Patent Publication Number: US-2013239566-A1

Title: Integrated electronic hydraulic brake system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 2012-0025409, filed on Mar. 13, 2012 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Embodiments of the present invention relate to an electronic hydraulic brake system, and more particularly, to an integrated electronic hydraulic brake system which is provided with an actuator, having a master cylinder and a pedal simulator, an electronic stability control (ESC) and a hydraulic power unit (HPU) in a single unit. 
     2. Description of the Related Art 
     In recent years, for the fuel efficiency and exhaust gas reduction, hybrid vehicles, fuel cell vehicles, and electric vehicles have been actively developed. Such a vehicle needs to be provided with a brake apparatus, that is, a brake apparatus of a brake system for vehicles. The brake apparatus for vehicle represents an apparatus configured to reduce the vehicle speed or to stop the vehicle while on the driving of a vehicle. 
     In general, the brake apparatus includes a vacuum brake generating a brake force by use of the suction pressure of an engine and a hydraulic brake generating a brake force by use of the hydraulic pressure. 
     The vacuum brake represents an apparatus that exhibits a large brake force by use of the pressure difference between the suction pressure of the vehicle engine and the atmospheric pressure at a vacuum booster. That is, when a driver steps a pedal, the vacuum brake generates an output significantly greater than the force applied to the pedal by a driver. 
     In order for the vacuum brake to form vacuum, a suction pressure of the engine of the vehicle needs to be provided to a vacuum booster, causing the fuel efficiency to be reduced. In addition, in order to form vacuum at the time of stopping the vehicle, the engine needs to be driven at all times. 
     In addition, since the fuel cell vehicle and the electric vehicle are not provided with engines, the general vacuum brake that amplifies the pedal effort between the brake operations is difficult to be applied to the fuel cell vehicle and the electric vehicle, and since the hybrid vehicle needs to be equipped with an idle stop function to enhance the fuel efficiency, the hydraulic brake is required. 
     That is, all the vehicles described above needs to implement the regenerative brake operation to enhance the fuel efficiency, and the hydraulic brake easily enables the regenerative braking operation. 
     Meanwhile, an electronic hydraulic brake system classified into the hydraulic brake is a brake system in which a driver steps a pedal and an electronic control unit senses the stepping on the pedal and supplies a fluid pressure to a master cylinder, and thus the brake fluid pressure to wheel cylinders (not shown) on respective wheels so as to generate a brake force. 
     Referring to  FIG. 1 , the electronic hydraulic brake system includes an actuator  1  including a master cylinder  1   a  to control the brake fluid pressure being delivered to a wheel cylinder  20 , a booster  1   b , a reservoir  1   c  and a pedal simulator  1   d , an electronic stability control (ESC)  2  individually controlling the respective wheels, and a hydraulic power unit (HPU)  3  including a motor, a pump, an accumulator and a control valve. 
     However, the respective units forming the electronic hydraulic brake system are separately provided and installed from each other, a large installation space needs to be ensured due to the limitation on the installation space in the vehicle, and also the weight of the vehicle is increased. In this regard, there is a need for an electronic hydraulic brake system capable of ensuring the vehicle stability at the braking operation, enhancing the fuel efficiency and the proper stepping operation while improving the braking performance. 
     In addition, the pedal simulator  1   d  is an apparatus that receives a pressure generated by the food effort of a brake pedal (not shown) to press a piston (not shown) and a spring (not shown) that are provided at an inside a simulation chamber (not shown) so as to provide a stepping operation according to the reaction to the compression of the spring. Such a conventional pedal simulator  1   d  is provided as a dry type. The dry type is implemented as a pneumatic structure including a simulation chamber having a piston and a spring exposed to the air. Accordingly, the movement of piston causes a friction and a long period of time of use of the pedal simulator reduces the durability and increases the possibility for foreign substance to be introduced. 
     Accordingly, a large amount of researches is conducted to develop an electronic hydraulic brake system provided with a simple configuration, ensuring an easy control, facilitating implementing a brake force even at a malfunction, improving the durability of a pedal simulator and preventing foreign substances from being introduced. 
