Quality 101: Back to Basics with 3-D Optical Measurement
It is a known fact: fixed coordinate measuring machines (CMMs) generate high accuracy measurements and can be automated, but they are expensive and offer no portability. In comparison, conventional articulated arms offer increased versatility and portability, but they are highly sensitive to environmental conditions; require a rigid setup; and are ill-adapted for shop-floor conditions. Simply put, the arm solutions eliminated CMM downsides, but also rid CMMs of two of their main advantages: stability of the machine and rigidity of the measurement setup.
Optical 3-D measuring devices were developed in recent years mainly to compensate for these flaws, and they are bound to play an increasing role in the world of quality control and metrology. Their increased shop-floor accuracy, decreased sensitivity to environment and higher measurement pace make them strong contenders to traditional quality control devices.
The success of 3-D measurement solutions that combine optics and sensors lies in the fact that they offer the benefit of optics-such as speed, portability, reduced sensitivity to the measurement environment and higher measurement volume-while remaining compatible with known, proven procedures. However, most of these optical measurement solutions still use mechanical technologies that present serious limitations in production environments. They require extreme stability throughout the measurement process, in addition to control templates and heavy bases which are costly, cumbersome and mostly inflexible.
3-D optical measurement solutions sidestep these limitations by enabling automatic positioning and continuous device measurement: the self-positioning concept or dynamic referencing.
How Do 3-D Optical Measurement Devices Work?The basic principle of this technology is that the optical CMM’s cameras track the position of the part and that of a 3-D scanner or touch probe simultaneously in a locked reference model, which makes measuring possible in all conditions. Factors such as instability in the machine or part setup, vibrations or thermal variations will have absolutely no effect on the output measurement accuracy.
The core component of these systems is the dual camera sensor, which continuously measures the position of passive and wireless reflectors located in the measuring volume through optical triangulation. First, the sensor measures the position of targets, which have been previously fixed on a part or on a fixture. This way, the machine reference system is completely locked onto the part, which creates a stable alignment, even if the part, the fixture or the sensor is moved. In the same way, the sensor can measure the position of numerous targets placed on an optical CMM 3-D scanner and accurately locate the latter within the measurement volume. The localization is relative to the part itself, thanks to the targets placed on it.
The same dual-camera sensor can be used with an arm-free portable CMM and work to localize the wireless probing apparatus. Therefore, the exact position of the tip sphere’s center also can be processed, and all the functionalities of a manual CMM become available to the operator without the requirement for stability.
Technologies Behind an Optical CMMThe first step is image processing. The challenge is to detect, with the highest accuracy possible, the 2-D position of each reflector.
It is not an easy task to accurately localize small dots at a few meters distance to obtain a resolution of a few micrometers. Based on advanced mathematics and physics, highly sophisticated algorithms have been developed to provide 2-D localization of the reflectors with a resolution that can be a thousand times higher than the cameras’ basic resolution.
Once all the 2-D coordinates from the images are processed, it is possible to compute the 3-D coordinates of each targets using the basics of photogrammetry. By using the 3-D coordinates of targets placed on the same object, it becomes possible to compute the position of this object, such as the probe, the scanner, a fixture or the part to be measured.
3-D Measurement for Everyday ApplicationsNow, we have a 3-D optical CMM-and can proceed to 3-D measurement as with any other type of CMM-but the difference is that we now have a better alternative for micrometer- precision 3-D measurements.
With dynamic referencing, 3-D optical measurement technologies deliver increased accuracy, particularly in shop floor conditions. The accuracy of an optical CMM is more than just a specification on a technical sheet. It will remain constant despite the quality of the setup, the environmental conditions or the operator’s level of experience.
This technology is highly innovative, but also is based on extensive knowledge and proven technologies. This is a result of more than 20 years of research in image processing and optical triangulation fields and takes advantage of the photogrammetry, which has been commonly used in the aerospace industry for years. For these reasons, 3-D optical measurement technologies are bound to play an increasing role in the world of QC and metrology.