Patent Publication Number: US-11046294-B2

Title: Electronic brake system and method for operating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application Nos. 2017-0127433 and 2017-0127510, respectively filed on Sep. 29, 2017 and Sep. 29, 2017 in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Embodiments of the present disclosure relate to an electronic brake system and a method for operating the same, and more particularly to an electronic brake system for generating braking force using an electrical signal corresponding to a displacement of a brake pedal, and a method for operating the same. 
     2. Description of the Related Art 
     A brake system for braking of a vehicle is essentially mounted to a vehicle, and various brake systems have recently been proposed to secure safety of a driver and passengers. 
     Conventionally, when a driver depresses a brake pedal, a conventional brake system is designed to supply hydraulic pressure needed for braking to wheel cylinders using a booster mechanically connected to the brake pedal. However, as the demand of users who desire to implement various braking functions according to vehicle driving environments is rapidly increasing, an electronic brake system provided with a hydraulic-pressure supply device has recently been developed and rapidly come into widespread use. Once a driver pushes a brake pedal, the hydraulic-pressure supply device of the electronic brake system senses a displacement of the brake pedal through a pedal displacement sensor, and receives an electric signal indicating the driver&#39;s braking intention from the pedal displacement sensor, such that hydraulic pressure needed for braking is supplied to wheel cylinders. 
     When the electronic brake system is in a normal operation mode, a displacement of the brake pedal depressed by the driver is converted into an electric signal, the electric signal is supplied to the hydraulic-pressure supply device, and the hydraulic-pressure supply device is electrically operated and controlled based on the electric signal, such that hydraulic pressure needed for braking is formed and supplied to wheel cylinders. Since the electronic brake system can be electrically operated and controlled as described above, the electronic brake system can implement complicated and various braking actions. However, if technical issues occur in electronic components of the electronic brake system, hydraulic pressure needed for braking is not stably formed, there is a high possibility of threatening the safety of a driver and passengers who ride in the vehicle. 
     Therefore, if any one of various electronic components embedded in the vehicle abnormally operates or if it is impossible to control the abnormal electronic component, the electronic brake system enters an abnormal operation mode. In this case, there is needed a mechanism in which an operation state of the brake pedal depressed by the driver is directly interoperable with wheel cylinders. That is, during the abnormal operation mode of the electronic brake system, hydraulic pressure needed for braking needs to be immediately formed in response to a pedal effort of the brake pedal depressed by the driver, and the hydraulic pressure needs to be directly supplied to wheel cylinders. 
     Meanwhile, as the demand of users who desire to use eco-friendly vehicles is rapidly increasing, hybrid vehicles are becoming more and more popular with consumers. Generally, a hybrid vehicle converts kinetic energy generated by vehicle deceleration into electric energy, stores the electric energy in a battery, and supplementarily uses the stored energy during vehicle driving, resulting in increased fuel efficiency. As a result, hybrid vehicles have become prevalent and more popular with consumers. 
     In order to increase an energy recovery rate, the hybrid vehicle is designed to recover energy using a generator or the like during braking or deceleration of the vehicle, such that this braking operation is referred to as a regenerative braking operation. However, during regenerative braking, this regenerative braking mode may unavoidably affect distribution of brake force applied to a plurality of vehicle wheels, such that oversteer, understeer, or slippage of the wheels may occur, resulting in reduction of vehicle driving stability. 
     CITED REFERENCE 
     Patent Document 
     European Registered Patent No. EP 2 520 473 A1 (Honda Motor Co., Ltd.), (Nov. 7, 2012) 
     SUMMARY 
     Therefore, it is an aspect of the present disclosure to provide an electronic brake system for stably distributing and providing brake pressure to wheels of a vehicle during regenerative braking of the vehicle, and a method for operating the same. 
     It is another aspect of the present disclosure to provide an electronic brake system for efficiently braking a vehicle in various driving situations, and a method for operating the same. 
     It is another aspect of the present disclosure to provide an electronic brake system for implementing driving stability of a vehicle, and a method for operating the same. 
     It is another aspect of the present disclosure to provide an electronic brake system capable of stably generating high brake pressure, and a method for operating the same. 
     It is another aspect of the present disclosure to provide an electronic brake system for improving performance and operational stability of a product, and a method for operating the same. 
     It is another aspect of the present disclosure to provide an electronic brake system for improving product durability by reducing load applied to electronic components, and a method for operating the same. 
     It is another aspect of the present disclosure to provide an electronic brake system for reducing the size of a product and the number of electronic components of the product, and a method for operating the same. 
     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 disclosure, an electronic brake system includes a hydraulic-pressure supply device configured to generate a hydraulic pressure by operating a hydraulic piston using an electric signal that is output in response to a displacement of a brake pedal, as well as to include not only a first pressure chamber formed at one side of the hydraulic piston movably disposed in a cylinder block, but also a second pressure chamber formed at the other side of the hydraulic piston, and a hydraulic control unit configured to include not only a first hydraulic circuit to control a hydraulic pressure applied to two wheel cylinders, but also a second hydraulic circuit to control a hydraulic pressure applied to two other wheel cylinders. The hydraulic control unit includes a first hydraulic passage configured to communicate with the first pressure chamber, a second hydraulic passage branched from the first hydraulic passage, a third hydraulic passage connected to the second hydraulic circuit while being branched from the first hydraulic passage, a fourth hydraulic passage configured to communicate with the second pressure chamber, fifth and sixth hydraulic passages that are branched from an intermediate part of the fourth hydraulic passage and are then linked again to the fourth hydraulic passage, a seventh hydraulic passage configured to connect a second hydraulic passage to a third hydraulic passage, and an eighth hydraulic passage configured to connect both the second hydraulic passage and the fourth hydraulic passage to the first hydraulic circuit. 
     The hydraulic control unit may include a first valve provided in the second hydraulic passage to control flow of a pressure medium, a second valve provided in the third hydraulic passage to control flow of a pressure medium, a third valve provided in the fifth hydraulic passage to control flow of a pressure medium, a fourth valve provided in the sixth hydraulic passage to control flow of a pressure medium, a fifth valve provided in the seventh hydraulic passage to control flow of a pressure medium, and a sixth valve provided in the eighth hydraulic passage to control flow of a pressure medium. 
     Each of the first, third, fifth, and sixth valves may be provided as a solenoid valve to control bidirectional flow of the pressure medium. The second valve may be provided as a check valve that allows only flow of the pressure medium flowing from the first pressure chamber to the second hydraulic circuit. The fourth valve may be provided as a check valve that allows only flow of the pressure medium flowing from the second pressure chamber to the eighth hydraulic passage. 
     A generator may be provided in two wheel cylinders of the first hydraulic circuit. 
     The electronic brake system may further include a first dump passage configured to connect the first pressure chamber to a reservoir storing a pressure medium, a second dump passage configured to connect the second pressure chamber to the reservoir, a first dump valve provided in the first dump passage to control flow of a pressure medium, and provided as a check valve that allows only flow of a pressure medium flowing from the reservoir to the first pressure chamber, a second dump valve provided in the second dump passage to control flow of a pressure medium, and provided as a check valve that allows only flow of a pressure medium flowing from the reservoir to the second pressure chamber, and a third dump valve provided in a bypass passage connected parallel to the second dump valve on the second dump passage so as to control flow of a pressure medium, and provided as a solenoid valve that controls bidirectional flow of a pressure medium flowing between the reservoir and the second pressure chamber. 
     The electronic brake system may further include a master cylinder configured to include first and second master chambers and first and second pistons respectively provided in the first and second master chambers, as well as to discharge a pressure medium in response to a pedal effort of the brake pedal, and a reservoir passage configured to connect the reservoir to the master cylinder. 
     The reservoir passage may include a first reservoir passage configured to connect the first master chamber to the reservoir, a second reservoir passage configured to connect the second master chamber to the reservoir, a reservoir check valve provided in the first reservoir passage to control flow of a pressure medium, and configured to allow only flow of a pressure medium flowing from the reservoir to the master chamber, and an inspection valve provided in a bypass passage connected parallel to the reservoir check valve on the first reservoir passage so as to control flow of a pressure medium, and provided as a solenoid valve that controls bidirectional flow of a pressure medium flowing between the first master chamber and the reservoir. 
     The electronic brake system may further include a first backup passage configured to connect the first master chamber to the first hydraulic circuit, a second backup passage configured to connect the second master chamber to the second hydraulic circuit, a first cut valve provided in the first backup passage to control flow of a pressure medium, and a second cut valve provided in the second backup passage to control flow of a pressure medium. 
     The electronic brake system may further include a simulation device connected to the master cylinder to provide a reaction force corresponding to a pedal effort of the brake pedal, and a simulator valve configured to open or close a passage disposed between the master cylinder and the simulation device. 
     The first hydraulic circuit may include first and second inlet valves that respectively control hydraulic pressures applied to two wheel cylinders, and first and second outlet valves that respectively control hydraulic pressures discharged from two wheel cylinders to the reservoir. The second hydraulic circuit may include third and fourth inlet valves that respectively control hydraulic pressures applied to two other wheel cylinders, and third and fourth outlet valves that respectively control hydraulic pressures discharged from two other wheel cylinders to the reservoir. 
     One end of the eighth hydraulic passage may be connected to the second hydraulic passage, and the other end of the eighth hydraulic passage may be connected to the first hydraulic circuit, such that the fourth hydraulic passage is linked to an intermediate part of the eighth hydraulic passage. The sixth valve may be disposed between one position where the sixth valve is connected to the second hydraulic passage on the eighth hydraulic passage and another position where the sixth valve is linked to the fourth hydraulic passage on the eighth hydraulic passage. 
     The second hydraulic passage and the fourth hydraulic passage may be linked to each other. The eighth hydraulic passage may connect a linked position of the second hydraulic passage and the fourth hydraulic passage to the first hydraulic circuit. 
     A method for operating the electronic brake system includes performing a normal operation mode. The normal operation mode may be classified into a low-pressure mode for providing a relatively low hydraulic pressure and a high-pressure mode for providing a relatively high hydraulic pressure according to a level of a hydraulic pressure flowing from the hydraulic-pressure supply device to the wheel cylinders, and may control the low-pressure mode and the high-pressure mode to be sequentially carried out according to the level of the hydraulic pressure flowing from the hydraulic-pressure supply device to the wheel cylinders. 
     The low-pressure mode includes opening the second, seventh, and eighth valves, and supplying a hydraulic pressure, that is formed in the first pressure chamber by forward movement of the hydraulic piston, to the first hydraulic circuit and the second hydraulic circuit. 
     The high-pressure mode may include opening the second, seventh, and eighth valves, after lapse of the low-pressure mode, supplying some parts of the hydraulic pressure formed in the first pressure chamber by forward movement of the hydraulic piston to the first hydraulic circuit and the second hydraulic circuit, opening the third valve, and supplying some parts of a remaining hydraulic pressure of the hydraulic pressure formed in the first pressure chamber to the second pressure chamber. 
     A process of releasing the low-pressure mode may include opening the second, seventh, and eighth valves, and forming a negative pressure in the first pressure chamber by backward movement of the hydraulic piston, and allowing the pressure medium of each of the first hydraulic circuit and the second hydraulic circuit to be collected in the first pressure chamber. 
     A process of releasing the high-pressure mode may include opening the second, seventh, and eighth valves, forming a negative pressure in the first pressure chamber by backward movement of the hydraulic piston, and allowing the pressure medium of each of the first hydraulic circuit and the second hydraulic circuit to be collected in the first pressure chamber, opening the fourth valve, and supplying the pressure medium of the second pressure chamber to the first pressure chamber. 
     A method for operating the electronic brake system includes performing a normal operation mode provided with a regenerative braking mode in which two wheel cylinders provided at the first hydraulic circuit perform a regenerative braking mode using the generator. The regenerative braking mode closes the sixth valve and prevents a hydraulic pressure from flowing into the first hydraulic circuit. 
     A method for operating the electronic brake system includes performing an abnormal operation mode. The abnormal operation mode includes opening the first cut valve in a manner that the first master chamber communicates with the first hydraulic circuit, and opening the second cut valve in a manner that the second master chamber communicates with the second hydraulic circuit. 
     A method for operating the electronic brake system includes performing an inspection mode in which presence or absence of a leak in the master cylinder or in the simulator valve is confirmed. The inspection mode includes closing the inspection valve and the second cut valve, and opening the first cut valve, supplying a hydraulic pressure generated by activation of the hydraulic-pressure supply device to the first master chamber, and comparing an estimated pressing-medium hydraulic pressure value scheduled to be generated based on a displacement of the hydraulic piston with a hydraulic pressure value of a pressure medium supplied to the first master chamber. 
    
    
     
       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 an electronic brake system according to a first embodiment of the present disclosure. 
         FIG. 2  is a graph illustrating characteristics of hydraulic pressures and regenerative brake pressures of wheel cylinders during regenerative braking based on the electronic brake system according to a first embodiment of the present disclosure. 
         FIG. 3  is a hydraulic circuit diagram illustrating the electronic brake system for providing brake pressure of a low-pressure mode by forward movement of a hydraulic piston according to a first embodiment of the present disclosure. 
         FIG. 4  is a hydraulic circuit diagram illustrating a rear-wheel regenerative braking state of the electronic brake system according to a first embodiment of the present disclosure. 
         FIG. 5  is a hydraulic circuit diagram illustrating the electronic brake system for providing brake pressure of a low-pressure mode by forward movement of a hydraulic piston according to a first embodiment of the present disclosure. 
         FIG. 6  is a hydraulic circuit diagram illustrating the electronic brake system for providing brake pressure by backward movement of a hydraulic piston according to a first embodiment of the present disclosure. 
         FIG. 7  is a hydraulic circuit diagram illustrating the electronic brake system for releasing brake pressure of a high-pressure mode by backward movement of a hydraulic piston according to a first embodiment of the present disclosure. 
         FIG. 8  is a hydraulic circuit diagram illustrating the electronic brake system for releasing brake pressure of a low-pressure mode by backward movement of a hydraulic piston according to a first embodiment of the present disclosure. 
         FIG. 9  is a hydraulic circuit diagram illustrating the electronic brake system for releasing brake pressure by forward movement of a hydraulic piston according to a first embodiment of the present disclosure. 
         FIG. 10  is a hydraulic circuit diagram illustrating an abnormal operation state of the electronic brake system according to a first embodiment of the present disclosure. 
         FIG. 11  is a hydraulic circuit diagram illustrating an operation state of the electronic brake system staying in an inspection mode according to a first embodiment of the present disclosure. 
         FIG. 12  is a hydraulic circuit diagram illustrating an electronic brake system according to a second embodiment of the present disclosure. 
         FIG. 13  is a hydraulic circuit diagram illustrating the electronic brake system for providing brake pressure of a low-pressure mode by forward movement of a hydraulic piston according to a second embodiment of the present disclosure. 
         FIG. 14  is a hydraulic circuit diagram illustrating an operate state of the electronic brake system for providing brake pressure by forward movement of at least one hydraulic piston, and at the same time implementing a rear-wheel regenerative braking state according to a second embodiment of the present disclosure. 
         FIG. 15  is a hydraulic circuit diagram illustrating an operate state of the electronic brake system for providing brake pressure of a high-pressure mode by forward movement of at least one hydraulic piston according to a second embodiment of the present disclosure. 
         FIG. 16  is a hydraulic circuit diagram illustrating the electronic brake system for providing brake pressure by backward movement of a hydraulic piston according to a second embodiment of the present disclosure. 
         FIG. 17  is a hydraulic circuit diagram illustrating the electronic brake system for releasing brake pressure of a high-pressure mode by backward movement of a hydraulic piston according to a second embodiment of the present disclosure. 
         FIG. 18  is a hydraulic circuit diagram illustrating the electronic brake system for releasing brake pressure of a low-pressure mode by backward movement of a hydraulic piston according to a second embodiment of the present disclosure. 
         FIG. 19  is a hydraulic circuit diagram illustrating the electronic brake system for releasing brake pressure by forward movement of a hydraulic piston according to a second embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. The embodiments to be described below are provided to fully convey the spirit of the present disclosure to a person skilled in the art. The present disclosure is not limited to the embodiments disclosed herein and may be implemented in other forms. In the drawings, some portions not related to the description will be omitted and will not be shown in order to clearly describe the present disclosure, and also the size of the component may be exaggerated or reduced for convenience and clarity of description. 
