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Model-Based Design for Embedded Systems- P60

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Model-Based Design for Embedded Systems- P60: This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. | 576 Model-Based Design for Embedded Systems g at tn-1 0.5hn and tn-1 0.75hn we must fire but not postfire these actors. Postfiring the actors would erroneously commit them to state updates before we know whether the step size hn is valid. Thus in effect the solver must provide them with tentative inputs at each tag one tag for each of these time values as shown in Equations 17.5 and 17.6 and find a fixed point at that tag. But it must not commit the actors to any state changes until it is sure of the step size. Avoiding invocation of the postfire method successfully avoids these state changes as long as all actors conform to the actor abstract semantics. This mechanism is similar to that used in Simulink where the model_update method is not invoked until a simulation step is concluded. We can now see that CT operates similar to DE models with the only real difference being that in addition to using an event queue to determine the advancement of time we must also consult an ODE solver. The same fireAt mechanism that we used in DE would be adequate but for efficiency we have chosen to use a different mechanism that polls relevant actors for their constraints on the advancement of time and aggregates the results. In our implementation any actor can assert that it wishes to exert some influence on the passage of time by implementing a ContinuousStepSizeController interface. All such actors will be consulted before time is advanced. The Integrator actors implement this interface and serve as proxies for the solver. But given this general mechanism there are other useful actors that also implement this interface. For example the LevelCrossingDetector actor implements this interface. Given a CT input signal it looks for tags at which the value of the signal crosses some threshold given as a parameter. If a step size results in a crossing ofthe threshold the actor will exert control overthe step size reducing it until the time of the crossing is identified to some .