Speaker
Mr
W. J. O'Connor
(School of Mechanical and Materials Engineering. University of College Dublin)
Description
We consider the problem of guidance and control of a completely passive, target, piece of debris, using an actively-controlled, “chaser” spacecraft, connected to the debris by an elastic tether. Compared with robotic capture, the use of an elastic tether reduces the need for precise chaser-target coordination during capture, reduces collision risk during capture, simplifies subsequent de-tumbling, makes for lighter and mechanically simpler systems, and limits subsequent jerk with its potentially destructive consequences.
The target debris is likely to be tumbling initially, so the chaser must achieve active de-tumbling before, or during, further manoeuvres of the “stack” (debris-tether-chaser), such as imposing a delta-V in order to de-orbit. The preceding task, of attachment of the tether to the debris, could be achieved in various ways, including by net capture or harpoon. A novel net capture technique will be briefly presented, but the techniques considered here will apply regardless of the attachment method. Note that some proposed capture methods require a separate de-tumbling prior to capture, whereas methods using an elastic tether can de-tumble after capture, thereby reducing mission complexity, uncertainty and mass.
The first control challenge, then, is precisely due to debris tumbling prior to capture. If there are energy loss mechanisms on board, the debris rotation will probably be mainly around the axis of maximum moment of inertia. If so, and if the dominant debris rotation axis is approximately aligned with the tether axis, its main effect after capture will be a twisting action on the tether. If the rotation axis is approximately perpendicular to the tether, the debris will tend to wind up along the tether. In either case, the chaser needs to respond so as to reduce the tumbling towards zero over time, while imposing control and avoiding tether entanglement and debris-chaser collisions. The general case can be a dynamic combination of these two extreme cases (with spin, nutation and precession), with possible slippage during any wrapping.
Aside from de-tumbling, a further control requirement is to change the velocity of the debris-tether-chaser stack by a target mount in a target direction, while again avoiding entanglement, collisions, and uncontrolled pendular and libation motions, both during and after this delta-V manoeuvre. There may also be a subsequent requirement to allow the stack to coast for a relatively long period while ensuring the tether stays fully extended, to avoid entanglement during the period, and to be ready for a subsequent stack manoeuvre.
The paper will show how wave-based control easily meets all these requirements using a single control strategy, combining position control and active vibration damping. The chaser-debris interaction is modelled as two-way wave motion, longitudinal (stretching) or rotational (twisting), travelling back and forth along the tether between chaser and debris. It can be shown that the effect of the control is to make the actuator behave as a viscous damper (motion absorber) to wave motion travelling in the tether from the debris towards the chaser. In this way when the tether is under tension the debris experiences a force and torque due to the tether which appears to have a viscous damper to ground at the far end, causing it to tend exponentially towards its natural (unstretched) length, and/or zero-twist condition. Meanwhile the chaser can simultaneously impose a desired velocity (or attitude) using an outgoing wave. The details of this combined control action and its implementation will be explained. Its advantages over classical control approaches include being simple to implement; very robust to wide variations in the debris parameters (which may be unknown); requiring minimal sensing; and all sensing can be done at the chaser end.
The technique is tested in a detailed computer simulation of a complete de-tumbling of an arbitrarily shaped structure undergoing arbitrary tumbling motion. Then a complete de-orbit of the Envisat satellite to a target location is simulated and tested, and parameters such as fuel requirements assessed. The control technique was also tested on hardware using two model hovercrafts, one remotely controlled.
Keywords: Debris removal, elastic tether, stack control, de-tumbling, mechanical waves, GNC
REFERENCES
[1] “CDF study report e.deorbit”, Technical report, European Space Agency, Sept. 2012.
[2] W. O’Connor. Wave-like modelling of cascaded, lumped, flexible systems with an arbitrarily moving boundary, Journal of Sound and Vibration, 330 (13):3070–3083, 2011.
[3] Aslanov, V., and Yudintsev, V., ”Dynamics of large space debris removal using a tethered space tug”, Acta Astronautica, Vol. 91, May 2013, doi: 10.1016/j.actaastro.2013.05.020.
[4] Aslanov, V., and Yudintsev, V., ”Behavior of tethered debris with flexible appendages”. Acta Astronautica, Vol. 104, July 2014, doi:10.1016/j.actaastro.2014.07.028.
[5] Schaub, H., Valery, T., Jasper, L., Seubert, C., and Yutkin, E., ”Tethered tug for large low earth debris removal”. Advances in the Astronautical Sciences, AIAA, 2012.
[6] Schaub, H., and Jasper, L., ”Input shaped large thrust maneuver with a tethered debris object”. Acta Astronautica, Vol. 96, Nov. 2013.
| Applicant type | Co-author |
|---|
Primary author
Mr
W. J. O'Connor
(School of Mechanical and Materials Engineering. University of College Dublin)
Co-authors
Ms
Deborah Hayden
(School of Mechanical and Materials Engineering. University of College Dublin)
Mr
Sean Cleary
(School of Mechanical and Materials Engineering. University of College Dublin)
