Passive Local Force Feedforward Control in Bilateral Telemanipulation with External Disturbances

Abstract

A key characteristic of bilateral telemanipulation is the desire to provide haptic feedback to the human operator to aid in performing a given telemanipulation task. For this purpose, force-torque sensors are often employed to measure the environment forces at the end effector, which then serve as inputs for the control law of the system. However, the passivity analysis of the bilateral controllers relying on the environment forces often don’t consider that some interaction forces with the environment might not be measured by the force-torque sensor. In this thesis, the effects of these external disturbances on the passivity of local force feedforward are investigated, which is often employed in bilateral controllers. Based on a practical scenario possibly encountered during robotic surgery, it is shown that external disturbances may lead to a loss of passivity and make the system unstable. Therefore, a series of algorithms based on the Time-Domain Passivity Approach (TDPA) are proposed and evaluated in this thesis, enhancing the standard formulation in multiple aspects. First, to reduce conservativeness, the passivity observer is designed with respect to the power-scaling relationship between the robot and the environment according to the inertia scaling factor realized by the local force feedforward control. Second, the feedforward gain is modulated to ensure passivity in cases when damping injection by itself is insufficient. Third, the restoration of the passivity is distributed in time to reduce the discontinuities in the commanded force, leading to the proposal of the novel passivization method termed compliantly controlled passivity in this thesis. The proposed controllers with gain modulation successfully stabilized the example system, and the novel compliant passivity controller achieved passivization without discontinuities in the commanded force. The proposed passivization of the external disturbances is expected to be applicable to a wider range of bilateral control architectures, and the novel passivity controller may be used to improve TDPA in other contexts as well.