       FIG. 1  is a hydraulic circuit diagram illustrating an electronic brake system  1  according to a first embodiment of the present disclosure. 
     Referring to  FIG. 1 , the electronic brake system  1  may include a master cylinder  20  to pressurize and discharge a pressure medium (e.g., brake fluid) included therein according to a pedal effort of a brake pedal  10  depressed by a driver of a vehicle, a reservoir  30  formed to communicate with the master cylinder  20  to store the pressure medium, one or more wheel cylinders  40  to perform braking of respective wheels RR, RL, FR, and FL upon receiving hydraulic pressure generated by the pressure medium, a simulation device  50  to provide the driver with reaction force corresponding to a pedal effort of the brake pedal  10 , a hydraulic-pressure supply device  100  to generate hydraulic pressure of a pressure medium by mechanically operating upon receiving an electric signal indicating the driver&#39;s braking intention from a pedal displacement sensor  11  sensing displacement of the brake pedal  10 , a hydraulic control unit  200  to control hydraulic pressure applied to the wheel cylinders  40 , and an electronic control unit (ECU) (not shown) to control the hydraulic-pressure supply device  100  and various valves based on hydraulic pressure information and pedal displacement information. 
     The master cylinder  20  may be configured to have at least one chamber, such that the master cylinder  20  may pressurize and discharge the pressure medium therein. The master cylinder  20  may include a first master chamber  20   a , a second master chamber  20   b , a first piston  21   a  provided in the first master chamber  20   a , and a second piston  22   a  provided in the second master chamber  20   b.    
     The first master chamber  20   a  may include the first piston  21   a  connected to the input rod  12 , and the second master chamber  20   b  may include the second piston  22   a . The first master chamber  20   a  may communicate with a first hydraulic port  24   a  through which a pressure medium (i.e., fluid) is input and output. The second master chamber  20   b  may communicate with a second hydraulic port  24   b  through which a pressure medium (i.e., fluid) is input and output. For example, the first hydraulic port  24   a  may be connected to a first backup passage  251 , and the second hydraulic port  24   b  may be connected to a second backup passage  252 . 
     The master cylinder  20  according to a first embodiment of the present disclosure may include two master chambers  20   a  and  20   b  configured to be independent of each other, such that the master cylinder  20  may secure safety in the event of malfunction. For example, the first master chamber  20   a  of the two master chambers  20   a  and  20   b  may be connected to the rear left wheel RL and the rear right wheel RR, and the other master chamber  20   b  may be connected to the front left wheel FL and the front right wheel RR of the vehicle, such that braking of the vehicle remains possible even when one of the two master chambers malfunctions. 
     Alternatively, differently from the drawings, one of the two master chambers may be connected to the rear right wheel RR and the front left wheel FL, and the other master chamber may be connected to the rear left wheel RL and the front right wheel FR. One of the two master chambers may be connected to the front left wheel FL and the rear left wheel RL, and the other master chamber may be connected to the rear right wheel RR and the front right wheel FR. In other words, wheels connected to the master chambers of the master cylinder  20  may be located at various positions. 
     A first spring  21   b  may be disposed between the first piston  21   a  and the second piston  22   a  of the master cylinder  20 , and a second spring  22   b  may be disposed between the second piston  22   a  and one end of the master cylinder  20 . That is, the first piston  21   b  may be contained in the first master chamber  20   a , and the second piston  22   b  may be contained in the second master chamber  20   b.    
     The first spring  21   b  and the second spring  22   b  may be compressed by the first piston  21   a  and the second piston  22   a  that move in response to change of displacement of the brake pedal  10  depressed by the driver. When the driver takes their foot off the brake pedal  10  to release the pedal effort applied to the brake pedal  10 , the first spring  21   b  and the second spring  22   b  may be expanded by elastic force, such that the first piston  21   a  and the second piston  221  can move back to original positions thereof. 
     Meanwhile, the brake pedal  10  may be coupled to the first piston  21   a  of the master cylinder  20  through the input rod  12 . The input rod  12  may be directly coupled to the first piston  21   a , or may closely contact the first piston  21   a . Therefore, the brake pedal  10  depressed by the driver may directly pressurize the master cylinder  20  without a pedal free stroke region. 
     The first master chamber  20   a  may be connected to the reservoir  30  through a first reservoir passage  61 , and the second master chamber  20   b  may be connected to the reservoir  30  through a second reservoir passage  62 . The first reservoir passage  61  may be provided with a check valve  64  that allows a pressure medium to flow from the reservoir  30  to the first master chamber  20   a  and prevents a pressure medium from flowing from the first master chamber  20   a  to the reservoir  30 . In other words, the check valve  64  may be provided to allow the pressure medium to flow in only one direction from the reservoir  30  to the first master chamber  20   a.    
     In addition, the first reservoir passage  61  may be provided with an inspection passage  63  connected parallel to the check valve  64 . In more detail, the inspection passage  63  may be provided as a bypass passage on the first reservoir passage  61 , such that a front end of the check valve  64  is connected to a rear end of the check valve  65  through the inspection passage  63  acting as a bypass passage. Inspection passage  63  provided as the bypass passage may include an inspection valve  60  configured to control flow of a pressure medium. 
     The inspection valve  60  may be implemented as a bidirectional valve to control flow of a pressure medium between the reservoir  30  and the master cylinder  20 . The inspection valve  60  may be implemented as a normally open (NO) solenoid valve that remains open in a normal state and is then closed upon receiving a closing signal from an electronic control unit (ECU). A detailed function and operation of the inspection valve  60  will hereinafter be described. 
     The master cylinder  20  may include two sealing members respectively disposed before and after the first reservoir passage  61 , and two other sealing members respectively disposed before and after of the second reservoir passage  62 . Each of the four sealing members may be formed in a ring shape protruding from the inner surface of the master cylinder  20  or the outer circumference of the piston  21   a  or  22   a.    
     The simulation device  50  may be connected to a first backup passage  251  so as to provide reaction force corresponding to a pedal effort of the brake pedal  10 . The simulation device  50  may provide reaction force corresponding to a pedal effort of the brake pedal  10  depressed by the driver, and may provide pedal feel to the driver, such that the brake pedal  10  can more precisely operate and braking force can also be precisely adjusted as intended by the driver. 
     Referring to  FIG. 1 , the simulation device  50  may include a simulation chamber  51  to store a pressure medium discharged from the first hydraulic port  24   a  of the master cylinder  20 , a reaction force piston  52  provided in the simulation chamber  51 , a pedal simulator provided with a reaction force spring  53  elastically supporting the reaction force piston  52 , and a simulator valve  54  connected to a front end of the simulation chamber  51 . 
     The reaction force piston  52  and the reaction force spring  53  may be installed to have a predetermined range of displacement within the simulation chamber  51  by the pressure medium flowing into the simulation chamber  51 . The simulator valve  54  may connect the first master chamber  20   a  of the master cylinder  20  to the front end of the simulation chamber  51 , and the rear end of the simulation chamber  51  may be connected to the reservoir  31 . Therefore, the simulation chamber  51  may receive a pressure medium from the reservoir  31  even when the reaction force piston  52  moves back to the original position thereof, such that the simulation chamber  51  may always be fully filled with the pressure medium. 
     Meanwhile, the reaction force spring  53  is merely an example capable of supplying elastic force to the reaction force piston  52 , and may be implemented as any of other examples capable of storing elastic force therein. For example, the reaction force spring  53  may be formed of a material such as rubber, or may include various members formed in a coil or plate shape to store elastic force therein. 
     The simulator valve  54  may be provided in a passage through which the first master chamber  20   a  of the master cylinder  20  is connected to the front end of the simulation chamber  51 . The simulator valve  54  may be implemented as a normally closed (NC) solenoid valve that remains closed in a normal state. The simulator valve  54  is open when the driver applies a pedal effort to the brake pedal  10  by depressing the brake pedal  10 , such that a pressure medium stored in the first master chamber  20   a  may flow into the simulation chamber  51 . 
     A simulator check valve  55  connected parallel to the simulator valve  54  may be provided in a flow passage through which the first master chamber  20   a  of the master cylinder  20  is connected to the front end of the simulation chamber  51 . In more detail, the simulation device  50  may be connected to the first master chamber  20   a  of the master cylinder  20  through a flow passage branched from the first backup passage  251 . The simulator check valve  55  may allow a pressure medium to flow from the simulation chamber  51  to either the first master chamber  20   a  or the first backup passage  251 , and may prevent the pressure medium from flowing from the first master chamber  20   a  or the first backup passage  251  to the simulation chamber  51 . Therefore, when the driver depresses the brake pedal  10 , the pressure medium stored in the first master chamber  20   a  may flow into the simulation chamber  51  through the simulator valve  54 . When the driver takes their foot off the brake pedal  10 , the pressure medium stored in the simulation chamber  51  may flow into either the first master chamber  20   a  or the first backup passage  251  through the simulator valve  54  or the simulator check valve  55 , simulator pressure can rapidly return. In addition, when hydraulic pressure of the simulation chamber  51  is higher than hydraulic pressure of the pressure medium flowing in either the first master chamber  20   a  or the first backup passage  251 , the pressure medium may be discharged from the simulation chamber  51  to either the first master chamber  20   a  or the first backup passage  251  through the simulation check valve  55 , such that the simulation device  50  can rapidly return to a ready state. 
     The pedal simulator  50  may operate as follows. If a pedal effort is applied to the brake pedal  10  by the driver of the vehicle, the simulation valve  54  is open, a pressure medium stored in the first master chamber  20   a  is provided to the front side (i.e., a left side of the reaction force piston in  FIG. 1 ) of the reaction force piston  52  included in the simulation chamber  51 , such that the reaction force piston  52  compresses the reaction force spring  53  and thus proper pedal feel is provided to the driver. In this case, the pressure medium filling the rear side (i.e., a right side of the reaction force piston in  FIG. 1 ) of the reaction force piston  52  of the simulation chamber  51  may flow into the reservoir  30 . Thereafter, if the driver takes their foot off the brake pedal  10  to release the pedal effort applied to the brake pedal  10 , the reaction force spring  53  is expanded by elastic force such that the reaction force piston  52  may move back to an original position thereof. The pressure medium filling the front side of the reaction force piston  52  of the simulation chamber  51  may be discharged to the first master chamber  20   a  or the first backup passage  251  through the simulator valve  54  or the simulator check valve  55 . In this case, the rear side of the reaction force piston  52  provided in the simulation chamber  51  may receive the pressure medium from the reservoir  30 , such that the simulation chamber  51  may be fully filled with the pressure medium again. 
     As described above, the simulation chamber  51  is always filled with the pressure medium. Therefore, frictional force of the reaction force piston  52  is minimized during operation of the simulation device  50 , such that durability of the simulation device  50  can be improved and foreign materials from the outside can be prevented from flowing into the simulation device  50 . 
     Meanwhile, several reservoirs  30  may be shown in  FIG. 2 , and the respective reservoirs  30  may be denoted by the same reference number. However, the reservoirs  30  may be implemented as the same or different components. For example, the reservoir  30  connected to the pedal simulator  50  may be identical to the reservoir  30  connected to the master cylinder  20 , or may store a pressure medium therein in a different way from the reservoir  30  connected to the master cylinder  20 . 
     The hydraulic-pressure supply device  100  may mechanically operate by receiving an electrical signal indicating the driver&#39;s braking intention from the pedal displacement sensor  11  sensing displacement of the brake pedal  10 , such that hydraulic pressure caused by the pressure medium may occur. 
     The hydraulic-pressure supply device  100  may include a hydraulic-pressure providing unit  110  to supply pressing-medium pressure to wheel cylinders  40 , a motor  120  to produce rotational force according to an electrical signal from the pedal displacement sensor  11 , and a power switching unit  130  to convert rotational motion of the motor  120  into rectilinear motion and to provide the rectilinear motion to the hydraulic-pressure providing unit  110 . In this case, the hydraulic-pressure providing unit  110  may also operate by pressure supplied from a high-pressure accumulator, instead of using driving force supplied from the motor  120 . 
     The hydraulic-pressure providing unit  110  may include a cylinder block  111 , a hydraulic piston  114 , one or more sealing members  115 , and a drive shaft  133 . The cylinder block  111  may have a pressure chamber to store a pressure medium supplied thereto. The hydraulic piston  114  may be provided in the cylinder block  111 . The sealing member  115  may be disposed between the hydraulic piston  114  and the cylinder block  111  to seal the pressure chamber. The drive shaft  133  may transfer power from the power switching unit  130  to the hydraulic piston  114 . 
     The pressure chamber may include a first pressure chamber  112  located at a front side (i.e., a forward direction, see a left side of the hydraulic piston in  FIG. 1 ) of the hydraulic piston  114 , and a second pressure chamber  113  located at a rear side (i.e., a backward direction, see a right side of  FIG. 1 ) of the hydraulic piston  114 . That is, the first pressure chamber  112  may be divided by the cylinder block  111  and the front end of the hydraulic piston  114 , and may have a volume changeable according to movement of the hydraulic piston  114 . The second pressure chamber  113  may be divided by the cylinder block  111  and the rear end of hydraulic piston  114 , and may have a volume changeable according to movement of the hydraulic piston  114 . The first pressure chamber  112  may be connected to a first hydraulic passage  211  through a first communication hole  111   a  formed at the cylinder block  111 . The second pressure chamber  113  may be connected to a fourth hydraulic passage  214  through a second communication hole  111   b  formed at the cylinder block  111 . 
     The sealing member may include a piston sealing member  115  and a drive-shaft sealing member. The piston sealing member  115  may be disposed between the hydraulic piston  114  and the cylinder block  111  to seal a gap between the first pressure chamber  112  and the second pressure chamber  113 . The drive-shaft sealing member may be disposed between the drive shaft  113  and the cylinder block  111  to seal a gap between the second pressure chamber  113  and the opening of the cylinder block  111 . Hydraulic pressure or negative pressure of the first and second pressure chambers  112  and  113  affected by forward or backward movement of the hydraulic piston  114  may be blocked by the piston sealing member  115 , so that the resultant hydraulic pressure or negative pressure of the first and second pressure chambers  112  and  113  can be transmitted to the first and fourth hydraulic passages  211  and  214  without leaking to the second pressure chamber  113 . Hydraulic pressure or negative pressure of the second pressure chamber  113  affected by forward or backward movement of the hydraulic piston  114  may be blocked by the drive-shaft sealing member, so that the resultant hydraulic pressure or negative pressure of the second pressure chamber  113  may not leak to the outside of the cylinder block  111 . 
     The first pressure chamber  112  may be connected to the reservoir  30  through the first dump passage  116 , such that the first pressure chamber  112  may receive a pressure medium from the reservoir  30  and store the received pressure medium or may transmit the pressure medium of the first pressure chamber  112  to the reservoir  30 . The second pressure chamber  113  may be connected to the reservoir  30  through the second dump passage  117 , such that the second pressure chamber  113  may receive a pressure medium from the reservoir  30  and store the received pressure medium or may transmit the pressure medium of the second pressure chamber  113  to the reservoir  30 . To this end, the first dump passage  116  may communicate with the first pressure chamber  112  through a third communication hole  111   c  formed in the cylinder block  111 , and may be connected to the reservoir  30 . The second dump passage  117  may communicate with the second pressure chamber  113  through a fourth communication hole  111   d  formed in the cylinder block  111 , and may be connected to the reservoir  30 . 
     The motor  120  may produce driving force according to an electric signal from the ECU. The motor  120  may include a stator  121  and a rotor  122 , and may rotate in a forward or backward direction using the stator  121  and the rotor  122 , such that the motor  120  may produce power or force through which displacement of the hydraulic piston  114  occurs. A rotational angular speed and a rotation angle of the motor  120  may be precisely controlled by a motor control sensor (MPS). The motor  120  is well known to those skilled in the art, and as such a detailed description thereof will herein be omitted for convenience of description. 
     The power switching unit  130  may convert rotational force of the motor  120  into rectilinear movement. For example, the power switching unit  130  may include a worm shaft  131 , a worm wheel  132 , and a drive shaft  133 . 
