Measuring Glass Thickness

Accuratelyand reliably measuring glass thickness on a moving target is a difficultproblem. The challenge is not only consistently detecting the glass, but alsotaking reliable measurements without needing to hold the glass very still andat precisely the correct angle.

But now, new laser displacement sensor designsare providing greater reliability, fewer dropouts and bad readings, greateraccuracy and the ability to measure at a faster rate. For glass industryapplications, these advancements have provided a significant breakthrough byeliminating a limiting factor in measuring glass.

"Laser displacement sensors arebetter on glass than they have ever been," says Michael Montgomery, assistanttechnical marketing manager for KeyenceCorp. "It used to be that when you put a laser displacement sensor on aflat piece of glass, the glass had to be perfectly flat."

He says that with advances on the receiverside, laser displacement sensors are more forgiving on angle changes. It usedto be the standard question to ask when specifying a laser displacement sensorwas "what target are you looking at?" and "is the target something you canactually see?"

"With better optics in the sensors,we are able to keep a more precise beam spot on the receiving element,"Montgomery says. "Doubling the number of usable pixels on the receiving elementitself also provides more accuracy. A smaller beam spot doesn't necessarilyhelp the application but, if you have more pixels that are also smaller, thesmaller beam spot provides a greater level of accuracy."

Traditionally in glass industry applications,operators would stop the production line while the measurements were performedat predetermined locations using contact sensors on the glass or designers usedstrategically located, laser displacement sensors. Displacement sensors enablednon-contact measurements to be made without stopping the process, allowing formore products to be produced in the same amount of time.

Non-contact measurement also offers a moreaccurate solution because the material is not scratched or damaged during themeasurement and multiple points can be measured simultaneously, furtherincreasing throughput. Plus by using software setup tools, the task ofcoordinating the measurements into thickness calculations and getting the datainto a useable form has been greatly simplified.

But when faced with the cost of thesensor, customers were often concerned that there may be a less-costlysolution. Specifically, "why would I need a sensor with such a fast samplingrate? What do I gain?"

Montgomery says that for measurementapplications on moving targets, the two most important factors are speed andaccuracy. The accuracy of the sensor gets you into the range of the measurementspecification and tolerance requirement, and the speed keeps you there. Generally,the accuracy of the sensor needs to be about 10 times better than the tolerancerequirement.

On products close to the tolerance limits,the sensor is designed to make intelligent decisions and not falsely accept badparts as good and/or reject good parts as bad. Montgomery says this featurealone will decrease false rejects and significantly increase throughput.

The second critical feature is theadvantage of speed. As long as a target is stationary or "static," the samplerate and amount of averaging doesn't make much difference above a certain point.Most laser displacement sensors are pulsed and measure at a certain samplingrate (number of measurements/unit time). The resulting data samples are thenaveraged to obtain a reasonably stable measurement.

"The noise band, which is stationary, issometimes called the static resolution, and any displacement within this bandmay go unnoticed," Montgomery says. "There would have to be a displacementabove this level to register as a change. Because the target is static, staticresolution can be a very small number."

Once the target starts to move, othervariables become involved and the "dynamic resolution" can become a muchdifferent value. These variables include color changes and variations, surfaceroughness, how much laser light is absorbed into the material and how much isreflected, etc. As individual samples are taken across the surface of thetarget, the sensor is, in effect, averaging measurements over that surface. Evenif the part is moving slowly, each sample from the sensor may be on a completelydifferent spot. With a faster sampling rate, more measurement samples that are closertogether can be taken on the surface. Better dynamic resolution increases theaccuracy which, in turn, reduces waste and increases throughput.

Advancements in the basic design oflaser displacement sensors, compared to previous models, are making thedifference especially in applications such as glass handling. New digitalsignal processors are able to track changes in received light and make adjustmentseight times faster. A better laser transmission lens system allows the laserspot size to be more consistent throughout the measurement range.

Enhancements in Keyence's Ernostar receivinglens system creates a smaller, more defined laser spot on the receivingelement, and RS-CMOS receiving elements offer faster clock speeds and twice thenumber of pixels.

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