TRW Cam Gears manufacture the steering wheel racks for Ford cars. These have a variable gear ratio along their length allowing greater sensitivity near the centre of the travel for high speed driving and less sensitivity at the ends for turning sharply in either direction. These variable ratio gear racks are difficult to make. Ford require quality control testing of all racks.
Pan Controls built a control system for driving a small test rig to inspect the gear ratios of these racks. A servo-motor and leadscrew are used to move the rack longitudinally. This movement is measured by a linear position encoder. The rack turns a steering column shaft and this turning is measured by a rotary encoder. Finally a third probe-type encoder measures any sideways displacement of the steering shaft. The rotation and displacement of the steering column are then compared with the longitudinal position using some custom software. The relationships are compared with tolerance limits and displayed on a screen. The software decides if the component passes or fails this test. A dot matrix printout is used to produce a test certificate with graphical plots of the effective gear ratio, shaft displacement, date, reference number, and pass/fail decision. Part of a typical test certificate is reproduced in the following figure.
So far, two similar rigs have been built. The first is used for development work in a laboratory and the second for production testing on a factory shopfloor.