1171 Hydromechanical antilock brake system ABS suitable for cars SCS Lucas Girling

This hydro-mechanical antilock braking system has two modular units, each consisting of an integrated flywheel decelerating sensor, cam operated piston type pump and the brake pressure modulator itself (Fig. 11.38). Each modulator controls the adjacent wheel brake and the diagonally opposite rear wheel via an apportioning valve. The modular flywheel sensor is driven by a toothed belt at 2.8 times the wheel speed. The flywheel sensor deter-

Lucas Girling Brake System
Fig. 11.38 Stop control braking system (SCS) layout

mines when the front wheel is approaching a predetermined deceleration. In response to this the modulator reduces the pressure in the respective brake circuits. When the wheel speeds up again, the pump raises that pressure in order to bring the braking force back to a maximum level. This sequence of pressure reduction and build-up can be up to five times a second to avoid the wheel locking and also to provide the necessary deceleration of the car.

Braking as normal (Fig. 11.39(a)) Under normal braking conditions, the master cylinder fluid output is conveyed to the wheel brakes through the open cut-off valve. The dump valve is closed and the pump piston is held out of engagement from the rotating eccentric cam by the return spring.

Brake pressure reducing (Fig. 11.39(b)) When the deceleration of the front wheel, and therefore the drive shaft, exceeds a predetermined maximum (the wheels begin to lock), the flywheel overruns the drive shaft due to its inertia. The clutch balls then roll up their respective ramps, forcing the flywheel to slide inwards and causing the dump valve lever to tilt and open the dump valve. The fluid pressure above the deboost piston drops immediately. The much higher brake line pressure underneath the deboost piston and the pump piston forces the pump piston against its cam and raises the deboost piston. Fluid above both pistons is displaced back to the reservoir via the dump valve. The effect of the deboost piston rising is to close the ball cut-off valve so that the master cylinder pipe line fluid output and the wheel cylinder pipe line input become isolated from each other. As a result, the sealed chamber space below the deboost piston is enlarged, causing a rapid reduction in the fluid pressure delivered to the wheel cylinders and preventing the wheels connected to this brake circuit locking.

Brake pressure increasing (Fig. 11.39(c)) The pressure reduction resulting from the previous phase releases the brakes and allows the wheel to accelerate to the speed of the still decelerating flywheel. When the drive shaft and the flywheel are at roughly equal speeds, the clutch balls roll down their respective ramps, enabling the dump valve lever return spring to slide the flywheel over. The dump valve lever then pivots and closes the needle-type dump valve. The flywheel is again coupled to

Fig. 11.39 (a-c) Antilock braking system (ABS) for front wheel drive

the drive shaft so that its speed rises with the drive shaft. At the same time the pump piston commences to build up pressure above the deboost piston by the action of the pump inlet and outlet valves. The output pressure generated by the pump pushes the deboost piston downward and, because the space underneath the deboost piston forms part of the brake pipe line circuit leading to the wheel cylinders, the total fluid volume is reduced. The brake pipe line pressure will be restored in steps due to the pump action until the downward movement of the deboost piston stem once again opens the cut-off valve. The pump piston then disengages and thereafter further pressure rise in the brake pipe line will be provided by the master cylinder in the normal way.

11.7.2 Hydraulic-electric antilock brake system (ABS) suitable for cars (Bosch) (Figs 11.40 and 11.41)

Speed sensor and excitor (Fig. 11.40) The speed sensor uses the variable reluctance magnetic sensing principle, whereby a cylindrical permanent magnetic core with a coil wire wound around it, mounted on the stationary hub carrier, axle casing or back plate, produces a magnetic field (flux) which overlaps the rotating excitor ring. The exci-tor may be of the tooth ring or rib-slot ring type attached to the rotating wheel hub or drive shaft. A number of teeth or slots are arranged radially which, with the speed of rotation of the road wheel, determine the frequency of the signal transmitted to the electronic-control unit. As the wheel and excitor revolve, the teeth and gaps or ribs and slots of the excitor pass through the magnetic field of the sensor. The coil wrapped around the mag-

Fig. 11.40 Magnetic speed sensor and excitor

netic cone senses the changing intensity of the magnetic field as the teeth or ribs pass through the flux lines and so an alternating voltage is induced in the coil, whose frequency is proportional to the speed of the rotating wheel. The voltage is transmitted to the control unit whenever the road wheels are rotating, regardless of whether the brakes are applied.

