Abstract:
Disclosed herein is an electro-hydraulic brake system. The electro-hydraulic brake system includes a master cylinder to transmit brake oil, a high-pressure accumulator to transmit brake oil independently of the master cylinder, first and second circuits connected respectively to the master cylinder, at least one first wheel to be braked by the first circuit, and at least one second wheel to be braked by the second circuit. The first circuit is connected to the high-pressure accumulator, to control the first wheel. The second circuit is connected to the first circuit, to control the second wheel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 2009-0069422, filed on Jul. 29, 2009 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Embodiments of the present invention relate to an electro-hydraulic brake system to control hydraulic brake force according to a change in regenerative brake force. 
     2. Description of the Related Art 
     A hybrid electric vehicle includes more than one power source, such as an electric motor (drive motor) and an internal combustion engine, and selectively uses power of the engine or the electric motor according to the load and velocity of the vehicle. The motor also functions to convert the remaining energy into electric energy. Thus, the hybrid electric vehicle may achieve high fuel efficiency and low environmental pollution. 
     In the above described hybrid electric vehicle, drive wheels of the vehicle are rotated by the electric motor that is operated by electric energy during traveling. In this case, the utilization efficiency of electric energy in the electric motor may be very important. To this end, if a vehicle driver commands deceleration or braking, the electric motor functions as a generator to generate electric energy. The generated electric energy is stored in a capacitor. While the electric motor functions as a generator, brake force is applied to the wheels of the vehicle. This is referred to as regenerative braking. In conclusion, the brake force applied to the wheels is the sum of regenerative brake force generated by the electric motor and hydraulic brake force generated by a hydraulic mechanism. 
     In other words, driver requested braking may be satisfied by generating only the hydraulic brake force that corresponds to a difference between the regenerative brake force generated by the electric motor and brake force demanded by the driver. 
     SUMMARY 
     Therefore, it is an aspect of the present invention to provide an electro-hydraulic brake system in which each circuit may brake wheels by use of a high-pressure accumulator for control of hydraulic brake force. 
     Additional aspects of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     In accordance with an aspect of the present invention, an electro-hydraulic brake system includes a master cylinder to transmit brake oil, a high-pressure accumulator to transmit brake oil independently of the master cylinder, a first circuit connected to the master cylinder and the high-pressure accumulator, a second circuit connected to the master cylinder and the first circuit, a first wheel to be braked by the first circuit, and a second wheel to be braked by the second circuit, wherein the first circuit includes a first flow-rate control valve to control a flow rate of the brake oil transmitted from the high-pressure accumulator into the first circuit, and the second circuit includes a second flow-rate control valve to control a flow rate of the brake oil introduced into the second circuit. 
     The electro-hydraulic brake system may further include a first inlet path provided with the first flow-rate control valve and connecting the high-pressure accumulator and the first circuit to each other, and a second inlet path provided with the second flow-rate control valve and connecting the first circuit and the second circuit to each other. 
     The electro-hydraulic brake system may further include a first backup path connecting the master cylinder and the first circuit to each other, a second backup path connecting the master cylinder and the second circuit to each other, a first mode switching valve to control connection between the first backup path and the master cylinder, and a second mode switching valve to control connection between the second backup path and the master cylinder. 
     The electro-hydraulic brake system may further include a first pressure sensor provided at an exit side of the first flow-rate control valve to measure pressure of oil introduced into the first circuit, and a second pressure sensor provided at an exit side of the second flow-rate control valve to measure pressure of oil introduced into the second circuit. 
     The electro-hydraulic brake system may further include a third pressure sensor provided at an entrance side of the first mode switching valve to measure brake pedal pressure. 
     The first mode switching valve, the second mode switching valve, the first flow-rate control valve and the second flow-rate control valve may be arranged in a row, to control transmission of the brake oil to the wheels. 
