Examining the Passive Stiffness Workspace Using Variable Stiffness Robots

Abstract

Passive stiffness control is a method for managing contact forces and dynamics between a robotic manipulator and its environment. Compliance control is typically implemented in redundant serial manipulators using a force torque sensor and active software algorithms. However, time delays in these algorithms can cause large impulse forces between the manipulator and its environment. For applications with limited computation power, large time delays, and low damping, such as In-space Servicing Assembly and Manufacturing (ISAM) and Active Debris Removal (ADR), these effects can cause a manipulator to push away or tip off the target, preventing successful capture.

This thesis examines the implementation of passive stiffness control in a redundant serial manipulator using Variable Stiffness Actuators (VSA). Unlike traditional robot actuators, VSAs have an adjustable stiffness element in series with the primary joint position/control motor to generate varying end-effector position and stiffness. These adjustable springs act as low pass force filters to increase the actuator robustness against external loads at the cost of positioning accuracy. Different optimization algorithms are used to vary the VSA joint stiffness to achieve a desired Cartesian stiffness matrix. However, there are severe limitations with the passive joint to Cartesian stiffness mapping performance over the whole robotic workspace due to the significant kinematic configuration dependence.

Given this limitation, this work attempts to answer for a single, well-defined task, are there regions of the workspace where a prescribed level of passive stiffness realization can be achieved? Or for a mobile robot, can we plan trajectories within a region of the workspace to improve realization performance? This is done by first examining the implementation of three passive stiffness realization methods, each with increasing performance. Next, the idea of Successful Task/Stiffness Trajectories and Success Task/Stiffness Regions are introduced as a way to examine the workspace dependency of the passive stiffness realization. Finally, the application of passive stiffness control for ISAM and ADR applications is studied, and unique design objectives for the manipulator are proposed.