Systems Modeling Language Feature Set

SysML makes use of nine primary diagram taxonomies in its graphical modeling language. The package diagram allows for representation of model elements. This provides a clear overview of the organization of the model. Requirement diagrams allow for explanation of requirements along with the requirement hierarchy throughout the product. Requirements are text strings that qualitatively describe the product. Activity diagrams demonstrate the product behavior based on input/output sequences. This is similar to state machine diagrams except state machine diagrams are based on event-driven state transition [1].




Sequence diagrams demonstrate the message exchange in a given system. Use case diagrams allow for stating the system functionality with regards to external systems. Block diagrams allow for structural composition (both internal and external) to be defined. This also allows for showing hierarchy of components. In the case of internal block diagrams, system interrelations and internal interfaces will be defined. Parametric diagrams allow for clearly defining constraints in terms of other parameters [1].

SysML Activity Diagram [1]
For example, consider a common pneumatic tool air compressor. This system could be broken down using SysML. The package diagram organization would contain the air compressor requirements, behavior, structure, and parametric properties. The requirements would be further broken down into the specifications for the air compressor. The requirements would include features such as the power requirement, air compression rate requirement, maximum pressure requirement, and aspects of portability, flow rate, relative reliability, and storage capacity.

The activity diagram for the air compressor’s behavior (shown above) shows the expected operation of an air tool that is connected to the air compressor in question. All interactions are defined including the suction the air compressor draws of the atmosphere. The primary function of the air compressor shown here is to compress air as a service for the air tool.

The actual structure of the air compressor can be easily described using a hierarchical block diagram. The air compressor uses a motor controller, a motor, a pump, and a tank to achieve a full compression and storage cycle. All of these items ultimately achieve the objective of attaining a specified air flow rate out of the compressor.

From a parametric standpoint, the air compressor receives a given flow rate from several constraints. The constraints are the volume of the tank, differential pressure across the vessel compared to atmosphere, power of the motor, efficiency of the air pump system, and the air flow rate provided based on the command speed from the motor controller [1]. These parameters can be parametrized for use in either SysML modeling or imported into an axiomatic design matrix as design parameters [2].

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References:

[1] Freidenthal, S., Moore, A., Steiner, R., 2012, A Practical Guide to SysML: The Systems Modeling Language, Elsevier Ltd, Amsterdam.

[2] Suh, N. P., 1995, “Axiomatic Design of Mechanical Systems,” J. Mech. Des., 117(B), p. 2.

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