Patent Publication Number: US-2016236665-A1

Title: Electronic brake system and method for controlling the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 200______-______, filed on ______ ______, 200______, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an electronic brake system of a vehicle and, more particularly, to an electronic brake system for drag reduction which may help achieve increased fuel efficiency for the vehicle by solving a drag problem that may occur when a piston for generating a braking force is not completely returned to its original position after a pedal is released in an electronic braking system. 
     2. Discussion of Related Art 
     In recent years, the development of a hybrid vehicle, a fuel cell vehicle, an electric vehicle, and the like has been actively conducted in order to improve fuel efficiency and reduce exhaust gas. In such a vehicle, it is essential that a brake system, that is, a vehicle brake is installed. Here, the vehicle brake refers to an apparatus that functions to stop or reduce the speed of a running vehicle. 
     An electro-hydraulic brake which is one kind of hydraulic brakes is a brake system in which a control unit detects a driver stepping on a pedal and supplies hydraulic pressure to a master cylinder and thereby transmits brake hydraulic pressure to the wheel cylinder of each wheel and generates a braking force. 
       FIG. 1  is a hydraulic circuit diagram schematically showing a configuration of the above-described conventional electronic brake system for a vehicle. 
     Referring to  FIG. 1 , the conventional electronic brake system for the vehicle typically includes a master cylinder  20  that generates a fluid pressure in accordance with a stepping force on a brake pedal  10 , wheel cylinders  40  that receive the fluid pressure generated by the master cylinder  20  and perform braking of each of the wheels RR, RL, FR, and FL, and a pedal displacement sensor  11  that detects a displacement of the brake pedal  10 . 
     In addition, the conventional electronic brake system for the vehicle includes a pressure supply device that is operated by receiving a driver&#39;s braking intension as an electrical signal from the pedal displacement sensor  11 , a hydraulic control unit  60  that performs braking of wheels using a force generated by the pressure supply device, first and second switching valves  62  and  64  that are connected in series to a flow passage for connecting the pressure supply device and the hydraulic control unit  60 , and a simulator  70  that is connected to the master cylinder  20  to provide a reaction force to the brake pedal  10 . 
     The pressure supply device  50  includes a pressure chamber  51  in which a predetermined space is formed so that oil is received and stored therein, a piston  52  and a spring  53  which are provided inside the pressure chamber  51 , a motor  54  that generates a rotational force by the electrical signal of the pedal displacement sensor  11 , a motion conversion unit  55  that converts the rotational motion of the motor  54  into a linear motion thereof, and a hydraulic flow passage  56  that connects a reservoir  57  and the pressure chamber  51  so as to supply oil to the pressure chamber  51 . Here, the signal detected by the pedal displacement sensor  11  is transmitted to an ECU (electronic control unit, not shown), and the ECU controls the motor  54  and valves provided in the brake system. 
     As for the operation of the pressure supply device in the above-described conventional electronic brake system, the motor  54  is operated when a driver steps on the brake pedal  10  in an early phase of braking, and the motion conversion unit  55  converts the rotational force of the motor  54  into the linear motion and thereby pressure the pressure chamber  51 . In this case, the pressure chamber  51  may be connected to the reservoir  57  by the hydraulic flow passage  56  so that oil is provided in a stored state, and fluid pressure may be generated in accordance with the linear motion of the motion conversion unit  55  for converting the rotational motion into the linear motion. In this instance, a check valve  58  for preventing a backflow of the pressure of the pressure chamber  51  is provided in the hydraulic flow passage  56 . Such a check valve  58  functions to allow oil to be suctioned into the pressure chamber  51  and stored therein when the piston  52  is returned as well as functioning to prevent the backflow of the pressure of the pressure chamber  51 . 
     However, when the stepping force is applied to the brake pedal  10  so that the pressure supply device is operated and then the stepping force applied to the brake pedal  10  is released, the piston  52  is returned to its original position by a repulsive force of the spring  53 , but in this case, a problem of drag occurs in which the piston  52  is not completely returned to its original position. 
