 Anti-lock braking system 3
We are proceeding strictly according to the diagram here. You are driving towards the left vertical main line (y-axis) at a speed of 80 km/h. You have spotted an obstacle and after a certain reaction time you have fully
pressed the brake pedal. Its effect now begins exactly when you cross the main line.
The speed of the wheel in question here drops significantly more than the reference speed displayed above it, which you can safely equate with the driving speed of the truck. It is developed by the ABS control unit from the
wheel speeds of the wheels, omitting any sudden changes, and passed on to the speedometer unit, for example.
Below you can see how much the brake pressure has increased. The control unit interprets the strong change in wheel speed as too much slip and regulates down the brake pressure exactly at the first line, first stronger
and then significantly weaker. It then drops to a fairly constant value for a short time.
The whole action resulted in the wheel speed adapting back to the reference speed, albeit with a certain delay. The control unit has also finished its intervention. According to Wabco, the whole process took less than half a
second.
But it's no use because you didn't really notice it and you can't react that quickly. So you're still on the foot brake. As if the control unit had become more critical, this time it reacts to a comparatively smaller deviation from the
target speed than the first time (third auxiliary line).
However, it does not intervene so sharply in the brake pressure, but only achieves a certain pause and because the reduction in the wheel speed continues at the top, an attempt is made to reduce down the brake
pressure more, but with a noticeable delay. Almost 0.2 seconds pass before it is low enough and the wheel speed increases again.
Shortly after the fourth line, the time has come. While the brake pressure was still falling, the control unit has finished intervening. But the effect can be clearly seen above. The next time the wheel speed drops, the process
repeats itself. The control unit intervenes carefully again, but only causes the brake pressure to remain constant, before then intervening more strongly and for longer.
However, this is not always the case. If you take the decreasing reference speed into account, two wheel speeds that oscillate downwards by the same amount represent a reduction in the deflection. The length of the
more energetic reduction in the brake pressure is not always the same, as can be seen in the penultimate control phase of the control unit, and the brake pressure is significantly lower.
Finally, it is just clear that more action was needed here. Just look at the increase in slip at the top of the wheel speed. Here, the first stage of pressure reduction is very short and the second very long. The control unit
carries out both controls in less than 0.1 seconds.
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