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#215 Design Image Training - 31: Designing Reliable Automation Device

Category : Hints on designing
October30, 2015

An automation device is comprised of the mechanism unit and drive unit with many components assembled along with other units including sensing/control units. This section explains the reliable design of an automation device that has a complex structure as described above.

  • "Reliability" is a technical term defined in the JIS glossary. It is defined as "a characteristic of a system, device, or component thereof expressed as a probability that it will perform its required function under the given conditions within the specified operating periods".
  • "Reliability" is expressed by a multiplication formula (series model) of the reliability of all the assembly components comprising the automation device.

    Reliability of the automation device

  • Since the reliability decreases by the number of components multiplied, it is more effective to decrease the number of components than to increase the reliability of individual component.

    Examples:
    * The reliability of the automation device with 10 components (90% reliability) assembled is R = 0.910 = 0.35, which is extremely low.
    * Improving the reliability of 10 components to 93%: R = 0.9310 = 0.48
    * Decreasing the number of components to eight by eliminating two components: R = 0.98 = 0.43
    * Even if the reliability of each component is 99.9%, the overall reliability will be calculated as R = 0.9991000 = 0.368 for 1,000 pieces.
    * For the system design requiring 100% reliability, such as a space satellite, the parallel model will be adopted instead of the series model for its redundancy.

  • The table below shows the major failure factors for the assembly mechanism unit and sensing/control units (◎ indicates an extremely high ratio)

    Figure

  • As you can see from the above table, minimizing the number of components, especially the sensor components, is the important factor in designing a highly-reliable automation device.
  • It is also important to adopt a design that is highly reliable and easy to maintain ("good accessibility" in other words) during the operating period.

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