Closed Loop Control of a Shape Memory Alloy Actuation System for Variable Area Fan Nozzle

摘要

Shape Memory Alloys have been used in a wide variety of actuation applications. A bundled shape memory alloy cable actuator, capable of providing large force and displacement has been developed by United Technologies Corporation (patents pending) for actuating a Variable Area fan Nozzle (VAN). The ability to control fan nozzle exit area is an enabling technology for the next generation turbofan engines. Performance benefits for VAN engines are estimated to be up to 9% in Thrust Specific Fuel Consumption (TSFC) compared to traditional fixed geometry designs. The advantages of SMA actuated VAN design are light weight and low complexity compared to conventionally actuated designs. To achieve the maximum efficiency from a VAN engine, the fan nozzle exit area has to be continuously varied for a certain period of time during climb, since the optimum nozzle exit area is a function of several flight variables (flight Mach number, altitude, etc). Hence, the actuator had to be controlled to provide the time varying desired nozzle area. A new control algorithm was developed for this purpose, which produced the desired flap area by metering the resistive heating of the SMA actuator. Since no active cooling was used, reducing overshoot was a significant challenge of the controller. A full scale, 2 flap model of the VAN system was built, which was capable of simulating a 20% nozzle area variation, and tested under full-scale aerodynamic load in NASA Langley Jet Exit Test facility. The controller met all the requirements of the actuation system and was able to drive the flap position to the desired position with less than 2% overshoot in step input tests. The controller is based on a adaptive algorithm formulation with logical switches that reduces its overshoot error. Although the effectiveness of the controller was demonstrated in full-scale model tests, no theoretical results as to its stability and robustness has been derived. Stability of the controller will have to be investigated for the next stage of technology readiness.