Anti lock Braking System Description

Goals of Anti Lock braking

Braking stability (freedom from wheel lock induced 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. The ABS system will not prevent skidding if stability is lost for reasons other than wheel lock, such as exceeding the maximum cornering speed.

Steerability ( no break away from chosen line during avoidance action )

This means that even during a panic brake application the driver will ordinarily 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.

Note, however, that the system cannot prevent the front or rear end from breaking away during emergency avoidance action because of such factors unrelated to wheel lock such as exceeding the maximum cornering speed.

Optimum Braking Distances

It is a well known fact that a rolling wheel can transfer higher frictional forces than a locked (skidding) one so that, all other things being equal , a car equipped with ABS should achieve a shorter braking distance than the same car without ABS would in a locked wheel stop.

To sum up, the fundamental task of the anti locking system is to prevent accidental wheel locking , thus, help to maintain the vehicles stability and steerability whenever the brakes are applied.

In order to achieve this objective, the ABS has to operate extremely quickly and accurately. This can only be ensured by recourse to a complex electronic control system. The input signals needed to permit the electronics to regulate operation of the brakes are derived from changes in the speed of wheel rotation and the degree of wheel slip.

ABS utilizes a 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 characteristics of the hydraulic dual circuit have not been altered and conform in every respect with non ABS equipped cars.

"Select low" refers to the regulating action that is governed by whichever wheel is closer to locking up or skidding. This ensures that the lower available braking force at the wheel is used as a basis for determining brake action to prevent individual rear wheel lock up. The other wheel, therefore, can then 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.

Theory of ABS operation

Suitable signal sources for use as guide values are road speed, wheel acceleration or deceleration and wheel slip.

When a vehicle is braked or accelerated, frictional forces must be transmitted between the tires and the road surface. If a brake effort 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 a mathematical form

µ = VF - VR   x 100 (%)
          VF

Where:
VF = Vehicle speed
VR = Peripheral speed of wheel
µ = Slip

ABS operation

A pulse generator ( sensor or speed detector and gear wheel ) provides the necessary information on the tire / road situation, and transmits its signals in proportion to wheel speed to the central electronic control unit. This processes the signals from the wheels and decides whether and to what extent the hydraulic unit should begin to influence the normal braking pressure build up.

The signals from the speed detector (sensor) are transmitted to the electronic computer. 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.

Continuous comparison between wheel slip, wheel acceleration and deceleration and the threshold values are stored in the electronic memory enables the dynamic condition of each wheel to be monitored.

If the retardation threshold and/ or the slip threshold are exceeded, 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 then reached. After this, if necessary , pressure reduction or pressure increase signals are transmitted.

The hydraulic unit is supplied with three kinds of instruction signals from the electronic control unit. Pressure retention, pressure reduction and pressure increase. 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.

Vehicle speed, wheel speed and wheel deceleration are the governing factors for the regulating cycle during a regulated brake application.

Driving cars equipped with ABS

As described in the section ABS Operation, the ABS warning light lights up when the ignition is turned ON and should go out when the engine starts.

Then the ABS is activated and the BITE program begins to operate. If the ABS warning light goes out, the system is fully operational.

Once the car exceeds a speed of 7.5 mph (12 km/h), The ABS is capable of regulating brake applications. Below 4 mph (6km/h) the regulating action ceases. The anti locking system does not start to operate again until the car has again begun to move at a speed higher than 7.5 mph (12 km/h).

Important

The sole purpose of the ABS is to assist the driver when braking in an emergency. It increases active safety and helps avoid accidents and collisions.

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 BMW as a means of confirming that the ABS is in action.

Electronic control unit

A multi channel electronic computer receives the electrical signals for the pulse generators. These signals are proportional to the speed at which the wheels are rotating. From them the computer derives the acceleration, deceleration and slip values. Logic circuits then integrate these values and generate control instructions for the electromagnetic valves in the hydraulic pump assembly.

Signal processing in the computer determines the regulation action of the system. Complex integrated circuits of the digital type are used to ensure the necessary high level of operating reliability and narrow tolerances.

The electronic control unit can be divided into three functional zones

Signal processing has the task of accepting the wheel speeds as they arrive from the pulse generators and deriving from them the regulating values such as peripheral deceleration or acceleration at the wheel and the slip information in a form suitable for the logic circuits.

