The problem

No mechanical structure is completely rigid, so when we were approached by a customer asking for one, we were a little surprised. A box was needed, to hold optical components for scanning a distant object in 3D. Because the technology relies on measuring phase differences in light, the mechanical structure needed to hold the components stationary to within a few microns and micro radians. That’s pretty close to rigid. The product had different options, so the mechanical structure also had to be modular and quickly assembled without any special tools. The customer wanted to improve on an existing carbon fibre model. Coupland Bell consultants were asked to produce a concept design, in conjunction with a design agency.

Our solution

Carbon fibre was the clear choice for a structure that would be reasonably light, very rigid and be as thermally stable as we could reasonably make it. The main problem was that a projector would be mounted in a central box, resting on a tripod stand and a camera would be mounted in a separate module cantilevered from its side. The camera could be mounted on either side. This aspect of the design had already been fixed. This made the loads asymmetric with the supports, introducing a twist that would be hard to control to within micro radians.

Our consultants solution was to mount the projector within a cradle. The cradle and camera support would be separately mounted to a rigid structure on the tripod. This allowed the projector and camera to move together under gravity and thermal loads and balanced the twisting effect. Various different concepts for the mechanical structures were tried using FEA, leading to a simple cost-effective solution. For the quick assembly joints, our consultants modelled dove tails, fir trees, nested cylinders and tapered joints. The performance of each was quantified, leading to a table comparing the performance, advantages and disadvantages of each type, helping our customer decide which to use.