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8. SYSTEM DESIGN
•This field is largely overlooked, not because it is unimportant, but because most engineers are educated in selected disciplines, and this is an interdisciplinary topic.
•Most of the successful uses of this topic have been by computer system designers, electrical engineers, and aerospace designers.
•The basic set of problems in any system design are,
-what functions are needed
-how do the modules interact
-what do we do when a module fails
-how reliable is a system
-etc.
8.1 SYSTEM FAILURE
8.1.1 Introduction
•Basic concept, anticipate things going wrong, and determine what to do ahead of time.
•Key terms,
backup - a secondary system that can be used to replace the primary system if it fails.
fail operational - even when components fail, the system continues to operate within specifications.
fail safe - when the system fails, it does not cause damage, and possibly allows continued operation outside of specification.
failure tolerant - in the event that one system component fails, the entire system does not fail.
prime - a main system that is responsible for a task.
redundant - secondary systems that run in parallel with the prime, and will be able to hot swap if the prime fails.
time critical - the system has a certain response time before a failure will occur.
• essential components in these systems are,
monitoring systems - check for sanity or failure of systems. The purpose of these system is detection and reporting of failures.
emergency control functions - these functions will switch control when faults are detected. In some cases this might include human intervention, and be triggered automatically. These systems are intended to eliminate or reduce the effects of a failure.
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•safety criticality might be categorized as below, Criticality I - Catastrophic
-causes human disability/death
-causes loss of equipment Criticality II - Critical
-causes major human injury
-lose use of emergency system
-major damage to essential equipment Criticality III - Marginal
-minor human injury
-major damage to emergency system
-minor damage to essential equipment
•safing is a process whereby a system that has failed is shut down appropriately (i.e., actuators shut down, brakes applied, or whatever is appropriate to the situation).
•safing paths often include,
-braking equipment
-removal of power to actuators
-consideration of complete power failure
-operator control should be available, even when automated systems are in place
-multiple safing paths should be available
•the operator will be a good decision maker. Possible options include,
-safing procedures
-attempt to manually repair
-ignore
•techniques used are,
-checksums
-parity bits
-software interlocks
-watch dog timers
-sample calculations
•The role of various reliability programs can be related to a product life cycle.
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product |
engineering |
design |
prototype |
planning |
design |
analysis |
testing |
reliability |
design for |
reliability |
reliability |
management |
reliability |
analysis |
modelling |
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accident |
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reliability |
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reliability |
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modelling |
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8.1.2 The Theory of Module Reliability and Dependability
•Dependability is a combination of,
-reliability - the probability that a system operates through a given operation specification.
-availability - the probability that the system will be available at any instant required.
•Failure rate is the expected number of failures per unit time, and is shown with the constant (lambda), with the units of failures per hour.
•Basically,
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d |
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----R( t) |
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z( t) = –---------------dt |
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R( t) |
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1 – Q( t) |
where, |
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R( t) = |
the reliability, the chance that the system will fail betwee n[ t0, t] |
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z( t) = |
the failure rate |
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Q( t) = |
unreliability = 1 – R( t) |
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• The bathtub curve shows typical values for the failure rate.
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• The basic reliability equation can be rearranged, eventually leading to a compact expression,
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z( t) = – |
dtR( t) |
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R( t) = e |
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During the useful life of the product, we can approximate the failure rate as linear, as reflected by the relation below,
∫ z( t) dt ≈ λ t
R( t) = e–λ t = The ExponentialwLruFail
• MTTF (Mean Time To Failure) - this is the expected time before a failure.
∞
E[ X] = ∫ xf( x) dx
–∞
where,
X = a random variable
E[ X] = expected value
f( x) = a probability density function
MTTF = ∫∞ tf( t) dt
0
Given the probability density function, and using integration by parts we can find the relationship between the MTTF, and reliability.
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