When implementing Part Average Testing (PAT), the buffer size and moving average offset can be set to unique values for each calculation by adding 'bufferSize' or 'patLimit' to the calc button's Parameters. Please reference the PAT Application Note ( https://roos.com/docs/RBEH-BL4VJP?Open) for more information.
iPat uses a rolling window of data to calculate the Robust Mean and Sigma values. (See Figure 1) The window size is user-defined but is typically set to 100. That is to say the 100 most recent prior test results will be used to calculate the Mean and Sigma for the current device under test. The default rolling window size should be set with the "patBufferSize" symbol. This default value can be changed in a particular calc button by setting the 'bufferSize' Parameter.
Moving Average Offset
The default for the limit display in the datalog is the average of the last N parts +/- 6 sigma, so the raw data can be tracked. This can be changed by adding an attribute 'patLimit' to the calc button. For PAT, the value stored to the datalog is equal to the "OutputValue - (average of last N)", so it behaves like a moving average offset of the data. (See Figure 2) The calculated offset is expected to have a value around "0.0". The value in the 'patLimit' parameter changes the number of previous devices used to adjust the offset. This value defaults to '6', which is inherited from automotive industry standard.
Patch 247.18 (2020-07-12) or later, the limits on the PAT data save can now be one sided or asymmetric. 'patLimit' parameter can be added to change the data's moving average offset.
Patch 290.18 (2020-07-12) or later, change "ampleonPAT" calculation to fix the limit out to+/- 6 sigma, and allow one sided limits on PAT. 'bufferSize' can be added to change the size of the buffer.
How to Implement Custom PAT Parameters:
- Navigate to a test panel that contains a "Pat" Calc button.
- Choose Parameters... from the right mouse button menu on the System Method ...Pat... button (see Figure 3).
- Choose Dictionary > Add, and enter 'bufferSize' and/or 'patLimit' as the new key name and choose OK (see Figure 4 & Figure 5).
- Replace "Nil" with the desired value (i.e. "50") in the value pane.
- Choose Save from the right mouse button menu in the same value pane. (see Figure 6)
- Close the Inspecting: IdentityDictionary window by clicking the close icon in the upper right corner.
- Repeat this process for each PAT calc button that requires a different bufferSize or patLimit value.
Note: The testplan must still be saved for this change to become persistent.
Figure 1: bufferSize Parameter Sets the Rolling Window
image credit: Pat Implementation & Operation, page 4
Figure 2: patLimit Parameter Sets the Moving Average Offset 'N" Value
image credit: Pat Implementation & Operation, page 3
Figure 3: Parameters menu
Figure 4: Adding a Parameter
Figure 5: Setting Parameter Name
Figure 6: Setting Value and Saving