Implement Statistical Process Control for Versatile Turn Signal Flasher

Inspired by early instruction on statistical process control, I noted that we had ongoing continuous problems with the turn signal flasher flash rate and duty cycle.  The flasher does two rates – turn, and hazard.  To meet the SAE J590, there rates each have minimum and maximum rate limits, but also a minimum and maximum ratio between the two rates.  We had constant ongoing high fallout rate on the production line, something like 30-50%, and were having to change component values continuously.

First, we did a process study.  We took 33 units off of one batch and carefully measured their characteristics.  When we analyzed them, to our surprise, we found that, although the spread of the rates was too wide to meet our requirements, it wasn’t that wide of a variation – it was primarily the process mean that was far off.  The resulting Cpk did not perfectly meet our requirements, but it was close enough that we could improve our results considerably!

I undertook a careful analysis of the results of the study.  I determined that changing the timing capacitor and resistor values, and using higher precision parts, should bring the results into J590 acceptable limits.

We ran another batch, and to our delight, almost every unit passed.

After that, I was still called upon periodically to address characteristics going outside the limits, but it was far less often.  We found that often, this was as a result of batch-to-batch variations in the ICs or capacitors.