Measuring equipment for automotive components and assemblies offers portability, noncontact inspection, and high data volumes
By Michael Tolinski
Automotive manufacturing is a world of unexpected delays caused (despite best efforts) when parts simply don't fit together properly. With multiple product variations and changeovers compounding the issue, fit and finish problems must be identified and corrected quickly to avoid production bottlenecks.
To this end, inspection and measurement practices in automotive production face sometimes competing interests: the need for quick measurement methods (and automated inspection when practical), for large volumes of accurate measurement data in useful form, and for movable shop-floor systems that can be taken to the part or area of interest.
Noncontact measurement techniques are helping manufacturing personnel find answers to these needs. Laser and vision-based inspection systems can supply huge amounts of data about form, fit, and contour in only a few minutes. The systems are being integrated with touch-probe measuring systems--or replacing them. Meanwhile, improved CMMs and software have been developed in response to the need for speed.
One approach to spotting problems early-on is to acquire more useful measurement data during inspection. Touch-probe methods are limited to discrete measurements, from which it's sometimes difficult to see the big picture about what went wrong. One solution is noncontact, laser-based surface scanning, which acquires millions of points to define a surface.
Various systems for real-time laser-probe surface inspection (RTLI) are being tweaked by vendors for use by major automakers. Essentially, these systems consist of three components. The first is a laser scanner that can gather over 15,000 points/sec with an accuracy of less than 100 µm. This scanner is usually attached to a multiaxis portable CMM (PCMM) to locate the scanned points in space. Data points are then sent to software and compared with the geometric model or CAD data using PCs, which are now powerful enough to handle these millions of points.
Providers of RTLI technology claim other benefits from the large numbers of points generated during scanning. Points can be displayed as color-coded, archivable "weather maps" of a part's surface, showing the position and degree of deviation from tolerance. These maps allow trends to be analyzed--such as areas on a molded part that indicate tool wear. They also allow "what if" analyses that show which tolerances are critical and which are excessive. And RTLI is said to be taking over the conventional CMM's role for making crucial first-article inspections.
Romer CimCore (Wixom, MI) has provided RTLI systems for production environments. Offering an example of RTLI's usefulness, the company points to an application of laser-scanning for a General Motors SUV assembly line. GM had found a problem with the fit between the back rear-window glass (which is slightly curved) and the center high-mount stop lamp. Laser scanning and a Romer 3000i PCMM gathered surface points and fed the data to PowerInspect software from Delcam Inc. (Windsor, Ontario). The software converted the data to a mesh of triangles (as in STL data files) to compare with CAD data to show deviations from nominal in the mating surfaces between the two parts.
For applications like this, RTLI is said to be good for measuring continuous surfaces--and particularly useful for sculpted and contoured parts. And providers argue that the larger the inspected object, the more cost-effective RTLI becomes.
But this doesn't mean laser scanning eliminates the need for touch-probe inspection. Some companies, such as Leica Geosystems (Lawrenceville, GA), provide systems that combine the two. The company's laser trackers work together with its T-Probe and T-Scan products for measuring sheetmetal, tooling, or fixtures, says Leica's Metrology Division regional manager for automotive, Joel Martin. "The T-Probe gives you the functionality to do touch-probe measurements on hard fixtures, while the T-Scan gives you the ability to do high-density scanning over a very large volume."
The company estimates that the laser tracker and T-Probe working in tandem provide 50% inspection-time savings compared to using the laser tracker alone. This is because the T-Probe can operate without CMM arms or wires as a "walkaround CMM" for measuring hard-to-access holes, slots, and features on complex tooling and large fixtures. "With the T-probe, we can set up a single station and measure the complete tool, or sometimes multiple tools from one set of locations, from the measurement station,"Martin says.
The probe's reported accuracy is 0.0023" (60 µm) over a measurement space of 56' (17 m). The T-Scan has also been shown to be accurate for acquiring surface data; benchmark studies by DaimlerChrysler and Peugeot resulted in measurements that deviated only 20 µm from standard CMM measurements, says Martin.
The combined systems align their measurement data with the vehicle's coordinate system, allowing sheetmetal-style applications. "We actually probe the vehicle to take the four-way and two-way setup holes and fit into the coordinate system for the vehicle the same way you would with a CMM, yet still are able to add all the high-definition scanning data on top of it in one system and one piece of software."
