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366 Multibody Systems Approach to Vehicle Dynamics
a steering test simulation. Typical output is shown in Figure 6.40 where
the steering wheel angle is plotted on the x-axis and the road wheel angle
is plotted on the y-axis. The three lines plotted represent the steering ratio
test for the suspension in the static (initial model set up here), bump and
rebound positions.
Having decided on the suspension modelling strategy and how to manage
the relationship between the handwheel rotation and steer change at the
road wheels the steering inputs from the driver and the manoeuvre to be
performed need to be considered.
6.12.3 Steering inputs for vehicle handling manoeuvres
The modelling of steering inputs suggests for the first time some representation
of the driver as part of the full vehicle system model. Any system can
be considered to consist of three elements – the ‘plant’ (the item to be controlled),
the input to the plant and the output from the plant (Figure 6.41).
Inputs to the system (i.e. handwheel inputs) are referred to as ‘open loop’ or
‘closed loop’. An open loop steering input requires a time dependent rotation
to be applied to the part representing a steering column or handwheel in
the simulation model. In the absence of these bodies an equivalent translational
input can be applied to the joint connecting a rack part to the vehicle
body or chassis, assuming a suspension linkage modelling approach has
been used. Examples will be given here where the time dependent motion is
based on a predetermined function or equation to alter the steering inputs or
a series of measured inputs from a vehicle on the proving ground.
Any system can be considered to consist of three elements – the ‘plant’ (the
item to be controlled), the input to the plant and the output from the plant
(Figure 6.41).
When a closed-loop controller is added to the system, its goal is to allow
the input to the plant to be adjusted so as to produce the desired output. The
desired output is referred to as the ‘reference’ state; a difference between
the actual output and the reference is referred to as an ‘error’ state. The
goal of the control system is to drive the error to zero.
We can consider an example of a closed loop steering input that requires a
torque to be applied to the handwheel or steering column such that the
vehicle will follow a predetermined path during the simulation. A mechanism
must be modelled to measure the deviation of the vehicle from the path
and process this in a manner that feeds back to the applied steering torque.
Error
Input
Reference
Plant
Output
Gain
Fig. 6.41
grey
Feedback
An open-loop system, in black, is a subset of a closed-loop system, in