External Torque Estimation Using Higher Order Sliding-Mode Observer for Robot Manipulators

On-line monitoring of external torque/force is receiving much attention with the increasing demand in industries, where robot manipulators are required to co-operate with humans, assemble product parts or perform certain tasks that involve the interaction with fragile objects. Estimation of external torque when robot manipulator interacts with the external environment is investigated in this paper. A higher-order sliding mode (SM) based observer is designed to estimate the external torque on-line against nonlinear friction. Moreover, a Luenberger observer is designed to stabilize the general momenta error dynamics by using sum of squares (SOS) programming. Lyapunov analysis is considered to demonstrate the stability of overall system and convergence of angular position tracking errors. Simulation as well as experimental results using 7-DOF Sawyer robot under various practical scenarios demonstrate the effectiveness of proposed scheme.