Studies of braking behaviour in emergency situations show that the majority of drivers apply too little force to the brake pedal. As a result, valuable stopping distance is “wasted” because the maximum theoretically possible brake force is not achieved.

The aim of HBA is to increase the brake pressure during an emergency braking manoeuvre to such an extent that as short a stopping distance as possible is achieved while preserving the steerability

of the vehicle.
HBA is an expanded version of ESC and is integrated on all current Audi models. It cannot be deactivated by the driver for safety reasons.

Sequence of an HBA-controlled braking manoeuvre

(driver is braking).
In an emergency braking situation, the driver depresses the brake pedal at a much higher speed and with much greater force than during an anticipatory braking manoeuvre (comfort braking).  The consequence is a sharp rise in brake pressure. During a comfort braking manoeuvre the rate of pressure build-up is within a range of approximately 30-60 bar/s,

whereas the corresponding figure in an emergency braking situation is a multiple of this value. The control logic recognises the emergency braking situation by evaluating the rate of pressure rise. The pressure sensors integrated in the ESC hydraulic unit supply this information. The subsequent control operation consists of three phases.

Phase 1

A defined limit for the rate of brake pressure build-up initiated by the driver is exceeded and the emergency braking situation is recognised. Active build-up of brake pressure commences.

For this purpose, the high-pressure control valves and the changeover valves are activated. The recirculating pump is activated and draws in brake fluid through the open high-pressure control valves. The brake pressure applied by the driver is further increased by the active build-up of brake pressure at all four wheel brakes. The objective is to initiate ABS-controlled braking as quickly as possible.

Phase 2

When the ABS deploys, the vehicle is decelerated at the best theoretically possible rate of brake deceleration while preserving full steerability. ABS-controlled braking is provided by the “brake pressure buildup”, “brake pressure holding” and “brake pressure reduction” functions.

In each brake circuit the wheel with the lower brake pressure is controlled by the assigned intake valve, while the wheel with the higher brake pressure is controlled by the assigned changeover valve.

In the example given here, the brake pressure in the front right wheel brake is higher than in the rear left wheel brake. The front right brake pressure should not be increased any further by the “brake pressure holding” function, but the rear left brake pressure can continue to increase. The corresponding intake valve for the front right wheel is activated, thereby disconnecting the pump from the wheel brake. Pressure can be further increased at the rear left wheel via the open intake valve. To maintain a constant brake pressure, the corresponding intake valve is also closed.

Phase 3

The driver reduces the pressure on the brake pedal. This is detected by evaluating the pressure signal from the brake pressure sensor, and the HBA control operation is cancelled.

HBA in vehicles with Adaptive Cruise Control (ACC)

In vehicles with ACC the traffic situation ahead of the ACC vehicle is “observed” continuously by the radar sensors. This is also the case even if ACC has not been activated by the driver. If a situation with a higher accident hazard potential is detected, the conditions for activation of HBA are changed.

In this case, the function is activated earlier, i.e. at a lower rate of brake pressure build-up. This more sensitive deployment strategy allows even shorter stopping distances in emergency braking siuations.