ABS Non-2E System
Anti-lock Brake System
OUTLINE
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
ABS component parts
The ABS consists of four tone wheels (5), four ABS sensors (4) , an electronic control unit (8), a hydraulic control unit (7), a G sensor (6) and a warning light (10).
ABS System Diagram
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. ELECTRONIC CONTROL UNIT (E.C.U.)
The electronic control unit is a digital type that utilizes LSI elements to achieve compact structure and improve circuit reliability.
It consists of an arithmetic circuit, control circuit, signal input-output circuits, a safety circuit and a regulated power circuit. All these circuits are housed in the case located on the PC board.
Figure 34 shows a block diagram of the electronic circuits. When the ABS sensor sends an alternating voltage corresponding with the wheel speed to the input amplifier circuit, the signal voltage is converted into a rectangular wave which is sent to the digital signal generating circuit. (This circuit receives two channels of signal at a time.)
The LSI circuit, which consists of approximately 16,000 transistors,computes wheel speed in relation to the signal sent from the ABS sensor. It then emits the required control signal as a result of computation. This circuit also contains a safety circuit for monitoring purposes. The control signal emitted from the LSI circuit is then sent to the current-control and amplification circuits where a signal is produced to operate the magnet valve of the hydraulic control unit.
The memory circuit, which serves to memorize system failure, and monitor the regulated power circuit and others, is housed in a separate IC. When the E.C.U. power is applied with the ignition switch "ON" the safety circuit begins to monitor electronic circuits, sensors, the hydraulic control unit, etc. If any circuits or units malfunction, a warning light (dual circuit design) comes on to warn the driver of a problem. The LED in the ECU illuminates to show a trouble code. The brake system then functions as a conventional brake system in place of the ABS.
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.
5. HYDRAULIC CONTROL UNIT
The hydraulic control unit is a fluid pressure control assembly which is composed of an electric motor, a plunger pump, a damper, a housing,a magnet valve and a relay.
There are two types of hydraulic control unit. One is manufactured by NIPPON ABS under license from ROBERT BOSCH, and the other is manufactured by ROBERT BOSCH. Both hydraulic control units are very similar to each other. The BOSCH type hydraulic control unit does not have the F-valve but has brake pedal "kick back" instead.
NIPPON ABS
During normal braking - When the brake pedal is depressed, fluid pressure is delivered from master cylinder (1) to magnet valve (4) via F-valve (2) while moving the ball up. Since current does not flow through the magnet valve, fluid pressure is delivered to wheel cylinder (12) so that normal braking force occurs.
Fluid pressure delivered to the F-valve (2) also reaches outlet valve (8) which is sealed by the ball.
Accumulator (11) has its spring set to act by high fluid pressure,and not by normal fluid pressure. When current does not flow through the magnet valve, the outlet port is sealed so that the oil passage to reservoir (5) is not linked.
When the brake pedal is released, master cylinder (1) fluid pressure will decrease. Wheel cylinder (12) pressure will then return to the master cylinder (1) while pushing the ball of check valve (3). At this point, F-valve (2) is moved up by the ball to prevent wheel cylinder (13) pressure from returning to the master cylinder (1).
However, when master cylinder (1) pressure drops below approximately 981 kPa (10 kg/cm2, 142 psi), the F-valve (2) return spring is moved to the left to push the ball up. This allows a slight amount of residual pressure - applied to check valve (3) - to be delivered to wheel cylinder (12) via magnet valve (4) .
When the brake pedal is slightly released while the ABS is operating,excess wheel cylinder fluid pressure returns to the master cylinder via check valve (3) so that wheel cylinder fluid pressure is balanced with master cylinder fluid pressure.
During the time the excess wheel cylinder fluid pressure returns to the master cylinder, the driver may feel a pedal "kickback".
This is not an indication of a problem.
Pressure-decrease action with ABS in operation. When the wheels begin to lock during brake application, the E.C.U. emits an instruction so that a current of 4.8 to 6 amperes (which decreases fluid pressure) flows through magnet valve (4)
This closes the inlet port of the magnet valve (4) and opens the outlet port so that wheel cylinder (12) pressure is delivered to reservoir (5) via the outlet port. Since current also flows through the motor (10) (simultaneously when the current flows through the magnet valve), motor (10) will start to activate the plunger pump (7) via the bearing (9), provided with an eccentric cam, connected to it. Brake fluid pressure delivered to reservoir (5) then passes through inlet valve (6) and is increased by plunger pump (7). Increased brake fluid then passes through inlet valve (6) and is stored in accumulator (11). At this point, the increased brake fluid pressure is sealed by the ball of valve F (2) to shutout the fluid pressure in master cylinder (1): Fluid pressure in wheel cylinder (12) also shuts out the fluid pressure in the master cylinder (1) by means of check valve (3).
