EXPANDING VISION

NON-CONTACT INSPECTION KEEPS PACE WITH PRODUCTION.

The first optical comparators, known as "shadowgraphs," entered the factory when customers' needs for identical parts became more critical. The shadowgraph presented a magnified shadow of a part that allowed easy comparison to an accurate profile of the part, usually in the form of an overlay. The operator would use a micrometer head to move the X- and Y-axes to approximate the horizontal and vertical deviation between the overlay and the part. Making note of any discrepancies, the operator went back to his machine, reworked the part and repeated the process until the part was within tolerance. It was essentially a simple go/no go, pass/fail test.

Today, the optical comparator is a reliable factory stalwart that workers find easy to understand and use, whether on the shop floor or in the inspection room. The transition from purely observational optical measurement to direct digital analysis began about 20 years ago. Modern comparators are equipped with better optics, readout devices, software interfaces, geometric measuring capabilities, and screens that range in size from 12 in. up to 40 in. Light path options are offered in horizontal or vertical orientations. Today, many optical comparators are configured with optical edge detection.

STEPPING UP TO VISION

Starrett Galileo EZ isan affordable, compactmanual visionsystem featuring livevideo camera feed.

While the comparator is a versatile measuring and inspection tool, it does have limitations when it comes to high surface illumination and magnification requirements. For these applications, the manual vision system is the next logical step up from a comparator.

A manual vision system provides superior sur face illumination and the flexibility of a zoom lens with magnification ranging from 10x to 240x. At one time, the manual vision system used a simple crosshair, CRT monitor and geometric readout. Those that offered edge detection required the use of complex computer systems. However, today, video edge detection is a common feature and operators that are familiar with a comparator find the accompanying electronics and readout intuitive and easy to learn. Popular features include the ability to grab images and archive them, add text, and upload data to a flash memory device.

AUTOMATED INSPECTION SPEEDS PRODUCTION

Growing production volume will inevitably require increased throughput and repeatability from the inspection process. With large batches, inspection often involves repetitive measurement routines that naturally lend themselves to automation. At the same time, increasingly tight tolerances may simply be incompatible with manual inspection and operator subjectivity. When inspection can no longer keep pace or accurately inspect complicated parts, an automated non-contact inspection process may be the answer.

For manufacturers with large batches of components, the inspection process can also be streamlined by arranging the work on pallets. The solution is in using fixtures to organize parts. By creating a fixture setup in rows and columns, the offset between parts will be a known and constant size in both the X- and Yaxes. This information is used to program a measuring routine appropriate for the fixtured parts. For some applications, though, the fixture could be as simple as a right-angled plate against which parts are placed one after the other.

THE NEED TO SEE MORE

As parts become smaller and more intricate, the demands on inspection systems become greater. Higher magnification and better illumination underscore a basic, yet enduring principle for Vision systems: if you cannot see it, you cannot measure it.

Lighting, for example, is essential for obtaining optimum contrast. Quadrant lighting provides individual control over the angle and direction of the light source. Through-the-lens lighting is frequently necessary for height measurements and for seeing down blind holes.

Automated vision systems use video edge detection to automatically establish points. Some systems are now able to take up to 300 data points with the click of a mouse, measuring a feature such as a circle or a line. In 2D scanning mode, the software can take up to 5,000 data points around the shape. The more data on the part, the more precise the comparison to the reference file

An operator using a traditional overlay to check parts can now import a 2D CAD file, create a tolerance, and using a video tool, scan around the part. By using this method, the operator can now collect approximately 5,000 data points. Then the system automatically fits the points to the CAD profile and tells the operator whether the part has passed or failed. This process is a little longer than the traditional way, but the operator is able to obtain infinitely more data for the comparison. The data points can be stored for future reference or inspections-- something that cannot be done with the overlay method. For parts with very tight tolerances, accuracy may also require the use of an automated vision system, some of which are capable of a working accuracy of less than four microns.

For example, medical instrument manufacturers are finding the inspection of small intricate parts, such as surgical blades and bone drills, have become so complex that they require the magnification and accuracy of a vision system. Optical comparators are still used for many shop floor inspections, while vision systems are providing measurement repeatability and increased productivity in the QC department.

NON-CONTACT MEASUREMENT CONTINUES TO EVOLVE

Inspector views magnification of acustom needle point on a Starrett HB400projector.

Successfully measuring complex parts requires the most suitable scanning method and the mathematical power to extract and compare key dimensional features. Today's advanced vision systems are integrating video and laser scanning with touch-probe inspection. This creates a multi-sensing vision system capable of switching between methods, depending on the needs of the part. With these new systems, the phrase we used earlier -- "If you cannot see it, you cannot measure it"-- is no longer a concern because if the vision cannot see it then you can probably measure the part using either the Touch Probe or the Laser.

The decision to go with a multi-sensing inspection system warrants careful analysis. Some parts are best suited to physical measurement using touch probe inspection, while other more intricate parts with 2D features are better suited for a vision system because of its ability to precisely find the part's edge and take a large number of data points. Then there are situations where assemblies can include both 2D and 3D features which require the capabilities of both the touch probe and the vision system. This is where the multi-sensor inspection system becomes essential. It enables the operator to use the touch probe when it is most appropriate and switch to the vision system as necessary, treating the entire inspection in a single routine.

For accurate results, fixturing needs to be considered when using automated multi-sensor measuring systems. Proper fixturing is critical for producing reliable measurements.

Fully-automated inspection is becoming reality today. Some vision systems can interface with pallet-loading robots. The vision system coordinates positioning the work stage, loading the work piece, measuring the part, and calling on the robot to present the next part for inspection.

In the future, it may be possible to interface inspection with a pallet loading robot and the machining center so that systems can be used to measure. They would then visualize modifications and communicate the new offsets and dimensions back to the machining center. Today's visual inspection goals will continue to be removing subjectivity from the production process, while improving accuracy, repeatability and throughput.

Non-contact measurement has evolved into a resource with many options, designed to suit a wide array of applications. Where illumination and magnification have become issues, the transition to a manual vision system is relatively easy and economical. Where throughput, speed, volume and repeatability issues loom, automated inspection systems provide an ideal solution.