Patent Publication Number: US-2023160447-A1

Title: Brake device for vehicle and apparatus and method for calibrating braking force of brake device to zero point

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 10-2021-0163301 filed on Nov. 24, 2021 and Korean Patent Application No. 10-2021-0166921 filed on Nov. 29, 2021 in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes. 
     BACKGROUND 
     1. Technical Field 
     Exemplary embodiments of the present disclosure relate to a brake device or system for a vehicle and an apparatus and method for calibrating a braking force of the brake device to a zero point, and more particularly, to a brake device for a vehicle and an apparatus and method for calibrating a braking force of the brake device to a zero point, which are capable of reducing an ineffective stroke and calibrating a point in time at which generation of a load by motor current is estimated by using a point in time at which a load is actually generated on brake pads. 
     2. Discussion of Related Art 
     In general, an electronic mechanical brake (EMB) for a vehicle presses a piston by converting a rotational force of a drive motor into a rectilinear motion by using a screw-nut mechanism. The pressed piston presses brake pads, which are friction members, against a wheel disc, thereby generating a braking force. 
     The electronic mechanical brake in the related art hardly estimates positions in respect to points in time at which the braking force is generated and released, only on the basis of control information applied to the motor. Further, it is impossible to calibrate the initial position in accordance with abrasion of the brake pads. 
     In addition, the electronic mechanical brake for a vehicle in the related art generates a load by pushing the brake pads rectilinearly by mechanically converting a rotation by controlling the electric current of the motor. 
     However, the electronic mechanical brake for a vehicle in the related art cannot estimate the positions in respect to the actual points in time at which the braking force is generated and released, only on the basis of the control information applied to the motor. Further, the electronic mechanical brake for a vehicle in the related art cannot estimate an actual braking force, during a braking operation performed by a driver, only by the operation of a load switch. 
     The background technology of the present disclosure is disclosed in Korean Patent No. 10-0456779 (registered on Apr. 29, 2004 and entitled ‘Brake Device for Vehicle’). 
     SUMMARY 
     Various embodiments are directed to a brake device or system for a vehicle, which is capable of reducing initial ineffectiveness, i.e., an ineffective stroke by resetting and restoring an initial point in time of an operation on the basis of a point in time at which a braking force is released by recognizing points in time at which the braking force is generated and released. 
     Various embodiments are also directed to an apparatus and method for calibrating a braking force of a brake device for a vehicle to a zero point, which are capable of calibrating a point in time at which generation of a load by motor current is estimated by using a point in time at which a load is actually generated on brake pads of a brake device for a vehicle. 
     In an embodiment, a brake device for a vehicle includes: a caliper body configured to surround a brake disc; a pair of brake pads disposed in the caliper body so as to face each other and respectively positioned at two opposite sides of the brake disc; a screw bar installed in the caliper body and configured to be rotatable by power from a motor unit; a nut unit configured to be rectilinearly moved by a rotation of the screw bar; a piston unit configured to surround an outer portion of the nut unit and press any one of the pair of brake pads by moving together with the nut unit; a bearing unit configured to support a rotation of the screw bar and be moved by a repulsive force of the screw bar; and a load switch unit installed in the caliper body, disposed to face the bearing unit, and configured to operate by being pressed by the bearing unit. 
     In addition, the brake device may further include a control unit configured to receive an electrical signal from the load switch unit and control an operation of the motor unit on the basis of the electrical signal. 
     In addition, the load switch unit may include: a housing part installed in the caliper body; a movable block part installed in the housing part configured to be moved by being pressed by the bearing unit, the movable block part being configured to be returned to an original position thereof by an elastic restoring force when the bearing unit is released; and a connector part configured to transmit the electrical signal to the control unit by coming into contact with the movable block part when the movable block part moves. 
     In addition, the movable block part may protrude to the outside of the housing part. 
     In addition, the movable block part may include: a movable block movably disposed in the housing part and configured to come into contact with the connector part; and an elastic member connected to the movable block, coupled to the connector part, and configured to elastically support the movable block. 
     In addition, the connector part may include: a connector main body coupled to the elastic member; and a contact point part mounted in the connector main body and configured to generate the electrical signal by coming into contact with the movable block. 
