Automotive Test Equipment

 

The Product:

 

The existing product was a piece of automotive test equipment that featured, among many other things, a tachometer.  Unlike the product described in “Cost Saving Data Acquisition System”, this design actually worked.

 

The Situation:

 

The tachometer design was analog-to-digital-to-analog-to-digital.  The production test featured adjusting two interactive pots.  Both pots had to be set at two different frequencies to calibrate the full measurement range.  The input range was only reliably accurate between the two set points.  It only stayed accurate until vibration bumped one of the pots out of adjustment.  Since this was a portable instrument, this was likely to happen eventually.  Operation was adequate, but marginal.  The second analog stage featured a barely adequate low pass filter.  The test process seemed to feature an unusual number of coffee breaks.  Nevertheless, the company planned to use this circuit for the next generation instrument.

 

The Goal:

 

The company didn’t see anything wrong with this situation, but there was definitely room for improvement.

 

The Result:

 

The new design featured an all-digital tachometer that had no adjustments at all.  Range was 100 to 20,000 RPM, with better than 0.1% accuracy across the range.  With no trim pots, the design was as rugged as a brick.  The parts cost was one dollar higher than the previous design, but the calibration labor was zero.  Test time was almost zero, since it only had to be tested at one point.  If the board was stuffed and soldered properly, the circuit just worked.