Impedance Control of Redundant Manipulators

Abstract

This thesis presents research work related to the impedance control of redundant manipulators. The main goal is to study the effects of redundancy on the dynamic behaviour of manipulators. Firstly, theoretical developments related to redundancy, dynamics and impedance control are presented in general form without restriction to any particular manipulator. The purpose is to keep the findings useful for other developments and continue the research on a wider scale. Later, the Water Hydraulic MANipulator (WHMAN), which consists of redundant degrees of freedom, is used for simulations and experiments. WHMAN is developed at the Department of Intelligent Hydraulics and Automation at Tampere University of Technology (IHA/TUT). The purpose of this manipulator is to provide assistance during the ITER divertor maintenance and it requires position and force control during these operations. The analytical model of WHMAN in the form of state space equations is used for mathematical analysis and numerical verification. The verified simulation model is used for the development and verification of controllers. The position controllers of the joints are developed using a linearized model and then fine-tuned using a nonlinear model and the actual manipulator. The results show that models of WHMAN can be utilized for control design and further study. Both position-based and equivalent force-based implementations of impedance controller were tested with the simulation model of WHMAN. The simulation results showed unstable behaviour of the manipulator for equivalent force-based implementation. Therefore only the position-based implementation was used in the experiments. The results show that, in the absence of linear mapping between joint-space and operational-space, the development of equivalent force-based impedance controller is not straightforward and requires separate design for the inner-loop force controller. The theoretical, simulation and experimental results show that the existence of redundant degrees of freedom can result in improving the dynamic performance of a manipulator and thus the impedance regulation capabilities.