Page 23 - Mechatronic Systems Modelling and Simulation with HDLs

12 2 PRINCIPLES OF MODELLING AND SIMULATION

Predictively valid Specification

Structurally valid Implementation

Figure 2.4 Level of validity and its significance for the design of a technical system

2.3.4 Relationship of design strategies to modelling

In the case of the top-down design sequence, modelling is used for the specification
of the desired behaviour or for the formulation of designs. In both cases the result
can be directly checked through simulation; there is no such thing as modelling
exclusively for the purpose of simulation. In this connection, an important classi-
fication of such models by their level of validity can be made, see Figure 2.4. For
a specification, predictive validity is sufficient — the manner in which the terminal
behaviour of the specified systems and modules is individually generated is not
relevant. A system design, on the other hand, ideally supplies a structurally valid
model that describes both the terminal behaviour and the inner structure.
By contrast, if a technical system is to be developed using a bottom-up design
sequence, then simulation can be used for checking the system design or parts of
it after the conclusion of the design phase. Modelling is thus not an integral part
of the design process; instead it is often performed exclusively for the purpose of
the simulation, which raises questions regarding the verification and validation of
the model.
Where modelling is used outside a design process we can differentiate between
the following two cases: structurally valid modelling in natural and social sciences
in order to gain understanding of a system; and replicatively valid modelling in the
field of training. The former plays only a lesser role in the consideration of technical
systems. The latter is used primarily for the imitation of familiar behaviour. A well-
known example is flight simulators that are used for the training of pilots in all
feasible operational situations. Such simulators are now available on the market
for almost all types of vehicle. But simulators can also be used for other types of
training. Preparation for the repair of the Hubble telescope involved a great deal
of expenditure on simulation due to the considerable costs and the narrow time
frame for such measures in space, see Loftin [237] and [242].

2.3.5 Modelling for the specification

The main purpose of a specification is to describe the desired behaviour of a system
to be developed and the associated boundary conditions. Classically, a specifica-
tion is available on paper, which is associated with a whole range of problems.