Everything is better with lasers!
Take large scale metrology, for example.
Although they began as inspection tools, laser trackers have evolved into one of the most versatile weapons in the quality professional’s arsenal.
As the price of laser trackers continues to decrease, with some systems falling below USD$100,000, more manufacturers will have to consider whether to add these tools to their metrological arsenal.
If you’re already at that point, this article can help you decide whether a laser tracker would be a valuable investment. If you’re entirely new to laser trackers, think of this as an introduction: an overview of how laser trackers work, how they compare to other metrology tools—such as coordinate measuring machines (CMMs) and laser scanners—and their applications.
How Laser Trackers Work
Laser Tracker Principles
Laser trackers take measurements using a spherical coordinate system, which means that any given point is specified by three numbers:
- The radial distance of the point from the tracker
- The polar or zenith angle, which measures elevation
- The azimuth angle, measured on a reference plane that passes through the tracker and is orthogonal to the zenith
The numbers for the zenith and azimuth angles are determined by means of two angular encoders which measure the orientation in the tracker’s gimbal along its mechanical axes. The radial distance of the point from the tracker is determined using an interferometer (IFM), an absolute distance meter (ADM) or a combination of the two.
Although IFM used to be the dominant technology in laser trackers, Zach Ryan, product manager, metrology for Automated Precision (API), explained that this is no longer the case: “Today, if you say it’s an IFM system, generally it’s an ADM and IFM system. There are no more IFM-only systems on the market.”
Laser trackers equipped with IFMs split their lasers in two: one beam travels directly to the interferometer, while the other travels to a target device in contact with the component being measured.
The two lasers interfere with one another inside the interferometer (hence the name), resulting in a cyclic change equal to one quarter of the laser’s wavelength—approximately 0.0158 microns—each time the target device changes its distance from the tracker.
By counting these cyclic changes—what’s known as fringe counting—the laser tracker can determine the distance the laser has travelled.
ADMs use infrared light from a semiconductor laser which bounces off the target device and re-enters the tracker. The infrared light is then converted into an electrical signal for time-of-flight analysis.
Initially, ADMs were not fast enough for continuous surface measurement, which is why they were only used to supplement interferometers.
“ADM isn’t as fast as IFM and probably won’t be for some time,” said Ryan, “but ADM is plenty fast for most applications. If you’re talking about high-speed dynamics in, say, machine control, IFM is still preferable.”
The primary advantage of ADM is that it can measure distances automatically, even if the beam has previously been broken.
In contrast, if the beam is broken while using an interferometer then the operator is forced to return the target device to a known position in order to reset the tracker.
Laser Tracker Operation
The basic laser tracker setup consists of the tracker itself, a target device and a computer running the tracker’s application software.
Accessories (optional on some trackers and standard on others) include:
- A remote control
- Air temperature sensors to help compensate for temperature fluctuations
- Material temperature sensors to help compensate for thermal expansion
- An inclinometer to measure the tracker’s orientation with respect to gravity
The most common target device is a retroreflector, which reflects the laser beam back in the direction from which it came. This is the same principle that explains why cats’ eyes appear to glow in the dark and it’s also the basis of the famous Apollo 11 lunar laser ranging experiment.
The most common retroreflector design is the spherically mounted retroreflector (SMR), which has the advantage of keeping the center of the retroreflector at a constant distance from the surface being measured.
Some laser trackers use a contact probe or even a handheld laser scanner in place of an SMR.
In the case of an SMR, the measurement sequence begins by positioning the target device in the home position at a known distance from the tracker. Once the two devices have been paired, the tracker keeps the laser centered on the target device by redirecting part of the returning laser to a position-sensing detector (PSD).
When the laser strikes the target device off center, the redirected part of the returning beam will also strike the PSD off center, causing the tracker’s mechanical axes to compensate by reorienting until the laser once again strikes the PSD (and the target device) in the appropriate place.
In cases where the target device is a contact probe or laser scanner, the target device can be paired with the tracker using a visible light or infrared camera system.
All of this can be done by a single operator, though it’s generally more efficient to have two: one stationed at the tracker and one taking measurements with the target device.
Laser Tracker Applications
The aerospace industry was an early adopter of laser tracker technology and is one of the biggest users of laser trackers today. It’s not difficult to see why. Manufacturing aircraft is the quintessential example of large scale metrology, involving in-place inspection of large parts and assemblies.
Jig component inspection and wing component assembly are two of the most common applications for laser trackers in the aerospace industry. Their portability makes them well suited to both tasks.
Moreover, given the industry’s tight tolerances and strict regulations, the high accuracy of laser trackers also weighs heavily in their favor.
Other applications for laser trackers in the aerospace industry include:
- Die inspections
- First article inspections
- Machine tool accuracy and repeatability diagnostics
Increasing standards have led to laser trackers being adopted in the automotive industry. Like the aerospace industry, the automotive involves the measurement of large parts and assemblies, such as automotive body-in-white measurements.
The automotive industry uses many similar tooling applications to those found in aerospace and can apply laser trackers to similar ends.
