Patent Publication Number: US-2020276964-A1

Title: Electric booster

Description:
TECHNICAL FIELD 
     The present invention relates to an electric booster that applies a braking force to a vehicle such as an automobile. 
     BACKGROUND ART 
     An electric booster configured to use an electric actuator is known as a booster (a brake booster) mounted on a vehicle such as an automobile. The electric booster can supply a brake hydraulic pressure to a wheel brake mechanism of the vehicle with use of the electric actuator. Now, PTL 1 discusses an electric booster configured to be able to acquire various brake characteristics by variably controlling a relative position between an input member displaceable according to an operation on a brake pedal and an assist member advanceable and retractable by the electric actuator. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Patent Application Public Disclosure No. 2011-235894 
     SUMMARY OF INVENTION 
     Technical Problem 
     Then, the electric booster like the example discussed in PTL 1 can acquire the various brake characteristics by changing the relative position between the assist member and the input member according to an amount of the operation on the brake pedal. However, an error may be generated between a relative position recognized by a control apparatus and an actual relative position due to an error of a sensor for detecting the relative position, a variation in a mechanical tolerance, and the like. Then, the brake characteristic may be changed according to this error (i.e., the brake characteristic may deviate from a desired brake characteristic). 
     Solution to Problem 
     An object of the present invention is to provide an electric booster capable of preventing the change in the brake characteristic. 
     According to one aspect of the present invention, an electric booster includes an input member configured to receive transmission of a part of a reaction force from a piston of a master cylinder coupled with a brake pedal, an assist member advanceable and retractable relative to this input member, an electric actuator configured to thrust the assist member forward by the movement of the input member, a reaction force distribution member configured to combine thrust forces of the input member and the assist member to transmit them to the piston of the master cylinder, and distribute the reaction force from the piston to the input member and the assist member, and a control device configured to detect a relative position between the input member and the assist member, and drive and control the electric actuator. The input member is subjected to a mechanical limitation on a displacement thereof relative to the assist member. The control device moves forward/backward the assist member independently of the movement of the input member and determines an abutment state between the input member and the assist member under the mechanical limitation based on the detected relative position, and corrects the relative position between the input member and the assist member to control the electric actuator. 
     The electric booster according to the one aspect of the present can prevent the change in the brake characteristic. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  schematically illustrates a vehicle on which an electric booster according to a first embodiment is mounted. 
         FIG. 2  is a vertical cross-sectional view illustrating the electric booster illustrated in  FIG. 1  in an enlarged manner. 
         FIG. 3  is a control block diagram illustrating the electric booster, a master cylinder, a wheel brake mechanism, and the like. 
         FIG. 4  are enlarged cross-sectional views each illustrating a reaction disk in a state elastically deformed between an input piston and a power piston, and an output rod. 
         FIG. 5  illustrates a characteristic line representing a relationship between an input rod load and a hydraulic reaction force. 
         FIG. 6  illustrates a characteristic line representing a change in the relationship between the input rod load and the hydraulic reaction force due to an error in a relative displacement amount. 
         FIG. 7  is a control block diagram specifically illustrating a relative displacement amount calculation processing portion illustrated in  FIG. 3 . 
         FIG. 8  are schematic half cross-sectional views illustrating motions of the power piston, the input member, the output rod, and the like. 
         FIG. 9  illustrates characteristic lines representing one example of changes in a position of the power piston and a position of the input member over time. 
         FIG. 10  illustrates a characteristic line representing one example of a relationship between the position of the input member and a detection error. 
         FIG. 11  are schematic half cross-sectional views illustrating motions of the power piston, the input member, the output rod, and the like according to a second embodiment. 
         FIG. 12  illustrate characteristic lines representing one example of changes in the position of the power piston and a motor electric current over time. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following description, an electronic booster according to embodiments will be described in detail with reference to the accompanying drawings based on an example in which this electric booster is mounted on a four-wheeled automobile. 
       FIGS. 1 to 10  illustrate a first embodiment. In  FIG. 1 , four wheels in total that include front left and right wheels  2 L and  2 R and rear left and right wheels  3 L and  3 R are mounted on a vehicle body  1  forming a main structure of a vehicle under (on a road surface side of) it. These wheels (i.e., the front wheels  2 L and  2 R and the rear wheels  3 L and  3 R) form the vehicle together with the vehicle body  1 . Front wheel-side wheel cylinders  4 L and  4 R are mounted on the front left and right wheels  2 L and  2 R, respectively. Rear wheel-side wheel cylinders  5 L and  5 R are mounted on the rear left and right wheels  3 L and  3 R, respectively. These individual wheel cylinders  4 L,  4 R,  5 L, and  5 R serve as a wheel brake mechanism (a frictional brake mechanism) that applies braking forces (frictional braking forces) to the respective wheels  2 L,  2 R,  3 L, and  3 R, and is, for example, constructed with use of, for example, a hydraulic disk brake or a drum brake. 
     A brake pedal  6  is provided on a dashboard side of the vehicle body  1 . The brake pedal  6  is operated by being pressed by a driver when a braking force is provided to the vehicle. At this time, each of the wheel cylinders  4 L,  4 R,  5 L and  5 R applies the braking force based on a brake hydraulic pressure to the wheel  2 L,  2 R,  3 L, or  3 R. A brake operation sensor  7  is provided on the brake pedal  6  (more specifically, an input member  32  of an electric booster  30 , which will be described below). The brake operation sensor  7  functions as an operation amount detector that detects an operation amount of the brake pedal  6  (a brake pedal operation amount) that is input by the driver. 
     The brake operation sensor  7  can be embodied by, for example, a stroke sensor (a displacement sensor) that detects a stroke amount (a pedal stroke) corresponding to a displacement amount of the brake pedal  6  (the input member  32 ). The brake operation sensor  7  is not limited to the stroke sensor, and can be embodied by various kinds of sensors capable of detecting the operation amount (a pressing amount) of the brake pedal  6  (the input member  32 ), such as a force sensor that detects a pedal pressing force (a load sensor), and an angle sensor that detects a rotational angle (a tilt) of the brake pedal  6 . In this case, the brake operation sensor  7  may be constructed with use of one (one kind of) sensor or may be constructed with use of a plurality of (a plurality of kinds of) sensors. 
     A detection signal of the brake operation sensor  7  (a brake operation amount) is output to an electric booster ECU  51  (hereinafter referred to as an ECU  51 ), which will be described below. The ECU  51  forms the electric booster  30 , which will be described below, together with the brake operation sensor  7  and the like. As will be described below, the ECU  51  outputs a driving signal to an electric motor  37  of the electric booster  30  based on the brake pedal operation amount of the brake operation sensor  7  (a first braking instruction value), thereby causing a hydraulic pressure (the brake hydraulic pressure) to be generated in hydraulic chambers  25  and  26  (refer to  FIG. 2 ) in a master cylinder  21  attached to the electric booster  30 . 
     Further, the ECU  51 , for example, also causes the hydraulic pressure to be generated in the master cylinder  21  when receiving an autonomous brake instruction (a second braking instruction value) via a vehicle data bus  12 , which will be described below. At this time, the ECU  51  can output the driving signal to the electric motor  37  of the electric booster  30  based on the autonomous brake instruction to cause the hydraulic pressure to be generated in the hydraulic chambers  25  and  26  in the master cylinder  21  independently of the operation performed on the brake pedal  6  by the driver. 
     The hydraulic pressure generated in the master cylinder  21  is supplied to each of the wheel cylinders  4 L,  4 R,  5 L, and  5 R via a hydraulic pressure supply apparatus  9 , and the braking force is applied to each of the wheels  2 L,  2 R,  3 L, and  3 R. Configurations of the master cylinder  21 , a reservoir  29 , the electric booster  30 , and the like illustrated in  FIGS. 2 to 4  will be described in detail below. 
     As illustrated in  FIG. 1 , the hydraulic pressure generated in the master cylinder  21  is supplied to the hydraulic pressure supply apparatus  9  (hereinafter referred to as an ESC  9 ) via, for example, a pair of cylinder-side hydraulic pipes  8 A and  8 B. The ESC  9  is provided between the master cylinder  21  and the wheel cylinders  4 L,  4 R,  5 L, and  5 R. The ESC  9  distributes and supplies the hydraulic pressure output from the master cylinder  21  via the cylinder-side hydraulic pipes  8 A and  8 B to the wheel cylinders  4 L,  4 R,  5 L, and  5 R via brake-side pipe portions  11 A,  11 B,  11 C, and  11 D, respectively. 
     The ESC  9  includes, for example, a plurality of control valves, a hydraulic pump, an electric motor, and a hydraulic control reservoir (none of them illustrated). The hydraulic pump increases a pressure of the brake fluid. The electric motor drives this hydraulic pump. The hydraulic control reservoir temporarily stores extra brake fluid therein. Opening/closing of each of the control valves and driving of the electric motor of the ESC  9  are controlled by a hydraulic pressure supply apparatus ECU  10  (hereinafter referred to as an ECU  10 ). 
     The ECU  10 , which corresponds to a first ECU, includes, for example, a microcomputer, a driving circuit, and an electric power source circuit. The microcomputer includes, for example, a memory including a flash memory, a ROM, a RAM, an EEPROM, and/or the like (none of them illustrated), in addition to an arithmetic device (a CPU). The ECU  10  is a hydraulic pressure supply apparatus control unit that electrically controls driving of the ESC  9  (each of the control valves and the electric motor thereof). An input side of the ECU  10  is connected to the vehicle data bus  12  and a hydraulic sensor  15 . An output side of the ECU  10  is connected to each of the control valves of the ESC  9 , the electric motor, and the vehicle data bus  12 . The ECU  10  controls the driving of each of the control valves, the electric motor, and the like of the ESC  9  individually. By this control, the ECU  10  performs control of reducing, maintaining, increasing, or pressurizing the brake hydraulic pressures to be supplied from the brake-side pipe portions  11 A,  11 B,  11 C, and  11 D to the wheel cylinders  4 L,  4 R,  5 L, and  5 R, respectively, for each of the wheel cylinders  4 L,  4 R,  5 L, and  5 R individually. 
     In this case, the ECU  10  can perform, for example, the following kinds of control (1) to (8) by controlling the actuation of the ESC  9 . 
     (1) braking force distribution control of appropriately distributing the braking force to each of the wheels  2 L,  2 R,  3 L, and  3 R according to a vertical load and the like when the vehicle is braked
 
