+ Limit Protection in Gas Turbine Engines

Limit protection is an essential functionality of the control system of an aircraft gas turbine engine. This functionality handles constraints such as surge avoidance, over-speed and over-temperature limits, combustion lean blowout limit, actuator magnitude and rate limits, etc. The violation of these limits can lead to disastrous consequences, such as surge, and has caused aircraft losses. The conventional approach to handling constraints through max and min fuel limiters, while reasonably well-understood, may be conservative and restrict unnecessarily the thrust response.

We have been developing approaches to limit protection based on the application of reference governors and extended command governors (Figure 1). Reference and extended command governors are add-on schemes to the nominal control design that are used to protect the closed-loop system against violating the pointwise-in-time state and control constraints. They operate by exploiting physical models of the constraints and predictively modifying the reference commands to closed-loop systems when necessary in order to avoid violating pointwise-in-time state and control constraints. These schemes remain inactive when no danger of constraint violation exist and they become active only when the closed-loop system needs to be protected against constraint violation. For gas turbine engine applications, the reference and extended command governors may also be viewed as algorithmic generalizations of the classical fuel topping governor type schemes.

Both reference governor and extended command governor are predictive nonlinear control schemes that differ in their respective prediction mechanism, with the extended command governor generally providing larger domain of attraction and better performance at the expense of the increase in the computing cost. Unlike Model Predictive Controllers, governors are add-on solutions and do not require replacing a nominal engine controller by a Model Predictive Controller. They also require less on-board computing power for implementation.

Figure 1: Reference and extended command governors for limit protection in closed-loop control systems are nonlinear control schemes that modify the original set-points, r(t) , to safe set-points, v(t), as necessary to guarantee that the prescribed state and control constraints, expressed as y(t) in Y, are enforced despite uncertainties/disturbances, w(t). Governors use current estimate of the state x(t) in determined the required modification.

A robust reference governor that can enforce surge margin constraints and non-conservatively handle inlet distortions/disturbances has been developed in [1], [2]. Figure 2 illustrates our approach in which we estimate online the ranges in which aggregated disturbance parameters may vary, and exploit the robust reference governor to modify EPR set-point to a robust H-infinity feedback controller, in coordination with adjusting Variable Stator Vane (VSV) and Variable Bleed Valve (VBV) positions in order to enforce constraints. Figure 3 illustrates the simulated responses of the engine when EPR command is modified by the robust reference governor to satisfy the surge margin constraints despite the effects of un-modeled and time-dependent inlet distortions. Recent studies based on simulation models have been conducted demonstrating that constraints in gas turbine engines can be enforced using reduced order, prioritized and decentralized reference governor designs [3], [4], [5]. In particular, the research into the decentralized reference governor theory in [4] has been motivated by its potential use as an enabling technology for distributed control of aircraft engines (see http://www.decwg.org/).

Figure 2: Robust reference governor modifying set-points to robust H-infinity controller while relying on real-time inlet distortion range estimates to enforce the constraints.

Figure 3: Simulated engine responses at 20,000 ft cruise conditions with steps in thrust command and step-wise changes in inlet distortions. The surge margin constraints are enforced by modifying EPR set-point to the feedback control system in coordination with adjusting Variable Stator Vane (VSV) and Variable Bleed Valve (VBV) positions.

References

[1] Weiss, A., Kolmanovsky, I.V., and Merill, W., "Incorporating risk into control design for emergency operation of turbo-fan engines," Proceedings of Infotech@Aerospace 2011, St. Louis, Missouri, March 29-31, 2011, AIAA Paper AIAA-2011-1591.

[2] Kolmanovsky, I.V., Jaw, L., Merrill, W., and H.-T. Tran, "Robust control and limit protection in aircraft gas turbine engines," Proceedings of the 2012 IEEE Multi-conference on Systems and Control, Dubrovnik Palace Hotel, Dubrovnik, Croatia, October 3-5, 2012.

[3] Tian, Y., and Kolmanovsky, I.V. "Reduced order and prioritized reference governors for limit protection in aircraft gas turbine engines," Proceedings of AIAA SciTech 2014, Baltimore, MD.

[4] Kalabic, U., and Kolmanovsky, I.V. "Decentralized constraint enforcement using reference governors," Proceedings of 2013 IEEE Conference on Decision and Control, Florence, Italy, 2013.

[5] Kolmanovsky, I.V., and Merill, W., "Limit protection in gas turbine engines based on reference and extended command governors," Proceedings of 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, Ohio, 2014.