Making measurements close to the point of production requires portability and ease-of-use
By Bruce Morey
"The trend is definitely toward moving metrology closer to production,” says Dave Armstrong, product manager for Hexagon Metrology's Romer (Carlsbad, CA) products. Witness the popularity of portable CMM measurement arms (PCMMs.) Romer claims an installed base in the thousands. Armstrong attributes their popularity in part to their improving accuracy. For instance, Romer recently released an 8' (2.4-m) version of the Infinite series of portable arms that is accurate to 24.5 µm. Within Romer's customer base, people are using PCMMs for everything from statistical checks on products and processes to (occasionally) 100% inspection. Using PC-DMIS software from Hexagon Metrology's Wilcox and Associates (North Kingstown, RI), customers create measurement programs—with a CAD model, if they choose. Used to inspect subsequent parts, the program guides the user through each inspection step.
Armstrong points out that while PCMMs might replace hand tools, from his perspective they do not necessarily replace stationary CMMs. "Portable arms in general are a complement to CMMs, rather than a replacement," he states. "Every system has its place. CMMs are great for batch work. If you have many parts developed in batch, it is much better to write a program for a CMM. If you have small one-offs, or small production runs, or if getting the part to the CMM takes a long time, then portable arms are perfect." For example, in 2008 Hexagon's Sheffield unit (Fond du Lac, WI) built the Discovery III CMM for the shop floor. Capable of using vision systems, laser systems, or touch probes, this device enhances accuracy and delivers a touch-probe accuracy to 3.9 µm in three different measuring volumes, the largest being 762 x 1016 x 609 mm.
"The top industries that have embraced portable CMMs like the FaroArm include aerospace, automotive, and medical. In general, anyone cutting chips," says Rob Sanville, product manager for Faro (Lake Mary, FL). According to him, users find that a portable device capable of taking on-machine, in-process measurements means that if there is an issue with a part, the issue can often be corrected immediately while the part remains fixtured in the machine. Faro also offers a variety of measuring arms, from the FaroArm Quantum in a 12' (3.7-m) version that can provide 60 µm of volumetric accuracy, to the PowerGage, which can deliver 5 µm of accuracy within its 1200-mm diam measuring volume. The PowerGage provides a capability to create inspection programs from CAD models using PowerInspect software from Delcam (Birmingham, UK).
An interesting variation on the PCMM is the Faro Gage, a 1200-mm arm that's also accurate to 5 µm. This arm does not include an interface to off-line programming software like PowerInspect. "The Faro Gage is made to be used out on the shop floor—near where the parts are made," says Sanville. "It does not necessarily replace hand tools, but does offer more consistency in measurement and, most importantly, provides written reports of the data." Romer's MultiGage PCMM, released in 2008, is a similar device. Also measuring to 5 µm within its 1200-mm-long arm, the MultiGage is offered by Romer as a flexible replacement for hand-held tools. With its version of PC-DMIS, MultiGage takes common 3-D measurements, such as dimensions, features, and constructions—it even mimics a height gage.
Another variation on portable-arm CMMs is the class of systems known as optical CMMs, which use a stationary camera system to track targets on a portable measuring device. An example is the K-Series optical CMM from Metris (Brighton, MI). It uses LEDs mounted on handheld devices called SpaceProbes that are triangulated from a distance by a device using three linear CCD cameras. Triangulating the position of the LEDs on the probe, the Metris K610 provides volumetric accuracy to 60 µm. It's described as a flexible, easy-to-use system that can be employed over a large inspection volume—whole cars for instance—without the need for 'leapfrogging' points of origin, as is required for portable arms. The K-Series also features a dynamic reference point that makes it possible to measure unstable, dynamic parts.
Portable CMM arms may have limitations in a production environment. "For high-value, complicated parts produced in relatively high volume, you are going to want an automated solution," explains Mark Hoefing, senior vice president of the industrial business unit at Perceptron (Plymouth, MI). Perceptron has been putting metrology in-line with production processes for 28 years, starting in automotive body assembly. The company's offerings have improved in both flexibility and ease-of-use, as well as accuracy—and these improvements are the sort that is always in demand, according to Hoefing. Their AutoGauge robotic measurement system also is more flexible than traditional in-line fixed gaging, a standard choice for many years for in-line metrology. When a process is challenged by frequent production changeovers, an automated system like AutoGauge only requires revised programming to begin measuring new parts.
The company offers robotic, fixed-sensor, and hybrid versions of its AutoGauge product for even greater flexibility. While fixed-sensor systems are best suited for high production rates with low model mix, the robotic system is best for lines with high model mix. There are tradeoffs. "Robots offer increased flexibility, but they add 2–3 sec per measurement," according to Hoefing (when compared to traditional fixed sensor solutions). Why? "The measurements are sequential rather than simultaneous." In any configuration, the system delivers approximately 200 µm volumetric accuracy (six sigma). The robotic solution is calibrated during initial installation, and temperature variations are compensated automatically in-cell, typically during the material-transfer stage of the cycle.
