Modeling Redundancy User Guide — Introduction and Redundancy in RBD Analysis

Version: 3.9.1

1 Nomenclature

Term	Definition
FCM	Fuzzy Cognitive Mapping
FMECA	Failure Mode, Effects & Criticality Analysis
MADE	Maintenance Aware Design Ecosystem
MTTF	Mean Time to Failure
RAM	Reliability, Availability & Maintainability
RBD	Reliability Block Diagram

2 Introduction

This guide is aimed at testing and documenting the effects of redundancy in a MADE model using FCM simulation. Redundant arrangements commonly occur in engineering systems to safeguard typically hazardous or critical failures, or to increase the general reliability of a system or subsystem.

"Q: How can the user model the effects of a redundant arrangement in MADE?"

The effects of redundancy may be investigated from the start of the functional model definition all the way through to the reliability reporting. This guide outlines the basic approach to the modeling of redundant items in a system using MADE. Although every type of redundancy arrangement is not specifically considered, this guide intends to show how redundancy can be considered, modelled, changed, analyzed and documented. The most important considerations when treating redundancies in MADE are to understand the type of redundancy being considered, and how to define it and the related components in the model.

RBD analyses, response simulation and FMECA reporting can also be used to experiment and document the effects of redundancy in the system. This allows the flexibility to design, modify and compare alternate system configurations in terms of its expected availability.

"A: MADE can model the effects of redundancy in a system by defining the failure response(s), criticality and reliability of the redundant items."

Figure 1 ("The effects of redundancy in MADE") shows a triangular relationship: "The Effects of Redundancy" (center) connected to three areas — Redundancy in Response Simulation, Redundancy in RBD Analysis, Redundancy in FMECA Reporting.

3 Redundancy in RBD Analysis

With a redundancy arrangement, the reliability of the redundant group of items should act to increase the overall system or platform reliability. This effect is captured through RBD analysis.

For this section of the guide please ensure the RAM Module is selected in the main toolbar.

Figure 2 ("Redundancy in RBD Analysis Process"): Step 1 (Reliability Definition) → Step 2 (RBD Modeling) → Step 3 (RBD Reporting).

3.1 Reliability Definition (Step 1)

To see the effects of redundancy on the reliability of the system, it is first useful to define a failure rate estimate (inverse of MTTF) so the numbers being calculated are more meaningful. In this step, we will enter the failure rate for each redundant component which will be seen in later steps of this section.

To edit the reliability data for redundant components:

1. In the main menu select Analyses → Criticality & Reliability Editor
2. Set Element Selection drop-down menu to Reliability
3. Expand the Redundant System/Subsystem in the tree diagram
4. Select a redundant item
5. Enter a Failure Rate or MTTF
6. Repeat Steps 3-5 as necessary

Figure 3 ("Reliability editor") shows the "Criticality & Reliability Editor" with Element Selection = Criticality (shown in this figure, though the section describes setting it to Reliability), a tree (Vehicle System > Coupling > Mechanical - rotational Angular velocity ×2, Driveline, Power Generation, Vehicle), and a "Function Flow Criticality" panel: Selected Profile = PHMT Risk Assessment Criteria (FAA), Criticality Method = ARP 4761 (Rev A), with sliders for Difficulty of Detection (10.0, "Very High"), Probability/Occurrence (1.0, "Extremely Improbable"), Severity (10.0, "Catastrophic"), Controllability (10.0, "Controllability 3"), Software Control (MIL-STD-882E) (0.0).

3.2 RBD Modeling (Step 2)

The most important step in RBD redundancy analysis is to define the reliability groupings and form an RBD model to represent the effects of redundancy on reliability. MADE automatically converts the functional model into RBD blocks and may adopt the layout of the original model if desired.

The MADE RBD tool allows users to define different types of redundancy including:

• Series Groups
• Parallel Groups
• K/N Groups
• Standby Redundancy Groups (Hot/Cold)
• Complex Groups

By assigning the appropriate reliability grouping in the RBD model the reliability of the system will change and the effects may be seen. (For this process, the continuation of the parallel grouping will be investigated.)