(2) anti-lock brake control of preventing each of the wheels  2 L,  2 R,  3 L, and  3 R from being locked (slipped) by automatically adjusting the braking force provided to each of the wheels  2 L,  2 R,  3 L, and  3 R when the vehicle is braked
 
(3) vehicle stabilization control of stabilizing a behavior of the vehicle, by preventing or reducing understeer and oversteer while detecting a sideslip of each of the wheels  2 L,  2 R,  3 L, and  3 R when the vehicle is running to thus appropriately automatically control the braking force to be applied to each of the wheels  2 L,  2 R,  3 L, and  3 R regardless of the operation amount of the brake pedal  6 
 
(4) hill start aid control of aiding a start by maintaining a braked state on a slope
 
(5) traction control of preventing each of the wheels  2 L,  2 R,  3 L, and  3 R from idly spinning, for example, when the vehicle starts running
 
(6) adaptive cruise control of maintaining a predetermined distance to a vehicle running ahead
 
(7) traffic lane departure avoidance control of maintaining the vehicle within a traffic lane
 
(8) obstacle avoidance control of avoiding a collision with an obstacle located in a direction in which the vehicle is traveling (a collision avoidance system)
 
     The ESC  9  directly supplies the hydraulic pressure generated in the master cylinder  21  to the wheel cylinders  4 L,  4 R,  5 L, and  5 R at the time of, for example, a normal operation in response to the brake operation performed by the driver. On the other hand, for example, the ESC  9  maintains the hydraulic pressures in the wheel cylinders  4 L,  4 R,  5 L and  5 R by closing a control valve for the pressure increase when performing the anti-lock brake control or the like, and discharges the hydraulic pressures in the wheel cylinders  4 L,  4 R,  5 L, and  5 R as if releasing them to the hydraulic control reservoir by opening a control valve for the pressure reduction when reducing the hydraulic pressures in the wheel cylinders  4 L,  4 R,  5 L, and  5 R. 
     Further, the ESC  9  actuates the hydraulic pump by the electric motor with a control valve for the supply closed, thereby supplying the brake fluid discharged from this hydraulic pump to the wheel cylinders  4 L,  4 R,  5 L, and  5 R when increasing or pressurizing the hydraulic pressures to be supplied to the wheel cylinders  4 L,  4 R,  5 L, and  5 R to perform, for example, the stabilization control (electronic stability control) when the vehicle is running. At this time, for example, the brake fluid in the reservoir  29  is supplied from the master, cylinder  21  side toward an intake side of the hydraulic pump. 
     The vehicle data bus  12  is a communication network between vehicle ECUs called a V-CAN that is mounted on the vehicle. More specifically, the vehicle data bus  12  is a serial communication portion that establishes multiplex communication among a large number of electric apparatuses (for example, among the ECU  10 , an ECU  16 , and the ECU  51 ) mounted on the vehicle. Electric power is supplied from an in-vehicle battery  14  to the ECU  10  via an electric power line  13 . Electric power is also supplied from the in-vehicle battery  14  to the ECU  16  and the ECU  51 , which will be described below, via the electric power line  13 . In  FIG. 1 , a line with two slash marks added thereto indicates an electricity-related line such as a signal line and an electric power source line. 
     A hydraulic sensor  15  is provided in, for example, the cylinder-side hydraulic pipe  8 A between the master cylinder  21  (the first hydraulic chamber  25  thereof) and the ECU  9 . The hydraulic sensor  15  is a hydraulic pressure detection portion that detects the pressure (the brake hydraulic pressure) generated in the master cylinder  21 , i.e., a hydraulic pressure in the cylinder-side hydraulic pipe  8 A. The hydraulic sensor  15  is electrically connected to the ECU  10  of the ESC  9 . A detection signal of the hydraulic sensor  15  (a hydraulic value) is output to the ECU  10 . The ECU  10  outputs the hydraulic value detected by the hydraulic sensor  15  to the vehicle data bus  12 . The electric booster ECU  51 , which will be described below, can monitor (acquire) the hydraulic value generated in the master cylinder  21  by receiving the hydraulic value from the ECU  10 . 
     The ECU  10  and the ECU  51  may be connected to each other via a communication line (a signal line) provided separately from the vehicle data bus  12 , such as a communication line called an L-CAN capable of establishing communication between in-vehicle ECUs (i.e., the communication network between vehicle ECUs), and be configured to transmit and receive the hydraulic value of the hydraulic sensor  15  via this communication line, although this configuration is not illustrated in  FIG. 1 . In other words, the electric booster ECU  51  can acquire the hydraulic value detected by the hydraulic sensor  15  from the ECU  10  via the communication network between vehicle ECUs (the vehicle data bus  12  or the communication line). 
     The autonomous brake ECU  16  (hereinafter referred to as the ECU  16 ) is connected to the vehicle data bus  12 . The ECU  16 , which corresponds to a second ECU, is an autonomous brake control unit that outputs an autonomous brake instruction (an autonomous brake braking instruction value). The ECU  16  also includes a microcomputer similarly to the ECU  10  and the ECU  51 , which will be described below, and is connected to the ECUs  10  and  51  and the like via the vehicle data bus  12 . 
     Now, the ECU  16  is connected to, for example, an eternal world recognition sensor  17 . The external world recognition sensor  17  forms an object position measurement device that measures a position of an object located around the vehicle, and can be embodied by a camera such as a stereo camera and a single camera (for example, a digital camera), and/or a radar such as a laser radar, an infrared radar, and a millimeter-wave radar (for example, a light emitting element such as a semiconductor radar and a light receiving element that receives it). The external world recognition sensor  17  is not limited to the camera and the radar, and may be embodied by various kinds of sensors (a detection device, a measurement device, and a radiodetector) capable of recognizing (detecting) a state of the external world, which is a neighborhood around the vehicle. 
     The ECU  16  calculates, for example, a distance to an object located in front of the vehicle based on a result (information) of the detection by the external world recognition sensor  17 , and also calculates the autonomous brake braking instruction value corresponding to the braking force (the braking hydraulic pressure) to apply based on this distance, the present running speed of the vehicle, and the like. The calculated autonomous brake braking instruction value is output from the ECU  16  to the vehicle data bus  12  as the autonomous brake instruction. 
     In this case, for example, upon acquiring the autonomous brake braking instruction value (a second braking instruction value) via the vehicle data bus  12 , the electric booster ECU  51 , which corresponds to a third ECU, drives the electric motor  37  of the electric booster  30  based on this acquired autonomous brake braking instruction value. In other words, the electric booster  30  can apply the braking force to each of the wheels  2 L,  2 R,  3 L, and  3 R by causing the hydraulic pressure to be generated in the master cylinder  21  to increase the pressure in each of the wheel cylinders  4 L,  4 R,  5 L, and  5 R based on the autonomous brake braking instruction value (the autonomous brake). 
     Next, the master cylinder  21 , the reservoir  29 , and the electric booster  30  will be described with additional reference to  FIG. 2 , along with  FIG. 1 . 
     The master cylinder  21  is actuated by the brake operation performed by the driver. The master cylinder  21  is a cylinder device that supplies the brake hydraulic pressure to the wheel cylinders  4 L,  4 R,  5 L, and  5 R, which apply the braking force to the vehicle. As illustrated in  FIG. 2 , the master cylinder  21  includes a tandem-type master cylinder. More specifically, the master cylinder  21  includes a cylinder main body  22 , a primary piston  23 , a secondary piston  24 , the first hydraulic chamber  25 , the second hydraulic chamber  26 , a first return spring  27 , and a second return spring  28 . 
     The cylinder main body  22  is formed into a closed bottomed cylindrical shape having an opened end on one side thereof (for example, a right side in a horizontal direction in  FIG. 2  and a rear side in a longitudinal direction of the vehicle) and a bottom portion on the other side thereof (for example, a left side in the horizontal direction in  FIG. 2  and a front side in the longitudinal direction of the vehicle) in an axial direction (the horizontal direction in  FIG. 2 ). The cylinder main body  22  is attached at the opening end side thereof to a booster housing  31  of the electric booster  30 , which will be described below. First and second reservoir ports  22 A and  22 B connected to the reservoir  29  are provided on the cylinder main body  22 . Further, first and second supply ports  22 C and  22 D, to which the cylinder-side hydraulic pipes  8 A and  8 B are connected, are provided on the cylinder main body  22 . The first and second supply ports  22 C and  22 D are connected to the wheel cylinders  4 L,  4 R,  5 L, and  5 R via the cylinder-side hydraulic pipes  8 A and  8 B, and the like. 
     The primary piston  23  includes a bottomed rod insertion hole  23 A on one axial side thereof and a bottomed spring containing hole  23 B on the other axial side thereof. The spring containing hole  23 B is opened to an opposite side from the rod insertion hole  23 A (opened to the other side), and one side of the first return spring  27  is disposed in the spring containing hole  23 B. The rod insertion hole  23 A side of the primary piston  23  protrudes outward from the opening end side of the cylinder main body  22 , and an output rod  48 , which will be described below, is inserted in the rod insertion hole  23 A in an abutment state. 
     The secondary piston  24  is formed into a bottomed cylindrical shape, and is closed at a bottom portion  24 A formed on one axial side thereof that faces the primary piston  23 . A spring containing hole  24 B, which is opened to the other axial side, is formed at the secondary piston  24 , and one side of the second return spring  28  is disposed in the spring containing hole  24 B. 
     The first hydraulic chamber  25  is defined between the primary piston  23  and the secondary piston  24 . The second hydraulic chamber  26  is defined between the secondary piston  24  and a bottom portion of the cylinder main body  22 . The first and second hydraulic chambers  25  and  26  are formed so as to be axially spaced apart from each other in the cylinder main body  22 . 
     The first return spring  27  is positioned in the first hydraulic chamber  25 , and is arranged between the primary piston  23  and the secondary piston  24 . The first return spring  27  biases the primary piston  23  toward the opening end side of the cylinder main body  22 . The second return spring  28  is positioned in the second hydraulic chamber  26 , and is arranged between the bottom portion of the cylinder main body  22  and the secondary piston  24 . The second return spring  28  biases the secondary piston  24  toward the first hydraulic chamber  25  side. 
     For example, when the brake pedal  6  is operated by being pressed, the primary piston  23  and the secondary piston  24  are displaced toward the bottom portion side of the cylinder main body  22  in the cylinder main body  22  of the master cylinder  21 . At this time, when the first and second reservoir ports  22 A and  22 B are blocked by the primary piston  23  and the secondary piston  24 , respectively, the brake hydraulic pressure (an M/C pressure) is generated from the master cylinder  21  by the brake fluid in the first and second hydraulic chambers  25  and  26 . On the other hand, when the operation on the brake pedal  6  is released, the primary piston  23  and the secondary piston  24  are displaced toward the opening portion side of the cylinder main body  22  by the first and second return springs  27  and  28 , respectively. 
     The reservoir  29  is attached to the cylinder main body  22  of the master cylinder  21 . The reservoir  29  is configured as a hydraulic oil tank that stores the brake fluid therein, and replenishes (supplies and discharges) the brake fluid into each of the hydraulic chambers  25  and  26  in the cylinder main body  22 . As illustrated in  FIG. 2 , when the first reservoir port  22 A is in communication with the first hydraulic chamber  25  and the second reservoir port  22 B is in communication with the second hydraulic chamber  26 , the brake fluid can be supplied or discharged between the reservoir  29  and the hydraulic chambers  25  and  26 . 
     On the other hand, when the first reservoir port  22 A is disconnected from the first hydraulic chamber  25  by the primary piston  23  and the second reservoir port  22 B is disconnected from the second hydraulic chamber  26  by the secondary piston  24 , the supply and the discharge of the brake fluid are stopped between the reservoir  29  and the hydraulic chambers  25  and  26 . In this case, the brake hydraulic pressure (the M/C pressure) is generated in the hydraulic chambers  25  and  26  of the master cylinder  21  according to the displacements of the primary piston  23  and the secondary piston  24 , and this brake hydraulic pressure is supplied from the first and second supply ports  22 C and  22 D to the ESC  9  via the pair of cylinder-side hydraulic pipes  8 A and  8 B. 
     The electric booster  30  as an electric brake apparatus is provided between the brake pedal  6  and the master cylinder  21 . The electric booster  30  serves as a boosting mechanism (a booster) that transmits the brake operation force (the pressing force) to the master cylinder  21  while powering up this force by driving the electric motor  37  according to the brake pedal operation amount (the pressing amount), which corresponds to the first braking instruction value, when the operation of pressing the brake pedal  6  is performed by the driver. In addition thereto, the electric booster  30  serves as an autonomous brake application mechanism that autonomously applies the braking force (the autonomous brake) even without the brake operation (the pedal operation) performed by the driver. 
