Anti lock breaking is a braking system used in cars to prevent wheel slippage/lockup upon braking. It enables the driver to still be able to turn the car whilst braking in order to possibly avoid an object, instead of the car losing traction and veering straight, ultimately causing a crash.
ABS is integrated in the normal brake system, in the sense it still uses the same disc brakes as a conventional non ABS car. The difference is when the brakes are applied hard in an ABS equipped car, the ABS causes the brake pressure to modulate up to 20 times per second, whereas in a non ABS equipped car the brake pressure is constant, therefore the wheels just lock up, and traction is lost.
ABS's main components:
1. Speed Sensors
- Speed sensors are used to detect how fast the wheels are spinning upon acceleration or deceleration. The sensors are usually reluctor type sensors using a magnetic pick up that sends a signal to the ABS controller/ECU in the form of a/c voltage
- They are usually placed behind the hubs, and although they have shields protecting them, they can still get corroded, or metal fillings can get stuck in them causing incorrect voltages
2. G - Sensor
- Monitors the rate of deceleration or lateral acceleration in order for the ECU to calculate the precise condition of the road
3. Actuators
- These are solenoids that are turned on or off by the ABS control relays. They control the hydraulic brake pressure from the master cylinder to the brake calipers in response to the ABS ecu's signals
4. ABS ECU
- This is the brains of the ABS, sending signals to the outputs such as the actuators, in response to the inputs like the speed/G sensors
5. ABS Warning Light
- This is used to show when the ABS is in operation, or whether it is malfunctioning in which it will blink displaying a fault code
Slip Tolerance
Slip tolerance refers to the amount of slip/skidding the car undergoes when its wheels loose traction. This is calculated by the speed and G sensors, sending inputs back to the ecu to adjust the actuators accordingly
ABS's actions can be seen in three steps:
1. Hold mode
This is initiated when wheel lock up is sensed by the ECU. Hydraulic pressure in the brakes is held at a constant by a pressure hold valve turned on and pressure reduction valve off (ensuring there is no brake fluid in the reservoir), until the wheel to vehicle speed is calculated, in which case 2 of the following steps will come into faze.
2. Pressure reduction mode
This is the reason for "pulsation" felt in the brake pedal, and activates the ABS warning light.
When the slip tolerance of the car exceeds 20 percent, the ECU will continue to activate the pressure hold valve, causing the flow of fluid from the master cylinder to be stopped. The pressure reduction valve will also then be activated causing brake fluid to flow into the reservoir, in which the brake pressure will decrease rapidly. At the same time, the ABS pump will be on, pumping fluid into the master cylinder.
3. Pressure increase mode
When the ECU senses slip tolerance above 8 percent in response to the speed sensor, brake pressure is increased by deactivating both the pressure hold valve and the pressure reduction valve.
Different types of ABS
1. Four channel
A four channel system incorporates a speed sensor for each wheel, in which braking to all individual wheels can be applied.
2. Three channel
A three channel system is much like the four channel, except the rear wheels share one speed sensor, usually located in the differential or transmission. An average of both wheels is calculated by the speed sensor. Using one sensor for the rear wheels means less complications, and cheaper production
3. One channel (rear-wheel ABS)
This is the most simple ABS system, consisting of just one speed sensor for the rear wheels, like a three channel, and no speed sensors for the front wheels. This is common place for vehicles that carry a heavy load, such as pick up trucks or vans.
Here's a good video by Bosch explaining ABS and its advantages:
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