Antilock Brakes / Traction Control Systems: Description and Operation: Fundamentals and Basics
Description
Fig. 3 Anti-Lock Brake System. Accord:
Features/Construction/Operation
In a conventional brake system, if the brake pedal is depressed excessively, the wheels can lock before the vehicle comes to a stop. In such a case, the stability of the vehicle is reduced if the rear wheels are locked, and maneuverable of the vehicle is reduced if the front wheels are locked, creating an extremely unstable condition.
The Anti-Lock Brake system modulates the pressure of the brake fluid applied to each caliper, thereby preventing the locking of the wheels, whenever the wheels are likely to be locked due to excessive braking. It then restores normal hydraulic pressure when there is no longer any possibility of wheel locking.
Features
- Increased braking stability can be achieved regardless of changing driving conditions.
- The maneuverability of the vehicle is improved as the system prevents the front wheels from locking.
- When the anti-lock brake system goes into action, a kick-back is felt on the brake pedal.
- The anti-lock brake system is equipped with a self-diagnosis function. When an abnormality is detected, the anti- lock brake system indicator light comes on and the LED display on the control unit blinks. The location of the system's trouble can be diagnosed from the frequency of the LED display blinks.
- This system has individual control of the front wheels and common control ("Select Low") for the rear wheels. ("Select Low") means that the rear wheel that would lock first (the one with the lowest resistance to lock-up) determines anti-lock brake system activation for both rear wheels.
- The system has a fail-safe function that allows normal braking if there's a problem with the anti-lock brake system.
Construction
In addition to the conventional braking system, the anti-lock brake system is composed of: gear pulsers attached to the rotating part of individual wheels; speed sensors, which generate pulse signals in correspondence to the revolution of the gear pulsers; control unit, which controls the working of the anti-lock brake system by performing calculations based on the signals from the individual speed sensors and the individual switches; modulator unit, which adjusts the hydraulic pressure applied to each caliper on the basis of the signals received from the control unit; an accumulator, in which high-pressure brake fluid is stored; a pressure switch, which detects the pressure in the accumulator and transmits signals to the control unit; a power unit, which supplies the high-pressure working fluid to the accumulator by means of a pump; a motor relay for driving the power unit; a fail-safe relay, which cuts off the solenoid valve ground circuit when the fail-safe device is at work; an indicator light.
Master Cylinder
1. Construction
A tandem master cylinder is adopted for the master cylinder in order to improve safety of the braking system. In addition, a center valve method is introduced so as to match the anti-lock brake system operation. The master cylinder has one reservoir tank which is connected to the cylinder sections by two small holes. It has two pistons-primary and secondary, which are criss-cross connected with the calipers so that the fluid pressure works separately on each system (front right wheel & rear left wheel, and front left wheel & rear right wheel. A stop bolt for controlling movement of the primary piston is provided at the side of the master cylinder body. A reed switch for detecting the brake fluid volume is also provided on the cap of the reservoir tank.
2. Operation
When the brake pedal is depressed, the secondary piston is pushed through the brake booster and the center valve B is closed so that the fluid pressure is generated on the secondary side. At the same time, the primary piston is pushed by the secondary fluid pressure and the center valve A is closed so that braking fluid pressure is generated both on the primary and secondary sides. When the brake pedal is released, the primary and secondary pistons are returned to the original position by the brake fluid pressure and piston spring.
3. Responses when fluid is leaking
(1) In case of leaking from the primary system
Since the fluid pressure on the primary side does not rise, the primary piston is pushed by the fluid pressure of the secondary piston and the tension of the piston spring until the end hits on the cylinder, the braking is performed by the fluid pressure on the secondary side.
(2) In case of leaking from the secondary system
The secondary piston does not produce fluid pressure, keeps moving ahead, hits on the end surface of the primary primary piston so that the primary piston is pushed under the same condition as an ordinary rod. Therefore, the braking is conducted by the fluid pressure on the primary side.
Speed Sensor
The speed sensor is a contactless type and it detects the rotating speeds of a wheel. It is composed of a permanent magnet and coil. When the gear pulsers attached to the rotatory parts of each wheel (front wheel: outboard joint of the driveshaft, rear: hub bearing unit) turn, the magnetic flux around the coil in the speed sensor alternates, generating voltages with frequency in proportion to wheel rotating speed. These pulses are inputted into the control unit and the control unit identifies the wheel speeds.
