What Is A Car Antilock Braking System And How It Works

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What is the ABS System

The anti-lock braking system works on a simple theory where a skidding wheel – where the contact patch on the tyre slides relative to the road – offers less traction as compared to a non-skidding wheel. It often happens that when you are stuck on ice but the wheels are spinning, you know you don’t have any traction at all and that happens because the contact patch on tyre is sliding relative to the ice instead of the road. Here the anti-lock brakes are beneficial in two major ways: one is that you will stop faster than usual brakes and the other is that it will enable you to steer while you are stopping.

The main components of ABS System

The ABS system consists of four main components:

  • Speed sensors
  • Pump
  • Valves
  • Controller

­Now let’s have a look at what they are and how they work so we may have a better understanding of the system.

Speed Sensors

As the humans use their senses to know what to do and when to do, almost same law works for the cars. For the anti-lock braking system to know when the wheel is about to lock up, it needs some way or sense. The speed sensors are used to provide this information; they are normally located at each wheel or sometimes in some cases, in the differential.

Valves

For each ABS controlled brake, there is a brake line which has valve on it. The position of the valve differs according to the systems but there are three common positions:

  • Valve’s first position is open where the pressure comes from the master cylinder and passes right through to the brake.
  • Valve’s second position is called ‘block’ where the blocked line isolates the brake from the master cylinder. Due to the blockage, the pressure does not rise and the driver has to push the brakes even harder.
  • In third position, some of the pressure from the brake is released through valve.

Pump

Valves make it possible to release the pressure from the brakes, once it’s done, there is a need to put that pressure back in – that’s what the pump is for, it puts the pressure back up in a line when the valves reduce it.

Controller

As the name defines, controller is a computer in the car. Its prime objective is to watch the speed sensors and control the valves accordingly.

How the ABS Work

The ABS systems have so many control algorithms and a lot of different variations. Here we’re going to have a look at the simplest system and its operation.

First of all, the controller is responsible of monitoring the speed sensors at all times. What it does is that it looks for the decelerations in the wheel whether they are out of the ordinary. As a wheel gets to lock up, a rapid deceleration is noticed and it stops the car in less than a second where normal cars take about five seconds to stop from a 60-0 mph, and that’s under ideal conditions.

The ABS controller is very well aware of that such a rapid deceleration is almost impossible, so what it does is, that it reduces the pressure to that specific brake until an acceleration is seen; then the pressure is increased until again the deceleration is seen. But all this increase and decrease happens so fast that you merely notice any significant change in the speed. This helps the tyre to slow down at the same pace as the car does. In this scenario the brakes will keep the tyres near to the point where they will start to lock up, enabling the system to have maximum braking power without much of a fuss.

One thing you will feel by pushing the brake pedal is the pulsing while the brakes are operating, it happens when the valves open and close rapidly – even in some system this cycle can be up to 15 times per second.

What are the types of Anti-Lock Brakes

Different schemes are used in anti-lock braking systems highly dependent on the brakes type in use. Here we will consider the number of channels – individually how many valves are controlled – and how many speed sensors are used.

Four-channel, four-sensor ABS

Four-channel, four-sensor ABS is the best scheme so far. Each of the four wheels has its own speed sensor and separate valve on them. The best thing about this scheme is that controller individually monitors each wheel to insure maximum braking force is achieved.

Three-channel, three-sensor ABS

The three-channel, three-sensor ABS scheme is mostly applied on pickup trucks with four-wheel ABS where each of the front wheels has a valve and speed sensor and one sensor and one valve for both rear wheels. The rear axle hosts the speed sensor for the rear wheels.

Both of the front wheels are controlled individually for maximum braking force whereas the rear wheels are bother monitored together. Sometimes it happens that one of the rear wheels locks up thus cause reduced brake effectiveness.

One-channel, one-sensor ABS

Pickup trucks with rear-wheel ABS is the major application of this scheme. With one speed sensor placed in the rear axle and one valve to control both rear wheels, this scheme works exactly the same as of the three-channel’ rear end.

Both of the rear wheels starts to lock up even before the ABS kicks in, and both of the rear wheels are monitored together. This system is also prone to lock only one wheel thus reducing the effectiveness of the brake.

One brake line going to both rear wheels through a T-fitting is the main identification of this scheme. You can also spot the speed sensor near the differential on the rear axle housing along with an electric connection.


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