     In addition, the movable block may include: a movable block main body elastically supported by the elastic member; and a conductor plate installed in the movable block main body and configured to come into contact with the contact point part. 
     In addition, the contact point part may include: a contact point protrusion mounted in the connector main body and configured to come into contact with the conductor plate; and a terminal connected to the contact point protrusion and configured to transmit the electrical signal to the control unit, the electrical signal being generated between the contact point protrusion and the conductor plate when the conductor plate comes into contact with the contact point protrusion. 
     In an embodiment, an apparatus for calibrating a braking force of a brake device for a vehicle to a zero point includes: a load switch unit configured to output an electrical signal when a brake pad actually presses a brake disc and a screw bar is moved rearward by a repulsive force; a motor current measurement unit configured to measure motor current of a motor that rotates the screw bar so that the brake pad presses the brake disc; and a control unit configured to estimate, for each motor current, an estimated load applied to the screw bar during a process of operating the motor and calibrate the estimated load to a zero point on the basis of the electrical signal. 
     The control unit according to the present disclosure may calibrate the estimated load to the zero point on the basis of the estimated load estimated at a point in time at which the electrical signal is inputted. 
     When the estimated load is not 0 at the point in time at which the electrical signal is inputted, the control unit according to the present disclosure may calibrate the estimated load at the point in time at which the electrical signal is inputted to 0. 
     The control unit according to the present disclosure may calibrate all the estimated loads for the respective motor currents on the basis of the zero point calibrated at the point in time at which the electrical signal is inputted. 
     The control unit according to the present disclosure may determine that a motor current monitoring function of the motor current measurement unit fails when the estimated load is maintained to be 0 for a predetermined time or longer after the electrical signal is inputted. 
     In an embodiment, a method of calibrating a braking force of a brake device for a vehicle to a zero point includes: measuring, by a motor current measurement unit, motor current of a motor when the motor rotates a screw bar so that a brake pad presses a brake disc and detecting, by a control unit, an estimated load estimated as being applied to the screw bar for each motor current of the motor measured by the motor current measurement unit; outputting, by a load switch unit, an electrical signal when the brake pad actually presses the brake disc and the screw bar is moved rearward by a repulsive force; and performing, by the control unit, zero point calibration on the estimated load on the basis of a point in time at which the electrical signal is inputted when the electrical signal is inputted from the load switch unit. 
     The performing of the zero point calibration on the estimated load according to the present disclosure may include performing the zero point calibration on the estimated load on the basis of the estimated load estimated at the point in time at which the electrical signal is inputted. 
     The performing of the zero point calibration on the estimated load according to the present disclosure may include calibrating the estimated load at the point in time at which the electrical signal is inputted to 0 when the estimated load at the point in time at which the electrical signal is inputted is not 0. 
     In the performing of the zero point calibration on the estimated load according to the present disclosure, the control unit may calibrate all the estimated loads for the respective motor currents on the basis of the zero point calibrated at the point in time at which the electrical signal is inputted. 
     The method according to the present disclosure may further include determining, by the control unit, that a motor current monitoring function of the motor current measurement unit fails when the estimated load is maintained to be 0 for a predetermined time or longer after the electrical signal is inputted. 
     According to the brake device for a vehicle according to the present disclosure, the control unit may use the load switch unit to recognize the points in time at which the braking force is generated and released. Therefore, the control unit does not always return the position of the screw bar to the initial point in time at which the braking force is generated, but resets the initial point in time of the operation on the basis of the point in time at which the braking force is released, and then returns the position of the screw bar to the reset point in time, thereby reducing initial ineffectiveness, i.e., an ineffective stroke. 
     In addition, in the present disclosure, the initial point in time of the operation of the screw bar is reset in accordance with abrasion of the brake pad, which makes it possible to reduce the ineffective stroke during the subsequent braking operation. 
     The apparatus and method for calibrating the braking force of the brake device for a vehicle to the zero point according to the present disclosure may calibrate the point in time at which the generation of the load by the motor current is estimated by using the point in time at which the load is actually generated on the brake pads of the brake device for a vehicle, thereby reducing the initial ineffectiveness and improving precision in calculating the output load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a brake device for a vehicle according to an embodiment of the present disclosure. 
         FIG.  2    is a cross-sectional view taken along line A-A′ in  FIG.  1   . 