Control Unit:
The control unit consists of a main function section, which controls the operation of anti-lock brake system, and sub-function, which controls the pump motor and "self-diagnosis."
1. Main Function
The main function section of the control unit performs calculations on the basis of the signals from each speed sensor and controls the operation of the anti-lock brake system by putting into action the solenoid valves in the modulator unit for each front brake and for the two rear brakes.
2. Sub-function
The sub-function section gives driving signals to the pump motor and also gives "self-diagnosis" signals, necessary for backing up the anti-lock brake system.
1. Self-Diagnostic Function
Since the anti-lock brake system modulates the braking pressure when a wheel is about to lock, regardless of the driver's intention, the system operation and the braking power will be impaired if there is a malfunction in the system. To prevent this possibility, at speeds above 6 km/h, the self diagnosis function, provided in the sub-function of the control unit, monitors the main system functions. When an abnormality is detected, the anti-lock brake system indicator light goes on. There is also a check mode of the self-diagnosis system itself; when the ignition switch is first turned on, the anti- lock brake system indicator light comes on and stays on for a few seconds after the engine starts, to signify that the self-diagnosis system is functional.
2. Fail-Safe Function
When abnormality is detected in the control system by the self-diagnosis, the solenoid operations are suspended by turning off the relay (fail safe relay) which disconnects the ground lines of all the solenoid valves to inhibit anti- lock brake system operations. Under these conditions, the braking system functions just as an ordinary one, maintaining the necessary braking function. When the anti-lock brake system indicator light is turned on, it means the fail-safe is functioning.
Modulator Unit
Modulators for each wheel and solenoid valves are integrated in the modulator unit. The modulators for front and rear brakes are of independent construction and they are positioned vertically for improved maintainability. The modulators for rear brakes are provided with a PCV function (Proportioning Control Valve) in order to prevent the rear wheel from locking when the anti-lock brake system is malfunctioning or not activated. The solenoid valve features quick response (5 ms or less). The inlet and outlet valves are integrated in the solenoid valve unit. There are three solenoid valves provided, each one for the front-right wheel, for the front-left wheel and for the rear wheels.
Accumulator
The accumulator is a pneumatic type which accumulates high pressure brake fluid fed from the pump incorporated in the power unit. When the anti-lock brake system operates, the accumulator feeds high pressure brake fluid to the modulator valve via the inlet side of the solenoid valve.
Pressure Switch
The pressure switch monitors the pressure accumulation (pressure from the pump) in the accumulator and is turned off when the pressure becomes lower than a prescribed level. When the pressure switch is turned off, the switching signal is sent to the control unit. Upon receiving the signal, the control unit activates the pump motor relay to operate the motor. If the pressure doesn't reach the prescribed value, the anti-lock brake system indicator light comes on.
Operation
When the pressure in the accumulator rises, the Bourdon tube in the pressure switch deforms outwards. When the free end of the Bourdon tube moves more than the prescribed amount, the micro switch is activated by the force of the spring attached to the sensing lever. When the pressure in the accumulator decreases due to anti-lock brake system operations, the Bourdon tube moves in the direction opposite to the one described above, and the microswitch is eventually turned off. Upon receiving this signal, the control unit activates the motor relay to operate the motor.
Power Unit
The power unit consists of a motor and a plunger pump. Since an eccentric bearing is positioned on the end of the motor shaft, the rotation of the motor provides the reciprocating motion of the plunger. The brake fluid is thus pressurized and fed to the accumulator.
As the motor rotates more and the pressure in the accumulator exceeds the prescribed level, the pressure switch is turned on. Approx. 3 seconds after receiving the ON-signal, the control unit stops the motor relay operation. In this state, the pressure in the accumulator reaches 230 kg/cm2.
If the pressure doesn't reach the prescribed value after the motor has continuously operated for 120 seconds or more, the control unit stops the motor and activates the anti-lock brake system indicator light.
Anti-Lock Brake System Indicator Light
This warning system turns on the anti-lock brake system indicator light when one or more of the below described abnormalities is detected.
- When the operating time of the motor in the power unit exceeds 120 seconds.
- When vehicle running time exceeds 30 seconds without releasing the parking braking brake lever.
- When one of the rear wheels is locked during running.
- When absence of speed signals from anyone of the four speed sensors is detected.
- When the activation time of all solenoids exceeds a given time or an open circuit is detected in the solenoid system.
