3-D Laser Scanning Accelerates Inspection
March 26, 2008
Dimensional error margins that compound too enthusiastically throughout the parts assembly process introduce reduced product quality and, subsequently, lengthy corrective actions. To avoid this, accurate and efficient metrology solutions keep a close eye on component and assembly geometry until the final product is proofed. The use of touch-probe measurement in part-to-CAD (computer-aided design) inspection and reverse engineering applications offers accurate results, but faces limitations in terms of inspection throughput and freeform inspection capability.
3-D laser scanning, which has matured during the past 10 years, is on the verge of revolutionizing the micrometrology market in automotive, aerospace and many other markets. Laser scanners accurately capture parts of various shape, size and material in a fraction of the time required for touch-probe measurement, resulting in increased inspection productivity. In addition, they acquire hundreds of thousand or millions of points across the entire geometry of the scanned object, making it possible to accurately describe freeform surfaces and digitize complete components.
The surface information is used for part-to-CAD inspection or to reverse engineer CAD models from the physical object. Having a full digital model of the test specimen means that any type of feature or surface inspection can still be done at any time without having to redo the measurement. Last but not least, compared to touch probes that can potentially scratch fragile components or press flexible parts, laser scanning is entirely noncontact.
During operation, 3-D laser line scanners beam a wide laser stripe on the surface of the part being inspected. A camera captures the projected laser stripe and converts it into thousands of 3-D measurement points using triangulation and digital imaging. The scanner itself is mounted on a so-called localizer that is used to determine the absolute position of the scanner in 3-D space. By combining the scanner measurement points with the scanner position coming from the localizer, accurate 3-D coordinates of the scanned surface are determined. A range of localizers are possible, ranging from articulated arms to traditional coordinate measuring machines (CMMs) up to industrial robots.
The digital capability of recent scanner generations allows scanned surfaces to be displayed on screen in real-time, and dynamically adapt sensor performance according to varying surface material, color and reflectivity. To suit operators’ specific inspection needs, laser scanner solutions are available for different measurement volumes, accuracy classes and in handheld, CMM and robotic configurations.
Convenience of Handheld ScanningHandheld laser scanners are either mounted on an articulated measurement arm or tracked by an optical tracking system (optical CMM). Both manual systems handle on-site troubleshooting tasks just as easily as in-depth dimensional inspection on the production line. The compact size and low weight of handheld scanners enable operators to run metrology jobs at other divisions or plants, or even at the customer’s site.
Where articulated measurement arms offer a limited action radius, an optical CMM covers a larger measurement area in which the operator can freely walk around while manually operating the scanner. The optical CMM dynamically tracks the position and orientation of the scanner as well as the object, providing a metrology-enabled workplace that even fits an entire car.
In the early stages of product development, handheld laser scanners are often used to digitize rapid prototypes or clay models. Besides part-to-CAD inspection, a manual laser scanner comes in handy when physical shape modifications crafted by the designer need to be fed back into CAD. Without leaving marks on the clay model, it provides a detailed operator-independent description of the surface, thanks to the high number of measured points.
A head light, a suspension part, a wheel rim, a bumper or a plastic air filter box-each individual die or mold component can be easily checked to ensure dimensional specifications are actually met. The creation of graphic full-part comparison reports accelerates this qualitative evaluation, and guarantees that material shrinkage or spring-back effects are controlled correctly. After vehicles are assembled, handheld 3-D scanning is widely used to run accurate and efficient flush-and-gap verification.
Among numerous reverse engineering applications, handheld laser scanners provide value when digitizing the geometric space envelope that is available, instead of relying on CAD files that may no longer be up-to-date. The acquired 3-D scans digitally reflect the available space that is available to design the structural connection between tow bar and a vehicle chassis, or powerful customized turbo chargers that fit into tuned sports cars.
AccuracyWhen top accuracy and repetitive inspection means are required for the job, dedicated 3-D laser scanners for mechanical CMMs come into play. The availability of complete packages that include CMM scanner hardware and software allow most existing CMMs to be retrofitted with 3-D laser scanning capability.
