Comparison of control allocation methods in the presence of Failures for the High Altitude Performance Demonstrator

This paper deals with the application of control allocation concepts to the High Altitude Performance Demonstrator (HAPD) unmanned aircraft studied at CIRA (Centro Italiano Ricerche Aerospaziali). Three different techniques aimed at preserving control performance on the three axes in the presence of multiple actuators, and possible faults are compared. An equivalent classical set of three virtual command surfaces is obtained and a Proportional Integral flight control system with H-infinity performance is first designed. A control allocator is then added to obtain a suitable distribution of the control action on the available surfaces also in the presence of failures. A first technique is based on the off-line calculation of a bank of control allocation matrices taking into account a family of aircraft linearized models which are representative of the operating conditions; a second technique is based on the on-line solution of a Quadratic Programming problem taking into account also control input saturations. The use of H-infinity controllers, scheduled with the possible actuator faults, is finally analyzed which directly assumes all the healthy control surfaces available to guarantee a suitable effort distribution. A comparison of the performance exhibited by the three techniques is made by means of numerical simulations involving the nonlinear mathematical model of the HAPD aircraft.