Antilock braking system & components

• ABS brake system
   Antilock braking system operation
• Principles of ABS braking
• ABS master cylinder
• Hydraulic control unit
• Wheel speed sensors 
• ABS electronic control unit

ABS brake system

ABS brakes

The antilock braking system is designed to prevent wheels locking or skidding, no matter how hard brakes are applied, or how slippery the road surface.

The primary components of the ABS braking system are:

• Electronic control unit (ECU)
• Hydraulic control unit or modulator
• Power booster & master cylinder assembly
• Wheel sensor unit

 

Electronic control unit (ECU)

The ECU is located inside the vehicle. It receives signals from the sensors in the circuit and controls the brake pressure at the road wheels according to the data analyzed by the Unit.

 

Hydraulic control unit or modulator

The location of the Hydraulic Control Unit, or Modulator, varies from manufacturer to manufacturer. Some locate it under the fender or hood.
It receives operating signals from the ECU to apply or release the brakes under ABS conditions.

 

Power booster & master cylinder assembly

The power booster and master cylinder assembly is mounted on the firewall and is activated when the driver pushes down on the brake pedal. It provides the power assistance required during braking.

 

Wheel sensor unit

The wheel sensor unit consists of a tooth rotor that rotates with the road wheels and a pick-up that is located in the wheel hub.

Antilock braking system operation

Applying brakes too hard, or on a slippery surface, can cause the wheels to lock. When wheels lock, steering control is lost and, in most cases, it produces longer stopping distances. The antilock braking system prevents wheels locking or skidding, no matter how hard brakes are applied, or how slippery the road surface. Steering stays under control and stopping distances are generally reduced.
It consists of a brake pedal, a master cylinder, wheel speed sensors, the electronic control unit or ECU, and the hydraulic control unit, also called a hydraulic modulator.
The wheel speed sensor consists of a notched or toothed rotor that rotates with each wheel, and a pickup. As the wheel turns, a small voltage pulse is induced into the pickup and sent to the electronic control unit. When the brakes are applied, the wheel's speed of rotation changes. This sends a new signal to the ECU.
If the control unit detects that a wheel might lock, it sends a signal to the hydraulic control unit. In a 3-channel system, the hydraulic control unit uses 3 solenoid valves to control brake pressure and prevent them locking.
The valves are in series with the brake master cylinder and the brake circuits. One operates for each of the front wheels and one controls both rear wheels. At the start of a journey, the ABS automatically checks itself. Any failure in the system lights up a warning light in the dash-panel.

History

The German firm of Robert Bosch GmbH had been developing anti-lock braking technology since the 1930s, but the first production cars using Bosch's electronic system became available in 1978. They first appeared in trucks and German limousines from Mercedes-Benz. Systems were later introduced on motorcycles.
Anti-lock braking systems were first developed for aircraft. An early system was Dunlop's Maxaret system, introduced in the 1950s and still in use on some aircraft models. This was a fully mechanical system. It saw limited automobile use in the 1960s in the Ferguson P99 racing car, the Jensen FF and the experimental all wheel drive Ford Zodiac, but saw no further use; the system proved expensive and in automobile use somewhat unreliable. A purely mechanical system developed and sold by Lucas Girling was factory-fitted to the Ford Fiesta Mk III. It was called the Stop Control System.

