Switching mechanism analysis and motion control

Switching mechanism analysis and motion control

The behavior of a complex mechanism was correctly predicted from a detailed simulation. The mechanism analysis led directly to useful changes to the motion control strategy.

Infrared “night vision” systems often have a field-of-view switching mechanism. This mechanism electromechanically changes the front-end “afocal” lense assembly in front of the infrared camera. Changing the lenses changes the magnification of the optical system and allows the operator to switch from a “wide field” (zoom out) view for best target search to a “narrow field” (zoom in) view for target definition and possible weapon sighting.

The FOV switcher shown is an unusual design, in that it combined mirrors and lenses in a folded arrangement. The standard afocals found in most switchers are straight-through lense assemblies, similar in concept to the lenses found in 35mm camera bodies. The folded arrangement shown was necessary to fit into the extremely tight space provided by the host vehicle. With the switch folded down, the sight would look through a large diameter, high-zoom, narrow-field afocal. The switcher would pop-up to interrupt the incoming light and direct it through a lower magnification wide-field afocal.
The program schedule did not allow for a trial-and-error prototype phase. The mechanism had to work out-of-the-box. A dynamic modeling effort was undertaken to produce a dynamic model and real-time simulation of the mechanism. A translation tool was employed to directly extract geometry, connectivity, and mass property information from the CAD model of the mechanism. The motor and motor drive were modeled along with g-forces from the vehicle environment.

The model was particular tricky to simulate. In order to deploy within the available vertical “shaft space” allowed, several unusual elements were incorporated into the mechanism. These included a curved sliding contact element and a contacting “trip mechanism” to pop up and pop down the upper mirror. This change of configuration required a different set of equations of motion for the different operating regions of the mechanism. Momentum and energy in the simulation had to be correctly accounted for during transitions from one equation set to another.

The results of the simulation indicated that mechanism would deploy within the specified time period in almost all its operational scenarios. The simulation predicted that there was one condition, however, in which the mechanism would not only fail to deploy within the required time but would actually stall and not deploy at all. The program office and customer accepted this as a small risk. Interestingly, several years after deployment, field trouble reports came back to the factory reporting that the mechanism failed to deploy in the exact scenario predicted by the simulation. Fortunately, a simple motor upgrade, not possible under the original program schedule, rectified the problem.

keywords: mechanism design, mechanism simulation, mechatronics, motion control