Design of anti-lock braking system based on Simulink

1. Introduction

An anti-lock braking system (ABS) is an automotive safety system that allows the wheels of a motor vehicle to maintain traction contact with the road surface based on driver input when braking, preventing the wheels from locking and avoiding uncontrolled skidding. Anti-lock braking systems generally provide improved vehicle control and reduce stopping distances on dry and slippery roads. The system modulates brake line pressure independently of pedal force to bring wheel speed back into the range of slip levels required for optimal braking performance.

2. Composition of ABS system

The main components of the ABS braking system include:

(1) Electronic Control Unit (ECU)

It receives signals from sensors in the circuit and controls the brakes. The ECU assists the vehicle operator by regulating wheel slip to prevent wheel locking.

(2) Hydraulic control unit or regulator

It receives an operating signal from the ECU to apply or release the brakes under ABS conditions. It executes the command using three solenoid valves connected in series with the master cylinder and brake circuit - one valve for each front wheel hydraulic circuit and one valve for the two rear wheels. Therefore, the brakes can be activated by controlling the hydraulic pressure.

(3) Power booster and master pump assembly

Activated when the driver depresses the brake pedal. The master cylinder converts the applied pedal force into hydraulic pressure, which is delivered to all four wheels simultaneously. It also provides the power assistance required when braking.

(4) Wheel sensor unit

The speed sensor consists of a magnet wrapped in a coil and a toothed sensor ring. The electric field generated by the contact between the magnet and the toothed ring produces an AC voltage. The frequency of the voltage is proportional to the wheel speed. It monitors the wheel speed and transmits this data to the ABS control module.

3. Working principle of ABS

How ABS works

If the wheel speed sensor signals a lock, the ECU sends an electric current to the hydraulic unit . This energizes the solenoid valve. The action of this valve isolates the brake circuit from the master cylinder. This stops the brake pressure on that wheel from rising and keeps it constant. It allows the wheel speed to increase and the slip to decrease;

When speed increases, the ECU reapplies brake pressure to limit wheel slip to a specific value;

The hydraulic control unit controls the brake pressure in each wheel cylinder based on input from the system sensors. This results in control of the wheel speed.

4. Mathematical model of ABS

Wheel speed Simulink block diagram of anti-lock braking system :

Overall system block diagram:

5. Simulation Example

Initial vehicle speed: