PRISM Forum - Message Replies
Topic: PRISM Questions and Answers
Topic Posted by: SRC
Date Posted: Wed Jan 12 8:33:33 US/Eastern 2000
Topic Description: Welcome to the PRISM forum! Please feel free to post your questions and comments about the PRISM assessment software here.
Posted by: Fargneil Ausenhaus
Date posted: Thu Jul 29 15:15:49 US/Eastern 2004
I am wondering how one is to use this software to account for several stages of a products life span. We must incorporate shelf time, transport, launch, and operation of a device but I see no way to do this all in one system. Furthermore, how do we accomodate the proper proportions of the overall life span for each stage, ie how do I make the travel time 1/20 of the overall lifetime? Thank you for whomever's assistance.
Subject: Assessing Multiple Usage Profiles
Reply Posted by: David Dylis
Organization: Reliability Analysis Center (RAC)
Date Posted: Mon Aug 9 10:01:13 US/Eastern 2004
When assessing the reliability of an item that has different environmental and/or usage profiles it is recommended that each of these profiles be modeled separately using the PRISM software. After each profile is modeled the user will need to combine the results (external to the PRISM software) using basic reliability equations (e.g., Reliability=e^(-Lambda*T).
PRISM allows the user to make changes to the "Environment" and/or "Operating Profile" parameters associated with a reliability prediction. The ability to allow or not allow a global change to the "Environment" and "Operating Profile" parameter values are defined by the user with the "Change Control" option on the "Item Information" folder tab of a component or an assembly. The parameter value changes are modified at the branch level above where the affected assembly and component branches reside. As a result, no change control options are available for the "System" branch of the tree, since there is no higher branch in the tree to define the changed parameter values.
The "Change Control" options defined within PRISM are:
• Allow (Default)
• Don't allow
Allow: The component or assembly will accept the absolute value of the changed parameter from the next higher level assembly or system. For example, if the system-level branch operating temperature is changed from 90°C to 80°C, the operating temperature of each of its assemblies and components defined with the "Allow" option will be changed to 80°C, regardless of what its value was before.
Don't allow: For the "Environment" or "Operating Profile" parameters the component or assembly will not accept a change to any of its parameters, i.e., they will remain fixed regardless of any global changes made at the next higher branch of the System Outline Tree.
The steps for performing a change are straightforward. The user will first highlight the system or assembly branch in the System Outline Tree for which the change in "Environment" and/or "Operating Profile" parameter values will be made. Once the change(s) is/are made, the user clicks on the "Trickle Down" button to flow the changes down to those subordinate assemblies and components whose "Change Control" option is defined as "Allow". Once the changes are "trickled down", the user must remember to re-summarize the prediction (using the Summarize" button on the appropriate item "Failure Rate" folder tab), before saving the changes to the database.