ABS 2S BRAKING SYSTEM

The aim of this system is to modulate the braking system preventing the wheels from locking regardless of the road conditions.

As we are already aware, if one or more of the wheels lock with a subsequent loss in road holding, it becomes much more difficult to keep the vehicle under control and stopping distances are increased. The ABS system eliminates these problems allowing all drivers, even the less expert ones, to react quickly and decisively even when faced with the most dangerous conditions.

THEORY OF QPERATION BEHIND ABS

The system consists of:

• four magnetic inductive sensors which register the wheel speed; an integrated electronic and hydraulic control unit which houses the following components;
  • electronic control unit ,
  • four solenoid valves ,
  • solenoid valve relay;
  • pump control relay;
• self-diagnostic connector; brake switch (which also activates the stop lights) that signals to the system the state of braking.

If the driver brakes just to slow down or even if he applies the brakes harder, the system will not intervene as long as the skid coefficient ( = difference between the vehicle speed and the wheel peripheral speed, expressed as a percentage) remains within the accepted limits.

If, on the other hand, one or more of the wheels decelerates beyond a certain value, which on the basis of complex calculations is taken to be the value at which the wheels will start to lock, the ABS system is automatically activated. The system is divided into three cyclic stages:

• Maintains pressure;
• Releases pressure;
• Increases pressure.

To maintain the pressure, the electronic control unit acts on the solenoid valve of the wheel which is starting to lock and interrupts the flow of brake fluid between the pump and the caliper; in this way the pressure is maintained. At this point another command is sent to the solenoid valve affected which starts to reduce the pressure. At the same time the fluid bled from the brakes is recovered by the scavenge pump and a series of small accumulators which take it back upstream the solenoid valve.

The pressure reduction stage will continue until the inductive sensors signal an increase in wheel speed which is enough to bring the skid coefficient back within the prescribed limits. Once this threshold, established by the control unit on the basis of the vehicle's own properties, has been exceeded the reduction stage is interrupted and pressure is increased once again.

The brake fluid, recovered during the reduction phase, can be sent back to the brake caliper through a series of load impulses supplied by the solenoid valve which tend to decelerate the wheel again to a pre-established value. By repeating these three stages cyclically, the brake pressure can be modulated in order to keep the skid value within optimal limits. The ABS system is automatically cut out by releasing the brake pedal or when the vehicle speed falls below 3 MPH thus allowing the wheels to lock completely.

It should be noted that unlike the front wheels, the rear wheels are controlled by the SELECT LOW technique. This means that the command is sentto the rearwheel solenoid valve synchronously and is determined by the wheel which tends to lock the most. Although this command may prevent one of the rear wheels from operating at full efficiency, it has no detrimental effect on vehicle stability, steering and braking.