Gravity compensation suspension systems are essential to support space
structures during tests on Earth, but also impose constraints on the structures
that have the effect of changing their behaviour. The deployable structure
used for the present study is a model of a rigid panel type solar array
that is able to deploy and retract automatically. A computational
and experimental study of the interaction of this structure with a manually
adjustable suspension system, during quasistatic deployment tests, is presented.
A methodology is established for modelling this interaction, for predicting
the effects of suspension system adjustments, and for optimisation of the
suspension system through these adjustments. It was found that some
improvements can be achieved by manual adjustment, but further optimisaiton
requires an active system. The two significant substructures of the
active system are: a deployable support mechanism that mirrors the
structure, and seven suspension devices that contain force and displacement
transducers and control the length of the support cables by a screw and nut
actuator. A theoretical representation of the active suspension system
is established in an analogous way to that for the passive system.
Two different gravity compensation strategies, displacement control and force
control, are implemented. Experiments carried out with the active suspension
system are presented for these two schemes and their gravity compensation
capability is evaluated and compared. It was found that gravity can
be compensated by controlling the forces in the suspension elements to between
10% and 20% of g, however by controlling the displacements, the compensation
is more than twice as bad. The latter strategy was, in practice, only
marginally better than the passive system. Better results would have
been possible with actuators having a higher positioning accuracy.
This research has shown that a system with a self-deploying overhead structure,
and active vertical suspension elements is a good concept for multi-point
gravity compensation and should be further developed.
[Cambridge University | CUED | Structures Group | Geotechnical Group]