Answer:
Reliability serves as a measure of quality of the product and service design. The quality associated with a product often increases with the dependability of the product customer experience.
Example: Patients expect the hospitals to have competent staff. People expect mobile networks to be congestion free etc.
One of the emerging disciplines is Design for Reliability (DFR) that refers to the process of designing reliability into products. This process encompasses several tools and practices and describes the order of their deployment that an organization needs to have in place in order to drive reliability into their products. Typically, the first step in the DFR process is to set the system’s reliability requirements. Reliability must be “designed in” to the system. During system design, the top-level reliability requirements are then allocated to subsystems by design engineers and reliability engineers working together.
Reliability design begins with the development of a model. Reliability models use block diagrams and fault trees to provide a graphical means of evaluating the relationships between different parts of the system. These models incorporate predictions based on parts-count failure rates taken from historical data. While the predictions are often not accurate in an absolute sense, they are valuable to assess relative differences in design alternatives.
One of the most important design techniques is redundancy. This means that if one part of the system fails, there is an alternate success path, such as a backup system.
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 very high and the actual value for the maximum reliability is usually dictated by technological or financial constraints. In deciding on a value to use for the maximum achievable reliability, the current state of the art of the component in question and other similar factors will have to be considered. In the end, a realistic estimation based on engineering judgment and experience will be necessary to assign a value to this input. One must note that the time associated with this maximum achievable reliability is the same as that of the overall system reliability goal. Almost any component can achieve a very high reliability value, provided the mission time is short enough.
