The Switch Block
Parameters:
Description:
The Switch block can be thought to direct either the first or the third input port to its output port. The second input port takes the decision which of the input ports is actually propagated: if the value of the second input port is bigger than a given level, the Switch propagates the first input port, otherwise the third one.
Contrary to PowerDEVS, where this block is an atomic model, ModelicaDEVS makes use of the fact that the DEVS formalism allows for hierarchical models: by putting a multi-component model into one of the predefined block hull templates it can be used as a normal block for further models. The interior life of the Switch block looks as shown in the following figure:
Let us have a look at the purpose of each of the internal blocks.
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The Constant block represents the level to which the second port is compared. Note that since the level is set by the user by means of a parameter (parameter Real level=0 "Switching level.";), this value has to be passed to the Constant block which yields the following declaration: Sources.Constant Constant1(v=level).
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The two Comparator blocks evaluate if the value of the second input port is above or below the specified level. To this end, the upper comparator emits an output value of 1 if the value of the second port is bigger than the level and 0 otherwise. The lower Comparator emits an output value of 1 if the value of the second port is smaller or equal than the level and 0 otherwise. In other terms, the Comparators declare which of the two switch input signals (port 1 or port 3) to propagate to the output port: if for instance, the upper comparator emits 0, the lower one by definition emits 1 and vice-versa.
Note the settings of the output values of the Comparators: Comparator1.vU = 1, Comparator1.vL = 0, Comparator2.vU = 0, Comparator2.vL = 1.
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The Multiplier blocks receive the input of the first and the third port, respectively, and a value of the Comparators that determines whether the particular input signal has to be multiplied by 0 or 1. If, for instance, the input of the second Switch port is smaller or equal than the specified level, the upper Comparator emits 0 and the lower one emits 1. Hence, the signal of the third port would be forwarded since its value is multiplied by 1 whereas the upper one is zeroed-out by multiplying its value by 0.
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The Add block merges the two outputs coming from the Multiplier blocks (one of which is always zero and the other one carries the value from one of the input ports of the Switch block).
The two pictures below illustrate the behaviour of a Switch block.
The first picture shows the trajectory of the second input port (a sine signal plotted in blue) and the switching level (red) given by the parameter level. The crossing instants of the sine signal with the switching level determines the time instants when the Switch has to flip, thereby changing the source (port 1 or port 3) for its output port.
The second picture shows the first input port (blue) that is simply a constant value at v=3 and the third input port that is a sine signal that starts only at time t=0.8. The output of the Switch block is then plotted in red. It can be easily seen that when the sine signal in the first picture is bigger than the switching level, the output is constant at the value 3, and when the sine signal drops below the switching level, the Switch output equals the trajectory of the sine signal of the third input port.
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