Antilock Brakes / Traction Control Systems: Description and Operation
SYSTEM OUTLINE1. ABS OPERATION
If the brake pedal is depressed suddenly, the ABS controls the hydraulic pressure of the wheel cylinders for all the four wheels to automatically avoid wheel locking and ensure the directional and steering stability of the vehicle. If the brake pedal is depressed suddenly, the skid control ECU with actuator controls the solenoids in the actuators using the signals from the sensors to move the brake fluid to the reservoir in order to release the braking pressure applied to the wheel cylinder. If the skid control ECU with actuator detects that the fluid pressure in the wheel cylinder is insufficient, the ECU controls the solenoids in the actuators to increase the braking pressure.
2. TRACTION CONTROL OPERATION
The traction control system controls the engine torque, the hydraulic pressure of the driving wheel cylinders, slipping of the wheels which may occur at start or acceleration of the vehicle, to ensure an optimal driving power and vehicle stability corresponding to the road conditions.
3. VSC OPERATION
Unexpected road conditions, vehicle speed, emergency situation, and any other external factors may cause large under- or over-steering of the vehicle. If this occurs, the VSC system automatically controls the engine power and wheel brakes to reduce the under- or over-steering.
To reduce large over-steering :
If the VSC system determines that the over-steering is large, it activates the brakes for the outer turning wheels depending on the degree of the over-steering to produce the moment toward the outside of the vehicle and reduce the over-steering.
To reduce large under-steering :
If the VSC system determines that the under-steering is large, it controls the engine power and activates the rear wheel brakes to reduce the under-steering.
Traction control SW
The traction control SW is used to stop the VSC function. After the engine is started, the VSC system is stopped (turned off) and the TRAC OFF (Except Korea) or TRC OFF (Korea) indicator light lights up. When the traction control SW is pressed again, the VSC system enters the stand-by mode. If the engine is stopped and restarted, the VSC system enters the stand-by mode regardless of the traction control SW.
4. MUTUAL SYSTEM CONTROL
To efficiently operate the VSC system at its optimal level, the VSC system and other control systems are mutually controlled while the VSC system is being operated.
Engine throttle control
The engine power does not interfere with the VSC brake control by controlling the opening of the throttle and reducing the engine output.
Engine control and electronically controlled transmission control
The strong braking force does not interfere with the braking force control of the VSC system by turning off the accel. and reducing changes in the driving torque at shift-down.
VSC system operation indication
The SLIP indicator light flashes and the buzzer sounds intermittently to warn the driver that the current road is slippery, while the VSC system is being operated.
5. FAIL SAFE FUNCTION
If an error occurs in the skid control ECU with actuator, sensor signals, and/or actuators, the skid control ECU with actuator inhibits the brake actuator control and inputs the error signal to the engine control module. According to the error signal, the brake actuator turns off the solenoid and the engine control module rejects any electronically controlled throttle open request from the VSC system. As a result, the vehicle functions regardless of the ABS, TRAC, and VSC systems.
System Diagram Part 1:
System Diagram Part 2:
SYSTEM DIAGRAM
SYSTEM DESCRIPTION
1. SYSTEM DESCRIPTION
HINT:
- The skid control ECU is a single unit with ABS & TRACTION ACTUATOR.
- The skid control ECU also works as a TPMS (Tire Pressure Monitor System) ECU.
- The yaw rate sensor and deceleration sensor are combined and in a single unit. This unit communicates with the skid control ECU through CAN communication.
a. ABS(Anti-lock Brake System)
The ABS helps prevent the wheels from locking when the brakes are applied firmly or when braking on a slippery surface.
Operation description
The skid control ECU detects wheel lock condition by receiving vehicle speed signals from each speed sensor, and sends control signals to the pump motor and solenoid valve. The pump motor and solenoid valve avoid wheel lock by controlling the oil pressure of each wheel cylinder.
The ABS warning lamp comes on when the ABS system is malfunctioning.
b. EBD (Electronic Brake force Distribution)
The EBD control utilizes ABS, realizing proper brake force distribution between front and rear wheels in accordance with driving conditions.
In addition, during cornering braking, it also controls the brake forces of the right and left wheels, helping to maintain vehicle behavior.
Operation description
The skid control ECU receives the speed signal from each speed sensor to detect the slip condition of the wheels and sends the control signal to the solenoid.
The solenoid valve controls the oil pressure of each wheel cylinder and splits the control power properly between the front and rear wheels and the right and rear wheels.
Both of the ABS warning lamp and the brake warning lamp come on to indicate a malfunction in the EBD system.
c. BA (Brake Assist)
The primary purpose of the brake assist system is to provide an auxiliary brake force to assist the driver who cannot generate a large enough brake force during emergency braking, thus helping to maximize the vehicle's brake performance.
Operation description
The skid control ECU receives the signal from the stop lamp switch and the oil pressure signal from the master cylinder pressure sensor to determine whether brake assist is necessary or not. If brake assist is deemed necessary, the ECU sends control signals to the pump motor and solenoid. The pump and the solenoid valve then control the pressure applied to each wheel cylinder. The ABS warning lamp comes on to indicate a malfunction in the brake assist system.
d. TRAC (Traction Control)
The TRAC system helps prevent the drive wheels from slipping if the driver presses down on the accelerator pedal excessively when starting off or accelerating on a slippery surface.
Operation description
The skid control ECU detects vehicle's slip condition by receiving signals from the speed sensor and ECM via CAN communication. The skid control ECU controls engine torque with the ECM, vehicle position with suspension control ECU via CAN communication, and oil pressure with the pump and solenoid valve. The SLIP indicator lamp blinks when the system is operating. TRAC OFF switch stops traction control operation.
e. VSC (Vehicle Stability Control)
The VSC system helps prevent the vehicle from slipping sideways as a result of strong front wheel skid or strong rear wheel skid during cornering.
Operation description
The skid control ECU determines vehicle condition by receiving signals from the speed sensor, yaw rate and deceleration sensor and steering angle sensor. The skid control ECU controls engine torque with the ECM, vehicle position with suspension control ECU via CAN communication, and oil pressure with the pump and solenoid valve. The warning light comes on and the buzzer sounds when the system is operating.
2. ABS with EBD & BA & TRAC & VSC OPERATION
a. Skid control ECU calculates vehicle stability tendency based on the signal of sensors, speed sensor, yaw rate and deceleration sensor and steering angle sensor. And it judges whether the control of engine output torque by electronic control throttle and of wheel brake pressure by brake actuator will operate or not by the calculation results.
b. The SLIP indicator blinks and the VSC buzzer sounds to inform the driver that the VSC system is operating. The SLIP indicator also blinks when TRAC is operating, and the operation being performed is displayed.
3. FAIL SAFE FUNCTION
a. When a failure occurs in the ABS with BA & TRAC & VSC systems, the ABS warning light and the VSC warning light turns on and the ABS with BA & TRAC & VSC operations are prohibited. In addition to this, when there is a failure that disables the EBD operation, the brake warning light also comes on and the EBD operation is prohibited.
b. If some control is prohibited due to a malfunction during it's operation, that control will be cut off gradually not to change stability of vehicle suddenly.
4. PRE-COLLISION BRAKE ASSIST SYSTEM (w/PRE-COLLISION BRAKE ASSIST SYSTEM)
If a collision (front collision) is determined to be unavoidable, the pre-collision brake assist system receives the brake assist standby signal from the distance control ECU and enters brake assist standby mode. If the STP signal is input, the pre-collision brake assist system improves deceleration performance by increasing pressure according to the amount of brake pedal effort.