A Novel Gravity Compensation System for Space Robots

Abstract

Realistic experimental investigation of the behavior of space robots requires simulation on earth of the micro-gravity environment existing in space. This is particularly true due to the typically long reach and high structural flexibility of manipulators designed for low- or zero-gravity operation. Two-dimensional testbeds, using air bearings or suspension systems, may be satisfactory in some cases, but do not allow 3-D testing. Neutral-buoyancy testing is often used, but complicates the manipulator design and testing, and introduces inertial and viscous effects that can substantially affect dynamic behavior. Other "gravity-compensation" schemes have been implemented, but typically require high power, sophisticated sensing and precise control, and have restrictive bandwidth limits. An approach which obviates most of these disadvantages is to suspend the robot with a cable from an electromechanical system that passively generates a vertical counterbalance force, while tracking the robot's horizontal motion actively or passively to minimize horizontal disturbances. We have implemented two such systems that enable realistic testing of our Self Mobile Space Manipulator (SM2), a robot being developed to walk on the exterior structure of the Space Station Freedom. This paper provides an overview of our gravity-compensation systems including mechanical hardware, sensing and electronics, real-time control strategy and software, and experimental results.