An alternative to laser-based systems, camera-based "photogrammetric" systems gather line-of-sight data and convert the information into 3-D coordinate measurements. Photogrammetry uses triangulation to calculate a point's position, based on digital photographs of the part taken from at least two locations. The method is said to be useful in industrial settings where vibration, unstable floors, or extreme temperatures harm the performance of other sensitive metrology devices.
The V-Stars system of Geodetic Systems Inc. (Melbourne, FL) is being used by one automaker, BMW, in automotive assembly environments. It's composed of a hand-held digital camera, a laptop computer, and software for image processing. The resulting 3-D data can be aligned with an object coordinate system or compared with previous measurements or CAD data. This system self-calibrates at the time of measurement, allowing an accuracy of 25 - 50 µm (0.001-0.002") for a 3-m (10') object--accuracy is comparable to that achieved by a PCMM, says the company.
A vision-based approach is also used by CogniTens Inc. (Wixom, MI) in its OptiCell and Optigo-200 systems for production-floor measurement. Based on technology that's "derivative" of photogrammetry, the first system is for repetitive automatic measurements, while the second is for portable on-the-spot measurements and comparisons to CAD data.
As with all measurement tools, the main purpose of new technology is to cut overall cost. In automotive, this means shortening the vehicle development cycle by providing accessible, usable dimensional data--and by getting rid of excess hardware. For example, the OptiCell reportedly allows savings by replacing conventional checking fixtures, which have been used for decades as a standard gaging method in the automotive industry.
But don't expect familiar CMM touch-probing and hard-gage checking to become obsolete any time soon. Fixed-position CMMs and articulated-arm PCMMs each have their appropriate use (see Quality Scan in the July issue of Manufacturing Engineering), and each type of system is keeping pace with the auto industry's needs.
For one thing, the size of fixed-position CMMs appears to be limited only by the size of the part being measured. For example, Mitutoyo America Corp. (Aurora, IL) has introduced its Car Body Measuring System that's literally just that; with a measurement range of 6 X 1.6 X 2.4 m. Entire car and light-truck bodies can be placed on its platform between two measurement-arm pylons. The high-speed CNC CMM can be fitted with a variety of probes--contact, noncontact, laser, or vision--for measuring speeds of 5 mm/sec.
Meanwhile, portable CMMs are being put to some new uses. Magna Automotive Testing (MAT; Livonia, MI) uses a FaroArm PCMM from Faro Technologies Inc. (Lake Mary, FL) around and under prototype vehicles to check the ergonomics of auto interiors. By digitizing interior surfaces (and with the help of a 167-lb [75.8-kg] dummy in the driver's seat), engineers spot ergonomic problems before start of production by calculating sight lines and determining the relative positions of the headliner, steering-wheel, and controls.
MAT also uses the PCMM to solve problems with vehicle dynamics. An example is given by Todd Hovey, group leader for quality and inspection at MAT. "Customers of a particular car model were experiencing a high-frequency vibration that was transmitted through the steering column." The manufacturer found that all drive-train components were in balance, but the PCMM found the real cause. "We raised the vehicle and digitized all the rotating parts of the drive train and their supports." Analysis of the data showed that at certain speeds, a resonance would indeed feed back through the steering column--a correctable problem.
And then there's the issue of CMM speed, which system providers are increasing with combined software and hardware upgrades. Since announcing the 500 mm/sec scanning speed of its Renscan5 software on a five-axis CMM (Tech Front in Manufacturing Engineering, July 2005), Renishaw Inc. (Hoffman Estates, IL) reports a successful demonstration of the system for making form-measurement scans of a cylinder bore. Scanning time was cut from 90 to 2.5 sec when engineers compared a conventional CMM to a CMM running Renscan5 motion control software and the company's "ultra-high-speed" REVO measurement head.
With the conventional CMM, engineers performed a typical helical scan of an 80-mm cylinder bore using a standard scanning probe at 10 mm/sec. The CMM moved in all three axes to keep the probe in contact with the cylinder, keeping head movement slow to avoid inertial error. Then the same kind of scan was performed using the intermediate-level Renscan DC software, which permitted a speed of 100 mm/sec, or a 12-sec cycle.
