The four-channel bilateral system that is a hybrid of position and force in the acceleration dimension based on a disturbance observer can be divided into two modal spaces: common and differential modal spaces. In the four-channel bilateral system, bilateral position control is implemented in the differential modal space, and bilateral force control is implemented in the common modal space, respectively. On the other hand, In the three-channel system, unilateral position control is implemented in the slave side, and bilateral force control is implemented in the common modal space. This paper analyzes the performance of position control and force control in four-channel and three-channel systems under time delay. According to the analysis, this paper proposes a method of velocity difference damping that utilizes the velocity difference between the robot and the robot model. This method improves the performance and stability of bilateral force control in the common modal space without deteriorating the position control. The validity of the proposed control system is confirmed by some experimental results.
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