Variable Speed Control Moment Gyroscopes
Advanced attitude actuators could offer an agility option to future generation satellites. But their coupling with the spacecraft flexible modes (inducing vibrations) could be a critical show stopper. The maximum possible accuracy achievable during an attitude tracking maneuver using variable speed control moment gyroscopes (VSCMG) as actuators, is severely limited by the spacecraft flexibility.
Project overview:
Reaction wheels have been extensively used to actuate the torque demand of a space structure. Their capability is though quite limited and higher torque demand, like the one present on large structure in space, and requires a different approach. Control Moment Gyroscopes (CMG) have been proposed and implemented for this purpose in large structures such as the International Space Station. The problems connected to the singular configurations of these devices have created quite some problems in the past. By allowing the wheels to vary their spin speed, while re-orientating the spin axis, a VSCMG is obtained. The ACT studied the VSCMG from a mathematical modeling point of view, and developed and tested numerically steering laws, which were able to maximize the advantages of these devices. A double gimbal VSCMG would in fact be able, alone, to perform three-axis control of a platform realizing higher slew rates than a standard system of three reaction wheels. In order to achieve the highest possible accuracy though, the flexibility of the structure has to be accounted for.
The ACT looked into using the flexibility of the structure to actuate high precision pointing requirements. In this approach, the structure deformation is being controlled either by piezo-electric materials or via artificial muscles. For the mathematical modeling of the actuator, we utilized results on artificial muscles obtained during the Ariadna study "EAP-based artificial muscles as an alternative to space mechanisms", studied in cooperation with the Universities of Pisa and Reading.