SPC Data Aids Calibration

Manufacturingengineers use statistical process control (SPC) techniques tohelp keep a process within control limits. Defects such as a missing componentor an upside-down connector are easy to identify. Measuring an electricalquantity provides a wider range of results and involves instruments thatrequire calibration. Most articles about SPC seem to either assume perfect measurements or assume someone else will discussinstrument calibration.

In my opinion, information gathered by SPC tools can offer cluesthat help determine when an instrument might need recalibration prior to itsscheduled trip to a cal lab. No one wants to reject good products only becausea DVM went out of calibration.

When you measure, say, avoltage at a test point on many of the same product and plot voltage versusnumber of units, you will see a graph similar to the one shown. The graphincludes the X-bar, or average, value for a good product. You can use this typeof SPC graph to determine how closely the measured voltages match the expectedresult. I purposely exaggerated the 35 voltage points to make their valuedistinct and easy to see.

This graph also includesinformation about the distribution of the voltage values around the 20Vaverage. On its own, the distribution shows nothing abnormal for this smallsample. I artificially set an upper-control-limit (UCL) and a lower-controllimit (LCL). SPC software would calculate the UCL and LCL from gathered data.In this type of graph, signs of process problems can include:

1. Eight or more sequentialpoints above or below the average.
2. Seven or more points in anup or down "ramp."
3. Seven or more sequentialpoints with the same value.
4. Fourteen or moresequential points that alternate above or below the average.
5. Any point above or below theUCL or LCL lines.
6. Two sequential pointsabove or below A plus or minus 2s (standard deviation).
7. Four out of fivesequential points that fall outside A plus or minus s on the same side of the average.(Ref. 1, 2)

All these characteristics can arise from production variables,depending on what you measure. For electrical measurements, recurrence of thefirst three situations indicates a need to examine instrument measurementaccuracy and consider the need for recalibration.

SPC Data Aids Calibration_A