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158 Model Driven Systems Development with Rational Products
Parametrics
The SysML Parametric Diagram (PD) is used to represent the usage of
constraint blocks as constraint properties. Syntactically the PD is actually is
similar to IBD. In a PD, constraint properties are connected to each other through
the parameters defined by their constraint block. In turn they connect to other
properties in the context of their owning block. These other properties must be
directly bound to parameters of the constraint properties because they can only
play a “feeding role” to the constraints parameters in a PD.
An example of a PD is shown in Figure 7-10 on page 160. Constraint
properties are represented by boxes with rounded corners. In this diagram,
both the sensor and windshield parts compute an operational range that is
compared by the property named
compare. These values are also fed to the
part representing the configuration file (bottom of the figure). If the sensor and
the windshield are compatible, the flag
IsCalibrated (exposed as a port) is set
to true. The verification of the calibration requirement is hence reduced to
testing the value of this port. The system is therefore resilient to changes in
windshield and sensor characteristics.
The usage of the constraint blocks WindshieldIREffectiveRange and
SensorEffectiveRange can be seen in the diagrams of Figure 7-8 and
Figure 7-9, respectively. They are nested in the parts named RainSensor and
CarWindshield (see comments in the figure).
An attractive aspect of constraint blocks is that they provide a reusable
mechanism to define types of constraints. Hence the same constraint can be
used several times in the model. It is important to note that a constraint does
not specify which variable is an input or an output. Values are assigned by the
context and a numerical solver will provide results for the variables of the
system.
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See the work by Peak et al. on constraints for more details: Peak RS, Friedenthal S, Moore A,
Burkhart R, Waterbury SC, Bajaj M, Kim I, Experiences Using SysML Parametrics to Represent
Constrained Objectbased Analysis Templates. 2005. 7th NASA-ESA Workshop on Product Data
Exchange (PDE): The Workshop for Open Product & System Lifecycle Management (PLM/SLiM),
Atlanta. See also http://www.pslm.gatech.edu/topics/sysml/