Wheel cylinder (12) fluid pressure will then decrease to prevent the brake pedal from kicking back while it is being controlled to accommodate vehicle deceleration.
Pressure "hold" action with ABS in operation. When wheel cylinder (12) fluid pressure is decreased or increased to the optimum point, the controller emits an instruction so that a hold current of 1.9 to 2.3 amperes flows through magnet valve (4).The inlet and outlet ports will then be closed. At this point, the controlled fluid pressure is held in the wheel cylinder (12), the fluid pressure increased by the pump (which increases the decreased wheel cylinder) is stored in accumulator (11), the fluid pressure increased by brake pedal depression force is held in master cylinder (1) and the fluid pressure discharged from the pump is held in reservoir (5).
Pressure-increase action with ABS in operation. When current flowing through magnet valve (4) is interrupted (OFF) by an instruction emitted from the E.C.U., the fluid pressure of wheel cylinder (12) is increased. (At this point, internal passages of the magnet valve function is the same as in a conventional brake system.) When the magnet valve (4) is OFF, the inlet port is opened and the outlet port is closed. High fluid pressure is then delivered from accumulator (11) to wheel cylinder (12) via the inlet port so that wheel cylinder pressure is increased. Since check valve (3) and F-valve (2) remain sealed by the fluid pressure of master cylinder (1) and high fluid pressure of accumulator (11) respectively, only wheel cylinder (12) fluid pressure is increased. Accordingly, a sensation of brake pedal "pull" will not occur.
BOSCH ABS
During normal braking. When the brake pedal is depressed, the brake fluid in the master cylinder (1) goes through the magnet valve (4) and is delivered to the wheel cylinder (12), so that a normal braking force occurs. At the same time, the fluid pressure also reaches the outlet valve (8) which is sealed by the ball. At this time, current is not passing through the magnet valve (4), and so the oil passage to reservoir (5) is clogged by the magnet valve (4) and the outlet valve (8).
When the brake pedal is released, the fluid pressure in the master cylinder (1) will decrease. The fluid in the wheel cylinder (12) is pushed back. This fluid goes through the magnet valve (4), at the same time, as going through the check valve (3) by pushing the inner ball. Then, it returns to the master cylinder (12).
Pressure-decrease action with ABS in operation. When the wheels begin to lock during brake operation, the E.C.U. emits an instruction so that a current of 4.8 to 6 amperes flows through magnet valve (4). This operation closes the inlet port of the magnet valve (4) and opens the outlet port so that wheel cylinder (12) pressure is delivered to reservoir (5) via the outlet port. Brake fluid pressure delivered to reservoir (5) then passes through inlet valve (6) and is increased by plunger pump (7). This causes"kick back" of the brake pedal. Damping unit (11) reduces the pulse flow caused by plunger pump (7). The fluid pressure in wheel cylinder (12) is shut off from the fluid pressure in master cylinder (1) by means of check valve (3). This prevents master cylinder pressure from flowing into the wheel cylinder. Wheel cylinder (12) fluid pressure will then decrease to prevent the wheel lock.
Pressure "hold" action with ABS in operation. When wheel cylinder (12) fluid pressure is decreased or increased to the optimum point, the controller emits an instruction so that a hold current of 1.9 to 2.3 amperes flows through magnet valve (4) .The inlet and outlet ports will then be closed. At this point, the controlled fluid pressure is held in the wheel cylinder (12), the fluid pressure increased by the pump (which increases the decreased wheel cylinder) is stored in accumulator (11), the fluid pressure increased by brake pedal depression force is held in master cylinder (1) and the fluid pressure discharged from the pump is held in reservoir (5).
Pressure-increase action with ABS in operation When current flowing through magnet valve (4) is interrupted (OFF) by an instruction emitted from the E.C.U., the fluid pressure of wheel cylinder (12) is increased. At this time, the magnet valve position is the same as the normal braking position. When magnet valve (4) is OFF, the inlet port is opened and the outlet port is closed. High fluid pressure caused by the force on the brake pedal is then delivered to wheel cylinder (12) via the inlet port so that wheel cylinder pressure is increased.