Fast measurement without penetration

A non-contact measurement system has been developed that solves the problem of how to measure the height of transparent glass assembly items. Dean Palmer reports A major optoelectronics company, who has asked to remain nameless, faced a problem on its production line recently. Automation engineers at the plant had to solve the problem of how to measure the height of a device that was transparent without touching it. The problem stemmed from the fact that production targets at the plant had to be met, which meant automating the assembly line. In the optoelectronics industry, automating an assembly line isn't straightforward. The nee for accurate placement and alignment are key to product performance and many of the components being assembled are highly sensitive to damage or contamination. One such component was a small glass device used to accurately align one of the key parts in the final assembly. Equipment was needed that would be able to hold a carrier containing a number of devices. It needed to go to specific places on the holder and measure a step on each part. This step needed to be recorded and associated with a location, depending on the size of it. In each of these locations were different sized glass parts, each manufactured to suit different step sizes. The requirement was then to mount the corresponding glass part on to the assembly so that all the finished step sizes were exactly the same. This then needed to be qualified post mounting of the glass. The company turned to industrial non-contact measurement specialist Scantron based in Taunton. Scantron's 3D surface profilometer, Proscan. This unique technology seemed ideal for measuring displacement. A measurement solution was then developed whereby the operator placed the holder onto the Proscan stage using a vacuum mount to hold the devices true and still. The operator then simply presses a 'Go' button and Proscan automatically begins its measurement cycle. The table moves so that the sensor is in the correct start position to begin measuring. The step is then measured, with the resulting size recorded. This routine is repeated for all the devices on the holder, where on completion, the software automatically assigns a glass part location to each device. Once the glass has been fitted, the holder is placed back on to Proscan and the sequence is run again to qualify that all the step heights are now within the specified allowable tolerance. Two issues were key to the project's success. First, whatever solution was offered had to be fast. And second, the solution had to measure onto glass without penetration. Non-contact measurement systems are generally seen as faster than contact methods as they do not have to wait for the probes to move up and down to take measurements. But some non-contact measurement systems need to penetrate the object being measured, using lasers which rely on the reflected light from a surface to function. Using Scantron's chromatic sensor overcame both these problems though. Because the sensor is non-contact, it is certainly fast, but it can also be set up so as not to be affected by penetration. Although light from the sensor does penetrate the transparent object, it reflects a spot of a certain frequency from the upper surface and a different spot with a different frequency from the lower surface. Using this information, it was possible to set Proscan so that it could only see the upper spot and ignore the lower one. The new system has proven to be such a success that the optoelectronics company is now also considering combining the measurement data and location information with a robotic pick-and-place station, to fully automate the glass mounting process.