Hybrid Force Position Control System Design Of Posture Alignment

Generally the posture alignment mechanism of aircraft components which is part of flexible aircraft assembly system consists several three-axis localizers, which constitute redundant drive spatial parallel mechanism. There exist perpendicularity error on each localizer’s axis and parallelism error between multiple axes. If the moving trajectory of aircraft components is designed according to theory model of localizers, it may cause interaction between components and localizers. Since components are structure of weak stiffness, it may cause posture alignment error and internal force which will lead to deformation or destruction of components. In this paper, a hybrid force/position control strategy is introduced to avoid internal force caused by localizers interaction and improve posture alignment accuracy. The paper’s content includes:First, the motion theory of posture alignment mechanism is explained and the inverse kinematics of the mechanism is introduced. With the consideration of perpendicularity error and parallelism error of localizers, the error model of posture alignment mechanism is built. Then hybrid force position control strategy is introduced, that is, some axes are selected under position control and the others are selected under force control.The select rationality of force and position controlled axes is analysed based on screw theory. The selection strategy is optimized by the method of kinematic ellipsoids and stiffness ellipsoids and evaluation index based on torque dexterousness and stiffness condition number is introduced.The axes under force control and position control respectively are selected for a real posture alignment mechanism. The desire contact force of localizers is analysed and the structure of hybrid control system is described. Relative calculation indicates that hybrid control strategy may improve posture accuracy and reduce internal force significantly under the given posture alignment condition when0.05mm/m perpendicularity error exists on each localizer and the maximum parallelism error is0.1mm/m and the averaged parallelism error is0.07mm/m between localizers.The design of hybrid force position control system is implemented based on position servo system of localizer. The simulation of control system indicates that the force controller with inner-loop position controller follows the desire support force signal properly. The performance with no oscillation shows the dynamic force error is below0.05N, and the static force error converges to0. Experiment carried on single axis indicates that the torque controller follows the desire torque signal properly and the static torque error is below0.02Nm, which is applicable for posture alignment control.