         FIG.  3    is an enlarged view of part A in  FIG.  2   . 
         FIG.  4    is a half cross-sectional perspective view of the brake device for a vehicle according to the embodiment of the present disclosure. 
         FIG.  5    is an exploded perspective view of a main part of the brake device for a vehicle according to the embodiment of the present disclosure. 
         FIG.  6    is a perspective view illustrating a load switch unit of the brake device for a vehicle according to the embodiment of the present disclosure. 
         FIG.  7    is an exploded perspective view of the load switch unit of the brake device for a vehicle according to the embodiment of the present disclosure. 
         FIG.  8    is a perspective view of  FIG.  7    when viewed in another direction. 
         FIG.  9    is a view illustrating a state in which the load switch unit operates when the brake device for a vehicle according to the embodiment of the present disclosure performs a braking operation. 
         FIG.  10    is an enlarged view of part B in  FIG.  9   . 
         FIG.  11    is a block configuration view of an apparatus for calibrating a braking force of the brake device for a vehicle to a zero point according to the embodiment of the present disclosure. 
         FIG.  12    is a flowchart of a method of calibrating a braking force of the brake device for a vehicle to a zero point according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a brake device or system for a vehicle and an apparatus and method for calibrating a braking force of the brake device or system to a zero point according to an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. Here, thicknesses of lines illustrated in the drawings, sizes of constituent elements, or the like may be exaggerated for clarity and convenience of description. In addition, the terms used below are defined in consideration of the functions thereof in the present disclosure and may vary depending on the intention of a user or an operator or a usual practice. Therefore, such terms should be defined based on the entire contents of the present specification. 
       FIG.  1    is a perspective view of a brake device for a vehicle according to an embodiment of the present disclosure,  FIG.  2    is a cross-sectional view taken along line A-A′ in  FIG.  1   ,  FIG.  3    is an enlarged view of part A in  FIG.  2   ,  FIG.  4    is a half cross-sectional perspective view of the brake device for a vehicle according to the embodiment of the present disclosure,  FIG.  5    is an exploded perspective view of a main part of the brake device for a vehicle according to the embodiment of the present disclosure,  FIG.  6    is a perspective view illustrating a load switch unit of the brake device for a vehicle according to the embodiment of the present disclosure,  FIG.  7    is an exploded perspective view of the load switch unit of the brake device for a vehicle according to the embodiment of the present disclosure,  FIG.  8    is a perspective view of  FIG.  7    when viewed in another direction,  FIG.  9    is a view illustrating a state in which the load switch unit operates or is activated when the brake device for a vehicle according to the embodiment of the present disclosure performs a braking operation, and  FIG.  10    is an enlarged view of part B in  FIG.  9   . 
     Referring to  FIGS.  1  to  10   , a brake device  1  for a vehicle according to an embodiment of the present disclosure includes a caliper body  100 , a pair of brake pads  200 , a screw bar  300 , a nut unit  400 , a piston unit  500 , a bearing unit  600 , a load switch unit  700 , and a control unit  800  (illustrated in  FIG.  11   ). The caliper body  100  surrounds a brake disc  10 . The caliper body  100  includes a first caliper body  110  and a second caliper body  120 . 
     The screw bar  300 , the nut unit  400 , the piston unit  500 , the bearing unit  600 , the load switch unit  700 , and a motor unit  900  (illustrated in  FIG.  11   ) are mounted in the first caliper body  110 . The second caliper body  120  is connected to the first caliper body  110  and mounted to face the pair of brake pads  200 . 
     The pair of brake pads  200  is disposed in the caliper body  100  so as to face each other and respectively positioned at two opposite sides of the brake disc  10 . Specifically, the brake pads  200  each include a backplate  210  and a friction member  220 . The backplate  210  faces the piston unit  500  to be described below. The backplate  210  is pressed by the piston unit  500 . 
     The friction member  220  is coupled to a surface of the backplate  210  facing the brake disc  20 . The friction member  220  may come into contact with the brake disc  10 . 
     The screw bar  300  is installed in the caliper body  100  and may be rotated by power from the motor unit  900 . The screw bar  300  has a rod shape and is inserted into the nut unit  400  to be described below. An external thread (not illustrated) is provided on an outer portion of the screw bar  300 , and the screw bar  300  is rotated by a rotational force transmitted from the motor unit  900  to be described below. 
