Application of optimal control to structural load alleviation control systems
Feedback laws based upon optimal control theory were derived. and these resulted in a reduction of the structural loads on the wing of a simulated aircraft. Various models of the aircraft dynamics were used. the most complete being of order 79. This model included rigid body motion, structural flexibility effects, unsteady aerodynamics, gust dynamics and actuator dynamics. The structural effects were characterised by the first fifteen bending modes. The subject aircraft studied, was considered to employ active ailerons and elevators and was subjected to manoeuvre commands and simulated atmospheric turbulence. Extensive numerical tests have shown that feedback laws derived from reduced dimension models performed comparably with the feedback law based on the most complete model. Tests were made on feedback laws ranging from order 79 to order 5. It was. however. not possible to reduce the number of feedback variables below five as this then affected the stability of the aircraft. The law based upon five state variable feedback was given the designation 'safety law'. One of the consequences of operating under the action of the 'safety law' was that the same level of load reduction could not be achieved as was obtained whenever a full state feedback law was employed. In addition 'safety law' operation was often marked by large transient oscillations of the wing root bending moment and it was considered that this would subsequently affect the fatigue life of the structure. An observer design was then investigated which reconstructed the complete state vector from a selection of measurements of the sensor signals appropriate to the 'safety law'. Results have shown that it is possible to achieve a practical implementation of such a scheme which will possess all the attendant advantages of full state feedback control. A consequence of reducing the strength of the wing of the aircraft as a result of employing an active load alleviation scheme is that a considerable degree of reliability of the control system, higher than that of both the basic airframe and its propulsive system, will be required. Because the use of hardware redundancy techniques as a means of providing the required degree of reliability would be expensive, software redundancy techniques suggest an attractive alternative. One example of how software redundancy may be employed is demonstrated in respect of , checking the analogue feedback gain controller used in the aircraft to implement linear feedback. It is shown how a-microprocessor may be effectively employed to introduce a surrogate gain should one or more of the channels of the controller fail.