     The worm shaft  131  may be integrated with a rotational shaft of the motor  120 . At least one worm may be formed at the outer circumference of the worm shaft  131  in a manner that the worm shaft  131  is meshed with the worm wheel  132  so that the worm wheel  132  can rotate. The worm wheel  132  may be meshed with the drive shaft  133  so that the drive shaft  133  performs rectilinear motion. The drive shaft  133  is connected to the hydraulic piston  114 , such that the hydraulic piston  114  may slidably move within the cylinder block  111 . 
     In more detail, a signal sensed by the pedal displacement sensor  11  due to displacement of the brake pedal  10  may be transmitted to the ECU, and the ECU may operate the motor  120  in one direction so that the worm shaft  131  may also rotate in one direction. Rotational force of the worm shaft  131  may be transmitted to the drive shaft  133  through the worm wheel  132 , and the hydraulic piston  114  connected to the drive shaft  133  moves forward, so that hydraulic pressure may occur in the first pressure chamber  112 . 
     In contrast, when a pedal effort is removed from the brake pedal  10 , the ECU may operate the motor  120  so that the worm shaft  131  may rotate in the opposite direction. Accordingly, the worm wheel  132  may also rotate in the opposite direction, and the hydraulic piston  114  connected to the drive shaft  133  moves backward, thereby generating negative pressure in the first pressure chamber  112 . 
     Hydraulic pressure and negative pressure may also occur in other directions opposite to the above-mentioned directions as necessary. In other words, a signal sensed by the pedal displacement sensor  11  due to displacement of the brake pedal  10  may be transmitted to the ECU, and the ECU may operate the motor  120  in an opposite direction so that the worm shaft  131  may also rotate in the opposite direction. Rotational force of the worm shaft  131  may be transmitted to the drive shaft  133  through the worm wheel  132 , and the hydraulic piston  114  connected to the drive shaft  133  moves backward, so that hydraulic pressure may occur in the second pressure chamber  113 . 
     In contrast, when a pedal effort is removed from the brake pedal  10 , the ECU may operate the motor  120  in one direction so that the worm shaft  131  may also rotate in one direction. Accordingly, the worm wheel  132  may also rotate in the opposite direction, and the hydraulic piston  114  connected to the drive shaft  133  moves forward, thereby generating negative pressure in the second pressure chamber  113 . 
     As described above, according to a rotation direction of the worm shaft  131  affected by driving of the motor  120 , hydraulic pressure may occur in the first pressure chamber  112  or negative pressure may occur in the second pressure chamber  113 . Information as to whether to brake the vehicle using hydraulic pressure or information as to whether to release braking using negative pressure may be determined by controlling several valves. A detailed description thereof will hereinafter be described. 
     Although not shown in the drawings, the power switching unit  130  may also be formed of a ball-screw-nut assembly. For example, the power switching unit  130  may include a screw that is integrated with a rotational shaft of the motor  120  or rotates with the rotational shaft of the motor  120 , and a ball nut that is screw-coupled to the screw in a restricted rotation state and performs rectilinear motion according to rotation of the screw. The above-mentioned ball-screw-nut assembly is well known to those skilled in the art, and as such a detailed description thereof will herein be omitted. In addition, the power switching unit  130  may be implemented not only as the ball-screw-nut assembly, but also as any structure capable of converting rotational force into rectilinear motion without departing from the scope and spirit of the present disclosure. 
     The hydraulic control unit  200  may be provided to control hydraulic pressure applied to wheel cylinders  40 , and the ECU may be provided to control the hydraulic-pressure supply device  100  and various valves based on hydraulic pressure information and pedal displacement information. 
     The hydraulic control unit  200  may include a first hydraulic circuit  201  to control flow of hydraulic pressure applied to two wheel cylinders  40 , and a second hydraulic circuit  202  to control flow of hydraulic pressure applied to two other wheel cylinders  40 . The hydraulic control unit  200  may include a plurality of flow passages and a plurality of valves to control hydraulic pressure flowing from the hydraulic-pressure supply device  100  to the wheel cylinders  40 . 
     Referring back to  FIG. 1 , the hydraulic control unit  200  will hereinafter be described. 
     Referring to  FIG. 1 , the first hydraulic passage  211  may be provided to connect the first pressure chamber  112  to the first and second hydraulic circuits  201  and  202 . The first hydraulic passage  211  may be branched into a second hydraulic passage  212  and a third hydraulic passage  213 , and the third hydraulic passage  213  may be connected to the second hydraulic circuit  202 . As a result, hydraulic pressure generated by the first pressure chamber  112  according to forward movement of the hydraulic piston  114  may be transmitted to the second hydraulic circuit  202  through the first hydraulic passage  211  and the third hydraulic passage  213 , and may be transmitted to the first hydraulic circuit  201  through the first hydraulic passage  211 , the second hydraulic passage  212 , and an eighth hydraulic passage  218  to be described later. 
     The second hydraulic passage  212  may be provided with a first valve  231  to control flow of a pressure medium, and the third hydraulic passage  213  may be provided with a second valve  232  to control flow of a pressure medium. 
     The first valve  231  may be implemented as a bidirectional valve to control flow of the pressure medium received through the second hydraulic passage  212 . The first valve  231  may be implemented as a normally closed (NC) solenoid valve that remains closed in a normal state and is then open upon receiving an opening signal from the ECU. 
     The second valve  232  may be implemented as a check valve that allows a pressure medium to flow from the first pressure chamber  112  to the second hydraulic circuit  202  and prevents the pressure medium from flowing from the second hydraulic circuit  202  to the first pressure chamber  112 . That is, the first valve  232  may allow hydraulic pressure of the first pressure chamber  112  to flow into the second hydraulic circuit  202 , and may prevent hydraulic pressure of the second hydraulic circuit  202  from leaking to the first pressure chamber  112  through the third hydraulic passage  213 . 
     The fourth hydraulic passage  214  may connect the second pressure chamber  113  to the first and second hydraulic circuits  201  and  202 . The fourth hydraulic passage  214  may be branched into a fifth hydraulic passage  215  and a sixth hydraulic passage  216 . The fifth hydraulic passage  215  and the sixth hydraulic passage  216  may be linked again to the fourth hydraulic passage  214 . Both ends of a seventh hydraulic passage  217  may respectively communicate with a rear end of the first valve  231  of the second hydraulic passage  212  and a rear end of the second valve  232  of the third hydraulic passage  213 , such that the second hydraulic passage  212  may be connected to the third hydraulic passage  213 . The eighth hydraulic passage  218  may connect the second hydraulic passage  212  to the first hydraulic circuit  201  and the fourth hydraulic circuit  214 . To this end, one end of the eighth hydraulic passage may be connected to the rear end of the first valve  231  of the second hydraulic passage  212 , the other end of the eighth hydraulic passage may be connected to the first hydraulic circuit  201 , and an intermediate part of the eighth hydraulic passage may be linked to the fourth hydraulic passage  214 . 
     The fifth hydraulic passage  215  may be provided with a third valve  233  to control flow of a pressure medium, and the sixth hydraulic passage  216  may be provided with a fourth valve  234  to control flow of a pressure medium. 
     The third valve  233  may be implemented as a bidirectional valve to control flow of the pressure medium flowing through the fifth hydraulic passage  215 . The third valve  233  may be implemented as a normally closed (NC) solenoid valve that remains closed in a normal state and is then open upon receiving an opening signal from the ECU. 
     The fourth valve  234  may be implemented as a check valve that allows a pressure medium to flow from the fourth hydraulic passage  214  communicating with the second pressure chamber  113  to the eighth hydraulic passage  218  and prevents the pressure medium from flowing from the eighth hydraulic passage  218  to the fourth hydraulic passage  214 . That is, the fourth valve  234  may prevent hydraulic pressure of the first or second hydraulic circuit  201  or  202  from leaking to the second pressure chamber  113  through the sixth hydraulic passage  216  and the fourth hydraulic passage  214 . 
     Since the fifth and sixth hydraulic passages  215  and  216  are branched from the fourth hydraulic passage  214  and then meet again the fourth hydraulic passage  214 , the third valve  233  and the fourth valve  234  may be arranged parallel to each other. 
     The seventh hydraulic passage  217  may be provided with a fifth valve  235  to control flow of a pressure medium, and the eighth hydraulic passage  218  may be provided with a sixth valve  236  to control flow of a pressure medium. 
     The fifth valve  235  may be implemented as a bidirectional valve to control flow of the pressure medium flowing through the seventh passage  217  (for example, flow of the pressure medium flowing between the second hydraulic passage  212  and the third hydraulic passage  213  respectively communicating with both ends of the seventh hydraulic passage  217 ). The fifth valve  235  may be implemented as a normally closed (NC) solenoid valve that remains closed in a normal state and is then open upon receiving an opening signal from the ECU. 
     The sixth valve  236  may be implemented as a bidirectional valve to control flow of the pressure medium flowing through the eighth hydraulic passage  218 . In more detail, the sixth valve  236  may be arranged between a first point where the second hydraulic passage  212  is connected to the eighth hydraulic passage  218  and a second point where the fourth hydraulic passage  214  meets the eighth hydraulic passage  218 . 
     The sixth valve  236  may be disposed between the pressure chamber of the hydraulic pressure generator and at least one wheel cylinder to be used for regenerative braking, such that the sixth valve  236  may selectively connect the pressure chamber to the corresponding hydraulic circuit or may selectively sever such connection between the pressure chamber and the corresponding hydraulic circuit, such that only some parts of hydraulic pressure of the pressure medium may be transmitted to the corresponding wheel cylinder. For example, as shown in  FIG. 1 , the sixth valve  236  may be provided in the eighth hydraulic passage  218  between the first pressure chamber  112  and the first hydraulic circuit  201  provided with the wheel cylinders  40  of the rear wheels RL and RR in which rear-wheel regenerative braking is implemented, such that the sixth valve  236  may selectively connect the first pressure chamber  112  to the first hydraulic circuit  201  or may selectively sever such connection between the first pressure chamber  112  and the first hydraulic circuit  201 , and thus only some parts of hydraulic pressure of the pressure medium can be transmitted to the rear wheel cylinders  40 . A detailed description thereof will hereinafter be described. 
     The sixth valve  236  may be implemented as normally closed (NC) solenoid valves that remain closed in a normal state and are then open upon receiving an opening signal from the ECU. 
     By the above-mentioned passages and valves, hydraulic pressure produced in the second pressure chamber  113  by backward movement of the hydraulic piston  114  may be supplied to the second hydraulic passage  212  through the fourth hydraulic passage  214  and the fifth and sixth hydraulic passages  215  and  216 , and may be supplied to the third hydraulic passage  212  through the fourth hydraulic passage  214 , the fifth and sixth hydraulic passages  215  and  216 , the eighth hydraulic passage  218 , and the seventh hydraulic passage  217 , such that hydraulic pressure produced in the second pressure chamber  113  may be transmitted to the first hydraulic circuit  201  and the second hydraulic circuit  202 . 
     Furthermore, both ends of the seventh hydraulic passage  217  may respectively communicate with and be respectively connected to the rear end of the first valve  231  of the second hydraulic passage  212  and the rear end of the second valve  232  of the third hydraulic passage  213 . As a result, when the first valve  231  or the second valve  232  abnormally operates, the fifth and sixth valves  235  and  236  are opened, such that hydraulic pressure produced in the first pressure chamber  112  may be stably transmitted to each of the first hydraulic circuit  201  and the second hydraulic circuit  202 . In contrast, the seventh hydraulic passage  217  may operate to open the third valve  233 , the fifth valve  235 , and the sixth valve  236 , such that hydraulic pressure produced in the second pressure chamber  113  may be stably transmitted to the first hydraulic circuit  201  and the second hydraulic circuit  202 . 
     The first, fifth, and sixth valves  231 ,  235 , and  236  are open when a pressure medium is taken out from the wheel cylinders  40  and then flows into the first pressure chamber  112  in a manner that hydraulic pressure applied to the wheel cylinders  40  is released, because the second valve  232  provided in the third hydraulic passage  213  is implemented as a check valve for allowing the pressure medium to flow only in one direction. 
     The first hydraulic circuit  201  and the second hydraulic circuit  202  of the hydraulic control unit  200  will hereinafter be described. 
     The first hydraulic circuit  201  may control hydraulic pressure of wheel cylinders  40  installed in the rear right wheel RR and the rear left wheel RL. The second hydraulic circuit  202  may control hydraulic pressure of other wheel cylinders  40  installed in the front right wheel FR and the front left wheel FL. 
     The first hydraulic circuit  201  may be connected to the first hydraulic passage  211  and the second hydraulic passage  212  so as to receive hydraulic pressure from the hydraulic-pressure supply device  100 , and the second hydraulic passage  212  may be branched into two passages that are respectively connected to the rear right wheel RR and the rear left wheel RL. Likewise, the second hydraulic circuit  202  may be connected to the first hydraulic passage  211  and the third hydraulic passage  213  so as to receive hydraulic pressure from the hydraulic-pressure supply device  100 , and the third hydraulic passage  213  may be branched into two passages that are respectively connected to the front right wheel FR and the front left wheel FL. 
     The first and second hydraulic circuits  201  and  202  may include a plurality of inlet valves  221  ( 221   a ,  221   b ,  221   c ,  221   d ) to control flow of the pressure medium and hydraulic pressure. For example, the first hydraulic circuit  201  may be provided with two inlet valves  221   a  and  221   b  connected to the second hydraulic passage  212  such that the two inlet valves  221   a  and  221   b  may respectively control hydraulic pressures applied to two wheel cylinders  40 . The second hydraulic circuit  202  may be provided with two inlet valves  221   c  and  221   d  connected to the third hydraulic passage  213  such that the two inlet valves  221   c  and  221   d  may respectively control hydraulic pressures applied to the wheel cylinders  40 . 
     The inlet valves  221  may be arranged upstream of the wheel cylinders  40 . The inlet valves  221  may be implemented as normally open (NO) solenoid valves that remain open in a normal state and are then closed upon receiving a closing signal from the ECU. 
     The first and second hydraulic circuits  201  and  202  may include check valves  223   a ,  223   b ,  223   c , and  223   d  connected parallel to the inlet valves  221   a ,  221   b ,  221   c , and  221   d . The check valves  223   a ,  223   b ,  223   c , and  223   d  may be provided in bypass passages by which front ends and rear ends of the respective inlet valves  221   a ,  221   b ,  221   c , and  221   d  are connected to one another in the first and second hydraulic circuits  201  and  202 . The check valves  223   a ,  223   b ,  223   c , and  223   d  may allow a pressure medium to flow from the wheel cylinders  40  to the hydraulic-pressure providing unit  110  and prevents the pressure medium from flowing from the hydraulic-pressure providing unit  110  to the wheel cylinders  40 . The check valves  223   a ,  223   b ,  223   c , and  223   d  may allow hydraulic pressure of the pressure medium applied to the wheel cylinders  40  to be rapidly discharged. Alternatively, during abnormal operation of the inlet valves  221   a ,  221   b ,  221   c , and  221   d , the check valves  223   a ,  223   b ,  223   c , and  223   d  may allow hydraulic pressure of the pressure medium applied to the wheel cylinders  40  to flow into the hydraulic-pressure providing unit  110 . 
     The first and second hydraulic circuits  201  and  202  may further include a plurality of outlet valves  222  ( 222   a ,  222   b ,  222   c ,  222   d ) connected to the reservoir  30  so as to improve performance or throughput when braking of the wheel cylinders  40  is released. The outlet valves  222  may be respectively connected to the wheel cylinders  40  so as to control flow of the pressure medium discharged from the wheel cylinders  40  of the respective wheels RR, RL, FR, and FL. That is, the outlet valves  222  may sense brake pressures of the respective wheels RR, RL, FR, and FL. If decompression braking is needed, the outlet valves  222  may be selectively open to control decompression of the wheel cylinders  40 . 
     The outlet valves  222  may be implemented as normally closed (NC) solenoid valves that remain closed in a normal state and are then open upon receiving an opening signal from the ECU. 
