Page 57 - DMGT501_OPERATIONS_MANAGEMENT

Unit 2: Product and Service Design

Notes
Example: An automobile brake light might use two light bulbs. If one bulb fails, the
brake light still operates using the other bulb.
Redundancy significantly increases system reliability, and is often the only viable means of
doing so. However, redundancy is difficult and expensive, and is therefore limited to critical
parts of the system. Another design technique, physics of failure, relies on understanding the
physical processes of stress, strength and failure at a very detailed level. Then the material or
component can be re-designed to reduce the probability of failure. Another common design
technique is component de-rating: Selecting components whose tolerance significantly exceeds
the expected stress, as using a heavier gauge wire that exceeds the normal specification for the
expected electrical current.

Improving Reliability

There are two suggested approaches for improving the reliability of a system: fault avoidance
and fault tolerance. Fault avoidance is achieved by using high-quality and high-reliability
components and is usually less expensive than fault tolerance. Fault tolerance, on the other
hand, is achieved by redundancy. Redundancy can result in increased design complexity and
increased costs through additional weight, space, etc.

Before deciding whether to improve the reliability of a system by fault tolerance or fault
avoidance, a reliability assessment for each component in the system should be made. Once the
reliability values for the components have been quantified, an analysis can be performed in
order to determine if that system's reliability goal will be met. If it becomes apparent that the
system's reliability will not be adequate to meet the desired goal at the specified mission
duration, steps can be taken to determine the best way to improve the system's reliability so that
it will reach the desired target.
We need to answer some basic questions before getting down to improving the system's reliability.
How much does each component need to be improved for the system to meet its goal? How
feasible is it to improve the reliability of each component? Would it actually be more efficient
to slightly raise the reliability of two or three components rather than radically improving only
one?
In order to answer these questions, costs must be analyzed. Cost does not necessarily have to be
in monetary terms. It could be described in terms of non-monetary resources, such as time. By
associating cost values to the reliabilities of the system's components, one can find an optimum
design that will provide the required reliability at a minimum cost. There is always a cost
associated with changing a design due to change of vendors, use of higher-quality materials,
retooling costs, administrative fees, etc. The cost as a function of the reliability for each component
must be quantified before attempting to improve the reliability. Otherwise, the design changes
may result in a system that is needlessly expensive or over-designed. Developing the "cost of
reliability" relationship will give the engineer an understanding of which components to improve
and how to best concentrate the effort and allocate resources in doing so. The first step will be to
obtain a relationship between the cost of improvement and reliability.
The preferred approach would be to formulate the cost function from actual cost data. This can
be done from past experience. If a reliability growth program is in place, the costs associated
with each stage of improvement can also be quantified. Defining the different costs associated
with different vendors or different component models is also useful in formulating a model of
component cost as a function of reliability.

For the purposes of reliability optimization, we also need to define a limiting reliability that a
component will approach, but not reach. The costs near the maximum achievable reliability are

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