Representing Logical Relationships Between Components

In nearly all multi-component systems, logical relationships will exist between the components (e.g., "in order for component A to operate, component B or component C must be operating"). These relationships can be represented within the reliability elements by defining either a requirements-tree A logic tree in a Reliability Module element that that must evaluate to True in order for the component to operate. or a fault-tree A logic tree in a Reliability Module element that that must evaluate to False in order for the component to operate.. In a requirements-tree, the top level node must evaluate to true for the component to operate, and for a fault-tree, the top level node must evaluate to false for the component to operate. These trees are used to model the logical relationships between components, and to propagate the effects of changes in element status to affected components.

In the case of the Computer element discussed in the previous topics, it requires an operational CPU, motherboard and hard disk in order to function properly. In the case of the CPU, it needs its fan to be operational in order to function properly. Of course, each of these subcomponents can have its own set of failure modes. Let's assume for the purpose of this example that all of the failure modes for the Computer (except one) are assigned to the subcomponents. One failure mode (Power Supply Failure) is assigned to the parent element (the Computer), since we have decided not to explicitly represent the power supply as another element.

GoldSim can easily represent these relationships, but it provides a slightly different approach to fault trees than many reliability engineers will be used to. In particular, there is no need to develop a "global" requirements- or fault- tree for the entire system. The user is only required to define the immediate relationships between components. In this case, the computer's requirements tree would include four nodes, indicating that in order for the Computer to operate, the CPU, Hard Disk and Motherboard must all be operating, and the Computer itself must not have failed due to the Power Supply failure mode:

Note that these are listed as "Internal Requirements" because the components and the failure mode are internal to the Computer. Failure modes for the Computer itself are added automatically to this list by GoldSim. Any sub-components need to be added manually (as appropriate).

"External Requirements" can be used to specify requirements external to the Computer (e.g., that electricity is available).

After we build a model, we can run it. We will discuss that in the next topic.