Page 26 - Mechatronic Systems Modelling and Simulation with HDLs

2.4 MODEL DEVELOPMENT 15

relatively costly and requires a comprehensive understanding of the system. On
the other hand, such models can be adapted to the actual system over a wide
range by modifying parameters. If, for example, a system is to be driven by a DC
motor, various makes can be included in the simulation by the use of the applicable
parameters. These ‘generic’ models thus cover a whole class of components. As
an alternative to modelling on the basis of physical behaviour the other option is
to take measured data and feed this into models. This is also called experimental
modelling and is used if physical modelling is not implementable or the resulting
model is too complex for the desired purpose. Typically, however, experimental
modelling has to be repeated every time one of the components in question is
altered. Both in the case of physical and experimental modelling the models are
generally formulated on the basis of equations and assignments, i.e. consequently
formulated in the form of text.
In addition to a simulation, an emulation may also come into consideration
under certain speed requirements. This has different characteristics for electronics
and mechanics. In the field of digital electronics the term emulator is used to mean
a device that can take on the function of any desired digital circuit, see for example
Bender and Kaiser [25]. This function is based upon a number of programmable
chips, for example so-called FPGAs, the logic functions of which are stored in a
local RAM and can thus be modified. Currently up to a hundred thousand gate
functions can be stored on a single FPGA. With regard to speed, FPGAs, and
thus emulators, are generally significantly slower than dedicated hardware, but
are, however, faster than a simulation by orders of magnitude. The emulation
of analogue electronics and mechanics on the other hand is based upon signal
processors, so-called DSPs, that are optimised for analogue signal processing, see
for example Huang et al. [155] or Georgiew [116]. So differential equation models
of mechanical components can again be calculated faster than is the case for a
simulator by orders of magnitude.
Since modelling is a difficult process, and prone to errors, in some cases real
components are embedded into a simulation, see for example Helld¨ orfer et al.
[136] or Le et al. [219]. This is also called ‘hardware in the loop’. This does not
mean that the entire system is constructed as an electronic bread-board assembly or
mechanical prototype, instead usually just one component is fitted. Alternatively,
the environment of the system to be developed can be included in real form.
The rest of the system is modelled in the classical manner, so that simulated and
real behaviour are mixed together. The advantage of this is that the modelling
and its validation can be dispensed with for the real hardware in the simulation
loop. However, the principle disadvantage is that the real components have to be
fully installed in the laboratory and adequately fitted with actuators and sensors
in order to ensure the main inputs and outputs. Furthermore, the simulation of
the remainder of the system must in this case take place in real time, which may
involve considerable cost, depending upon the system. Alternatively, this real time
simulation can be replaced by an emulation to speed things up.