     In other words, the electric booster  30  causes the brake hydraulic pressure to be generated in the master cylinder  21  by driving the electric motor  37  according to the autonomous brake instruction, which corresponds to the second braking instruction value (for example, from the ECU  16 ). Due to this configuration, the electric booster  30  can supply the brake hydraulic pressure into each of the wheel cylinders  4 L,  4 R,  5 L, and  5 R regardless of the brake operation by the driver (regardless of whether the operation is present or absent), thereby autonomously applying the braking force (the autonomous brake). 
     The electric booster  30  includes the brake operation sensor  7  (refer to  FIGS. 1 and 3 ) as an operation amount detector, an input member  32 , an electric actuator  36 , an angle sensor  39  (refer to  FIGS. 1 and 3 ) as a movement amount detection portion, a power piston  45  as an assist member, a reaction disk  47  as a reaction force distribution member, and the ECU  51  as a control device. More specifically, the electric booster  30  includes the brake operation sensor  7 , the booster housing  31  as a housing, the input member  32 , the electric actuator  36 , the angle sensor  39 , the power piston  45 , the reaction disk  47 , an output rod  48 , the ECU  51 , and the like. 
     The booster housing  31  forms an outer shell of the electric booster  30 , and is fixed to, for example, a front wall of a vehicle compartment, which is the dashboard of the vehicle body  1 . The booster housing  31  includes a motor case  31 A, an output case  31 B, and an input case  31 C. The motor case  31 A contains therein the electric motor  37  and a part (a driving pulley  40 A side) of a speed reduction mechanism  40 , which will be described below. The output case  31 B contains therein the other portion (a driven pulley  40 B side) of the speed reduction mechanism  40 , a part (the other axial side) of a rotation-linear motion conversion mechanism  43  and the power piston  45 , the second return spring  46 , the output rod  48 , the reaction disk  47 , and the like. The input case  31 C closes openings of the motor case  31 A and the output case  31 B on one axial sides thereof, and also contains therein the other portion (one axial side) of the rotation-linear motion conversion mechanism  43  and the power piston  45 , an intermediate portion of the input member  32 , and the like. 
     An annular stopper member  31 D in abutment with a flange portion  33 B of the input member  32  is provided on an opening of the input case  31 C on one side thereof. Stopper pieces  31 D 1  (not illustrated in  FIG. 2 , and refer to  FIG. 8 ) protruding radially inward are provided on the stopper member  31 D at two circumferential locations (for example, two locations spaced apart from each other by 180 degrees). The flange portion  33 B of the input member  32  is brought into abutment with the stopper pieces  31 D 1  of the stopper member  31 D, by which the input member  32  is prohibited from being displaced toward the one axial side (the rear side, and the right side in  FIG. 2 ) more than that. In other words, the stopper member  31 D (the stopper pieces  31 D 1  thereof) serve as a step (a positioning step X 1  and refer to  FIG. 8 ) that positions the input member  32  by being brought into abutment the flange portion  33 B of the input member  32  when the input member  32  is displaced toward the rear side (the right side in  FIG. 2 ), which corresponds to the one axial side. 
     The input member  32  is provided axially movably relative to the booster housing  31 , and is connected to the brake pedal  6 . A part of a reaction force from the primary piston  23  of the master cylinder  21  coupled with the brake pedal  6  is transmitted to the input member  32 . To fulfill this function, the input member  32  includes an input rod  33  and an input piston  34 . The input rod  33  and the input piston  34  are inserted through inside the rotation-linear motion conversion mechanism  43  and the power piston  45  in a concentrically connected state. In this case, one axial side of the input rod  33  protrudes from the input case  31 C of the booster housing  31 . Then, the brake pedal  6  is coupled to the one axial side of the input rod  33  that corresponds to a protrusion end thereof. 
     On the other hand, the other axial side of the input rod  33  includes a spherical portion  33 A formed on a distal end thereof, and this spherical portion  33 A is inserted in the power piston  45 . The annular flange portion  33 B is provided at an axial center of the input rod  33 . The flange portion  33 B protrudes radially outward along an entire circumference. A first return spring  35  is provided between this flange portion  33 B and the power piston  45 . The first return spring  35  constantly biases the input member  32  (the input rod  33 ) relative to the power piston  45  toward the one axial side. 
     The input piston  34  is fittedly inserted in the power piston  45  axially movably (slidably) relative to the power piston  45 . The input piston  34  includes a piston main body  34 A and a pressure reception portion  34 B. The piston main body  34 A is provided so as to face the input rod  33 . The pressure reception portion  34 B is provided so as to protrude from this piston main body  34 A toward the other axial direction. A recessed portion  34 C is provided on one axial side of the piston main body  34 A at a position corresponding to the spherical portion  33 A of the input rod  33 . The spherical portion  33 A of the input rod  33  is fixed in the recessed portion  34 C with use of a method such as crimping. 
     On the other hand, a distal end surface of the pressure reception portion  34 B serves as an abutment surface abuttable against the reaction disk  47 . For example, when the vehicle is not braked without the brake pedal  6  operated, a predetermined space is formed between the distal end surface of the pressure reception portion  34 B and the reaction disk  47 . When the brake pedal  6  is operated by being pressed, the distal end surface of the pressure reception portion  34 B and the reaction disk  47  are brought into abutment with each other, and a thrust force of the input member  32  (the pressing force) is applied to the reaction disk  47  (refer to  FIG. 4 ). 
     The electric actuator  36  is actuated when the hydraulic pressure is supposed to be generated from the master cylinder  21 , and applies the braking hydraulic pressure to each of the wheel cylinders  4 L,  4 R,  5 L, and  5 R of the vehicle. In this case, the electric actuator  36  thrusts the power piston  45  as the assist member forward by the movement of the input member  32 . In other words, the electric actuator  36  causes the power piston  45  to be moved in the axial direction of the master cylinder  21  and applies a thrust force to this power piston  45 . As a result, the power piston  45  axially displaces the primary piston  23  (and the secondary piston  24 ) in the cylinder main body  22  of the master cylinder  21 . 
     The electric actuator  36  includes the electric motor  37 , the speed reduction mechanism  40 , a cylindrical rotational member  41 , and the rotation-linear motion conversion mechanism  43 . The speed reduction mechanism  40  slows down the rotation of this electric motor  37 . The rotation slowed by this speed reduction mechanism  40  is transmitted to the cylindrical rotational member  41 . The rotation-linear motion conversion mechanism  43  converts the rotation of this cylindrical rotational member  41  into the axial displacement of the power piston  45 . The electric motor  37  is constructed with use of, for example, a DC brushless motor, and includes a rotational shaft  37 A, a rotor (not illustrated), and a stator (not illustrated). The rotational shaft  37 A functions as a motor shaft (an output shaft). The rotor is, for example, a permanent magnet attached to this rotational shaft  37 A. The stator is, for example, a coil (an armature) attached to the motor case  31 A. An end portion of the rotational shaft  37 A on one axial side thereof is rotatably supported by the input case  31 C of the booster housing  31  via a roller bearing  38 . 
     The electric motor  37  is provided with the angle sensor  39  (refer to  FIGS. 1 and 3 ) called a resolver or a rotational angle sensor. Then angle sensor  39  detects a rotational angle (a rotational position) of the electric motor  37  (the rotational shaft  37 A thereof), and outputs a detection signal thereof to the ECU  51 . The ECU  51  performs feedback control on the rotational position of the electric motor  37  (i.e., the displacement of the power piston  45 ) according to this rotational angle signal. Then, based on the rotational angle of the electric motor  37  that is detected by the angle sensor  39 , a movement amount (a displacement amount or a position) of the power piston  45  can be calculated by using a speed reduction ratio of the speed reduction mechanism  40 , which will be described below, and a linear displacement amount per unit rotational angle of the rotation-linear motion conversion mechanism  43 . 
     Therefore, the angle sensor  39  forms the movement amount detection portion that detects the movement amount of the power piston  45  (a power piston position). The movement amount detection portion is not limited to the angle sensor  39  including the resolver, and may be embodied by, for example, a rotary potentiometer. Further, the angle sensor  39  may detect the rotational angle after the speed is slowed down by the speed reduction mechanism  40  (for example, a rotational angle of the cylindrical rotational member  41 ) instead of the rotational angle (the rotational position) of the electric motor  37 . Alternatively, for example, a displacement sensor (a position sensor) that directly detects the linear displacement (the axial displacement) of the power piston  45  may be used instead of the angle sensor  39  that indirectly detects the movement amount of the power piston  45 . Alternatively, the linear displacement of a linear motion member  44  of the rotation-linear motion conversion mechanism  43  may be detected with use of a displacement sensor. 
     The speed reduction mechanism  40  is configured as, for example, a belt speed reduction mechanism. The speed reduction mechanism  40  includes the driving pulley  40 A, the driven pulley  40 B, and a belt  40 C. The driving pulley  40 A is attached to the rotational shaft  37 A of the electric motor  37 . The driven pulley  40 B is attached to the cylindrical rotational member  41 . The belt  40 C is wound around between them. The speed reduction mechanism  40  transmits the rotation of the rotational shaft  37 A of the electric motor  37  to the cylindrical rotational member  41  while slowing down this rotation at the predetermined speed reduction ratio. The cylindrical rotational member  41  is rotatably supported by the input case  31 C of the booster housing  31  via a roller bearing  42 . 
     The rotation-linear motion conversion mechanism  43  is configured as, for example, a ball-screw mechanism. The rotation-linear motion conversion mechanism  43  includes the cylindrical (hollow) linear motion member  44  provided axially movably via a plurality of balls on an inner peripheral side of the cylindrical rotational member  41 . For example, the liner motion member  44  can form the assist member together with the power piston  45 . The power piston  45  is inserted inside the linear motion member  44  from an opening of the linear motion member  44  on the other axial side thereof. A flange portion  44 A is provided at a position closer to an end portion of the linear motion member  44  on one axial side thereof. The flange portion  44 A protrudes radially inward along an entire circumference. One end portion (a rear end portion) of the power piston  45  is in abutment with a surface (a front-side surface) of this flange portion  44 A on the other side. Due to this abutment, the linear motion member  44  can be displaced to the other axial side (the front side) integrally with the power piston  45  on inner peripheral sides of the input case  31 C and the cylindrical rotational member  41 . 
     The power piston  45  is actuated (axially moved) by the electric actuator  36  to generate the hydraulic pressure in the master cylinder  21  (apply the brake hydraulic pressure to each of the wheel cylinders  4 L,  4 R,  5 L, and  5 R). The power piston  45  forms the assist member advanceable toward and retractable from the input member  32 , and is axially thrust (moved) forward by the electric actuator  36 . The power piston  45  includes an outer cylindrical member  45 A, an inner cylindrical member  45 B, and an annular member  45 C. 
     The outer cylindrical member  45 A of the power piston  45  is provided inside the linear motion member  44  axially displaceably (slidably movably) relative to this linear motion member  44 . The inner cylindrical member  45 B is provided inside the outer cylindrical member  45 A. An end surface (one end surface) of the inner cylindrical member  45 B on one axial side (a rear side) thereof is in abutment with the annular member  45 C together with one end surface of the outer cylindrical member  45 A. The input piston  34  of the input member  32  is fittedly inserted inside the inner cylindrical member  45 B axially relatively movably (slidably movably). 
     A flange portion  45 B 1  is formed on the other axial side (the front side) of the inner cylindrical member  45 B. The flange portion  45 B 1  protrudes radially inward along an entire circumference. This flange portion  45 B 1  (a surface thereof on the other side) faces (confronts) the reaction disk  47  together with the pressure reception portion  34 B of the input piston  34 . On the other hand, the flange portion  45 B 1  (a surface thereof on one side) serves as a step (a step X 2  on the other side) brought into abutment with the input piston  34  of the input member  32 , for example, when the input member  32  is displaced relative to the power piston  45  toward the front side (the left side in  FIG. 2 ), which corresponds to the other axial side. 