The logic circuits which receive the signals as stated above, generate the actual adjustment instructions such as pressure build up, pressure hold or pressure reduction, which are passed to the solenoid valves on a hydraulic unit.

Faults (e.g. deriving form the pulse generators) which can arise as a result of normal manufacturing tolerances are filtered out before processing, before they pass to the amplifier stage.

The safety circuit is used to monitor the system before each use 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 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 :

If an error is detected, the error memory is activated, the ABS warning light comes on, the ABS is rendered non operative and any braking which is necessary takes place by conventional means.

Hydraulic pump assembly

The hydraulic pump assembly is located inside the engine compartment and either reduces the brake fluid pressure in the wheel brake calipers during the regulating cycle, or keeps it constant as appropriate. The hydraulic pump assembly cannot by itself raise raking pressure, so that as a result the pressure which the brake master cylinder has built up cannot be exceeded. (Please not this is only relevant to the systems used on older cars, such as the E30. New systems include pressure pumps to allow individual activation of brakes.)

The hydraulic unit consists primarily of four 3 way solenoid valves( one for each wheel brake cylinder , rear brakes count as one ), two reservoirs ( one for each brake circuit) and the return flow pump. When the brake application starts, the solenoid valves are de energized and the brake fluid can flow from master cylinder to calipers.

To built up pressure, the connection between master cylinder and caliper is re opened. Since the master cylinder is still at increased pressure at this stage, the pressure in the caliper also builds up.

As pressure builds up and the regulating action commences, the valve switches to its central position - the pressure hold phase. This blocks the passage from the brake master cylinder to the calipers, so that effective pressure at the wheel brake cylinders cannot rise any higher

In the subsequent pressure reduction phase the valve shifts to its third position. The connection between master cylinder and caliper remains interrupted, and brake fluid is now permitted to escape from the caliper to the reservoir. The return flow pump which starts as soon as the regulating action is initiated delivers this fluid back to the master cylinder.

Pulse Generators.

Rotating movement of the road wheels is detected by inductive sensors and the resulting electrical signal is transmitted to the electronic control unit.

For the front axle the signal transmitters are arranged axially and require no adjustment. They are installed in the stub axles, with the gear wheel ( which generates the actual impulses) pressed into the front axle hub

At the rear axle, the sensors are attached to the wheel hubs between the two wheel bearings and the pulse generating gear wheel is integrated into the spacing sleeve used on the wheel bearings.

The speed detectors consist primarily of a magnetic core and coil. The pole is surrounded by a magnetic field  As the wheel revolves, the teeth of the gear wheel move through the magnetic field. This varies the magnetic flux and induces a voltage in the coil of the speed detector.

The frequency of the pulses generated by the successive teeth is proportional to the speed of the rotation of the road wheel

Notes on maintenance and repair

The anti locking brake system is fundamentally maintenance free. It is monitored continuously by the built in safety circuit and the monitoring circuits. If a fault is detected, the ABS warning light will come on and the ABS itself becomes non operational.

If work is performed on the brake system, for instance, renewing the brake pads or calipers, brake discs, hoses or the tandem master cylinder with brake booster servo, or if the parking brake has to be repaired, it is sufficient afterwards to simply check that the ABS is operating.

Caution:

Notes

Various methods of brake pressure regulation are feasible, so that anti locking systems differ according to the methods or their combinations used and the equipment thus installed.

A fundamental distinction is made between anti lock devices for the rear axle alone and those for all four wheels. In addition, the regulating system can operate on both brakes of each axle together or separately, or both methods con be combined. Furthermore, axle regulation can employ the "select low" principle, in which the wheel with the poorer level of traction or transmission of forces determines the cut in point, or alternatively , the "select high" principle, in which the signal form the wheel with the higher level of traction is adopted.

As stated earlier , E30's use individual wheel control at the front wheels and the "select low" principle at the rear axle, with a detector in each rear wheel. "Select low" is considered to offer better stability.

Special note about 325iX vehicles

Engine braking torque is reduced during low friction / poor traction ABS operation and at road speeds above 15 mph. On cue from the ABS control unit , the Motronic control unit (DME) will position the idle control valve to limit engine braking to an acceptable level. This info is transferred from Pin 3 of the ABS control unit to pin 50 of the DME.

E30 M3 Repair Manual V2.7.  Copyright ©1999-2001 Koala Motorsport & Brett Anderson