     The motor unit  900  transmits the power to the screw bar  300 . The motor unit  900  includes a motor  910  and a gear module  920 . The gear module  920  includes a plurality of gears, any one of the plurality of gears is connected to the motor  910 , and another of the plurality of gears is connected to the screw bar  300 . That is, the gear module  920  receives the rotational force from the motor  910  and rotates the screw bar  300 . 
     The nut unit  400  is rectilinearly moved by the rotation of the screw bar  300 . The nut unit  400  surrounds an outer portion of the screw bar  300  and selectively moves toward the brake pad  200  or away from the brake pad  200  depending on the rotation direction of the screw bar  300 . 
     Specifically, when the screw bar  300  rotates in a predetermined direction, the nut unit  400  moves toward the brake pad  200  by converting the rotational motion of the screw bar  300  into the rectilinear motion. On the contrary, when the screw bar  300  rotates in a direction opposite to the predetermined direction, the nut unit  400  moves away from the brake pad  200 . 
     The piston unit  500  is shaped to surround an outer portion of the nut unit  400  and presses any one of the pair of brake pads  200  by moving together with the nut unit  400 . The piston unit  500  has a cylindrical shape, a portion of the piston unit  500  into which the nut unit  400  is inserted is opened, and a portion of the piston unit  500  facing the brake pad  200  is closed. 
     The piston unit  500  moves together with the nut unit  400  to apply a pressure to the brake pad  200  by being pressed by the nut unit  400  or to release the pressure applied to the brake pad  200  by being released from the nut unit  400 . The piston unit  500  moves toward the brake pad  200  by being pressed by the nut unit  400  or moves away from the brake pad  200 . 
     When the piston unit  500  presses the brake pad  200 , the brake pad  200  comes into contact with the brake disc  20 , thereby generating a braking force. That is, when the piston unit  500  presses the backplate  210  of the brake pad  200 , the friction member  220  of the brake pad  200  comes into contact with the brake disc  20 . 
     The bearing unit  600 , which is a thrust bearing, supports the rotation of the screw bar  300  and is moved by a repulsive force of the screw bar  300 . Specifically, when the piston unit  500  comes into close contact with the brake pad  200  and presses the brake pad  200 , the repulsive force is applied to the screw bar  300 , and the screw bar  300  is moved in a predetermined direction (e.g., a rearward direction). 
     Therefore, the bearing unit  600  moves rearward and presses the load switch unit  700 . That is, the control unit  800  to be described below may measure a point in time at which the braking force is generated. 
     The load switch unit  700  is installed in the caliper body  100 , disposed to face the bearing unit  600 , and operated by being pressed by the bearing unit  600 . 
     The load switch unit  700  includes a housing part  710 , a movable block part  720 , and a connector part  730 . The housing part  710  is installed in the first caliper body  110  of the caliper body  100 . 
     The housing part  710  has a ring shape and an installation hole  711  in which the movable block part  720  to be described below is installed. A passing hole  712  is formed at a central portion of the housing part  710 , and the screw bar  300  passes through the passing hole  712 . 
     The movable block part  720  is installed in the housing part  710  and moved by being pressed by the bearing unit  600 . When the pressure applied to the bearing unit  600  is released, the movable block part  720  is returned to an original or initial position thereof by an elastic restoring force. 
     When the bearing unit  600  is moved by the repulsive force of the screw bar  300  and presses the movable block part  720 , the movable block part  720  moves rearward and comes into contact with the connector part  730  to be described below. In this case, a point in time at which the repulsive force of the screw bar  300  is generated is a point in time at which the braking force is generated as the piston unit  500  comes into close contact with the brake pad  200  and presses the brake pad  200 . 
     Thereafter, when the braking force is released as the piston unit  500  releases the brake pad  200 , the bearing unit  600  releases the movable block part  720 , and the bearing unit  600  is returned to the original position thereof by an elastic restoring force of elastic members  722  to be described below. This point in time is a point in time at which the braking force is released. 
     The movable block part  720  protrudes to the outside of the housing part  710 . Therefore, the movable block part  720  may be pressed when the bearing unit  600  is moved by the repulsive force of the screw bar  300 . 
