Source: Webel IT Australia ( Darren Kelly)
On p.81 the IBD Figure 59: Internal structure of hydraulics system shows identical initial values 9.8 for the value property gravity for two distinct usages of Tank in the context of ConnectedTanks. Similarly, identical values 10.0 for fluidDensity are shown.
Unless the Tanks are subject to separate gravitational forces (unlikely) this appears to break Single Source of Truth. It is in any case not an ideal example (given that some users might blindly copy the approach on real-world engineering projects).
The specification doesn't in fact explicitly state that in the particular example all initial values are handled by Slots on InstanceSpecifications as initial value carries, but does seem to imply it here (and elsewhere):
SysML initial values specify property values for components used in internal block diagrams. Figure 59
shows initial values for fluid density, gravity, tank surface area, pipe radius, pipe length, and dynamic
viscosity of the fluid (properties defined in Subannex A.4.4).
In any case, at least gravity and probably also fluidDensity, are better handled as simple Property::defaultValue assignments for their corresponding value properties in block Tank. At least in the MagicDraw/Cameo tool these then appear to "shine through" (Translucency) in the initialValues compartment on IBDs, alongside any context-specific values for different value properties.
Indeed leaving only fluidLevel (typically initially different between Tanks) and tankSurfaceArea (possibly different between Tanks) handled via Slots on InstanceSpecifications, while handling gravity and fluidDensity through Property::defaultValue assignments, would offer a good contrast between the approaches. This might require an additional diagram.
Reported: SysPhS 1.1 — Mon, 15 Feb 2021 12:00 GMT
Updated: Tue, 16 Feb 2021 10:20 GMT
- Figure 59_ Internal structure of hydraulics system.webel.SYSPHS12-15.annotated.png 451 kB (image/png)
- Figure 59_ Internal structure of hydraulics system.webel.values.png 361 kB (image/png)
- Figure 60_ Hydraulics blocks, ports, & component properties.png 918 kB (image/png)
SYSPHS12 — The handling of initial values for usages of Tank within ConnectedTanks for Figure 59 breaks Single Source of Truth for value property 'gravity'
- Key: SYSPHS12-15
- OMG Task Force: SysPhS 1.2 RTF