Operation

The anti-lock brake controller is also known as the CAB (Controller Anti-lock Brake).
A typical ABS is composed of a central electronic unit, four speed sensors (one for each wheel), and two or more hydraulic valves on the brake circuit. The electronic unit constantly monitors the rotation speed of each wheel. When it senses that one or more wheel is rotating slower than the others (a condition that will bring it to lock) it moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel. Applying brakes too hard, or on a slippery surface, can cause the wheels to lock. When wheels lock, steering control is lost and, in most cases, it produces longer stopping distances.
The antilock braking system prevents wheels locking or skidding, no matter how hard brakes are applied, or how slippery the road surface. Steering stays under control and stopping distances are generally reduced.
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It consists of a brake pedal, a master cylinder, wheel speed sensors, the electronic control unit or ECU, and the hydraulic control unit, also called a hydraulic modulator.
The wheel speed sensor consists of a notched or toothed rotor that rotates with each wheel, and a pickup. As the wheel turns, a small voltage pulse is induced into the pickup and sent to the electronic control unit. When the brakes are applied, the wheel’s speed of rotation changes. This sends a new signal to the ECU.
If the control unit detects that a wheel might lock, it sends a signal to the hydraulic control unit.
In a 3-channel system, the hydraulic control unit uses 3 solenoid valves to control brake pressure and prevent them locking.
The valves are in series with the brake master cylinder and the brake circuits. One operates for each of the front wheels and one controls both rear wheels.
At the start of a journey, the ABS automatically checks itself. Any failure in the system lights up a warning light in the dash-panel.
An anti-lock braking system (commonly known as ABS, from the German name "Antiblockiersystem" given to it by its inventors at Bosch) is a system on motor vehicles which prevents the wheels from locking while braking. The purpose of this is twofold: to allow the driver to maintain steering control and to shorten braking distances.

Effectiveness

On high-traction surfaces such as bitumen, whether wet or dry, most ABS-equipped cars are able to attain braking distances better (i.e. shorter) than those that would be possible without the benefit of ABS. A moderately-skilled driver without ABS might be able, through the use of cadence-braking, to match the performance of a novice driver with an ABS-equipped vehicle. However, for a significant number of drivers, ABS will improve their braking distances in a wide variety of conditions. The recommended technique for non-expert drivers in an ABS-equipped car, in a typical full-braking emergency, is to press the brake pedal as firmly as possible and, where appropriate, to steer around obstructions. In such situations, ABS will significantly reduce the chances of a skid and subsequent loss of control—particularly with heavy vehicles.
In gravel and snow, ABS tends to increase braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring. Some ABS controllers reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain control of the car rather than go into a skid—though loss of control remains more likely on soft surfaces like gravel or slippery surfaces like snow or ice.
When activated, the ABS causes the brake pedal to pulse noticeably. As most drivers rarely or never brake hard enough to cause brake lockup, and a significant number rarely bother to read the car's manual, this may not be discovered until an emergency. When drivers do encounter an emergency that causes them to brake hard and thus encounter this pulsing for the first time, many are believed to reduce pedal pressure and thus lengthen braking distances, contributing to a higher level of accidents than the superior emergency stopping capabilities of ABS would otherwise promise. Some manufacturers have therefore implemented "brake assist" systems that determine the driver is attempting a crash stop and maintain braking force in this situation. Nevertheless, ABS significantly improves safety and control for drivers in on-road situations if they know not to release the brakes when they feel the pulsing of ABS.
It is worth noting that the heavier a vehicle is, the more it will benefit from ABS. This is particularly true of vehicles with less-sophisticated hydraulic braking systems where fine control is not as easy as with the more developed braking systems. Conversely, lighter vehicles, especially sports cars with highly-developed braking systems without ABS can outbrake comparable vehicles even with ABS.

Traction control

The ABS equipment may also be used to implement traction control on acceleration of the vehicle. If, when accelerating, the tire loses traction with the ground, the ABS controller can detect the situation and apply the brakes to reduce the acceleration so that traction is regained. Manufacturers often offer this as a separately priced option even though the infrastructure is largely shared with ABS. More sophisticated versions of this can also control throttle levels and brakes simultaneously, leading to what is termed Electronic Stability Control or what Bosch terms the Electronic Stability Program (ESP).

Principles of ABS braking

The ABS controls braking forces by controlling the vehicle's brake system hydraulic pressure to prevent the road wheels from locking up during heavy braking forces. The braking force is controlled by a combination of factors. These include:

• Slip ratio - difference that occurs between the road wheel speed and the vehicle speed;
• Friction Coeffiecient - road surface condition;
• Road wheel speed; and
• Vehicle speed.

