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What is "redundancy," exactly? As far as I know a system/component is redundant if critical components are being duplicated to ensure a higher safety level while reducing availability.

Lets take a look at a sensor. If it is a redundant sensor it has 2 channels, right? I have seen that some manufacturers say that their components have triple redundancy with 3 channels. How is this possible? As far as I get this triple redundancy means that there should be 4 channels working independently.

Example here.

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Wikipedia says

In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the form of a backup or fail-safe.

In many safety-critical systems, such as fly-by-wire and hydraulic systems in aircraft, some parts of the control system may be triplicated, which is formally termed triple modular redundancy (TMR).

and

In reliability engineering, dual modular redundancy (DMR) is when components of a system are duplicated, providing redundancy in case one should fail.

I think your confusion lies within the formal terms. If you think of a redudant system as a DMR system the term TMR makes more sense and takes away your confusion about the sensor.

So if you add redundancy to the system (there is no such thing "single" redundancy) you can choose a DMR system or a TMR system. Usually people would think of a DMR system unless specified otherwise. So to keep consistent with your terms, what you would define as single redundant is the same as a DMR system. And then, if you have 3 modules in total, you speak of a TMR system. It all refers to the total number of modules, not the number of added modules.

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  • $\begingroup$ Thanks for your reply. So if triplicated means triple modular redundancy and redundancy means the duplication of components the Term "Double Redundancy" does not exist? Right? Or what does it mean than? I dont get the difference between (Single) Redundancy, DMR and TMR. $\endgroup$ Mar 22, 2016 at 12:52
  • $\begingroup$ I couldn't edit my comment and just saw your edit. I tried to be more specific in the end. Hope this helps. $\endgroup$
    – idkfa
    Mar 22, 2016 at 15:48
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OK, in short, redundancy means duplication of critical elements, where the back-up element kicks in to take over in the event of system failure.

But where is the logic of calling such a back-up a redundancy? Redundancy, in its "everyday" meaning, has negative connotations. Yet here its connotations are anything but negative. So where is the logic?

OK, so perhaps the element that conks out becomes redundant, hence redundant. But isn't that stretching it a bit too far?

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