- When solenoid output is not detected in the simulated anti-lock brake system operation carried out during running at speed of 10 km/h or more. To check the indicator light bulb, the light is activated when the ignition switch is turned on. It is turned off after the engine is started if there is no abnormality in the system.
Operation
Ordinary Braking Function
In ordinary brake operations, the cut-off Out valve in the modulator is open to transmit the hydraulic pressure from the master cylinder to the brake calipers via chamber A and chamber B. Chamber C is connected to the reservoir through the outlet valve which is normally open. It is also connected to the hydraulic pressure source (pump, accumulator, pressure switch, etc.) via the inlet valve which is normally closed. Chamber D serves as an air chamber. Under these conditions, the pressures of chambers C and D are maintained at about atmospheric pressure, permitting regular braking operations.
If brake inputs (force exerted on brake pedal) are excessively large and a possibility of wheel locking occurs, the control unit operates the solenoid valve, closing the outlet valve and opening the inlet valve. As a result, the high pressure is directed into chamber C, the piston is pushed upward, causing the slide piston to move upward and the cut-off valve to close.
As the cut-off valve closes, the flow from the master cylinder to the caliper is interrupted, the volume of chamber B, which is connected to the caliper, increases, and the fluid pressure in the caliper declines.
When both of the two valves, inlet and outlet, are closed (when only the outlet valve is activated) the pressure in the caliper is maintained constant.
When the possibility of wheel locking ceases, it is necessary to restore the pressure in the caliper. The solenoid valve is therefore turned off (outlet valve: open, inlet valve closed).
2. Slide Piston Function
When the car is used on rough roads where the tires sometimes lose adhesion, the anti-lock brake system may function excessively, causing an excessively large volume of brake fluid to flow into chamber C. As this occurs, the piston is moved excessively, resulting in an abnormal loss of pressure in the chamber B. In order to overcome this problem, the slide piston is kept in a proper position by spring force to prevent the pressure in chamber B from becoming negative.
3. Kickback
When the anti-lock brake system is functioning, the piston moves upward, the volume of chamber B increases, and the fluid pressure on the caliper side is reduced. At the same time, the volume of chamber A is reduced and the brake fluid is returned to the master cylinder. When the brake fluid is pushed back to the master cylinder, the driver can feel the functioning of the anti-lock brake system because the brake pedal is kicked back.
4. PCV (Proportioning Control Valve) Function
In the modulator for the rear wheels, the diameters of the piston and the slide piston are distinctly different. This provides a PCV (Proportioning Control Valve) function to prevent the rear wheels from locking during an emergency stop.
(1) Before the Turning Point
When the fluid pressure from the master cylinder is below the turning point, the cut-off valve is always pushed downward by the force of the slide piston and its spring. Under these conditions, there is a gap between the cut-off valve shoulder and the sleeve. Chamber A and chamber B are therefore connected through the gap. The pressure from the master cylinder flows into the rear calipers through chamber A and chamber B.
(a) When the fluid pressure from the master cylinder reaches the turning point, the force on the slide piston overcomes the force of spring, causing the slide piston to travel upward. The cutoff valve, previously being in contact with the bottom of the slide piston, then moves upward and the cut-off valve shoulder hits the sleeve, blocking the fluid passages (the fluid pressure at this point is called the turning point).
(2) After the turning point
As the fluid pressure from the master cylinder further increases, the pressure in chamberA becomes higher, causing a force to push down the large diameter portion of the piston. Consequently, the slide piston comes down, the cutoff valve is pushed downward by the bottom of the slide piston, allowing chambers A and B to connect momentarily. As this occurs, pressure in chamber B increases, the slide piston is pushed upward, the cut-off valve goes up, and the connection between chamber A and chamber B is blocked again. As described above, when the pressure in the master cylinder is above the turning point, the slide piston reduces the pressure in the rear caliper to the prescribed pressure by repeating these processes.
Let the terms be as follows:
F = Set load of spring
A1 = Sectional area of cylinder at the slide piston
A2 = Sectional area of cylinder measured at the large diameter portion of the piston
A3 = Sectional area of cylinder measured at the small diameter portion of the piston
A4 = Sectional area of the cut out valve sealing part
Po = Input fluid pressure from the master cylinder
At the turning point,
Po = F/(A1 - A2)-
Since then, the reduction rate of pressure S in relation to an increase in Po is expressed by;
S = (A2 - A3 - A4)/(A1 - A3 - A4)