During normal braking, as the rotational speed of the wheel falls, no electric current flows from the ECU to the hydraulic unit. The solenoid valve is not energised. The brake master cylinder hydraulic pressure is applied to the brake unit, and the ABS is not involved. However, even though the ABS is passive during normal braking, its control module is constantly monitoring for rapid deceleration of any of the wheels.
If a wheel-speed sensor signals severe wheel deceleration - which means the wheel is likely to lock up - the ECU sends a current to the hydraulic unit. This energises the solenoid valve. The action of the valve isolates the brake circuit from the master cylinder. This stops the braking pressure at that wheel from rising, and keeps it constant.
If the sensors signal the wheel is still decelerating too rapidly, the ECU sends a larger current to the hydraulic unit. The armature moves even further and opens the valve. It opens a passage from the brake circuit. Brake fluid is sent from the brake circuit back to the master cylinder. Pressure in the brake caliper circuit is reduced so that the wheel is braked less heavily.
If the wheel sensors indicate that lowering the brake pressure is letting the wheel accelerate again, the ECU stops sending current to the hydraulic unit and de-energizes the solenoid valve. This lets the pressure increase, so that the wheel is again decelerated.
This cycle repeats itself about 4 to 6 times per second.
It is normal in an ABS for the valves in the hydraulic control unit to keep changing position as they change the brake pressure that’s being applied. These changes in position may cause rapid pulsations to be felt through the brake pedal.

ABS master cylinder

The master cylinder is connected to the brake pedal via a pushrod.
The ABS master cylinder is similar to the tandem master cylinder used in divided systems. It has a primary piston, and a secondary piston.
The secondary piston incorporates a centre valve. This controls the opening and closing of a supply port drilling in the piston. At rest, the supply port is open and connects the reservoir with the front brake circuits. The primary piston still has an inlet port, and a compensating port.
When the brake is applied, the primary piston moves, which closes its compensating port. Fluid pressure in the primary circuit rises. It acts with the primary piston spring, to move the secondary piston forward, closing the centre valve. Pressure builds in the secondary circuit. Pressure keeps building in both circuits, and applies the brakes in both circuits.
If the secondary circuit fails, the secondary piston is forced to the end of the cylinder. When it reaches the end, pressure builds in the primary circuit.
If the primary circuit fails, the primary piston contacts the secondary piston, and pushes it to operate the secondary circuit.
In normal operation when the pedal is released, the springs in the master cylinder push the pistons back more quickly than the fluid can flow back from the wheel brake units. This creates a low pressure area in front of each piston. Such low-pressures can cause air to be drawn into the system.
To prevent this, there are recuperating grooves in the primary piston and the seal. Fluid at atmospheric pressure flows through the inlet port, and past these grooves. When the primary piston is returned fully, any extra fluid coming back from the brake units displaces fluid into the reservoir, through the compensating port.
In the secondary circuit, fluid also at atmospheric pressure is forced back into the inlet port. The inlet port connects with the supply port drilling in the piston. Any difference in pressure lifts the centre valve from its seat, and lets fluid enter the chamber ahead of the secondary seal, and prevents low pressures developing.
When the piston has returned to the “rest” position, the seal is pulled off its seat by the action of the link and spring. This lets fluid still returning from the wheel units displace fluid back to the reservoir.
If braking conditions are such that the hydraulic modulator must return brake fluid to the master cylinder, then, for the front brake circuits, fluid is returned to the front section. This forces the secondary piston back, against the force of the primary piston spring, and the rear brake pressure. If enough fluid returns, the centre valve opens, and allows fluid to return to the reservoir.
If fluid is returned from the rear brake circuit, the secondary and primary pistons tend to be forced apart.
The amount of fluid that returns to the master cylinder is determined by the degree of anti-lock braking control. With approximately 4 to 6 ABS control cycles per second, the rapid changes in pressure cause pulsations that can be felt by the driver at the brake pedal.