     The movable block part  720  includes a movable block  721  and the elastic members  722 . The movable block  721  is movably disposed in the housing part  710  and comes into contact with the connector part  730 . 
     The movable block  721  includes a movable block main body  721   a  and a conductor plate  721   d.  The movable block main body  721   a  is elastically supported by the elastic members  722 . The movable block main body  721   a  includes a first movable block main body  721   b  and a second movable block main body  721   c.    
     The first movable block main body  721   b  is movably disposed in the installation hole  711  of the housing part  710 . The second movable block main body  721   c  is connected to the first movable block main body  721   b,  and the conductor plate  721   d  to be described below is installed at one side of the second movable block main body  721   c.    
     The conductor plate  721   d  is installed on the movable block main body  721   a  and comes into contact with contact point parts  732 . The conductor plate  721   d  is installed on the second movable block main body  721   c  of the movable block main body  721   a  and brought into contact with the connector part  730  by the movement of the movable block  721 . 
     The elastic member  722 , which is a spring, is connected to the movable block  721 . The elastic member  722  is coupled to the connector part  730  and elastically supports the movable block  721 . Therefore, when the pressing force applied to the movable block  721  by the bearing unit  600  is released, the movable block  721  may be returned to the original position thereof. 
     The connector part  730  transmits an electrical signal to the control unit  800  by coming into contact with the movable block part  720  when the movable block part  720  moves. 
     The connector part  730  includes a connector main body  731  and the contact point parts  732 . The elastic members  722  are coupled to the connector main body  731 . The connector main body  731  includes a first connector main body  731   a,  a coupling plate  731   b,  and a second connector main body  731   c.    
     The contact point parts  732  to be described below are installed in the first connector main body  731   a.  The coupling plate  731   b  is connected to the first connector main body  731   a  and coupled to the elastic members  722 . The second connector main body  731   c  has a cylindrical shape. The second connector main body  731   c  is connected to the coupling plate  731   b  and surrounds the contact point part  732 . 
     The contact point part  732  is mounted in the connector main body  731  and generates the electrical signal by coming into contact with the movable block  721 . The contact point part  732  is provided in plural, and the plurality of contact point parts  732  is mounted in the first connector main body  731   a  of the connector main body  731  and spaced apart from one another. 
     The contact point part  732  includes a contact point protrusion  732   a  and a terminal  732   b.  The contact point protrusion  732   a  is mounted in the connector main body  731  and comes into contact with the conductor plate  721   d.  The contact point protrusion  732   a  is provided in plural, and the plurality of contact point protrusions  732   a  is mounted in the first connector main body  731   a  of the connector main body  731  and spaced apart from one another. 
     The terminal  732   b  is connected to the contact point protrusion  732   a.  When the conductor plate  721   d  comes into contact with the contact point protrusions  732   a,  the terminals  732   b  transmit the electrical signal, which is generated between the contact point protrusions  732   a  and the conductor plate  721   d,  to the control unit  800  to be described below. The terminal  732   b  is provided in plural, and the plurality of terminals  732   b  is respectively connected to the contact point protrusions  732   a.  The second connector main body  731   c  of the connector main body  731  surrounds an outer portion of the terminal  732   b.    
     The control unit  800  receives the electrical signal from the load switch unit  700  and controls the operation of the motor unit  900  on the basis of the electrical signal. Specifically, on the basis of the electrical signal transmitted from the load switch unit  700 , the control unit  800  may predict the point in time at which the braking force is generated. 
     On the contrary, when the control unit  800  does not receive the electrical signal from the load switch unit  700 , the control unit  800  may predict the point in time at which the braking force is released. The control unit  800  may predict the point in time at which the braking force is generated and the point in time at which the braking force is released and calibrate the position of the screw bar  300  by controlling the operation of the motor unit  900 . 
     In this case, the control unit  800  does not always return the position of the screw bar  300  to the initial point in time at which the braking force is generated, but resets the initial point in time of the operation on the basis of the point in time at which the braking force is released, and then returns the position of the screw bar  300  to the reset point in time, thereby reducing initial ineffectiveness, i.e., an ineffective stroke. Moreover, the initial point in time of the operation of the screw bar  300  is reset in accordance with abrasion of the brake pad  200 , which makes it possible to reduce the ineffective stroke during the subsequent braking operation. 
