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Stracener_EMIS 7305/5305_Spr08_02.05.08
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Systems ReliabilityModeling & Analysis
Series and Active Parallel Configurations
Dr. Jerrell T. Stracener, SAE Fellow
Leadership in Engineering
EMIS 7305/5305Systems Reliability, Supportability and Availability Analysis
Systems Engineering ProgramDepartment of Engineering Management, Information and Systems
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System Reliability Models
•Series Configurations•Parallel or Redundant Configurations
Active Parallel
r-out-of-n
Standby•Series-Parallel and Parallel-Series Configurations•General
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System Reliability Models
• The reliability definitions, concepts and models presented apply at any level of a system, from a single discrete component up to and including the entire system.
• Systems reliability deals with the reliability of the end-item system and is based on the system configuration and component failure rates as well intended service usage
• There are two basic types of reliability configurationsSeriesParallel or Redundant
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Terminology and Notation
• Path: A physical means for accomplishing a given function.
• Element: The basic system level under discussion. An element may be a Component, an Assembly, an Equipment, a Line Replaceable Unit (LRU), a Subsystem or a System
• Block: A logical representation of an Element.
• Reliability Block Diagram: A logical representation of a System, Subsystem, or Assembly in terms of its Elements.
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Series Reliability Configuration
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Series Reliability Configuration
• Simplest and most common structure in reliability analysis.
• Functional operation of the system depends on the successful operation of all system components Note: The electrical or mechanical configuration may differ from the reliability configuration
Reliability Block Diagram
• Series configuration with n elements: E1, E2, ..., En
• System Failure occurs upon the first element failure
E1 E2 En
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Series Reliability Configuration: Math Models
System Reliability
Rs(t) = P(A1) P(A2|A1) P(A3|A1A2) ... P(An|A1A2 ... An-1)
Where RS(t) is system reliability, i.e. The probability of systemsuccess for time t, given that the system was ‘up’ at t = 0 andP(Ai|A1 A2 ... Ai-1) is the conditional probability of event A occurring (i.e., element Ei survives for time t), given that eventsA1, A2, ... And Ai-1 have occurred (i.e. Elements E1, E2, ... and Ei-1 have survived for time t, for i = 1, 2, ..., n
•Product Rule of System Reliability
if the n elements are independent
n
1ii
n
1iiS )t(R)A(P)t(R
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System Reliability Models - Series Reliability Configuration
• Simplest and most common structure in reliability analysis.
• Functional operation of the system depends on the successfuloperation of all system components Note: The electrical or mechanical configuration may differ from the reliability configuration
• Block Diagram For Series Reliability Configuration with n elements: E1, E2, ..., En
Since a single path exists, the failure of any element in thesystem interrupts the path and causes the system to fail.
E1 E2 En
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System Reliability Models - Series Configuration: Product Rule
• Rs(t) = P(A1) P(A2|A1) P(A3|A1A2) ... P(An|A1A2 ... An-1)
Where RS(t) is system reliability, i.e. The probability of systemsuccess for time t, given that the system was ‘up’ at t = 0 andP(Ai|A1 A2 ... Ai-1) is the conditional probability of event A occurring (i.e., element Ei survives for time t), given that eventsA1, A2, ... And Ai-1 have occurred (i.e. Elements E1, E2, ... And Ei-1 have survived for time t, for i = 1, 2, ..., n
if the n elements are independant
n
1ii
n
1iiS )t(R)A(P)t(R
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System Reliability Models - Series Configuration
General time to element failure distributions• System reliability
n
1ii )t(Ht
S e)t(R
Where Hi(t) is the cumulative failure rate of element i, for i = 1, 2, ... n
• System mean time to failure
0
)(1 dteMTTFn
ii tHt
S
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Series Reliability Configuration with Exponential Distribution
•Reliability Block Diagram
•Exponential distributions of element time to failure
Ti ~ E(i) for i = 1, 2, ... n
• System reliability
tS
Se)t(R
SS
SMTTF
1
Where the system failure rate is
• System mean time to failure
n
1iiS )t(
E1 E2 En
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Series Reliability Configuration with Exponential Distribution
•Reliability Block Diagram
•Exponential distributions of element time to failure
Ti ~ E() for i = 1, 2, ... n
•System reliability tnS e)t(R
Which is the same as the expected time to the first failure, E(T1),when n identical items are put into service
nMTTFS
• System mean time to failure
E1 E2 En
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Series Reliability Configuration with Weibull Distribution
•Reliability Block Diagram
•Weibull distribution of element time to failure
Ti ~ W(i,i) for i = 1, 2, ... n
• System reliability in
1i i
t
S e)t(R
• System failure rate
