Understanding the True Dynamics of a Space Manipulator from Its Testing with Air-Bearing based Support Equipment

Abstract

A space manipulator can easily handle a large payload with a much larger mass than the manipulator itself in a 3D space. However, physical test of a long space manipulator handling a large-mass payload on the ground, even in a 2D space, is very difficult due to the gravity effect. A common approach for such testing is to use a support equipment with air-bearing pads to support the manipulator and the payload such that the robot can operate in a horizontal floor like floating in a microgravity environment. However, such a test methodology suffers a major problem: the air-bearing based support equipment is large and massive when the manipulator is long (e.g., 10 meters or longer) and/or its payload is heavy (e.g., 500 kg or heavier), which adds significant inertial loads to the robot when it maneuvers with the test system. As a result, the manipulator has to make significantly more effort than it needs for the same task in space. Moreover, the attached air-bearings along with the support structures also change the dynamics characteristics of the manipulator such as the natural frequencies, stiffness, damping as well as the performance of the robot’s closed-loop control system. The problem may not be so significant when the manipulator is small (say, less than 5 meters long) but it basically invalidates the test when the manipulator is large such as the Space Station Remote Manipulator System (SSRMS). In this paper we present a study to understand the true dynamics of the tested space manipulator. In the study we developed a general model of the extra load exerted on the manipulator by air-bearing based support equipment. Understandably, modeling the support equipment is much easier and more accurate than modeling the manipulator because the former is just a simple and passive structure. Then, we use this load model to decouple the dynamics of the tested manipulator from the rest of the tested system. With such an approach, we can understand the true dynamics of the space manipulator although it has to be constrained to a massive air-bearing based support equipment for testing.