Parameter Identification And Control In Post-capture Of A Tumbling Target By A Dual-arm Space Robot

The capture of a target by a space robot,as one of the enabling techniques for on-orbit servicing,is the foundation and precondition of the on-orbit servicing task,such as repairing,refueling,and orbital debris removal.The capture process includes a series of operations,which may be divided into four phases: pre-capture,contact,post-capture and compound stabilization.Compared to a space robot with a single arm,a dual-arm space robot is more flexible and has the ability to conduct coordinated operations,which makes it a better choice when capturing a non-cooperative tumbling target.The unknown dynamic parameter and initial angular momentum of the target makes the post-capture control complicated.To improve the precision and stability of control in compound stabilization and following operations,it's necessary to complete dynamic parameter identification of the target in post-capture phase.The paper mainly studies control and parameter identification in post-capture of a tumbling target by a dual-arm space robot.The main contents and contributions are shown as follows:Firstly,for the control in post-capture of a tumbling target by a dual-arm space robot,a multi-stage control strategy is proposed to solve the problem of dynamic parameter identification,target berthing,base reorientation and stabilization.The control in post-capture phase is divided into three stages according to the multi-stage control strategy.In stage one,the coupling motion between the base and two arms of the dual-arm space robot is used to keep the attitude of the base still,and the measured data is used to conduct the dynamic parameter identification.The joint trajectory planning and tracking for target berthing is achieved in state two.In state three,the distributed momentum control is proposed to transfer the angular momentum to the auxiliary arm and reorient the base.Secondly,the reactionless control for a dual-arm space robot in post-capture phase has been researched.The reactionless control method,designed based on momentum conservation,reaction null space theory and recursive least squares adaptive filtering,is used to control the motion of the two arm of the dual-arm space robot to keep the attitude of the base still while the dynamic parameter and initial angular momentum of the target is unknown.The wide range of motion of the capture arm can provide rich measurement data for the dynamic parameter identification.Furthermore,the joint trajectory planning method for target berthing and the distributed momentum control method has been researched.The paper uses sinusoidal function with quintic polynomial being the argument for the parameterization of the joint trajectory.The coefficients of the polynomial is determined by the initial and terminal constraints of joint positions,velocities and accelerations.Because this trajectory planning method can provide a closed-form expression for the joint trajectory,it satisfies the requirements for real-time on-line trajectory planning.The distributed momentum control method based on the angular momentum conservation principle can make system state on a designed sliding surface,accomplishing the transfer of angular momentum to the auxiliary arm and the reorientation of the base.Finally,dynamic parameter identification technique of the captured tumbling target has been researched.The linear regression model for parameter identification is constructed according to the linear momentum conservation equation and the increment form of the angular momentum conservation equation,and the QRD-RLS algorithm with the measurement data in the reactionless control stage is used to estimate the dynamic parameter of the target.Besides,a novel online optimization method for the excitation trajectories is proposed in the paper.The method projects the excitation input to the null space of the coupling inertia matrix of the dual-arm space robot,satisfying the PE condition without disturbing the attitude of the base.