This paper tackles a friction compensation problem without using a friction model. The unique feature of the proposed friction observer is that the nominal motor-side signal is fed back into the controller instead of the measured signal. By doing so, asymptotic stability and passivity of the controller are maintained. Another advantage of the proposed observer is that it provides a clear understanding for the stiction compensation which is hard to be captured in model-free approaches. This allows to design observers that do not overcompensate for the stiction. The proposed scheme is validated through simulations and experiments.
This paper is basic study on vibration control, disturbance rejection and friction compensation in robots with flexible driving systems. Firstly, the system gain characteristic at the antiresonance frequency is introduced to evaluate the vibratory behavior of the control system. Secondly, the joint torque negative feedback which has a good effect on vibration suppression is discussed, while it is shown that the property of disturbance rejection is easily deteriorated when using high-gain joint torque feedback. Based on the assignment of pole-distribution, the relations between the vibration suppression and the disturbance rejection are analyzed. Thirdly, a feedforward compensation control based on a friction observer is proposed, it is shown that the tracking error is effectively decreased by this friction compensation. Lastly, a position control system considering the trade-off between vibration suppression and disturbance rejection is presented, and experimental results are also given. In this paper, a disturbance observer based control algorithm is proposed for industrial robots having flexible joints. The joint flexibility of the robot is modeled as a two mass system. We study on the practical issues for implementing disturbance observer based control scheme in flexible joint robots. For industrial robots, generally the sensors are located on the motor side. If we construct disturbance observer using motor side dynamics, due to the zero dynamics, disturbance observer cannot directly reject the disturbance at the link side. To solve this problem, we propose a dual observer that estimates disturbance and states simultaneously. Using the proposed dual observer, we construct full state feedback controller. The effectiveness of the proposed control scheme for disturbance rejection and robustness is demonstrated by numerical simulation and experiment using HILS (Hardware In the Loop Simulation) system. The authors propose a novel control method based on two observers to suppress the vibration of a flexible joint, even under fast motion. One observer is a disturbance observer and is used for the realization of an acceleration controller of the motor axis. The other observer estimates the link velocity and is effective for the suppression of torsional vibration. The disturbance observer estimates the total sum of the disturbance torques which are imposed on the motor axis. Feedback compensation by this estimated disturbance torque increases the servo stiffness and the robustness of the motor portion. The feedback of the estimated link velocity totally suppresses the torsional vibration of the flexible joint even under fast motion. The proposed control was implemented in a 16-b microprocessor with a digital signal processor for a laboratory test. > 