     Meanwhile, a first dump valve  241  may be provided in the first dump passage  116  to control flow of a pressure medium, and a second dump valve  242  may be provided in the second dump passage  117  to control flow of the pressure medium. Referring back to  FIG. 1 , the first dump valve  241  may be implemented as a check valve that allows the pressure medium to flow from the reservoir  30  to the first pressure chamber  112  and prevents the pressure medium from flowing from the first pressure chambers  112  to the reservoir  30 . The second dump valve  242  may be implemented as a check valve that allows the pressure medium to flow from the reservoir  30  to the second pressure chamber  113  and prevents the pressure medium from flowing from the second pressure chamber  113  to the reservoir  30 . That is, the first dump valve  241  may allow the pressure medium to flow from the reservoir  30  to the first pressure chamber  112 , and may prevent the pressure medium from flowing from the first pressure chambers  112  to the reservoir  30 . The second dump valve  242  may allow the pressure medium to flow from the reservoir  30  to the second pressure chamber  113 , and may prevent the pressure medium from flowing from the second pressure chamber  113  to the reservoir  30 . 
     In addition, the first dump passage  117  may be provided with a bypass passage connected parallel to the second dump valve  242 . In more detail, the bypass passage may be provided as a detour (i.e., a bypass route) on the second dump passage  117  such that a front end of the second dump valve  242  is connected to a rear end of the second dump valve  242  through the bypass passage. The bypass passage may include a third dump valve  243  configured to control flow of a pressure medium between the second pressure chamber  113  and the reservoir  30 . 
     The third dump valve  243  may be implemented as a bidirectional valve to control flow of a pressure medium between the second pressure chamber  113  and the reservoir  30 . The third dump valve  243  may be implemented as a normally open (NO) solenoid valve that remains open in a normal state and is then closed upon receiving a closing signal from an electronic control unit (ECU). 
     The hydraulic-pressure providing unit  110  of the electronic brake system  1  according to the first embodiment of the present disclosure may operate in a double-acting manner. 
     In more detail, hydraulic pressure produced in the first pressure chamber  112  by forward movement of the hydraulic piston  114  may be transmitted to the first hydraulic circuit  201  through the first hydraulic passage  211 , the second hydraulic passage  212 , and the eighth hydraulic passage  218 , thereby braking the wheel cylinders  40  installed in the rear right wheel RR and the rear left wheel RL. In addition, hydraulic pressure produced in the first pressure chamber  112  by forward movement of the hydraulic piston  114  may be transmitted to the second hydraulic circuit  202  through the first hydraulic passage  211  and the third hydraulic passage  213 , thereby braking the wheel cylinders  40  installed in the front right wheel FR and the front left wheel FL. 
     Likewise, hydraulic pressure produced in the second pressure chamber  113  by backward movement of the hydraulic piston  114  may be transmitted to the first hydraulic circuit  201  through the fourth hydraulic passage  214 , the fifth hydraulic passage  215 , and the sixth hydraulic passage  216 , thereby braking the wheel cylinders  40  installed in the rear right wheel RR and the rear left wheel RL. In addition, hydraulic pressure produced in the second pressure chamber  113  by backward movement of the hydraulic piston  114  may be transmitted to the second hydraulic circuit  202  through the fourth hydraulic passage  214 , the fifth and sixth hydraulic passages  215  and  216 , the eighth hydraulic passage  218 , and the seventh hydraulic passage  217 , thereby braking the wheel cylinders  40  installed in the front right wheel FR and the front left wheel FL. 
     Negative pressure produced in the first pressure chamber  112  by backward movement of the hydraulic piston  114  may suction the pressure medium from the wheel cylinders  40  installed in the rear right wheel RR and the rear left wheel RL, such that the pressure medium may move back from the first hydraulic circuit  201  to the first pressure chamber  112  through the eighth hydraulic passage  218 , the second hydraulic passage  212 , and the first hydraulic passage  211 . In addition, the negative pressure produced in the first pressure chamber  112  by backward movement of the hydraulic piston  114  may suction the pressure medium from the wheel cylinders  40  installed in the front right wheel FR and the front left wheel FL, such that the pressure medium may move back from the second hydraulic circuit to the first pressure chamber  112  through the third hydraulic passage  213  and the first hydraulic passage  211  or may move back from the second hydraulic circuit to the first pressure chamber  112  through the third hydraulic passage  213 , the seventh hydraulic passage  217 , the second hydraulic passage  212 , and the first hydraulic passage  211 . 
     The electronic brake system  1  according to the first embodiment of the present disclosure may include a first backup passage  251  and a second backup passage  252 , each of which is configured to directly transmit the pressure medium discharged from the master cylinder  20  to the wheel cylinders  40  during abnormal operation of the electronic brake system  1 , resulting in braking of the vehicle. 
     The first backup passage  251  may connect the first hydraulic port  24   a  of the master cylinder  20  to the first hydraulic circuit  201 , and the second backup passage  252  may connect the second hydraulic port  24   b  of the master cylinder  20  to the second hydraulic circuit  202 . In more detail, the first backup passage  251  may be linked to front ends of the first and second inlet valves  221   a  and  221   b  in the first hydraulic circuit  201 , and the second backup passage  252  may be linked to front ends of the third and fourth inlet valves  221   c  and  221   d  in the second hydraulic circuit  202 . 
     The first backup passage  251  may be provided with the first cut valve  261  for controlling flow of the pressure medium, and the second backup passage  252  may be provided with the second cut valve  262  for controlling flow of the pressure medium. The first and second cut valves  261  and  262  may be implemented as normally open (NO) solenoid valves that remain open in a normal state and are then closed upon receiving a closing signal from the ECU. 
     Therefore, hydraulic pressure supplied from the hydraulic-pressure supply device  100  when the first and second cut valves  261  and  262  are closed may be supplied to the wheel cylinders  40  through the first and second hydraulic circuits  201  and  202 . Hydraulic pressure supplied from the master cylinder  20  when the first and second cut valves  261  and  262  are open may be supplied to the wheel cylinders  40  through the first and second backup passages  251  and  252 . In this case, the plurality of inlet valves  221   a ,  221   b ,  221   c , and  221   d  remain open, so that operation states of the inlet valves  221   a ,  221   b ,  221   c , and  221   d  need not be changed. 
     Meanwhile, the electronic brake system  1  according to the first embodiment may include a backup-passage pressure sensor PS 1  to sense hydraulic pressure of the master cylinder  20 , and passage pressure sensors PS 21  and PS 22  to sense hydraulic pressure of at least one of the first hydraulic circuit  201  and the second hydraulic circuit  202 . For example, the backup-passage pressure sensor PS 1  may be provided at the front end of the first cut valve  262  on the first backup passage  261 , thereby sensing hydraulic pressure produced in the master cylinder  20 . The passage pressure sensors PS 21  and PS 22  may be provided at the front end of the inlet valve  221  of at least one of the first hydraulic circuit  201  and the second hydraulic circuit  202 , thereby sensing hydraulic pressure applied to the first hydraulic circuit  201  and hydraulic pressure applied to the second hydraulic circuit  202 . Although the drawings have disclosed that the passage pressure sensors PS 21  and PS 22  are respectively provided in the first hydraulic circuit  201  and the second hydraulic circuit  202  for convenience of description, the scope or spirit of the present disclosure is not limited thereto, and it should be noted that the number of passage pressure sensors may also be set to 1 or any other number so long as hydraulic pressure applied to each of the hydraulic circuits  201  and  202  can be sensed. 
     Meanwhile, as the demand of users who desire to use eco-friendly vehicles is rapidly increasing, hybrid vehicles having superior fuel efficiency are becoming more and more popular with consumers. Generally, a hybrid vehicle converts kinetic energy generated by vehicle deceleration into electric energy, stores the electric energy in a battery, and uses a motor as an auxiliary driving source of the vehicle. In order to increase an energy gain factor, the hybrid vehicle is designed to recover energy using a generator (not shown) or the like during braking or deceleration of the vehicle, such that this braking operation is referred to as a regenerative braking operation. However, during regenerative braking, not only a brake hydraulic pressure caused by hydraulic pressures applied to four wheels of the vehicle, but also a regenerative brake pressure produced by the generator or the like is additionally applied to the four wheels, such that cooperative control between a brake hydraulic pressure caused by the hydraulic-pressure supply device and a regenerative brake pressure is needed for stable braking obtained by constant brake force applied to four wheels. 
       FIG. 2  is a graph illustrating characteristics of hydraulic pressures and regenerative brake pressures of wheel cylinders during regenerative braking based on the electronic brake system according to the first embodiment of the present disclosure. 
     Referring to  FIG. 2 , if an energy recovery device such as a generator  41  is installed in the rear wheels RL and RR of the first hydraulic circuit  201  as shown in  FIG. 1 , a brake hydraulic pressure corresponding to a braking level desired by the driver is produced by the hydraulic-pressure supply device, the entire brake force of the front wheels receiving only a brake hydraulic pressure caused by hydraulic pressure may be increased and maintained in the same manner as in the brake hydraulic pressure. However, according to the rear wheels needed for implementation of regenerative braking, the entire rear-wheel brake force corresponding to the sum of a brake hydraulic pressure caused by the hydraulic-pressure supply device and a regenerative brake pressure caused by the generator  41  should be identical to the entire front-wheel brake force or the brake hydraulic pressure desired by the driver. Therefore, as soon as the vehicle starts regenerative braking, the sixth valve  236  of the hydraulic control unit  200  is closed, such that a brake hydraulic pressure flowing from the hydraulic-pressure supply device to the rear wheels may be kept constant. Simultaneously, a regenerative brake pressure caused by the energy recovery device such as a generator  41  may increase, such that the entire rear-wheel brake force may be identical to the entire front-wheel brake force or a brake hydraulic pressure desired by the driver. A detailed description thereof will be given later with reference to  FIG. 4 . 
     A method for operating the electronic brake system  1  according to the first embodiment of the present disclosure will hereinafter be described. 
     The electronic brake system  1  according to the first embodiment may allow the hydraulic-pressure supply device  100  to be used in a low-pressure mode and a high-pressure mode in different ways. The hydraulic control unit  200  may operate in different ways according to the low-pressure mode and the high-pressure mode. The hydraulic-pressure supply device  100  may use the high-pressure mode, such that the hydraulic-pressure supply device  100  can provide a high hydraulic pressure without increasing an output level of the motor  120 , resulting in reduction in load applied to the motor  120 . As a result, the production cost and weight of the brake system can be reduced and stable brake force can be obtained, resulting in an increase in durability and operational reliability of the brake system. 
     If the hydraulic piston  114  moves forward by driving of the motor  120 , hydraulic pressure may occur in the first pressure chamber  112 . As the hydraulic piston  114  gradually moves forward from an initial position thereof, (i.e., as an operation stroke of the hydraulic piston  114  gradually increases), the amount of a pressure medium flowing from the first pressure chamber  112  to the wheel cylinders  40  is gradually increased, such that a brake pressure is also increased. However, there is an effective stroke in the hydraulic piston  114 , such that a maximum pressure caused by forward movement of the hydraulic piston  114  may be present in the hydraulic piston  114 . 
     In this case, a maximum pressure of the low-pressure mode may be lower than a maximum pressure of the high-pressure mode. However, compared with the low-pressure mode, the high-pressure mode may have a smaller pressure increase rate per stroke of the hydraulic piston  114 , because the entire pressure medium discharged from the first pressure chamber  112  is partially transmitted to the second pressure chamber  113  without being fully transmitted to the second pressure chamber  113 . A detailed description thereof will be given later with reference to  FIG. 5 . 
     Therefore, during an initial braking stage in which braking response characteristics are considered important, the electronic brake system  1  may use the low-pressure mode in which a pressure increase rate per stroke is high. During the latter braking stage in which a maximum brake pressure is considered important, the electronic brake system  1  may use the high-pressure mode in which a maximum pressure is high. 
       FIG. 3  is a hydraulic circuit diagram illustrating the electronic brake system  1  for providing brake pressure of the low-pressure mode by forward movement of the hydraulic piston  114  according to the first embodiment of the present disclosure.  FIG. 4  is a hydraulic circuit diagram illustrating the rear-wheel regenerative braking state of the electronic brake system  1  in the brake pressure providing state of  FIG. 3 .  FIG. 5  is a hydraulic circuit diagram illustrating the electronic brake system  1  for providing brake pressure of the high-pressure mode by forward movement of the hydraulic piston  114  according to a first embodiment of the present disclosure. 
     Referring to  FIG. 3 , when the driver depresses the brake pedal  10  in the initial braking stage, the motor  120  may rotate in one direction, rotational force of the motor  120  may be transmitted to the hydraulic-pressure providing unit  110  by the power switching unit  130 , the hydraulic piston  114  of the hydraulic-pressure providing unit  110  moves forward, such that hydraulic pressure may occur in the first pressure chamber  112 . Hydraulic pressure discharged from the first pressure chamber  112  may be transmitted to the wheel cylinders  40  respectively provided to four wheels through the first hydraulic circuit  201  and the second hydraulic circuit  202 , such that braking force occurs in the wheel cylinders  40 . 
     In more detail, hydraulic pressure supplied from the first pressure chamber  112  may be directly transmitted to the wheel cylinders  40  provided in the first hydraulic circuit  201  not only through the first hydraulic passage  211  connected to the first communication hole  111   a , but also through the second hydraulic passage  212  and the eighth hydraulic passage  218 . In this case, the first and second inlet valves  221   a  and  222   b  respectively installed in two passages branched from the first hydraulic circuit  201  may remain open, and the first and second outlet valves  222   a  and  222   b  installed in passages branched from two passages branched from the first hydraulic circuit  201  may remain closed, such that hydraulic pressure is prevented from leaking to the reservoir  30 . 
     In addition, hydraulic pressure supplied from the first pressure chamber  112  may be directly transmitted to the wheel cylinders  40  provided in the second hydraulic circuit  202  not only through the first hydraulic passage  211  connected to the first communication hole  111   a , but also through the third hydraulic passage  213 . In this case, the third and fourth inlet valves  221   c  and  222   d  respectively installed in two passages branched from the second hydraulic circuit  202  may remain open, and the third and fourth outlet valves  222   c  and  222   d  installed in passages branched from two passages branched from the second hydraulic circuit  202  may remain closed, such that hydraulic pressure is prevented from leaking to the reservoir  30 . 
     In this case, the first valve  231 , the fifth valve  235 , and the sixth valve  236  may transition to the open state, such that the second hydraulic circuit  212 , the seventh hydraulic passage  217 , and the eighth hydraulic passage  218  may be open. Since the seventh hydraulic passage  217  and the eighth hydraulic passage  218  are opened, hydraulic pressure supplied from the first pressure chamber  112  may be transmitted to the second hydraulic circuit  202  after sequentially passing through the first hydraulic passage  211 , the second hydraulic passage  212 , and the seventh hydraulic passage  217 , or may also be transmitted to the first hydraulic circuit  201  after sequentially passing through the first hydraulic passage  211 , the third hydraulic passage  212 , the seventh hydraulic passage  217 , and the eighth hydraulic passage  218 . 
     The third valve  233  may remain closed, such that the fifth hydraulic passage  215  can be blocked. As a result, hydraulic pressure produced in the first pressure chamber  112  may be prevented from flowing into the second pressure chamber  113  through the fifth hydraulic passage  215 , such that a pressure increase rate per stroke of the hydraulic piston  114  may be improved. Therefore, the electronic brake system  1  may obtain a rapid braking response in the initial braking stage. 
     When hydraulic pressure of the pressure medium occurs by the hydraulic-pressure supply device  100 , the first and second cut valves  261  and  262  provided in the first and second backup passages  251  and  252  may be closed, such that hydraulic pressure discharged from the master cylinder  20  is prevented from flowing into the wheel cylinders  40 . Hydraulic pressure produced in the master cylinder  20  according to a pedal effort of the brake pedal  10  may be transmitted to the simulation device  50  connected to the master cylinder  20 . In this case, the simulator valve  54  provided at the front end of the simulation chamber  51  may be opened, such that hydraulic pressure discharged from the first master chamber  20   a  of the master cylinder  20  may be transmitted to the front side of the reaction force piston  52  disposed in the simulation chamber  51  through the simulator valve  54 . As a result, the reaction force spring  53  may be compressed by movement of the reaction force piston  52 , and a reaction force corresponding to a pedal effort of the brake pedal may occur by elastic restoring force of the reaction force spring  53 , resulting in formation of proper pedal feel for the driver. 