     Hereinafter, an operation and an effect of the brake device for a vehicle according to the embodiment of the present disclosure will be described with reference to  FIGS.  2 ,  3 ,  9 , and  10   . 
     To brake the vehicle, the screw bar  300  is rotated in the predetermined direction by the power from the motor unit  900 . When the screw bar  300  rotates in the predetermined direction, the nut unit  400  and the piston unit  500  move toward the brake pad  200 , and the piston unit  500  presses the brake pad  200 . 
     The brake pads  200  selectively contact or come into contact with the brake disc  10 , such that the braking force is generated. In this case, as the piston unit  500  presses the brake pad  200 , a repulsive force is applied to the piston unit  500  and the nut unit  400 , and the repulsive force is also applied to the screw bar  300 . 
     The bearing unit  600  is moved rearward by the repulsive force of the screw bar  300 , and the bearing unit  600  presses the load switch unit  700 , such that the load switch unit  700  operates. That is, the load switch unit  700  is operated or activated by a load of the bearing unit  600 . 
     The load switch unit  700  transmits an electrical signal to the control unit  800 . The control unit  800  may predict the point in time at which the braking force is generated, on the basis of the electrical signal transmitted from the load switch unit  700 , and display the prediction result on a monitor unit (not illustrated). 
     Thereafter, when the screw bar  300  is rotated in the direction opposite to the predetermined direction by the power from the motor unit  900  at the time of releasing the braking operation, the nut unit  400  and the piston unit  500  move away from the brake pads  200 , and the brake pads  200  are released. 
     When the brake pads  200  are released, the bearing unit  600  is returned to the original position thereof, the bearing unit  600  stops pressing the load switch unit  700 , and the load switch unit  700  stops operating. 
     The load switch unit  700  does not transmit the electrical signal to the control unit  800 , and the control unit  800  may predict the point in time at which the braking force is released and display the prediction result on the monitor unit (not illustrated). 
     As described above, the control unit  800  uses the load switch unit  700  to predict the point in time at which the braking force is generated and the point in time at which the braking force is released, and calibrates the position of the screw bar  300  by controlling the operation of the motor unit  900  on the basis of the prediction result. 
     The control unit  800  does not always return the position of the screw bar  300  to the initial point in time at which the braking force is generated by operating the motor unit  900 , but resets the initial point in time of the operation on the basis of the point in time at which the braking force is released, and then returns the position of the screw bar  300  to the reset point in time of the operation. 
     Therefore, it is possible to reduce the initial ineffectiveness, i.e., the ineffective stroke. Moreover, the initial point in time of the operation of the screw bar  300  is reset in accordance with abrasion of the brake pad  200 , which makes it possible to reduce the ineffective stroke during the subsequent braking operation. 
       FIG.  11    is a block configuration view of an apparatus for calibrating a braking force of the brake device for a vehicle to a zero point according to the embodiment of the present disclosure. 
     Referring to  FIG.  11   , the apparatus for calibrating the braking force of the brake device for a vehicle to the zero point according to the embodiment of the present disclosure includes the load switch unit  700 , the control unit  800 , the motor unit  900 , a motor current measurement unit  1000 , and an output unit  1100 . 
     In this case, because the load switch unit  700  and the motor unit  900  are identical to those of the above-mentioned embodiment, detailed descriptions thereof will be omitted. 
     The motor current measurement unit  1000  measures motor current, which is applied as the motor  910  operates, and inputs the measured motor current to the control unit  800 . 
     The output unit  1100  outputs a result of monitoring the motor current. For example, when the control unit  800  determines that a motor current monitoring function fails, the output unit  1100  notifies an occupant or the like of the failure of the motor current monitoring function through an image or sound on the basis of a control signal of the control unit  800 . In this case, the occupant or the like may recognize the failure of the function. 
     The output unit  1100  may be, but not particularly limited to, a cluster (not illustrated) of a vehicle. 
     The control unit  800  rotates the screw bar  300  by operating the motor  910 . The nut unit  400  rectilinearly moves as the screw bar  300  is rotated by the power from the motor unit  900 . Depending on the rotation direction of the screw bar  300 , the nut unit  400  moves toward the brake pad  200  to press the brake pad  200  or moves away from the brake pad  200  to release the brake pad  200 . 
