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Modelling and assembly of the full vehicle 367
As the simulation progresses the torque is constantly modi-fied based on
the observed path of the vehicle and the desired trajectory. Such an input is
referred to as closed loop since the response is observed and fed back to the
input, thus closing the control loop.
To return to the open-loop case, we can consider an example of an open
loop manoeuvre for a steering input where we want to ramp a steering input
of 90 degrees between 1 and 1.5 seconds of simulation time. Using an
MSC.ADAMS solver statement the function applied to the steering motion
would be:
FUNCTION STEP(TIME, 1, 0, 1.5, 90D)
In a similar manner if we wanted to apply a sinusoidal steering input with
an amplitude of 30 degrees and a frequency of 0.5 Hz we could use:
FUNCTION 30D * SIN(TIME*180D)
For the lane change manoeuvre described earlier the measured steering
wheel angles from a test vehicle can be extracted and input as a set of XY
pairs, which can be interpolated using a cubic spline fit. A time history plot
for the steering inputs is shown in Figure 6.42 for lane change manoeuvres
at 70 and 100 km/h.
By way of example the MSC.ADAMS statements which apply the steering
motion to the steering column to body revolute joint and the spline data are
shown in Table 6.3 for a 100 km/h lane change. The x values are points in
time and the y values are the steering inputs in degrees. In the absence of
measured data it is possible to construct an open loop single or double lane
change manoeuvre using a combination of nested arithmetic IF functions
with embedded step functions with some planning and care over syntax.
Note that for a fixed steering input a change in vehicle configuration will
produce a change in response so that the vehicle fails to follow a path.
For a closed loop steering manoeuvre a torque is applied to the steering
column, or a force to the steering rack if the column is not modelled, that
will vary during the simulation so as to maintain the vehicle on a predefined
120.0
STEERING INPUT – LANE CHANGE MANOEUVRE
Steering wheel angle (deg)
80.0
40.0
0.0
40.0
80.0
120.0
0.0
1.0
2.0
3.0
4.0 5.0
Time (s)
Fig. 6.42 Steering input for the lane change manoeuvre at 70 km/h (dashed
line) and 100 km/h (solid line)