     The passage pressure sensors PS 21  and PS 22  for sensing hydraulic pressure of at least one of the first hydraulic circuit  201  and the second hydraulic circuit  202  may sense hydraulic pressure applied to wheel cylinders  40 , and may control the hydraulic-pressure supply device  100  based on the sensed hydraulic pressure, such that the amount or hydraulic pressure of the pressure medium applied to the wheel cylinders  40  can be controlled. Moreover, during regenerative braking of the rear wheel cylinders  40  of the first hydraulic circuit  201 , the ECU may determine whether to close the sixth valve  236  based on pressure information sensed by the passage pressure sensor P 21 , and may also determine a closing start time of the sixth valve  236  based on the pressure information sensed by the passage pressure sensor P 21 . In addition, if hydraulic pressure applied to the wheel cylinders  40  is higher than a target pressure value corresponding to the pedal effort of the brake pedal  10 , at least one of the first to fourth outlet valves  222  is open such that the resultant hydraulic pressure may be controlled to correspond to the target pressure value. 
     The rear-wheel regenerative braking operation of the electronic brake system  1  according to the first embodiment of the present disclosure will hereinafter be described with reference to the attached drawings. 
     Referring to  FIG. 4 , during the initial braking stage in which pressure of the low-pressure mode is provided, if the driver depresses the brake pedal  10 , the motor  120  may rotate in one direction, rotational force of the motor  120  may be transmitted to the hydraulic-pressure providing unit  110  by the power switching unit  130 , the hydraulic piston  114  of the hydraulic-pressure providing unit  110  moves forward, such that hydraulic pressure may occur in the first pressure chamber  112 . Hydraulic pressure discharged from the first pressure chamber  112  may be transmitted to the wheel cylinders  40  respectively provided at four wheels through the first hydraulic circuit  201  and the second hydraulic circuit  202 , resulting in occurrence of braking force. 
     In addition, the first valve  231 , the fifth valve  235 , and the sixth valve  236  may transition to the open state, such that the second hydraulic circuit  212 , the seventh hydraulic passage  217 , and the eighth hydraulic passage  218  may be open. Since the seventh hydraulic passage  217  and the eighth hydraulic passage  218  are opened, hydraulic pressure supplied from the first pressure chamber  112  may be transmitted to the second hydraulic circuit  202  via the third hydraulic passage  213  after sequentially passing through the first hydraulic passage  211 , the second hydraulic passage  212 , and the seventh hydraulic passage  217 , or may also be transmitted to the first hydraulic circuit  201  after sequentially passing through the first hydraulic passage  211 , the third hydraulic passage  213 , the seventh hydraulic passage  217 , and the eighth hydraulic passage  218 . 
     Thereafter, when the ECU determines that regenerative braking is driven in the rear wheels (for example, in the wheel cylinders  40  of the first hydraulic circuit  201 ), the ECU may calculate the magnitude of a brake hydraulic pressure calculated in response to a difference between a brake pressure requested by the driver (hereinafter referred to as a driver-requested brake pressure) and a regenerative braking pressure, the first hydraulic circuit  201  may close the sixth valve  236  after applying hydraulic pressure corresponding to the corresponding pressure level to the rear wheel cylinders  40 , such that transmission (or delivery) of hydraulic pressure is prevented. Accordingly, a brake hydraulic pressure of the rear wheels in which regenerative braking has occurred may be less than in a non-operation state of the regenerative-braking. 
     The ECU may stably control a brake hydraulic pressure flowing from the hydraulic-pressure supply device  100  to the rear wheel cylinders  40  of the first hydraulic circuit  201  using the passage pressure sensor PS 21  that senses a hydraulic pressure of the first hydraulic circuit  201 . In more detail, the ECU may allow the passage pressure sensor PS 22  to sense a brake hydraulic pressure applied to the front wheel cylinders  40  of the second hydraulic circuit  202  that receives only a brake hydraulic pressure caused by hydraulic pressure produced from the hydraulic-pressure supply device  100 , may compare the sensed brake hydraulic pressure with the brake hydraulic pressure applied to the rear wheel cylinders  40  of the first hydraulic circuit  201 , and may more precisely control a rear-wheel brake hydraulic pressure that needs to be blocked or reduced by the rear wheel cylinders  40  of the first hydraulic circuit  201  during regenerative braking. 
     As described above, during rear-wheel regenerative braking, the ECU may control operation of the sixth valve  236 , such that a brake hydraulic pressure applied to the rear wheel cylinders  40  of the first hydraulic circuit  201  can be stably adjusted according to a regenerative braking pressure. As a result, a brake pressure or braking force can be evenly applied to four wheels of the vehicle, such that stability in vehicle braking is increased and oversteer or understeer of the vehicle is prevented, resulting in increased driving stability of the vehicle. 
     Meanwhile, if the ECU desires to reduce some parts of hydraulic pressure applied to the first hydraulic circuit  201  so as to facilitate regenerative braking, the ECU may open the first and second outlet valves  222   a  and  222   b  such that some parts of the hydraulic pressure applied to the wheel cylinders  40  can be directly discharged to the reservoir  30 . The ECU may open the third valve  233 , such that some parts of the hydraulic pressure of the first hydraulic circuit  201  are supplied to the second pressure chamber  113  after sequentially passing through the eighth hydraulic passage  218 , the fifth hydraulic passage  215 , and the fourth hydraulic passage  214 . The hydraulic pressure supplied to the second pressure chamber  113  may also be discharged to the reservoir  30  through the third dump valve  243 . 
     The hydraulic-pressure supply device  100  of the electronic brake system  1  according to the first embodiment may transition from the low-pressure mode shown in  FIGS. 3 and 4  to the high-pressure mode shown in  FIG. 5  before the hydraulic piston  114  moves forward by a maximum distance. 
     Referring to  FIG. 5 , if a hydraulic pressure sensed by each of the passage pressure sensors PS 21  and PS 22  is higher than a predetermined pressure level, the ECU may transition from the low-pressure mode to the high-pressure mode. During the high-pressure mode, the third valve  233  may transition to the open state, such that the fifth hydraulic passage  215  may be open. Therefore, some parts of the hydraulic pressure produced in the first pressure chamber  112  may be transmitted to the second pressure chamber  113  after sequentially passing through the first hydraulic passage  211 , the second hydraulic passage  212 , the eighth hydraulic passage  218 , and the fifth hydraulic passage  215 , such that the resultant hydraulic pressure can allow the hydraulic piston  114  to move farther forward and load applied to the motor  120  can also be reduced. 
     In the high-pressure mode, some parts of the pressure medium discharged from the first pressure chamber  112  may flow into the second pressure chamber  113 , such that the pressure increase rate per stroke is reduced. However, some parts of a hydraulic pressure produced in the first pressure chamber  112  may allow the hydraulic piston  114  to move farther forward, such that a maximum pressure of the pressure medium can be increased. The reason why the maximum pressure of the pressure medium is increased is that the drive shaft  133  passes through the second pressure chamber  113  so that a volume change rate per stroke of the hydraulic piston  114  is relatively smaller in the second pressure chamber  113  than in the first pressure chamber  112 . 
     In addition, as the hydraulic piston  114  moves farther forward, a hydraulic pressure of the first pressure chamber  112  is increased, force needed for backward movement of the hydraulic piston  114  affected by the increased hydraulic pressure of the first pressure chamber becomes stronger, such that load applied to the motor  120  is also increased. However, the fifth hydraulic circuit  215  is open under control of the third valve  233 , and some parts of the pressure medium discharged from the first pressure chamber  112  are transmitted to the second pressure chamber  113 , such that a hydraulic pressure may also occur in the second pressure chamber  113 , resulting in reduction of load applied to the motor  120 . 
     In this case, the third dump valve  243  may transition to a closed state. Since the third dump valve  243  is closed, the pressure medium in the first pressure chamber  112  may rapidly flow into the second pressure chamber  113  having a negative pressure, such that a hydraulic pressure may also occur in the second pressure chamber  113 . However, the third dump valve  243  is kept open as necessary, such that the pressure medium stored in the second pressure chamber  113  may flow into the reservoir  30 . 
     A method for supplying a brake pressure to the wheel cylinders  40  by backward movement of the hydraulic piston  114  will hereinafter be described. 
       FIG. 6  is a hydraulic circuit diagram illustrating the electronic brake system  1  for providing brake pressure by backward movement of the hydraulic piston  114  according to the first embodiment of the present disclosure. Referring to  FIG. 6 , during the initial braking stage, if the driver depresses the brake pedal  10 , the motor  120  may rotate in an opposite direction, rotational force of the motor  120  may be transmitted to the hydraulic-pressure providing unit  110  by the power switching unit  130 , the hydraulic piston  114  of the hydraulic-pressure providing unit  110  moves backward, such that hydraulic pressure may occur in the second pressure chamber  113 . Hydraulic pressure discharged from the second pressure chamber  113  may be transmitted to the wheel cylinders  40  respectively provided at four wheels through the first hydraulic circuit  201  and the second hydraulic circuit  202 , resulting in occurrence of braking force. 
     In more detail, hydraulic pressure from the second pressure chamber  113  may be directly transmitted to the wheel cylinders  40  mounted to the first hydraulic circuit  201  not only through the fourth hydraulic passage  214  connected to the second communication hole  111   b , but also through the open fifth hydraulic passage  215  and the sixth hydraulic passage  216 . In this case, the first and second inlet valves  221   a  and  221   b  may remain open, and the first and second outlet valves  222   a  and  221   b  may remain closed, such that hydraulic pressure can be prevented from leaking to the reservoir  30 . 
     In addition, hydraulic pressure from the second pressure chamber  113  may sequentially pass through the fourth hydraulic passage  214  connected to the second communication hole  111   b , the open fifth hydraulic passage  215 , the sixth hydraulic passage  215 , the open eighth hydraulic passage  218 , and the open seventh hydraulic passage  217 , such that the resultant hydraulic pressure is directly transmitted to the wheel cylinders  40  of the second hydraulic circuit  202  through the third hydraulic passage  213 . In this case, the third and fourth inlet valves  221   c  and  221   d  may remain open, and the third and fourth outlet valves  222   c  and  222   d  may remain closed, such that hydraulic pressure is prevented from leaking to the reservoir  30 . 
     In this case, the third valve  233  may transition to the open state such that the fifth hydraulic passage  215  is open. The fourth valve  234  is provided as a check valve for allowing the pressure medium to flow from the second pressure chamber  113  to the wheel cylinders  40 , such that the sixth hydraulic passage  216  may also be open. In addition, the fifth valve  235  and the sixth valve  236  may also transition to the open state in a manner that hydraulic pressure is also transmitted to the wheel cylinders  40  of the second hydraulic circuit  202 , such that the seventh hydraulic passage  217  and the eighth hydraulic passage  218  may be open. 
     Meanwhile, the first valve  231  may remain closed such that the second hydraulic passage  212  can be blocked. As a result, hydraulic pressure from the second pressure chamber is prevented from leaking to the first pressure chamber  112  through the second hydraulic passage  212 , such that a pressure increase rate per stroke of the hydraulic piston  114  is increased such that a rapid braking response can be achieved in the initial braking stage. 
     The third dump valve  243  may transition to a closed state. Since the third dump valve  243  is closed, hydraulic pressure of the pressure medium can rapidly and stably occur in the second pressure chamber  113 , and hydraulic pressure from the second pressure chamber  113  may be discharged only to the fourth hydraulic passage  214 . 
     A method for releasing brake pressure in a normal operation state of the electronic brake system  1  according to the first embodiment of the present disclosure will hereinafter be described with reference to the attached drawings. 
       FIG. 7  is a hydraulic circuit diagram illustrating the electronic brake system  1  for releasing brake pressure of the high-pressure mode by backward movement of the hydraulic piston  114  according to the first embodiment of the present disclosure.  FIG. 8  is a hydraulic circuit diagram illustrating the electronic brake system  1  for releasing brake pressure of the low-pressure mode by backward movement of a hydraulic piston  114  according to the first embodiment of the present disclosure. 
     Referring to  FIG. 7 , when a pedal effort applied to the brake pedal  10  is released, the motor  120  produces rotational force in an opposite direction to the braking rotation direction and transmits the rotational force to the power switching unit  130 , the worm shaft  131 , the worm wheel  132 , and the drive shaft  133  of the power switching unit  130  may rotate in the opposite direction to the braking rotation direction, such that the hydraulic piston  114  moves back to an original position thereof. As a result, hydraulic pressure from the first pressure chamber  112  may be released, and a negative pressure may occur in the first pressure chamber  112 . Simultaneously, the pressure medium discharged from the wheel cylinders  40  may be transmitted to the first pressure chamber  112  through the first and second hydraulic circuits  201  and  202 . 
     In more detail, a negative pressure produced in the first pressure chamber  112  may release a pressure from the wheel cylinders  40  mounted to the first hydraulic circuit  201  after passing through the eighth hydraulic passage  218 , the second hydraulic passage  212 , and the first hydraulic passage  211 . In this case, the first and second inlet valves  221   a  and  221   b  respectively installed in two passages branched from the first hydraulic circuit  201  may remain open, and the first and second outlet valves  222   a  and  222   b  respectively installed in two passages branched from the first hydraulic circuit  200  may remain closed, such that the pressure medium of the reservoir  30  is prevented from flowing into the first pressure chamber  112 . 
     In addition, a negative pressure produced in the first pressure chamber  112  may release a pressure from the wheel cylinders  40  mounted to the second hydraulic circuit  202  after passing through the third hydraulic passage  213  connected to the first communication hole  111   a , the seventh hydraulic passage  217 , the second hydraulic passage  212 , and the first hydraulic passage  211 . In this case, the third and fourth inlet valves  221   c  and  221   d  respectively installed in two passages branched from the second hydraulic circuit  202  may remain open, and the third and fourth outlet valves  222   c  and  222   d  respectively installed in two passages branched from the second hydraulic circuit  202  may remain closed, such that the pressure medium of the reservoir  30  is prevented from flowing into the first pressure chamber  112 . 
     Meanwhile, the first valve  231  may transition to the open state and the third valve  233  may also transition to the open state, such that the fifth hydraulic passage  215  is open. In addition, the fifth and sixth valves  235  and  236  may transition to the open state, such that the seventh and eighth hydraulic passages  217  and  218  are also open and the first pressure chamber  112  can communicate with the second pressure chamber  113 . 
     That is, in order to form a negative pressure in the first pressure chamber  112 , the hydraulic piston  114  should move backward. However, when hydraulic pressure of the pressure medium is present in the second pressure chamber  113 , unexpected resistance may occur in backward movement of the hydraulic piston  114 . Therefore, the first, third and sixth valves  231 ,  233  and  236  may transition to the open state in a manner that the first pressure chamber  112  may communicate with the second pressure chamber  113 , such that the pressure medium of the second pressure chamber  113  may flow into the first pressure chamber  112 . 
     In this case, the third dump valve  243  may transition to the closed state. Since the third dump valve  243  is closed, the pressure medium of the second pressure chamber  113  may be discharged only to the fourth hydraulic passage  214 . However, the third dump valve  243  may remain open as necessary, such that the pressure medium of the second pressure chamber  113  may also flow into the reservoir  30 . 
     In addition, when a negative pressure applied to each of the first and second hydraulic circuits  201  and  202  is measured to be higher than a target pressure release value corresponding to a released pedal effort of the brake pedal  10 , at least one of the first to fourth outlet valves  222  is open, such that the resultant pressure may be controlled to correspond to the target pressure value. In addition, the first and second cut valves  261  and  262  respectively installed in the first and second backup passages  251  and  252  may be closed, such that a negative pressure produced in the master cylinder  20  may not be transmitted to the hydraulic control unit  200 . 
     Meanwhile, during a high-pressure mode shown in  FIG. 7 , not only a pressure medium stored in the wheel cylinders  40  but also a pressure medium stored in the second pressure chamber  113  may be supplied to the first pressure chamber  112  due to a negative pressure produced in the first pressure chamber  112  affected by backward movement of the hydraulic piston  114 , such that a pressure reduction rate of the wheel cylinders  40  is at a low level. Therefore, it may be difficult to rapidly release brake pressure in the high-pressure mode. As a result, the operation for releasing brake pressure of the high-pressure mode may be used only in a high-pressure situation of the brake pressure. In order to rapidly release a brake pressure that is equal to or less than a predetermined brake pressure, the operation mode may transition to the operation for releasing brake pressure in the low-pressure mode shown in  FIG. 8 . 
