2. Introduction to Redundancy Techniques

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2. Introduction to Redundancy Techniques

• Redundancy
• Implies the use of hardware, software,
  information, or time beyond what is needed for
  normal system operation.
• Has a strong impact on a system in the areas of
  performance, size, weight, power consumption, and
  reliability.

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2. Introduction to Redundancy Techniques

• 2.1 Hardware Redundancy

• Passive
• ? Based on the concept of fault masking to hide
  the occurrence of faults and prevent the faults
  from resulting in errors (developed around the
  concept of majority voting)
• ? Do not provide for faults detection, but
  simply mask them
• Active, or Dynamic
• ? Attempts to achieve fault tolerance by means
  of fault detection, fault location,
  reconfiguration, and recovery (property of fault
  masking is not obtained there is no attempt to
  prevent faults from producing errors within the
  system)
• ? More suitable for applications where
  temporary, erroneous results are acceptable, as
  long as the system reconfigures and regains its
  operational status in a satisfactory length of
  time
• Hybrid
• ? Combines the attractive features of both the
  Active and the Passive approaches

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2. Introduction to Redundancy Techniques

• 2.1 Hardware Redundancy

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2. Introduction to Redundancy Techniques

• 2.1 Hardware Redundancy

• Voting at Several Levels within
• N-Modular Redundancy (NMR) Systems

• 3 independent temperature sensors perform a vote
  on the 3 sensor values. Next, calculate the
  amount of heat/cooling by means of 3 separate
  modules, and then vote on the calculations to
  determine a result.
• X
• 3 independent sensors sample the temperature,
  perform the calculations, and then provide a
  single vote on the final result.
• Difference between the two approaches ? fault
  containment voting at the sensors will mask and
  contain the effects of an eventual sensor fault.

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2. Introduction to Redundancy Techniques

• 2.1 Hardware Redundancy

• HW Voting x SW Voting ?
• The availability of processor to perform the
  voting
• The speed at which voting must be performed
• The criticality of space, power, and weight
  limitations
• The of different voters that must be provided
• The flexibility required of the voter with
  respect to future changes in the system

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2. Introduction to Redundancy Techniques

• 2.1 Hardware Redundancy

• In practical applications of voting, 3 results in
  a TMR system may not completely agree, even in a
  fault-free environment
• e.g., A/D converters in sensors may produce
  quantities that disagree in the least-significant
  bits. This disagreement can propagate into larger
  discrepancies after computation, which can
  significantly affect the voting process.

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2. Introduction to Redundancy Techniques

• 2.1 Hardware Redundancy

• Solution ? Mid-Value select Technique
• A TMR system selects the value that lies in the
  middle of the others