Synchronous language elements are added to Modelica as an alternative to normal when-clauses to making modeling of complex sampled systems safer and easier.
// Discrete controller
when Clock() then
E*dc.xd=A*previous(dc.xd)+B*dc.yd;
dc.ud=C*previous(dc.xd)+D*dc.yd;
end when;
// hold controller output:
plant.u=hold(dc.ud);
// Plant
0=f(der(plant.x),plant.x,plant.u);
plant.y=g(plant.x);
// Sample continuous signal
dc.yd=sample(plant.y, Clock(3));
In this example dc.xd and dc.ud
are Clocked variables, and only defined when the Clock is active
(every 3rd second). At time instants where the associated clock is
not active, the value of a clocked variable can be inquired by
using an explicit cast operator, e.g., hold.