Vision and multisensor inspection go mainstream

Jan. 14, 2011
Software advances make vision and multisensor technology an everyday tool for inspecting and analyzing mechanical components.

Miniaturization and advanced materials are cutting costs and improving the utility of all sorts of mechanical and electromechanical products. Examples include handheld digital devices, medical implants, miniature plastic-gear drives, diesel-fuel injectors, and compressor blades. Manufacturing engineers are thereby looking for ways to measure and analyze such components quickly and accurately during product development and production.

The exclusive use of contact-inspection systems is no longer an option for many kinds of parts. Conventional CMM probes, for example, even with 1-mm tips, cannot access small blind holes or tiny features. In other instances, complex geometries prevent probes from reaching critical points. In addition, soft, pliable, and dual-durometer materials that easily deform, as well as mirror finishes that may be damaged by contact, also make poor candidates for tactile inspection.

In the past, when these sorts of components were a rarity, measuring microscopes were a suitable choice. However, the increasing number of parts with small and inaccessible features along with requirements in some industries for 100% inspection have turned microscope inspection into part-validation bottlenecks.

Fortunately, current vision and multisensor systems, which might include devices such as microprobes, laser scanners, and chromatic white lights, let users rapidly collect vast amounts of dimensional information for design analyses and subsequent part validation. The systems use CAD-based programming and inspection software to operate in 2, 2.5, and 3D modes, collecting data that is useful not only for validating dimensions, but also for analyzing designs and manufacturing processes.

During the past five years, inspection-equipment developers have invested a lot of time in developing software that includes proprietary algorithms for accurately capturing images and transforming them into discrete data points that can be automatically compared to nominals in CAD models. These efforts have pushed vision and multisensor equipment onto the shop floor and away from the dedicated inspection laboratory. The advanced systems are as easy to use as a typical CMM.

Algorithms augment optics

A big barrier to the primary use of vision and multisensor devices in advanced metrology has been the perception that adjusting systems for appropriate lighting, contrast, and edge-detection sensitivity took specialized knowledge beyond that of average users. While this once may have been true, it is no longer the case. Many powerful new software algorithms effectively automate these important adjustments to provide consistent inspections from part to part and one vision machine to another.

A legitimate concern has been the subjectivity of making manual adjustments to set contrast. Optimized contrast substantially improves inspection accuracy by improving the vision system’s capability to detect edges and compensate for the tendency of light to bend around the edges of cylindrical surfaces, thereby shortening measured distances. Today, special algorithms automate the adjustment of contrast levels. At the touch of a button, the algorithm makes a series of rapid iterative adjustments until it reaches the best contrast.

Also, differences in light sources (for example, halogen or LED) used to illuminate parts and ambient lighting in different locations was another source of vision-measurement variability. However, it is now straightforward to correct for these variations. Current inspection software lets users compensate for these effects just as they would calibrate a probe on a CMM.

Additionally, because camera probes do not touch the edge they are measuring, edge detection must rely on the accurate interpretation of the data the vision software receives from the camera. Advanced vision-inspection software can fine-tune algorithms to account for both the part surface and illumination. This lets the software accurately find each feature edge.

Generally, inspection software uses a dominant-edge algorithm to select the edge of a part —especially when using a device containing built-in illumination — and this approach works well. But when measuring top-lit parts with a high-surface finish, this method is problematical. In these cases, a specific-edge algorithm is preferable. It detects features of interest based on contrast, shape, and location. Another example: Grind marks on the part might confuse a camera using top lighting. Here, the software might apply another type of algorithm that chooses the most dominant edge out of possible candidates in the camera’s field of view.

No more bottlenecks

Until recently, quality assurance (QA) was one of the biggest bottlenecks in product development for small high-tech components. Advances in vision and multisensor software technology now let many harried QA technicians keep up with the frenetic pace of development. Here are a few reasons why.

