Page 20 - Mechatronic Systems Modelling and Simulation with HDLs

2.3 FIELDS OF APPLICATION 9

model may serve to evaluate the power of a computer, which accesses its hard
drive with a probability of x% and its tape deck with a probability of y%. Models
containing at least one random variable are classified as stochastic. All others are
called deterministic.
A further option for the classification of models is the consideration of the
‘outside world’ of a model. If the model is isolated from the outside world and
thus has no inputs and outputs, then it is called autonomous. All other models
are called non-autonomous. An autonomous model produces a movement in the
state space from itself, without taking in and producing data, whereas a non-
autonomous model primarily converts values at the inputs into the outputs based
upon the current state.
A final option for the classification of models is represented by the question of
whether or not time crops up explicitly in the model equations. In the former case
the model is time-variant, in the latter time-invariant.

2.3 Fields of Application

2.3.1 Introduction

If technical systems are to be developed, two main fields of application can be
identified for the simulation: The validation of specifications and the verification of
designs. In the ideal case the specification or design will be available immediately
in model form, so that nothing stands in the way of direct simulation. Hitherto
this has mainly been the case in the design of digital electronics using hardware
description languages. Otherwise, modelling must take place first to bring about
the transition from an arbitrary description to a simulatable model.
The use of modelling and simulation is closely linked to the underlying design
processes. These can be roughly divided in accordance with their design direction
into top-down and bottom-up design flows. In what follows these will be briefly
introduced and characterised by their influence upon modelling.

2.3.2 Bottom-up design

Bottom-up design is the classic method of development of electronics and mechan-
ics, see Figure 2.2. The initial starting point is a specification, which is typically
drawn up in natural language. Then the basic components, e.g. transistors, resistors,
capacitors or springs, masses, shock absorbers, joints, etc. are added and combined
successively to form ever more complex and abstract creations until a complete
design emerges. This takes place on a structural level, so that the only thing that is
determined each time is which submodules make up a module and how these are