ABS 2E System
Anti-lock Brake System-2E
OUTLINE
The ABS-2E has been developed for exclusive use on automatic transmission vehicles.
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 as well as shortening the braking distance.
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*1 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."
COMPONENTS
The ABS consists of four tone wheels (5), tour ABS sensors (4), an ABS control unit (8), a hydraulic control unit (6) and a warning light (9).
ABS component parts.
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 which is determined by the following equation:
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 the above image.
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.
CONSTRUCTION AND OPERATION
1. ABS SENSOR
The ABS sensor detects wheel speed and consists of a permanent magnet, coil, tone wheel, etc. The magnetic flux produced by the permanent magnet varies with the tone wheel (which rotates together with the wheel) and the sensor emits an alternating voltage corresponding with the wheel speed by electromagnetic induction.
2. ABS CONTROL UNIT (ABS C/U)
The ABS control unit determines the state of the wheels based on input signals from the ABS sensor, and optimally increases or decreases brake fluid pressure by controlling the electric current flowing through magnet valves built into the hydraulic unit.
ABS C/U performs logical computation and monitoring Ion each of two 8-bit microcomputers, creating a fail safe system by which system failures can be detected. This ABS C/U detects open circuits by sending an electrical current through the sensor system. An upper limit is established with respect to the sensor signal input characteristics, and if an input value exceeds that limit it is considered a failure. Image above shows [a marginal level] a failure.
A maximum of 3 trouble codes are stored in the EEP ROM and if 3 or more areas fail, then only the 3 most recent failures are stored. The trouble codes remain stored until they are erased.
The trouble codes can be determined by the flashing of the ABS warning light located in the combination meter. This ABS C/U (ABS-2E) induces a sequence control pattern and facilitates the checking of the hydraulic unit.
3. 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"- b0"), the control unit transmits a "hold" signal to hold the brake fluid pressure in wheel cylinder at that point. At the same time, the control unit 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 unit then transmits a "decrease" 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 "+ b10"at point "C" (brake fluid pressure), the control unit transmits a"hold" signal to hold the brake fluid pressure at that point. When wheel acceleration setting value "+ b10" is exceeded and when brake fluid pressure is at point "D", the control unit judges that wheel lock-up will not occur and then transmits an "increase" signal to increase brake fluid pressure.
When wheel acceleration drops below "+ b0" at point"E" (which occurred due to a brake fluid pressure increase), the repetition of the "hold" and "increase" signals takes place a at constant cycle.
When wheel deceleration exceeds "- b0", at point "F" of the brake fluid pressure, the control unit immediately transmits a "decrease" signal to decrease brake fluid pressure.
4. ABS WARNING LIGHT
When a signal system or the ABS control unit (ABS C/U) becomes inoperative, the warning light in the instrument panel comes on to indicate that the system or control unit is malfunctioning. At the same lime, current flowing through the hydraulic control unit is interrupted so that the brake system functions as a conventional brake system. The circuit through which the warning light comes on utilizes a dual system design.
If the warning light comes on upon detection of a system malfunction, call a trouble code and identify it using the warning light.
Figure 8:
5. HYDRAULIC CONTROL UNIT
The hydraulic control unit is a fluid pressure control assembly which consists of an electric motor, plunger pump, plunger piston, housing, magnet valve and a relay. The magnet valve moves to three positions, as shown in figure 8. When it is moved to position "1", (current "OFF"), the master cylinder and brake system are connected to each other. With the valve moved to position "2" (part of current flow "ON"), all passages are closed. At position "3" (full-current flow "ON"), master cylinder ports are closed and the line between the hydraulic unit reservoir is connected to the brake system.
As wheel deceleration exceeds the specified deceleration setting during braking, the magnet valve moves to position "2" to hold brake fluid pressure. Further deceleration increases moves the magnet valve to position "3" so that brake fluid pressure is released and, at the same time, the motor starts to drive the plunger pump When wheel deceleration stops, the magnet valve is held at position "2" to maintain brake fluid pressure.
1) During normal braking (Electric current is OFF). When the brake pedal is depressed, fluid pressure is generated in the master cylinder and is routed to each wheel cylinder [FR (Front right), FL (Front left), and RR (Rear right)] passing through each magnet valve [FR, FL, and RR (Rear right)], causing the brake to work. Pressurized fluid passes through the port of plunger piston, activating the RL (Rear left) wheel cylinder. Releasing the brake pedal allows fluid pressure in the reverse direction.
2) Pressure "decrease" action with ABS in operation [Electric current (4.8 - 6.0 A) fully ON].
If one wheel shows signs of locking, magnet valves are controlled under the state of decompression (full electric current) by the ABS C/U, and the inlet port of each magnet valve is closed while the outlet port is opened. Brake fluid from the FL, FR, and RR wheel cylinders is released through the outlet port to the reservoir, which decreases the braking force. The unequal fluid pressure in the pressure chamber at the right side of the plunger piston results in the reduction of the fluid pressure in the RL wheel cylinder by the magnet valve, which energizes the plunger piston to move to the left. Upon closing the pressure chamber port on the left side, the side chamber of the RL wheel cylinder is decompressed. When the magnet valve is controlled at full electric current, the ABS C/U simultaneously sends an electric current to the motor relay thus driving the motor. Brake fluid in the reservoir is pumped to the damper chamber of the plunger piston by the plunger pump where it dampens pulsating pressure and then returns to the master cylinder (at this time, the brake pedal feels as if it is pressed back.).
3) Pressure "hold" action with ABS in operation (Half electric current).
When the fluid pressure in the wheel cylinders is optimally reduced (or increased), the magnet valve is maintained in a "holding state" (half electric current) in which both inlet and outlet port are closed. Fluid pressure in the FL, FR, and RR wheel cylinder sides from the magnet valve is held in this controlled state. Thus, the fluid pressure generated in the wheel cylinder side from the magnet valve corresponds to a force depressing the brake pedal.
Since fluid pressure in the RL wheel cylinder is controlled by the plunger piston, it is kept equal to the pressure in the RR wheel cylinder.
4) Pressure increase action with ABS in operation (Electric current is OFF).
As soon as wheel lock is avoided, electric current in the magnet valve is cut off as instructed by the electronic controller.
Thus, fluid pressure becomes constant from the master cylinder through the FL, FR, and RR wheel cylinders. The fluid pressure in the master cylinder generated by depressing the brake pedal can pass directly to the FL, FR, and RR wheel cylinders and reenergize the braking force.
When an electric current in the R magnet valve cuts off, the fluid entering the plunger piston pressure chamber on the right side is compressed. Then, from an equalized position, the plunger piston moves to the right side reducing the capacity of the pressure chamber on the left side (compressed). Thus, the pressure in the RL wheel cylinder increases. Further movement of the plunger piston opens the plunger piston port, and the fluid pressure of the master cylinder is directly applied (at this time, the brake pedal feels as if it is being pulled in.).