     The annular member  45 C is fixedly attached to an opening of the inner cylindrical member  45 B on the one axial side thereof by being threadably engaged therewith. A flange portion  45 C 1  is formed on an axially intermediate portion of the annular member  45 C. The flange portion  45 C 1  protrudes radially outward along an entire circumference. The flange portion  44 A of the linear motion member  44  is in abutment with one side surface of this flange portion  45 C 1 . On the other hand, the outer cylindrical member  45 A and the inner cylindrical member  45 B are in abutment with the surface of the flange portion  45 C 1  on the other side. Further, the annular member  45 C includes a cylindrical portion  45 C 2  extending toward the other axial side inside the inner cylindrical member  45 B. The cylindrical portion  45 C 2  (a surface thereof on the other side) serves as a step (a step X 3  on one side) brought into abutment with the input piston  34  (the piston main body  34 A) of the input member  32 , for example, when the input member  32  is displaced relative to the power piston  45  toward the rear side (the right side in  FIG. 2 ), which corresponds to the one axial side. 
     The second return spring  46  is provided between the outer cylindrical member  45 A of the power piston  45  and the output case  31 B of the booster housing  31 . The second return spring  46  constantly biases the power piston  45  in a braking release direction. Due to this configuration, the power piston  45  is returned to an initial position illustrated in  FIG. 2  due to a driving force from the rotation of the electric motor  37  to a braking release side and the biasing force of the second return spring  46  when the brake operation is released. 
     The reaction disk  47  is the reaction force distribution member provided between the input member  32  (the input piston  34 ) and the power piston  45  (the inner cylindrical member  45 B), and the output rod  48 . The reaction disk  47  is formed into a disk-like shape with use of an elastic resin material such as rubber, and is in abutment with the input member  32  and the power piston  45 . The reaction disk  47  transmits, to the output rod  48 , the pressing force (the thrust force) transmitted from the brake pedal  6  to the input member  32  (the input piston  34 ) and the thrust force transmitted from the electric actuator  36  to the power piston (the inner cylindrical member  45 B) (a booster thrust force). In other words, the reaction disk  47  distributes and transmits a reaction force P (refer to  FIG. 4 ) of the brake hydraulic pressure generated in the master cylinder  21  to the input member  32  and the power piston  45 , as the reaction force distribution member. 
     For example, when the brake pedal  6  is pressed, the power piston  45  is moved toward the reaction disk  47  side by the electric actuator  36  according to this pressing. At this time, the reaction disk  47  is elastically deformed as illustrated in  FIGS. 4(A) and 4(B) , which will be described below. In other words, the reaction disk  47  is elastically deformed between the flange portion  48 A of the output rod  48 , and the inner cylindrical member  45 B of the power piston  45  and the input member  32  (the pressure reception portion  34 B of the input piston  34 ). In  FIG. 4 , the shape of the inner cylindrical member  45 B of the power piston  45 , the shape of the pressure reception portion  34 B of the input piston  34 , and the like are schematically illustrated compared to  FIG. 2 . 
     The output rod  48  functions to output the thrust force of the input member  32  and/or the thrust force of the power piston  45  to the master cylinder  21  (the primary piston  23  thereof). The output rod  48  includes a large-diameter flange portion  48 A provided on one end side thereof. The flange portion  48 A is fitted to the inner cylindrical member  45 B of the power piston  45  from outside while sandwiching the reaction disk  47  therebetween. The output rod  48  axially presses the primary piston  23  of the master cylinder  21  based on the thrust force of the input member  32  and/or the thrust force of the power piston  45 . 
     Now, the rotation-linear motion conversion mechanism  43  has back-drivability, and can cause the cylindrical rotational member  41  to be rotated from the linear motion (the axial movement) of the linear motion member  44 . As illustrated in  FIG. 2 , when the power piston  45  is retracted (maximumly retracted) to a return position (an initial position), the linear motion member  44  is brought into abutment with the closed end side of the input case  31 C (the stopper member  31 D). This closed end (a side surface of the stopper member  31 D) functions as a stopper that regulates the return position of the power piston  45  via the linear motion member  44 . 
     The flange portion  44 A of the linear motion member  44  is in abutment with the power piston  45  (the annular member  45 C thereof) from the rear side (the right side in  FIG. 2 ). This allows the power piston  45  to be advanced alone separately from the linear motion member  44 . That is, for example, suppose that the electric booster  30  has some abnormality, such as a malfunction of the electric motor  37  due to a disconnection or the like. In this case, the linear motion member  44  is returned to the retracted position together with the power piston  45  due to the spring force of the second return spring  46 . This can contribute to prevention of a brake drag. 
     On the other hand, when the braking force is applied, the hydraulic pressure can be generated in the master cylinder  21  by displacing the output rod  48  toward this master cylinder  21  side via the reaction disk  47  based on the forward movement of the input member  32 . At this time, when the input member  32  is advanced by a predetermined amount, the front end of the piston main body  34 A of the input piston  34  is brought into abutment with the inner cylindrical member  45 B (the flange portion  45 B 1  thereof) of the power piston  45 . As a result, the hydraulic pressure can be generated in the master cylinder  21  based on the forward movements of both the input member  32  and the power piston  45 . 
     The speed reduction mechanism  40  is not limited to the belt speed reduction mechanism, and may be constructed with use of another type of speed reduction mechanism such as a gear reduction mechanism. Further, the rotation-linear motion conversion mechanism  43 , which converts the rotational motion into the linear motion, can also be constructed with use of, for example, a rack and pinion mechanism. Further, the speed reduction mechanism  40  does not necessarily have to be provided. For example, the electric booster  30  may be configured in such a manner that the cylindrical rotational member  41  is rotated directly by the electric motor, with the rotor of the electric motor provided at the cylindrical rotational member  41  and the stator of the electric motor also disposed around the cylindrical rotational member  41 . Further, in the embodiment, the rotation-linear motion conversion mechanism  43  and the power piston  45  are prepared as different members from each other, but may be prepared while a part of each of them is integrated. For example, the power piston  45  and the linear motion member  44  of the rotation-linear motion conversion mechanism  43  may be integrated with each other. In other words, the assist member can be formed by the “power piston  45 ” and the “linear motion member  44  prepared as a member different from or a member integrated with the power piston  45 ”. 
     Next, the electric booster ECU  51  will be described. 
     The ECU  51 , which controls the electric booster  30 , includes, for example, a microcomputer, a driving circuit, and an electric power source circuit. The microcomputer includes, for example, a memory including a flash memory, a ROM, a RAM, an EEPROM, and/or the like (none of them illustrated), in addition to an arithmetic device (a CPU). The ECU  51  is an electric booster control unit that electrically drives and controls the electric motor  37 . As illustrated in  FIG. 1 , an input side of the ECU  51  is connected to the brake operation sensor  7 , the angle sensor  39 , and the vehicle data bus  12 . The brake operation sensor  7  detects the operation amount (or the pressing force) of the brake pedal  6 . The angle sensor  39  detects the rotational position of the electric motor  37  (the movement amount of the power piston  45  corresponding thereto). The vehicle data bus  12  provides and receives a signal to and from the ECU  10  or  16  of another vehicle apparatus. On the other hand, an output side of the ECU  51  is connected to the electric motor  37  and the vehicle data bus  12 . 
     The ECU  51  drives the electric motor  37  so as to increase the pressure in the master cylinder  21  according to, for example, the detection signal output from the brake operation sensor  7  (the brake pedal operation amount, i.e., the input member position) and the autonomous brake instruction from the ECU  16  (the autonomous brake braking instruction value). More specifically, the ECU  51  moves (displaces) the power piston  45  by controlling the electric actuator  36  (the electric motor  37 ) based on the first braking instruction value (an input member position) based on the operation performed on the brake pedal  6 . In this case, the ECU  51  detects the relative position between the input member  32  and the power piston  45 , and drives and controls the electric actuator  36  (the electric motor  37 ). Further, the ECU  51  moves (displaces) the power piston  45  by controlling the electric actuator  36  (the electric motor  37 ) based on the second braking instruction value (the autonomous brake instruction) input from the vehicle data bus  12  serving as the communication network between apparatuses of the vehicle. 
     In other words, the ECU  51  variably controls the braking hydraulic pressure to generate in the master cylinder  21  by driving the electric motor  37  based on the input member position or the autonomous brake instruction to move the power piston  45 . As illustrated in  FIG. 3 , which will be described below, a motor driving circuit  52  and a control signal calculation processing portion  53  are mounted inside the ECU  51 . The ECU  51  supplies an electric current to the electric motor  37  via the motor driving circuit  52  based on a driving signal calculated by the control signal calculation processing portion  53 . 
     Then, when the electric current is supplied from the ECU  51  to the electric motor  37 , the rotational shaft  37 A of the electric motor  37  is rotationally driven. The rotation of the rotational shaft  37 A is slowed down by the speed reduction mechanism  40 , and is converted into the linear displacement of the liner motion member  44  (the displacement in the horizontal direction in  FIG. 2 ) by the rotation-linear motion conversion mechanism  43 . The linear motion member  44  is cylindrical, and contains the power piston  45  therein displaceably in the horizontal direction in  FIG. 2  integrally with the power piston  45 . The second return spring  46  is placed at the distal end side of the power piston  45  between it and the booster housing  31 , and the power piston  45  is biased retractably in the same direction as the linear motion member  44  integrally with the linear motion member  44  when the linear motion member  44  is linearly displaced rightward in  FIG. 2 . 
     The reaction disk  47 , which is the elastic member, is attached at the distal end of the power piston  45  (the inner cylindrical member  45 B thereof), and the displacement of the power piston  45  is transmitted to the primary piston  23  of the master cylinder  21  via the reaction disk  47 . The reaction disk  47  combines the thrust forces of the input member  32  and the power piston  45 , and transmits them to the primary piston  23  of the master cylinder  21 . Along therewith, the reaction disk  47  distributes the reaction force from the primary piston  23  derived from the brake hydraulic pressure generated in the master cylinder  21  to the input member  32  and the power piston  45 . 
     In  FIG. 2 , the primary piston  23  does not block the route for supplying the brake fluid that connects the reservoir  29  and the master cylinder  21  to each other, and the hydraulic pressure is not generated inside the master cylinder  21  (the hydraulic chambers  25  and  26 ). From this state, the hydraulic pressure can be generated in the master cylinder  21  by driving the electric motor  37 , displacing the primary piston  23  leftward in  FIG. 2 , blocking the route for supplying the brake fluid that connects the reservoir  29  and the master cylinder  21  to each other, and further displacing the primary piston  23 . 
     The power piston  45  has the cylindrical shape as a whole, and the input member  32  is inserted through inside the power piston  45 . The input member  32  is installed slidably independently of the displacement of the power piston  45  relative to this power piston  45 , and contactably at the distal end thereof with the reaction disk  47 . In addition, the steps for limiting the displacement relative to the input member  32  (i.e., the other-side step X 2  and the one-side step X 3 ) are provided at portions inside the power piston  45  that slide on the input member  32 . For example, when the driver presses the brake pedal  6  without the electric motor  37  driven, the input member  32  is advanced, and the piston main body  34 A of the input piston  34  is brought into abutment with the other-side step X 2  (the side surface on the flange portion  45 B 1 ) of the inner cylindrical member  45 B of the power piston  45 . 
     As a result, the power piston  45  is separated from the linear motion member  44  and is advanced together with the input member  32 , thereby being able to generate the hydraulic pressure in the master cylinder  21 . On the other hand, when the power piston  45  is thrust forward by driving the electric motor  37  without the brake pedal  6  pressed by the driver, the one-side step X 3  (the end surface of the cylindrical portion  45 C 2 ) of the annular member  45 C of the power piston  45  is brought into abutment with the piston main body  34 A of the input piston  34 . As a result, the input member  32  is thrust forward integrally with the power piston  45 . 
     Further, the first return spring  35 , which serves as an input spring, is provided between the input member  32  (the input rod  33 ) and the power piston  45  or the linear motion member  44  (between the input rod  33  and the power piston  45  in  FIG. 2 ). A load of the first return spring  35  varies according to the relative position between the input rod  33  of the input member  32  and the power piston  45 . The first return spring  35  is mounted so as to apply a load to the input member  32  in a direction for returning the brake pedal  6  to an initial position (a direction for axially separating the input rod  33  and the power piston  45  away from each other). 