An automated, in-line, 100% inspection system is not for every situation, as Hoefing is quick to point out. A number of factors beyond volume, value, and complexity contribute to the decision, including tolerances, stability of the process, and the relative cost of gaging. When economics cannot justify 100% in-line inspection, but manual measurement is too slow, he advises an in-between solution. A single laser scanner mounted on a robot and surrounded by as many as 10 separate part fixtures satisfies this middle ground, for example. Manual labor moves parts onto the fixtures. Measurements sent to a database are analyzed as needed. "These systems are typically operated by personnel who may know very little about metrology," says Hoefing. "More customers are setting up this kind of automated cell on the plant floor. Another feature is that users can adjust their sample rate. For example, during launch-mode, there is a lot of variation and they need to measure more parts, perhaps hundreds a day. Conversely, in steady-state production they may want to do only five parts a day."
One way to improve robotic metrology would be to not rely on the robot's accuracy for localization, thus eliminating initial calibration and in-process temperature compensation. Metris does so by using its K-series optical CMM with a robot mounting an MMD laser scanner, according to Darian Butt, project engineer for Metris's Integration Services and Technology division. The result is their K-Robot series. Instead of calibrating the robot so that its path is predictable, the system tracks position using LEDs mounted on the robot's head. An optical CMM triangulation device—the same one used in its K-Series optical CMM—then measures these LEDs. "Users can program the robot off-line in any one of several packages, or with the robot's own OLP package," says Butt. It provides 100 µm (two sigma) accuracy in a measurement volume of 17 m3. Other advantages of this concept include the ability to measure in-process while the part is in motion, if the part or a fixture holding it is instrumented with LEDs. "If the part is moving, whether linearly, rotating, or even vibrating, we can scan it and relate all the data, because the K-series optical CMM is a high-frequency measurement device," Butt explains.
While there are advantages to an in-line automated system built using commercially available robots, Metris is looking even further ahead. Their Robotic CMM Arm (RCA) looks to improve access, portability, and accuracy over their own K-Robot solution. One disadvantage of the K-Robot is the need for line-of-sight between the robot head and the triangulation cameras. Peeking inside a car body, for instance, is not practical. Another challenge is that robots are, in general, heavy and difficult to move. Moreover, while there are plenty of people who know how to program robots, programming using well-known metrology systems—like Metris' own CAMIO system—would offer its own advantages.
By essentially automating a PCMM, Metris is targeting accuracy of around 50 µm (two sigma) throughout the entire measurement volume, according to Chris Marriott, vice president for engineering, Metris UK (Darby, UK). "By automating a PCMM, Metris is targeting accuracy of less than 100 µm with its largest units—or significantly less in smaller, lighter units." Other advantages include lower total cost than a robotic system, as well as programming using standard CMM software. "The RCA is mobile, but not manportable. The unit weighs 120 kg, heavier than a PCMM but much lighter than a robot. To move it, you can use a pallet shifter or a jigged crane with a lifting arm on it, or other methods specific to the application," observes Marriott. "The project is in the beta-test stage with our partners, Tata Motors and the University of Warwick, under a $2M grant from the Technology Strategy Board of the UK." He sees applications in general inspection, including on-machine-tool part verification, and initial samples or inprocess manufacturing, including body-in-white inspection, fixture assemblies, or welded fabrications.
"Throughout metrology there is a move to simplify, automate, provide better record keeping, and provide more data," explains Jim Clark, Metris vice president. "Not simply replicate what hand gages could do. Users want real, current data to provide management with the tools required to change production before you make a mistake."
Specialized Systems Go to the Part
Makers of specialized, high-accuracy metrology equipment are continuing to shop-harden their equipment while making it easier to use, as both qualities are needed to get closer to the production line. When measuring roundness or cylindricity to fractions of microns—think automotive fuel injectors or implantable medical devices—specialpurpose form testers such as those supplied by Mahr Federal (Providence, RI) represent value.
"Putting metrology on the shop floor has been talked about ever since I have been in the business. It has not gone as fast as anyone expected," remarks Pat Nugent, vice president, metrology systems for Mahr Federal. A 20-year veteran in the metrology trenches, he has noted more urgency about shop-floor metrology in the last few years. According to Nugent, people originally underestimated both the harshness of the shop environment and the ease-of-use needed if production workers were to use systems rather than metrology professionals. "We do not need something that's more accurate. We need something that is easier for everybody to use and still achieve that level of accuracy," he explains. In response, Mahr Federal recently released their EasyForm 3.0 software, the latest upgrade. The company describes the software as providing an intuitive touch screen interface that guides operators through measurement setup and operation. A teach-in mode combines steps for repetitive multifeature measurements.
Another notable point is the lack of push for integrating these kinds of instruments with CAD models. While Mahr Federal offers an off-line programming tool that imports a CAD model called MarSim, hands-on is the preferred approach, according to Nugent. "Our customers like the teach-in approach using our touch screen interface that guides operators through measurement setup and operation," he explains.
This article was first published in the November 2009 edition of Manufacturing Engineering magazine.