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
"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
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.
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