     Referring to  FIG. 8 , when the brake pressure is released in the low-pressure mode, the third dump valve  233  transitions to the open state or remains open, without closing the fifth hydraulic passage  215  affected by the third valve  233  that remains closed or transitions to the closed state, such that the second pressure chamber  113  may communicate with the reservoir  30 . 
     When the brake pressure is released in the low-pressure mode, a negative pressure produced in the first pressure chamber  112  may be used only to recover (or retrieve) the pressure medium of the wheel cylinders  40 , such that a pressure reduction rate per stroke of the hydraulic piston  114  may be increased more than in the other case in which a brake pressure is released in the high-pressure mode. In this case, hydraulic pressure produced in the second pressure chamber  113  by backward movement of the hydraulic piston  114  may transition to the open state, such that most of the hydraulic pressure may be transmitted to the reservoir  30  staying in an atmospheric pressure state without passing through the fourth valve  234 . 
     Differently from  FIG. 8 , it may be possible to release a brake pressure of the wheel cylinders  40  even when the hydraulic piston  114  moves forward. 
       FIG. 9  is a hydraulic circuit diagram illustrating the electronic brake system for releasing brake pressure by forward movement of the hydraulic piston  114  according to the first embodiment of the present disclosure. 
     Referring to  FIG. 9 , when a pedal effort applied to the brake pedal  10  is released, the motor  120  produces rotational force in an opposite direction to the braking rotation direction and transmits the rotational force to the power switching unit  130 , the worm shaft  131 , the worm wheel  132 , and the drive shaft  133  of the power switching unit  130  may rotate in the opposite direction to the braking rotation direction, such that the hydraulic piston  114  moves forward to an original position thereof. As a result, hydraulic pressure from the second pressure chamber  113  may be released, and a negative pressure may occur in the second pressure chamber  113 . Simultaneously, the pressure medium discharged from the wheel cylinders  40  may be transmitted to the second pressure chamber  113  through the first and second hydraulic circuits  201  and  202 . 
     In more detail, a negative pressure produced in the second pressure chamber  113  may release a pressure from the wheel cylinders  40  mounted to the first hydraulic circuit  201  after passing through the fourth hydraulic passage  214  connected to the second communication hole  111   b , the fifth hydraulic passage  215 , and the second hydraulic passage  212 . In this case, the first and second inlet valves  221   a  and  221   b  respectively installed in two passages branched from the first hydraulic circuit  201  may remain open, and the first and second outlet valves  222   a  and  222   b  respectively installed in the passages branched from the first hydraulic circuit  201  may remain closed, such that the pressure medium of the reservoir  30  is prevented from flowing into the second pressure chamber  113 . 
     In addition, a negative pressure produced in the second pressure chamber  113  may release a pressure from the wheel cylinders  40  mounted to the second hydraulic circuit  202  after passing through the fourth hydraulic passage  214  connected to the second communication hole  111   b , the fifth hydraulic passage  215 , the seventh hydraulic passage  217 , the eighth hydraulic passage  218 , and the third hydraulic passage  213 . In this case, the third and fourth inlet valves  221   c  and  221   d  respectively installed in two passages branched from the second hydraulic circuit  202  may remain open, and the third and fourth outlet valves  222   c  and  222   d  respectively installed in two passages branched from the second hydraulic circuit  202  may remain closed, such that the pressure medium of the reservoir  30  is prevented from flowing into the second pressure chamber  113 . 
     In this case, the third valve  233  may transition to the open state such that the fifth hydraulic passage  215  is open. Thereafter, the fifth and sixth valves  235  and  236  may also transition to the open state, such that the seventh and eighth hydraulic passes  217  and  218  may be open. 
     In this case, the third dump valve  243  may transition to the closed state, such that a negative pressure produced in the second pressure chamber  113  may quickly retrieve the pressure medium from the wheel cylinders  40 . 
     In addition, when a negative pressure applied to each of the first and second hydraulic circuits  201  and  202  is measured to be higher than a target pressure release value corresponding to a released pedal effort of the brake pedal  10 , at least one of the first to fourth outlet valves  222  is open, such that the resultant pressure may be controlled to correspond to the target pressure value. In addition, the first and second cut valves  261  and  262  respectively installed in the first and second backup passages  251  and  252  may be closed, such that a negative pressure produced in the master cylinder  20  may not be transmitted to the hydraulic control unit  200 . 
     When the electronic brake system  1  according to the first embodiment of the present disclosure abnormally operates, the operation states of the electronic brake system  1  will hereinafter be described. 
       FIG. 10  is a hydraulic circuit diagram illustrating an abnormal operation state of the electronic brake system  1  according to the first embodiment of the present disclosure. 
     Referring to  FIG. 10 , when the electronic brake system  1  abnormally operates, individual valves are controlled to return to the initial braking stage in which the valves do not operate. Thereafter, when the driver depresses the brake pedal  10 , the first piston  21   a  connected to the brake pedal  10  may move forward, and the second piston  22   a  may also move forward by movement of the first piston  21   a . Therefore, hydraulic pressure may occur in the pressure medium stored in the first and second master chambers  20   a  and  20   b , the hydraulic pressure produced in the first and second master chambers  20   a  and  20   b  may be transmitted to the wheel cylinders  40  through the first and second backup passages  251  and  252 , resulting in formation of braking force. 
     In this case, the first and second cut valves  261  and  262  provided in the first and second backup passages  251  and  252  may be implemented as normally open (NO) solenoid valves. The inlet valves  221  provided in the first and second hydraulic circuits  201  and  202  may be implemented as normally open (NO) solenoid valves. The simulator valve  54  and the outlet valves  222  may be implemented as normally closed (NC) solenoid valves, such that the hydraulic pressure produced in the first and second master chambers  20   a  and  20   b  of the master cylinder  20  can be immediately transmitted to four wheel cylinders  40 , such that braking stability is improved and rapid braking is carried out. 
     An inspection mode of the electronic brake system  1  according to the first embodiment of the present disclosure will hereinafter be described. 
     The inspection mode may include a first inspection mode for inspecting the presence or absence of a leak in the master cylinder  20 , a second inspection mode for inspecting the presence or absence of a leak in the simulation device  50 , and a third inspection mode for inspecting the presence or absence of air in the master cylinder  20 . The electronic brake system  1  according to the first embodiment may perform the inspection mode before the vehicle starts driving or during traveling or stopping of the vehicle, such that the electronic brake system  1  may periodically or frequently inspect the presence or absence of device malfunction. 
       FIG. 11  is a hydraulic circuit diagram illustrating the electronic brake system  1  for inspecting the presence or absence of a leak either in the master cylinder  20  or in the simulator valve, or for inspecting the presence or absence of air in the master cylinder  20  according to the first embodiment of the present disclosure. 
     Referring to  FIG. 11 , when the electronic brake system  1  abnormally operates, individual valves are controlled to return to the initial braking stage in which the valves do not operate. Not only the first and second cut valves  261  and  262  installed in the first and second backup passages  251  and  252 , but also the inlet valves  221  provided to the front end of the wheel cylinders  40  mounted to four wheels RR, RL, FR, and FL may be opened, such that hydraulic pressure can be immediately transmitted to the wheel cylinders  40 . 
     In this case, the simulator valve  54  may remain closed, such that hydraulic pressure applied to the wheel cylinders  40  through the first backup passage  251  is prevented from flowing into the reservoir  30  through the simulation device  50 . Therefore, hydraulic pressure discharged from the master cylinder  20  when the driver depresses the brake pedal  10  may be transmitted to the wheel cylinders  40 , resulting in implementation of stable braking. 
     However, if there is a leak either in the master cylinder  20  or in the simulator valve  54 , some parts of hydraulic pressure discharged from the master cylinder  20  may leak to the reservoir  30  through the simulator valve  54 , such that it is impossible to produce braking force intended by the driver, resulting in reduction in vehicle braking stability. 
     In addition, even when air is present in the master cylinder  20 , the same issues as described above may also occur. If air is present in the master cylinder  20 , pedal feel provided to the driver may be reduced. Nevertheless, if the driver recognizes a state of the reduced pedal feel as a normal operation state and the operation mode transitions to a fallback mode, braking performance of the vehicle may be deteriorated. 
     If hydraulic pressure discharged from the hydraulic-pressure supply device  100  flows into the reservoir  30  and thus a pressure loss occurs, it is difficult to recognize whether a leak occurs in the master cylinder  20  or the simulator valve  54  or it is difficult to recognize whether air is present in the master cylinder  20 . Therefore, during the inspection mode, the inspection valve  60  is closed such that a hydraulic circuit connected to the hydraulic-pressure supply device  100  may be implemented as a closed circuit. In other words, the inspection valve  60 , the simulator valve  54 , and the outlet valves  222  are closed to block a passage between the reservoir  30  and the hydraulic-pressure supply device  100 , resulting in formation of a closed circuit. 
     During the inspection mode, the brake system  1  may supply hydraulic pressure only to the first backup passage  251  connected to the simulation device  50  from among the first and second backup passages  251  and  252 . Therefore, in order to prevent hydraulic pressure from flowing into the master cylinder  20  via the second backup passage  252 , the second cut valve  262  may transition to the closed state. In addition, the third valve  233  for interconnecting the first hydraulic circuit  201  and the second hydraulic circuit  202  is controlled to be closed, such that the hydraulic pressure from the first pressure chamber  112  is prevented from leaking to the second pressure chamber  113 . 
     Referring to  FIG. 11 , during the inspection mode, individual valves of the electronic brake system  1  according to the first embodiment are controlled to return to the initial braking stage in which the valves do not operate, the first to fourth inlet valves  221   a ,  221   b ,  221   c , and  221   d  and the second cut valve  262  may transition to the closed state, and the first cut valve  261  may remain open, such that hydraulic pressure produced in the hydraulic-pressure supply device  10  may be transmitted to the master cylinder  20 . 
     The inlet valves  221  are controlled in the closed state, such that the hydraulic pressure from the hydraulic-supply supply device  10  is prevented from flowing into the wheel cylinders  40 . The second cut valve  262  is controlled in the closed state, such that the second cut valve  262  may prevent the hydraulic pressure of the hydraulic-pressure supply device  100  from being discharged along the second backup passage  262 . The inspection valve  60  may transition to the closed state, such that the inspection valve  60  may prevent the hydraulic pressure supplied to the master cylinder  20  from leaking to the reservoir  30 . 
     Specifically, when inspecting the presence or absence of air in the master cylinder  20 , the inlet valves  221  may be controlled to be closed, thereby preventing the hydraulic pressure from flowing into the wheel cylinders  40 . There is a very small variation in hydraulic pressure affected by the air present in the first master chamber  20   a  of the master cylinder  20 , such that it is preferable that hydraulic-pressure interference caused by the wheel cylinders  40  be minimized. 
     During the inspection mode, the ECU may generate hydraulic pressure using the hydraulic-pressure supply device  100 , and may analyze a pressure value of the master cylinder  20  measured by the backup-passage pressure sensor PS 1 , such that the ECU may determine the presence or absence of a leak either in the master cylinder  20  or in the simulation valve  54  or may determine the presence or absence of the air in the master cylinder  20 . The ECU may compare a pressing-medium hydraulic pressure value estimated to be generated by a displacement of the hydraulic piston  114  with the inner pressure of the first master chamber  20   a  measured by the backup-passage pressure sensor PS 1 , such that the ECU may diagnose the presence or absence of a leak or air in the master cylinder  20 , and may also diagnose the presence or absence of a leak in the simulator valve  54 . In more detail, the ECU may compare a first hydraulic pressure value of the first pressure chamber  112 , that is calculated and estimated based on either the displacement of the hydraulic piston  114  or the rotation angle measured by the motor control sensor (MPS), with a second hydraulic pressure value actually measured by the backup-passage pressure sensor PS 1 . 
     If two hydraulic pressure values (i.e., the first hydraulic pressure value and the second hydraulic pressure value) are identical to each other, the ECU may determine the absence of a leak in the master cylinder  20  or in the simulation valve  54  and may also determine the absence of the air in the master cylinder  20 . In contrast, when the first hydraulic pressure value is lower than the second hydraulic pressure value, this means that some parts of hydraulic pressure of the pressure medium supplied to the first master cylinder  20   a  are lost, such that the ECU may determine the presence of a leak either in the master cylinder  20  or in the simulator valve  54  or the presence of air in the master cylinder  20 , and may inform the driver of the result of determination. 
     An electronic brake system  2  according to the second embodiment of the present disclosure will hereinafter be described. 
     In the following detailed description of the electronic brake system  2  according to the second embodiment, the remaining parts other than other constituent elements denoted by different reference numbers not shown in the electronic brake system  1  of the first embodiment are identical to those of the electronic brake system  1  of the first embodiment, and as such a detailed description thereof will herein be omitted to avoid redundant description thereof. 
       FIG. 12  is a hydraulic circuit diagram illustrating the electronic brake system  2  according to a second embodiment of the present disclosure. 
     Referring to  FIG. 12 , a first hydraulic passage  311  of the electronic brake system  2  according to the second embodiment may be provided to connect the first pressure chamber  112  to the first and second hydraulic circuits  201  and  202 , and the first hydraulic passage  311  may be branched into a second hydraulic passage  312  and a third hydraulic passage  313 . The third hydraulic passage  313  may be connected to the second hydraulic circuit, and the second hydraulic passage  312  may be connected to the first hydraulic circuit  201  through an eighth hydraulic passage  318  to be described later. As a result, hydraulic pressure produced in the first pressure chamber  112  by forward movement of the hydraulic piston  114  may be transmitted to the first hydraulic circuit  201  through the first hydraulic passage  311 , the second hydraulic passage  312 , and the eighth hydraulic passage  318 , and may then be transmitted to the second hydraulic circuit through the first hydraulic passage  311  and the third hydraulic passage  313 . 
     The second hydraulic passage  312  may be provided with a first valve  331  to control flow of a pressure medium, and the third hydraulic passage  313  may be provided with a second valve  332  to control flow of a pressure medium. 
     The first valve  331  may be implemented as a bidirectional valve to control flow of the pressure medium received through the second hydraulic passage  312 . The first valve  331  may be implemented as a normally closed (NC) solenoid valve that remains closed in a normal state and is then open upon receiving an opening signal from the ECU. 
     The second valve  332  may be implemented as a check valve that allows a pressure medium to flow from the first pressure chamber  112  to the second hydraulic circuit  202  and prevents the pressure medium from flowing from the second hydraulic circuit  202  to the first pressure chamber  112 . That is, the first valve  332  may allow hydraulic pressure of the first pressure chamber  112  to flow into the second hydraulic circuit  202 , and may prevent hydraulic pressure of the second hydraulic circuit  202  from leaking to the first pressure chamber  112  through the third hydraulic passage  313 . 
     The fourth hydraulic passage  314  may communicate with the second pressure chamber  113 , and may be linked to the second hydraulic passage  312 . The fourth hydraulic passage  314  may be branched into a fifth hydraulic passage  315  and a sixth hydraulic passage  316 . The fifth hydraulic passage  315  and the sixth hydraulic passage  316  may be linked again to the fourth hydraulic passage  314 . In addition, the eighth hydraulic passage  318  may connect the linked position of the second hydraulic passage  312  and the fourth hydraulic passage  314  to the first hydraulic circuit  201 , such that the second hydraulic passage  312  and the fourth hydraulic passage  314  may be connected to the first hydraulic circuit  201 . Both ends of the seventh hydraulic passage  317  may respectively communicate with a rear end of the first valve  331  of the second hydraulic passage  312  and a rear end of the second valve  332  of the third hydraulic passage  313 , such that the second hydraulic passage  312  may be connected to the third hydraulic passage  313 . 
     The fifth hydraulic passage  315  may be provided with a third valve  333  to control flow of a pressure medium, and the sixth hydraulic passage  316  may be provided with a fourth valve  334  to control flow of a pressure medium. 
     The third valve  333  may be implemented as a bidirectional valve to control flow of the pressure medium flowing through the fifth hydraulic passage  315 . The third valve  333  may be implemented as a normally closed (NC) solenoid valve that remains closed in a normal state and is then open upon receiving an opening signal from the ECU. 
