Example 3 - Allocating Design Reliability to Subsystems
Background
This example will show you how to use the 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. Lambda Predict 3 includes five allocation models: Equal, AGREE, Feasibility of Objectives, ARINC and Repairable Systems.
For this example, assume that you have two MIL-HDBK-217F systems for which you will be apportioning the design reliability requirements. The first system is for a vehicle. You decide to create a simple model with four subsystems, each of which contains one external component: an engine, a drive line suspension chassis, a transmission and an electrical system. From previous testing, you know the failure rate for each component and you enter that information when you create the system.
The second system is for an AEW radar set that contains five subsystems: a power supply, a transmitter, a receiver, a control unit and a moving target. Because you will be experimenting with various allocation scenarios, you decide to add blocks for the subsystems, but not to add any additional components or enter any failure data at this time.
In the example file, each system is in a separate prediction with the vehicle system in the ARINC prediction and the AEW radar set in the AGREE prediction.
[Download Lambda Predict 3 Example File (*.lp3)]
Analysis
After creating the system, with the ARINC (vehicle) prediction open, Lambda Predict looks like this.
The vehicle system as set up in Lambda Predict.
Using the Equal and ARINC Allocation Models
For the vehicle system, you will use the Equal and ARINC allocation models to apportion the system reliability.
Step 1: Click the Vehicle system in the System Hierarchy panel and choose Tools > Allocation. The Allocation window displays, with the EQUAL allocation type displaying by default and a reliability goal of 90% at 8760 hours.
The Equal Allocation model apportions reliability equally among all subsystems to meet a user-defined reliability goal for a specified operation time.
Step 2: For each component, select the Include check box. Note that each component has the same allocated reliability value, in this case 0.9740, as shown next.
The allocation results from using the Equal Allocation model.
Next, you use the ARINC Allocation model to see the similarities and differences between it and the EQUAL Allocation model.
Step 3: In the Allocation Type field, choose ARINC.
Step 4: For each component, select the Include check box. The ARINC apportionment looks like the following.
The allocation results from using the ARINC Allocation model with equal failure rates.
The ARINC Allocation model assumes that all subsystems are in series and have constant failure rates. It meets the defined reliability goal by weighing the individual failure rates of the subsystems.
By default, the Present Failure Rate column displays in white, which indicates that you can enter the failure rates that determine the weighing factors. This gives you the ability to see how different failure rates affect the system.
As an alternative, you can select the Use Failure Rate From Current Project check box in the Control Panel to use the calculated failure rates from the prediction. In this case, the Present Failure Rate column displays in gray, which indicates that you cannot change values in it.
Step 5: Click the Use Failure Rate From Current Project check box. Notice how the values in the other columns have changed, as shown next.
The allocation results from using the ARINC Allocation model using the system failure rates.
When using the EQUAL Allocation model, each component has an allocated failure rate of 3.0069 FMPH. Similarly, when using the ARINC Allocation model with identical present failure rates, the allocated failure rates are also 3.0069 FMPH. However, when the ARINC Allocation model is used with the unequal present failure rates given in the prediction, the components with the higher present failure rates are allocated higher failure rates.
Step 6: Close the Allocation window.
Using the AGREE Allocation Model
For the AEW radar set system, you decide to use the AGREE Allocation model.
Step 1: Open the AGREE prediction and then open the Allocation window.
Step 2: In the Allocation Type field, choose AGREE. The Allocation window looks like the one shown next
The initial view of the AGREE Allocation model.
The AGREE model takes into account additional data for each subsystem when calculating subsystem reliability goals, including complexity and importance. The Importance Factor indicates how critical the block is to the overall system operation, expressed as a decimal 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.
You know from your data what the operating time, importance factor and number of subelements for each system are, as shown next.
| Name | Operating Time |
Importance Factor |
Number of Subelements |
| Power Supply | 12 | 1 | 35 |
| Transmitter | 12 | 1 | 91 |
| Receiver | 12 | 1 | 88 |
| Control | 12 | 1 | 231 |
| Moving Target | 6 | 0.25 | 88 |
Step 3: For each component, select the Include check box and enter the component values. The values in the Reliability Allocated and other calculated columns (gray background) will be updated based on your inputs, as shown next.
The allocation results from using the AGREE Allocation model with the component data.
You can then plot the data.
Step 4: Click the Plot icon in the Allocation utility Control Panel. The Allocation window looks like the one shown next:
The AGREE Allocation model plot.
The plot displays the apportioned failure rate goals that you just calculated in the Allocation utility.
