Digital video improves metrology system performance
Incorporating a multi-megapixel machine-vision camera into a video microscope creates a metrology instrument that rivals CMM precision for Digital Precision Corp. Photo: Toshiba Teli America.
The current generation of high-resolution digital video cameras is making it possible to achieve measurement precision with video microscopes that was previously achievable only with coordinate measuring machines (CMMs). With this new camera technology, video metrology equipment can make many of the same non-destructive surface-shape measurements faster and with similar accuracy, the main limitation being that video metrology works best for measuring flat objects in two dimensions.
CMMs achieve high measurement resolution by contacting component surfaces with a mechanical probe held by a robotic arm or gantry. Encoders in the robotic system determine exactly where the probe tip's center is in three-dimensional space, and the CMM's host computer uses that information to determine the contact point between the probe and the part's surface. By probing dozens of points over the part, the CMM builds up a 3-D model of the part's shape matching the accuracy and precision of the encoders' resolutions.
Video microscopes—which use a video camera and a precision lens to capture an image and then store it on a host computer for display or analysis—capture a far larger number of points that CMMs ever could, and do it in a fraction of a second.
Using a high-quality lens system calibrated for the field of view (FOV), image-analysis software running on the system's host computer can isolate and measure all the edges visible in the image. It then calculates the position of each edge in two dimensions. Such a system's advantages are much higher measurement speed and much better coverage of the part's features. The system's disadvantage is difficulty in measuring parts with high relief in the third dimension.
When challenged with issues of performance and price in the design of newly-ordered video metrology machines, Digital Precision Corp. turned to Toshiba Teli America for cameras that would accurately capture video images of various size work pieces. These images in turn would be transferred for analysis using Digital Precision's proprietary software.
'After reviewing the two designs,' recalls Al Sabeh, a vice president at Toshiba Teli America, 'I recommended incorporating our new CSB4000F four-megapixel CMOS camera with FireWire and 2,008 x 2,047 [pixel] resolution.'
The CSB4000F's ultra-high resolution qualified it for use with large FOV measurement and inspection. Coupled with an FOV microscope, the camera can measure larger size parts, such as 50 x 50 mm, with better than 5 micron accuracy online or off-line without an expensive stage.
Digital Precision incorporated the camera into a modified version of their DP1-FV video microscope system (shown in Figure 1). The camera was the main modification to the system. The digital camera's greater than 4 Mpx resolution made it possible to achieve much higher measurement precision than a standard analog camera.
The new design still uses a MVZL macro video zoom lens with close-up attachment manufactured by Optem (now Qioptiq), and ZView DMP-2000 image capture and analysis software. The lens has a 6:1 optical zoom range from 18-108 mm effective focal length, and a f/2.5 maximum aperture, making it appropriate for metrology applications at working distances from 5.5 to 11-in. (140-279 mm). The software provides features that lend it to automated-metrology applications, such as one-touch calibration, image enhancement to clean up noisy images, and exporting of images and analysis results to other computers.
The design has now been used in a number of applications, such as paper quality control and label-quality inspection. According to Yan Yu, marketing manager at Digital Precision, current applications have moved quality control off-line, but they are looking at in-line applications as well. 'It is a simple matter of reconfiguring the system for automated applications,' she says.
Sabeh, from Toshiba-Teli agrees: 'One of the DMP2000 software's advantages is its ability to link with other applications, such as CAD package and statistical process control (SPC) software, in real time.'
C.G. Masi , Senior Editor, Control Engineering
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