     The fourth valve  334  may be implemented as a check valve that allows a pressure medium to flow from the fourth hydraulic passage  314  communicating with the second pressure chamber  113  to the eighth hydraulic passage  318  and prevents the pressure medium from flowing from the eighth hydraulic passage  318  to the fourth hydraulic passage  314 . That is, the fourth valve  334  may prevent hydraulic pressure of either the second hydraulic passage  312  or the eighth hydraulic passage  318  connected to the second hydraulic passage  312  from leaking to the second pressure chamber  113  through the sixth hydraulic passage  316  and the fourth hydraulic passage  314 . 
     Since the fifth and sixth hydraulic passages  315  and  316  are branched from the fourth hydraulic passage  314  and then meet again the fourth hydraulic passage  314 , the third valve  333  and the fourth valve  334  may be arranged parallel to each other. 
     The seventh hydraulic passage  317  may be provided with a fifth valve  335  to control flow of a pressure medium, and the eighth hydraulic passage  318  may be provided with a sixth valve  336  to control flow of a pressure medium. 
     The fifth valve  335  may be implemented as a bidirectional valve to control flow of the pressure medium flowing through the seventh hydraulic passage  317 . In the same manner as in the sixth valve  336 , the fifth valve  335  may be implemented as a normally closed (NC) solenoid valve that remains closed in a normal state and is then open upon receiving an opening signal from the ECU. 
     The sixth valve  336  may be implemented as a bidirectional valve to control flow of the pressure medium flowing through the eighth hydraulic passage  318 . Specifically, the sixth valve  336  may be disposed between the pressure chamber of the hydraulic pressure generator and at least one wheel cylinder to be used for regenerative braking, such that the sixth valve  336  may selectively connect the pressure chamber to the corresponding hydraulic circuit or may selectively sever such connection between the pressure chamber and the corresponding hydraulic circuit, such that only some parts of hydraulic pressure of the pressure medium may be transmitted to the corresponding wheel cylinder. For example, as shown in  FIG. 12 , the sixth valve  336  may be provided in the eighth hydraulic passage  318  between the first pressure chamber  112  and the first hydraulic circuit  201  provided with the wheel cylinders  40  of the rear wheels RL and RR in which rear-wheel regenerative braking is implemented, such that the sixth valve  336  may selectively connect the first pressure chamber  112  to the first hydraulic circuit  201  or may selectively sever such connection between the first pressure chamber  112  and the first hydraulic circuit  201 , and thus only some parts of hydraulic pressure of the pressure medium can be transmitted to the rear wheel cylinders  40  of the first hydraulic circuit  201 . A detailed description thereof will hereinafter be given. 
     The sixth valve  336  may be implemented as normally closed (NC) solenoid valves that remain closed in a normal state and are then open upon receiving an opening signal from the ECU. 
     By the above-mentioned passages and valves, hydraulic pressure produced in the second pressure chamber  113  by backward movement of the hydraulic piston  114  may be supplied to the first hydraulic circuit  201  through the fourth hydraulic passage  314 , the fifth and sixth hydraulic passages  315  and  316 , and the eighth hydraulic passage  318 , and may be supplied to the second hydraulic circuit  202  through the third hydraulic passage  313  after sequentially passing through the fourth hydraulic passage  314 , the fifth and sixth hydraulic passages  315  and  316 , and the seventh hydraulic passage  317 . 
     Furthermore, both ends of the seventh hydraulic passage  317  may respectively communicate with and be respectively connected to the rear end of the first valve  331  of the second hydraulic passage  312  and the rear end of the second valve  332  of the third hydraulic passage  313 . As a result, when the first valve  331  or the second valve  332  abnormally operates, hydraulic pressure produced in the first pressure chamber  112  may be stably transmitted to each of the first hydraulic circuit  201  and the second hydraulic circuit  202 . 
     The first, seventh, and eighth valves  331 ,  337 , and  338  are open when a pressure medium is taken out from the wheel cylinders  40  and then flows into the first pressure chamber  112  in a manner that hydraulic pressure applied to the wheel cylinders  40  is released, because the third valve  333  provided in the third hydraulic passage  313  is implemented as a check valve for allowing the pressure medium to flow only in one direction. 
     The first hydraulic circuit  201  and the second hydraulic circuit  202  of the hydraulic control unit  200  will hereinafter be described. 
     The first hydraulic circuit  201  may control hydraulic pressure of wheel cylinders  40  installed in the rear right wheel RR and the rear left wheel RL. The second hydraulic circuit  202  may control hydraulic pressure of other wheel cylinders  40  installed in the front right wheel FR and the front left wheel FL. 
     The first hydraulic circuit  201  may be connected to the first hydraulic passage  311 , the second hydraulic passage  312 , and the eighth hydraulic passage  318  so as to receive hydraulic pressure from the hydraulic-pressure supply device  100 , and the second hydraulic passage  312  may be branched into two passages that are respectively connected to the rear right wheel RR and the rear left wheel RL. Likewise, the second hydraulic circuit  202  may be connected to the first hydraulic passage  311  and the third hydraulic passage  313  so as to receive hydraulic pressure from the hydraulic-pressure supply device  100 , and the third hydraulic passage  313  may be branched into two passages that are respectively connected to the front right wheel FR and the front left wheel FL. 
     The first and second hydraulic circuits  201  and  202  may include a plurality of inlet valves  221  ( 221   a ,  221   b ,  221   c ,  221   d ) to control flow of the pressure medium and hydraulic pressure. For example, the first hydraulic circuit  201  may be provided with two inlet valves  221   a  and  221   b  connected to the second hydraulic passage  312  such that the two inlet valves  221   a  and  221   b  may respectively control hydraulic pressures applied to two wheel cylinders  40 . The second hydraulic circuit  202  may be provided with two inlet valves  221   c  and  221   d  connected to the third hydraulic passage  313  such that the two inlet valves  221   c  and  221   d  may respectively control hydraulic pressures applied to the wheel cylinders  40 . 
     The inlet valves  221  may be arranged upstream of the wheel cylinders  40 . The inlet valves  221  may be implemented as normally open (NO) solenoid valves that remain open in a normal state and are then closed upon receiving a closing signal from the ECU. 
     The first and second hydraulic circuits  201  and  202  may include check valves  223   a ,  223   b ,  223   c , and  223   d  connected parallel to the inlet valves  221   a ,  221   b ,  221   c , and  221   d . The check valves  223   a ,  223   b ,  223   c , and  223   d  may be provided in bypass passages by which front ends and rear ends of the respective inlet valves  221   a ,  221   b ,  221   c , and  221   d  are connected to one another in the first and second hydraulic circuits  201  and  202 . The check valves  223   a ,  223   b ,  223   c , and  223   d  may allow a pressure medium to flow from the wheel cylinders  40  to the hydraulic-pressure providing unit  110  and prevents the pressure medium from flowing from the hydraulic-pressure providing unit  110  to the wheel cylinders  40 . The check valves  223   a ,  223   b ,  223   c , and  223   d  may allow hydraulic pressure of the pressure medium applied to the wheel cylinders  40  to be rapidly discharged. Alternatively, during abnormal operation of the inlet valves  221   a ,  221   b ,  221   c , and  221   d , the check valves  223   a ,  223   b ,  223   c , and  223   d  may allow hydraulic pressure of the pressure medium applied to the wheel cylinders  40  to flow into the hydraulic-pressure providing unit  110 . 
     The first and second hydraulic circuits  201  and  202  may further include a plurality of outlet valves  222  ( 222   a ,  222   b ,  222   c ,  222   d ) connected to the reservoir  30  so as to improve performance or throughput when braking of the wheel cylinders  40  is released. The outlet valves  222  may be respectively connected to the wheel cylinders  40  so as to control flow of the pressure medium discharged from the wheel cylinders  40  of the respective wheels RR, RL, FR, and FL. That is, the outlet valves  222  may sense brake pressures of the respective wheels RR, RL, FR, and FL. If decompression braking is needed, the outlet valves  222  may be selectively open to control decompression of the wheel cylinders  40 . 
     The outlet valves  222  may be implemented as normally closed (NC) solenoid valves that remain closed in a normal state and are then open upon receiving an opening signal from the ECU. 
     A method for operating the electronic brake system  2  according to the second embodiment of the present disclosure will hereinafter be described. 
     The electronic brake system  2  according to the second embodiment may allow the hydraulic-pressure supply device  100  to be used in a low-pressure mode and a high-pressure mode in different ways. 
       FIG. 13  is a hydraulic circuit diagram illustrating the electronic brake system  2  for providing brake pressure of the low-pressure mode by forward movement of the hydraulic piston  114  according to the second embodiment of the present disclosure.  FIG. 14  is a hydraulic circuit diagram illustrating the rear-wheel regenerative braking state of the electronic brake system  3  in the brake pressure providing state of  FIG. 13 .  FIG. 15  is a hydraulic circuit diagram illustrating the electronic brake system  2  for providing brake pressure of the high-pressure mode by forward movement of the hydraulic piston  114  according to the second embodiment of the present disclosure. 
     Referring to  FIG. 13 , when the driver depresses the brake pedal  10  in the initial braking stage, the motor  120  may rotate in one direction, rotational force of the motor  120  may be transmitted to the hydraulic-pressure providing unit  110  by the power switching unit  130 , the hydraulic piston  114  of the hydraulic-pressure providing unit  110  moves forward, such that hydraulic pressure may occur in the first pressure chamber  112 . Hydraulic pressure discharged from the first pressure chamber  112  may be transmitted to the wheel cylinders  40  respectively provided to four wheels through the first hydraulic circuit  201  and the second hydraulic circuit  202 , such that braking force occurs in the wheel cylinders  40 . 
     In more detail, hydraulic pressure supplied from the first pressure chamber  112  may be directly transmitted to the wheel cylinders  40  provided in the first hydraulic circuit  201  not only through the first hydraulic passage  311  connected to the first communication hole  111   a , but also through the second hydraulic passage  312  and the eighth hydraulic passage  318 . In this case, the first and second inlet valves  221   a  and  222   b  respectively installed in two passages branched from the first hydraulic circuit  201  may remain open, and the first and second outlet valves  222   a  and  222   b  installed in passages branched from two passages branched from the first hydraulic circuit  201  may remain closed, such that hydraulic pressure is prevented from leaking to the reservoir  30 . 
     In addition, hydraulic pressure supplied from the first pressure chamber  112  may be directly transmitted to the wheel cylinders  40  provided in the second hydraulic circuit  202  not only through the first hydraulic passage  311  connected to the first communication hole  111   a , but also through the third hydraulic passage  313 . In this case, the third and fourth inlet valves  221   c  and  222   d  respectively installed in two passages branched from the second hydraulic circuit  202  may remain open, and the third and fourth outlet valves  222   c  and  222   d  installed in passages branched from two passages branched from the second hydraulic circuit  202  may remain closed, such that hydraulic pressure is prevented from leaking to the reservoir  30 . 
     In this case, the first valve  331  and the sixth valve  336  may transition to the open state, such that the second hydraulic passage  312  and the eighth hydraulic passage  318  may be open. The fifth valve  335  may also transition to the open state, such that the seventh hydraulic passage  317  may be open. Since the seventh hydraulic passage  217  and the eighth hydraulic passage  218  are opened, hydraulic pressure supplied from the first pressure chamber  112  may be transmitted to the second hydraulic circuit  202  after sequentially passing through the first hydraulic passage  311 , the second hydraulic passage  312 , and the seventh hydraulic passage  317 , or may also be transmitted to the first hydraulic circuit  201  after sequentially passing through the first hydraulic passage  311 , the third hydraulic passage  312 , and the seventh hydraulic passage  317 . 
     The third valve  333  may remain closed, such that the fifth hydraulic passage  315  can be blocked. As a result, hydraulic pressure produced in the first pressure chamber  112  may be prevented from flowing into the second pressure chamber  113  through the fifth hydraulic passage  315 , such that a pressure increase rate per stroke of the hydraulic piston  114  may be improved. Therefore, the electronic brake system  2  may obtain a rapid braking response in the initial braking stage. 
     The passage pressure sensors PS 21  and PS 22  for sensing hydraulic pressure of at least one of the first hydraulic circuit  201  and the second hydraulic circuit  202  may sense hydraulic pressure applied to wheel cylinders  40 , and may control the hydraulic-pressure supply device  100  based on the sensed hydraulic pressure, such that the amount or hydraulic pressure of the pressure medium applied to the wheel cylinders  40  can be controlled. Moreover, during regenerative braking of the rear wheel cylinders  40  of the first hydraulic circuit  201 , the ECU may determine whether to close the sixth valve  336  based on pressure information sensed by the passage pressure sensor P 21 , and may also determine a closing start time of the sixth valve  336  based on the pressure information sensed by the passage pressure sensor P 21 . In addition, if hydraulic pressure applied to the wheel cylinders  40  is higher than a target pressure value corresponding to the pedal effort of the brake pedal  10 , at least one of the first to fourth outlet valves  222  is open such that the resultant hydraulic pressure may be controlled to correspond to the target pressure value. 
     The rear-wheel regenerative braking operation of the electronic brake system  2  according to the second embodiment of the present disclosure will hereinafter be described with reference to the attached drawings. 
     Referring to  FIG. 14 , during the initial braking stage in which pressure of the low-pressure mode is provided, if the driver depresses the brake pedal  10 , the motor  120  may rotate in one direction, rotational force of the motor  120  may be transmitted to the hydraulic-pressure providing unit  110  by the power switching unit  130 , the hydraulic piston  114  of the hydraulic-pressure providing unit  110  moves forward, such that hydraulic pressure may occur in the first pressure chamber  112 . Hydraulic pressure discharged from the first pressure chamber  112  may be transmitted to the wheel cylinders  40  respectively provided at four wheels through the first hydraulic circuit  201  and the second hydraulic circuit  202 , resulting in occurrence of braking force. 
     In addition, the first valve  331  and the sixth valve  336  may transition to the open state, such that the second hydraulic circuit  312  and the eighth hydraulic passage  318  may be open. Thus, hydraulic pressure supplied from the first pressure chamber  112  may sequentially pass through the first hydraulic passage  311 , the second hydraulic passage  312 , and the eighth hydraulic passage  318 , such that the resultant hydraulic pressure may be transmitted to the first hydraulic circuit  201  provided with the wheel cylinders  40  of the rear wheels RL and RR. Alternatively, hydraulic pressure supplied from the first pressure chamber  112  may sequentially pass through the first hydraulic passage  311  and the third hydraulic passage  313 , such that the resultant hydraulic pressure may be transmitted to the second hydraulic circuit  202  provided with the wheel cylinders  40  of the front wheels FR and FL. 
     Thereafter, when the ECU determines that regenerative braking is driven in the rear wheels (for example, in the wheel cylinders  40  of the first hydraulic circuit  201 ), the ECU may calculate the magnitude of a brake hydraulic pressure calculated in response to a difference between a driver-requested brake pressure and a regenerative braking pressure, the first hydraulic circuit  201  may close the sixth valve  336  after applying hydraulic pressure corresponding to the corresponding pressure level to the rear wheel cylinders  40 , such that transmission (or delivery) of hydraulic pressure is prevented. Accordingly, a brake hydraulic pressure of the rear wheels in which regenerative braking has occurred may be less than in a non-operation state of the regenerative-braking. 
     The ECU may stably control a brake hydraulic pressure flowing from the hydraulic-pressure supply device  100  to the rear wheel cylinders  40  of the first hydraulic circuit  201  using the passage pressure sensor PS 21  that senses hydraulic pressure of the first hydraulic circuit  201 . In more detail, the ECU may allow the passage pressure sensor PS 22  to sense a brake hydraulic pressure applied to the front wheel cylinders  40  of the second hydraulic circuit  202  that receives only a brake hydraulic pressure caused by hydraulic pressure produced from the hydraulic-pressure supply device  100 , may compare the sensed brake hydraulic pressure with the brake hydraulic pressure applied to the rear wheel cylinders  40  of the first hydraulic circuit  201 , and may more precisely control a rear-wheel brake hydraulic pressure that needs to be blocked or reduced by the rear wheel cylinders  40  of the first hydraulic circuit  201  during regenerative braking. 
     As described above, during rear-wheel regenerative braking, the ECU may control operation of the sixth valve  336 , such that a brake hydraulic pressure applied to the rear wheel cylinders  40  of the first hydraulic circuit  201  can be stably adjusted according to a regenerative braking pressure. As a result, a brake pressure or braking force can be evenly applied to four wheels of the vehicle, such that stability in vehicle braking is increased and oversteer or understeer of the vehicle is prevented, resulting in increased driving stability of the vehicle. 
