Page 18 - Mechatronic Systems Modelling and Simulation with HDLs

2.1 INTRODUCTION 7

Verification

Implemen-
Conceptual tation Executable
model model

Analysis Simulation
Qualification Validation

Reality

Figure 2.1 Model generation, simulation, validation and verification in context

to the executable model. Model validation, on the other hand, should tell us whether
the executable model is suitable for fulfilling the envisaged task within its field
of application. In other words: Verification ensures the system is modelled right,
whereas validation is all about modelling the right system. Various degrees of
validity can be defined for a model:

Replicative validity
A model is replicatively valid if it moves along tracks that have already been
marked out by measurements upon the real system. This is the lowest level of
validity. Such models may, for example, be used in the field of training to teach
people to use a real system by means of virtual experiments.

Predictive validity

A model is predictively valid if it ‘predicts’ data that are not extracted from the
system until later. So, for example, simulations supply important information on
the functionality of a circuit even before it has been constructed in the form of
a chip or board. It is also perfectly possible to mix predictively valid component
models with replicatively valid models if measurement data is available for the
modelling of some components but not for others. A predictively valid model is
also replicatively valid.

Structural validity
A model is structurally valid if it not only describes the outward behaviour of
a real system accurately enough, but also imitates the internal processes for the