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Stihl, Inc. manufactures chain saws and other gas powered products. When production of a new type of chain saw started, there was a need to change the quality control testing procedures. The company already had PLC based test stands in place that had been used to test past models. The new requirement for the chain saws was to be able to accelerate from 3000 to 11000 RPM in .6 seconds or less. The PLC system was reprogrammed to perform this task. However, when production started a large number of chain saws (approx. 30%) were failing the test, but knowledgeable engineers could listen to the tests, and knew these chain saws were in fact good units. The problem was that the scan time of the PLCs was too slow to adequately capture the fast acceleration curve. The acceleration data had to be collected at a faster rate.
Stihl contacted SDS to solve the problem. The solution to the problem had to use one PC to perform the tests for all five stands and the development time had to be kept to an absolute minimum since the chain saws were already in full scale production.
The solution was to retain the PLCs, since they were performing other important functions, and have a PC based system performing the acceleration test for all five test stands.
A DAS-1600 data acquisition board was chosen because it had enough A/D channels and fast enough acquisition rate, along with 24 lines of digital IO that would easily interface into a SSIO-24 solid state relay board to communicate with the PLC through 24VDC IO lines.
By communicating with the PLCs through the digital IO lines, the system acquires the acceleration data at the proper time from each test stand and reports back a pass/fail result to the PLCs. The acceleration curves for each test performed during the last week are saved to disk and a summary file is created for all units that are tested. The software is written in Visual Basic and runs under Windows 95. The system development was completed in less than one week.