ABS System Description
ABS SYSTEM DESCRIPTION
The anti-lock brake system (ABS) is designed to prevent wheels from locking due to excessive brake efforts.
Main functions of the ABS are:
- To reduce braking distance by avoiding wheels from locking
- To improve stability of steering and handling by avoiding slip during braking
The Sephia incorporates a 4-sensor/4-channel ABS system so that each front wheel is controlled independently and the rear wheels are controlled by a select low function (hydraulic pressure is controlled on the side that slip first occurred).
TYPES OF OPERATION
Under normal conditions, the standard brake system performs until locking of a wheel is detected by the ABS control unit.
During standard braking, inlet valves are normally opened and outlet valves are normally closed.
When the ABS control unit detects a wheel lock, the hydraulic unit controls each valve to maintain, increase or decrease pressure.
The ABS control unit detects the speed and the deceleration rate of each wheel with wheel speed sensors. When braking, the wheel speed is decreased and difference between wheel speed and vehicle speed is generated. If deceleration rate of some wheels exceeds expected value (Point A), ABS control unit determines wheel lock has occurred and opens the corresponding outlet solenoid valves to reduce brake fluid pressure. If locking pressure on the wheels is exceeded (Point B) and wheel speed reaches point C, then the ABS control unit closes the outlet valves of the corresponding wheels to increase fluid pressure.
For vehicles with ABS the symptoms in the following table can occur, but indicate normal operation of the ABS.
PRINCIPLE OF OPERATION
Standard braking
1. Non-ABS braking
When the brake pedal is depressed, the power brake unit closes the central valve of the master cylinder and generates pressure on the brake hydraulic pressure line. This opens the solenoid inlet valve, closes the outlet valve, and transmits hydraulic pressure from the master cylinder to the wheel cylinders.
ABS Braking
2.1 Maintaining pressure
After the ABS control unit detects wheel lock due to excessively increased pressure, the ABS control unit closes the inlet valve to avoid further increase in hydraulic pressure.
2.2 Decreasing pressure
If wheel lock continues, the ABS control unit opens the corresponding outlet valve so that brake fluids flow to the pressure accumulator. This reduces pressure only for wheels that are locked.
2.3 Increasing pressure
When wheels have returned to normal speed from locking, the ABS control unit stops operating the solenoid valve and returns to standard braking operation.
PRINCIPLE OF ELECTRONIC BRAKE FORCE DISTRIBUTION (EBD) OPERATION
EBD operating (Non-ABS braking)
When the brake pedal is depressed, the electronic brake force distribution (EBD) in ABS electronic control unit opens the solenoid outlet valve of the rear wheel to prevent the rear wheel locking first (due to excessively increased pressure). As a result, the rear wheel doesn't increase the brake fluid pressure any longer.
If the brake fluid pressure of the rear wheel is decreased again, the EBD closes the outlet valve of rear wheel to increase the brake fluid pressure. It can improve driving stability as the EBD distributes the brake fluid pressure regularly.
During the EBD operating, the ABS system doesn't operate.
The Electronic brake force distribution (EBD) is a part of the ABS electronic control unit which controls the wheel slip under braking condition.
The ABS system can be operated more effectively as the EBD controls the rear wheel slip in the partial braking range.
- To optimize the real brake force.
- To optimize adhesion coefficient utilization.
- To maintain driving stability, when braking.
During abrupt braking, the center of gravity (G) of vehicle is moved to the front wheel side and then the rear wheel is locked earlier than the front wheel. As a result, the braking distance may be longer and extreme steering high speed or high speed cornering may cause vehicle spinning.
After detecting the rear wheel locking first the EBD controls the rear wheel electrically to perform the ideal brake force distribution.