Example 3 - Allocating Design Reliability to Subsystems
Download Example File for Version 9 (*.rsgz9)
This example demonstrates how to use Lambda Predict's Allocation utility to logically apportion a product's design reliability into lower level design criteria such that the cumulative reliability still meets the system requirements. The software includes five allocation models: Equal, AGREE, Feasibility of Objectives, ARINC and Repairable Systems.
The example file contains sample system configuration data for all prediction standards supported by Lambda Predict. You can open any of the sample projects, but only the systems that are analyzed with the prediction standard you have purchased will be displayed. To demonstrate the tools, we will use the MIL-217 system as an example. The instructions are generally the same for all prediction standards (unless noted otherwise), so you can follow along with the example regardless of which system is available to you.
Assume that you have two systems for which you will be apportioning the design reliability requirements:
- System 1 consists of four subsystems. You create a simple model with four blocks, each of which contains one component. From previous testing, you know the failure rate for each component and you enter that information when you create the system.
- System 2 consists of five subsystems. Because you will be experimenting with various allocation scenarios, you decide to add just blocks for the subsystems and not add any additional components or enter any failure data at this time.
The following pictures show the system configurations for Systems 1 and 2 in the sample MIL-217 project.
Using the Equal and ARINC Allocation Models
For System 1, we will use the Equal and ARINC allocation models to apportion the system reliability.
Step 1: Click the top-level item in the System Hierarchy panel and choose Prediction Tools > Allocation.
In the Allocation window, click the drop-down list on the control panel and choose the Equal allocation type. Enter 0.9 for the reliability goal and 8,760 hours for the operating time.
Step 2: Select the Include check box for each item, as shown next, then click the Calculate icon on the control panel.
The results show that each item has the same allocated reliability value, in this case 0.9740. The Equal Allocation model apportions reliability equally among all items to meet the specified reliability goal for the specified operation time.
Step 3: Next, we will use the ARINC allocation model and compare its results with the EQUAL allocation model. On the control panel, choose the ARINC allocation type.
Step 4: Select the Include check box for each item. The ARINC allocation model assumes that all items are in series and have constant failure rates. The model meets the defined reliability goal by weighing the individual failure rates of the items. As shown in the following picture, items with higher "present failure rates" are allocated higher failure rates.
Step 5: Change the weights of the failure rates by entering the value 0.5 in the Present Failure Rate column for each item, as shown next. This will distribute the failure rates equally among the items. The results show that the allocated failure rate of each item is 3.0069 FPMH. This is equal to the results we obtained from the Equal allocation model.
To return to using the failure rates from the current prediction analysis, select the Use failure rate from current prediction check box on the control panel. Note that if you keep this option selected, you will not be able to edit the values in the Present Failure Rate column.
Using the AGREE Allocation Model
For System 2, we will use the AGREE allocation model.
Step 6: Open the "System 2" prediction folio, and then open the Allocation window.
Step 7: On the control panel, choose the AGREE allocation type, then select the check box for each item.
The AGREE model takes into account additional data for each subsystem when calculating subsystem reliability goals, including complexity and importance. The values in the Importance Factor column indicate how critical each block is to the overall system operation. The values are expressed as decimals from 0 to 1, where 0 indicates that the block's failure will not cause any problem for the system and 1 indicates that the block's failure will definitely cause system failure.
Enter the data from the following table into the corresponding columns in the Allocation window.
The following picture shows the completed setup and the results.
Step 8: To display a plot of the apportioned failure rate goals that were calculated by the utility, click the Plot icon on the control panel. The following picture shows the plot.