Seat Belt Testing

A New England assembly firm  uses Labtech Notebook and Labtech Control for  SPC (statistical process control) in manufacturing. This firm has installed this application on one of several PCs that control and monitor a large machine that assembles seat  belts.  Here is the Build-Time setup.

At the end of the assembly process, the machine mates (clicks together) the finished seat-belt fasteners and tests them. The QC test measures the force resisting the mating of the fasteners. This force curve has a characteristic shape for  well-manufactured belts. Defects in the metal parts such as burrs or grooves cause the force curve to deviate from normal. The assembly firm has found that 100 points of force data are sufficient to characterize the entire curve (the jagged black line in the upper righthand plot in the VISION display). The upper and lower curves are the upper and lower  bounds of acceptability. The acquisition run must collect the 100-point profile and compare each point against the upper and lower bounds. If even one point is outside these bounds, the part is rejected.

 Here is what the VISION display looks like while the setup is running:

In order to keep up with the manufacturing process, the 100 points and the comparisons should run at a 1-kHz rate. To  implement this task, the test system performs data acquisition of the force profile with a PC-based data-acquisition card. To start an acquisition run, a digital input on the card reads a digital output from the PLC (programmable logic controller) that controls the machine's functions.

 At the left of the screen are SPC Shewart X-bar and sigma charts derived from a single summary data value characterizing each seat-belt closure (in this case, the average force). These values are updated in real time so the operator can monitor them and adjust the machine to continually improve process quality. If the process exceeds either the upper or lower control limits, the system plays a WAV file that gives the operator an audio message indicating what's gone out of spec. Finally, at the lower right, the system accumulates data on the product reject rate.

 Many manufacturers are finding that QC machine monitors are not only requirements for ISO 9000 and total quality management programs, but if these monitors incorporate excellent graphics, they can become a competitive edge that makes the difference in winning a sale over a competition. Also, there is an ever-increasing trend toward networking this  type of monitor as part of an overall production control system for the plant and the manufacturing enterprise.

Notes on running the Seatbelt setup:

 The Seatbelt setup should be run with the interrupt-driven scheduler.  If your machine is slow and you get error 300, set Abort on Clock Overrun to "No".  The small amount of jitter caused by slower PCs  does not invalidate the profile.

Click here to download the actual LABTECH Seatbelt Setup used in this application.  Just unzip these files and then copy them into your LABTECH Program subdirectory.

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