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Redundancy (engineering)
 
 
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In engineering
Engineering

Engineering is the discipline and profession of applying Technology and science knowledge and utilizing natural laws and physical resources in order to design and implement materials, structures, machines, devices, systems, and process that safely realize a desired objective and meet specified criteria....
, redundancy is the duplication of critical components of a system with the intention of increasing reliability of the system
System

System is a set of interacting or interdependent entities, real or abstract, forming an integrated whole.The concept of an "integrated whole" can also be stated in terms of a system embodying a set of relationships which are differentiated from relationships of the set to other elements, and from relationships between an element of the se...
, usually in the case of a backup or fail-safe
Fail-safe

Fail-safe or fail-secure describes a device or feature which, in the event of Failure mode, responds in a way that will cause no harm or at least a minimum of harm to other devices or danger to personnel....
.

In many safety-critical systems, such as fly-by-wire and hydraulic systems in aircraft
Aircraft

An aircraft is a vehicle which is able to flight by being supported by the air, or in general, the atmosphere, of a planet. Examples include balloons, airplanes and helicopters....
, some parts of the control system may be triplicated. An error in one component may then be out-voted by the other two. In a triply redundant system, the system has three sub components, all three of which must fail before the system fails.






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In engineering
Engineering

Engineering is the discipline and profession of applying Technology and science knowledge and utilizing natural laws and physical resources in order to design and implement materials, structures, machines, devices, systems, and process that safely realize a desired objective and meet specified criteria....
, redundancy is the duplication of critical components of a system with the intention of increasing reliability of the system
System

System is a set of interacting or interdependent entities, real or abstract, forming an integrated whole.The concept of an "integrated whole" can also be stated in terms of a system embodying a set of relationships which are differentiated from relationships of the set to other elements, and from relationships between an element of the se...
, usually in the case of a backup or fail-safe
Fail-safe

Fail-safe or fail-secure describes a device or feature which, in the event of Failure mode, responds in a way that will cause no harm or at least a minimum of harm to other devices or danger to personnel....
.

In many safety-critical systems, such as fly-by-wire and hydraulic systems in aircraft
Aircraft

An aircraft is a vehicle which is able to flight by being supported by the air, or in general, the atmosphere, of a planet. Examples include balloons, airplanes and helicopters....
, some parts of the control system may be triplicated. An error in one component may then be out-voted by the other two. In a triply redundant system, the system has three sub components, all three of which must fail before the system fails. Since each one rarely fails, and the sub components are expected to fail independently, the probability of all three failing is calculated to be extremely small. Redundancy may also be known by the terms "majority voting systems" or "voting logic".

Forms of redundancy

There are four major forms of redundancy, these are:
  • Hardware redundancy, such as DMR
    Dual modular redundant

    A machine which is Dual Modular Redundant has duplicated elements which work in parallel to provide one form of redundancy. A typical example is a complex computer system which has duplicated nodes, so that should one node fail, another is ready to carry on its work....
     and TMR
    Triple modular redundancy

    In computing, triple modular redundancy is a fault tolerant form of N-modular redundancy, in which three systems perform a process and that result is processed by a voting system to produce a single output....
  • Information redundancy, such as Error detection and correction
    Error detection and correction

    In mathematics, computer science, telecommunication, and information theory, error detection and correction has great practical importance in maintaining data integrity across noisy channels and less-than-reliable storage media....
     methods
  • Time redundancy, including transient fault detection methods such as Alternate Logic
  • Software redundancy such as N-version programming
    N-version programming

    N-version programming , also known as multiversion programming, is a method or process in software engineering where multiple functionally equivalent programs are independently generated from the same initial specifications ....


Calculating the Probability of System Failure

Each duplicate component added to the system decreases the probability of system failure according to the formula:

P =

where:
  • n - number of components
  • - probability of component i failing
  • P - the probability of all components failing (system failure)


This formula assumes independence of failure events (ie. any single component is sufficient to keep the system running).

See also

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  • Common mode failure
    Common mode failure

    A common mode failure occurs when events are not statistics. That is, one event causes multiple systems to fail.An example is when all of the pumps for a fire sprinkler system are located in one room....
  • Data redundancy
    Data redundancy

    Data redundancy sometime refers to in computer data storage, is a property of some disk arrays which provides Fault tolerant design, so that all or part of the data stored in the array can be recovered in the case of disk failure....
  • Double switching
    Double switching

    Double switching is the practice of using a multipole switch to close or open both the positive and negative sides of a Direct current electrical circuit, or both the hot and neutral sides of an Alternating current circuit....
  • Fault tolerant design
  • Radiation hardening
    Radiation hardening

    Radiation hardening is a method of designing and testing electronic components and systems to make them resistant to damage or malfunctions caused by high-energy subatomic particles and electromagnetic radiation, such as would be encountered in outer space, high-altitude flight, around nuclear reactors, or during warfare....

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  • Reliability engineering
    Reliability engineering

    Reliability engineering is an engineering field, that deals with the study of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time....
  • Reliability theory of aging and longevity
    Reliability theory of aging and longevity

    Reliability theory of aging and longevity is a scientific approach aimed to gain theoretical insights into mechanisms of biological aging and species survival patterns by applying a general theory of systems failure, known as reliability theory....
  • Safety engineering
    Safety engineering

    Safety engineering is an applied science strongly related to systems engineering and the subset System Safety Engineering. Safety engineering assures that a life-critical system behaves as needed even when pieces fail....
  • Self-healing ring
    Self-healing ring

    A self-healing ring, or SHR, is a common configuration in telecommunications Transmission systems. SDH, SONET and Wavelength Division Multiplexing systems are often configured in self-healing rings....
  • MTBF


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