     During normal braking, the first mode switching valve and the second mode switching valve may be kept in a closed state, and the first flow-rate control valve and the second flow-rate control valve may transmit hydraulic pressure generated in the high-pressure accumulator to the first circuit and the second circuit, and during emergency braking, the first mode switching valve and the second mode switching valve may be kept in an open state, to transmit hydraulic pressure generated in the master cylinder to the first circuit and the second circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a hydraulic circuit diagram illustrating a non-braking mode of an Electro-Hydraulic Brake (EHB) system in accordance with an embodiment of the present invention; 
         FIG. 2  is a hydraulic circuit diagram illustrating a braking mode of the electro-hydraulic brake system in accordance with the embodiment of the present invention under normal operation of the system; and 
         FIG. 3  is a hydraulic circuit diagram illustrating a braking mode of the electro-hydraulic brake system in accordance with the embodiment of the present invention under abnormal operation of the system. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiment of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  is a hydraulic circuit diagram illustrating a non-braking mode of an electro-hydraulic brake system in accordance with the embodiment of the present invention. 
     Referring to  FIG. 1 , the electro-hydraulic brake system (hereinafter, referred to as “EHB” system) in accordance with the embodiment may include a brake pedal  10  to be operated by a driver upon braking, a master cylinder  11  to which force from the brake pedal  10  is transmitted, a reservoir  12  located above the master cylinder  11  to store oil therein, wheel cylinders  20   a ,  20   b ,  20   c  and  20   d  to perform braking of respective wheels RR, RL, FR and FL using the oil transmitted from the reservoir  12 , and a hydraulic control unit  30  (hereinafter, referred to as “HCU”) provided between the master cylinder  11  and the wheel cylinders  20   a ,  20   b ,  20   c  and  20   d.    
     The driver may decelerate or stop a traveling vehicle by pressing the brake pedal  10 . 
     The master cylinder  11  is located to receive the oil from the reservoir  12  located thereabove, and to discharge the oil downward through an exit  11   a  thereof. The discharged oil is introduced into the HCU  30 . 
     The HCU  30  may include a first circuit  41  to control the rear wheels RL and RR and a second circuit  42  to control the front wheels FL and FR. 
     The first circuit  41  may include a first inlet path  43  connected to the wheel cylinders  20   a  and  20   b . In addition, the first circuit  41  may include, for example, a flow-rate control valve  45 , inlet valve  47 , pressure reducing valve  61  and pressure sensor  53 . The second circuit  42  may include a second inlet path  44  connected to the wheel cylinders  20   c  and  20   d . In addition, the second circuit  42  may include, for example, a flow-rate control valve  46 , inlet valve  48 , pressure reducing valve  62  and pressure sensor  54 . 
     The HCU  30  may include a pump  63 , motor  64  and high-pressure accumulator  65 . An exit of the high-pressure accumulator  65  may be connected to the first inlet path  43  of the first circuit  41 . The first circuit  41  and second circuit  42  may be connected to each other via a connection path  40 , to allow brake oil of the high-pressure accumulator  65  to pass through the first circuit  41  and second circuit  42  in sequence. In this way, the brake oil having passed through the first inlet path  43  of the first circuit  41  may be transmitted to the second inlet path  44  of the second circuit  42  through the connection path  40 . 
     The pump  63  functions to pump the oil transmitted from the reservoir  12  at a high pressure to create brake pressure. To this end, the motor  64  is connected to the pump  63  to provide the pump  63  with drive power. 
     The high-pressure accumulator  65  is provided at an exit side of the pump  63  and temporarily stores high-pressure oil generated by operation of the pump  63 . A first pressure sensor  51  is provided at an exit side of the high-pressure accumulator  65  to measure the pressure of the oil from the high-pressure accumulator  65 . If the measured oil pressure is lower than a preset pressure, the pump  63  is operated to suction the oil of the reservoir  12  and to charge the high-pressure accumulator  65  with the oil. 
     The first flow-rate control valve  45  is of a normal close type and is normally kept in a closed state. The first flow-rate control valve  45  is opened when the driver pushes the brake pedal  10 , allowing the brake oil stored in the high-pressure accumulator  65  to be introduced into the first circuit  41 . 
     The third pressure sensor  53  is provided at an exit side of the first flow-rate control valve  45  to measure the pressure of oil introduced into the first circuit  41 . The opening rate of the first flow-rate control valve  45  is regulated based on the measured oil pressure, to control the flow rate of brake oil introduced into the first circuit  41 . 