Economically very relevant is that laser scanning accelerates CMM feature inspection. Instead of numerous indexing head rotations and multiple CMM axes displacements that are needed to operate a touch probe, the CMM scanner performs inspection along straightforward linear and polygon motion paths. Every second pinched off from the inspection cycle is multiplied by the number of cycles run on the CMM, adding up to time savings.
Simplified motion paths of the CMM scanner also mean more straightforward off-line CMM programming. This compares favorably with tactile inspection where elaborated programming effort is required to define a rather lengthy sequence of touch sensor movements and measurements.
CMM laser scanners are suited for detailed sheet metal feature inspection. For this purpose, laser scanners with multiple laser stripes and cameras have been developed. For example, a CMM scanner with three-stripes, each shifted 120 degrees from one another, views the inspected surface simultaneously from three different angles. This eliminates the need to scan certain surface areas a second time using a different scanner perspective. As this further reduces scanner motion and rotation, multistripe laser scanners accelerate feature inspection of sheet metal as well as molded metal and plastic parts. From the acquired point clouds, specialized software automatically detects the features, calculates their characteristics and graphically reports CAD deviations.
In the assembly process of sheet metal parts, CMM laser scanning is deployed to scan the shape of mating surfaces in order to verify whether they will fit together correctly. A better alternative than putting both parts on a master buck is to virtually assemble the parts by scanning their mating surfaces and verifying the connection on a graphic software display.
As a matter of fact, these scanners successfully deal with sheet metal, castings, composites, injection molds, foams and even glass. Detailed 3-D scans also assist in the development and repair of expensive die and mold production equipment. In this regard, detailed measurement of tools and first parts are very important in tuning and verifying the dimensional quality of stamped, casted or injection-molded parts. Point cloud data acquired through laser scanning represent a valuable resource because scan data is used for realistic FE meshes for thermal, strength and dynamic simulations, in case CAD data is missing or not up-to-date.
Power of Inline Robotic Laser ScanningIn production facilities for automotive parts, manual or CMM-driven geometric inspection tasks may slow down manufacturing throughput. In response to the growing need for accurate inline inspection solutions, laser scanners also have found their way into robotic metrology solutions. The problem with most robotic scanning solutions on the market is that their inspection precision ultimately depends on the motion accuracy of the robot itself. As robots typically lack position and path accuracy, they are simply not suitable for 3-D metrology.
The latest innovation in robotic laser scanning is combining a laser scanner installed on an industrial robot with an optical CMM. The purpose of the optical CMM is to dynamically track the accurate position and orientation of the laser scanner. This information is essential in this robotic laser scanning approach, as it obsoletes cyclic robot calibration and eliminates the influence of robot warm-up, drift and backlash. As a result, this solution transforms industrial robots into highly accurate and efficient inline metrology solutions.
As the optical CMM tracks the robot scanner entirely independently, it does not require positional data from the robot. This simplifies the user interface and radically reduces communication overhead between robot and metrology. Inline robotic laser scanning with metrology-level accuracy fits inspection applications where objects need to be scanned in their entirety, such as sheet metal body panels and body-in-white as well as forged or molded parts.
Altogether, laser scanning is an exciting and promising technology that has already proven great value in a wide range of metrology applications, in particular in the automotive industry. Also in other production environments across other industries-such as household appliances, plastic parts, turbine blades-where components are inspected for optimum assembly, 3-D laser scanning will further gain importance and undoubtedly increase its application reach in combination with tactile inspection methods. Q
Quality OnlineFor more information on laser measurement, visit www.qualitymag.com to read these articles:
- “Holographic Measurement Sheds Light on Complex Shapes”
- “How to Measure in Artificial Atmospheric Pressure Environments with Laser Trackers”
- “In-Process Measurement Makes Sense”
- “Practical Multisensor Metrology”