Gravity Compensation for Impedance Control of Legged Robots Using Optimizationless Proportional Contact Force Estimation

Abstract

Impedance control of humanoid robots, a form of compliant control, allows them to move in a fashion similar to humans and increase the safety of interactions with humans or the environment. In low stiffness impedance control, gravitational forces will cause the robot to deviate significantly from the desired position. Thus, a gravity compensation term in the joint motor torque command is required to counteract gravitational forces. Ground reaction forces are sometimes used to estimate the gravity compensation torque required for each joint. In this paper, a novel method to estimate contact forces by using model mass properties and relative force and torque sensor data of each contact point with respect to all loaded limbs is proposed. This simple and straightforward method, called the proportional method, is able to resolve internal forces arising from closed-loop kinematic chains in multi-contact situations, for example the double support phase of bipedal robots, without optimization. The proposed method is also more robust to sensor error and is able to implicitly distinguish between gravitational and external forces for impedance control. Simulations and experiments using the humanoid robot HRP-4 are performed to validate the proposed method.