n
1i
1i
i
iS t)t(h
i
E1 E2 En
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System Reliability Models - Series Configuration
• System cumulative failure rate
0
SS dt)t(RMTTF
• System mean time to failure rate
t
0
n
1ii
1
S dt)t(ht
1t)t(H
i
i
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System Reliability Models - Series Configuration
Weibull distribution of element time to failure Ti ~ W(,) for i = 1, 2, ... n
• System reliability
tn
S e)t(R
• System failure rate
1
S
tn)t(h
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System Reliability Models - Series Configuration
• System cumulative failure rate
1
1
nMTTF
/1S
• System mean time to failure rate
1
S
nt)t(H
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Example: Series Reliability Configuration
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Parallel Reliability Configuration
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Parallel Reliability Configuration – Basic Concepts
• Definition - a system is said to have parallel reliability configuration if the system function can be performed by any one of two or more paths
• Reliability block diagram - for a parallel reliability configuration consisting of n elements, E1, E2, ... En
E1
E2
En
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Parallel Reliability Configuration
• Redundant reliability configuration - sometimes called a redundant reliability configuration. Other times, the term ‘redundant’ is used only when the system is deliberately changed to provide additional paths, in order to improve the system reliability
• Basic assumptions
All elements are continuously energized starting at time t = 0
All elements are ‘up’ at time t = 0
The operation during time t of each element can be describedas either a success or a failure, i.e. Degraded operation orperformance is not considered
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Parallel Reliability Configuration
System success - a system having a parallel reliability configuration operates successfully for a period of time t if at least one of the parallel elements operates for time t without failure. Notice that element failure does not necessarily mean system failure.
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Parallel Reliability Configuration
• Block Diagram
• System reliability - for a system consisting of n elements, E1, E2, ... En
n
jiij
ji
n
1iiS )t(R)t(R)t(R)t(R
n
ii
nk
n
kjiijk
ji tRtRtRtR1
1 )()1...()()()(
if the n elements operate independently of each other and where Ri(t) is the reliability of element i, for i=1,2,…,n
E1
E2
En
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System Reliability Model - Parallel Configuration
• Product rule for unreliabilities
n
iiS tRtR
1
)(11)(
•Mean Time Between System Failures
0
SS (t)dtRMTBF
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Parallel Reliability Configuration
s
p=R(t)
s
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Parallel Reliability Configuration with Exponential Distribution
Element time to failure is exponential with failure rate
• Reliability block diagram:
• Element Time to Failure Distribution
with failure rate for i=1,2.
• System reliability
• System failure rate
t
t
S e2
e12)t(h
ttS eetR 22)(
E1
E2
θE~Tiθ
1λ
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Parallel Reliability Configuration with Exponential Distribution
• System Mean Time Between Failures:
MTBFS = 1.5
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Parallel Reliability Configuration
Exponential distributions of element time to failure
Ti ~ E(i) for i = 1, 2
• System reliability
• System failure rate
t)(tt
t)(21
t2
t1
S 2121
2121
eee
e)(ee)t(h
tttS
2121 eee)t(R
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Notice that hS(t) is an increasing function of t even though thefailure rate of each element is constant
• System mean time between failures
21
2121SMTBF
Parallel Reliability Configuration - continued
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System Reliability Models - Parallel Configuration
Exponential distributions of element time to failureTi ~ E(i) for i = 1, 2, ... n
• System reliability
n
1i
tS
ie11)t(R
• System mean time between failure MTBFS =
n
1ii
1n
kjik j i kji
jij i ji
n
1i i
1)1(...
111
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System Reliability Models - Parallel Configuration
Exponential distributions of element time to failureTi ~ E() for i = 1, 2, ... n
• System reliability
ntS )e1(1)t(R
• System mean time between failures
n
1iS i
11MTBF
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System Reliability Models - Parallel Configuration
Weibull distribution of element time to failure
Ti ~ W(i,i) for i = 1, 2
•System reliability
2β
2
1β
1
2β
2
1β
1θ
t
θ
t
θ
t
θ
t
S eee)t(R
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System Reliability Models - Parallel Configuration
Weibull distribution of element time to failure Ti ~ W(,) for i = 1, 2
• System reliabilityββ
θ
t2
θ
t
S ee2)t(R
• System failure rate
t
t1
S
e2
e1t2)t(h
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System Reliability Models - Parallel Configuration
Weibull distribution of element time to failure
Ti ~ W(,) for i = 1, 2
• System mean time between failures
1
S 2
12
11MTBF
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nelements
melements
nelements
melements
nmpR 11
mnpR 11
System Reliability Models - Parallel Configuration