The road wheel speed measured by the speed sensor provides the wheel deceleration and wheel acceleration signals for the electronic-control unit. The merging and processing of the individual wheel speed sensor signals by the control unit provide a single reference speed which is roughly the vehicle speed. A comparison of any individual wheel speed with the reference speed supplies the wheel to road slip (wheel tending to lock) signal.

Electronic-control unit (Fig. 11.41(a)) The function of the electronic-control unit is to receive, amplify, process, compute and energize the individual solenoid control valves. That is, to evaluate the minimum wheel deceleration and maximum wheel acceleration for optimum braking and accordingly supply the energizing current to the individual solenoid control valves so they can regulate the necessary wheel cylinder pipe line pressures.

Hydro/electric modulator (Fig. 11.41(a)) This unit combines the solenoid control valves; one for each wheel, an accumulator for each of the dualbrake circuits and a twin cylinder return flow pump driven from an electric motor. The solenoid valve switches half or fully on and off through the control unit's solid-state circuits, causing the master cylinder to wheel cylinder fluid supply to be interrupted many times per second. The reduced pressure accumulator rapidly depressurizes the wheel cylinder pipe line fluid when the solenoid valve opens the return passage, due to the diaphragm chamber space instantly enlarging to absorb the outflow of fluid. The return flow pump, with its inlet and outlet ball valves, transfers fluid under pressure from the reducer accumulator to the master cylinder output leading to the brake cylinders. By these means, the wheel cylinder fluid pressure is matched to the optimum braking severity relative to the condition of the road surface.

In the following description of the anti-skid system operating, only one wheel is considered for simplicity.

{b) Pressure hold position

Fig. 11.41 (a-c) Antilocking brake system (ABS) for cars

{b) Pressure hold position

Fig. 11.41 (a-c) Antilocking brake system (ABS) for cars

Normal braking conditions (Fig. 11.41(a)) Under normal braking conditions, the solenoid is disengaged and the armature valve is held in its lowest position by the return spring. When the brakes are applied, fluid flows unrestricted from the master cylinder to the wheel cylinder via the solenoid piston armature type valve central passage. This continues until the required pressure build-up against the caliper piston produces the desired retardation to the vehicle.

Pressure hold (Fig. 11.41(b)) When the wheel deceleration approaches some predetermined value, the speed sensor signals to the computer control unit the danger of the wheel locking. The control unit immediately responds by passing a small electric current to the appropriate solenoid valve. Accordingly, the solenoid coil is partially energized. This raises the armature valve until it blocks the flow of fluid passing from the master cylinder to the wheel cylinder pipe line. The fluid pressure in the pipe line is now held constant (Fig. 11.42).

Pressure reducing (Fig. 11.41(c)) Should the wheel sensor still signal an abnormally rapid speed reduction likely to cause the wheel to lock, the control unit increases the supply of current to the solenoid coil, causing the armature valve to lift

Time in seconds

Fig. 11.42 Typical antilock brake system (ABS) pressure, wheel and vehicle speed characteristics with respect to time o

Time in seconds

Fig. 11.42 Typical antilock brake system (ABS) pressure, wheel and vehicle speed characteristics with respect to time still further to a position where it uncovers the return flow passage. The 'hold' line pressure collapses instantly because the highly pressurized fluid is able to escape into the pressure reducer accumulator. At the same time as the accumulator is being charged, surplus fluid is drawn from the accumulator into the return flow pump via the inlet valve whence it is discharged back into the appropriate pressurized master cylinder output pipe line. Consequently, the reduction in pressure (Fig. 11.42) permits the wheel to accelerate once again and re-establish its grip with the road surface. During the time fluid is pumped back into the master cylinder output pipe line, a light pressure pulsation will be experienced on the foot pedal by the driver due to the cyclic discharge of the pump.

Pressure increasing (Fig. 11.41(a)) Once the wheel rotational movement has changed from a deceleration back to acceleration, the sensor signals to the control unit to switch off the solenoid valve current supply. The return spring instantly snaps the solenoid valve into its lowest position and once again the fluid passage between the master cylinder output pipe line and the wheel caliper cylinder pipe line is re-established, causing the brake to be re-applied (Fig. 11.42). The sensitivity and response time of the solenoid valve is such that the pulsating regulation takes place four to ten times per second.