     Next,  FIG. 3  illustrates a configuration and signals regarding the operation for generating the hydraulic pressure by the electric booster  30 , and processing performed by the control signal calculation processing portion  53  inside the electric booster ECU  51 . 
     As illustrated in  FIG. 3 , the ECU  51  of the electric booster  30  includes the motor driving circuit  52  and the control signal calculation processing portion  53 . The motor driving circuit  52  controls the electric current to supply to the electric motor  37  based on the driving signal output from the control signal calculation processing portion  53  (an electric current feedback control portion  62 , which will be described below), by which the rotation of the electric motor  37  is controlled. The rotation of the electric motor  37  (the rotational shaft  37 A) is slowed down by the speed reduction mechanism  40  and is also converted into the linear displacement by the rotation-linear motion conversion mechanism  43 , thereby causing the power piston  45 , which is the assist member, to be linearly displaced axially (the horizontal direction in  FIG. 2 ). 
     At this time, the electric current supplied to the electric motor  37  (the electric current flowing to the coil) is detected by an electric current sensor  52 A provided at the motor driving circuit  52  of the ECU  51 . Further, the rotational angle of the rotational shaft  37 A of the electric motor  37  (i.e., the motor rotational position) is detected by the angle sensor  39 . In this case, the displacement amount (the movement amount) of the power piston  45  can be calculated by using the rotational angle detected by the angle sensor  39 , the speed reduction ratio of the speed reduction mechanism  40 , and the linear displacement amount per unit rotational angle of the rotation-linear motion conversion mechanism  43 . The control signal calculation processing portion  53  of the ECU  51  can perform control in such a manner that the displacement amount of the power piston  45  matches a predetermined displacement amount, i.e., the power piston  45  is displaced to a predetermined position by calculating the driving signal with use of, for example, a known feedback control technique. The detected angle may be the rotational angle after the speed is slowed down instead of the rotational angle of the rotational shaft  37 A (the rotor). Further, the displacement sensor that directly detects the linear displacement of the power piston  45  may be used instead of the angle sensor  39 . 
     As illustrated in  FIG. 3 , the control signal calculation processing portion  53  of the ECU  51  includes a brake operation input portion  54 , a relative displacement amount calculation processing portion  55 , an addition portion  56 , an autonomous brake instruction calculation processing portion  57 , a selection portion  58 , an angle input portion  59 , a position feedback control portion  60 , an electric current input portion  61 , and the electric current feedback control portion  62 . An input side of the brake operation input portion  54  is connected to the brake operation sensor  7 , and an output side thereof is connected to the addition portion  56 . The brake operation input portion  54  amplifies the detection signal output from the brake operation sensor  7 , and also outputs this amplified detection signal to the addition portion  56  as an input member position (a brake pedal operation amount) Xir. 
     The relative displacement amount calculation processing portion  55  functions to calculate, for example, a relative displacement amount ΔXcom, which is a target value of a distance (a relative displacement amount ΔX illustrated in  FIG. 4 ) from a contact surface (a PR contact surface) between the inner cylindrical member  45 B of the power piston  45  and the reaction disk  47  to the distal end surface of the input member  32  (the pressure reception portion  34 B of the input piston  34 ). In other words, the relative displacement amount calculation processing portion  55  sets the relative displacement amount ΔXcom that should be held (maintained) between the PR contact surface and the distal end surface. An output side of the relative displacement amount calculation processing portion  55  is connected to the addition portion  56 , and the relative displacement amount ΔXcom set by the relative displacement amount calculation processing portion  55  is output to the addition portion  56 . The relative displacement amount ΔXcom is a value set so as to be able to acquire a pedal feeling desired for the driver (a control target value), and may be set to a constant value (a fixed value) or may be set to a variable value varying according to a change in a driving situation, such as a change in the vehicle speed. 
     An input side of the addition portion  56  is connected to the brake operation input portion  54  and the relative displacement amount calculation processing portion  55 , and an output side thereof is connected to the selection portion  58 . The addition portion  56  adds the relative displacement amount ΔXcom output from the relative displacement mount calculation processing portion  55  to the input member position Xir output from the brake operation input portion  54 . The addition portion  56  outputs the added value (Xir+ΔXcom) to the selection portion  58  as a “power piston position instruction at the time of the pedal operation”. 
     An input side of the autonomous brake instruction calculation processing portion  57  is connected to the vehicle data bus  12 , and an output side thereof is connected to the selection portion  58 . For example, the autonomous brake instruction output from the ECU  16  via the vehicle data bus  12  is input to the autonomous brake instruction calculation processing portion  57 . The autonomous brake instruction is input to, for example, the autonomous brake instruction calculation processing portion  57  as the hydraulic value to generate in the master cylinder  21 . The autonomous brake instruction calculation processing portion  57  calculates the power piston position corresponding to the input autonomous brake instruction (the hydraulic value) based on, for example, a brake characteristic (characteristic data) indicating a relationship between the hydraulic pressure generated in the master cylinder  21  (the hydraulic value) and the position of the power piston  45 , i.e., a “hydraulic pressure P-power piston position X characteristic”. The brake characteristic of the autonomous brake instruction calculation processing portion  57  is stored in the memory of the ECU  51 . The autonomous brake instruction calculation processing portion  57  outputs the calculated power piston position to the selection portion  58  as a “power piston position instruction at the time of the autonomous brake”. 
     An input side of the selection portion  58  is connected to the addition portion  56  and the autonomous brake instruction calculation processing portion  57 , and an output side thereof is connected to the position feedback control portion  60 . The selection portion  58  compares the “power piston position instruction at the time of the pedal operation” output from the addition portion  56  and the “power piston position instruction at the time of the autonomous brake” output from the autonomous brake instruction calculation processing portion  57 , and also selects a larger one of them. The selection portion  58  outputs the selected position instruction to the position feedback control portion  60  as the “power piston position instruction”. 
     An input side of the angle input portion  59  is connected to the angle sensor  39 , and an output side thereof is connected to the position feedback control portion  60 . The angle input portion  59  amplifies the detection signal output from the angle sensor  39 , and also outputs this detection signal (i.e., the detection signal as a result of detecting the movement position of the power piston  45 ) to the position feedback control portion  60  as an actual power piston position Xpp. 
     An input side of the position feedback control portion  60  is connected to the selection portion  58  and the angle input portion  59 , and an output side thereof is connected to the electric current feedback control portion  62 . Based on the “power piston position instruction” output from the selection portion  58  and the actual power piston position Xpp output from the angle input portion  59 , the position feedback control portion  60  calculates, for example, a difference (a positional difference) between them, and also outputs an electric current instruction to the electric current feedback control portion  62  so as to reduce this difference. 
     An input side of the electric current input portion  61  is connected to the electric current sensor  52 A, and an output side thereof is connected to the electric current feedback control portion  62 . The electric current input portion  61  amplifies the detection signal output from the electric current sensor  52 A (the electric current signal flowing to the electric motor  37 ), and also outputs this detection signal to the electric current feedback control portion  62  as an actual electric current value. 
     An input side of the electric current feedback portion  62  is connected to the position feedback control portion  60  and the electric current input portion  61 , and an output side thereof is connected to the motor driving circuit  52 . Based on the electric current instruction output from the position feedback control portion  60  and the actual electric current (the detection signal) output from the electric current input portion  61 , the electric current feedback control portion  62  outputs the driving signal (i.e., the driving signal for driving the electric motor  37 ) to the motor driving circuit  52  so as to reduce a difference between them. The electric motor  37  is driven (rotated) based on the driving signal output from the motor driving circuit  52 . 
     Next, the processing and the operation of the electric booster  30  for generating the hydraulic pressure in the master cylinder  21  based on the operation performed on the brake pedal  6  by the driver will be described. 
     When there is neither the operation performed on the brake pedal  6  by the driver nor the autonomous brake instruction (the autonomous brake instruction value=0), the electric booster ECU  51  calculates the power piston position instruction serving as the instruction directed to the position of the power piston  45  in the following manner. That is, in this case, the ECU  51  calculates such a power piston position instruction that the power piston  45  maintains the relative displacement between the power piston  45  and the input member  32  so as not to block the route for supplying the brake fluid that connects the reservoir  29  and the master cylinder  21  to each other and so as to prohibit the distal end of the input member  32  (the distal end of the pressure reception portion  34 B of the piston member  34 ) from contacting (abutting against) the reaction disk  47 . Then, the ECU  51  outputs the driving signal to the electric motor  37  so as to maintain this position. 
     More specifically, the detection signal of the brake operation sensor  7  is converted into the input member position Xir by the brake operation input portion  54 . The addition portion  56  adds the relative displacement amount ΔXcom from the power position desired to be maintained to the converted input member position Xir. When the autonomous brake instruction is not input, a value calculated from the addition is selected by the selection portion  58 , and is also input from the selection portion  58  to the position feedback control portion  60  while being set as the “power piston position instruction”. The position feedback control portion  60  calculates the “electric current instruction” in such a manner that the calculated “power piston position instruction” and the “power piston position Xpp” calculated by converting the detection signal of the angle sensor  39  match each other, and outputs the calculated instruction to the electric current feedback control portion  62 . The electric current feedback control portion  62  calculates the motor driving signal in such a manner that the calculated “electric current instruction” and the “electric value” calculated by converting the detection signal of the electric current sensor  52 A match each other. For example, a known feedback control technique can be used for the calculation of such a motor driving signal. 
     Now, the relative displacement amount ΔXcom added to the input member position Xir is calculated by the relative displacement amount calculation processing portion  55 . The relative displacement amount ΔXcom is a value for setting the distance from the contact surface (the PR contact surface) between the power piston  45  (the inner cylindrical member  45 B) and the reaction disk  47  to the distal end of the input member  32  (the pressure reception portion  34 B of the input piston  34 ) as an arbitrary value. More specifically, the relative displacement amount ΔXcom is determined in consideration of relationships between the dimensions of the components forming the electric booster  30  and respective origins of the input member position Xir and the power piston position Xpp recognized by the ECU  51  (the positions where they are in abutment with the booster housing  31 ). 
     In the embodiment, for simplification, assume that the relative displacement amount ΔXcom is the distance itself from the contact surface (the PR contact surface) between the power piston  45  and the reaction disk  47  to the distal end of the input member  32  (the distal end of the input member). Due to this method, the position of the power piston  45  can be displaced so as to keep the distance between the distal end of the input member and the PR contact surface at the arbitrary relative displacement amount ΔX independently of the brake pedal operation amount (i.e., the input member position). Therefore, the power piston  45  can be displaced according to displacing the input member  32  by operating the brake pedal  6 . In this manner, the power piston  45  is displaced by the brake pedal operation, by which the primary piston  23  is moved via the reaction disk  47 . As a result, the route for supplying the brake fluid that connects the reservoir  29  and the master cylinder  21  to each other is blocked, and the hydraulic pressure is generated in the master cylinder  21 . 
