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 showstopper
The maximum possible accuracy achievable during an attitude tracking manouvre using VSCMG (or CMG) as actuators, is severely limited by the spacecraft flexibility. See Izzo, D. and Pettazzi, L., "Command shaping for a flexible satellite platform controlled by advanced fly-wheels systems", Acta Astronautica, Vol 60, No.10-11, pp 820-827, 2007.
Reaction wheels (RW) have been extensively used to actuate the torque demand of a space structure. Their capability is though quite limited and a higher torque demand, such as that of a large structure in space, requires different devices. Control Moment Gyroscopes (CMG) have been proposed and implemented for this purpose in large structures such as the ISS. 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 reorientating the spin axis a Variable Speed Control Moment Gyroscope (VSCMG) is obtained. The ACT studied these devices from a mathematical modeling point of view and developed and tested numerically steering laws able to maximise 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. To achieve the highest possible accuracy, the flexibility of the structure has to be accounted for as these devices would probably be implemented on very large structures.
The final results from a modeling point of view may be found in the following research papers downloadable from the ACT publication pages:
1. Izzo D., Pettazzi L., Valente C.: "A comparison between Models Representing Flexible Spacecrafts", 6th International Conference On Dynamics and Control of Systems and Structures in Space, Riomaggiore, Italy, 18-22 July, 2004.
2. Izzo D., Bevilacqua R., Pettazzi L.: "Taking into account flexibility in Attitude Control", 6th International Conference On Dynamics and Control of Systems and Structures in Space, Riomaggiore, Italy, 18-22 July, 2004.
3. Izzo, D., Pettazzi, L.: "Command Shaping for a Flexible Satellite Platform Controlled by Advanced Fly-Wheel Systems ", Acta Astronautica, Vol 60, No.10-11, pp 820-827, 2007.
Following the results obtained in this preliminary phase of the study the ACT is currently working on concepts to alleviate the phenomenon. The work brought forth the idea of using the flexibility of the structure to actuate high precision pointing requirments. This idea, the structure deformation being controlled either by piezo-electric materials or via artificial muscles, is under investigation by using the mathematical models developed and the results on artificial muscles that the ACT has gathered during the completed Ariadna study "EAP-based artificial muscles as an alternative to space mechanisms", studied in cooperation with the Universities of Pisa and Reading.