11.7.3 Air ¬°electric antilock brake system (ABS) suitable for commercial vehicles (WABCO)

The antilock brake system (ABS) consists of wheel sensors and excitors which detect the deceleration and an acceleration of individual wheels by generating alternating voltages the frequency of which are proportional to the wheel speed (Fig. 11.43(a)).

Sensors on each wheel (Fig. 11.40) continually measure the wheel speed during braking and this information is transmitted to an electronic (processor) control unit which senses when any wheel is about to lock. Signals are rapidly relayed to solenoid control valve units which quickly adjust the brake air line pressure so that the wheels are braked in the optimum slip range.

Each wheel is controlled according to the grip available between its tyre and the road. By these means, the vehicle is brought to a halt in the shortest time without losing vehicle stability and steer-ability.

Fig. 11.43 (a-d) Antilock brake system for commercial vehicles (ABS)

Pressure increasing (Fig. 11.43(a)) When the foot pedal is depressed, initially both solenoids are switched off so that their armatures are moved to their outermost position by the return springs. Consequently the first solenoid's inlet valve (I) is closed and its exhaust valve (I) is open whereas the second solenoid valve's inlet valve (II) is open and its exhaust valve (II) is closed.

Under these conditions, pilot chamber (I) is exhausted of compressed air so that air delivered from the foot valve enters the solenoid control valve unit inlet port and pushes open diaphragm

(I) outlet passage, enabling compressed air to be supplied to the wheel brake actuator. At the same time pilot chamber (II) is filled with compressed air so that diaphragm (II) closes off the exhaust passage leading to the atmosphere. As a result, the foot pedal depression controls the rising air pressure (Fig. 11.44) delivered from the foot valve to the wheel actuator via the solenoid control valve unit.

Pressure reducing (Fig. 11.43(b)) As soon as wheel deceleration or wheel slip threshold values are exceeded, the sensor transmits this information to the electronic-control unit which signals to the solenoid valve unit to reduce the wheel actuator pipe line air pressure.

Both solenoids are energized. This opens inlet valve (I) whilst inlet valve (II) is closed and exhaust valve (II) is opened. The open inlet valve (I) allows air to enter and pressurize pilot chamber (I) so that diaphragm (I) closes the outlet passage, thus preventing any more air from the foot valve passing through to the outlet passage port.

At the same time, solenoid (II) closes inlet valve

(II) and opens exhaust valve (II). This exhausts air from pilot chamber (II), permitting compressed

/1 / L

Reducing |

Fig. 11.44 Air/electric antilock brake system (ABS) pressure/time characteristics o

Time (see)

Fig. 11.44 Air/electric antilock brake system (ABS) pressure/time characteristics air from the wheel actuator to push open diaphragm (II) outlet exhaust passage, causing the air pressure in the actuator pipe line to reduce quickly (Fig. 11.44).

Pressure hold (Fig. 11.43(c)) When the road wheel acceleration reaches some predetermined value, the sensor relays this information to the electronic-control unit, which in turn signals the solenoid control valve unit to hold the remaining pipe line actuator pressure.

Solenoid (I) remains energized but solenoid (II) is de-energized. Therefore solenoid (I) inlet valve (I) and exhaust valve (I) remain open and closed respectively. Inlet valve (II) allows compressed air into pilot chamber (I) so that diaphragm (I) closes the outlet passage leading to the wheel actuator pipe line.

Conversely, solenoid (II) is now de-energized causing its return spring to move the armature so that the inlet valve (II) opens and exhaust valve (II) closes. Compressed air from the foot valve now flows through the open inlet valve (II) along the passage leading to the underside of diaphragm (II), thus keeping the outlet exhaust passage closed. Compressed air at constant pressure (Fig. 11.44) is now trapped between both closed diaphragm outlet passages and the wheel actuator pipe line. This pipe line pressure is maintained until the sensor signals that the wheel is accelerating above its threshold, at which point the electronic-control unit signals the solenoid control valve to switch to its rising pressure mode.

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Responses

  • kalvin
    Where does the pressurized fluid from master cylinder flow through the pipe lines?
    3 years ago

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