     SUMMARY 
     Therefore, it is an aspect of the present invention to provide an integrated electronic hydraulic brake system having a simple configuration thereof so as to improve the safety on the braking operation and the installation efficiency on the vehicle, and during the brake operation, providing a stable stepping action while enhancing the fuel efficiency by supporting the regenerative brake. 
     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 one aspect of the present invention, an integrated electronic hydraulic brake system for vehicles includes a master cylinder, a reservoir, two hydraulic circuits, an accumulator, a pump, a motor, a flow control valve and a pressure reducing valve, a balance valve, a first shut off valve and a second shut off valve, a pedal simulator and a simulation valve. The master cylinder may be configured to generate a fluid pressure according to a pedal effort of a brake pedal. The reservoir may be coupled to an upper part of the master cylinder so as to store oil. The two hydraulic circuits each may be connected to two wheels. The accumulator may be configured to store a predetermined level of pressure. The pump may be configured to draw the oil through a hydraulic pipe connected to the reservoir and discharge the drawn oil to the accumulator to form a pressure at the accumulator. The motor may be configured to drive the pump. The flow control valve and a pressure reducing valve may be connected to one of the two hydraulic circuits so as to control a pressure being transmitted from the accumulator to wheel cylinders installed on the respective wheels. The balance valve may be provided between the two hydraulic circuits to control a connection between the two hydraulic circuits. The first shut off valve and the second shut off valve may be installed between the master cylinder and the two hydraulic circuits to block a fluid pressure according to a pedal effort of a driver. The pedal simulator may be connected to the master cylinder to provide a reaction force of the brake pedal. The simulation valve may be installed at a rear end of the pedal simulator. The simulation valve may be connected to the reservoir such that oil is filled into an inside the pedal simulator through the simulation valve. 
     Each of the flow control valve and the pressure reducing valve may be provided as a single high capacity valve serving as a normally close type solenoid valve that is maintained in a closed state at normal times. 
     The balance valve may be a normally close type solenoid valve that is maintained in a closed state at normal times, and during a brake operation, may be open based on pressure information. 
     The integrated electronic hydraulic brake system for vehicles may further include a simulation check valve. The simulation check valve may be provided between the pedal simulator and the simulation valve, wherein a pressure at a rear end of the simulation valve according to the pedal effort of the brake pedal may be transmitted only through the simulation valve, and at the time of releasing of the pedal effort of the brake pedal, oil may be drawn through the simulation check valve and stored in the pedal simulator. 
     The simulation check valve may be provided as a pipe-purpose check valve having no spring such that a residual pressure of the pedal simulator may be returned at the time of releasing the pedal effort of the brake pedal. 
     Each of the hydraulic circuits may include a normally open type solenoid valve, a normally closed type solenoid valve and a return path. The normally open type solenoid valve may be disposed at a upstream side of the wheel cylinder to control a fluid pressure being transmitted to the wheel cylinder. The normally closed type solenoid valve may be disposed at a downstream side of the wheel cylinder to control a fluid pressure being discharged from the wheel cylinder. The return path may connect the normally closed type solenoid valve to the hydraulic pipe. 
     Each of the first shut off valve and the second shut off valve may be provided as a normally open type solenoid valve that is maintained in an open state at normal times, and at the time of a normal braking, may be operated to be closed. 
     A pulsation attenuation device configured to minimize a pressure pulsation may be formed on a path connecting the fluid control valve and the pressure reducing valve to one of the two hydraulic circuits. 
     As described above, the integrated electronic hydraulic brake system in accordance with the present disclosure is provided with an actuator including a master cylinder and a pedal simulator, and various valves and sensors serving as an electronic stability control (ESC) and a hydraulic power unit (HPU) in a single block, thereby easily securing the installation space and decreasing the weight thereof while facilitating the assembly process. 
     In addition, in order to supply or release pressure with respect to two hydraulic circuits, two hydraulic circuits are connected using a balance valve, and the pressure is controlled through a single flow control valve and a single pressure reducing valve, thereby facilitating the control of pressure while improving the control characteristics. 