     Meanwhile, if the ECU desires to reduce some parts of hydraulic pressure applied to the first hydraulic circuit  201  so as to facilitate regenerative braking, the ECU may open the first and second outlet valves  222   a  and  222   b  such that some parts of the hydraulic pressure applied to the wheel cylinders  40  can be directly discharged to the reservoir  30 . 
     The hydraulic-pressure supply device  100  of the electronic brake system  2  according to the second embodiment may transition from the low-pressure mode shown in  FIGS. 13 and 14  to the high-pressure mode shown in  FIG. 15  before the hydraulic piston  114  moves forward by a maximum distance. 
     Referring to  FIG. 15 , if hydraulic pressure sensed by each of the passage pressure sensors PS 21  and PS 22  is higher than a predetermined pressure level, the ECU may transition from the low-pressure mode to the high-pressure mode. During the high-pressure mode, the third valve  333  may transition to the open state, such that the fifth hydraulic passage  315  may be open. Therefore, some parts of the hydraulic pressure produced in the first pressure chamber  112  may be transmitted to the second pressure chamber  113  after sequentially passing through the first hydraulic passage  311 , the second hydraulic passage  312 , the fifth hydraulic passage  315 , and the fourth hydraulic passage  314 , such that the resultant hydraulic pressure can allow the hydraulic piston  114  to move farther forward and load applied to the motor  120  can also be reduced. 
     During the high-pressure mode, some parts of the pressure medium discharged from the first pressure chamber  112  may flow into the second pressure chamber  113 , such that the pressure increase rate per stroke is reduced. However, some parts of hydraulic pressure produced in the first pressure chamber  112  may allow the hydraulic piston  114  to move farther forward, such that a maximum pressure of the pressure medium can be increased. The reason why the maximum pressure of the pressure medium is increased is that the drive shaft  133  passes through the second pressure chamber  113  so that a volume change rate per stroke of the hydraulic piston  114  is relatively smaller in the second pressure chamber  113  than in the first pressure chamber  112 . 
     In addition, as the hydraulic piston  114  moves farther forward, hydraulic pressure of the first pressure chamber  112  is increased, force needed for backward movement of the hydraulic piston  114  affected by the increased hydraulic pressure of the first pressure chamber  112  becomes stronger, such that load applied to the motor  120  is also increased. However, the fifth hydraulic circuit  315  is open under control of the third valve  333 , and some parts of the pressure medium discharged from the first pressure chamber  112  are transmitted to the second pressure chamber  113 , such that hydraulic pressure may also occur in the second pressure chamber  113 , resulting in reduction of load applied to the motor  120 . 
     In this case, the third dump valve  343  may transition to a closed state. Since the third dump valve  343  is closed, the pressure medium in the first pressure chamber  112  may rapidly flow into the second pressure chamber  113  having a negative pressure, such that hydraulic pressure may also occur in the second pressure chamber  113 . However, the third dump valve  243  is kept open as necessary, such that the pressure medium stored in the second pressure chamber  113  may flow into the reservoir  30 . 
     A method for supplying a brake pressure to the wheel cylinders  40  by backward movement of the hydraulic piston  114  will hereinafter be described. 
       FIG. 16  is a hydraulic circuit diagram illustrating the electronic brake system  2  for providing brake pressure by backward movement of the hydraulic piston  114  according to the second embodiment of the present disclosure. Referring to  FIG. 16 , during the initial braking stage, if the driver depresses the brake pedal  10 , the motor  120  may rotate in an opposite direction, rotational force of the motor  120  may be transmitted to the hydraulic-pressure providing unit  110  by the power switching unit  130 , the hydraulic piston  114  of the hydraulic-pressure providing unit  110  moves backward, such that hydraulic pressure may occur in the second pressure chamber  113 . Hydraulic pressure discharged from the second pressure chamber  113  may be transmitted to the wheel cylinders  40  respectively provided at four wheels through the first hydraulic circuit  201  and the second hydraulic circuit  202 , resulting in occurrence of braking force. 
     In more detail, hydraulic pressure from the second pressure chamber  113  may be directly transmitted to the rear wheel cylinders  40  mounted to the first hydraulic circuit  201  not only through the fourth hydraulic passage  314  connected to the second communication hole  111   b , but also through the open fifth hydraulic passage  315 , the sixth hydraulic passage  316 , and the eighth hydraulic passage  318 . In this case, the first and second inlet valves  221   a  and  221   b  may remain open, and the first and second outlet valves  222   a  and  221   b  may remain closed, such that hydraulic pressure can be prevented from leaking to the reservoir  30 . 
     In addition, hydraulic pressure from the second pressure chamber  113  may sequentially pass through the fourth hydraulic passage  314  connected to the second communication hole  111   b , the open fifth hydraulic passage  315 , the sixth hydraulic passage  316 , and the seventh hydraulic passage  217 , such that the resultant hydraulic pressure is directly transmitted to the front wheel cylinders  40  of the second hydraulic circuit  202  through the third hydraulic passage  313 . In this case, the third and fourth inlet valves  221   c  and  221   d  may remain open, and the third and fourth outlet valves  222   c  and  222   d  may remain closed, such that hydraulic pressure is prevented from leaking to the reservoir  30 . 
     In this case, the third valve  233  may transition to the open state such that the fifth hydraulic passage  315  is open. The fourth valve  334  is provided as a check valve for allowing the pressure medium to flow from the second pressure chamber  113  to the wheel cylinders  40 , such that the sixth hydraulic passage  316  is also open. In addition, the sixth valve  336  may also transition to the open state in a manner that hydraulic pressure is also transmitted to the wheel cylinders  40  of the first hydraulic circuit  202 . 
     Meanwhile, the first valve  431  may remain closed. As a result, hydraulic pressure from the second pressure chamber is prevented from leaking to the first pressure chamber  112  through the second hydraulic passage  312 , such that a pressure increase rate per stroke of the hydraulic piston  114  is increased such that a rapid braking response can be achieved in the initial braking stage. 
     The third dump valve  243  may transition to a closed state. Since the third dump valve  243  is closed, hydraulic pressure of the pressure medium can rapidly and stably occur in the second pressure chamber  113 , and hydraulic pressure from the second pressure chamber  113  may be discharged only to the fourth hydraulic passage  314 . 
     A method for releasing brake pressure in a normal operation state of the electronic brake system  2  according to the second embodiment of the present disclosure will hereinafter be described with reference to the attached drawings. 
       FIG. 17  is a hydraulic circuit diagram illustrating the electronic brake system  2  for releasing brake pressure of the high-pressure mode by backward movement of the hydraulic piston  114  according to the second embodiment of the present disclosure.  FIG. 18  is a hydraulic circuit diagram illustrating the electronic brake system  2  for releasing brake pressure of the low-pressure mode by backward movement of the hydraulic piston  114  according to the second embodiment of the present disclosure. 
     Referring to  FIG. 17 , when a pedal effort applied to the brake pedal  10  is released, the motor  120  produces rotational force in an opposite direction to the braking rotation direction and transmits the rotational force to the power switching unit  130 , the worm shaft  131 , the worm wheel  132 , and the drive shaft  133  of the power switching unit  130  may rotate in the opposite direction to the braking rotation direction, such that the hydraulic piston  114  moves back to an original position thereof. As a result, hydraulic pressure from the first pressure chamber  112  may be released, and a negative pressure may occur in the first pressure chamber  112 . Simultaneously, the pressure medium discharged from the wheel cylinders  40  may be transmitted to the first pressure chamber  112  through the first and second hydraulic circuits  201  and  202 . 
     In more detail, a negative pressure produced in the first pressure chamber  112  may release a pressure from the rear wheel cylinders  40  mounted to the first hydraulic circuit  201  after passing through the eighth hydraulic passage  318 , the second hydraulic passage  312 , and the first hydraulic passage  311 . In this case, the first and second inlet valves  221   a  and  221   b  respectively installed in two passages branched from the first hydraulic circuit  201  may remain open, and the first and second outlet valves  222   a  and  222   b  respectively installed in two passages branched from the first hydraulic circuit  200  may remain closed, such that the pressure medium of the reservoir  30  is prevented from flowing into the first pressure chamber  112 . 
     In addition, a negative pressure produced in the first pressure chamber  112  may release a pressure from the front wheel cylinders  40  mounted to the second hydraulic circuit  202  after passing through the third hydraulic passage  313  and the seventh hydraulic passage  311 . In this case, the third and fourth inlet valves  221   c  and  221   d  respectively installed in two passages branched from the second hydraulic circuit  202  may remain open, and the third and fourth outlet valves  222   c  and  222   d  respectively installed in two passages branched from the second hydraulic circuit  202  may remain closed, such that the pressure medium of the reservoir  30  is prevented from flowing into the first pressure chamber  112 . 
     Meanwhile, the first, fifth, and sixth valve  331 ,  335  and  336  may transition to the open state and the third valve  333  may also transition to the open state, such that the fifth hydraulic passage  315  is open. As a result, the first pressure chamber  112  may communicate with the second pressure chamber  113 . 
     That is, in order to form a negative pressure in the first pressure chamber  112 , the hydraulic piston  114  should move backward. However, when hydraulic pressure of the pressure medium is present in the second pressure chamber  113 , unexpected resistance may occur in backward movement of the hydraulic piston  114 . Therefore, the first and third valves  331  and  333  may transition to the open state in a manner that the first pressure chamber  112  may communicate with the second pressure chamber  113 , such that the pressure medium of the second pressure chamber  113  may flow into the first pressure chamber  112 . 
     In this case, the third dump valve  243  may transition to the closed state. Since the third dump valve  243  is closed, the pressure medium of the second pressure chamber  113  may be discharged only to the fourth hydraulic passage  314 . However, the third dump valve  243  may remain open as necessary, such that the pressure medium of the second pressure chamber  113  may also flow into the reservoir  30 . 
     In addition, when a negative pressure applied to each of the first and second hydraulic circuits  201  and  202  is measured to be higher than a target pressure release value corresponding to a released pedal effort of the brake pedal  10 , at least one of the first to fourth outlet valves  222  is open, such that the resultant pressure may be controlled to correspond to the target pressure value. In addition, the first and second cut valves  261  and  262  respectively installed in the first and second backup passages  251  and  252  may be closed, such that a negative pressure produced in the master cylinder  20  may not be transmitted to the hydraulic control unit  200 . 
     Meanwhile, during a high-pressure mode shown in  FIG. 17 , not only a pressure medium stored in the wheel cylinders  40  but also a pressure medium stored in the second pressure chamber  113  may be supplied to the first pressure chamber  112  due to a negative pressure produced in the first pressure chamber  112  affected by backward movement of the hydraulic piston  114 , such that the pressure reduction rate of the wheel cylinders  40  is at a low level. Therefore, it may be difficult to rapidly release the brake pressure in the high-pressure mode. As a result, the operation for releasing the brake pressure of the high-pressure mode may be used only in a high-pressure situation of the brake pressure. In order to rapidly release the brake pressure that is equal to or less than a predetermined brake pressure, the operation mode may transition to the operation for releasing brake pressure in the low-pressure mode shown in  FIG. 18 . 
     Referring to  FIG. 18 , when the brake pressure is released in the low-pressure mode, the third dump valve  243  transitions to the open state or remains open, without closing the fifth hydraulic passage  315  affected by the third valve  333  that remains closed or transitions to the closed state, such that the second pressure chamber  113  may communicate with the reservoir  30 . 
     When the brake pressure is released in the low-pressure mode, a negative pressure produced in the first pressure chamber  112  may be used only to recover (or retrieve) the pressure medium of the wheel cylinders  40 , such that a pressure reduction rate per stroke of the hydraulic piston  114  may be increased more than in the other case in which a brake pressure is released in the high-pressure mode. In this case, hydraulic pressure produced in the second pressure chamber  113  by backward movement of the hydraulic piston  114  may transition to the open state, such that most of the hydraulic pressure may be transmitted to the reservoir  30  staying in an atmospheric pressure state without passing through the fourth valve  334 . 
     Differently from  FIG. 18 , it may be possible to release a brake pressure of the wheel cylinders  40  even when the hydraulic piston  114  moves forward. 
       FIG. 19  is a hydraulic circuit diagram illustrating the electronic brake system  2  for releasing brake pressure by forward movement of the hydraulic piston  114  according to the second embodiment of the present disclosure. 
     Referring to  FIG. 19 , when a pedal effort applied to the brake pedal  10  is released, the motor  120  produces rotational force in an opposite direction to the braking rotation direction and transmits the rotational force to the power switching unit  130 , the worm shaft  131 , the worm wheel  132 , and the drive shaft  133  of the power switching unit  130  may rotate in the opposite direction to the braking rotation direction, such that the hydraulic piston  114  moves forward to an original position thereof. As a result, hydraulic pressure from the second pressure chamber  113  may be released, and a negative pressure may occur in the second pressure chamber  113 . Simultaneously, the pressure medium discharged from the wheel cylinders  40  may be transmitted to the second pressure chamber  113  through the first and second hydraulic circuits  201  and  202 . 
     In more detail, a negative pressure produced in the second pressure chamber  113  may release a pressure from the rear wheel cylinders  40  mounted to the first hydraulic circuit  201  after passing through the eighth hydraulic passage  318 , the fourth hydraulic passage  314 , and the fifth hydraulic passage  315 . In this case, the first and second inlet valves  221   a  and  221   b  respectively installed in two passages branched from the first hydraulic circuit  201  may remain open, and the first and second outlet valves  222   a  and  222   b  respectively installed in the passages branched from the first hydraulic circuit  201  may remain closed, such that the pressure medium of the reservoir  30  is prevented from flowing into the second pressure chamber  113 . 
     In addition, a negative pressure produced in the second pressure chamber  113  may release a pressure from the front wheel cylinders  40  mounted to the second hydraulic circuit  202  after passing through the third hydraulic passage  313 , the seventh hydraulic passage  317 , the fourth hydraulic passage  314 , and the fifth hydraulic passage  315 . In this case, the third and fourth inlet valves  221   c  and  221   d  respectively installed in two passages branched from the second hydraulic circuit  202  may remain open, and the third and fourth outlet valves  222   c  and  222   d  respectively installed in two passages branched from the second hydraulic circuit  202  may remain closed, such that the pressure medium of the reservoir  30  is prevented from flowing into the second pressure chamber  113 . 
     In this case, the third valve  333  may transition to the open state such that the fifth hydraulic passage  315  is open. In addition, the third dump valve  243  may transition to the closed state, such that a negative pressure produced in the second pressure chamber  113  may quickly retrieve the pressure medium from the wheel cylinders  40 . 
     In addition, when a negative pressure applied to each of the first and second hydraulic circuits  201  and  202  is measured to be higher than a target pressure release value corresponding to a released pedal effort of the brake pedal  10 , at least one of the first to fourth outlet valves  222  is open, such that the resultant pressure may be controlled to correspond to the target pressure value. In addition, the first and second cut valves  261  and  262  respectively installed in the first and second backup passages  251  and  252  may be closed, such that a negative pressure produced in the master cylinder  20  may not be transmitted to the hydraulic control unit  200 . 
     As is apparent from the above description, the electronic brake system and the method for operating the same according to the embodiments of the present disclosure may stably distribute a brake pressure to wheels of a vehicle during regenerative braking of the vehicle. 
     The electronic brake system and the method for operating the same according to the embodiments of the present disclosure may improve driving stability of a vehicle. 
     The electronic brake system and the method for operating the same according to the embodiments of the present disclosure may stably and efficiently brake a vehicle in various driving situations. 
     The electronic brake system and the method for operating the same according to the embodiments of the present disclosure may stably generate high brake pressure. 
     The electronic brake system and the method for operating the same according to the embodiments of the present disclosure may increase performance and operational stability of a product. 
     The electronic brake system and the method for operating the same according to the embodiments of the present disclosure may provide stable brake pressure in an abnormal state of constituent elements or in a leakage state of a pressure medium. 
     The electronic brake system according to the embodiments of the present disclosure may be simplified in structure, may reduce the number of constituent elements, such that the size and weight of a product can be reduced. 
     The electronic brake system and the method for operating the same according to the embodiments of the present disclosure may improve product durability by reducing load applied to electronic components. 
     Although a few embodiments of the present disclosure 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.