Hydraulic control unit

The ABS Control Module or ECU, sends commands in the form of electrical signals to the hydraulic control unit. This unit executes them, using 3 solenoid valves, connected in series with the master cylinder and the brake circuits - 1 valve for each front wheel hydraulic circuit, and 1 for both of the rear wheels’ hydraulic circuit.
This is a simplified diagram of 1 solenoid valve operating on just 1 wheel. In normal, non-ABS braking, brake pedal force is transmitted to the master cylinder, then through the solenoid valve to the brake unit at the wheel. The signals from the wheel speed sensor show no tendency for the wheel to lock up, so the ECU is not sending any control current to the solenoid coil. The solenoid valve is not energised. And the hydraulic pressure from the master cylinder is supplied to the brake unit at the wheel.
When the control unit detects any lock-up tendency, perhaps from too-rapid wheel deceleration, it sends a command current to the solenoid coil. This causes the armature and valve to move upward, and isolate the brake circuit from the master cylinder. That keeps the pressure between the solenoid and the brake circuit constant - whether or not the master cylinder hydraulic pressure rises.
If the sensors signal continuing excessive wheel deceleration, the Control Module sends a larger current to the solenoid valve. This lowers the braking pressure by moving the armature up further, opening a passage from the brake circuit to an accumulator - a temporary reservoir for any brake fluid that flows out of the wheel brake cylinders because of the fall in pressure. A return pump sends this brake fluid back to the master cylinder.
If the sensors then signal the lower pressure has let the wheel speed up, the ECU stops all command current, which de-energises the solenoid valve. The pressure rises, and the wheel is again slowed down.
No matter the phase of operation, pressure in the circuit can never rise above master cylinder pressure.

Input and Output Signals - ABS Operation

The ABS system is electronically controlled by the system's Electronic Control Unit (ECU) or sometimes referred to as the Electronic Control Module (ECM). For the system to control the vehicle's braking capacity, the system must receive various signals and react to these values when applying the brakes. The microprocessor analyses the signals and then sends an electrical signal to the appropriate solenoid and modulator - hydraulic control unit and to the pump motor.
Inputs are from:

• Road wheel sensors - these inform the ECu of the individual road wheel speed. This allows identification of any road wheel that is rotating too slow; too fast during a heavy brake application.
• Brake light switch - this informs the ECU that the driver has applied the brakes. The ABS then monitors the road wheel speed during the brake application.
• Ignition switch - informs the ECU that the vehicle is about to be started. This signal instigates a self-diagnostic process to commence. In addition, it is the power supply for the ABS system.
• Alternator - informs the ECU that the engine is running as part of its self-diagnostic process.

The outputs controls:

• Solenoids and Hydraulic Control Unit - varies the brake application pressures in the applicable channel to control the road wheel speed to the most effective braking rotational speed.
• Pump Motor - operates the hydraulic pump that supplies pressure to the accumulator; or to return fluid to the reservoir. This is dependent of the type of ABS system used.
• ABS Warning Light - alerts the driver that the self-diagnostic system has found the ABS system to be functioning correctly. The dash warning light will go out. If the system detects a malfunction in the system, the dash light will be illuminated. This will cause the system to revert back to non-ABS assisted braking.
• Check Connector - diagnostic output connector.

Wheel speed sensors

A wheel sensor consists of a toothed rotor that rotates with the wheels, and a pickup. As each tooth of the rotor passes the pickup, a small voltage is induced in the pickup.
These pulses are sent as input signals to the electronic control unit which processes them, to operate the hydraulic control unit.

ABS electronic control unit

The electronic control unit receives signals from different sources. A switch at the brake pedal provides a brake-operating signal. Another in the ignition system signals the engine is operating. This sets off the automatic check the ABS conducts every time the engine starts.
Another input is from the wheel speed sensors. These signals are used to control the hydraulic control unit and anticipate wheel lock. If a wheel starts to lock, the electronic control unit operates the solenoid valves to reduce hydraulic pressure appropriately