SPC block

This block calculates a number of statistics used in statistical process control over a moving data window.

SPC block

Description

The SPC block produces statistics useful for monitoring and reducing the variation in a process. These statistics are produced for a chosen field over a moving data window. Data is resampled over this window, resulting in time-weighted statistics. Samples of bad quality are ignored. Output is generated for two ports: the Window port and the Subgroup port.

SPC block

Block Type

Statistical block (moving window)

Input ports

The SPC block takes as input the various KPIs (Key Performance Indicators) available to the Selected KPI field in the Properties page. These must be of type double or integer.

Output ports

The block produces output statistics for the window (output to the Window port) and for the subgroup (output to the Subgroup port).

The output statistics are calculated using a moving window. The output fields start updating as soon as there are at least 2 subgroups in the window with at least 2 data points of good quality per subgroup.

Timestamp, Quality, and Value

Timestamp: The timestamps of all output statistics are set to the end of the window over which the calculation is performed. This may be before the time at which the block executes.
Quality: The quality of the Cp- and Pp- statistics may be affected by the quality of the LSL and USL inputs. Otherwise, if the input data quality is good then the output data quality is good. Consult the SPC block data quality overview to see how the SPC block treats incoming data quality.
Value: The output values are the SPC statistics for the Window port and Subgroup port described above.

Window port

Statistics produced on this port are applicable to the window you have defined on the SPC Properties page.

The window statistics produced are:

Cp
Cpk
Cpl
Cpu
Estimated Sigma
Maximum value of the KPI field over the calculation window
Minimum value of the KPI field over the calculation window
Mean of Subgroup Means
Mean of Subgroup Ranges
Mean of Subgroup Sigmas
Pp
Ppk
Ppl
Ppu
Process Mean
Process Sigma
Range Center Line
Range Lower Control Limit
Range Sigma
Range Upper Control Limit
Sigma Center Line
Sigma Lower Control Limit
Sigma of Sigmas
Sigma of Subgroup Means
Sigma of Subgroup Ranges
Sigma of Subgroup Sigmas
Sigma Upper Control Limit
Xbar Center Line
Xbar Lower Control Limit
Xbar Sigma
Xbar Upper Control Limit

Subgroup port

Statistics produced on this port are applicable to the subgroup you have defined on the SPC Properties page.

The subgroup statistics produced are:

Range
Sigma
Xbar

Parameter port

The parameter port is used for variable LSL and USL limits. The parameter port does not have to be used. The block reads its parameters from the parameter port, if it is configured to do so. When an SPC calculation occurs and outputs are produced and a USL or LSL was provided on the parameter port, only the current values of the parameter port fields are used at execution time for the entire calculation window.

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