The Space Station Alpha is the most significant international space project of this century and the largest international technology development project ever undertaken. The space robot manipulators will be a substantial part of the space station and will perform tasks such as assembly as well as maintenance of the station. Therefore the robot manipulators need a very sophisticated real-time control capability for gross and fine motions (i.e. compliant motions) during various operations. Moreover, the proposed dual-arm robot system servicing the Space Station requires automated motion coordination, synchronization of the arms, and controlled mechanical interaction with fixed and moving objects involved in various tasks.

The paper deals with on-line obstacle avoidance in an unstructured environment based on the force strategy. The obstacle avoidance is considered as a control problem. We discuss three approaches regarding the sensors used to detect the obstacles. First we investigate how obstacles can be avoided without using any sensors to detect them. To solve the problem we make use of a very basic principle: an action causes a reaction. For backdrivable manipulators we propose a controller which ensures stiff behaviour in the task space and compliant behaviour in the null space. Using such control the tracking capabilities in the task space can be preserved and the redundant degrees of freedom are used to avoid the obstacle after the collision. The tactile sensors are proposed to be used as the alternative for stiff systems. A tactile sensor detects an obstacle and the controller generates the avoiding motion. Last, we present a virtual forces approach which can be applied to the systems with proximity sensors. The objective is to assign each point on the body of the manipulator, which is close to the obstacle force component in a direction that is away from the obstacle. The proposed formulation avoids the problem of singular configurations and is very suitable when many obstacles are present in the neighbourhood of the manipulator. The computational efficiency of the proposed algorithms allows real-time application in a unstructured or time-varying environment. The efficiency of the proposed control algorithms is illustrated by simulations of highly redundant planar manipulators and by experiments on a four link planar manipulator.