     The first inlet valve  47  is of a normal close type and is normally kept in a closed state. The first inlet valve  47  is opened when the driver pushes the brake pedal  10 , allowing the brake oil in the first circuit  41  to be transmitted to the corresponding wheel cylinder. 
     The first pressure reducing valve  61  is of a normal open type and is normally kept in an open state. The first pressure reducing valve  61  is closed when the driver pushes the brake pedal  10 , to prevent the brake oil from being transmitted to the corresponding wheel cylinder. 
     The second flow-rate control valve  46  is of a normal close type and is normally kept in a closed state. The first flow-rate control valve  45  is opened when the driver pushes the brake pedal  10 , allowing the brake oil having passed through the first flow-rate control valve  45  of the first circuit  41  to be introduced into the second circuit  42 . 
     The fourth pressure sensor  54  is provided at an exit side of the second flow-rate control valve  46  to measure the pressure of oil introduced into the second circuit  42 . The opening rate of the second flow-rate control valve  46  is regulated based on the measured oil pressure, to control the flow rate of the brake oil introduced into the second circuit  42 . 
     The second pressure reducing valve  62  is of a normal open type and is normally kept in an open state. The second pressure reducing valve  62  is closed when the driver pushes the brake pedal  10 , to prevent the brake oil from being transmitted to the corresponding wheel cylinder. 
     The HCU  30  may include a return path  60  that connects an entrance of the pump  63  to the wheel cylinders  20   a ,  20   b ,  20   c  and  20   d . The return path  60  may be provided with pressure reducing valves  61  and  62  that discharge the oil of the wheel cylinders  20   a ,  20   b ,  20   c  and  20   d  to the entrance of the pump  63 . The oil discharged from the pressure reducing valves  61  and  62  may be temporarily stored in the reservoir  12 . 
     A first backup path  71  and second backup path  72  may be provided between the exit  11   a  of the master cylinder  11  and the wheel cylinders  20   a ,  20   b ,  20   c  and  20   d . These backup paths  71  and  72  may be used when the EHB system breaks down. The first backup path  71  may be provided with a first mode switching valve  73  to open or close the first backup path  71 , and the second backup path  72  may be provided with a second mode switching valve  74  to open or close the second backup path  72 . 
     The first mode switching valve  73  is of a normal open type and is normally kept in an open state. The first mode switching valve  73  is closed during normal braking, to prevent hydraulic pressure generated in the master cylinder  11  from being transmitted to the first circuit  41 . On the other hand, if an emergency braking situation, such as breakdown of the system, occurs, the first mode switching valve  73  is opened to transmit the hydraulic pressure of the master cylinder  11  to the first circuit  41 . 
     The second mode switching valve  74  is of a normal open type and is normally kept in an open state. The first mode switching valve  73  is closed during normal braking to prevent hydraulic pressure generated in the master cylinder  11  from being transmitted to the second circuit  42 . On the other hand, if an emergency braking situation, such as breakdown of the system, occurs, the second mode switching valve  74  is opened to transmit the hydraulic pressure of the master cylinder  11  to the second circuit  42 . 
     A second pressure sensor  52  to measure the pressure of oil from the master cylinder  11  may be provided between the first mode switching valve  73  and the exit  11   a  of the master cylinder  11 . The backup path  71  may be intercepted by the first mode switching valve  73  during normal braking, and the second pressure sensor  52  may be used to determine driver requested braking. 
     A pedal simulator  80  may be provided between the second pressure sensor  52  and the master cylinder  11 , to generate pedal force of the brake pedal  10 . 
     The pedal simulator  80  includes a simulation chamber  81  to store the oil discharged from the exit  11   a  of the master cylinder  11  and a simulator valve  82  provided at an entrance of the simulation chamber  81 . The simulation chamber  81  contains a piston  83  and elastic member  84  so as to be displaced within a constant range by the oil introduced thereinto. The simulator valve  82  is of a normal close type and is normally kept in a closed state. The simulator valve  82  is opened when the driver pushes the brake pedal  10 , allowing the brake oil to be transmitted into the simulation chamber  81 . 