     In addition, a pedal simulator is connected to a reservoir, and a simulation valve is provided to control the connection between the pedal simulator and the reservoir, so that oil is stored in the pedal simulator and the durability of the pedal simulator is improved while preventing the foreign substances from being introduced. 
     In addition, a simulation check valve having no spring is provided, so that the residual pressure is minimized, and even if the pressure is randomly adjusted during the brake operation, the pedal operation being delivered to the driver is stably maintained. 
     In addition, the brake operation is performed even at the malfunction of the brake system, so that the application to the electric vehicles, fuel cell vehicles and hybrid vehicles is easily achieved. 
     In addition, regardless of the existence or the operation of an engine, a brake force desired by a driver is implemented, so that the fuel efficiency is enhanced. 
     In addition, when compared to a conventional negative pressure type booster, the integrated electronic hydraulic brake system in accordance of the present disclosure has a simple configuration, and different from a vacuum brake, the suction pressure of the engine is not used, so that the fuel efficiency of the vehicles is enhanced. In addition, such a simple configuration of the electronic hydraulic brake system enables the application to a compact size vehicle. 
    
    
     
       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 view schematically illustrating a conventional electronic hydraulic brake system; 
         FIG. 2  is a hydraulic circuit diagram of an integrated electronic hydraulic brake system in accordance with one embodiment of the present disclosure at a non-braking operation; 
         FIG. 3  is a hydraulic circuit diagram of an integrated electronic hydraulic brake system in accordance with one embodiment of the present disclosure at a normal operation; and 
         FIG. 4  is a hydraulic circuit diagram of an integrated electronic hydraulic brake system in accordance with one embodiment of the present disclosure at an abnormal operation. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 2  is a hydraulic circuit diagram of an integrated electronic hydraulic brake system in accordance with one embodiment of the present disclosure. 
     Referring to  FIG. 2 , the integrated electronic hydraulic brake system includes a brake pedal  10  manipulated by a driver at a braking operation, a master cylinder  110  to which the force is transmitted from the brake pedal  10 , a reservoir  115  coupled to an upper part of the master cylinder  110  so as to store oil, two hydraulic circuits HC 1  and HC 2  each connected to two of wheels RR, RL, RF and FL, an accumulator  120  to store a predetermined level of pressure, a pump  121  to draw the oil through a hydraulic pipe  119  connected to the reservoir  115  and discharge the drawn oil to the accumulator  120  to form a pressure at the accumulator  120 , a motor  122  to drive the pump  121 , a flow control valve  131  and a pressure reducing valve  132  that are connected to one of the two hydraulic circuits HC 1  and HC 2  so as to control a pressure being transmitted from the accumulator  120  to wheel cylinders  20  installed on the respective wheels FL, FR, RL and RR, a balance valve  150  provided between the two hydraulic circuits HC 1  and HC 2  to control a connection between the two hydraulic circuits HC 1  and HC 2 , a first shut off valve  163  and a second shut off valve  164  installed between the master cylinder  110  and the two hydraulic circuits HC 1  and HC 2  to block the fluid pressure according to the pedal effort of a driver, a pedal simulator  170  connected to the master cylinder  110  to provide a reaction force of the brake pedal, and a simulation valve  176  installed on an oil path  179  connecting the pedal simulator  170  to the reservoir  115 . 
     In addition, the integrated electronic hydraulic brake system may further include pressure sensors  101 ,  102 , and  103  disposed at a designated position on a path forming the system so as to measure the pressure generated during the brake operation. 
     The master cylinder  110 , the reservoir  115 , and the pedal simulator  170  are grouped in a single entity into which the functionalities of the ESC and HPU are incorporated, so that the weight of the integrated electronic hydraulic brake system in accordance with the present disclosure is reduced and the installation space is secured. 
