An Adaptive Compensation Strategy of Control Surfaces Free-Play

A technique aimed to neutralize the presence of free-play effects in a control surface actuation chain is presented. The procedure is based on the inversion of a function approximating such a nonlinearity. A simple yet robust on-line adaptive algorithm is proposed to identify the free-play parameters, i.e. free-play width and the equivalent control stiffness. To achieve such a compensation, the only measures required are those of the control surface defection and the hinge torque. The procedure is then coupled with an other adaptive control law to drastically reduce possible residual limit cycle oscillations in closed loop associated to the free-play presence. Within such a framework, the proposed compensation can be interpreted as a control augmentation which can be easily extended to multiple control surfaces. Even if only numerical analyses are considered in this work, measurement noise is taken into account, and both free-play and friction are simulated in the control surface actuation system, aiming to increase the reliability of the presented results. In addition, several off-design conditions are analyzed, e.g. varying the free-play width and friction amplitude. The proposed methodology is applied to a three degrees of freedom airfoil in transonic regime. This case is characterized by highly nonlinear unsteady aerodynamic loads, producing significant shock motions and large limit cycle oscillations at a relatively high frequency, resulting so in a challenging test for the proposed approach.