Reliability Engineering

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Reliability Engineering

• Richard C. Fries, PE, CRE
• Corporate Manager, Reliability Engineering
• Baxter Healthcare
• Round Lake, Illinois

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Definition of Reliability

• The probability, at a desired confidence level,
• that a device will perform a specified function,
• without failure,
• under stated conditions,
• for a specified period of time

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Customers Definition of Reliability

• A reliable product
• One that does what the customer wants,
• when the customer wants to do it

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Reliability Basics

• Reliability cannot be tested into a product
• It must be designed and manufactured into it
• Testing only indicates how much reliability
• is in the product

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Purpose of the Reliability Group

• Determine the weaknesses in a design
• AND correct them
• before the device goes to the field

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Areas Covered by Reliability

• Electrical
• Mechanical
• Software
• System

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Electrical Reliability
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Mechanical Reliability
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Theoretical Software Reliability
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Practical Software Reliability
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System Reliability
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Set the Reliability Goal

• Based on similar equipment
• Used as the basis for a reliability budget
• Listed as Mean Time Between Failures (MTBF) in
  hours or cycles
• MTBF the time at which 63 of the units in the
  field will have failed
• Minimum goal is ten years with a 98 reliability

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Parts Count Prediction

• Uses MIL-HDBK-217
• Indicates whether the design approximates the
  reliability goal
• Indicates those areas of the design with high
  failure rates

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Chemical Compatibility

• Test plastics with typically used chemical agents
  (alcohol, anesthetic agents, cleaning agents)
• Cleaning agents are the worst

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Force Puller
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Component Testing

• Cycle/life testing of individual components
• Comparison of multiple vendors of components
• Determine applicability for the intended use

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Philosophy of Testing

• Test to have the units pass
• Test with the addition of stresses to check the
  margins of functionality

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Types of Tests

• Time terminated, failed parts replaced
• Time terminated, no replacement
• Failure terminated, failed parts replaced
• Failure terminated, no replacement
• Test until first failure
• Test until all samples fail

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Determining Sample Size

• Uses Chi-Square table
• SS Chi-square Value(MTBF goal)/2
• Chi-square value includes confidence level and
  degrees of freedom 2f2
• Component testing 90 confidence level
• Life testing 95 confidence level

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Sample Calculation

• Want to test valves to be used for 2,000,000
  cycles per year with a 10 failure rate after 10
  years
• Reliability e(-t/MTBF)
• MTBF -t/ln Reliability
• -20,000,000/ln 0.90
• 389,914,514 cycles

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Sample Calculation

• MTBF 389,914,514 cycles
• Number of Samples Number of Cyles
• 10 89,777,817
• 50 17,955,563
• 100 8,977,782

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Component Test Setup
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Component Test Setup
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Component Test Setup
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Calculating Sample MTBF

• MTBF ( of samples)(length of test)
• of failures

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Calculating MTBF Where No Failures Occur

• A sample MTBF cannot be calculated
• A lower one-sided confidence limit is calculated
  and the MTBF stated to be greater than that
  number
• One-sided limit 2(units)(test time)
• Chi square value for
  the
• confidence limit and 2
  
• degrees of freedom

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Sample Calculation for a No Failure Test

• 10 valves are tested for 10,000 cycles with no
  failures. Calculate using a 90 confidence
  level.
• One-sided limit 2(10)(10,000)
• 4.605
• 43,431 cycles
• MTBF gt 43,431 cycles

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HALT

• Acronym for Highly Accelerated Life Testing
• Used to find the weak links in the design and
  fabrication process
• Usually performed during the design phase

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HALT Testing

• Possible stresses that can be applied
• random vibration
• rapid temperature transitions
• voltage margining
• frequency margining
• The product is stressed far beyond its
  specifications
• The test can be set up to find the destruct limits

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HALT Chamber
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Goal of HALT Testing

• Overstress the product
• Quickly induce failures
• By applying the stresses in a controlled, stepped
  fashion, while continuing monitoring for
  failures, the testing results in the exposure of
  the weakest points in the design
• This test, if successful, will expose weak points
  in the design

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Environmental Testing

• Operating temperature/humidity
• Storage temperature/humidity
• EMC
• Surges/transients
• Brown-outs
• Electrocautery
• Cell phones
• ESD
• Altitude

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Environmental Testing

• Autoclave
• Shock
• Vibration
• Shipping
• Tip testing
• Threshold testing

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Temperature Chamber
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Walk-In Temperature Chamber
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Autoclave Testing
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Customer Misuse

• Excess weight on tabletop
• Fluid spillage
• Cross connection of wires
• Pulling unit by non-pulling parts
• Wrong order of pressing keys
• Knowing how to operate the unit without reading
  the manual

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Making a Design Foolproof

• The biggest mistake engineers make
• when trying to make a design
• completely foolproof
• is underestimating the ingenuity
• of complete fools

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Failure Analysis

• Failure device does not operate according to
  its specification
• Determine root cause of the failure
• Suggest methods to address the failure

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Prototype Front Panel
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Plastic Structure
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Plastic Structure
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Autoclave Testing
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Manifold Port
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Prototype Port
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Life Testing

• Operate the device in its typical environment and
  application
• Use appropriate on/off cycles
• Can be used to verify the reliability goal or a
  specific period of time, such as the warranty
  period

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Tracking Reliability Growth in the Field

• Collect manufacturing data on how many units were
  manufactured by month
• Collect field failure data, by month
• Develop a reliability growth chart

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Reliability Growth Example
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Reliability Growth Example
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Reliability Growth Example
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The Reliability Group

• You make it,
• Well break it