     An Electric Control Unit (ECU) (not shown) to control electrically operative constituent elements and the HCU  30  containing the above described pedal simulator  80  may be combined to define a single compact Hydro Electronic Control Unit (HECU). 
     In the EHB system in accordance with the embodiment of the present invention, the simulator to create the pedal force as well as the motor, pump, accumulator, various valves and sensors are contained in the HCU in the form of a single block and thus, a master cylinder having a simulator only for the EHB system may be unnecessary. In this way, any generalized master cylinder may be used. 
     In addition, the simulator of the HCU is installed to protrude outward. This may prevent deterioration in reliability and durability of the EHB system due to abrasion of a seal inside the master cylinder and also, may reduce a space for arrangement of the master cylinder. 
     Hereinafter, operation of the EHB system in accordance with the embodiment of the present invention will be described in detail. 
       FIG. 2  is a hydraulic circuit diagram illustrating a braking mode of the EHB system under normal operation of the system. 
     Referring to  FIG. 2 , in a normal braking mode, if the driver pushes the brake pedal  10 , the first and second mode switching valves  73  and  74  are kept in a closed state to intercept the first and second backup paths  71  and  72 . 
     The pedal simulator  80  provides tactile feedback to the driver through the pedal. The force applied to the pedal  10  by the driver is transmitted to the pedal simulator  80 , creating tactile feedback. 
     A control unit may sense brake force demanded by the driver from information, such as pressure applied to the pedal  10  by the driver, etc. In addition, the magnitude of regenerative brake force may be input into the control unit. The control unit may calculate the magnitude of frictional brake force corresponding to a difference between the brake force demanded by the driver and the regenerative brake force, and consequently, may determine the magnitude of increased or decreased pressure at the wheels. 
     If the driver pushes the brake pedal  10  at an initial braking stage, the vehicle is sufficiently braked by the regenerative brake force and thus, may be controlled to generate no frictional brake force. That is, since the backup paths  71  and  72  are intercepted, the hydraulic pressure of the master cylinder  11  is not transmitted to the wheel cylinders  20   a ,  20   b ,  20   c  and  20   d.    
     Thereafter, an operation of regulating the frictional brake force according to a change in the regenerative brake force may be performed. The regenerative brake force is changed according to the charging rate of a battery or the velocity of the vehicle. For example, the regenerative brake force exhibits a rapid decrease below a predetermined velocity. To deal with this situation, it may be necessary to increase the frictional brake force. 
     The first flow-rate control valve  45  may control the flow rate of brake oil transmitted from the high-pressure accumulator  65  to the first circuit  41  through the first inlet path  43 . The second flow-rate control valve  46  may control the flow rate of brake oil transmitted from the first inlet path  43  of the first circuit  41  to the second inlet path  44  of the second circuit  42  through the connection path  40 . 
     Since no regenerative brake force is generated, the frictional brake force equal to brake force demanded by the driver may be generated. That is, the first flow-rate control valve  45  and second flow-rate control valve  46  may supply the brake oil of the high-pressure accumulator  65  toward the wheel cylinders  20   a ,  20   b ,  20   c  and  20   d.    
       FIG. 3  is a hydraulic circuit diagram illustrating a braking mode of the EHB system under abnormal operation of the system. 
     Referring to  FIG. 3 , assuming abnormal operation of the EHB system, the first and second mode switching valves  73  and  74  are kept in an open state for emergency braking, allowing the brake oil of the master cylinder  11  to be directly transmitted to the wheel cylinders  20   a ,  20   b ,  20   c  and  20   d  through the backup paths  71  and  72 . That is, if the driver pushes the brake pedal  10 , the hydraulic pressure of the master cylinder  10  may be transmitted to the wheel cylinders  20   a ,  20   b ,  20   c  and  20   d  through the backup paths  71  and  72 . 
     As is apparent from the above description, in accordance with an embodiment of the present invention, a single circuit is allotted to front wheels and a single circuit is applied to rear wheels. This circuit control may reduce costs. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.