Modeling Constraints with Parametrics

Parametric models capture the constraints on the properties of the system; expressed as equations; uses bdd to define constraint blocks; the header follows format:

par [model element type] model element name [diagram name]

[6] How are constraint properties represented on a parametric diagram? ANS: 7.2

[7] How are constraint parameters represented on a parametric diagram? ANS: 7.2

[1] What is the diagram kind of a parametric diagram? ANSWER = par


Constraint = SysML can do it graphically but based on the language used to enable the constraint (OCL, Java);

Constraint block = encapsulates constraint properties in constraint block, like how parts encapsulated by block;

Constraint parameters = special property used in constraint expressions; can be of variety types

Two characteristics of constraint properties: order and uniqueness;

[2] If a constraint parameter is ordered, what does that imply about its values? ANS = members of collection are mapped to values of positive integer; noted by keyword ordered

[3] If a constraint parameter is unique, what does that imply about its values? ANS = no duplicates in values, all unique

Noted by keyword unique (look at Fig. 7.4)

[4] How are constraint parameters represented on a block definition diagram? ANS = Figure 7.4

Another property is derived à marked by keyword derive and means its from other values;

[5] How is the composition of constraints represented on a block definition diagram? ANS:


[8] What are the semantics of a binding connector? ANS = binding connectors express equality relationships between parent and sibling constraint properties; when bound the elements must be the same – therefore modeler can build complex equations pulling variety of properties;


[9] How can constraint blocks be used to constrain the value properties of blocks? ANS = on BDD draw composite associations between block with values needing constraint to the constraint block;



[10] A block “Gas” has two value properties, “pressure ” and “volume, ” that vary inversely with respect to each other. Create an appropriate constraint block to represent the relationship and use it in a parametric diagram for “Gas” to constrain “pressure ” and “volume. ” ANS = similar to Figure 7.9

[11] What are the two approaches to specifying parametric models that include time-varying properties? ANS = 1 is to treat time as implicit in the expression; this help reduce diagram clutter as shown in Fig 7.11. ANS 2. Represent time as separate property; specifies unit and dimensions; as shown in Figure 7.12


Constraint blocks to constrain item flows:




Using Constraints for Block or System Analysis

Analysis context = contains both block and constraint blocks to perform analysis; also libraries of constraint blocks; here the constraint blocks may be analysis models that maybe complex;

[12] How are composite associations and reference associations typically used in an analysis context? ANS = composite associations are used between the analysis context and the analysis model (or any other constraint blocks); noted as solid diamond. Reference association is used between analysis system and the block




Using Constraints for MOEW and Trade Study analysis

[13] What is a measure of effectiveness and what is it used for? ANS: measure of effectiveness (MOES) = based on trade study to calculate values to see how well it satisfies a requirement. This is evaluated using an object function (cost function / utility function) which gives results for each alternative compared to a preferred solution.

[14] What is an objective function and how is it represented on a block definition diagram and a parametric diagram? ANS: object function is special constraint block that expresses an objective function with parameters bound to a set of MOES. Set of solutions can also be represented as separate blocks in BDD:


The MOE are noted in 7.16; these are what was used in trade study for calculation;