SPC Histogram

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A Statistical Process Control (SPC) histogram is a graphical representation used to display the distribution of a set of data points in order to assess the stability and quality of a process. It is a type of bar chart that shows the count of data points falling within certain intervals. Each bar represents a bin, and its height corresponds to the frequency of data points in that bin. It can be used in a variety of scenarios including:

1. Monitor Process Variability: By analyzing the shape and spread of the histogram, you can identify whether the process is in control (stable) or out of control (unstable).

2. Detect Deviations: Histograms can reveal if the process is producing data within acceptable limits or if there are shifts or drifts in the process that lead to variations.

3. Spot Outliers: Any data points that fall outside the expected range of the histogram might be considered outliers and warrant further investigation.

4. Evaluate Process Capability: Histograms help assess whether a process meets specifications and customer requirements.

5. Quality Improvement: Analyzing the histogram can guide efforts to minimize variation and improve the process over time.

Starting the Control

In PI Vision, click on "Assets" at the top left to expand the corresponding area.

In the upper area click on "weruSPCHistogram" to select the control.

Then you can search for the desired PI tag and add it to the display using Drag&Drop.

Functionality of the Control

The control accepts all data points in the configured time period and sorts the values independently of time into a configurable number of intervals. A value range of +/- 5 sigma from the average value is displayed. The black line corresponds to the normal distribution. The blue coloring in the background corresponds to that from the SPC Trend Control.

Under "Plot Time" you can select whether the PI Vision time range or statically the last n data points are to be used. "Number of bins" defines how many sections the x-axis will be divided into. "Sigma Calculation" offers two different methods to calculate sigma.

Under Center Line, another value can be defined as the center line instead of the average. Upper and Lower Control Limit define the value of +/- 3 Sigma. Upper and Lower Specification Limit have no influence on the calculations and define the specification limits.

All five values can be specified statically as a number or dynamically as PI Tag (format \\piserver\pitag).

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Changing the Size of the Control

In case that the legend or some part of the control is not visible, it is necessary to adjust the size of the control. To do so, the first step is to change into edit mode (Icon in the upper right corner) and in a second step to click on the control. The eight appearing dots around the control can be used to adjust the size that everything is visible.

Removing the Control

Before removing the control it is necessary to switch into edit mode (Icon in the upper right corner). Thereafter it is possible to click on the control and press the delete key to remove the control.