     Now, the reaction disk  47  made of the elastic member is little elastically deformed when the hydraulic pressure is not generated in the master cylinder  21  and the force transmitted from the primary piston  23  to the reaction disk  47  via the output rod  48  (i.e., the reaction force P) is small. In this case, the distance between the distal end of the input member  32  (the pressure reception portion  34 B of the input piston  34 ) and the reaction disk  47  is approximately equal to the distance from the contact surface between the power piston  45  and the reaction disk  47  to the distal end of the input member  32  (the pressure reception portion  34 B). 
     However, when the hydraulic pressure is generated inside the master cylinder  21  and a large force starts to be transmitted from the primary piston  23  to the reaction disk  47  via the output rod  48 , the reaction disk  47  is compressed due to the reaction force P illustrated in, for example,  FIG. 4(A)  and is elastically deformed so as to partially bulge to the inside of the power piston  45 . In other words, the reaction disk  47  partially bulges into the power piston  45  so as to reduce the distance toward the distal end of the input member (the pressure reception portion  34 B). 
     Then, when the reaction force P increases according to the increase in the hydraulic pressure in the master cylinder  21  and the deformation amount of the reaction disk  47  increases, the distance between the bulging portion of the reaction disk  47  and the distal end of the input member  32  (the pressure reception portion  34 B) reduces. Further, when the reaction force P increases as illustrated in  FIG. 4(B) , the reaction disk  47  and the distal end of the input member  32  (the distal end surface of the pressure reception portion  34 B) are eventually brought into contact with each other. At this time, the reaction force P transmitted to the reaction disk  47  according to the generated hydraulic pressure stars to be distributed at a ratio between a “contact area between the power piston  45  and the reaction disk  47 ” and a “contact area between the input member  32  and the reaction disk  47 ”, and be transmitted to each of them. 
       FIG. 5  illustrates a relationship between an input rod load applied to the input rod  33  of the input member  32  in the course of the generation of the hydraulic pressure in the master cylinder  21  (i.e., the pressing force on the brake pedal  6 ) and a hydraulic reaction force (a load) applied to the primary piston  23  (the output rod  48 ) that is generated due to the increase in the hydraulic pressure. Next, this relationship will be described with reference to  FIG. 5 . The driver, who operates the brake pedal  6  by pressing it, receives a hydraulic reaction force of zero until the hydraulic pressure is generated in the master cylinder  21 , and the pressing force on the brake pedal  6  (the input rod load) during that is equal to the load f 1  (refer to  FIG. 5 ) of the first return spring  35  determined based on the amount of the displacement relative to the power piston  45 . When the power piston  45  is displaced in the forward direction due to the actuation (the driving) of the electric actuator  36  according to the displacement of the input member  32 , the hydraulic pressure starts to be generated in the master cylinder  21 . However, until the distal end of the input member  32  is brought into contact with the reaction disk  47 , the hydraulic reaction force is not transmitted from the master cylinder  21  to the input member  32 , and therefore the input rod load is kept at the load f 1  exerted by the first return spring  35 . 
     After that, when the hydraulic pressure further increases in the master cylinder  21 , the distal end of the input member  32  is brought into contact with the reaction disk  47 . As a result, the hydraulic reaction force from the master cylinder  21  is distributed to the reaction force (the load) transmitted to the power piston  45  and the reaction force transmitted to the input member  32 , and suddenly rises to a reaction force value P 1  as indicated by a characteristic line  49  illustrated in  FIG. 5 . At this time, while the input rod load on a horizontal axis is kept at the load f 1 , the hydraulic reaction force on a vertical axis rises to the reaction force value P 1 . 
     In the characteristic line  49  illustrated in  FIG. 5 , the hydraulic reaction force on the vertical axis corresponds to a deceleration of the vehicle, and the input rod load is proportional to the pressing force on the brake pedal  6 . Therefore, this characteristic is felt by the driver as a characteristic that raises the deceleration of the vehicle while keeping the pedal pressing force at the beginning of pressing the brake pedal  6  (the load f 1 ) (a jump-in characteristic). This jump-in characteristic is a characteristic appearing when the vehicle starts to be braked (the deceleration is started), and therefore is desired to be kept constant on the same vehicle. 
     A jump-in hydraulic pressure that creates this jump-in characteristic is a hydraulic pressure when the reaction disk  47  and the input member  32  are brought into contact with each other (the reaction force value P 1 ), and is changed according to not only a deformation characteristic (a characteristic according to the elastic deformation) of the reaction disk  47  but also the relative displacement amount ΔX between the input member  32  and the power piston  45 . Therefore, it becomes possible to intentionally change the jump-in characteristic by intentionally changing the relative displacement amount ΔX depending on the vehicle. 
     However, the relative displacement amount ΔX is calculated based on the position of the power piston  45  (the power piston position) calculated by converting the value detected by the angle sensor  39  and the position of the input member  32  (the input member position) calculated by converting the value detected by the brake operation sensor  7 . Therefore, the calculated relative displacement amount may contain an error from the actual relative displacement amount due to a mechanical tolerance or a sensor error. Then, such an error may prohibit the intended relative displacement amount ΔX from being realized, thereby causing an unintended change in the jump-in characteristic. 
       FIG. 6  illustrates a change in the relationship between the input rod load and the hydraulic reaction force (the change in the jump-in characteristic) due to the error in the relative displacement amount ΔX. For example, when the actual relative displacement amount is larger than the relative displacement amount recognized by the ECU  51  due to the mechanical tolerance, the sensor error, or the like, the jump-in hydraulic pressure increases. In other words, when the distance between the distal end of the input member  32  and the reaction disk  47  is large, a greater hydraulic reaction force is required to allow them to contact each other, and therefore the jump-in hydraulic pressure exceeds the reaction force value P 1  as represented by a characteristic line  49 A indicated by a broken line in  FIG. 6 . On the other hand, when the actual relative displacement amount is smaller than the relative displacement amount recognized by the ECU  51 , the jump-in hydraulic pressure reduces. In other words, when the distance between the distal end of the input member  32  and the reaction disk  47  is small, a smaller hydraulic reaction force is required to allow them to contact each other, and therefore the jump-in hydraulic pressure falls below the reaction force value P 1  as represented by a characteristic line  49 B indicated by a broken line in  FIG. 6 . 
     Therefore, in the first embodiment, the relative position between the input member  32  and the power piston  45  is measured and the error due to the sensor error or the mechanical tolerance is estimated to prevent such an unintended change in the jump-in characteristic (the brake characteristic). Then, the relative position (the relative displacement amount) ΔXcom for determining the movement amount of the power piston  45  with respect to the operation amount of the input member  32  is corrected based on a result of this estimation (an estimated error). 
     More specifically, the ECU  51  controls the electric actuator  36  (the electric motor  37 ) while estimating the error due to the sensor error or the mechanical tolerance by the relative displacement amount calculation processing portion  55 , and correcting the relative position between the input member  32  and the power piston  45  (the relative displacement amount ΔXcom) based on this estimated error. This correction may be made to the position of the input member  32  (the input member position) that is detected by the brake operation sensor  7  or may be made to the position of the power piston  45  (the power piston position) that is detected by the angle sensor  39 . 
       FIG. 7  illustrates the relative displacement amount calculation processing portion  55  according to the embodiment. The relative displacement amount calculation processing portion  55  includes a base relative displacement amount calculation processing portion  63 , a relative displacement correction amount calculation processing portion  64 , and an addition portion  65 . The base relative displacement amount calculation processing portion  63  calculates a base value of the relative displacement amount as a base relative displacement amount ΔXcom.base. The base relative displacement amount ΔXcom.base is a value set from, for example, a calculation, an experiment, a test, or a simulation. The basic relative displacement amount calculation processing portion  63  may output a constant value as the base relative displacement amount ΔXcom.base, or may output a value variable according to the displacement amount or the displacement speed of the input member  32 , the hydraulic value generated in the master cylinder  21 , the deceleration of the vehicle, the vehicle speed, or the like. In this case, the hydraulic value, the deceleration of the vehicle, and the vehicle speed may be acquired by providing a sensor for detecting them to the ECU  51  (directly connecting the sensor and the ECU  51  to each other). Further, a signal transmitted from the ECU (for example, the ECU  10 ) of the other vehicle system connected via the vehicle data bus  12  may be used. 
     In this manner, the base relative displacement amount ΔXcom.base calculated by the basic relative displacement amount calculation processing portion  63  is output from the basic relative displacement amount calculation processing portion  63  to the addition portion  65 . The addition portion  65  adds a relative displacement correction amount ΔXcor calculated by the relative displacement correction amount calculation processing portion  64  to the base relative displacement amount ΔXcom.base, and outputs a result of this addition as the relative displacement amount ΔXcom. 
     Next, the relative displacement correction amount ΔXcor calculated by the relative displacement correction amount calculation processing portion  64  will be described with reference to operation diagrams of  FIG. 8  and a chronological characteristic line diagram of  FIG. 9 .  FIG. 8  each illustrate a schematic (simple) half portion of a layout of the components of the electric booster  30  illustrated in  FIG. 2 . 
       FIG. 8  illustrate states in which the power piston  45  is thrust toward the master cylinder  21  side by driving the electric motor  37  without operating the brake pedal  6  as three stages in order from the top. The top diagram among  FIG. 8 , “(A) WAITING STATE” indicates a state in which the brake pedal  6  is not operated by the driver and the autonomous brake instruction is neither input from the vehicle data bus  12 , i.e., a state waiting for the pressing of the brake pedal  6  and the autonomous brake instruction. This waiting state is a state in which the linear motion member  44  (the power piston  45 ) is held at a waiting position, which is a predetermined position, by driving the electric motor  37 . The waiting state corresponds to such a state that, for example, the vehicle is powered on (an ignition switch is switched on), by which the actuation of the electric booster  30  including the ECU  51  is completed. 
     More specifically, when the vehicle is powered off, the linear motion member  44  is in an initial state (in an abutment state or at an origin) in abutment with the stopper member  31 D of the booster housing  31  integrally with the power piston  45  based on the elastic force of the second return spring  46 . The waiting state is a state in which the power piston  45  is thrust forward (advanced) by actuating the ECU  51  by a predetermined amount and the linear motion member  44  and the stopper member  31 D are separated from each other by a predetermined amount from this initial state. This separation by the predetermine amount is set with the aim of avoiding undershooting of the actual position with respect to the control instruction and collision between the linear motion member  44  and the stopper member  31 D when a return of the power piston  45  to the waiting state is abruptly attempted, such as when the driver suddenly releases the brake pedal  6 . 
     Now, assume that a position at which the input member  32  is in abutment with the booster housing  31  (the stopper pieces  31 D 1  of the stopper member  31 D) and cannot be retracted more than that is detected as an origin (0) with respect to the input member position Xir recognized by the control signal calculation processing portion  53 . Further, assume that a position at which the power piston  45  (more specifically, the linear motion member  44  together with the power piston  45 ) is in abutment with the booster housing  31  (the side surface of the stopper member  31 D) and cannot be retracted more than that is detected as an origin (0) with respect to the power piston position Xpp recognized by the control signal calculation processing portion  53 . A position in the waiting state (a waiting position) is set to a value larger than zero in the embodiment, but the waiting state may be set to zero. Further, in this case, the relative displacement amount ΔX, which corresponds to the distance from the distal end of the power piston  45  (more specifically, the storage surface of the reaction disk  47 ) to the distal end of the input member  32 , can be calculated from the following equation, an equation 1. In this equation, Crd represents a value specific to the apparatus that is acquired from the dimension of the component forming the electric booster  30 , and a design value out of consideration of the tolerance can be used as the dimension of the component. 
       Δ X=Xpp−Xir+Crd   [EQUATION 1]
 