For example, in the creation and inspection of a die, a laser tracker can be used to digitize the surface of a clay model. Once the resulting point-cloud is used to mill a die, the tracker can again be used to measure both the die and the stamped parts it produces.
Trackers can even be used to control machine tools directly. This can help to ensure that final products meet their specifications, thereby reducing machine rework and production downtime as well as eliminating redundant testing.
Other applications for laser trackers in the automotive industry include:
- Alignment of hinge lines and body components
- Adjustments of industrial robots
- Deformation and dynamic measurement
Beyond the aerospace and automotive industries, there are many others that utilize laser trackers on a regular basis.
Virtually any application that involves large scale metrology has a place for laser trackers. For example, they offer an efficient and cost-effective option for aligning metal extrusion presses.
“There’s been a lot of growth in the service market,” said Ryan.
“People are servicing parts in machines that have rotating shafts where you don’t need a laser scanner, since you don’t have complex geometries. Instead, you have large parts that are cylindrical or with prismatic geometries and laser trackers are great for that. I spoke to someone recently who started using laser trackers in 2002 in paper mills and he said the growth in that industry has been astronomical. There are obviously very specialized technologies for measuring paper mill roller alignments, but they can’t be used to lay out your shop floor or build a machine. Laser trackers are almost like the Swiss Army knives of metrology.”
Laser Trackers vs. Other Metrology Tools
Laser Trackers vs. Coordinate Measuring Machines
The core principles underlying laser trackers and CMMs are essentially the same, as Sutphin explained:
“Whereas a CMM has a scale along the X, Y and Z, the tracker has two rotary encoders and then the distance component with the laser beam. It’s essentially the same thing—it’s just keeping track of where things are at as you touch. I think of it as just a big CMM. The difference is that my part doesn’t have to fit inside the CMM, so the part can be much bigger because I have a bigger volume.”
The size of the part being measured is one of the biggest factors when comparing laser trackers to CMMs, since a CMM has a much more limited measuring volume. Portable arms can offer a bit more flexibility, but still not as much as a laser tracker.
“It really depends on how big your part is,” said Rina Molari-Korgel, chairman of the executive committee of the Coordinate Metrology Society. “If you have something that’s tabletop or a little bigger, that’s perfect for a portable arm. If you have something that’s about car-sized or bigger then you definitely need a laser tracker.”
However, although a laser tracker is much more flexible than a CMM, it’s also much less accurate.
“A laser tracker is not as accurate as a CMM, not a chance,” said Sutphin. “The interferometer distance component is extremely accurate, down to the submicron level, but we still have angles to deal with. So you don’t always get the benefit of just the IFM; you have the angle encoders and angle uncertainty as well.”
Given their role in large scale metrology, laser trackers might seem like the quintessential shop floor tools. However, as Sutphin explained, it’s not uncommon to find them in quality labs as well.
“We see them in both places,” he continued. “It’s interesting. We have a bunch of customers where everything stays in the quality lab but they like to measure the parts with a laser tracker. There’s one in particular out in California that gave a presentation at CMSC [Coordinate Metrology Society Conference] a couple years ago and he made the comment that he has parts that are probably small enough to use with a portable arm or a CMM, but they already have laser trackers so they make do rather than buying another piece of machinery.”
This shows that laser trackers are versatile as well as flexible. Although some CMMs can operate on the shop floor, they need to be specifically designed to do so. In contrast, laser trackers can work equally well in the shop floor or the quality lab.
Laser Trackers vs. Laser Scanners
While laser trackers are similar to CMMs in principle, their portability puts them closer to laser scanners in practice. Laser trackers and laser scanners are both portable, highly precise instruments with large working envelops.
One of the primary differences between laser trackers and laser scanners lies in the number of data points they collect, as Molari-Korgel explained:
“If I want to scan a surface with a laser tracker at a little bit less than 20 feet, we’re talking about a thou and a half—plus or minus 3 sigmas. So that’s pretty good, but it’s not like a laser scanner where I’m getting a hundred thousand points in one second.”
Hence, in terms of the number of data points gathered, laser trackers don’t even come close to scanners. However, with the right metrology technique, laser trackers can still provide a large set of data points.
“A laser tracker takes individual points, but I can tell the tracker, ‘Take a point every time I move the target 30 thousandths of an inch,’ and it will take a point,” said Molari-Korgel. “So I can scrub a surface and that’s basically taking data that’s going to give me a plethora of points on that surface.”
It should also be noted that while laser scanners are fully noncontact metrology tools, laser trackers often depend on measuring the location of a retroreflector which makes contact with a part via a probe.
Laser Trackers: From Inspection to Manufacturing
Although they began as inspection tools, laser trackers have evolved considerably in their relatively short lifetime. Today, laser trackers can be found in quality labs and on shop floors, being used for in-line measurements, industrial robot control and aligning machine tools. The unique combination of speed, precision and flexibility makes laser trackers a worthwhile addition to the quality professional’s tool kit.
For more information, check out our white paper: How to Avoid 3 Common Mistakes When Using Laser Trackers.