Measurement: Should Laser Trackers be in Your Arsenal of Metrological Tools?
The relentless pace of technological advancement has been a two-edged sword for Talleres de Guernica in the Basque country of Spain. In the one hand, the trend has been tighter tolerances on ever-larger chucks and jaws that the Guernica-based company makes for machine tools. In fact, the trend led recently to an order from a manufacturer of wind turbines for a 65-ton chuck with tolerances of 0.05-millimeter.
The problem was that the 7-meter diameter exceeded the maximum that the company normally sees by three and a half times. “We did not have a lot of experience with such dimensions,” explains Manu Fuentes, quality manager. “But we knew that manufacturing mistakes would cost us a lot of time and money.”
Fortunately for him, technological evolution generates as many solutions as it does problems. While looking for tools to measure the large parts to the required tolerances, Fuentes was able to find what he needed in the latest generation of laser trackers. “Measuring with other tools, especially with analog ones, is impossible for us,” he says.
In his search, he discovered what a small, but growing number of his counterparts at other manufacturing companies also have been finding. Laser trackers are no longer an emerging technology that requires lots of development dollars and highly skilled operators to use. Advancements in the technology have made these instruments much easier to install and operate, making them practical, and even attractive, for a much wider range of applications beyond the initial ones in aerospace.
For this reason, Talleres bought a portable Leica Absolute Tracker AT401 from Hexagon Metrology (North Kingston, RI). “We can put it on the chuck and be ready to measure within a couple of moments,” reports Fuentes. Because the system is battery powered and wireless, one operator can measure critical features-including holes, edges and flatness-between manufacturing operations. The operator moves a reflector to the point to be measured and presses a button to log the measurement.
Besides the unit’s IP54-rated protective casing, an innovation that makes the tracker this easy to use is the vendor’s PowerLock technology, which uses a vision system to find the beam and reestablish contact automatically. “When the beam is broken, the camera inside the laser tracker becomes active, broadcasting an infrared light and receiving the reflections from the reflector in the operator’s hand,” says Joel Martin, Hexagon’s product manager for its Lecia laser trackers. “Upon finding the laser beam, the camera directs it onto the reflector.” The one in the AT401 has a field of view that is a little more than 10 degrees, allowing it to see about a meter square from four or five meters away.
The Need for SpeedTo enhance the performance of other models in its line of trackers, Hexagon also has developed what it calls an absolute interferometer, which combines an absolute distance meter (ADM) with a conventional interferometer. Although laser trackers have been based on interferometers since their inception more than 20 years ago, conventional interferometers have the disadvantage of being relative point-to-point measurement devices. “They need an absolute starting point, a home position,” explains Martin. “You cannot interrupt the beam through the entire measurement sequence because it’s all relative.”
To avoid the tedious process of recapturing the beam and starting the measurement process over again from zero after each interruption, manufacturers tried using ADMs instead in their trackers. Although ADMs solved the problem of having to return to the starting point each time, the first generation of the mid ’90s was slow and inaccurate. To avoid these limitations, as well as those of interferometers, Hexagon introduced its first absolute interferometer in 2009. “It gives us the ease of an ADM’s workflow (by being able to capture the beam anywhere) and the accuracy and high-speed dynamics of an interferometer,” notes Martin.
Other manufacturers have invested in different concepts to overcome the limitations of ADMs. Faro Technologies Inc. (Lake Mary, FL), for example, has developed an ADM that exploits the relationship among three slightly different wavelengths over 60 meters. The result is that only one set of electronics is necessary for both coarse and fine measurements. Because older designs required constantly switching between two sets, they were slower, more expensive and more susceptible to drifting. By eliminating the separate electronics for the coarse mode and by using predictive software, Faro’s new ADM allows the vendor’s Ion trackers to measure dynamically without an interferometer.
Easy to WieldDespite innovations like these, the core technology has not really changed very radically in recent years, according to Aaron Sabino, product manager at Automated Precision Inc. (Rockwell, MD). “ADMs have been around for almost 10 years now,” he notes. “They have been pretty much standard on all models.” He, therefore, thinks that the real news has been falling prices-below the $100,000 barrier in some cases-and the incorporation of sensors and software to make the devices easier to use and more versatile.
For example, besides reestablishing contact with lost laser beams, an onboard vision system also can find and measure more than one target in its field of view. “Basically, you pull up a window and press a button,” says Sabino, “and it finds all of the targets.”
Software can exploit onboard cameras in other ways, too. For example, it can take digital photographs for documentation in quality control records or for exchanging information visually during a troubleshooting process.
