Page 69 - Mechatronic Systems Modelling and Simulation with HDLs

58 3 MODELLING AND SIMULATION OF MIXED SYSTEMS

weaker in comparison to hardware description languages in general, and VHDL-
AMS in particular, so that digital electronics or software, as is demonstrated by
Scherber and M¨ uller-Schloer in [360], require the coupling of appropriate simula-
tors to the equation solver that underlies the language.
Perhaps the most important objection against domain-independent description
forms lies in the fact that it is necessary to start modelling up from scratch in
every domain. Alternatively, if we build up from a circuit or multibody simulator,
a large part of the system is already covered by the best available methodology.

3.5 Simulator Coupling

3.5.1 Introduction
The option of simulator coupling tackles the problem highlighted above in a
straightforward manner. Appropriate simulators are already available for the vari-
ous domains in the system and in the ideal case these would only have to exchange
their current simulation results. The use of simulator coupling can protect invest-
ments in models and facilitate the use of the best available simulator for a field.
However, simulator coupling is also associated with a whole range of problems.
For example, it generally requires access to the internals of the simulators involved,
which means that if commercial simulators are to be considered, the co-operation
of the provider in question is required. Furthermore, the coupled simulation forms
a very intricate software package, which is difficult to get to grips with. Perhaps
the most important disadvantage, however, lies in the synchronisation of two nor-
mally very different simulator cores. In the coupling of analogue electronics and
mechanics, differential equations are solved in both cases. However, their origin,
nature and formulation are very different. Furthermore, this form of co-simulation
is also associated with convergence problems, particularly in the case of a strong
coupling between two analogue solvers.

3.5.2 Simulator backplane

When coupling two simulators, the principle of ‘simulator backplane’ represents a
particularly systematic solution, see also Jorgensen and Odryna [171] or Maliniak
[255]. This principle is equally suited to the coupled simulation of exclusively
continuous, exclusively event-oriented or mixed systems. In principle, the simu-
lator backplane is a standardised procedure, see Kemp [187], for the inclusion of
simulators into an overall simulation, see Figure 3.11 from Zwoli´ nski et al. [441].
The main task of a backplane is to undertake a partitioning of the design data
before the actual simulation and to assign the individual parts of the simulators in
question. The backplane also looks after the synchronisation between the linked
simulators and the exchange of data. In the ideal case the backplane also has a
unified user interface with the associated output tools, but this tends to be rare.