train wheels. WSPER also avoids having
to spend hours setting up the paper tape
needed for low adhesion tests.
As the simulations ran, the noise of
compressed air venting from the brake
components in the equipment room
filtered through the door, bringing back
memories of Rail Engineer’s visit to RIDC
Melton, reported in issue 157 (November
2017), where variable-rate sanders were
being tested in low adhesion conditions
on a real train!
The results of the tests are shown in
figure 1.The sheer complexity, both of the
simulation and of brake control, is
illustrated by comparing test 1 with test- A screen shot from test 1 is shown
in figure 2. This shows a velocity:time
plot of an intermediate car on a
London Underground S Stock train with
electrodynamic and friction brakes on
clean dry rails with a brake demand of
0.75m/s^2. The plot shows the velocity
profile of each of the four axles, but, as
there is no wheel slide, these profiles are
superimposed on the actual velocity.
In comparison, test 4 illustrates a much
more complex situation, representing a
0.75m/s^2 brake demand on low adhesion.S stock is a seven-car or eight-car train,
with sanders on only two of the cars
(the first sander is in front of the trailing
bogie on car 2). As not all cars have any
or full benefit of sanders, it is necessary
to consider each car individually in order
to gain a picture of the behaviour of the
train.
Four plots are shown in figure 3 - from
top left to bottom right - a driving
car with no sander benefit, the first
intermediate car where only one bogie
benefits from the sander, the second
intermediate car with a sander and
finally the third intermediate car with no
sanders. It is immediately obvious that
the behaviour of the four wheelsets on
each car is different.
An example is illustrated in figure 4, an
enlargement of the bottom right plot on
figure 3 and is described in more detail.
The target velocity, with a slope of
0.75m/s2, is a straight orange line.
The actual speed of the train is the
white line above it, showing that the
train speed is exceeding the target
velocity.
Four “wiggly” lines in cyan, red,
green and yellow show individual wheel
velocity. As the adhesion is poor, the
wheels immediately slow relative to the
target velocity, braking more quickly
than the train and beginning to slip,
provoking WSP activity.
Because of the poor adhesion, the
actual velocity (white line) is reducing
more slowly than the target velocity.
At around four seconds after brake
initiation, sanding is initiated (point [a]
on the chart) and the slipping wheels
almost immediately resume nearly actual
train speed.
By about seven seconds, the actual
velocity, while higher than target
velocity, is decreasing at the same rateService brake, 7% g demand, level track- Dry rails
- Low adhesion
- Low adhesion + sand
- Low adhesion + sand + ATO (target
stopping distance based on nominal stopping
distance for 7% g demand on dry rail)
Stopping distance (m)369.
462
421
369.Sand used (kg)
For each sander0, 1.
0.66, 1.Figure 1 - simulated stopping distance against brake demand and adhesion.Figure 2 - Braking in dry conditions.1212 FE ATURE