     Hereinafter, the configuration and the function of each component forming the integrated electronic hydraulic brake system will be described in detail. First, the master cylinder  110  may be formed by at least one chamber to generate a fluid pressure, and is illustrated as being formed by two chambers in which a first piston  111  and a second piston  112  are formed, respectively. The master cylinder  110  is configured to generate a fluid pressure according to the pedal effort of the brake pedal  10 , and the chambers are connected to the two hydraulic circuits HC 1  and HC 2 , respectively. The master cylinder  110  is provided at an upper side with the reservoir  115  in which oil is filled, and at a lower side with an exit allowing oil to be discharged to the wheel cylinder  20 , which is installed on each of the wheels RR, RL, FR, and FL, through a first backup path  161  and a second backup path  162 . 
     Since the master cylinder  110  is provided with the two chambers which are connected to the two hydraulic circuits HC 1  and HC 2 , the operation safety is secured at a malfunction. For example, as shown on the drawing, a first hydraulic circuit HC 1  between the two hydraulic circuits HC 1  and HC 2  is connected to a front right wheel FR and a rear left wheel RL and a second hydraulic circuit HC 2  between the two hydraulic circuits HC 1  and HC 2  is connected to a front left wheel FL and a rear right RR. Alternatively, a first hydraulic circuit HC 1  between the two hydraulic circuits HC 1  and HC 2  may be connected to two front wheels FL and FR and a second hydraulic circuit HC 2  between the two hydraulic circuits HC 1  and HC 2  may be connected to two rear wheels RL and RR. As described, the two hydraulic circuits HC 1  and HC 2  are configured independent of each other, and even if one of the hydraulic circuits HC 1  and HC 2  is broken, the braking operation for vehicles may be possible. 
     Meanwhile, each of the hydraulic circuits HC 1  and HC 2  includes a path connecting to the wheel cylinder  20 , and a plurality of valves  141  and  142  is provided on the path to control the fluid pressure. As shown on the drawing, the plurality of valves  141  and  142  is divided into a normally open type (hereinafter, referred to as a NO type) solenoid valve  141  disposed at an upstream side of the wheel cylinder  20  to control the fluid pressure being transmitted to the wheel cylinder, and a normally closed type (hereinafter, referred to as a NC type) solenoid valve  142  disposed at a downstream side of the wheel cylinder  20  to control the fluid pressure being discharged from the wheel cylinder  20 . The opening/closing operation of the solenoid valves  141  and  142  is controlled by an electronic control unit (not shown) that is generally known in the art. 
     In addition, each of the hydraulic circuits HC 1  and HC 2  includes a return path  149  connecting the NC type solenoid valve  142  to the hydraulic pipe  119 . The return path  149  is connected to the hydraulic pipe  119  and an oil path  179 , which is to be described later. The return path  149  is configured to discharge the fluid pressure being transmitted to the wheel cylinder  20  such that the fluid pressure is transmitted to the reservoir  115  or is transmitted to the accumulator  120  through pumping of the pump  121 . 
     The balance valve  150  is installed between the two hydraulic circuits HC 1  and HC 2  to control the connection between the two hydraulic circuits HC 1  and HC 2 . The balance valve  150  is provided as a normally close type solenoid valve that is maintained in a closed state at normal times and is open based on pressure information. The balance valve  150  connects the two hydraulic circuits HC 1  and HC 2  to each other such that fluid pressure is supplied to the wheel cylinder  20  provided on each of the hydraulic circuits HC 1  and HC 2 . Detailed description of the balance valve  150  will be described later. 
     Meanwhile, reference numeral ‘ 11 ’ represents an input load installed on the brake pedal  10  so as to transmit a pedal effort to the master cylinder  110 . 
     The pump  121  is provided in at least one unit thereof so as to pump the oil being introduced from the reservoir  115  at high pressure, thereby forming a brake pressure. The motor  122  is provided at one side of the pump  121  to provide the pump  121  with a driving force. The motor  122  is driven by receiving the desire of a driver for a braking operation according to the pedal effort from a first pressure sensor  101  or a pedal displacement sensor that is to be described later. 
