Related Topics:
	Phase Diagrams

Operational Phase Blocks

In phase diagrams, operational phase blocks are used to represent any stage of the system's mission that is not dedicated exclusively to the execution of maintenance tasks. Operational phases are always defined by (linked to) a simulation RBD in the current project. Each operational phase has a fixed, predefined duration unless a failure occurs during the phase and then it depends on how the properties of the phase block have been configured.

Operational phase blocks can have unlimited incoming connections and up to two outgoing connections, which may include one success path and one failure path. The success path and the failure path must be different; if both success and failure of the block actually lead to the same outcome, there are two ways to model this:

• If the operational phase stops upon failure of the block and the simulation moves to the next phase along the success path, you can use a node block to model this configuration (i.e., the success path links directly to the next phase, and the failure path leads to a node block, which then links to the next phase).  

• If the operational phase continues for the specified duration despite failure and the simulation then moves to the next phase along the success path, do not create a failure path.

By default, the first connection that you create from an operational phase block is the success path and the second is the failure path. You can right-click either connector and choose Set to Success or Set to Failure to swap the two paths.

You can add an operational phase block to a phase diagram by choosing Phase > Phase > Add Operational Phase.

The Select Diagram for the Block window will appear. The list of available diagrams will show the simulation RBDs in the project. You can use the View area to sort the list hierarchically or alphabetically. The Filter field allows you to enter text to search for in the names of the available diagrams. If the diagram that the operational phase block will be linked to does not yet exist, click the Add RBD icon to add a new diagram to the project.

To open an operational phase block that already appears in the phase diagram, double-click the block or choose Phase > Settings > Block Properties to open the Phase Properties window.

The following properties are available to configure an operational phase block:

• Phase name and Description: By default, operational phase blocks are named according to the name of the diagram they represent. You can type new text either instead of or in addition to the default name. You can change the default names by choosing File > Manage Repository > Define Default Names.

The Phase name field must be populated. Description text is optional. Note that an asterisk is used (*) to represent default block names. The block name that is displayed in the diagram will replace the asterisk with the block's default name; this allows the block name to be updated dynamically.

• Phase Properties

• • Diagram allows you to select a simulation RBD in the current project that you want the phase block to represent. Note that common components across different RBDs are identified by name. In other words, a component with the exact same name in two RBDs is assumed to be the same component working in two different phases.

  • Phase duration allows you to specify how long the system will operate in the configuration defined for this phase. If a failure has not occurred by the end of the specified phase duration, the simulation will proceed along the success path leading from the phase block. If a failure occurs, the simulation will proceed along the failure path leading from the phase block.

  • Phase duty cycle allows you to specify a common duty cycle value for the entire RBD that the phase represents, thereby modeling situations where the actual usage of the RBD during system operation is not identical to the usage for which you have data (either from testing or from the field). This can include situations where the item:

  • • Does not operate continuously (e.g., a DVD drive that was tested in continuous operation, but in actual use within a computer accumulates only 18 minutes of usage for every hour the computer operates).

    • Is subjected to loads that are greater than or less than the rated loads (e.g., a motor that is rated to operate at 1,000 rpm but is being used at 800 rpm).

    • Is affected by changes in environmental stress (e.g., a laptop computer that is typically used indoors at room temperature, but is being used outdoors in tropical conditions).

In these cases, continuous operation at the rated load is considered to be a duty cycle of 1. Any other level of usage is expressed as a percentage of the rated load value or operating time. For example, consider the DVD drive mentioned above; its duty cycle value would be 18 min / 60 min = 0.3. A duty cycle value higher than 1 indicates a load in excess of the rated value.

If a duty cycle is specified for the phase and there are also duty cycles specified for blocks within the RBD, their effects are compounded. For instance, consider the aircraft example given earlier. During the take-off phase, the subsystems experience 1.5 times the normal stress, so you would use a phase duty cycle value of 1.5. We also know that the landing gear is not used continuously during take-off. Assume that the landing gear is actually in use only 30% of the time during take-off. Each landing gear block in the RBD, then, would have a duty cycle value of 0.3. For each block, the effects of the phase duty cycle and the block duty cycle are compounded, yielding an effective duty cycle value of 1.5 x 0.3 = 0.45.

• Phase Throughput properties allow you to specify the maximum output that the entire system can process during the particular phase. (See Throughput Analysis with Phase Diagrams.)

• Identifiers: The Comments field allows you to enter additional information about the block.

The Active Phase drop-down list at the bottom of the window allows you to choose another phase that you want to edit without the need to close and reopen the Phase Properties window. This option works in exactly the same way as the Active Block feature in the Block Properties window.