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Discrete Event Modeling

Physical modeling, in general, involves continuous variables and equations. In some cases, however, you can simplify the mathematical model of the system and improve simulation performance by treating certain changes in system behavior as discrete. Such an idealization assumes that system variables may only change values instantaneously and discontinuously at specific points in time.

An event is a conceptual notation that denotes a change of state in a system. Event modeling lets you perform discrete changes on continuous variables. The two most common applications of event modeling are:

  • Trigger-and-hold mechanism, such as a triggered delay. For example, a component has two inputs: u and x (triggered signal), and one output y. When and only when the triggered signal x changes value from false to true, output y is reset to the value of u at current time. y remains unchanged all other times.

  • Enabled component, acting on a principle similar to Simulink® enabled subsystem (Simulink). That is, the component has a control signal as input. If the control signal has a positive value, then the component holds certain states to the most recent value, or resets them. When the control signal is negative, the states change according to component equations.

The following constructs in Simscape™ language let you perform event modeling: event variables, events section, when clause, and edge operator.

Event Variables

Event variables are piecewise constant, that is, they change values only at event instants, and keep their values constant between events. You can declare internal component variables of type integer or real as event variables by setting the Event=true attribute.

For example, the following code declares two event variables: x (type real) and d (type integer).

variables (Event=true)
   x = 0;
   d = int32(0);
end

You can initialize event variables by using the initialevent operator. For more information, see initialevent.

You can also initialize event variables the same way as continuous variables, for example, through member component modification. The only difference is that initialization to event variables always has high priority. If you specify initialization priority other than high, the priority part gets ignored.

You update the values of the event variables in the events section of the component file, by using the when clause.

Event Data Type and edge Operator

The edge operator takes a scalar Boolean expression as input. It returns true, and triggers an event, when and only when the input argument changes value from false to true. The return type of edge is event type. Event data type is a special category of Boolean type, which returns true only instantaneously, and returns false otherwise.

The following graphic illustrates the difference between Boolean and event data types.

edge(b) returns true only when b changes from false to true.

To trigger an event on the falling edge of condition b, use edge(~b).

The data derivation rules between Boolean and event data types are:

  • edge(boolean) is event

  • ~event is boolean

  • (event && event) is event

  • (event && boolean) is event

  • (event || event) is event

  • (event || boolean) is boolean

You use the edge operator to define event predicates in when clauses.

Events Section and when Clause

The events section in a component file manages the event updates. The events section can contain only when clauses. The order of when clauses does not matter.

The when clause serves to update the values of the event variables. The syntax is

when EventPredicate
  var1 = expr1; 
  var2 = expr2; 
  ...
end

EventPredicate is an expression that defines when an event occurs. It must be an expression of event data type, as described in Event Data Type and edge Operator.

The variables in the body of the when clause must be declared as event variables. When the event predicate returns true, all the variables in the body of the when clause simultaneously get updated to the new values.

The order of the variable assignments in the body of the when clause does not matter, because all updates happen simultaneously. For example, if d1 and d2 are event variables initialized to 0,

when edge(time>1.0)
   d1 = d2 + 1;  
   d2 = d1 + 1;  
 end

is equivalent to:

when edge(time>1.0)
   d2 = d1 + 1;  
   d1 = d2 + 1;  
end

After the event, both d1 and d2 have a new value of 1, because they were both simultaneously updated by adding 1 to the old value of 0.

A when clause cannot update an event variable more than once within the same assignments list. However, two independent when clauses also may not update the same event variable. You must use an elsewhen branch to do this.

Branching of the elsewhen Clauses

A when clause can optionally have one or more elsewhen branches:

when EventPredicate
  var1 = expr1; 
  var2 = expr2; 
  ...
elsewhen EventPredicate
  var1 = expr3; 
  ...
end

Note

The default else branch in a when clause is illegal.

A common usage of elsewhen branches is to prioritize events. If multiple predicates become true at the same point in time, only the branch with the highest precedence is activated. The precedence of the branches in a when clause is determined by their declaration order. That is, the when branch has the highest priority, the last elsewhen branch has the lowest priority.

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