Structural Mechanisms

Future space projects involve the use of intelligent structures: a structural system whose geometric and inherent structural characteristics can be changed beneficially to meet practical requirements, either through remote commands or automatically in response of external environmental variations. Such a system incorporates actuators and sensors into the structure. From the viewpoint of structural engineering, a key element in this development is to develop robust structural concepts which provide for the capability to change configuration, i. e. structures able to produce different working geometries and also able to move from one configuration t o another. For example, the designers of deployable structures for satellite radars and antennas use standard structural components (rods, plates, shells) connected by pivoted or sliding joints and arranged in such a way as to form a structural mechanism. Unique features of a structural mechanism in comparison with conventional linkages such as 4-bar chains are that it is of much greater size, highly overconstrained and usually with a single internal degree of freedom. Also, sliding joints are avoided.

Mechanism design has always been a challenge to engineers. The problem can be stated as follows: "to find a mechanism topology which can produce a desired output movement for a given input movement." Usually, one relies on previous experience or on a CAD package, with a consisting of many different mechanism topologies from which a candidate mechanism can be selected and then proportioned to produce the desired motion. This approach cannot be applied to the design of structural mechanisms as it tends to produce impractically complex linkages whose joints are not suitable to carry loads included in large structural applications. In many cases, this approach simply does not work if the problem is over constrained, as the geometric constraints are non-linear and their number is too high. The multi-configuration requirements of intelligent structures add a further degree of difficulty.

In my current research, I have used an approach different from the one described above. I started with geometric compatibility equations and simplified them by using symmetry to reduce the size of the problem, then I made some assumptions to reduce the number of design parameters, and finally obtained a class of generic solutions. This approach worked extreamely well in discovering a number of different mechanisms, which are shown in next few sections, and gave me some insights into geometry of mechanisms. For the new multi-configuration problem, I plan to draw together my experience in dealing with geometric synthesis and also on and the earlier work in structural optimisation, as I believe that a new and systematic approach has to be taken in order to overcome the greater difficulty of multi-configuration problems.

This page is created by Zhong You, who is a university lecturer in the Department of Engineering Science of Oxford University. This is a link to his current address. You may e-mail zhong.you@eng.ox.ac.uk for further infomation.

Zhong was an EPSRC advanced fellow based at Cambridge University before moving to Oxford.