Reliability HotWire

Reliability HotWire

Issue 138, August 2012

Tool Tips

* How do I use ALTA 8’s Quick Parameter Estimator (QPE)?

The Quick Parameter Estimator (formerly called the “Parameter Experimenter”) is a useful tool whenever you need to define a model without being able to calculate the parameters of a data set. In ALTA 8, you can now estimate the parameters of the entire ALTA model, which is a combination of a life-stress relationship and life distribution. The QPE can be opened from various tools within the software (e.g., ALTA’s Monte Carlo tool), and it can be opened from the Ribbon by choosing Home > Tools > Quick Parameter Estimator (ALTA).

The first step is to estimate the parameters of the life distribution under normal use conditions. In the Model area, choose an ALTA model to solve the parameters for and specify the units that will be used for all time values. In this example, the Temperature-Humidity-Weibull (TH-Weibull) model has been selected. Then, in the Quantification Method area, choose whether you will provide one parameter and the unreliability at a given time or instead provide two unreliability points.

The following example uses one unreliability point (10% unreliability at 1,000 hours) and one parameter (beta = 1.4) to solve for the eta parameter of the Weibull life distribution at the product’s normal operating conditions (specified in the Use Stress Level area). Since TH-Weibull is a two-stress model, two use stress values are required. In this case, temperature = 315 K and relative humidity = 50% under normal usage conditions.

After you click Next> to proceed to the second page of the utility, you will see the characteristic life for the distribution at the use stress level, which was calculated using the inputs from the prior page. (In the case of Weibull, the characteristic life is the eta parameter, which is the time at which unreliability = 63.2%. For the exponential distribution, this would be the mean life, and for the lognormal distribution it would be the median life.)

This second page is used to calculate the parameters of the life-stress relationship that describes how the distribution’s scale parameter changes at different stress levels. To do this, you must enter characteristic life values at the required number of accelerated stress levels. Since a two-stress model is used in this example, a characteristic life value must be provided for two accelerated levels, as shown below. At a stress level of 333 K and 90% relative humidity, the analyst expects eta to be 3,000 hours, while at a stress level of 353 K and 80% relative humidity, the analyst expects 2,000 hours.

After you click Next> again, the last page of the QPE displays all the parameters of the TH-Weibull model. In this case, the following parameters are shown:

Depending on where you opened the tool from, you will be able to either copy the parameters to the Clipboard or automatically transfer them to the analysis where they’re needed.

 

* In Version 8, if I have a license for both Weibull++ and ALTA, can I choose to see only the Weibull++ interface (or only the ALTA interface)?

If you have a license for both Weibull++ 8 and ALTA 8 on the same computer, you can choose to see just one interface (i.e., just Weibull++ or just ALTA) or the combined interface (with both Weibull++ and ALTA) when you launch the software. You make the choice on the Other page of the Application Setup window (File > Application Setup).

Click an option from the Weibull++/ALTA View Mode drop-down list:

  • If you always want to see the Weibull++ and ALTA functionality combined in the same interface, choose Let the license decide.
  • If you would like to decide which interface to use each time you open the software, choose Select on startup. A window will be displayed where you can choose which interface to work with.

The following picture shows both the Application Setup option and the window that will be displayed upon launch if you have chosen Select on startup.