Reliability

Question 1

Describe a typical variation of hazard rate

Answer 1

‘bath tub’ curve with respect to time

Question 2

Why is the early age phase characterised by a high hazard rate

Answer 2

• irregularities in manufacturing process
• craftsmanship
• potentially prototype

Question 3

How to eliminate early age phase

Answer 3

equipment is subjected to ‘Reliability Shake-down Testing’ RST

Question 4

Describe typical RST cycle

Answer 4

• 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

Question 5

Describe deterministic approach to reliability

Answer 5

• interested in the physical process leading to failure - failure mechanism

Question 6

Give equation for MTTF

Answer 6

MTTF = To . e^(-Ea/nkT)

Ea/n = effective activation energy
k = Boltzmann's constant
T = absolute temp

Question 7

what does MTTF stand for

Answer 7

Mean time to failure

Question 8

Name three failure mechanisms in ICs

Answer 8

• corrosion
• electromigration
• purple plague

Question 9

Describe two types of corrosion

Answer 9

1. Anodic corrosion - is independent of temp.

2. Cathodic corrosion - depends on temp and has effective activation of around 0.5eV

Question 10

Describe electromigation

Answer 10

corrosion depending on temp. :

0.5eV < Ea/n < 0.8eV

Question 11

Describe purple plague

Answer 11

corrosion depending on temp:

Ea/n = 1eV

Question 12

Describe statistical reliability

Answer 12

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

Question 13

What is the failure distribution

Answer 13

The probability that the system fails at T before or at time t

F(t) = P(T

Question 14

What is the reliability function

Answer 14

The probability of survival for time interval [0,t]

R(t) = P(T>t) = 1 - F(t)

Question 15

What is the equation for failure probability density

Answer 15

f(t) = dF(t)/dt = -dR(t)/dt

Question 16

What is the equation relating MTTF with the reliability function

Answer 16

MTTF = ∫(inf / 0) [R(t)] dt

Question 17

What is the hazard rate

Answer 17

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)

Question 18

When is negative-exponential distribution adopted

Answer 18

• components fail independently of one another

- failures occur at random moments and with a constant hazard rate, z(t) = λ

Question 19

Describe reliability of series systems

Answer 19

• all components must function for the system to function properly

Question 20

Describe reliability of parallel systems

Answer 20

• only requires one component to be operational to perform its function
• allows for redundancy

Question 21

Describe reliability of m-out-of-n systems

Answer 21

• neither series nor parallel

- as title describes, at least m out of n component must function

Question 22

Describe failure detection and monitoring systems

Answer 22

• 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

Question 23

Describe non-adaptive majority voting system

Answer 23

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)

Question 24

Describe adaptive majority voting

Answer 24

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

Question 25

Fly-By-Wire

what signals are transmitted to/from the flight control computer(s)

Answer 25

• 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

Question 26

Fly-By-Wire

Tasks carried out by the flight control computers

Answer 26

• computation of flight control surface angles
• monitoring
• failure detection
• system reconfiguration in event of failure
• built in test

Question 27

What are the advantages of using fly-by-wire

Answer 27

• 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

Question 28

What are the disadvantages of using fly-by-wire

Answer 28

if computer fails then the whole aircraft fails

Question 29

Describe a well established redundant configuration

Answer 29

consists of 4 independent channels/quadruplex system:

• failures detected by cross comparison and majority voting
• failed channels are disconnected
• system can tolerate two failures

Question 30

What are common mode failures and give examples

Answer 30

a failure which can affect all channels simultaneously. e.g.

• lightning strike
• electro-magnetic interference
• fire, explosion
• design error
• incorrect maintenance

Question 31

Why are dissimilar systems used and give some different techniques of use:

Answer 31

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