     The control unit  800  may receive an electrical signal from the load switch unit  700  and control the operation of the motor unit  900  on the basis of the electrical signal. 
     In this case, the control unit  800  receives the motor current measured by the motor current measurement unit  1000  and estimates a load of the bearing unit  600  on the basis of the motor current. 
     The control unit  800  may detect an estimated load which is expected to be applied to the screw bar  300  for each motor current, by storing a load for each motor current through a form of a lookup table or calculating a load on the basis of the motor current. 
     The method of estimating the load of the bearing unit  600  on the basis of the motor current is not particularly limited. 
     Meanwhile, the control unit  800  receives the electrical signal from the load switch unit  700  as described above by the operation of the motor unit  900 . 
     That is, to brake the vehicle, the screw bar  300  is rotated in the predetermined direction by the power from the motor unit  900 , the nut unit  400  and the piston unit  500  move toward the brake pad  200 , and the piston unit  500  presses the brake pad  200 . 
     The braking force is generated as the brake pads  200  come into contact with the brake disc  10 . As the piston unit  500  presses the brake pad  200 , the repulsive force is applied to the piston unit  500  and the nut unit  400 , and the repulsive force is also applied to the screw bar  300 . 
     The bearing unit  600  is moved rearward by the repulsive force of the screw bar  300 , and the bearing unit  600  presses the load switch unit  700 , such that the load switch unit  700  operates or is activated and inputs the electrical signal to the control unit  800 . As described above, the electrical signal is inputted to the control unit  800  when a load is actually generated by the operation of the motor unit  900 . 
     Therefore, on the basis of the electrical signal inputted or received from the load switch unit  700  by the actual generation of the load, the control unit  800  may calibrate the zero point of the estimated load estimated on the basis of the motor current and calibrate an origin point of the braking force. That is, the control unit  800  performs zero point calibration on an initial braking force when the braking force is generated by actually controlling the motor  910 . 
     The point in time at which the electrical signal inputted from the load switch unit  700  is inputted is a point in time at which a load is actually generated. The control unit  800  calibrates the estimated load, which is estimated on the basis of the motor current, to 0 at the point in time at which the electrical signal is inputted from the load switch unit  700 , thereby performing the zero point calibration on the electric current estimated on the basis of the motor current. Therefore, the control unit  800  performs the zero point calibration on the initial braking force when the braking force is generated by actually controlling the motor  910 . 
     For example, when the motor operates as the control unit  800  applies the electric current to the motor, the motor current measurement unit  1000  measures the motor current. 
     In this case, the control unit  800  estimates the load on the basis of the measured motor current, and the estimated load may be larger than 0. 
     However, when the electrical signal is inputted from the load switch unit  700  in this process, it can be seen that the load estimated on the basis of the motor current is incorrect because the point in time at which the electrical signal is inputted is the point in time at which the load is actually generated. 
     Therefore, the control unit  800  calibrates the estimated load estimated on the basis of the motor current to 0 when the estimated load is larger than 0 at the point in time at which the electrical signal is inputted from the load switch unit. 
     Moreover, the control unit  800  may calibrate all the estimated loads for the respective motor currents by calibrating the lookup table or calculation formulas for estimating the load on the basis of the motor current. 
     A magnitude of the estimated load at the point in time at which the electrical signal is inputted may be an error. For example, the control unit  800  may calibrate all the estimated loads for the respective motor currents by subtracting the estimated load (error), which is estimated at the point in time at which the electrical signal is inputted, from the respective estimated loads in the lookup table. 
     Meanwhile, the control unit  800  determines that the motor current monitoring function of the motor current measurement unit  1000  fails when the estimated load estimated on the basis of the current motor current is maintained to be 0 for a predetermined time or longer in a state in which the electrical signal inputted from the load switch unit  700  exceeds 0. 
     Typically, the motor is operated by the motor current, and the piston is moved, before a circuit of the load switch unit  700  is closed. 
     Therefore, the control unit  800  determines that the motor current monitoring function fails when the estimated load estimated on the basis of the motor current is continuously 0 in the state in which the electrical signal is inputted from the load switch unit  700  (the electrical signal is larger than 0). Further, the control unit  800  outputs the failure of the motor current monitoring function through the output unit  1100 . 
