Requirements of ABS
A good way of considering the operation of a complicated system is to ask: ‘what must the system be able to do?’ In other words, what are they requirements? These can be considered for ABS under the following headings.
In the event of the ABS system failing the conventional brakes must still operate to their full potential. In addition, a warning must be given to the driver. This is normally in the form of a simple warning light.
Maneuverability must be maintained
Good steering and road holding must continue when the ABS system is operating. This is arguably the key issue, as being able to swerve around a hazard whilst still braking hard is often the best course of action.
Immediate response must be available
Even over a short distance the system must react such as to make use of the best grip on the road. The response must be appropriate whether the driver applies the brakes gently or slams them on hard.
Normal driving and maneuvering should produce no reaction on the brake pedal. The stability and steering must be retained under all road conditions. The system must also adapt to braking hysteresis when the brakes are applied, released and then re-applied. Even if the wheels on one side are on dry tarmac and the other side on ice, the yaw (rotation about the vertical axis of the vehicle) of the vehicle must be kept to a minimum and only increase slowly in order to allow the driver to compensate
Under normal braking this is proportional to pedal pressure but under control of the ABS it can be reduced, held or allowed to increase.
This is the actual result of changes in brake pressure, in other words the wheel speed, which then allows acceleration, deceleration and slip to be determined.
Disturbances such as the vehicle load, the state of the road, tire condition and brake system condition. From the wheel speed sensors the ECU calculates the following.
There are a few variations between manufacturers involving a number of different components. For the majority of systems, however, there are three main components.
- Wheel speed sensors.
- Electronic control unit.
- Hydraulic modulator.
Hydraulic modulator The hydraulic modulator has three operating positions.
- Pressure build-up – brake line open to the master cylinder.
- Pressure reducing – brake line open to the accumulator.
- Pressure holding – brake line closed.
Wheel speed instability occurs frequently and at random on rough roads. Due to this instability, brake pressure tends to be reduced more than it is increased, during ABS operation. This could lead to loss of braking under certain conditions. Adaptation to the conditions is therefore necessary to overcome this problem. An increase in brake pressure is made easier during hard re-acceleration of the wheel after an unstable instant. With modern soft suspension systems the axle may be subject to vibration. This can cause superimposed signals on the wheel speed sensors.
A novel approach to ABS has been developed which uses springs and a motor to produce the brake pressure conditions of reducing, holding or increasing. The potential advantage of this technique is that the response is smooth rather than pulsed.