Anti-lock Brake System

History of anti lock brake system:

     ABS was first developed for aircraft use in 1929 by the French automobile and aircraft pioneer Gabriel Voisin, as threshold braking on airplanes is nearly impossible.

These systems use a flywheel and valve attached to a hydraulic line that feeds the brake cylinders. The flywheel is attached to a drum that runs at the same speed as the wheel. In normal braking, the drum and flywheel should spin at the same speed. However, if a wheel were to slow down, then the drum would do the same, leaving the flywheel spinning at a faster rate. 

      In testing, a 30% improvement in braking performance was noted, because the pilots immediately applied full brakes instead of slowly increasing pressure in order to find the skid point. An additional benefit was the elimination of burned or burst tires.

- In 1972, four wheel drive Triumph 2500 Estates were fitted with Mullard electronic systems as standard. Such cars were very rare however and very few survive today.

-  In 1985 the Ford Scorpio was introduced to European market with a Teves electronic system throughout the range as standard. For this the model was awarded the coveted European Car of the Year Award in 1986.

- In 1988, BMW introduced the first motorcycle with an electronic-hydraulic ABS: the BMW K100. 

- Honda followed suit in 1992 with the launch of its first motorcycle ABS on theST1100 Pan European.

- In 2007, Suzuki launched its GSF1200SA (Bandit) with an ABS. In 2005, Harley-Davidson began offering ABS as an option for police bikes.

Effectiveness:

Braking distance from 80-0 km/h: locked wheels ABS
dry pavement -  45 m 32 m                                                                                    snow         -  53 m 64 m
     ice            - 255 m 404 m

Components:

Speed sensors:

     A speed sensor is used to determine the acceleration or deceleration of the wheel. These sensors use a magnet and a coil of wire to generate a signal. The rotation of the wheel or differential induces a magnetic field around the sensor. The fluctuations of this magnetic field generate a voltage into the sensor. Since the voltage inducted on the sensor is a result of the rotating wheel, this sensor can become inaccurate at slow speeds. The slower rotation of the wheel can cause inaccurate fluctuations in the magnetic field and thus cause inaccurate readings to the controller.

Valves:

     There is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions:

In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.

    In position two, the valve blocks the line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder.

In position three, the valve releases some of the pressure from the brake.

The majority of problems with the valve system occur due to clogged valves. When a valve is clogged it is unable to open, close, or change position. An inoperable valve will prevent the system from modulating the valves and controlling pressure supplied to the brakes.

Pump:

     The pump in the ABS is used to restore the pressure to the hydraulic brakes after the valves have released it. A signal from the controller will release the valve at the detection of wheel slip. After a valve release the pressure supplied from the user, the pump is used to restore a desired amount of pressure to the braking system. The controller will modulate the pumps status in order to provide the desired amount of pressure and reduce slipping.

Controller:

    The controller is an ECU type unit in the car which receives information from each individual wheel speed sensor, in turn if a wheel loses traction the signal is sent to the controller, the controller will then limit the brake force (EBD) and activate the ABS modulator which actuates the braking valves on and off.

Anti Lock Brake ABS Operation:

     The anti-lock brake system or  the ABS  control unit helps to maintain control and directional stability of an automobile  in case of extreme braking circumstances. This is achieved by controlling the rotational  speed of every wheel by metering the brake line pressure at the time of extreme  braking. The system works on most types of road surfaces and decreases the risk  of an accident and severity of an impact. Research has shown that an anti-lock brake  system can decrease the chance of a vehicle accident by 18%. ABS brake systems were  introduced in the late seventies and have enjoyed great technological advances since  that time.

     Not only does ABS provide non-skid functionality but it also supports electronic stability control, brake assist, traction control, etc. Recently, additional sensors  have been added to the system, gyroscopic sensors and steering wheel angle sensors.  Both synchronize to match the direction of the car with the direction of the steering  wheel. The wheel angle sensor also helps the ABS system control the outer wheels  to have a more positive braking effect when compared to the inner wheels on the  curve.

