Reliability HotWire

Reliability HotWire

Issue 121, March 2011

Tool Tips

*When working with Bellcore/Telcordia predictions in Lambda Predict 3, when should I use Method I, Method II or Method III?

Lambda Predict 3 allows you to use any of the three methods described in the Bellcore/Telcordia prediction standard. You set the method at the block (also called "unit") level, which then determines which other fields become enabled for the block and/or its assigned components (also called "devices").

  • Select Method I if you will be using only the generic failure rates that are proposed by the Bellcore/Telcordia prediction standard.
  • Select Method II if you have lab test data that will be combined with the generic failure rates given in the standard.
  • Select Method III if you have field data that will be combined with the generic failure rates given in the standard. If you use this method, you have three additional sub-methods to choose from:
    • Use Method III (a) if the field data is based on a similar unit or device that was used at the same temperature and stress conditions defined for the prediction.
    • Use Method III (b) if the field data is based on a similar unit or device, but it was used at different temperature and/or stress conditions than the ones defined for the prediction.
    • Use Method III (c) if the field data is based on a similar unit that was used at the same temperature and the field failure rate is known.

If you select Method I or the "Devices" option for Methods II or III, all of the inputs are made at the component level, and the block’s failure rate is equal to the sum of the component failure rates.

If you select the “Unit" option for Methods II or III, the inputs are made at the block level, and the block’s failure rate will also be calculated directly at the block level. This will be influenced by the failure rates of the assigned components, but it will not be a simple summation.

For information about the specific calculations performed by the software, please consult the Bellcore/Telcordia prediction standard.

*How can I create a plot that displays the results from more than one data sheet?

Weibull++ has a multiplot feature that gives you the ability to display in a single plot results from multiple data sheets. This provides an easy visual method to compare analyses. For example, you may wish to show the reliability plots of two product designs in the same plot, or compare a simulated data set with actual data obtained from fielded products. (Although this tooltip focuses on how to create multiplots in Weibull++, a similar feature is also available in ALTA, RGA and BlockSim.)

Before using the multiplot, you must first calculate the parameters of the data sets. Once that is completed, you can add a multiplot to either the project or the folio:

  • Add a multiplot to the project by choosing Project > Add Additional Plot > Add Multiplot. All the calculated data sheets in the project will be available for inclusion in the multiplot.
  • Add a multiplot to a standard folio by choosing Folio > Insert Multiplot Sheet. Only the calculated data sheets in the current folio will be available for inclusion in the multiplot.

Adding a multiplot will open a list of all the available data sheets, as shown next (data sheets will not appear in the list if the parameters have not been calculated). You can select to include data sheets from standard folios and warranty analysis folios. Select the data sheets you wish to include and click OK to create the plot.

ALT

The control panel in the multiplot is similar to the control panel in standard plots, except for two things:

  • The option to Select Data Sheets gives you the ability to add or remove data sets to be plotted.
  • If the selected data sheets have not been calculated with the same distribution, the probability and contour plots will not be available in the Plot Type drop-down list. This is because the scales on a probability plot and the axes of a contour plot vary for different types of distributions.