Finally, the cylinder was scanned with the REVO head in place. The head has two rotary axes of movement--vertical and horizontal--allowing it to scan more points in less time. In the demo, the CMM moved in one vector only at a constant velocity along the cylinder axis, as the head combined tilt in one axis with rotation in another to sweep the stylus around the inside wall of the cylinder. This helical scan of the bore reportedly took about 2.5 sec.
Five-axis inspection reportedly pays off for large, complex parts with many critical features, such as engine blocks. Dynamic errors are reduced, since the measuring head performs most of the motions for inspection instead of the larger mass of the CMM arm. This division of labor also allows the CMM to move at a more constant velocity, reducing inertial errors resulting from the repeated acceleration/deceleration of standard three-axis scanning.
The inspection of individual parts that would normally require hard gage fixtures is also being aided by new approaches to measurement. For body parts like car doors, expensive ring gages provide only a small number of manually measured points. Perceptron (Plymouth, MI) has shifted the gage's function to a robot-mounted sensor that reportedly inspects the door in less time with less tooling. Called the Flexible Ring Gauge, this system is also being targeted at hoods, decklids, and liftgates, the company says.
During each cycle, the laser-scanning sensor on the robot runs around the periphery of each part checking for proper form and fit. Hemming problems, in particular, can create obvious gaps on assembled vehicles, which is a reason why Perceptron targeted doors for this application. Since classic ring gages had been used for so long for measuring doors, the company sought to emulate the function of these gages with the robotic system.
After some trial and error development, the company created a system that promises a number of cost benefits. No hard ring gages means less lead time for fixtures, and measurements are automatic and more repeatable. And after the initial installed cost, which is comparable to the cost of hard gages for four doors, the cost of changing over to a new set of doors is relatively minor.
Along with body sheetmetal, engine and drive-train components can be measured with the newest scanners and probes. For example, camshafts and crankshafts are being measured with multipoint contact and vision-based gaging systems from Detroit Precision Hommel (DPH; Rochester Hills, MI), which integrate cameras to measure form and positional tolerances.
In production environments, DPH's Opticline noncontact and Gageline multipoint contact systems can reportedly measure up to 20 camshafts or crankshafts per hour with user-selectable measuring points along the length of the workpiece. The systems combine length and roundness measurements with a digital image projector to inspect up to 150 different features in less than a minute. In a single pass, they can reportedly measure contour, diameter, length, roundness, concentricity, flatness, parallelism, eccentricity, and other details for workpiece envelopes as large as 270 mm in diam and 1500-mm long, with measuring accuracy of ±1 µm.
In operation, the workpiece is clamped into a fixture that has a rotary drive and an angular encoder in the headstock. A measuring slide, consisting of a camera and light source, travels axially along the shaft, registering up to 2000 measurements/sec. Profiles of the workpiece are converted by the image-processing system into longitudinal and rotational measurements, which reveal deviations in shape on a CRT.
Surface roughness and finish of engine components is an area where automated inspection is critical for high-efficiency engines. DPH offers a roughness measurement system primarily for checking the surfaces of components like cylinder heads and crankcases. It reportedly increases measuring repeatability and consistency by eliminating operator influence on the measurement.
DPH's "Automotive" series of instruments can check shaft bearing surfaces, valve seats, and toothed wheels. For use on the factory floor, the multiaxis systems consist of a part-holding fixture mounted to a granite plate, and a measuring head with a stylus mounted on a traveling vertical column.
Surface finish can also be checked with laser-based, noncontact methods. Laser scanners can quickly find flaws in the finish of precisely machined parts, such as crankshafts or transmission shafts, offering 100%, go/no-go inspection, says IMPCO Machine Tools (Lansing, MI). The company's Opti-Scan can be installed within a machine tool or deployed as an off-line check.
The system uses several lasers positioned around a part fixture; each laser is aimed at a machined area of the workpiece. Reflected light quality shows whether a surface has been correctly finished. For calibration, the system is first shown a pre-finished master part and then a properly finished part, so it can quantify the differences between bad and good levels of finish.
One of the first applications for the Opti-Scan system uses 21 laser heads to check all machined journal surfaces of a six-cylinder diesel-engine crankshaft in less than two seconds. IMPCO says it would normally take up to an hour to check the part using conventional contact gaging; it would also require the parts to be laboratory-clean. The laser system reportedly works reliably amid the dirt and oil of the shop floor.
This article was first published in the September 2005 edition of Manufacturing Engineering magazine.