     Such a drag phenomenon is a phenomenon that occurs when a return movement of the piston  52  is blocked because the inside of the hydraulic system is not configured as a completely closed circuit due to characteristics of the check valve  58  provided on the hydraulic flow passage  56  for connecting the pressure chamber  51  and the reservoir  57 , so that a problem where a residual drag torque produced by such a drag phenomenon acts as a resistance that hinders the driving of the vehicle and therefore acts an important reason the fuel efficiency is reduced. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Korea Patent Publication No. 2013-0037874 (Apr. 17, 2013) 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an electronic brake system for drag reduction, in which a normally closed type solenoid valve may be provided on a hydraulic line connecting a cylinder of a pressure supply device for supplying fluid pressure for braking and a reservoir in the electronic brake system so that a hydraulic system may be implemented as a completely closed circuit system, and therefore a piston within the cylinder may be completely returned to the original position even when a brake pedal is released, thereby improving fuel efficiency of the vehicle. 
     According to an aspect of the present invention, there is provided a method for controlling an electronic brake system including a pressure supply device that includes a motor, a cylinder, and a piston that is installed so as to be moved forward and backward within the cylinder based on driving of the motor, a reservoir that supplies oil to the cylinder or receives a supply of oil from the cylinder, and a solenoid valve that is disposed on a hydraulic line formed between the reservoir and the cylinder, the method including: an ECU (electronic control unit) determining whether a pressure of a wheel cylinder of a vehicle needs to be reduced; the ECU controlling the motor so that the piston is movable backward within the cylinder based on information about whether the pressure of the wheel cylinder needs to be reduced; and the ECU applying a control signal for the solenoid valve to the solenoid valve. 
     Here, the electronic brake system may further include a pedal displacement sensor that detects a pedal operation of a driver, and whether the pressure of the wheel cylinder needs to be reduced is determined based on a variation of a pedal stroke detected by the pedal displacement sensor. 
     Also, the solenoid valve may be a normally closed type solenoid valve, and the ECU applying the control signal for the solenoid valve to the solenoid valve may include the ECU applying an open signal to the solenoid valve. 
     Also, after the ECU applying the control signal for the solenoid valve to the solenoid valve, the method for controlling the electronic brake system may further include: the ECU determining whether the piston is completely moved back within the cylinder based on a detection result of a rotation angle sensor for detecting a rotation angle of the motor; and the ECU controlling the motor to prevent the piston from being moved backward anymore. 
     According to another aspect of the present invention, there is provided an electronic brake system including: a motor that is driven based on a displacement of a pedal; a pressure supply device that includes a cylinder and a piston that is provided so as to be moved forward and backward within the cylinder based on driving of the motor; a reservoir that supplies oil to the cylinder or receives supply of oil from the cylinder; a solenoid valve that is disposed on a hydraulic line formed between the reservoir and the cylinder; and an ECU that controls the motor and the solenoid valve. 
     Here, another hydraulic line that is branched from the hydraulic line connecting the reservoir and the cylinder and connected to the cylinder may be further provided. 
     Also, the hydraulic lines and may be respectively connected to a portion of the cylinder at a front side of the piston and a portion of the cylinder at a rear side thereof. 
     Also, the solenoid valve may be a normally closed type solenoid valve. 
     Also, the ECU may apply an open signal to the solenoid valve based on information about whether pressure of a wheel cylinder of a vehicle needs to be reduced. 
     Also, the electronic brake system may further include a pedal displacement sensor that detects a pedal operation of a driver, wherein the ECU may determine whether the pressure of the wheel cylinder needs to be reduced based on a variation of a pedal stroke detected by the pedal displacement sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  is a hydraulic circuit diagram showing a configuration of a conventional electronic brake system for a vehicle; 
         FIG. 2  is a hydraulic circuit diagram showing an electronic brake system for a vehicle according to an embodiment of the present invention; and 
         FIG. 3  is a flowchart chronologically showing a method for controlling an electronic brake system for a vehicle according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 2  is a hydraulic circuit diagram showing an electronic brake system for a vehicle for reducing a brake drag according to the present invention. 
