which one is more subject to common cause failure: analog or software based system?



  • If a task can be done by both an analog system and a system based on a general purpose computer, which one is preferred regarding the common cause failure problem and why? For example, suppose the system is an aeroplane that you want to control its actuators. one way is to use analog modules near the actuator and the other way is to send the actuator signals to a computer and calculate the control signal there. If we have redundancies in both cases, which one is better considering common mode failures?



  • It depends

    The longer answer:

    There is no general answer to this question. Whether a mechanical solution or a software solution is more likely to fail depends on many factors, including but not limited to:

    • the nature of the problem - some problems are better solved by mechanical means where others are better solved by software.
    • the quality of the solution - a well-engineered and built mechanical solution will usually do a better job than a poorly designed and built software solution, and vice versa
    • the requirements of the problem - a problem requiring an always-available solution is potentially better served by a mechanical solution since software requires a power source that may not be available
    • the level of precision required - software precision levels can be iffy when dealing with numerical solutions due to the way software represent numerical operations. Mechanical solutions can be be problematic depending on their level of precision and margins of error.
    • the degree of repeatability required - software will usually repeat a series of actions with no deviation any number of times. Mechanical solutions may become less accurate over time with wear. Margins of error will determine which solution works better for the problem.
    • the problem domain - some domains are more suited to mechanical solutions, others are more suited to programmatic solutions.
    • the budget - if the budget lacks flexibility, you will be constrained to whichever solutions fit within budget.
    • type of precision needed - if the problem involves sound, analog solutions are better for accuracy because software solutions are unable to fully reproduce sound. Digital solutions invariably involve some loss of fidelity, but may be acceptable, depending on the requirements.
    • conversion factors - digital time is often an approximation, since analog time is based on solar time and does not resolve into simple easily divisible groupings (no computer with a binary basis can accurately represent 1/3 - they can only approximate to greater or lesser degrees of precision)
    • Extra, non-standard usage - you can't use a digital watch as an emergency compass. You can use an analog watch that way if it's kept accurate and you know the trick.

    If someone asks a question like this in an interview, your best option is to explain that it depends on circumstances, and give some examples where software or mechanical choices are better.



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