On the dynamics and control of ETS-7 satellite and its robot arm

ETS-7 is the 7th engineering test satellite of NASDA (National Space Development Agency of Japan) to be launched in 1997. The satellite has a 6-DOF robot arm for space robot technology experiments. Mass of the payload varies from a few kg to 400 kg and mass of the satellite is about 2.2t. Therefore a dynamic interaction between the robot arm motion and the satellite attitude motion is not negligible for satellite attitude stability and robot arm motion planning. Attitude of the satellite must be stabilized since a communication link with a ground control station is established by a dish antenna directed to a data relay satellite in the geostationary orbit. Two wing shaped solar panels must also be pointed to the Sun. It is difficult to control the satellite attitude and a robot arm in a combined mode since the current spaceborne computer does not have enough computational power to handle the complicated dynamics in real time. This paper proposes a distributed and cooperative control method which connects the robot control system and the satellite attitude control system. The robot control system controls the robot arm and estimates the reaction force to the satellite. The attitude control system will compensate this reaction force by the feedforward control. The robot control system will also check its motion plan not to produce large attitude error. This distributed and cooperative control method avoids excess onboard computational power and complicated interface between the two control systems. This method is applicable for both reaction (momentum) wheel control and gas jet thruster control