     Referring to  FIG. 2 , the electronic brake system  100  for the vehicle for reducing a brake drag according to an embodiment of the present invention includes a master cylinder  110  that generates fluid pressure for braking of wheels, a reservoir  120  that is coupled to an upper portion of the master cylinder  110  and stores oil, an input rod  106  that presses the master cylinder  110  in accordance with a stepping force on a brake pedal  102 , a wheel cylinder  130  that performs braking of each of the wheels RR, RL, FR, and FL by receiving a supply of the oil of the reservoir  120 , and a pedal displacement sensor  104  that detects a displacement of the brake pedal  102 . 
     In the master cylinder  110 , a first piston  111  and a second piston  113  are formed to have two hydraulic circuits and brought into contact with the input rod  106 . In this instance, one circuit of the two hydraulic circuits is connected to a front right (FR) wheel and a rear left (RL) wheel of the vehicle, and the other circuit is connected to a front left (FL) wheel and a rear right (RR) wheel of the vehicle. The reason the two circuits are independently configured in such a manner is to make braking of the vehicle possible even during a failure of one circuit. 
     The first piston  111  and the second piston  113  of the master cylinder  110  are elastically supported by a first spring  112  and a second spring  114 . When the first piston  111  and the second piston  113  are moved forward, the elastic forces of the first spring  112  and the second spring  114  are stored due to compression. 
     Meanwhile, the electronic brake system  100  according to the present invention includes a pressure supply device  140  that is operated by receiving a driver&#39;s braking intension as an electrical signal from the pedal displacement sensor  104  for detecting a displacement of the brake pedal  102 , a hydraulic control unit  150  that performs braking of wheels using a force generated by the pressure supply device  140 , first and second switching valves  152  and  154  that are connected in series to a flow passage for connecting the pressure supply device  140  and the hydraulic control unit  150 , and a pedal simulator  160  that is connected to the master cylinder  110  and provides a reaction force to the brake pedal  102 . 
     The pressure supply device  140  includes a cylinder  141  in which a predetermined space is formed so that oil is supplied from the reservoir  120  and stored therein, a piston  142  that is provided inside the cylinder  141  so as to be moved forward and backward, a spring  143  that provides an elastic reaction force to the piston  142 , a motor  144  that generates a rotational force for driving of the piston  142  by the electrical signal of the pedal displacement sensor  104 , a motion conversion unit  145  that converts a rotational motion of the motor  144  into a linear motion, and a hydraulic line that connects the reservoir  120  and the cylinder  141  to supply oil to the cylinder  141 . 
     Here, the signal detected by the pedal displacement sensor  104  is transmitted to an ECU (electronic control unit, not shown), and the ECU controls the motor  144  and valves provided in the brake system according to the present invention which will be described later. 
     As described above, the cylinder  141  is connected to the reservoir  120  by the hydraulic line  146  and stores supplied oil. In such a cylinder  141 , the piston  142  and the spring  143  for elastically supporting the piston  142  are provided. The piston  142  is connected to the motion conversion unit  145  so that the rotational motion of the motor  144  is converted into the linear motion by the motion conversion unit  145  to press the cylinder  141 , and the spring  143  takes on the role of returning the piston  142  to the original position. 
     When the motor  144  generates the rotational force due to a signal output from the ECU, the rotational force is generated by the ECU in a forward direction or a reverse direction. The rotational force generated by the motor  144  is converted into the linear motion by the motion conversion unit  145  to pressure the piston  142 , and thereby a fluid pressure may be generated. 
     In addition, a normally closed type solenoid valve  148  is provided on the hydraulic line  146  that connects the cylinder  141  and the reservoir  120 . Such a normally closed type solenoid valve  148  is closed in a normal state and opened when an open signal is applied from the ECU, thereby forming a flow passage between the cylinder  141  and the reservoir  120 . 
     Here, another hydraulic line  147  that is branched from the hydraulic line  146  is provided in the hydraulic line  146  connecting the cylinder  141  and the reservoir  120  so as to be connected to the cylinder  141 . 
     In this instance, the hydraulic lines  146  and  147  are configured to be respectively connected to a portion of the cylinder  141  at the front side of the piston  142  and a portion of the cylinder at the rear side thereof. 
