Reliability October 26, 2004

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ReliabilityOctober 26, 2004
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Today

• DFDC (Design for a Developing Country)
• HW November 2
• detailed design
• Parts list
• Trade-off
• Midterm November 4
• Factory Visit November 16th

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Midterm

• Presentation Purpose- a midcourse correction
• less than 15 minutes with 5 minutes discussion
• Approx. 7 power point slides- all should
  participate in presentation
• Show what you have done
• Show what you are going to do
• Discuss issues, barriers and plans for overcoming
  (procedural, team, subject matter, etc.
• Scored on originality, candor, thoughtfullness,
  etc. not on total amount accomplished
• Schedule today from 100 to 400 (speaker at 400
  PM)

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Reliability The probability that no (system)
failure will occur in a given time intervalA
reliable system is one that meets the
specifications Do you accept this?
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What do Reliability Engineers Do?

• Implement Reliability Engineering Programs across
  all functions
• Engineering
• Research
• manufacturing
• Testing
• Packaging
• field service

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Reliability as a Process module
INPUT
Reliability Assurance Module

• Reliability Goals
• Schedule time
• Budget Dollars
• Test Units
• Design Data

Product Assurance

• Internal Methods
• Design Rules
• Components Testing
• Subsystem Testing
• Architectural Strategy
• Life Testing
• Prototype testing
• Field Testing
• Reliability Predictions (models)

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Early product failure

• Strongest effect on customer satisfaction
• A field day for competitors
• The most expensive to repair
• Why?
• Rings through the entire production system
• High volume
• Long C/T (cycle time)
• Examples from GE (but problem not confined to
  GE!)
• GE Variable Power module for House Air
  Conditioning
• GE Refrigerators
• GE Cellular

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Early Product Failure

• Can be catastrophic for human life
• Challenger, Columbia
• Titanic
• DC 10
• Auto design
• Aircraft Engine
• Military equipment

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Reliability as a function of System
ComplexityWhy computers made of tubes (or
discrete transistors) cannot be made to work
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Three Classifications of Reliability Failure

• Old Remedy- Repair mentality
• Burn-in
• Maintenance
• In service testing

• Type
• Early (infant mortality)
• Wearout (physical degradation)
• Chance (overstress)

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Bathtub Curve
Infant Mortality
Useful life No memory No improvement No
wear-out Random causes
Wear out
Failure Rate /million hours
Time
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Reliability
Prob of dying in the next year (deaths/ 1000)
Age
From the Statistical Bulletin 79, no 1, Jan-Mar
1998
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Early failure causes or infant mortality (Occur
at the beginning of life and then disappear)

• Manufacturing Escapes
• workmanship/handling
• process control
• materials
• contamination
• Improper installation

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Chance Failures (Occur throughout the life a
product at a constant rate)

• Insufficient safety factors in design
• Higher than expected random loads
• Human errors
• Misapplication
• Developing world concerns

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Wear-out(Occur late in life and increase with
age)

• Aging
• degradation in strength
• Materials Fatigue
• Creep
• Corrosion
• Poor maintenance
• Developing World Concerns

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Failure Types

• Catastrophic
• Degradation
• Drift
• Intermittent

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Failure Effects(What customer experiences)

• Noise
• Erratic operation
• Inoperability
• Instability
• Intermittent operation
• Impaired Control
• Impaired operation
• Roughness
• Excessive effort requirements
• Unpleasant or unusual odor
• Poor appearance

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Failure Modes

• Cracking
• Deformation
• Wear
• Corrosion
• Loosening
• Leaking
• Sticking
• Electrical shorts
• Electrical opens
• Oxidation
• Vibration
• Fracturing

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Reliability Remedies

• Early
• Wearout
• Chance

• Quality manufacture/Robust Design
• Physically-based models, preventative
  maintenance, Robust design (FMEA)
• Tight customer linkages, testing, HAST

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Reliabilitysemi-empirical formulae
Early failure
pdf
k
Chance Failure
constant failure rate
mMTBF
Wear out
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Failures Vs time as a function of Stress
High Stress
Medium Stress
Low Stress
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Highly Accelerated Stress Testing

• Test to Failure
• Fix Failed component
• Continue to Test
• Appropriate for developing world?

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Duane Plot Reinertson p 237
x
Actual Reliability
x
x
Log Failures per 100 hours
x
x
Required Reliability at Introduction
x
x
x
x
x
x
x
x
x
Predicted
x
Log Cumulative Operating Hours
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Integration into the Product Development Process
FMEA- Failure Modes and Effects Analysis
Customer Requirements
Baseline data from Previous Products
Brainstorm potential failures
Summarize results (FMEA)
Use at Design Reviews
Update FMEA
Baseline data from Previous Products
Feed results to Risk Assessment Process
Probabilities developed through analysis
Develop Failure Compensation Provisions
Test Activity Uncovers new Failure modes
Failure prob- through test/field data
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Risk Assessment process

• Assess risk
• Program Risk
• Market Risk
• Technology Risk
• Reliability Risk
• Systems Integration Risk
• Devise mitigation Strategy
• Re-assess

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Fault Tree analysis
Seal Regulator Valve Fails
or
Valve Fails Open when commanded closed
Fails to meet response time
Excessive leakage
Regulates High
Regulates Low
Fails closed when commanded open
Excessive hysteresis
1
5
4
3
2
or
or
Excessive port leakage
Excessive case leakage
Fails to meet response time
Fails to meet response time
Next Page
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Fault Tree analysis (cont)
Valve Fails Open when commanded closed
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Valve Fails Open when commanded closed
or
Mechanical Failure Selenoid
Electrical Failure of Selenoid
Transient electro mechanical force
or
or
Open Circuit
Coil short Insulation
or
wear
Material selection
Solder Joint Failure
Wire Broken
seals
Material selection
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FMEA
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FMEA Root Cause Analysis
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Fault Tree Analysis- example
Example A solar cell driven LED
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Reliability Management

• Redundancy
• Examples
• Computers
• memory chips?
• Aircraft
• What are the problems with this approach
• 1. Design inelegance
• expensive
• heavy
• slow
• complex
• 2. Sub optimization
• Can take the eye off the ball of improving
  component and system reliability by reducing
  defects
• Where should the redundancy be allocated
• system
• subsystem
• board
• chip
• device

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Other best practices

• Fewer Components
• Small Batch Size (why)
• Better material selection
• Parallel Testing
• Starting Earlier
• Module to systems test allocation
• Predictive (Duane) testing
• Look for past experience
• emphasize re-use
• over-design
• e.g. power modules
• Best Understand the physics of the failure and
  model
• e.g. Crack propagation in airframes or nuclear
  reactors

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Other suggestions?