Where Microns Matter: Optical Measuring Systems

Smart Manufacturing Magazine March 2019
By Karen Haywood Queen

Optical measuring systems, which use light instead of touch, are becoming more widely used in manufacturing because of their faster speed, higher accuracy and ability to measure oddly shaped parts.

Optical measuring machines are used in all stages of manufacturing—for proof of concept, mass production and quality inspections. For times when both tactile and optical measurements are needed, manufacturers are offering more flexible, multi-sensor systems with laser, camera and tactile measuring options.

Optical metrology is proving its value in the aerospace, automotive, electronics and medical sectors, as well as in precision engineering across all sectors, manufacturing leaders say.

Optical metrology has a part to play both in proof of concept or first articles and when products move to mass production. With precision engineering, you have extremely small features on a part or features that can’t be accessed by traditional tactile programs methods or you don’t want to physically touch the part because of the material it’s made of.

Everyone is always looking at measuring things faster, with higher throughput and the same level of accuracy. Cell phones have smaller and smaller components. Even the glass that is on the devices is very intricate in different layers. We have sensors that can measure different layers of the glass thickness, from half a micron to five microns.

As printed circuit boards and other components are miniaturized, notably in the electronics industry, the ability to get accurate measurements will dictate whether the finished product will work.

Further, products in the electronics and injection molding industries are often characterized by dense clusters of small features not accessible to a tactile sensor. An image-processing sensor, on Optiv CMMs, simultaneously measures these densely arranged features within the camera’s field of view and does much faster than conventional tactile probing.

In precision engineering, you have very tight tolerances and the need for a large amount of data from what you’re measuring. When you’re using an optical probe, you’re getting thousands of points of data, and you can work within the software or export the data for analysis.

Medical sector especially promising
The medical sector has a high growth potential for optical measurements because of tough FDA regulations, the need for speed, strict accuracy requirements, oddly shaped components and the necessity of measuring parts without risking contamination.

Decades ago, the best way to measure the part was to actually make contact with the part. Nowadays, as video- and camera-based technology has increased, it is becoming more and more likely that you can get the same level of accuracy from measuring the part with non-contact methods.

For example, tactile measuring systems could contaminate an artificial hip. That hip also cannot be off spec by even a few microns.

Meantime, if the shifter knob on a car is slightly off, consequences aren’t dire. Users can choose the machine with the level of accuracy that works for their manufacturing requirements. Not surprisingly, greater accuracy correlates with higher cost.

Medical has had growth because of the strict requirements and the tighter accuracy that is needed. A lot of insertable instruments used have very tight tolerances and are difficult to measure. It’s not like you’re measuring a square part. We have three different configurations that allow for certain types of medical devices to be measured. It’s all about minimizing risk with the FDA. These are what medical companies are required to have documentation on tap for when they get audited by the FDA.

Optical measuring systems also make it easier to trace an audit trail. Managers can easily add an electronic signature to document their input so that it can be recorded and traced. When you are executing and inspecting a medical device, the whole history of the process, who did what, what lot it was, any issues, compliances and user access need to be tracked and logged in an audit log.

Not only can 100% inspection be realized, but inspection speed and throughput can also be dramatically increased due to automated system measurement routines and, depending on the application, palletized multi-part fixture inspection tables.

Ultimately, the goal in any good inspection program should be to attain maximum gage repeatability and reproducibility (GR&R) and, in the process, provide comprehensive data for statistical process control and traceability.

Reliable GR&R is possible with today’s non-contact video and multi-sensor systems and their advanced software. Current display readout and software measuring technology, such as Metlogix M3, provides full qualitative/quantitative profile analysis functions where an inspector can compare a part profile against a nominal CAD model and obtain an actual graphic representation of any deviation from the CAD file.

Speed of inspection is key

Speed also matters when all parts must be inspected. Without the necessary speed, the inspection process becomes a bottleneck during manufacturing. The ability to quickly measure a dense cluster of features also is a prerequisite for precise and repeatable evaluations of form and true position.

Pallet measurement in an automation setup, where several similar workpieces are measured at one time and the optical measurement is repeated in a loop, also helps speed up throughput.

AM will be affected
Additive manufacturing (AM) is another emerging sector for optical measuring. They are getting to the point where 3D printed parts can have the tolerance levels to require inspection systems like ours.

Optical measuring machines are deployed about half the time in production and the other half in quality inspections. On the production floor, temperature, humidity and vibrations can affect accuracy in measuring.

If the temperature changes dramatically when parts are getting measured, the measuring machine needs to account for that. Even more of a factor is vibration. That’s why people still send parts from the production floor to a quality lab for first inspection.

In the past, one challenge has been to find qualified operators with a good knowledge of measuring and programming logic. But, improved software has met those challenge.

Now, the biggest challenge on any automatic measuring machine is the creation of the inspection program. The ability to use the CAD model to define how users want to measure the part. The CAD model now has product and manufacturing information knowledge.

Other big issues include geometric dimensions and tolerances. To apply those tolerances, you have to program the machine correctly. Now the tolerances are embedded in the model and we can rely on our measurement software to report the right results. The software creates the whole program for you, including the measurement path and how to avoid hitting the part and fixture. The user doesn’t have to know as much.

Another challenge has been the need to measure multiple orientations of the same part. In typical optical measuring machines, the video and laser are always pointing down. Sometimes, though, users need to use multiple modalities, optical and tactile, during the manufacturing process. With that in mind, manufactures are using optical measuring machines that add an additional dimension via a tactile probe.

More recently, flexible measuring machines have been geared only toward measuring smaller parts that lend themselves well to benchtop machines. But small features now exist on larger parts. Think about a big turbine engine. Those are large products but they have intricate parts.

Instead of moving everything around to take measurements, you can measure everything at once [with] that snapshot by a video camera. When you incorporate a multi-multi-sensing system, laser and tactile on top of that, it can be hundreds of times faster. If you have thousands of features in that four-inch area, it can be thousands of times faster.

Many manufacturers add a tactile probe offset from their camera, which doesn’t give you true 3D compensation. But, by doing the opposite, the camera can calibrated through the entire 3D volume. That’s the benefit of being newer to the market.

Challenges remain. Some manufacturers are reluctant to try an unfamiliar technology or they simply don’t know these solutions exist. A lot of people are used to traditional CMMs. They’re familiar with tactile probes, less familiar with optical probes. People shy away because they don’t know the technology. But the learning curve is very low. The software for optical probing is the same as for tactile. Users don’t have to learn a new software platform.