By Philip Burgert
Sensing and measuring based on laser technology that offers tightly focused measuring-spot ranges and flexible mounting-distance options are finding their places in a wide range of applications that need very accurate alternatives to 3D vision.
Very-high-speed measurements with high resolution and repeatability are possible with lasers sensors. Other advantages include an ability to “reach” into difficult-to-access locations to provide precise metrology.
On the Spot
“A small spot size is really the only practical way to measure certain components or to measure an inside dimension, because lasers are used mainly for dimensional measurement; camera sensors, on the other hand, are used to determine the presence or absence of a part,” says David Lacey, president of system integrator Oryx Systems, Indian Trail, N.C. (www.oryxsystems.com). “When you get to actual metrology, that’s when the lasers really show off their strength.” Oryx builds custom and special-purpose measurement machines.
Lacey says, compared with proximity sensors, a key advantage of laser sensors is their stand-off distance. “They don’t have to be very close to the thing being measured,” he says. “They also have a good measurement range. Unlike cameras, of course, laser sensors don’t need special lighting. They can work in dark and shadows.”
Lacey notes that lasers sensors come in a variety of configurations. “Almost as many types as proximity switches,” he says. “They rival proximity because of the non-contact nature and the stand-off distance. But their greatest advantage is in doing high-accuracy, high-resolution measurements in non-contact mode looking for dimensions.”
Topography is area where lasers excel, says Lacey. “Vision systems can sense height but not as accurately as lasers can.” The non-contact nature of laser sensors is also an advantage for measuring parts that could easily be damaged, he notes. “To measure soft parts, for example, solder paste or metals such as lithium, you can’t touch the stuff with anything,” says Lacey. “The only way to do dimensional measurement is with a non-contact gauge and in most cases the lasers are more suitable than other types.”
Lasers also allow for measurement in limited access areas, he says, noting some custom-manufactured parts in automobile, aircraft, batteries and medical as examples, since they often are in unreachable locations (Figure 1). “They are very, very small. You’re looking down a hole and trying to measure the wall thickness at the bottom of the hole or measure the height of a feature down a hole,” he says. “There are many applications where a laser can excel at that.”
Mind the Details
Lasers can be difficult to use on a highly polished surface. In those cases a capacitive sensor that is less dependent on surface textures is a better solution, says Lacey. “But the capacitive sensor will measure even the smallest oil spot as well, whereas the laser doesn’t. There’s always a compromise.”
The spot size of the laser also sometimes works against it. With a really tiny spot size in the one-half micron range, it can pick up small, inconsequential deviations in the surface, which create problems. A larger area sensor tends to average and provide a smoother signal, says Lacey.
“With the correct application, the laser sensor can outperform these other devices for accuracy, repeatability and access to a small area,” he says. “But you still have to do the right signal processing and averaging to get data from the signals to process it properly. You simply can’t aim the laser and get a reading.”
Of the many laser-based applications that system integrator Vista Solutions (www.vistasolutions.ca), Windsor, Ontario, has developed, about 90% use laser sensors such as a displacement sensor for precise and accurate sensing results, says Aaron Bouchard, vision specialist. “We tend to always use the smallest laser point that you can get,” he says. “That would be the main advantage.”
The 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.