     In this case, the output unit  110  may warn of the failure of the motor current monitoring function through an image or sound. 
     Hereinafter, a method of calibrating a braking force of the brake device for a vehicle to a zero point according to the embodiment of the present disclosure will be described in detail with reference to  FIG.  12   . 
       FIG.  12    is a flowchart of a method of calibrating a braking force of the brake device for a vehicle to a zero point according to the embodiment of the present disclosure. 
     Referring to  FIG.  12   , the control unit  800  operates the motor  910  (S 100 ). 
     As the motor unit  900  operates, the motor current measurement unit  1000  measures the motor current of the motor  910  and inputs the measurement result to the control unit  800 . 
     The control unit  800  estimates the load on the basis of the motor current inputted from the motor current measurement unit  1000  (S 300 ). 
     In addition, when the motor unit  900  operates, the screw bar  300  is rotated by the power from the motor unit  900 , such that the nut unit  400  rectilinearly moves. 
     That is, to brake the vehicle, the screw bar  300  is rotated in the predetermined direction by the power from the motor unit  900 , the nut unit  400  and the piston unit  500  move toward the brake pad  200 , and the piston unit  500  presses the brake pad  200 . 
     The braking force is generated as the brake pads  200  come into contact with the brake disc  10 . As the piston unit  500  presses the brake pad  200 , the repulsive force is applied to the piston unit  500  and the nut unit  400 , and the repulsive force is also applied to the screw bar  300 . 
     The bearing unit  600  is moved rearward by the repulsive force of the screw bar  300 , and the bearing unit  600  presses the load switch unit  700 , such that the load switch unit  700  operates and inputs the electrical signal to the control unit  800 . 
     Therefore, the control unit  800  checks whether the electrical signal is inputted from the load switch unit  700  (S 400 ). When the electrical signal is inputted, the control unit  800  checks whether the estimated load, which is estimated as described above, is 0 (S 500 ). 
     When the check result in step S 500  indicates that the estimated load is 0, the control unit  800  determines whether the estimated load is maintained to be 0 for a predetermined time or longer (S 600 ). 
     When the determination result in step S 600  indicates that the estimated load is maintained to be 0, the control unit  800  determines that the motor current monitoring function fails and outputs the determination result through the output unit  110  (S 700 ). 
     When the determination result in step S 610  indicates that the estimated load is not maintained to be 0 and the estimated load varies, the control unit  800  determines that the estimated load is normal. 
     In contrast, when the check result in step S 500  indicates that the estimated load is not 0, the control unit  800  calibrates the estimated load, which is estimated on the basis of the motor current, to 0 at the point in time at which the electrical signal is inputted from the load switch unit  700  (S 800 ). 
     Further, the control unit  800  may calibrate all the estimated loads for the respective motor currents by calibrating the lookup table or calculation formulas for estimating the load on the basis of the motor current (S 900 ). 
     As described above, the apparatus and method for calibrating the braking force of the brake device for a vehicle to the zero point according to the embodiment of the present disclosure calibrate the point in time at which the generation of the load by the motor current is estimated by using the point in time at which the load is actually generated on the brake pads of the brake device for a vehicle, thereby reducing the initial ineffectiveness and improving precision in calculating the output load. 
     For example, the configurations described in the present specification may be implemented as methods or processes, devices, software programs, data stream, or signals. Even though the implementation of the single form is described (e.g., only the method is described), the described features may also be in other forms (e.g., devices or programs). The device may be implemented as appropriate hardware, software, firmware, and the like. For example, the method may be implemented by devices such as processors generally referring to processing devices including computers, microprocessors, integrated circuits, programmable logic devices, or the like. The processors also include communication devices such as computers, cellular phones, portable/personal information terminals (personal digital assistants (PDA)), and other devices that easily perform information communication with final users. 
     While the present disclosure has been described with reference to the embodiment illustrated in the drawings, the embodiment is described just for illustration, and those skilled in the art to the present technology pertains will understand that various modifications of the embodiment and any other embodiment equivalent thereto are available. Accordingly, the true technical protection scope of the present disclosure should be determined by the appended claims.