     The accumulator  120  is provided at an exit side of the pump  121  to temporarily store oil of a high pressure that is generated by the pump  210  driven. As described above, the accumulator  120  is disposed on a connection path  130  connecting the pump  121  to the flow control valve  131  to temporarily store the high pressure oil being discharged from the pump  121 . Although not shown, a check valve is installed between the pump  121  and the accumulator  120  to prevent the oil stored in the accumulator  120  from being flown backward. 
     A second pressure sensor  102  is provided at an exit side of the accumulator  120  to measure the oil pressure of the accumulator  120 . In this case, the oil pressure measured by the second pressure sensor  102  is compared with a predetermined pressure that is set by the electronic control unit (not shown), and the pump  121  is driven if the measured oil pressure is lower than the predetermined oil pressure, so that the oil in the reservoir  115  is drawn to be filled in the accumulator  120 . 
     In order to transmit the brake oil stored in the accumulator  120  by the operation of the pump  121  and motor  122  to the wheel cylinder  20 , the connection path  130  connected to one of the hydraulic circuits HC 1  and HC 2  is provided. On the drawing, the connection path  130  is illustrated as being connected to the first hydraulic circuit HC 1 . In addition, the flow control valve  131  and the pressure reducing valve  132  are provided on the connection path  130  so as to control the brake oil stored in the accumulator  120 . 
     Each of the flow control valve  131  and the pressure reducing valve  132  is provided as a normally close type solenoid valve that is maintained in a closed state at normal times. Accordingly, if a drives steps the brake pedal  10 , the flow control valve  131  is open, and then the brake oil stored in the accumulator  120  is transmitted to the wheel cylinder  20 . In this case, the brake oil being transmitted through the flow control valve  131  is transmitted to the first hydraulic circuit HC 1  connected to the connection path  130 , and at this time, the balance valve  150  connecting the two hydraulic circuits HC 1  and HC 2  to each other is operated to be open, so that the brake oil is transmitted to the second hydraulic circuit HC 2 . That is, the brake oil of the accumulator  120  is transmitted to each wheel cylinder  20  as the flow control valve  130  and the balance valve  150  are open. 
     Each of the flow control valve  131  and the pressure reducing valve  132  is provided in the form of a single valve configured to supply brake fluid pressure, and thus is provided as a high capacity valve. Although each of the flow control valve  131  and the pressure reducing valve  132  is illustrated as being provided in the form of a single valve, the present disclosure is not limited thereto. If a capacity is insufficient, each of the flow control valve  131  and the pressure reducing valve  132  may be provided in the form of a combination of two or more valves. 
     Meanwhile, a pulsation attenuation device  135  is installed on the connection path  130  connecting the flow control valve  131  to the first hydraulic circuit HC 1  to minimize the pressure pulsation. The pulsation attenuation device  135  is designed to temporarily store oil so as to attenuate the pulsation generated among the flow control valve  131 , the pressure reducing valve  132  and the NO type solenoid valve  141 . The pulsation attenuation device is generally known in the art, and thus the detailed description thereof will be omitted. 
     In addition, a third pressure sensor  103  is provided on the connection path  130  to sense the pressure being transmitted to the hydraulic circuit HC 1 . Accordingly, the pulsation attenuation device  135  is controlled to lower the pulsation according to the pressure of the brake oil being sensed by the third pressure sensor  103 . 
     In accordance with the present disclosure, a first backup path  161  and a second backup path  162  are provided that connect the master cylinder  110  to the two hydraulic circuits HC 1  and HC 2  when the integrated electronic hydraulic brake system is broken. A first shut off valve  163  is provided on the first backup path  161  to block the pressure of the master cylinder  110  according to the pedal effort of the driver, and a second shut off valve  164  is provided on the second backup path  162  to block the pressure of the master cylinder  110  according to the pedal effort of the driver. Each of the first and second shut off valves  163  and  164  is provided as a NO type solenoid valve that is maintained in an open state at normal times, and during a normal braking operation, is closed. The first backup path  161  is connected to the first hydraulic circuit HC 1  and the connection path  130  through the first shut off valve  163 , and the second backup path  162  is connected to the second hydraulic circuit HC 2  through the second shut off valve  164 . In particular, the first pressure sensor  101  is provided on the first backup path  161  to measure the oil pressure of the master cylinder  110 . Through such, at a normal braking operation, the backup paths  161  and  162  are blocked by the first shut off valve  163  and the second shut off valve  164  and the desire of the driver for brake operation is determined by the first pressure sensor  101 , and at an abnormal braking operation, the first shut off valve  163  and the second shut off valve  164  are in an open state, so the brake pressure generated from the master cylinder  110  is directly transmitted to the wheel cylinder  20 . 
