Engine Control by Classical Methods

Abstract

This chapter reviews and applies classical SISO design techniques (root locus and frequency domain loopshaping) to the problem of fan speed control using fuel flow rate as control input. A model-matching method is also described that is used in CMAPSS as a design tool. The shortcomings associated with the use of fixed linear compensation are illustrated with simulation examples.As expected, classical linear compensation is adequate only to govern the engine close to a fixed operating point, as defined by the current inlet conditions and desired thrust setpoint. Engine accelerations across wide fan speed ranges, as well as thrust regulation across changing inlet conditions are handled poorly when a fixed linear controller is used. Aside from nonlinearity and parametric changes in the plant, critical variables must be maintained within safety ranges.Linear compensation, however, is the basic building block of standard GTE control systems. Parametric changes and nonlinearity are addressed conventionally addressed with gain-scheduled linear compensators, while limit protection logic schemes are used to override the active linear regulator when a critical variable approaches its safety limit. In this chapter, three basic design approaches to thrust regulation by means of classical linear compensation are examined. Fuel flow rate is considered to be the only control actuator. At the end of the chapter, a CMAPSS simulation is presented that exposes the limitations of fixed-regulator schemes.