Back-stepping Control Based On Disturbance Observer For Free-floating Space Robot Under Dead-zone Effect

Problems like fuel consumption and unknown load often exist in the process of motion while space robot works in the outer space environment,which causes system's uncertainty.Because of the unknown dead-zone in the system,the controller fails to response,and it causes the instability of system.At the same time,the effects on the system's entire motion coming from flexible space robot system(flexible joint and flexible arm)are becoming increasingly obvious.In this context,this paper uses disturbance observer to design dynamic controllers for space robots under the situation of uncertain parameters,unknown dead-zone and flexible effects.Based on the law of linear momentum and angle momentum conservation and the second Lagrange method,the kinematic and dynamic models of the rigid single-arm and dual-arm space robot are sequentially derived.By regarding the flexible joint of space robot as a linear spring,the dynamic equation of flexible-joint space robot is derived.As for flexible-arm space robot,using Assumed mode method derives the dynamic model.These will provide a solid theoretical foundation for the following work.Considering the existence of dead-zone effect in the system's actuator,a L2 gain back-stepping controller based on the disturbance observer and dead-zone compensator is designed for rigid single-arm space robot with uncertain parameters to realize the desire trajectory 's tracking of space manipulator claw.The control scheme doesn't need to know the upper bound of system's uncertainties and the dead-zone parameters,which simplifies system's control.As the unknown parameter exists in the dual-arm space robot,a back-stepping adaptive control scheme based on the disturbance observer is designed for system whose carry's position and attitude are uncontrolled.The control scheme regards system's model error as disturbance,and it doesn't need to assume the low change rate of disturbance,which relaxes the restrictions on the change rate of disturbance.Since the flexible joints and uncertain parameters have a bad effect on the flexible-joint space robot,a L2 back-stepping control scheme combining disturbance observer with flexible compensator is designed to complete the accurate tracking of the desire trajectory.Moreover,through the compensations of the flexible joint compensator,the singular perturbation technique is successfully applied to the flexible-joint space robot.The main control target for the flexible-arm space robot is to realize the accurate tracking of desire trajectory and actively suppress the elastic vibration caused by flexible-arm.After the singular perturbation decomposition,a back-stepping adaptive control scheme is used to control the slow subsystem,which aims to achieve the exact tracking of the desire trajectory.And then,the elastic vibration is restrained by PD feedback control.The superimposition of these two schemes' control law makes tracking errors and vibration amplitudes tend to zero.Finally,the validity of the above control schemes is verified by the comparing of the MATLAB numerical simulation results.The simulation results show that the compensation of disturbance observer controller makes the system more accurately track the desired trajectory.