MBSE needs Truenumbers

Model-based systems engineering (MBSE) is the formalized application of modeling to support system requirements, design, analysis, verification and validation activities. SysML is the accepted modeling language for MBSE, and the mission is to replace traditional engineering documents with a family of object models.

This diagram is an example of a complete, well-defined SysML Activity Diagram. What’s missing are the labels, which are arbitrary from SysML’s point of view, but without which we can have no idea what the diagram is about. Together with the comments SysML encourages us to write, these are really what determine the engineering content of SysML.

So the reality of MBSE is that engineering is still 100% expressed in natural language, supported by meaningful diagrams, which is a pretty good description of the engineering documents MBSE sets out to replace. The value that MBSE adds (and it’s considerable) is that information once locked in pages and pages of free text becomes more organized and accessible.

We could do much more to support Systems Engineering if we captured this natural language content in an organized and accessible way. This one sentence is enough to describe vehicle braking:

“In order to minimize stopping distance, the ABS system of a vehicle prevents each wheel from locking by modulating the braking force.” 

It tells us more than the diagram.  The phrase “ABS system of a vehicle’” identifies the system of interest, and  “minimize stopping distance” tells us the purpose of the ABS.  The phrase “prevents each wheel from locking” tells us what the ABS does, and “modulating the braking force” describes how it does it.  To make these statements more accessible, we can factor the one sentence into three:

“minimizing stopping distance” is the purpose of the ABS system of vehicle”
“preventing each wheel from locking” is the function of the ABS system of vehicle”
“modulating braking force” is an implementation strategy of the ABS system of vehicle”

Managing truenumber sentences like these complements SysML models by cataloging and organizing the expressive language that must always be present to explain the meaning and context of any model.