Antilock Brakes / Traction Control Systems: Description and Operation
FEATURE
- This ABS 5.3i type incorporates the hydraulic control unit, ABS control module, valve relay and motor relay in one unit for better productivity and lightweight.
- The ABS (Anti-lock brake system) electrically controls brake fluid pressure to prevent wheel "lock" during braking on slippery road surfaces, thereby improving directional/steering stability.
- If the ABS becomes inoperative, the fail-safe system activates to ensure it acts as a conventional brake system. The warning light also comes on to indicate that the ABS is malfunctioning.
- The front-and-rear wheels utilize a 4-sensor, 4-channel control design: the front wheels have an independent control design*4 and the rear wheels have a select low control design*2.
*1: A system which independently controls fluid pressure to left and right front wheels.
*2: A system which provides the same fluid pressure control for the two rear wheels if either wheel starts to "lock."
FUNCTIONS OF SENSORS AND ACTUATORS
THEORY OF ABS CONTROL
When the brake pedal is depressed during operation, wheel speed as well as vehicle speed decreases. The difference which occurs between wheel speed and vehicle speed is called the "slip" phenomenon. The magnitude of this action is expressed by "slip" the ratio of which is determined by the following equation:
Slip ratio = Vehicle speed - Wheel speed / Vehicle speed x 100 %
When the "slip" ratio is 0 % vehicle speed equals wheel speed and the wheel rotates without any slippage. When the "slip" is 100 % the wheel locks and does not rotate (wheel speed = 0) although vehicle speed exists.
The relationship between the frictional force of a wheel in the fore-and-aft direction and the "slip" ratio is shown by two characteristic curves in figure.
These curves are determined by the relationship between the wheel and road surface. Where the same type of wheel are used; the curve shown by a solid line indicates wheels driven on asphalt or paved roads, the curve shown by dotted lines refers wheels subjected to slippery (snowy or icy) roads.
When different types of wheels are used, although the road surface is the same, these curves will change. In general, the frictional coefficient between wheel and road surface in relation to an increase in the "slip ratio" will reach the maximum value in the 8 - 30 % range and will tend to decrease after that.
ABS CONTROL CYCLE CURVES
As the brake pedal is depressed, brake fluid pressure increases correspondingly, which in turn decreases wheel speed. When brake fluid pressure reaches point "A" (where wheel deceleration exceeds "- bo"), the control module transmits signal to hold the brake fluid pressure in wheel cylinder at that point. At the same time, the control module computes a "dummy" vehicle speed. When the wheel speed drops below the slip ratio setting (= speed less than the dummy vehicle speed based on the predetermined value) at point "B" of the brake fluid pressure, the control module then transmits signal to prevent wheel lock-up. This causes the brake fluid pressure to decrease.
After brake fluid pressure is decreased, wheel acceleration increases. When it exceeds the wheel acceleration setting "+ bo" at point "C" (brake fluid pressure), the control module transmits signal to hold the brake fluid pressure at that point. When wheel acceleration setting value "+ b20" is exceeded and when brake fluid pressure is at point "D", the control module judges that wheel lock-up will not occur and then transmits signal to increase brake fluid pressure.
When wheel acceleration drops below "+b20" (point "E") (which occurs due to a brake fluid pressure increase), signals are sent so that "holding pressure" and "increasing pressure" may be cycled in a given interval.
When wheel deceleration exceeds "- b0", at point "F" of the brake fluid pressure, the control module immediately transmits signal to decrease brake fluid pressure.