Antilock Brakes / Traction Control Systems: Description and Operation
WARNING: Although the Antilock Brake System (ABS) provides benefits in an emergency in terms of stability, steerability and braking efficiency, it is not capable of defying the normal physical laws of motion. It remains the responsibility of the driver to suit his driving style and speed to road and climatic conditions, and the level of traffic encountered.
ABS Regulation:
PURPOSE
The task of the Antilock Brake System (ABS) is to prevent accidental wheel locking and thus maintain the vehicle's stability and steerability whenever the brakes are applied, even if the road surface is not consistent with effective braking.
Even the best of current conventional automobile brake systems cannot prevent unintentional wheel locking and skidding, for instance when a panic brake application is made and the driver exerts too much pressure on the brake pedal.
Driving Stability (Freedom From Skidding)
This involves maintaining the car's stability not only as brake pressure is gradually increased up to the wheel-lock limit, but also when brake pressure suddenly rises steeply, as in a panic brake application. It is assumed that stability is not lost for other reasons, for instance exceeding the maximum cornering speed.
Steerability (No Breaking Away From Chosen Line During Avoiding Action)
This means that even during a panic brake application the driver should still be able to steer around an obstruction or to apply the brakes while actually cornering, without affecting driving stability or the ability to steer the vehicle.
Optimum Braking Distances
A rolling wheel can transfer higher frictional forces than a locked (skidding) one so that with ABS the shortest possible braking distance under the conditions given is obtained.
ABS LAYOUT:
CONSTRUCTION
Components which make up ABS:
- Electronic Control Unit (Diagnosis Unit)
- The Hydraulic Pump Assembly
- The Pulse Generators (Speed Sensors With Gearwheel)
- Wiring Harness
With the four-wheel regulating system, with individual regulation at the front wheels and a "select-low" circuit for the rear axle with individual sensors at the rear wheels, the basically conventional brake system can be designed for optimum safety. The characteristics of the hydraulic dual circuit have not been altered and conform in every respect with standard-specification cars.
"Select-low" means that the regulating action is governed by whichever wheel is transmitting the least amount of power to the road surface.
This ensures that only the maximum available braking force at the wheel is used as a basis for determining brake action.
The other wheel is then approximately within the stable zone of the slip curve and can therefore accept greater lateral locating forces. This form of regulation was chosen for the rear axle because vehicle stability has to have the highest priority when the system's characteristics are chosen. Although in very exceptional circumstances, e.g. in snow, the stopping distance may be slightly longer, this has been accepted in view of the other advantages obtained.
ABS THEORY
Suitable signal sources for use as guide values are road speed, wheel acceleration or deceleration and wheelslip.
When a vehicle is braked or accelerated, frictional forces must be transmitted between the tires and the road surface. If a brake is applied to a rolling wheel a braking torque is developed, whereupon the tire distorts and slip occurs, so that the wheel rolls more slowly than the vehicle's road speed would lead one to expect.
This can be expressed in mathematical form "slip".
The wheel-slip graphs depend on road surface condition, the tires, vehicle speed, wheel load and possibly on slip angle when cornering. The diagrams also show specimen slip characteristics related to the road surface.
The rise from zero slip to maximum braking force is defined as the stable slip zone, with ABS regulation setting in close to the maximum end of this zone.
At high levels of wheel-slip - the unstable zone - the braking force coefficient drops, stable equilibrium is no longer present and the wheel soon locks and begins to skid.
Only in the special case of a snow-covered road is the braking force coefficient "slip" slightly higher, on account of the wedge of snow built up ahead of the tire.
However, the advantages of increased stability and steerability offered by the ABS undoubtedly outweigh this for all normal conditions.
Just as a given degree of peripheral wheel-slip is necessary whenever a braking force has to be transmitted, so each lateral locating force which the wheel is called upon to transmit or absorb causes a certain amount of lateral slip.
The main factor for braking regulation is the interaction of braking force (braking force coefficient "B") and lateral locating forces (lateral force coefficient "S").
Lateral force drops rapidly as peripheral wheel-slip increases, and that a locked wheel possesses no powers of location at all. The regulating range selected must therefore permit high braking forces together with good lateral locating forces.
ABS FUNCTION
When the ignition is switched "On" the ABS is activated and a test program checks the system. If the ABS warning light goes out, the system is fully operational.
Once the vehicle is in motion, speed sensors provide the necessary information of the tire/road situation, and transmit signals in proportion to wheel speed to the ABS control unit. After the car exceeds a speed of 12 km/h (7.5 mph), the ABS is capable of regulating brake applications. Below 6 km/h (3.5 mph) the regulating action ceases and does not resume operation again until the car moves at a speed higher than 12 km/h (7.5 mph).
Differentiation of the various speed signals enables wheel deceleration or acceleration to be identified. In addition, various logical criteria are applied to form the reference speeds from the individual speed signals.
A comparison then takes place between wheel speed and reference speed to obtain the slip signals by the ABS control unit.
Continuous comparison between wheel-slip, wheel acceleration and deceleration and the threshold values stored in the electronic memory enables the dynamic condition of each wheel to be monitored.
Pressure Build Up:
Pressure Hold:
Pressure Reduction:
Brake Pressure Regulation
If the retardation threshold and/or the slip threshold are exceeded (wheel lock), pressure regulation commences. The electronic control unit initially transmits the necessary signals to the hydraulic unit to maintain pressure in the brake circuits at the level the reached.
After this, if, necessary, pressure reduction or increase signals are transmitted. These regulating phases or regulating cycles are constantly repeated during the regulated brake application until the car comes to a standstill or until pedal pressure drops below the regulating system's cut-in point.
NOTE: Vehicle speed, wheel speed and wheel deceleration are the governing factors for the regulating cycle during a regulated brake application.
The regulating action of the anti-locking system takes place in fractions of a second. The driver will notice a pulsating effect through the brake pedal. At low speeds the noise of the hydraulic pump assembly in operation will also be heard.
These "feedback effects" from the system to the driver have been deliberately accepted by the vehicle manufacturer as a means of confirming that the ABS is in action and that the driver's speed or general approach is thus probably not in accordance with road conditions.
Safety Circuit And Monitoring Devices
In order to prevent the regulator from responding incorrectly, with potentially dangerous results, the ABS incorporates a safety circuit. The task of the safety circuit is to monitor the system before each journey and while in motion, to make sure that it remains operational.
BITE (Built-In Test Equipment) Test Cycle
This test cycle forms an active part of the safety circuit. On the one hand it checks operation of the two regulating modules by means of a predetermined test program, and on the other hand it transmits a test signal to ascertain that the built-in safety circuit is itself in working order.
The safety circuit and its monitoring devices check the following items:
- The presence of the general switching signal (cycle rhythm monitoring)
- The signal which switches the return-flow pump on and off, or the presence of this signal (return pump monitoring)
- Control of the valves
- The valve operating periods (logic monitoring). The maximum valve operating times are here monitored by a combination logic circuit; these maximum times are governed by physical considerations.
- In a 20-second rhythm, various items including the pump motor, control sensitivity signals, the test cycle, acceleration etc.
- Battery voltage (over-or under voltages at the electronic control unit).
If a fault is detected or one occurs when the system is in operation, the ABS monitor circuit shuts down the anti-locking function and thus ensures that conventional braking is not affected. An ABS warning light on the facia indicates to the driver that a fault has occurred or that the system has "failed safe".
Trailer Towing
When towing a trailer, it is recommended to make every effort to avoid entering the brake regulating zone, since the trailer's wheels may lock and render it unstable, so that the complete car and trailer outfit becomes difficult to control.