Send vision data to SPC software
By Jon Titus, Contributing Technical Editor -- Test & Measurement World, 11/1/2007 2:00:00 AM
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You expect a vision system to indicate a pass or fail condition after each inspection. But your system also might identify the types of failures, such as missing ICs, damaged capacitors, or nonfunctional displays. Using statistical process-control (SPC) software to collect and process information about the types of defects you encounter can give you better control over their causes and help you improve production yields.
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| A Pareto chart shows the defects that cause the most problems. Eliminating defects related to component #1 raises the ratio of products that pass inspection to about 77%. Test engineers can next eliminate defects related to component #2 and raise the ratio to better than 87%. |
You may already use home-grown statistical tools and even Excel spreadsheets to compute moving averages, standard deviations, range limits, and so on. A commercial SPC package formalizes the tools and techniques and provides information in standard charts that other engineers can understand. And commercial SPC packages use standard algorithms to help you keep processes under control and within specifications.
SPC software produces graphic information such as the Pareto chart, named after Italian economist Vilfredo Pareto (1884–1923), which in SPC applications plots defects from highest to lowest number. The Pareto chart shown in the figure illustrates the cumulative increase in the quality of products as engineers and equipment operators eliminate each defect. Elimination starts with the defect that causes the most problems.
SPC techniques also produce charts that plot how manufacturing variables change in production lots. A typical chart plots a variable, perhaps a critical dimension, from each lot of products against the expected average value. This type of graph also includes upper-control and lower-control limits. If a production lot’s average dimension exceeds these limits, part of the production process related to the dimension has gone out of control. Engineers don’t simply pull limits out of thin air or use those demanded by a marketing department; they use process statistics to determine limits based on normal distributions of defects.
You will find many other types of quality-measurement charts in commercial SPC software. But no matter how you look at SPC, the goal is not to simply plot data, but to identify the root causes of defects and eliminate them.
If you want to link inspection data from a vision system to SPC software, you must ensure the vision system can produce results in a format SPC software can accept. Many software packages provide tools that simplify this data-transfer step.
The QuickBuild tool within Cognex’s Vision Pro software, for example, lets engineers use comma-separated-value or tab-separated-value formats to transfer ASCII-encoded information to other applications. Likewise, data from National Instruments’ Vision Builder and LabView programs let engineers choose from many formats and use ActiveX controls to exchange data between applications. Software from both companies uses the TCP/IP protocol to transfer data to networked computers and servers.
| FOR FURTHER READING |
| For an introduction to SPC, step through Wayworld’s tutorial at www.wayworld.com. |
| Software supplier MoreSteam provides more details about SPC at www.moresteam.com/toolbox/t402.cfm. |
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