Reliability Flashcards
Describe a typical variation of hazard rate
‘bath tub’ curve with respect to time
Why is the early age phase characterised by a high hazard rate
- irregularities in manufacturing process
- craftsmanship
- potentially prototype
How to eliminate early age phase
equipment is subjected to ‘Reliability Shake-down Testing’ RST
Describe typical RST cycle
- soaking in chamber at 70 deg C
- rapidly cooling to -55 deg C in 20 mins and then soaking
- subjected to vibrations
typically done 20 times before delivery
Describe deterministic approach to reliability
- interested in the physical process leading to failure - failure mechanism
Give equation for MTTF
MTTF = To . e^(-Ea/nkT)
Ea/n = effective activation energy k = Boltzmann's constant T = absolute temp
what does MTTF stand for
Mean time to failure
Name three failure mechanisms in ICs
- corrosion
- electromigration
- purple plague
Describe two types of corrosion
- Anodic corrosion - is independent of temp.
2. Cathodic corrosion - depends on temp and has effective activation of around 0.5eV
Describe electromigation
corrosion depending on temp. :
0.5eV < Ea/n < 0.8eV
Describe purple plague
corrosion depending on temp:
Ea/n = 1eV
Describe statistical reliability
expected reliability of a future system is made on basis of:
- information of previously produced systems
- results of artificially accelerated reliability measurements of current components
What is the failure distribution
The probability that the system fails at T before or at time t
F(t) = P(T
What is the reliability function
The probability of survival for time interval [0,t]
R(t) = P(T>t) = 1 - F(t)
What is the equation for failure probability density
f(t) = dF(t)/dt = -dR(t)/dt
What is the equation relating MTTF with the reliability function
MTTF = ∫(inf / 0) [R(t)] dt
What is the hazard rate
the conditional probability of the system failing in the time interval [t,t+dt] on the condition it is still functioning on the t:
z(t) = f(t) / R(t)
∫(t / 0) [z(t)] dt = -ln(R(t) / R(0)
When is negative-exponential distribution adopted
- components fail independently of one another
- failures occur at random moments and with a constant hazard rate, z(t) = λ
Describe reliability of series systems
- all components must function for the system to function properly
Describe reliability of parallel systems
- only requires one component to be operational to perform its function
- allows for redundancy
Describe reliability of m-out-of-n systems
- neither series nor parallel
- as title describes, at least m out of n component must function
Describe failure detection and monitoring systems
- provided monitors have high reliability, n-1 failures can be tolerated in n redundant channels
- in high reliability requirement systems, monitoring is usually undertaken by dissimilar systems
- if too complex a system, monitoring might not be possible
Describe non-adaptive majority voting system
and m-out-of-n system where the majority of components must function for system to function
m = n/2 + 1 (even) m = (n+1)/2 (odd)
Describe adaptive majority voting
for n>3, each time a voter detects a failed channel, it is disconnected and is no longer part of the set of valid alternatives
Fly-By-Wire
what signals are transmitted to/from the flight control computer(s)
- signals from the pilot’s stick sensor
- signals from aircraft motion sensors via. gyros
- signals from the air data system via. airspeed & altitude
- control demand to the flight control surfaces servo actuation systems
Fly-By-Wire
Tasks carried out by the flight control computers
- computation of flight control surface angles
- monitoring
- failure detection
- system reconfiguration in event of failure
- built in test
What are the advantages of using fly-by-wire
- elimination of mechanical control runs
- consistent handling over a wide flight envelope
- automatic stabilisation
- carefree manoeuvring
- automatic integration of additional controls
- ability to exploit aerodynamically unstable aircraft
What are the disadvantages of using fly-by-wire
if computer fails then the whole aircraft fails
Describe a well established redundant configuration
consists of 4 independent channels/quadruplex system:
- failures detected by cross comparison and majority voting
- failed channels are disconnected
- system can tolerate two failures
What are common mode failures and give examples
a failure which can affect all channels simultaneously. e.g.
- lightning strike
- electro-magnetic interference
- fire, explosion
- design error
- incorrect maintenance
Why are dissimilar systems used and give some different techniques of use:
Dissimilar systems are there to take over in the event of a common mode failure.
- can use two different processors written in a different language
- use back-up analogue system
