Mechanism & Function - ABS Subaru Forester SH

FEATURES

1. To reduce the size and weight, this ABS has a hydraulic control unit, an ABS control module, a valve relay and a motor relay integrated into a single unit (called ABSCM & H/U).
2. The ABS electrically controls the brake fluid pressure to each wheel to prevent the wheel from locking during braking on slippery road surfaces, thereby enabling the driver to maintain the directional control.
3. If the ABS becomes inoperative, a fail-safe system is activated to ensure same level of braking performance as with a conventional brake system. In that case, the warning light comes on to indicate that the ABS is malfunctioning.
4. The ABS is a 4-sensor, 4-channel system. The system is a four-wheel independent control type, which controls each of the wheel brakes individually.

Scheme 15

FUNCTIONS OF SENSORS AND ACTUATORS

FUNCTIONS OF SENSORS AND ACTUATORS

Scheme 16

PRINCIPLE OF ABS CONTROL

When the brake pedal is depressed during driving, the wheel speed decreases and the vehicle speed does as well. The decrease in the vehicle speed, however, is not always proportional to the decrease in the wheel speed. The non-correspondence between the wheel speed and vehicle speed is called "slip" and the magnitude of the slip is expressed by the "slip ratio" which is defined as follows

Slip ratio = (Vehicle speed - Wheel speed)/Vehicle speed x 100%

When the slip ratio is 0%, the vehicle speed corresponds exactly to the wheel speed. When it is 100%, the wheels are completely locking while the vehicle is moving.

The braking effectiveness is represented by the "coefficient of friction" between the tire and road surface. The larger the coefficient, the higher the braking effectiveness. The diagram below shows the relationship between the coefficient of friction and the slip ratio for two different road surface conditions (asphalt-paved road and icy road), assuming that the same tires are used for both the conditions and the vehicles are moving forward. Although the braking effectiveness (coefficient of friction) depends on the road surface condition as shown and also on the type of the tire, its peak range generally corresponds to the 8 - 30% range of the slip ratio.

The ABS controls the fluid pressure to each wheel to maintain the slip ratio within this range.

Scheme 17

ABS WHEEL SPEED SENSOR

Each of the ABS wheel speed sensors detects the speed of the corresponding wheel. The sensor consists of a sensor body and a magnetic encoder. The magnetic encoder is a permanent magnet with N poles and S poles arranged alternately, and when this turns together with the wheel the magnetic field at the sensing area of the sensor body changes. The sensor body has a hall element and outputs signals at a frequency proportional to the wheel speed.

Scheme 18

EBD (ELECTRONIC BRAKE FORCE DISTRIBUTION)

The EBD system utilizes the function of the conventional ABS. This system prevents premature locking of the rear wheels by using electronic control instead of a proportioning valve. The system provides a feature to optimize the brake force distribution to the front and rear wheels according to change in loading conditions or displacement of the center of gravity during deceleration.

The ABSCM calculates the optimum brake force distribution from the difference in speed between the front and rear wheels. Based on the results, the ABSCM controls the rear wheel brake pressure through the ABS hydraulic unit (H/U) to achieve the optimum brake force distribution suitable for the driving condition.

The proportioning valve is disused on EBD equipped vehicles.

Scheme 19

ABS control module section (ABSCM)

The ABSCM contains two micro control modules (MCMs) which communicate with each other. Both the MCMs process the same program and each MCM monitors the other's outputs. If a mismatch occurs between their outputs, the ABSCM cuts off the system and activate the fail-safe function. The ABSCM can store a maximum of 3 diagnostic trouble codes (DTC) in a ROM. If more than 3 faults have occurred, only the 3 most recent failures are stored and others are erased. Trouble diagnostic trouble codes remain stored until they are internally or externally erased.

The ABSCM has a test routine (sequence control pattern) which facilitates checking of the hydraulic control unit.

1. ABS control

Using primarily the wheel speed data from each ABS wheel speed sensor and secondarily the vehicle deceleration rate data from the G sensor as parameters, the ABSCM generates a simulated vehicle speed when there is a risk of wheel lock-up. Using the simulated vehicle speed (called "dummy" vehicle speed) as a reference, the ABSCM determines the state of the wheel in terms of the tendency toward lock-up. If the result shows that the wheels are about to lock, the ABSCM issues commands to energize or de-energize the solenoid valves and activate the motor pump of the H/U to modulate the brake fluid pressures that act on the wheel cylinders, thereby preventing the wheels from locking.

The ABSCM controls the fluid pressure to each of the four wheels independently.

1. Functions available using SUBARU select monitor When the SUBARU select monitor is connected, the ABSCM allows it
2. To read out analog data
3. To read out ON/OFF data
4. To read out or erase diagnostic trouble codes
5. To read out data showing conditions under which a trouble code has been stored (freeze frame data)
6. To initiate ABS sequence control pattern
7. Indication functions Under the control of the ABSCM, the ABS warning light provides the following two indication functions
8. ABS fault alerting
9. Valve ON/OFF indication (when sequence control pattern is initiated)

Hydraulic control unit section (H/U)

The H/U is a fluid pressure controller consisting of, among others, a motor, solenoid valves, a housing and relays. It also constitutes passage of the two diagonally split brake circuits.

