Page 68 - Mechatronic Systems Modelling and Simulation with HDLs

3.4 DOMAIN-INDEPENDENT DESCRIPTION FORMS 57

Pin p, n;
Voltage v;
Current i;
equation
v = p.v -n.v;
0 = p.i + n.i;
i = p.i
end TwoPin;
...
model Resistor "Ideal electrical resistor"
extends TwoPin;
parameter Real R (unit="Ohm") "Resistance"
equation
R*i = v;
end Resistor;
model Capacitor "Ideal electrical capacitor"
extends TwoPin;
parameter Real C (unit="F") "Capacitance"
equation
C* der(v) = i;
end Capacitor;
...
Hardware description 3.2 Model of the components from Hardware description 3.1

3.4.4 Evaluation of domain-independent
description forms

From the examples shown above it is clear that bond graphs can describe both
analogue electronics and mechanics (and also a range of further domains) in com-
pact and graphic form. However, if we go beyond unidimensional mechanics and
passive electronics there are significant problems to be solved. Although the mod-
elling of transistors is also possible in principle using bond graphs, a meaningful
simulation of circuits of substantial complexity remains the exclusive preserve of
a dedicated circuit simulator. The same applies for three-dimensional multibody
mechanics. Moreover, bond graphs are in principle limited to continuous systems,
so that digital electronics and software cannot be illustrated using classical bond
graphs, or at least this cannot be done efficiently. Furthermore, every element must
be assigned a fixed causality prior to the simulation. This causality may alter dur-
ing a simulation, for example, if an electric motor becomes a generator, so that
such systems cannot be simply investigated using bond graphs. The same applies
in principle for block diagrams.
Domain-independent languages, and Modelica in particular, are broadly compa-
rable with analogue hardware description languages. However, they don’t have the
model basis of a circuit simulator. Furthermore, the event-oriented field is much