Off-line programming. In the past, users typically programmed vision systems using “teach-and-learn” methods. So, while they were developing inspection routines, the machines were not measuring parts. In contrast, today’s CAD-based vision software lets users develop part programs off-line, leaving the vision system free to measure. This does not mean that programmers have to work with unfamiliar software in an unfamiliar environment. The best of the new vision-metrology programs look the same in online and off-line modes and can generate part views that accurately simulate what the camera would see on an actual part.

One of the biggest advantages is off-line programming does not require the physical part. This lets QA personnel have programs ready to measure the part as soon as the prototype is complete. In many quarters, off-line programming is slashing days from product-development cycles.

Harmonious coordinates. Of course, vision systems can only measure what the camera can see. So, when something obstructs the camera’s view, another type of probe is necessary. In the past, this might require removing the part from the vision system and using a different inspection device.

Software used in advanced, multisensor systems lets users collect data using a combination of cameras and other probing technologies on the same machine. The most common alternatives are touch-trigger probes and white-light sensors. Sophisticated calibration techniques consolidate the inspections coming from these sensors into a single coordinate system. It makes no difference which type of probe collects the data. In addition, rotary and trundle tables can position the part at an ideal orientation for accurate inspection. Again, the software consolidates all the inspections into common coordinate systems.

Parametric programming. For manufacturers making families of parts, the capability to generate parametric programs for their vision systems is resulting in large productivity increases. A time-honored method used routinely by CMM programmers, parametric programming creates a set of rules that govern the generation of programs for every member of a part family. For each new member, the programmer simply changes the values in a table and the software creates the new part program.

Multiple feature captures. Software enhancements have also revolutionized the way vision systems collect data and, in the process, have boosted productivity. For example, MultiCapture Technology in PC-DMIS Vision software finds features that fit within the same field of view and then captures their inspection data simultaneously. Upon completing the inspections, the software drives the camera to the next cluster of features and measures them in the same way, continuing until the part is inspected. This approach lets users measure parts up to 35% faster.

Best-fitting results. It goes without saying that for parts to be commercially viable, manufacturers must maximize the production of good components. This is more easily said than done. But devising good inspection strategies goes a long way to making this possible. To meet this end, best-fit analysis, a tool long familiar to users of conventional CMMs, is being employed with great success on vision and multisensor machines.

Basically, best-fit algorithms use the information generated by the inspection software to evaluate related features as a group rather than as individual features. It then compares this “composite” feature to its design intent. This approach can substantially reduce the number of parts rejected during inspection, particularly when critical features of interest are complex and numerous. If not, it provides manufacturing engineers operations with actionable information that lets them rework the parts with greater precision and modify the manufacturing process to eliminate subsequent problems.

Enterprise metrology integration

Advanced inspection equipment can now operate within a broader enterprise metrology framework that allows for the integration of dimensional data and analyses into any phase of the manufacturing process from design through final inspection. Recent developments have further advanced this holistic approach.

Inspection planning. A new type of software lets engineers record their design intent in the form of inspection plans and attach them to their CAD files. They become an invaluable aid to part programmers in creating inspection programs. This “inspection-plan” approach only takes the designer a little additional time but lets vision system programmers complete their work up to 70% faster. Further, both designers and inspection-system programmers have access to “Change Management” tools. These bidirectional-software modules notify users of anything that affects the dimensional attributes of a part, whether design or manufacturing.

Robust data storage. Current systems capture enormous volumes of information, which provide a deep reservoir of data for analysis. High-tech manufacturers have started to generate vast data clouds for reverse engineering and for dimensional and statistical analysis. The need to manage this amount of data and make it readily available has spurred metrology-software vendors to rethink their data-storage strategies. Therefore, the latest releases of leading vision-metrology software support advanced, scalable, open-source database technologies.

Web–based reporting tools. Giving users of analytical tools access to raw data is only half the battle. Getting the inspection information where it needs to be quickly, securely, and in the right format is the other. To this end, software vendors have leveraged Internet technologies to build powerful, Web-based reporting and data-management software. This make it easy to collect data, configure reports, and disseminate actionable information for monitoring and improving manufacturing processes.

More at PC-DMIS Vision, Hexagon Metrology.

© 2011 Penton Media, Inc.

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