Temperature, vibration and other onboard sensors can report on the status of the machine and its environment and feed the relevant data to corrective software. In fact, API is packaging the ability to make use of such data into a technology platform that it calls Innovo. “We can detect whether the instrument is vibrating, whether the stand is set up correctly or whether there is air turbulence caused by an air conditioner,” says Sabino. He adds that API will continue to develop features as it receives feedback from its user base.
A Quick Warm UpIn a similar way, Faro Technologies is deploying a combination of temperature sensors and software to enhance the Smart Warm-up feature on its newest trackers. Because of simple physics, very slight distortions occur in every tracker from uneven heating as the laser, electronics and motors in them warm up. For this reason, most manufacturers recommend letting trackers warm up for an hour or two before taking measurements. Faro’s Smart Warm-up feature, however, shortens this process by sending current into the trackers’ bushless DC motors while they are stationary, thereby transforming them into heaters.
The problem, however, is that, until now, the warm-up period has been unpredictable. “The original version was designed to get you about 80% of the way warmed up in about 20 to 25 minutes,” says Ken Steffey, director of product management for Faro’s laser trackers. “For the same conditions, the new system may still take 25 to 30 minutes to warm up completely, but it tells you exactly when it will come to equilibrium.” Not only does the enhanced version eliminate the guesswork, but it also allows operators to plan their time precisely, letting them do other work until the tracker is ready for use.
Steffey credits this ability to mathematical and geometric modeling that Faro’s researchers have developed through computer simulation. Because of the steady evolution of computing technology, “smaller companies now have access to tools that only big ones like General Motors and Boeing had a decade ago,” he says. Consequently, using data from six temperature sensors inside the tracker, Faro’s embedded Smart Warm-up software can now decide how much and how long to heat each motor for bringing the unit to thermal equilibrium in the shortest time.
Because of time-fighting weapons like these, it is easy to see why users should take the time to consider whether the latest generation of laser trackers would be a good fit for their arsenals of metrological tools. Q
For more information on the companies mentioned in this article, visit their Web sites:
Automated Precision Inc., www.apisensor.com
Faro Technologies Inc., www.faro.com
Hexagon Metrology, www.hexagonmetrology.us
Tech TipsAdvancements in the laser trackers have made these instruments available for a range of applications beyond the initial ones in aerospace.
Besides reestablishing contact with lost laser beams, an onboard vision system also can find and measure more than one target in its field of view.
By eliminating the separate electronics for the coarse mode and by using predictive software, a new ADM allows the vendor’s Ion trackers to measure dynamically without an interferometer.
Application TipCheck the stability of your measuring environment periodically. An easy way to do this is to look at the standard deviation or root mean square of measurements taken of a distant point over 10 to 20 seconds, according to Ken Steffey at FARO Technologies. Because most trackers will take about 1,000 measurements per second, the practice will collect a suitable sample of 10,000 to 20,000 points. If the point is jumping around by 0.005 in. or so, then the environment is too unstable for a job that requires measuring within 0.001 in.
Although the culprit could be mechanical vibration, Steffey says that a more common cause is a temperature gradient in the air. If such gradients are large enough, they can refract light, including laser light. An example from common experience is the mirror-like surface that you might see over the road while driving on a hot summer day. In a similar way, a blast of cold or hot air from a vent or an open bay door can refract the laser beam enough to distort measurements.
Corrective action should prevent cold or hot masses of air from blowing across the measuring area. Possible remedies include redirecting vents, isolating the area with some kind of enclosure, or mixing the air with a fan.
Check the stability of your measuring environment periodically. Source: FARO Technologies
Measure by RadarAlthough a laser radar is not really a laser tracker, it is a close cousin that often vies for the attention of potential users. Its chief difference is that it does not calculate measurements based on the time that the beam takes to return to the device from a reflector sitting on the features to be measured. Instead, the laser radar relies on the phase shift in the frequency of the returning beam (see graph). Because the radar can bounce the beam directly off the part, it does not require an operator to put a reflector on the features, as a laser tracker does.
Consequently, not only can the radar simplify measuring hot or difficult-to-reach features, but it also is often faster and easier to automate. “With the laser radar, you can write a part program, let it go and come back when it’s done,” notes Jay Elepano, business development manager for large-scale metrology at Nikon Metrology Inc. (Brighton, MI), holder of the patent on the technology.
Consider the results of a study conducted by Nikon at an aerospace company. A particular application there required 14 trackers and 28 people about eight hours to make the required measurements. “We did it with three radars and two people in six hours,” says Elepano. He adds that the third generation of Nikon’s radar technology is four times faster than the second generation used in the study.