ELECTRIC HYDRAULIC BRAKE
Girish Kumar M
7 Mechanical


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ELECTRIC HYDRAULIC BRAKE
Girish Kumar M
7 Mechanical
INTRODUCTION
This is a system which senses the driver's will of braking through the pedal simulator and controls the braking pressures to each wheels. The system is also a hydraulic Brake by Wire system.


In the true sense of the definition, any EHB vehicle may be braked with an electrical “joystick” completely independent of the traditional brake pedal. Hydraulic fluid is used to transmit energy from the actuator to the wheel brakes.
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 supplied in full by the driver.
Input pedal force vs. Brake line pressure Output of a typical vacuum boosted vehicle.
Conventional vacuum boosted system

The conventional system utilizes a largely mechanical page link all the way from the brake pedal through the vacuum booster and into the master cylinder piston. Proportional assist is provided by an air valve acting in conjunction with the booster diaphragm to utilize the stored vacuum energy. The piston and seal trap brake fluid and transmit the hydraulic energy to the wheel brake.
Electro- Hydraulic Braking System

The driver’s input is normally interpreted by up to three different devices: a brake switch, a travel sensor, and a pressure sensor while an emulator provides the normal pedal “feel”. To prevent unwanted brake applications, two of the three inputs must be detected to initiate base brake pressure. The backup master cylinder is subsequently locked out of the main wheel circuit using isolation solenoid valves, so all wheel brake pressure must come from a high-pressure accumulator source.
Driver’s braking intent signals are sent to the ECU by the wire system
An algorithm translates the dynamically changing voltage input signals into the corresponding solenoid valve driver output current waveforms.

A pressure sensor at each wheel continuously “closes the loop” by feeding back information to the ECU so the next series of current commands can be given to the solenoid valves to assure fast and accurate pressure response.
To eliminate the possibility of boost failure due to electronic or mechanical faults, the ECU design, component redundancy is used throughout. This includes multiple wire feeds, multiple processors 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 consideration must be given to the ECU cover heat transfer properties, which could include the addition of cooling fins.
BASE NON-ISOLATED CIRCUIT

For failsafe operation, it becomes necessary to include an isolation valve between the pedal feel emulator -master cylinder (PFE-MC) assembly and wheel brake. Its functions include blocking the driver’s manual output pressure during a boosted apply as well as providing a vent path back to reservoir when the brakes are not activated.

A balance valve is placed between wheel brakes on each axle to prevent momentary pressure imbalance during panic-type base brake applies.

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


EHB’s 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 gas is kept separated from the brake fluid by either an elastomeric or metallic membrane or diaphragm.
The nitrogen gas will typically find its way through most elastomeric materials, and enter the molecular “pores” within the spaces of the pressurized brake fluid volume until all of the voids are filled. At that time,the equilibrium is re-established

Failure Mode Considerations
A carefully constructed accumulator with a small hole punctured in the diaphragm was installed in a vehicle. The brakes were subsequently applied and released at discreet intervals to study any change in operating characteristics.
Graph showing Vehicle Pedal Force / Pedal Travel


Figure is a plot of the brake system performance just one stroke after the failure occurrence. In this case, in the failed system backup mode, the PFE-MC assembly achieved the full travel of 120 mm with an input force of only 45 N.



The system is completely rebleeded
ISOLATED HYDRAULIC CIRCUIT

There is an isolation piston assembly between the pump circuit 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 wheel brake circuit. Any escaping nitrogen from the accumulator will have an unrestricted path back to reservoir in failsafe mode.
The amount of nitrogen gas, which can be permanently trapped, is limited to the drilled holes in the HCU housing, the clearance volume behind the isolation piston, and the volumes around the proportional control valves.
Pedal Force Emulator – Master Cylinder (PFE-MC)

The unit consists of a master cylinder with emulator piston and spring assembly. As the driver’s foot applies the brake pedal, an input push rod displaces the primary master cylinder piston, while at the same time the isolation valves in the HCU are commanded to close. This blocks both primary and secondary master cylinder outlet ports. The secondary piston becomes locked in place due to the trapped fluid. The fluid contained by the primary piston is displaced into the drill path, which leads to the emulator assembly. As pressure continues to build, the spring begins to deform under the load from the hydraulic pressure acting on the surface of the piston. This causes the brake pedal to move in proportion to the force exerted by the driver.
CONCLUSION

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


A carefully designed and implemented EHB system holds the promise of enabling the new brake-by-wire features while still reliably performing the everyday task of stopping the vehicle.
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