Continuous and Discrete Rate of Flow

Oct 20

Oct 20 Continuous and Discrete Rate of Flow

Rate is a stereotype that may be applied to an activity edge and “specifies the expected value of the number of objects and values that traverse the edge per time interval.” The SysML specification 1.6 (beta) warns that «rate» “does not refer to the rate at which a value changes over time.” Rather, «rate» is actually a stereotype with a single property of type InstanceSpecification. Because of this, the values of this property must be “instances of classifiers stereotyped by «valueType» or «distributionDefinition».” Although it might be intuitive to assign, for example, a rate of 5 (i.e., 5 meters per second) to a flow, you must first define the valueType with its associated quantityKind and unit, then create an instance specification of that valueType to assign as the value of the «rate» property.

Rate describes flow, which in SysML can be either continuous or discrete. Continuous flow means that “the increment of time between items approaches zero” whereas discrete means that “the increment of time between items is non-zero.” The nature of the flow through an activity parameter is indicated by applying either a «continuous» or «discrete» stereotype.

In the example, the activity parameter, Carrier Out, is stereotyped as «continuous» since it represents a continuous analog signal.On the other hand, Digital Out is stereotyped as «discrete» since it represents a discrete sequence of bits.In both cases, the activity edges entering the activity parameters have the «rate» stereotype applied along with a specific value of a valueType assigned as the rate.The Digital Out action pin has a FIFO ordering assigned to indicate the order in which data flows out of the action pin.