Examples provided here are purely for illustrating software features and functionality.

Example 3 – Allocating Design Reliability to Subsystems

Download Example File for Version 10 (*.rsgz10) or Version 9 (*.rsr9)

Background

Consider a chemical solution (e.g., ink formulation, medicine, etc.) that degrades with time. A quantitative measure of the quality of the product can be obtained. This measure (QM) is said to be around 100 when the product is first manufactured and decreases with product age. The minimum acceptable value for QM is 50. Products with QM equal to or lower than 50 are considered to be “out of compliance” or failed.

Engineering analysis has indicated that at higher temperatures the QM has a higher rate of decrease. Assuming that the product’s normal use temperature is 20ºC (or 293 K), determine the shelf life of the product via an accelerated degradation test.

For the purpose of this analysis, “shelf life” is defined as the time by which 10% of the products will have a QM that is out of compliance.

Example

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.

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.

Name	Operating Time	Importance Factor	Number of Sub-elements
Power Supply	12	1	35
Transmitter	12	1	91
Receiver	12	1	88
Control	12	1	231
Moving Target	6	0.25	88

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.