Deployable structures for masts and reflector antennas

by Zhong You

This dissertation presents several new concepts of deployable structures used for mast and antenna reflector applications.

First of all, a triangular pantographic mast has been proposed. Its deployable backbone is a three-dimensional pantograph of triangular cross-section, consisting of pairs of rods connected by a hinge in the middle. Cables are used for deploying and stiffening the pantograph. Deployment is due to an active cable whose overall length can be controlled by an electric motor. A special route for the active cable has been chosen, so that the remaining passive cables can become pretensioned after the structure is fully deployed. The general principles governing the route selection for the cables have been discussed. A 1.41 m high model of the mast has been built and tested. The results show that the proposed structure is both stiff and accurate, and its deployment process is highly repeatable.

Secondly, a concept for a deployable mesh reflector has been proposed. The reflector is based on a deployable edge frame that deploys and prestresses a 'rigid' cable network.

To form the edge frame, the concept of the pantographic mast has been extended to a circular form and, as a result, a family of deployable ring structures have been developed. For each member of this family, the special geometries which allow strain-free folding have been obtained and the implications of finite joint size have been considered. One of these rings has been chosen for the edge frame of the deployable mesh reflector. The structure is completed by a set of passive cables that become taut when the pantograph is deployed and are then pretensioned, together with one/two active cables that control the deployment of the pantograph and prestress the passive cables.

The last element of the mesh reflector is a cable network, for which two concepts have been proposed. They are made from tessellations of hexagons and triangles, upon which a reflective mesh could be attached. These two cable networks admit only two independent states of self-stress and thre is a particular global state of prestress that pretensions all of the cables. Tests on a 3.48 m diameter models have demonstrated the feasilibility of these concepts.

The key feature of the above structures, including the triangular pantographic mast, the ring structures and the cable networks, is that it is easy to set up the global state of prestress required for the successful implementation of these concepts.

Thirdly, the possible packaging methods and related deployment aspects of a collapsible rib tensioned surface antenna reflector have been investigated. The geometric constraint equations that govern the compact folding of flat membranes have been formulated and, based on these conditions, three folding schemes for the reflector have been proposed. These schemes have been extended to curved membranes. By these schemes, the membrane can be folded neatly either radially, towards the centre hub, or it can be wrapped around the hub. The geometric conditions underlying this approach guarantee that each folding pattern can be produced accurately, and is highly repeatable.

In association with one of three folding patterns, the dynamic deployment of a arib with a single fold has been studied. The large displacement, large rotation behaviour of a rib has been investigated experimentally, which led to a full characterisation of the moment/rotation relationship of the rib. A rigid-body analytical model has been developed, which is capable of capturing the key experimental observations and thus providing accurate simulations.

A sensitivity analysis tool based on the force method has been developed, for the optimisation of prestressed deployable structures. Furthermore, a shape adjustment method has been proposed, to remove unwanted shape distortion of the pretensioned cable network due to static loads and manufacturing errors.

Keywords: cable. cable network. deployable structure. folding. mast. membrane. pantograph. prestress. reflector. ring. sensitivity. shape. adjustment.