Compares established reliability requirements with state-of- the-art feasibility, and provides guidance in budget and schedule decisions.
The RBDA technique is a method for diagramming a system to determine the probability of successfully achieving a system requirement. RBDA provides an analytical solution to reliability problems in the form of Reliability Bock Diagrams (RBDs). The structure of a RBD defines the logical interaction of failures within a system. Assists in identification of, or lack of, redundant features and paths.
Identifies and ranks potential problem areas and suggest possible solutions.
As part of the RBDA, RBDs are among the first tasks completed during product definition. RBDs are constructed as part of the initial concept development. The RBDA is started as soon as the program definition exists, Completed as part of the requirements analysis, and In order to make decisions and trade-offs, continually expanded to a more detailed level as data becomes available
RBDA provides a basis for trade-off analysis and evaluate design alternatives.
Analyses & Integration
Constructing an RBD helps the Reliability Engineer develop a complete understanding of the systems functional relationships between hardware at various levels (system/subsystem/assembly/component ).
Individual blocks may represent single component failures, sub-system failures and other events that may contribute towards system failures.
Reliability of an individual sub-system block may be represented by a RBD at a lower hierarchical level.
Blocks are arranged in series and parallel arrangements/ paths from left to right. For a system to be successful in its operation at least one path must not fail beginning from a leftmost block to the rightmost block.
For example ,
In a series of 3 blocks A,B and C only one of the blocks failing would
eliminate the success path, ie, system failure.
In a parallel set of 3 blocks A,B and C all 3 would have to fail to eliminate the
3 success paths to result in a system failure
Basic Building Blocks of the RBDA
The Series and Parallel examples are shown and expressed in Boolean Algebra nomenclature, where the ‘+’ symbol represents OR logic, and the ‘·’ symbol represents AND logic
A system failure for A, B, and C blocks in series is represented by: A + B + C
A system failure for A, B, and C blocks in parallel is represented by: A · B · C
RBDs can be in a combination of series and parallel blocks. The system failure for a series and parallel combination of blocks A, B, C, D, E and F is represented by:
(A • D) + (A • E) + (A • F) + (D • B • C) + (E • B • C) + (F • B • C)
Reliability Block Diagram Analysis Application
When RBDA is applied to a design the benefits include:
- Facilitating an understanding of the system reliability by showing the relationships among its elements
- Providing a visualization of the flow of information within a system
- Assisting in identification of, or need of, redundant features and paths
- Providing inputs to other reliability methods, for example, Failure Modes and Effects Analysis ( FMEA ) / Failure Modes, Effects, and Criticality Analysis ( FMECA ), Reliability predictions ( MTBF ) and Fault Tree Analysis (FTA )
- Providing valuable input to facilitate design decisions and design trade-offs.
System Reliability Block Diagrams
The RBD below is an Auto-window system level RBD with partially updated sub-system blocks with prediction data and/or allocated MTBFs. Analytically calculated based on the time and failure rate assigned by the requirements
Calculation Method: Analytical
-- MTBF: 1923
-- Results at time 1000.00 --Reliability: 0.594521; Unreliability: 0.405479
- Shaded 3-D Blocks denote links to sub-system RBD as would be depicted using a typical RBDA Tool.
- Un-shaded Blocks would indicate no details are available, are yet to be developed at the higher levels, or no further details are required to a lower level.
Sub-system Reliability Block Diagrams
As sub-system RBD reliability values are updated, the system level reliability value is modified accordingly
- Battery Power Switching
-- MTBF: 25000
-- Reliability: 0.960789 ; Unreliability: 0.039211
-- Up Window Sense
-- MTBF: 20000
-- Reliability: 0.951229 ; Unreliability: 0.048771
As sub-system RBDs can be taken to multiple levels of data
-- Down Window Sense
-- MTBF: 3030 Calculation Method: Analytical
-- Results at time 1000.00 – Reliability: 0.718924 Unreliability: 0.281076
Dwn Window & Auto Rly
-- MTBF: 33333.3
-- Results at time 1000.00 --Reliability: 0.970446 Unreliability: 0.029554
Reliability Block Diagram Analysis (RBDA) has been performed on various types systems in all industries and applications. Listed below are a few actual examples of the types of systems and industries that RBDA has been performed:
A RBDA is usually performed with a FMEA, SWFMEA or Reliability Prediction Analysis to assist in organizing the system effects. It provides a system Reliability value.
RBDA Tools: RAM Commander RBDA/FTA/FMEA, RELEX-RBD/FTA/FMEA, RAPTOR, Reliasoft - BlockSim 7/XFMEA ; CAFTA
Past System Applications
International Space Station Systems
- Comm & Track
- Mechs & Structures
Logging While Drilling (LWD) Instrumentation
- LWD Negative Pulser / Positive Pulser
- Communication Interface Units