Typical ABS Brake System:

     The ABS assembly is made up of a central electronic unit, four solenoid valves  and two or more electric hydraulic pumps. The function of the electric hydraulic  pump is to supply brake fluid pressure to the braking system by forcing hydraulic  pressure to a reservoir located in the accumulator. The four solenoid pressure valves  control brake fluid pressure for each individual wheel. During an ABS operation  event, one or more of the solenoid valves dump brake line pressure to a particular  wheel allowing it to start turning.

     There are several types of ABS systems; some of the most popular models are based  from the Bosch ABS Actuator (BAA), Nippon-Denso ABS Actuator (NAA) systems. The  basic design and assembly is the same, the Nippon-Denso system has a separate solenoid  relay pack and ABS computer, where as in the Bosch system both the components are  combined. Also, there is an immediate connection between the wheel speed sensors  and the ECU (engine control unit), these systems are connected by the CAN (Controller  Area Network) bus, this system communicates by sending multiple signals to multiple  devices at any given time

     Each wheel has a dedicated wheel speed sensor which sends an electronic signal to the ABS controller, from  this signal the rotational speed of the wheel can be determined. The location of  wheel speed sensor may vary, on most vehicles the front sensor is made into the  steering knuckle, near the front wheel hub  or outer CV joint.  In the rear of the vehicle the sensor rings are bolted to the rear driver assembly  on the ring gear or attached to the outer axle flange on each side. Some even use  the vehicles speedometer sensor VSS (vehicle speed sensor) to read rear wheel speed.

Common Problems and Fixes:

     Today's brake systems are reliable and easy to maintain, most systems are designed for easy service and brake pad replacement. If the brake pedal becomes hard to push, there can be a possibility of a broken vacuum supply line or a failed power brake booster. It is also very important to use the correct type of brake  fluid; otherwise the brake system can malfunction. This can lead to problems such  as spongy feeling pedal and the brake pedal travel becoming excessive. Only the  highest quality brake fluid must be used in a brake system and must never be reused  and must always be discarded immediately after the fluid storage container is opened  to avoid moisture contamination. When adding brake fluid to the master cylinder  care must be taken make sure brake fluid is not spilled on the paint of the car,  brake fluid is very corrosive and will destroy the paint.

     Squealing brakes is an indication of a problem with the brake system. Most of  the time a squealing noise can be a warning that the brake pads are worn down and  need to be replaced. Other causes can be overheated brake pads or rotors. Sometimes  rust can get impregnated on the brake lining material and cause grumbling or squeaking  noises. When cleaning brake components prior to service precautions must be taken.  Most brake systems contain microscopic fibers that are extremely hazardous to your  health. While cleaning with brake system cleaner there is an increased chance of  inhaling these fibers that accumulate on the brake components. Cleaning should be  done in a well ventilated area, use an air filter respirator if possible for best  results.

      When the ABS system detects a problem a fault code is stored in the ABS system control unit. When an error occurs the ABS warning light located on the dashboard flashes or stays on steadily. The ABS computer stores a diagnostic code until the malfunction has been repaired. To retrieve ABS trouble codes on older vehicles can be tricky because of the variation  of retrieval methods; in this case a car repair manual is needed. On newer vehicles a diagnostic information connector  is located under the dash on the driver's side; a trouble code scanner can be attached to retrieve ABS codes similar to engine  trouble codes.

     After the problem has been repaired codes should erase automatically with no procedure from the scan tool in most cases. (Real time system) when the vehicle  has been moved about 6 feet and the brake pedal has been pressed for five seconds  the codes should clear. When repairs have been completed the ABS, MIL (malfunction indicator lamp) should not be illuminated, if the ABS light remains on there is  still a problem, re-scan the ABS computer for further repairs.