Good reliability requirements are going to drive our design decisions relating to the concept, the components, the materials, and other stuff. So, the moment to start defining reliability requirements is early in the design process. But, what makes a well-defined reliability requirement? There are five aspects it should cover: do you know what they are?
We'll describe what makes a good reliability requirement and examples of common (but not good) requirements.
One of the first steps in defining any good requirements is characterizing our product’s use and operating needs. This means knowing who is going to be using our product, in what way, and in what type of environment. This is one of the reasons why I promote an early usability engineering cycle. This can be done in tandem with a technical assessment of any new design. Both the usability engineering and technical assessment cycles can be part of the concept evaluation phase.
The 5 aspects of a good reliability requirement:
- measurement of time
- reliability at specific points in time
- a desired confidence level
- a definition of failure
- the operating and environmental conditions
Our example from this podcast: 99% reliability in system start-up to at least 300 rpm is required after 600 on-off cycles of operation with 95% confidence when operating in an environment with a temperature range of –15℃ to 40℃.
How does this example fit within our 5 aspects of a good reliability requirement?
- 99% reliability after 600 on-off cycles
- 95% confidence
- system start up less than 330 rpm
- temperature range of –15℃ to 40℃ (assuming no other environmental or user factors)
Which one of these aspects of reliability requirements have you struggled with in the past? Leave me a comment on this blog.
noMTBF.com A site dedicated to the eradication of the misuse of MTBF. There are posts and pages of examples explaining the pitfalls of this type of reliability requirement.
Reliability Requirements and Specifications is an article that steps through the mathematical implications of different types of reliability requirements and explains why they are good or not good. ["Reliability Requirements and Specifications." Reliability HotWire, iss. 80, Oct. 2007. Reliasoft Corporation. www.weibull.com/hotwire/issue80/relbasics80.htm. Accessed 29 Sep 2021.]