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Machine Vision for Process Control
When considering machine vision for automating manufacturing operations there are four categories of functions: gauging, guidance, sorting and inspection. Gauging and inspection, when not used in a sorting mode, are usually quality control operations. To get the maximum benefit from machine vision systems it is best to implement them as part of the process control rather than as quality control devices.
The phrase is often used "you can't inspect-in quality". Implementing machine vision inspection systems to detect manufacturing defects can point to problems in the manufacturing process, but cannot control them. Machine vision inspection is often used to find out what the problems are with a manufacturing operation in order to improve the process and avoid costly rejects. This is an expensive way to get satisfactory yields and throughput rates. It is costly to use higher precision mechanics and raw materials rather than controlling the process through feedback. Setup of the manufacturing equipment can be time consuming and can generate costly scrap parts. Machine vision has the greatest impact when it is part of the control system in the manufacturing process.
The most common control function implemented with machine vision is guidance. Robot guidance and alignment are two typical implementations. Robot guidance usually requires 3-D vision while alignment is typically 2-D. To align parts for processing, unique patterns must be identified during training that can be used by the vision system to register subsequent parts. The best results are obtained from specifically designed locating registration marks called fiducials. Solid circles are often preferred for their rotation insensitivity and easy centroid definition. Fiducial shape, size and location are defined for surface mount printed circuit boards for both board alignment and part placement in IPC-SM-782(1). To align a part in X, Y and Theta, or rotation, requires two fiducials or patterns. To get sufficient precision in Theta the two fiducials should not be in the same camera image. Either two cameras or a single moveable camera are required.
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(1) IPC-SM-782 Surface Mount Design and Land Pattern Standard, Institute for Interconnecting and Packaging Electronic Circuits |
