Original Date: 03/08/1999
Revision Date: 01/18/2007
Best Practice : Phase Shift Optical Triangulation
The key to drivetrain performance, in terms of noise and wear, is gear geometry accuracy. Both accuracy and cost, in turn, are dependent on the ability to perform precise measurements quickly and cheaply. Current practices employ coordinate measuring machines (CMMs) to inspect gears. Although accurate, this method is slow and provides sparse data, since it is dependent on physical contact with the gear. In response to this problem, the Applied Research Laboratory at the Pennsylvania State University (ARL Penn State) developed a non-contact, high-speed precision gear measurement system with sensitivity equal to CMMs, but with much higher speed and spatial data density.
The Phase Shifting Triangulation system, as shown in its current configuration in Figure 2-2, measures a gear surface and compares it to a master gear profile. To date, a surface profile accuracy of one micron, and a speed of one tooth per second have been demonstrated. The system provides output as an analytical report of gear error and as a 3-D topographical map. Developed for the Marine Corps Naval Air Depot in Cherry Point, North Carolina, the system can generate a 1000 x 1000 data point map in one second. The Phase Shifting Triangulation System utilizes a field measurement technique based on a four-step phase shift to measure the deformation of a projected grating. The amplitude of the deformation of the projected lines (stripes) is indicative of the surface differences. This data density (1000 x 1000 points compared to 3 x 3 points for CMMs) provides sufficient information to allow accurate predictions of noise and wear to be made a major step in cost and reliability enhancement.
To address real-world situations, the ARL Penn State enhanced the Phase Shifting Triangulation system with a recovery technique called Multiple Exposure Illumination Compensation. This technique can compensate for dirt or bright spots, which are points on gears where no data can be collected. These spots appear as white outs (flares) on phase maps. By performing multiple exposures at lower and lower illumination levels, this technique ultimately obtains good data at some illumination level for every point on the gear. By compiling the various maps at these levels, a compensated phase map is produced.
The Phase Shifting Triangulation system built for Cherry Point is based on comparative measurements against a master. The ARL Penn State has also built a system, based on an absolute measurement calibration, with a major supplier of gear inspection equipment. The current capability of inspecting spur gears can be expanded to include helical and spiral bevel gears. However, the system in its current configuration represents a major step forward in terms of speed, wear, and noise prediction, as well as the potential for affordability impact in drivetrain production.
Figure 2-2. Phase Shifting Triangulation System
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