Self-reconfigurable Modular Robot

Attitude Control in Space by Control Moment Gyros

Pipeline Inspection System


Attitude Control in Space by Control Moment Gyros

Reaction wheels are used for attitude control of current artificial sattelites in space. A reaction wheel generates torque by changing the rotation speed of its flywheel. For larger sattelites and a space station, Control Moment Gyros (CMGs) are advantageous in their high output torque and rapid response. The main difficulties of CMG systems are complicated control algorithms and their singularity problems.

A CMG contains a flywheel rotating in constant speed and a gimbal which supports the flywheel. By rotating the gimal, an output torque is generated. For three axis attitude control, no less than 3 CMGs are required and they must be controlled to genarate a required torque. This control argolithm needs calculation of a matrix and its inverse which is repeated frequently.

If the matrix is singular, inverse calculation is impossible. In this system state, all the possible torque vectors of the system are on a 2-dimensional plane in the 3 dimensional space. About the remaining direction, therefore, the system cannot generate any torque in this state. Thus, this kind of sinular state must be avoided for smooth attitude control.

By the recent research, it was shown that any CMG system has singularity for any given direction. This means even redundant systems have singularity problems. The singularity problem, however, has been expected to be solved by control using redundancy.

It was revealed that there are two types of singularity, passable and impassable by differential geometric study and passable singularity can be avoided by controls using redundancy. Some CMG systems of no less than 6 units only have passable singularity, hence can be controlled without meeting singularity.

The pyramid type CMG system of 4 units which has been widely studied is not the above case. It has impassable singularities hence its control is not easy. It was shown by a global geometric study that any control fails to avoid singularity if some large work space is considered. Based on this result, maximum work space for continuous sigularity avoidance control was introduced and actual argolithm for this space was proposed.

Those results by geometric analysis and the performance of the proposed control method were verified by using a ground simulation platform for attitude control.

A Geometric Study of Single Gimbal Control Moment Gyros - Singularity Problem and Steering Law -, Report of Mechanical Engineering Laboratory, No. 175, p.108, 1998.