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Modelling and assembly of the full vehicle 375
is an element of swings and roundabouts if choosing between the codes. In
general, codes like MSC.ADAMS have a history in very accurate simulation
of mechanical systems and can be coerced into representing control
systems. Codes like MATLAB and MATLAB/Simulink are the reverse;
they have a history in very detailed control system simulation and can be
coerced into representing mechanical systems. For this reason, a recent
development suggests using each code to perform the tasks at which it is
best; this is often referred to as ‘co-simulation’. The authors’ experiences
to date have been universally disappointing for entirely prosaic reasons –
the speed of execution is extremely poor and the robustness of the software
suppliers in dealing with different releases of each other’s product has been
somewhat inconsistent. The effort required to persuade the relevant software
to work in an area where it is weak is usually made only once and in
any case the additional understanding gained is almost always worthwhile
for the analyst involved. Until the performance and robustness of the software
improve, the authors do not favour co-simulation except for the most
detailed software verification exercises.
The next hurdle to be crossed is the representation of the intended behaviour
of the vehicle – the ‘reference’ states. Competition-developed lap simulation
tools use a ‘track map’ based on distance travelled and path curvature.
This representation allows the reference path to be of any form at all and
allows for circular or crossing paths (e.g. figures of eight) to be represented
without the one-to-many mapping difficulties that would be encountered
with any sort of y-versus-x mapping. Integrating the longitudinal velocity
for the vehicle gives a distance-travelled measure that shows itself to be tolerably
robust against drifting within simulation models. Using this measure,
the path curvature can be surveyed in the vicinity of the model.
Some authors favour the use of a preview distance for controlling the path
of the vehicle, with an error based on lateral deviation from the intended
path. However, there is usually a difficulty associated with this since the lateral
direction must be defined with respect to the vehicle. (Failure to anchor
the reference frame to the vehicle means that portions of the path approaching
90 degrees to the original direction of travel rapidly diverge to large
errors.) Projecting a preview line forward of the mass centre based on vehicle
centre line is unsatisfactory due to the body slip angle variations mentioned
previously. Either the proportional gain must be reduced to avoid
‘PIO’ type behaviour, which leads to unsatisfactory behaviour through
aggressive avoidance manoeuvres, or else some form of gain scheduling
must be applied. Alternatively, the preview vector can be adjusted for body
slip angle before it is used if oscilliatory behaviour is to be avoided. The
length of the preview vector must be adjusted with speed if reasonably consistent
behaviour is to be produced. For ‘normal’ driving this type of model
can produce acceptably plausible results but for manoeuvres such as the ISO
3888 Lane Change the behaviour becomes unacceptably oscilliatory particularly
after the manoeuvre.
An alternative method, used by the authors with some success for a variety
of extreme manoeuvres, is to focus on the behaviour of the front axle. This
model fits with one of the author’s (Harty) experience of driving at or near
the handling limit, particularly on surfaces such as snow where large body