Design and Evaluation of an Auto-Tuning Control System for an Altitude Test Facility

Abstract

Simulated altitude testing of large aircraft engines is a very expensive, but essential step in the development and certification of gas turbines used by commercial airlines. A significant contributor to the cost of this process is the time-intensive task of manually tuning the facility control system that regulates the simulated flight condition. Moreover, control system tuning must be performed each time the test conductor changes the flight condition. An adaptive control system that automatically performs this task can significantly reduce the costs associated with this type of engine testing. This paper examines the features of an auto-tuning controller architecture that contains both disturbance feedforward and PID feedback components in a two-input, two-output multivariable configuration. The paper reviews the underlying concepts of an auto-tuning system and contrasts its advantages/disadvantages with respect to other adaptive control techniques. The algorithm used to automatically tune the controller does not require a facility model. However, a nonlinear facility model was developed and used to substantiate a decoupled-loop design approach, to validate the controller design concept, and to evaluate the resulting adaptive control system design performance. This analysis and other practical design issues that impact the auto-tuning control system performance are addressed in the paper. The paper also presents results that illustrate the automatic tuning sequence and the disturbance rejection performance exhibited by this system during large engine transients at several key points in the flight envelope. The auto-tuning controller described in the paper was implemented at a Pratt & Whitney flight test facility used in the development of large, high bypass ratio gas turbines.