Measuring Glass Thickness

Accurately and reliably measuring glass thickness on a moving target is a difficult problem. The challenge is not only consistently detecting the glass, but also taking reliable measurements without needing to hold the glass very still and at precisely the correct angle. 

But now, new laser displacement sensor designs are providing greater reliability, fewer dropouts and bad readings, greater accuracy and the ability to measure at a faster rate. For glass industry applications, these advancements have provided a significant breakthrough by eliminating a limiting factor in measuring glass. 

"Laser displacement sensors are better on glass than they have ever been," says Michael Montgomery, assistant technical marketing manager for Keyence Corp. "It used to be that when you put a laser displacement sensor on a flat piece of glass, the glass had to be perfectly flat."

He says that with advances on the receiver side, laser displacement sensors are more forgiving on angle changes. It used to be the standard question to ask when specifying a laser displacement sensor was "what target are you looking at?" and "is the target something you can actually 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 element itself also provides more accuracy. A smaller beam spot doesn't necessarily help the application but, if you have more pixels that are also smaller, the smaller beam spot provides a greater level of accuracy."

Traditionally in glass industry applications, operators would stop the production line while the measurements were performed at predetermined locations using contact sensors on the glass or designers used strategically located, laser displacement sensors. Displacement sensors enabled non-contact measurements to be made without stopping the process, allowing for more products to be produced in the same amount of time.  

Non-contact measurement also offers a more accurate solution because the material is not scratched or damaged during the measurement and multiple points can be measured simultaneously, further increasing throughput. Plus by using software setup tools, the task of coordinating the measurements into thickness calculations and getting the data into a useable form has been greatly simplified.

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

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

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

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

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

Once the target starts to move, other variables become involved and the "dynamic resolution" can become a much different value. These variables include color changes and variations, surface roughness, how much laser light is absorbed into the material and how much is reflected, etc. As individual samples are taken across the surface of the target, the sensor is, in effect, averaging measurements over that surface. Even if the part is moving slowly, each sample from the sensor may be on a completely different spot. With a faster sampling rate, more measurement samples that are closer together can be taken on the surface. Better dynamic resolution increases the accuracy which, in turn, reduces waste and increases throughput.

Advancements in the basic design of laser displacement sensors, compared to previous models, are making the difference especially in applications such as glass handling. New digital signal processors are able to track changes in received light and make adjustments eight times faster. A better laser transmission lens system allows the laser spot size to be more consistent throughout the measurement range. 

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