SPC Charts
The SPC Chart processor function helps analyze process data over a specified window and generate insights into the process performance and capability. It calculates various statistical measures and assesses adherence to quality specifications using control charts and pre-defined rules.
- First, the target_mean (default 100), window size (default 50), and timer interval need to be set.
- The timer interval parameter ensures that even if the connected input tag is not polling, the SPC chart will still publish data at the specified interval.
- If the input is expected to publish at the expected rate, set the timer interval to 0 to disable the timer.
- The current mean and standard deviation (stdDev) are calculated based on the input data within the defined window.
- Upper Specification Limit (USL) and Lower Specification Limit (LSL) are computed based on the current_mean and standard deviation.
- The process capability (Cp) is calculated as the ratio of the difference between USL and LSL over six times the standard deviation (6σ).
- CPu (Upper Capability Index) and CPl (Lower Capability Index) are calculated to assess how centered the process is within the specification limits.
- CPu = (USL - avg) / 3σ, CPl = (avg - LSL) / 3σ
- Process performance (Cpk) is calculated by determining the smallest value between CPu and CPl.
- Optionally, if valid data is present, the Cpm and Cpkm (which use the target mean) will be computed.
- The chart will also output any Nelson Rule violations with a true flag indicating which rule number has been violated.
- The rules are as follows
- Rule1: One point is more than 3 standard deviations away from the mean.
- Rule2: Nine (or more) points in a row are on the same side of the mean.
- Rule3: Six (or more) points in a row are continually increasing or decreasing.
- Rule4: Fourteen (or more) points in a row alternate in direction, increasing then decreasing.
- Rule5: Two (or three) out of three points in a row are more than 2 standard deviations away from the mean in the same direction.
- Rule6: Four (or five) out of five points in a row are more than 1 standard deviation away from the mean in the same direction.
- Rule7: Fifteen points in a row are all within 1 standard deviation of the mean on either side of the mean.
- Rule8: Eight points in a row exist, but none within 1 standard deviation of the mean, and the points are in both directions from the mean.
Parameters | Details |
|---|---|
Window Size | Defines the number of data points considered for calculations. The window size affects the computation of the current mean, standard deviation, and specification limits. |
Target Mean | This represents the desired average value for the process. It serves as a reference point for evaluating process performance against expectations. |
Time Interval | Specifies the frequency at which the SPC chart publishes data. |
Note: When creating an analytics flow with SPC Charts processor, refer the Use the SPC Charts Function guide for more details.