Control Chart Setup
Explanation
This activity is used to setup statistical process control (SPC) for a specific
data point in a specific control plan. The setup is used both when creating
control charts manually and when they are created automatically using Real Time
SPC.
During the setup, a control chart type is chosen. There exists several
control chart types.
- p Chart - Gives information about the proportion of
nonconforming units.
- np Chart - Gives information about the number of nonconforming
units.
- c Chart - Gives information about the number of nonconformities.
- u Chart - Gives information about the number of nonconformities
per unit.
- Xbar and R Chart - Gives information about the average
and the range of the result set of each analysis selected in the graph.
- Xbar and S Chart - Gives information about the average
and the standard deviation of the result set of each analysis selected in
the graph.
- X and MR Chart - Gives information about individual
results and moving ranges for each analysis selected in the graph.
The available control chart types that can be chosen is depending on the
data type and the inspection type the specific data point has.
- If the data type is Attribute and the inspection type
is Percent the available chart types are p Chart
and u Chart.
- If the data type is Attribute and the inspection type
is Quantity the available chart types are np Chart,
c Chart, p Chart and u Chart.
- If the data type is Variable and the inspection type
is Percent the available chart type is X and MR
Chart.
- If the data type is Variable and the inspection type
is Quantity the available chart types are XBar
and R Chart, XBar and S Chart and X and MR Chart.
- If the data type is Categorical and the inspection
type is Percent or Quantity the available
chart type is p Chart.
Control charts are used to analyze a process or its outputs and take appropriate
actions to achieve and maintain a state of statistical control. A process is
in statistical control ("in control") when all special causes of variation
have been eliminated and only common causes remain, i.e., observed variation
can be attributed to a constant system of chance causes.
Several statistical tests (alarm rules) can be activated to determine whether
the control chart is "in control" or "out of control".
Out of Specification: An alarm rule violation will be triggered
when a point in the control chart is above Inner Max or Outer Max or below Inner
Min or Outer Min.
- Above Upper Control Limit: An alarm rule violation
will be triggered when a point in the control chart is above the Upper Control
Limit (UCL).
- Below Lower Control Limit: An alarm rule violation
will be triggered when a point in the control chart is below the Lower Control
Limit (LCL).
- 2 out of 3 beyond 2 sigma: An alarm rule violation
will be triggered when at least two out of three successive points in the
control chart on the same side of the centerline (CL) are beyond the 2 sigma
limit.
- 4 out of 5 beyond 1 sigma: An alarm rule violation
will be triggered when at least four out of five successive points in the
control chart on the same side of the centerline (CL) are beyond the 1 sigma
limit.
- 15 at or within 1 sigma: An alarm rule violation will
be triggered when 15 successive points in the control chart are at or within
1 sigma.
- 8 outside 2 sigma: An alarm rule violation will be
triggered when eight successive points in the control chart are outside
+2 or -2 sigma.
- 8 below or above the centerline: An alarm rule violation
will be triggered when eight successive points in the control chart are
below or above the centerline
- Runs up/down of length 8 or greater: An alarm rule
violation will be triggered when eight successive points in the control
chart are in ascending or descending order.
If an alarm rule violation occurs for an activated alarm rule, information
about the alarm rule violation will be displayed in the control chart.
It is also possible to decide if any specification limits should be displayed
in the chart and how to calculate the control chart control limits.
To set up SPC for a data point click Setup SPC Data on the
Control Plan Lines tab and enter the settings in the
Variable SPC Setup dialog. To see the default settings
of the SPC setup, click
Default Settings. To activate
the control chart setup click Save Settings.
Prerequisites
- A control plan with at least one control plan line must exist.
- Real Time SPC and/or SPC is enabled for the
control plan line.
System Effects
As a result of this activity, the control chart type is determined for
a specific data point in a specific control plan. Decisions are also made about
how to calculate control limits, whether to include specification limits in
the control chart, and whether one or many alarm rules should be activated.