     When the electric motor  37  is driven and the power piston  45  is linearly moved (thrust forward) from this state, the power piston  45  and the input member  32  are brought into abutment with the step for limiting the relative displacement as illustrated in “(B) ABUT AGAINST STEP” at the middle among  FIG. 8 . In other words, the one-side step X 3  of the power piston  45  (the end surface of the cylindrical portion  45 C 2  of the annular member  45 C) is brought into abutment with one end edge of the piston main body  34 A of the input member  32 . After that, when the electric motor  37  is further driven and the power piston  45  is thrust forward, the input member  32  is linearly moved (thrust forward) integrally with the power piston  45  with the power piston  45  and the input member  32  kept in abutment with the step as illustrated in “(C) FURTHER THRUST FORWARD” at the bottom among  FIG. 8 . In this state, i.e., the state in which the input member  32  is linearly moved integrally with the power piston  45 , the relationship between the power piston position Xpp and the input member position Xir ideally satisfies the following equation, an equation 2. In this equation, Cgap 1  is a value specific to the apparatus that is acquired from the dimension of the component forming the electric booster  30 . 
         Xpp−Xir=C gap1  [Equation 2]
 
       FIG. 9  illustrates temporal changes in the input member position Xir detected by the brake operation sensor  7  and the power piston position Xpp detected by the angle sensor  39  when the electric booster  30  operates from the state illustrated at the top to the state illustrated at the bottom among  FIG. 8 . As described above, at time 0 (the state illustrated at the top among  FIG. 8 ), the power piston position Xpp is larger than zero, and the input member position Xir matches zero. After that, as the power piston position Xpp increases (the power piston  45  is thrust forward), at time t 1 , the power piston  45  and the input member  32  are brought into abutment with each other, and, after that, the input member position Xir increases according to the increase in the power piston position Xpp. Ideally, the input member position Xir increases in such a manner that the power piston  45  and the input member  32  are in abutment with each other when Xpp reaches “Xpp=Cgap 1 ”, and keeps “Xir=Xpp−Cgap 1 ” after that as indicated by the above-described equation 2. 
     However, the detection error due to the sensor error or the like is contained in the detected input member position Xir and power piston position Xpp, and Cgap 1  is also different from the actual value due to the component tolerance or the like. These errors create an error in the relative displacement amount ΔX indicated in the equation 1, and this error may lead to the unintended error (the change) in the brake characteristic (the jump-in characteristic). 
     Therefore, in the embodiment, the relative displacement correction amount calculation processing portion  64  calculates the relative displacement correction amount ΔXcor with use of the power piston position Xpp and the input member position Xir detected at the time of the operation illustrated in  FIG. 8 . In this case, the relative displacement correction amount calculation processing portion  64  calculates an input member position Xir.ideal in the ideal state from the following equation, an equation 3 based on the above-described equation 1 with use of the power piston position Xpp detected when the power piston  45  is linearly moved (thrust forward). 
         Xir .ideal= Xpp−C gap1  [Equation 3]
 