     In this manner, the hydraulic lines  146  and  147  are formed so as to be respectively connected to the reservoir  120  at the front and rear sides of the cylinder  141 , and the normally closed type solenoid valve  148  is provided on the hydraulic line  146  connected to the portion of the cylinder  141  at the front side of the piston  142 , so that when the piston  142  moves forward and a pressure is added, the solenoid valve  148  may close the hydraulic line  146  so that a fluid pressure is normally generated. 
     Next, when the pressure on the brake pedal  102  is released so that the piston  142  moves backward, the solenoid valve  148  is opened in accordance with the open signal input from the ECU so that the flow passage connecting the cylinder  141  and the reservoir  120  form a completely closed circuit. Accordingly, the flow is maintained in a freely movable state, and therefore the piston  142  within the cylinder  141  may be completely returned to the original position with only the elastic repulsive force of the spring  143 . 
     Meanwhile, non-described reference numerals  132  and  134  are respectively a rotation angle sensor that detects a rotation angle of the motor  144  and a pressure sensor that detects fluid pressure of the cylinder  141 . 
     The hydraulic control unit  150  is constituted by two wheel braking circuits for receiving a supply of the fluid pressure so that braking is performed. The wheel cylinder  130  is provided in each of the wheels FR, FL, RR, and RL, and receives the supply of the fluid pressure so that braking is performed. At this point, a plurality of solenoid valves  151  and  153  for controlling fluid pressure are provided in the flow passage connected to the wheel cylinder  130 . Such opening/closing operations of the solenoid valves  151  and  153  are controlled by the ECU. 
     In addition, the first and second switching valves  152  and  154  which are connected in series to the flow passage connecting the pressure supply device  140  and the hydraulic control unit and control transmission of the fluid pressure to the wheel cylinder  130  by opening/closing operations of the switching valves  152  and  154  are provided in the flow passage connecting the pressure supply device  140  and the hydraulic control unit. The opening/closing operations of the first and second switching valves  152  and  154  are controlled by the ECU. 
     Two backup flow passages for forming a flow passage are formed between the master cylinder  110  and the wheel cylinder  130 , and shut-off valves  162  and  164  for opening and closing the backup flow passages are respectively provided in the backup flow passages. At this point, the backup flow passage is shut off by the shut-off valves  162  and  164  during braking of a vehicle by a driver. Also, the pedal simulator  160  for providing a reaction force in accordance with the stepping force on the brake pedal  102  is connected to the master cylinder  110 . 
     Hereinafter, operations of the electronic brake system according to the present invention will be described in detail. 
     Referring to  FIG. 2 , when braking starts by a driver, the electronic brake system detects a required braking level of the driver by information including the pressure on the brake pedal  102  with which the driver steps using the pedal displacement sensor  104  and the like. At this point, the ECU (not shown) drives the motor  144  by receiving an electrical signal output from the pedal displacement sensor  104 . 
     In addition, a rotational force in accordance with the driving of the motor  144  is converted into a linear motion by the motion conversion unit  145  to press the cylinder  141 . At this point, the normally closed type solenoid valve  148  provided in the hydraulic line  146  between the cylinder  141  and reservoir  120  is maintained in a closed state, and the piston  142  is pressed in accordance with the linear motion of the motion conversion unit  145  for converting the rotational motion of the motor  144  into the linear motion, and thereby a fluid pressure is generated. 
     In addition, the shut-off valves  162  and  164  provided in the backup flow passage connected to an outlet of the master cylinder  110  are closed, so that an oil pressure generated in the master cylinder  110  is prevented from being transmitted to the wheel cylinder  130 . Thus, the fluid pressure generated from the cylinder  141  is transmitted to each wheel cylinder  130  through the first and second switching valves  151  and  153  so that braking of the wheel is achieved. 
     Meanwhile, when braking of the wheel is achieved and then the pressure on the brake pedal  102  is released, the piston  142  within the cylinder  141  in the pressure supply device is moved backward and returned to the original position by the elastic repulsive force of the compressed spring  143 . At this point, the normally closed type solenoid valve  148  is opened by the open signal applied by the ECU so that the hydraulic line  146  between the cylinder  141  and the reservoir  120  is opened. 