1. The pump motor drives an eccentric cam which in turn moves the plunger pump to generate hydraulic pressure.
2. The housing accommodates the pump motor, solenoid valve and reservoir. It also constitutes a brake fluid passage.
3. The plunger pump, when operated, draws the brake fluid from the reservoir, lets the fluid in a wheel cylinder drain into the reservoir, and/or forces the fluid into the master cylinder.
4. The outlet solenoid valve is a 2-position type. It opens or closes the brake fluid passage between a wheel cylinder and the reservoir according to commands from the ABSCM.
5. The inlet solenoid valve is duty-controlled to reduce brake fluid pulsation for minimum ABS operation noise.
6. The reservoir temporarily stores the brake fluid drained from a wheel cylinder when pressure "decrease" control is performed.
7. The valve relay controls power supply to the solenoid valves in response to a command from the ABSCM. In normal (IG ON) condition, the relay is closed to supply power to the solenoid valves. When an error occurs in the system, the valve relay is turned OFF to keep the fluid pressure circuit in the normal mode (non-ABS mode).
8. The motor relay closes and supplies power to the pump motor in response to a command from the ABSCM during the ABS drive mode operations.
9. The H/U has four operating modes; normal mode (non-ABS mode), and three ABS active modes, i.e., "increase", "hold" and "decrease" modes.

DURING NORMAL BRAKING (ABS NOT ACTIVE)

Both the inlet and outlet solenoid valves are not energized.

This means that the inlet port of the inlet solenoid valve is open, whereas the outlet port of the outlet solenoid valve is closed. So the fluid pressure generated in the master cylinder is transmitted to the wheel cylinder, producing a brake force.

Note. For simplicity of explanation, operation of the hydraulic control unit is represented by operation of a single wheel circuit.

Scheme 20

PRESSURE "DECREASE" CONTROL (ABS ACTIVE)

Both the inlet and outlet solenoid valves are energized, which means that the inlet port is closed and the outlet port is open.

In this state, the wheel cylinder is isolated from the master cylinder but open to the reservoir, so the brake fluid in it can be drained into the reservoir, decreasing its pressure and reducing the braking force of the wheel.

The brake fluid collected in the reservoir is forced into the master cylinder by the pump.

During this phase of ABS operation, the pump motor continues operating.

Note. For simplicity of explanation, operation of the H/U is represented by operation of a single wheel circuit.

Scheme 21

PRESSURE "HOLD" CONTROL (ABS ACTIVE)

The inlet solenoid valve is energized, so the inlet port is closed.

On the other hand, the outlet solenoid valve is de-energized, so the output port is also closed. In this state, all the passages connecting the wheel cylinder, master cylinder and reservoir are blocked. As a result, the fluid pressure in the wheel cylinder is held unchanged.

During this phase of ABS operation, the pump motor continues operating.

Note. For simplicity of explanation, operation of the H/U is represented by operation of a single wheel circuit.

Scheme 22

PRESSURE "INCREASE" CONTROL (ABS ACTIVE)

Both the inlet and outlet solenoid valves are de-energized, which means that the inlet port of the inlet solenoid valve is open, whereas the outlet port of the outlet solenoid valve is closed. So the fluid pressure generated in the master cylinder is transmitted to the wheel cylinder and increased fluid pressure in the wheel cylinder applies the brake with a larger force.

During this phase of ABS operation, the pump motor continues operating.

Note. For simplicity of explanation, operation of the H/U is represented by operation of a single wheel circuit.

Scheme 23

ABS CONTROL CYCLE CURVES

Depressing the brake pedal increases the brake fluid pressure in each wheel cylinder, which in turn decreases the wheel speed (or increases the wheel deceleration rate). When the brake fluid pressure is increased to a level of point "A" of the brake fluid pressure curve in the diagram (at which the wheel deceleration rate exceeds threshold "-b 0 "), the ABSCM makes a pressure "hold" control. At the same time, the ABSCM calculates a "dummy" vehicle speed which is a reference speed it uses in the next stage of control.

When the wheel speed then drops below the slip ratio setting, i.e., a speed lower than the "dummy" vehicle speed by the predetermined value (at point "B" of the pressure curve), the ABSCM makes a control to prevent the wheel from locking, or a pressure "decrease" control.

As the wheel cylinder pressure decreases, the wheel speed starts increasing (or the wheel acceleration rate starts rising). When the wheel acceleration rate exceeds threshold "+b 10 " (at point "C" of the pressure curve), the ABSCM makes a pressure "hold" control. When the wheel acceleration rate exceeds threshold "+b 20 " (at point "D" of the pressure curve), the ABSCM recognizes that wheel lock-up will not occur and then makes a pressure "increase" control.

When the wheel acceleration rate drops below threshold "+b 20 ", (at point "E" of the pressure curve), the ABSCM starts pressure "hold" and "increase" control cycles at a given interval.

When the wheel deceleration rate then exceeds threshold "-b 0 " (at point "F" of the pressure curve), the ABSCM immediately makes a pressure "decrease" control.

Scheme 24

BRAKE FLUID PRESSURE DESCRIPTION

ABS WARNING LIGHT AND BRAKE WARNING LIGHT (ALSO FUNCTIONS AS EBD WARNING LIGHT)

When a fault occurs in the signal transmission system or the ABSCM, the ABS warning light and brake warning light in the combination meter come on. At the same time, the current to the hydraulic control unit is interrupted. The brake system then functions in the same manner as a system without ABS.

If the warning light comes on, one or more diagnostic trouble codes should be stored in the ABSCM. To identify these codes, use a Subaru select monitor.

Scheme 25

G SENSOR

The G sensor detects changes in the vehicle's acceleration/deceleration rate in the longitudinal direction.

A piezo-resistor is formed on the beam. When the mass area moves during acceleration and deceleration, the beam distorts causing the resistance of the piezo-resistor to change. This change is converted into change in voltage and sent to the ABSCM.

Scheme 26