Page 66 - Mechatronic Systems Modelling and Simulation with HDLs

3.4 DOMAIN-INDEPENDENT DESCRIPTION FORMS 55

here is that the structure of the circuit no longer corresponds with the structure of
the block diagram.

3.4.3 Modelling languages for physical systems

Languages such as ACSL [2], DSL [227], Dymola [90], [310] or Modelica [94],
[272] in particular deserve a mention. All these languages support the description of
physical systems. In what follows we will investigate Modelica in particular, as this
language includes the most up-to-date research results and furthermore is currently
being expanded to a standard, see [272] and [273]. An excellent introduction to
object-oriented modelling of mixed systems in general and of Modelica in particular
can be found in Otter [308].
Modelica is a language for the modelling of physical systems and was developed
specifically in order to support the exchange of models and the development of
libraries. Modelica does not insist upon an exclusively causal modelling, in which
cause and effect of every component have to be determined even before the sim-
ulation. The description of the models can also take place in the form of genuine
equations and not on the basis of assignments. Modelica supports the description of
continuous systems, which can be calculated on the basis of differential-algebraic
equation systems (DAE). In addition there are constructs for dealing with disconti-
nuities, which may occur in mechanical stops, or static to sliding friction transitions.
In principle it is also possible to use the discontinuities to describe event-oriented
processes, e.g. transitions in a state graph or the movement of markings in a Petri
net, but this possibility is limited by the underlying equation solver.
In principle, Modelica can be compared with an analogue hardware description
language, see also Tiller et al. [400]. Both structural and behavioural modelling is
possible. A particularly prominent feature of Modelica is object-orientation, which
is used, for example, to declare a model — or to be specific a model class — once
and instance it many times, with the option of setting certain parameters indi-
vidually for each instance. Similar concepts also exist in hardware description
languages, such as VHDL, with the possibilities of instancing and configuration.
Modelica also offers the option of transmission between model classes, so that
more complex model classes can easily be traced back to simpler ones.
To illustrate modelling in Modelica the description for an electronic circuit will
be given in what follows, see Figure 3.10 and [273].
The associated Modelica model is represented in Hardware description 3.1, with
key words shown in bold type. After the declaration of the circuit model the com-
ponents along with their main parameters are declared. At this level the equation
section specifies only the connectivity of the circuit.

model circuit
Resistor R1 (R=10);
Capacitor C (C=0.01);
Resistor R2 (R=100);