     In accordance with the present disclosure, the pedal simulator  170  is provided between the first pressure sensor  101  and the master cylinder  110  to form a pedal effort of the brake pedal  10 . 
     The pedal simulator  170  includes a simulation chamber  172  provided to store oil being discharged from the exit side of the master cylinder  110 , and the simulation valve  176  connected to a rear end of the simulation chamber  172 . The simulation chamber  172  includes a piston  173  and an elastic member  174  so as to form a predetermined range of displacement by the oil being introduced to the simulation chamber  172 . 
     The simulation valve  176  is installed on the oil path  179  connecting the rear end of the pedal simulator  170  to the reservoir  115 . In this case, the oil path  179  is connected to the reservoir  115  while being connected to the return path  149 . As shown on the drawing, an entry of the pedal simulator  170  is connected to the master cylinder  110 , the simulator valve  176  is mounted at the rear end of the pedal simulator  170  and an exit of the simulation valve  176  is connected to the return path  149 , which is connected to the reservoir  115 , through the oil path  179  so that pedal simulator  170 , that is, the interior space of the simulation chamber  172  is fully filled with oil. 
     The simulation valve  176  is provided in the form of a normally close type that is maintained in a closed state at normal times, and when a driver steps the brake pedal  10 , the simulation valve  176  is operated to be open. 
     In addition, a simulation check valve  175  is provided between the pedal simulator  170  and the master cylinder  110 , that is, between the pedal simulator  170  and the simulation valve  176 , and the simulation check valve  175  is configured to allow the oil to flow from the reservoir  115  to the simulation chamber  172 . The simulation check valve  175  is configured to allow the pressure at the rear end of the pedal simulator  170  according to the pedal effort of the brake pedal  10  to be transmitted only through the simulation valve  176 . That is, the piston  173  of the pedal simulator  170  compresses the spring  174 , and the oil in the simulation chamber  172  is transmitted to the reservoir  115  through the simulation valve  176  and the oil path  179 . At this time, oil is filled in the simulation chamber  172 , so the friction of the piston  173  is minimized during the operation of the pedal simulator  170 , and the durability of the pedal simulator  170  is improved, thereby providing a structure preventing the foreign substance from being introduced thereinto. 
     In addition, at the time of releasing the pedal effort of the brake pedal  10 , oil is supplied to the simulation chamber  172  through the simulation check valve  175 , and thus the return of the pressure of the pedal simulator  170  is achieved in a rapid manner. The simulation check valve  175  may be provided as a pipe-purpose check valve having no spring, so that the residual pressure of the pedal simulator  170  is returned at the time of releasing the pedal effort of the brake pedal  10 . 
     The integrated electronic hydraulic brake system is provided as a single block including an electronic control unit (ECU) that is electrically connected to the respective valves and sensors and control the valves and sensors, thereby leading to the compact structure. That is, the integrated electronic hydraulic brake system in accordance with the present disclosure is incorporated with the motor  122  and the pump  121  as well as the pedal simulator  170  configured to form a pedal effort of the brake pedal  10  in cooperation with the accumulator  120  and various valves and sensors in the form of a single block, thereby easily securing the installation space while decreasing the weight thereof. 
     Hereinafter, the operation of the integrated electronic hydraulic brake system in accordance with an embodiment of the present disclosure will be described in detail. 
       FIG. 3  is a hydraulic circuit diagram of an integrated electronic hydraulic brake system in accordance with one embodiment of the present disclosure at a normal operation. 