To create the RBD model:

• Right-click an item and select RBD Model
• Select all components and right-click the system model editor
• Select Group → Series Group
• Highlight the redundant items and right-click the system model editor
• Select Group → Parallel Group
• Verify the change in reliability from series to series with a parallel arrangement

The user may choose to combine RBD groups together, e.g. including a parallel group within a series group. This action will recalculate the group reliability value.

Figure 4 ("Redundant system RBD model") shows a worked driveline RBD model:

Series Group (top level, R(t)=0.9997854)
├── Parallel Group (R(t)=0.9999999)
│   ├── Series Group: Half Shaft F → Planetary Gearbox F → Wheel F → Wheel Resistance F
│   │     (each R(t)=0.9999285; combined R(t)=0.9997140)
│   └── Series Group: Half Shaft R → Planetary Gearbox R → Wheel Resistance R → Wheel R
│         (each R(t)=0.9999285; combined R(t)=0.9997140)
├── Driveshaft (R(t)=0.9999285)
├── Transmission (R(t)=0.9999285)
└── Differential (R(t)=0.9999285)

This demonstrates how a parallel redundancy sub-block (two mirrored front/rear wheel-drive series chains) is nested inside a larger overall series group alongside the non-redundant Driveshaft, Transmission, and Differential items, and how MADE displays the resulting reliability R(t) value at every level of the group hierarchy.

3.3 RBD Reporting (Step 3)

After the RBD model has been defined the results can be reported. The arrangement of the model and associated reliability data will be listed in the RBD report. The RBD report summarizes the effects of any redundant arrangements within a system, and any other groupings in the system to provide a visual display of the effects of the redundancy on the reliability of the system.

Using an RBD Delta reporting capability, the user can identify and quantify any overall increase in reliability of the system that results from introducing a redundancy.

To produce an RBD report:

1. Select Reports → Reliability Block Diagram Report, then select Next
2. Select checkboxes in the System Hierarchy to include items in the report
3. Select additional report options such as Screenshot Diagrams & Poor Performer Thresholds
4. Select Next to report formatting or Finish to generate the report

Figure 5 shows the Report Wizard report list (PDF) including Maintenance Actions Report (Abridged), Maintenance Cost Estimates Report; Mission Profile Definition group; Prognostics and Health Management group; Reliability & Availability > Reliability Block Diagram Report (selected), Reliability Allocation Report; Reliability-centered Maintenance group; Safety group. Description panel: "The Reliability Block Diagram or RBD report is PHMT defined and based on the RBD models in MADE. This report includes Failure Rates, MTTR, Grouping Types and Reliability information, including screenshots of the RBDs."

Figure 6 ("Report parameters page") shows the "RBD Report Parameters" page: a System Hierarchy checklist (Vehicle System > Series Group [Coupling, Driveline > Series Group (Differential, Driveshaft), Parallel Group (Front wheel configuration: Half Shaft Front, Planetary Gearbox Front, Wheel Front, Wheel Resistance Front; Rear wheel configuration: Half Shaft Rear, Planetary Gearbox Rear, Wheel Rear, Wheel Resistance Rear), Transmission], Power Generation [Control Unit, Diesel Engine > Series Group (Coupling 1, Engine, Injector Pump), K/N Redundancy Group (Air Filter, Primary Fuel Filter)]) all checked; options: "Include screenshot of diagrams?" (checked), "Poor Performers Threshold" (checked, radio options 0.9999999 / 0.95 / 0.90 / Custom), "Included Reliability Metrics" dropdown = "Baseline only"; Select All / Deselect All buttons.

Figure 7 ("Snapshot of RBD reports page") shows a generated "RELIABILITY BLOCK DIAGRAM REPORT" page for "Vehicle System > Series Group > Driveline": System Hierarchy, Item ID & Name (Driveline), LCN, Indenture Level (2), Baseline Reliability (0.999996), Baseline Inherent Availability (0.9999048), Baseline MTTF (194805.19), followed by the same nested Series/Parallel Group RBD diagram as Figure 4 (Rear wheel configuration and Front wheel configuration parallel groups, each containing 4-item series chains, nested within a top Series Group alongside Differential, Driveshaft, Transmission), each block annotated with its R(t) reliability value.