Advanced Model-Based System Engineering MBSE organizations are moving beyond the use of modeling to make “pretty pictures”. SysML parametrics are a way to add spreadsheet-like calculations to descriptive architectural diagrams. And roll-up calculations are among the most common use cases in systems engineering. Starting with a multi-level bill-of-materials or master equipment list, we want to calculate total mass, total cost, or roll-up any metric in general, based on the individual values of all the parts in the system. We also want to do that where the number of components (and levels of components) evolves over the project lifecycle, without having to rebuild the parametric model every time.
Check out our new tech note – SysML Parametrics for Roll-up Calculations (PDF) – that describes a number of approaches that can accomplish these goals using our parametric solvers – ParaMagic (MagicDraw), Melody (Rational Rhapsody), Solvea (Enterprise Architect), ParaSolver (PTC Integrity Modeler).
These approaches take advantage of SysML’s inheritance, multiplicity, redefinition, and recursion features, without eliminating the individuality of each component and assembly. The Technical Note steps through a series of methods with progressively more complexity and versatility.
1. The base level approach simply puts a constraint like a = b + c + d inside each assembly, adding the values of each of the assembly’s three parts.
2. The first step in adding flexibility is to use a constraint such as a = sum(b) to add the values for all the parts independent of the number of parts. This requires that all parts have a common supertype.
3. Making the constraint a property of the supertype reduces the number of parametric relations necessary for the roll-up down to one by using redefinition and recursion.
4. The final example treats the case where multiple roll-ups are needed, but not all over the same set of parts. Multi-level inheritance trees can enable this.
Not all calculations in systems engineering are complex, or require a powerful and expensive simulation tool. Roll-ups, unit conversion, and requirements verification are all valuable time-savers when embedded within the model. As SysML models seek interoperability with other engineering tools, translating between data models becomes a critical part of the process. Parametric solvers are becoming an important part of the system engineer’s toolbox.