1. Field of the Invention
This invention relates to the field of software, and more particularly to software based simulation tools that use process simulation techniques to assess the performance reliability of products that are made up of multiple components, each component having a reliability distribution.
2. Description of the Related Art
In general, products that have multiple components fail in their intended purpose when a component necessary for the product to perform the function for that purpose fails. By way of illustration, when an automobile, which has many components, has the mission of transporting a commuter to and from work suffers a failure of an electrical component that is necessary for the engine to operate, the automobile is stalled and unable to fulfill its mission. The auto might also serve other purposes, for example transporting people at night to social events. In that case, the failure or burnout of the headlamps of the car makes it unsafe to use at night, and so it cannot fulfill a subsidiary, but nonetheless important, intended purpose.
It would be useful to the user of a product to know, or to have some idea, when a component will fail that would create an inconvenience or that would impair the usefulness of the product. In markets where many manufacturers compete to make sell a consumer product, the manufacturer conducts tests to determine component reliability and sets specifications for manufacture that often reflect some reliability goals. Based on an understanding of the reliability of components, gathered from actual failures of components in use, or from test data, or both, a manufacturer can develop a “limited warranty” program. Such a program offers some compensation (cash, or component repair or replacement) to a purchaser whose product has a component failure before the component has reached a certain age or has been in use for a certain time period. Thus, for example, auto manufacturers offer “limited drivetrain warranties” based on the expected trouble free life (measured in mileage used, or age) of the drivetrain they supply. In doing so, manufacturers bear in mind that some few might fail before the warranty period, and that compensation then must be paid. But, manufacturers usually have carried out a cost benefit analysis, using statistical and financial techniques, from which they can determine that the number of failures in the warranty period and the cost of compensation is significantly outweighed by increased sales of their autos (attributable to the limited warranty), and the increased profits from these sales.
Of course, the consumer product model explained above does not apply to all kinds of products. For example, in the market for large commercial aircraft that carry hundreds of passengers, mission failure cannot be tolerated as readily as in the automotive industry. Accordingly, other techniques are used. In general, these are based on testing of components to determine rates of wear and component life, and observing component wear and life in service. Based on these, regular scheduled maintenance procedures are applied to replace each component on a routine basis before it reaches a point of failure. This technique is also used in other capital industries where the continuous reliable operation of large machines is necessary or critical.
There are also other areas where reliability of mission performance poses challenges. For example, some products are manufactured in bulk, and stored for a long period before their intended use. These products each a have a shelf life based on the rate of degradation of their components under storage conditions. For example, a pharmaceutical company may manufacture a medicine that contains several chemical compounds in batches at intervals and supply pharmacies from its inventory. Under these conditions, it is necessary to ensure that the beneficial chemical composition of the medicine not degrade during the storage period. Generally, pharmaceutical companies mark their products with an “expiration date” that is a date after which the company cannot guarantee that the medicine retains its potency. The pharmaceutical industry is not the only one faced with these issues of batch production and storage. Some industries have an added complication that their products have chemical, electronic and mechanical components or subsystems that are each prone to degradation with age, and mission performance reliability must be estimated in some manner.