Source chart and sampling

An SQC alarm point monitors the status of PI point values with regard to standard statistical tests. The next step in creating an SQC alarm is to specify the statistical calculations underlying the alarm point and the sampling methodology:

• Source Tag: The source tag provides data to which the pattern tests are applied. For a chart of individuals this can be any point on the same Data Archive server. For all other chart types, the source tag is created by the wizard and its data comprise appropriately aggregated samples of the raw data tag.

  Different statistical calculations and corresponding chart types require different types of source points. An Individuals chart allows any type of source point. EWMA chart types require a Performance Equation point. All other chart types require a Totalizer source point.

• Chart Type: Choose a chart type. One of eight charts may be selected to define the calculations performed and display the SQC results in PI ProcessBook SQC charts.

  The configuration options provided change depending on the selected chart type:

• Raw Data Tag: If you are creating a non-individuals type SQC alarm, then enter or search for the name of the point containing the raw data to be sampled.

• Event or Time Based Sampling: Select a sampling mode to choose how events are included in a sample group. The sampling configuration greatly affects the behavior of the source point.

  For event-based sampling, simply specify the number of Events per Sample. For time-based sampling, specify an offset from midnight and an interval of time to define windows used for sampling.

• Filter Expression: Enter an optional filter expression using PI Performance Equation syntax to filter out any values that you do not wish to include in sampling.

• Lambda and Scan Class: Enter values for these items when specifying an EWMA (exponentially-weighted moving average) chart, where the source point is a Performance Equation point of the following form:

  'RawDataTag' + if badval('SourceTag') then <Lambda> *
  PrevVal('RawDataTag') else <Lambda> * PrevVal('SourceTag')

• The Scan Class refers to a valid scan class of the Performance Equation Scheduler.