Statistical Process Control Guardbanding Methods for Inline Optical Screening Systems
Inline optical screening guardbanding absorbs measurement uncertainty into narrowed specification limits to guarantee target consumer risk levels.
Western Electric Rules
Meaning
Statistical process control frameworks use a specific set of decision guidelines on run charts to detect out-of-control conditions in manufacturing and administrative processes. Applying the Western Electric rules allows quality inspectors to identify non-random patterns and process drift before a system produces defective outputs. These guidelines divide a control chart into zones based on standard deviations from the process mean.
The rules do not apply to processes that are naturally unstable or those that follow a non-normal distribution, where they would generate excessive false alarms.
Chart Interpretation
Plotting the process output requires a control chart with a center line representing the mean, along with upper and lower control limits at three standard deviations. Under the Western Electric rules, the area between the mean and the control limits is divided into three equal zones of one standard deviation each. These zones are used to evaluate the distribution of consecutive points.
This structured layout makes it easy to spot trends.
Fluctuation Detection
Identifying a shift in the process mean involves looking for patterns of points within these zones. For example, a single point falling outside the three-sigma limit signals an out-of-control condition. Other rules look for runs of consecutive points on one side of the mean.
This helps in spotting slow process drift.
Signal Evaluation
Launching a corrective action whenever a minor trend appears can lead to over-adjustment and increased process variation. If a plant implements the Western Electric rules without training operators on how to handle signals, it will suffer from constant interruptions and unnecessary adjustments. The cost of reacting too early to a normal variation is a loss of overall process stability.
A careful application of the rules minimizes these false interventions.