electric hydraulic brake

8/7/2019 Electric Hydraulic Brake

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ELECTRIC HYDRAULIC BRAKE

Girish Kumar M

7 Mechanical


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INTRODUCTION

This is a system which senses the driver's will of brakingthrough the pedal simulator and controls the braking

pressures to each wheels. The system is also a hydraulicBrake by Wire system.

In the true sense of the definition, any EHB vehicle may bebraked with an electrical joystick completely

independent of the traditional brake pedal. Hydraulic fluidis used to transmit energy from the actuator to the wheelbrakes.


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HYDRAULIC DESIGN CONSIDERATIONS

EHB supplies a braking force proportional to

driver input, which reduces braking effort.The boost characteristics also contribute to

the pedal feel of the vehicle. If the boost

source fails, the system resorts to manual

brakes where brake input energy is suppliedin full by the driver.


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Input pedal force vs. Brake line pressure

Output of a typical vacuum boosted vehicle.


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Conventional vacuum boosted system


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The conventional system utilizes a largely

mechanical link all the way from the brakepedal through the vacuum booster and intothe master cylinder piston. Proportionalassist is provided by an air valve acting inconjunction with the booster diaphragm toutilize the stored vacuum energy. The pistonand seal trap brake fluid and transmit thehydraulic energy to the wheel brake.


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Electro- Hydraulic Braking System


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The drivers input is normally interpreted by up to threedifferent devices: a brake switch, a travel sensor, and a

pressure sensor while an emulator provides the normal pedalfeel. To prevent unwanted brake applications, two of the threeinputs must be detected to initiate base brake pressure. Thebackup master cylinder is subsequently locked out of themain wheel circuit using isolation solenoid valves, so allwheel brake pressure must come from a high-pressure

accumulator source. Drivers braking intent signals are sent to the ECU by the wire

system

An algorithm translates the dynamically changing voltage inputsignals into the corresponding solenoid valve driver outputcurrent waveforms.


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A pressure sensor at each wheel continuously closes the loopby feeding back information to the ECU so the next series of

current commands can be given to the solenoid valves toassure fast and accurate pressure response.

To eliminate the possibility of boost failure due to electronic ormechanical faults, the ECU design, component redundancy isused throughout. This includes multiple wire feeds, multipleprocessors and internal circuit isolation for critical valve drivers

Thermal robustness must also carefully be designed

Careful attention must be given to heat sinking, materials,circuit designs, and component selection. Special considerationmust be given to the ECU cover heat transfer properties, whichcould include the addition of cooling fins.


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BASE NON-ISOLATED

CIRCUIT


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For failsafe operation, it becomes necessary to include anisolation valve between the pedal feel emulator -master

cylinder (PF

E-MC) assembly and wheel brake. Its functionsinclude blocking the drivers manual output pressureduring a boosted apply as well as providing a vent pathback to reservoir when the brakes are not activated.

A balance valve is placed between wheel brakes on eachaxle to prevent momentary pressure imbalance during

panic-type base brake applies.

The accumulator circuit leads directly into the mastercylinder and wheel brake circuits through the apply valve


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EHBs utilize brake fluid stored in a central, gas

pressurized accumulator. The gas most commonly used is nitrogen due to its

relatively low cost and relative inertness. The nitrogen gasis kept separated from the brake fluid by either anelastomeric or metallic membrane or diaphragm.

The nitrogen gas will typically find its way through mostelastomeric materials, and enter the molecular poreswithin the spaces of the pressurized brake fluid volumeuntil all of the voids are filled. At that time,the equilibriumis re-established


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Failure Mode Considerations

A carefully constructed accumulator with a

small hole punctured in the diaphragm wasinstalled in a vehicle. The brakes were

subsequently applied and released at

discreet intervals to study any change in

operating characteristics.


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Graph showing Vehicle Pedal Force / Pedal

Travel


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Figure is a plot of the brake system performance just one strokeafter the failure occurrence. In this case, in the failed systembackup mode, the PFE-MC assembly achieved the full travel of120 mm with an input force of only 45 N.

At stroke number 114 of the brake pedal, the diagnostics of the ECUdetected a pressure out-of-bounds failure indicating base brake outputpressure was no longer able to follow the drivers brake pedal inputcommands. The system immediately reverted to the hydraulic failsafebackup mode.


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A significant percentage of the nitrogen made its wayinto the HCU-to-wheel brake circuit. The system was then bleed

between the HCU and wheel brakes.


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The system is completely rebleeded


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ISOLATED HYDRAULICCIRCUIT


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There is an isolation piston assembly between the pumpcircuit and each wheel brake circuit which will positively

stop nitrogen from entering the wheel brake circuit. Release valve is normally open. This provides an open flow

path back to reservoir, which is independent of the wheelbrake circuit. Any escaping nitrogen from the accumulatorwill have an unrestricted path back to reservoir in failsafemode.

The amount of nitrogen gas, which can be permanentlytrapped, is limited to the drilled holes in the HCU housing,the clearance volume behind the isolation piston, and thevolumes around the proportional control valves.


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Pedal Force Emulator Master Cylinder

(PFE-MC)


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The unit consists of a master cylinder with emulator pistonand spring assembly. As the drivers foot applies the brake

pedal, an input push rod displaces the primary mastercylinder piston, while at the same time the isolation valvesin the HCU are commanded to close. This blocks bothprimary and secondary master cylinder outlet ports. Thesecondary piston becomes locked in place due to thetrapped fluid. The fluid contained by the primary piston isdisplaced into the drill path, which leads to the emulator

assembly. As pressure continues to build, the springbegins to deform under the load from the hydraulicpressure acting on the surface of the piston. This causesthe brake pedal to move in proportion to the force exertedby the driver.


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CONCLUSION

EHB design allows system flexibility, inherent accumulatorprecharge isolation, and the ability to tune for optimum failedsystem stopping performance for all vehicle classes.

A carefully designed and implemented EHB system holds thepromise of enabling the new brake-by-wire features while still

reliably performing the everyday task of stopping the vehicle.


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THANK YOU

electric hydraulic brake

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