Considerable elasticity and nonlinear friction caused by harmonic transmission challenge the performance of flexible-joint manipulators. The uncertain dynamics of manipulator and the inadequate measurable states also limit the controller design. A new control method is proposed to address these problems, achieving the precise motion control of the flexible-joint manipulator. The method consists of three cascaded controllers: an adaptive controller, a torque-tracking controller, and a motor controller. The adaptive controller was adopted to generate the desired torque ensuring the robustness for uncertain dynamics. The torque-tracking controller derived the position compensation for motor control according to the torque error. As the elastic torque is under control, the vibration caused by harmonic drive can be eliminated. The motor was controlled based on poles-assignment method and friction compensation. The Kalman observer based on the Brownian motion model observed both velocity and the high-order derivatives of torque sensing. The stability of the control method was strictly proved. Calibration was performed on each joint to obtain the required joint stiffness and motor friction parameters. The control method was verified on a single joint and the frequency response of the system was obtained. The results show that the controller has good performance. The controller was realized on the self-developed seven-degree-of-freedom manipulator. The results reveal that the controller has high-precision tracking performance.
A space manipulator needs to exhibit high stiffness for tracking, and good compliance for capturing a tumbling satellite. However, the elasticity caused by harmonic drives challenges both the highprecision motion and the flexibility of the manipulator. In this paper, we propose a novel control method based on a robust torque-tracking controller to eliminate the unexpected vibration resulting from elasticity and reject the perturbation caused by nonlinear friction of the motor. In order to achieve the high-precision motion of the manipulator with uncertain dynamics, we combined the method with an improved adaptive method. The position-based impedance control, taking both the translational and rotational impedance into account, is added to achieve the soft-capture of a tumbling satellite. The stability of the proposed method has been strictly proven by the Lyapunov method. The space manipulator is mounted on a controlled chaser, forming a free-flying space robot. A loose coordinated control between the chaser and the manipulator is adopted in the task simulation. The simulations for verifying controller performance and the task simulation reveal that the manipulator has high-precision motion performance in the pre-capture phase, achieves soft-capture in the capturing phase and offers a reliable connection between the chaser and the target in the de-tumbling phase. INDEX TERMS Flexible-joint manipulator, capture of tumbling satellite, robust and adaptive control, free-flying space robot.
A novel control method is proposed to achieve high trajectory tracking precision, for flexible-joint manipulators. The method consists of three major parts: joint torque generator, joint torque tracker and motor position controller. The expected torque is generated by a PID controller based on the manipulator’s rigid dynamics model. In the torque tracker, motor position is corrected in both feedback and feedforward ways. Finally, the motor position controller is responsible to track the corrected motor trajectory to achieve the torque and position control. To suppress nonlinear friction, a disturbance observer is also implemented. The method is verified with a seven-DOFs manipulator. Simulation and experimental results show that, the proposed method is efficient and practical to suppress vibration caused by flexible transmission and disturbance due to friction. As result, high positioning accuracy is achieved in a certain wide working speed range. The no-load motion accuracy is better than 0.6 mm with a manipulator whose length is 1.8 meter, and the motion error is less than 3 mm with loading of four kilograms.
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