"A man's got to know his
limitations." This is one of the more memorable lines delivered by Clint
Eastwood in the movie "Magnum Force" and it possesses great wisdom. But how
does this relate to engineering system reliability?
What do we mean when we say that
a person is reliable? Is it possible to say that a person is reliable all the
time or just sometimes, in all
circumstances or in just some circumstances? The same questions need to be applied to an engineering system design
because reliability cannot be an after-thought.
As we become more dependent on
complex mechatronic systems, it is not sufficient to understand just how they
work; we must also understand how they fail. Fault-tolerant system design, not
just fault-tolerant component or subsystem design, has become paramount.
Reliability is the probability that an item performs a required function under
stated conditions for a stated period of time. So an engineer needs to define
the functions a system must perform, the boundary conditions under which the
system will operate and the time duration during which reliability is required.
To better understand
reliability, I spoke with Tim Kerrigan, fluid power consulting engineer at
Milwaukee School of Engineering's Fluid Power Institute, where he works to
ensure industrial and government systems are designed for reliability.
A physics-of-failure approach
to reliability is consistent with the model-based approach of modern
mechatronic system design. It uses modeling and analysis to design reliability
into a system, perform reliability assessments and focus reliability tests
where they will be most effective. The approach involves understanding and
modeling the potential failure mechanisms (e.g., fatigue, wear, temperature),
the failure sites and the failure modes (the activation of the failure mechanisms).
The failure modes of a
mechatronic system include those of mechanical, electrical, computer and
control subsystems, i.e., hardware and software failures. A physics-of-failure
approach can improve reliability, reduce the time to field systems, reduce
testing and costs, and increase customer satisfaction.
As mechatronic systems become
more complex, the interactions among the subsystems - mechanical, electrical,
computer and control - become more difficult to manage and the overall system
reliability is impacted by this integration. Therefore, an assessment of
overall system reliability must have an adequate margin for safety. An useful
analogy here is the feedback control system. It provides great benefits, but
feedback control systems can become unstable if there is an imbalance between
strength of corrective action (gain) and system dynamic lags (phase lags).
Model uncertainty is quantified by assuming that either gain changes or phase
changes occur and the tolerances of gain or phase uncertainty are the stability
margins, gain margin and phase margin. Real systems must have adequate
stability margins. Real systems must also have adequate reliability margins.
Mechatronics can enhance the
reliability and fault-tolerance of a system with prognostics, diagnostics and
built-in test capabilities. The additional sensors and control elements must be
very reliable and do add additional cost. But the long-term cost of
unreliability is huge compared to the initial design cost of reliability. In
addition, designing for reliability enhances energy efficiency and
sustainability. Reliability and fault-tolerance is a competitive advantage in
the commercial market and an absolute requirement in the health care, military
and transportation sectors.
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