Figure 2: This lab evaluation shows a laser sensor checking the depth of a machined notch on a forging. The unique setup involves a 30° reflective angle because a parallel configuration could not be used.Source: VISTA SOLUTIONSThe accuracy of lasers is an important advantage, says Bouchard, who notes one application Vista Solutions developed involves measuring the inside of a forged housing by spinning a displacement sensor with a servo motor inside the forging and capturing analog values of the points in space (Figure 2). &#;It&#;s providing the distance from the sensor and we know where it is when we spin it,&#; Bouchard says. &#;We&#;re plotting those points to determine whether or not the shape of this forging is correct.&#;
Along with other machine-vision systems, Vista Solutions uses displacement laser sensors in determining the 3D position of objects in manufacturing systems. &#;We do a lot of 2D robot guidance and we&#;ll sometimes need a third dimension,&#; says Bouchard. &#;Rather than use a full-blown 3D pickup system with a lot of development costs, for the right application you can use a 2D camera and two or three displacement sensors to get the plane of the part so you&#;ll know where it is in space and you can adjust your robot to pick it up. It&#;s really a complementary technology that can be much less expensive.&#;
Vista Solutions also uses laser presence sensors for critical timing in applications that look for a part moving on a manufacturing line. The laser sensor provides much greater accuracy because, as the part passes, it only has to break the small laser dot rather than a larger photo-eye surface, says Bouchard. He notes that the cost of laser sensing might be a little higher than some other solutions, so depending on the applications an inductive sensor or photo-eye might be a preferable way to achieve presence detection or other results.
Lasers also can be best to detect, for example, the presence of a rubber seal on a black part when there&#;s poor contrast between the two pieces, says Bouchard. &#;You throw a laser line across it and you actually can see it with just a standard 2D camera,&#; he says. &#;You&#;re not doing any measurement calculations based on an algorithm. You&#;re just checking to make sure it is present based on a laser line skipping up rather than running straight across.&#;
Rick Bondy, product manager for Sick (www.sickusa.com), says that in the past most photoelectric sensors used infrared light sources and the wavelength was not visible to the human eye. &#;It made alignment very difficult and trial by error in most applications,&#; he says. &#;With visible-red-light-emitting-diode sources, installation and setup improved significantly, since the installer could see the light hitting the target.&#;
The problem with both of these light sources was the control over the physical size of the light spot at varying distances, he adds. Similar to the light from a flashlight tending to flair out as it exits the housing of the flashlight, the optical-light-beam diameter of a photoelectric sensor also gets larger further from the lens.
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Controlling the size of the light spot size is especially critical in manufacturing applications that call for smaller machine footprints on the factory floor, in which targets are getting smaller, says Bondy.
Laser light sources enable factory automation applications that require precise, pin-point light-spot sizes to be developed reliably and at competitive cost, states Bondy. &#;Most lasers used today in factory automation are Class I or Class II lasers, as regulated by government agencies, and are considered safe due to their extremely low average power output,&#; he says.
Typical applications include product height/width, diameter, thickness, roundness, eccentricity, and profiling, notes Bob Hosler, regional product sales director for Keyence of America (www.sensorcentral.com), adding that the laser technology that has become very popular recently is two-dimensional, laser-displacement sensing. This technology essentially combines laser distance measurement (triangulation) with vision technology involving either two-dimensional charge-coupled device (CCD)or complementary metal&#;oxide semiconductor (CMOS) receiving elements to create a displacement profile of the target and enabling stable defect inspection or gauging without problems frequently associated with machine vision, such as inconsistent lighting and part variation, explains Hosler.
In packaging applications, lasers can be used to count a magazine of unfolded corrugated cardboard cases or cartons or to verify if minor and major flaps are fully inserted, says Jeff Allison, product manager for Pepperl+Fuchs (www.pepperl-fuchs.com). He notes that material-handling applications often employ laser displacement sensing for absolute positioning of automated storage and retrieval or printing applications can use laser sensors to count individual sheets of paper in a sheet-fed press.
Philip Burgert is a freelance writer, specializing in technical trade media.
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