be abstracted from the baroreceptor *Corresponding author. "tCurrent address: Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803-7303, U.S.A. reflex and employed in chemical process applications. Many chemical processes contain output measure- ments that are similar in character to the blood pres- sure measurements described above. The measure- ment of the controlled output (the primary measure- ment) often has unfavorable dynamic responses to input changes (for example, it may be delayed, or it may exhibit inverse response); on the other hand, a measure- ment of a different process output (the secondary measurement) that has more favorable dynamic charac- teristics is often available. High-performance, fault-toler- ant control can be achieved by designing a parallel control system in which the primary and secondary measurements are used in a coordinated fashion. 1.2. Related control techniques As in the baroreceptor reflex, measurements of a secondary process output are often used to improve the load response of the primary output in chemical process applications. The most common approach based on this concept is cascade control, which uses an intermediate process output that responds faster to certain disturbances than the controlled output. The output signal of the outer (master) controller serves as the setpoint for the inner (slave) controller [e.g. Bal- chen and Mumme (1988), Luyben (1990) and Seborg et al. (1989)]. In practice, PI controllers are used in both the inner and outer control loops, and the inner control loop is tuned first. An internal model control (IMC) design method for cascade controllers is discussed by Morari and Zafiriou (1989). Due to the controller 931