     Then, the hydraulic lines  146  and  147  each connected to the reservoir  120  at the front and rear sides of the cylinder  141  are all opened so that the flow passage connecting the cylinder  141  and the reservoir  120  forms a completely closed circuit. Accordingly, the flow is maintained in a freely movable state, and therefore the piston  142  within the cylinder  141  easily achieves a complete return to the original position by the elastic repulsive force of the spring  143  and a circulation force of the flow in accordance with the elastic repulsive force. 
     In this manner, the normally closed type solenoid valve  148  instead of an existing check valve is provided in the hydraulic line  146  connecting the cylinder  141  and the reservoir  120  so that the brake system is implemented as a completely closed circuit, and therefore a complete return of the piston  142  may be achieved and a thereby improved fuel efficiency of the vehicle due to reduced occurrence of drag may be expected. 
     Hereinafter, a method for controlling the electronic brake system according to the present invention will be described with reference to  FIG. 3 .  FIG. 3  is a flowchart chronologically showing a method for controlling an electronic brake system for a vehicle according to the present invention. 
     The method for controlling the electronic brake system according to the present invention relates to the above-described method applied to the electronic brake system according to the present invention, wherein operation S 100  in which the ECU determines whether the pressure of the wheel cylinder  130  of the vehicle is required to be reduced is first performed as shown in  FIG. 3 . The ECU determines whether the pressure of the wheel cylinder  130  is required to be reduced based on a variation of a pedal stroke detected by the pedal displacement sensor  104 . Specifically, when a driver applies a pressure to the pedal and then removes from the pedal, a displacement of the pedal is changed, and the ECU determines that the pressure of the wheel cylinder  130  needs to be reduced based on the variation. 
     When the ECU determines that the pressure of the wheel cylinder  130  needs to be reduced, operation S 200  is performed in which the ECU controls the motor  144  so that the piston  142  can be moved backward within the cylinder  141 . 
     Next, operation S 300  is performed in which the ECU applies to the solenoid valve  148  a control signal for the solenoid valve  148  disposed on the hydraulic line formed between the reservoir  120  and the cylinder  141 . Specifically, the ECU applies the open signal to the solenoid valve  148 . 
     In this case, the flow passage connecting the cylinder  141  and the reservoir  120  forms a completely closed circuit, so that the flow is maintained in a freely movable state, and therefore the piston  142  within the cylinder  141  may be completely returned to the original position with only the elastic repulsive force of the spring  143 . 
     Meanwhile, the method for controlling the electronic brake system according to the present invention may further include operation S 400  in which the ECU determines whether the piston  142  is completely moved backward within the cylinder  141  and operation S 500  in which the ECU controls the motor  144  to prevent the piston  142  from being moved backward anymore, after the operation of applying to the solenoid valve  148  the control signal for the solenoid valve  148 . 
     Specifically, when the motor  144  applies power to cause the piston  142  to be continuously moved backward even though the piston  142  is completely moved back within the cylinder  14 , an overloading is exerted on the motor  144 , and therefore operations S 400  and S 500  are performed to prevent a damage of the motor  144  and unnecessary power consumption caused by the overload. Meanwhile, it is preferable that the ECU determines whether the piston  142  is completely moved back within the cylinder  141  based on the detection result of a rotation angle sensor for detecting the rotation angle of the motor  144 . 
     As described above, according to the electronic brake system according to the present invention having the above-described configuration, the normally closed type solenoid valve that is closed in a normal state and opened when the open signal is applied from the ECU instead of the conventional check valve may be provided on the hydraulic line connecting the cylinder of the pressure supply device for supplying a fluid pressure for braking to the wheel cylinder and the reservoir, so that the electronic brake system may be implemented as a completely closed circuit, and therefore the piston may be completely returned to the original position within the cylinder when the brake pedal is released, and thus, a thereby normal brake operation may be performed wherein an improved fuel efficiency due to the reduced brake drag may be expected. 
     It will be apparent to those skilled in the art that various modifications can be made to the above-described exemplary embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers all such modifications provided they come within the scope of the appended claims and their equivalents.