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.

  

• 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)

  

• 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.