     Referring to  FIG. 3 , when a driver starts the brake operation, the amount of desired brake operation of a driver is sensed through the pressure information of the brake pedal  10  stepped by the driver, through the first pressure sensor  101  or a pedal displacement sensor. The electronic control unit (not shown) may receive the magnitude of the amount of regenerative brake, and the magnitude of the amount of friction brake is calculated according to the difference between the amount of desired brake operation and the amount of regenerative brake operation, thereby determining the magnitude of increase or decrease of pressure at the wheel side. 
     In detail, in the beginning of the braking operation, if a driver steps the brake pedal  10 , the braking operation is sufficiently achieved by the regenerative brake, and thus a control is made to prevent the friction brake from occurring. Accordingly, a pressure reduction of brake oil is required so as to prevent the fluid pressure, which is generated at the master cylinder  110  after being transmitted from the brake pedal  10 , from being transmitted to the wheel cylinder  20 . In this case, by opening the pressure reducing valve  132  so that the fluid pressure formed at the connection path  130  is discharged to the reservoir  115  through the return path  149  so as to prevent pressure from being formed at the wheels RR, RL, FR, and FL while maintaining the pressure of the brake pedal. 
     Thereafter, a process of adjusting the amount of friction brake according to the change in the regenerative brake is performed. The amount of regenerative brake varies with the charging status of a battery or the vehicle speed. If the vehicle speed is below a predetermined speed, the amount of the regenerative brake is rapidly decreased. In order to cope with such a condition, the flow control valve  131  may control the flow rate of the brake oil being transmitted from the accumulator  120  to the connection path  130 . 
     Thereafter, the amount of regenerative brake is not present, so the braking operation is performed according to a general brake condition. 
     Meanwhile, since the connection path  130  is connected only to the first hydraulic circuit HC 1 , the NC type balance valve  150  controlling the connection between the two hydraulic circuits HC 1  and HC 2  is operated to be open such that the pressure is transmitted to the two hydraulic circuits HC 1  and HC 2 . 
     In addition, the pressure generated by the pressing of the master cylinder  110  according to the pedal effort of the brake pedal  10  is transmitted to the pedal simulator  170  connected to the master cylinder  110 . In this case, the simulation valve  176  installed on the oil path  179  connecting the rear end of the pedal simulator  170  to the reservoir  115  is operated to be open, so that the oil filled in the simulation chamber  172  is transmitted to the reservoir  115  through the simulation valve  176 . In addition, the pressure corresponding to the weights of the piston  173  and the spring  174  supporting the piston  173  may provide the driver with a proper stepping sensation through the simulation chamber  172 . In addition, at the time of releasing the pedal effort of the brake pedal  10 , the oil is refilled into the simulation chamber  172  through the simulation check valve  175 , thereby ensuring the return of pressure of the pedal simulator  170  in a rapid manner. 
       FIG. 4  is a hydraulic circuit diagram of an integrated electronic hydraulic brake system in accordance with one embodiment of the present disclosure at an abnormal operation. 
     Referring to  FIG. 4 , when the integrated electronic hydraulic brake system is not normally operated, the pressure is transmitted through the first backup path  161  and the second backup path  162  to the wheel cylinder  20 , thereby implementing the brake force. In this case, each of the first shut off valve  163  and the second shut off valve  164 , which are installed on the first backup path  161  and the second backup path  162 , and the solenoid valve  141  of the two hydraulic circuits HC 1  and HC 2  is provided as a normally open type solenoid valve, and each of the flow control valve  131 , the pressure reducing valve  132  and the balance valve  150  is provided as a normally close type solenoid valve, so that the fluid pressure is directly transmitted to the wheel cylinder  20 . Accordingly, a stable braking is achieved, thereby improving the safety on the brake operation. 
     Meanwhile, the master cylinder  110  has a reduced inner circumference when compared to a general master cylinder so as to maximize the mechanical braking performance according to the pedal effort of the brake pedal  10 . That is, the master cylinder  110  has an inner circumference smaller than that of a general master cylinder, but may exit a sufficient brake force through the brake oil stored in the reduced inner circumference. 
     Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.