Inverse dynamic simulation and computer animation of extravehicular activity (EVA)

Abstract

Computational multi-body dynamics were used to simulate astronaut extravehicular activity (EVA) tasks. Two actual EVAs were simulated: manipulation of the Spartan astrophysics payload on STS-63 and attempts at capturing a spinning Intelsat VI satellite on STS-49. This research effort fills a current gap in quantitative analysis of EVA by employing computational dynamics, with emphasis on Kane's method, to solve the equations of motion for the dynamics of the astronaut's body segments and other interacting objects. The simulation approach can be divided into six phases: (1) model design, (2) system description, (3) equation formulation, (4) inverse kinematics, (5) inverse dynamics, and (6) data display with animation. The Spartan simulation is performed using a relatively simple seven segment astronaut body model with 6 degrees of freedom and motion restricted to a single plane. Results of the Spartan simulation reveal how an analyst might predict difficulties imposed by task specifications requiring violation of physiological limits, and modify the protocol so that the tasks objectives are humanly achievable. The more complex Intelsat simulation, using a 12 segment astronaut body model with 31 degrees of freedom, and interacting capture bar and satellite objects, each with 6 degrees of freedom, reveals greater challenges in terms of motion control and numerical integration. Interaction between the capture bar and satellite is modeled by means of constraint forces imposed at two contact points and achieves realistic motion of the two objects. Collision between the capture bar and Intelsat produces high acceleration spikes, which when used to perform prescribed motion of the astronaut body model lead to instabilities in the motion integration and high joint torque values. An initial attempt at controlling these instabilities produces improved transient behavior, but is unable to avoid eventual divergence. Future approaches to this problem, such as Baumgarte stabilization, are suggested.