Gravity Compensation Systems for Deployable Structures

What are gravity compensation systems?

Every space structure must undergo an extensive ground validation test program to ensure in-orbit reliability and performance in space. During testing on Earth, however, the structural behaviour is altered by the effect of gravity. In order to obtain reliable predictions for the behaviour of a structure in space, it is therefore necessary to minimise the influence of gravity during ground tests by means of a suitable gravity compensation system.

How to simulate zero-gravity?

Various methods exist:

Physical methods:
- drop towers
- parabolic flight manoeuvres
Buoyancy methods:
- buoyancy in water
- water floats
- helium balloons
Pneumatic methods:
- air cushions
- air bearings
Mechanical methods:
- counterweights, pulleys, cables, trolleys, rails, ...
- Zero Spring Rate Mechanisms
- passive and active suspension systems

For supporting the large-scale 1D, 2D or even 3D motion of deployable structures multi-point suspension systems are the most suitable concept.

What is the problem with suspension systems?

Deployable structures are due to their large and light built very flexible. Also their structural properties often change considerably during the shape transformation. That makes it difficult for the suspension system to carry exactly the weight of the structure throughout the entire range of motion without imposing extraneous constraints and hence producing internal stresses which distort the static and dynamic properties and therefore the deployment behaviour of the structure.

How to solve the problem?

Adjustable support systems provide the ability to adapt for the changing structural configuration during deployment. By optimisation of the support system adjustments during the shape transformation via active control the interaction between structure and gravity compensation system can be minimised.

An example:
Suspension system of a rigid panel solar array

The solar array structure:
The unfortunately somewhat wobbly and blurred animation shows the laboratory model of the solar array during deployment and retraction. The solar array is constructed from rigid panels which are hinged together such that the array folds like a concertina. Deployment and retraction is achieved with two active cables which are wound around pulleys located at either end of the hinge shafts. The cables are connected to a single motorised drum which is driven by a stepper motor controlled with a PC. animation

Overall length of solar array: about 2m

The suspension system:

As the photo of the fully retracted configuration shows, the weight of the array is supported at seven points with one single-point and three double-point suspension elements. The suspension elements consist of steel cables with strain-gauged turnbuckles which are connected to the hinges at one end and to horizontal tubes at the other end. The tubes are linked via rods to trolley pairs which run on horizontal rails. With the turnbuckles the length of the cables can be manually adjusted and with the strain gauges the tension force in the suspension cables is measured.

Experimental observation:
Experiments have revealed that the variation of the suspension forces in the cables during deployment due to the changing stiffness of the structure and unavoidable imperfections in the experimental set-up is surprisingly high indicating substantial interaction between structure and suspension system.

Computational modelling:
Simulations with computational structural models of the array and the suspension elements have shown that the measured suspension forces differ significantly from the forces predicted for optimal gravity compensation and that the distribution of the weight of the array amongst the suspension points can be optimised by length adjustments at the suspension cables.

Effect of manual length adjustment at the suspension system:
The following graphs show the suspension cable forces at the seven suspension points (node 1-7) throughout deployment. The deployment angle is zero and 90 deg. in the fully deployed and retracted configuration, respectively.

		The first graph gives the variation of suspension forces which is required for optimal gravity compensation according to computational results.
		The second graph shows one out of many other possible suspension force variations which can be measured during deployment depending on the initial length adjustments at the suspension cables. According to the difference between the measured and required suspension forces the necessary changes for the length adjustment at the suspension cables are calculated from the computational models.
		The third graph shows the measured suspension forces after the length of the suspension cables has been changed. The corrections have been carried out for the fully deployed configuration (deployment angle equals zero deg.). The improvement of the suspension force distribution for the considered configuration is clearly visible.

Conclusion:
Since with the present passive suspension system only manual length adjustments are possible, the required corrections can only be carried out for one particular deployment configuration. In order to achieve a suspension force distribution for optimal gravity compensation throughout deployment, it is essential that the length of the suspension cables can be actively controlled during the shape transformation of the solar array structure.

Actively controlled suspension system:
The passive experimental set-up has been converted into an active system equipped with sensors and actuators, see the movies next.
Deployment (click here) Details (click here)

[ Cambridge University | CUED | Structures Group | DSL ]

Last updated 6th April, 2001