     In this equation, the value calculated with use of the component design value out of consideration of the tolerance can be used as Cgap 1 . A difference between the input member position Xir.ideal in the ideal state that is calculated from the above-described equation 3 and the detected input member position Xir can be calculated from the following equation, an equation 4 as a detection error Xerr 1 . 
         Xerr 1= Xir−Xir -ideal  [Equation 4]
 
       FIG. 10  illustrates a relationship between the input member position Xir and the detection error Xerr 1  that are detected and calculated in  FIG. 9 , respectively. As illustrated in this diagram,  FIG. 10 , it is considered that, when the detection error Xerr 1  has a positive value, the “detected input member position Xir has a larger value than the actual input member position” or the “detected power piston position Xpp has a smaller value than the actual power piston position”. In either case, it is considered that the relative displacement mount ΔX calculated with use of this detected value has a larger value than the actual relative displacement amount. Therefore, a result of inverting the sign of this calculated detection error Xerr 1  is set as ΔXcor, and is calculated from the following equation, an equation 5. 
       Δ Xcor=−Xerr 1  [Equation 5]
 
     As illustrated in  FIG. 7 , the relative displacement correction amount calculation processing portion  64  outputs ΔXcor calculated from this equation 5 to the addition portion  65  as the relative displacement correction amount ΔXcor. In other words, the relative displacement amount calculation processing portion  55  adds the relative displacement correction amount ΔXcor to the base relative displacement amount ΔXcom.base, and determines that this added value is the relative displacement amount ΔXcom. This method allows the actual relative displacement amount to approach the base relative displacement amount ΔXcom.base. 
     The calculated detection error Xerr 1  is directly used as the correction amount ΔXcor in the first embodiment, but an average of results of measuring it a plurality of times may be used as the correction amount. Alternatively, only a part of them may be employed according to a tendency of a variation calculated based on a result of an actual dimension of the manufactured component. For example, a maximum value may be imposed on the detection error Xerr 1  (a maximum value of the correction amount may be set). Further, the characteristic with respect to the input member position Xir may be expressed as a function approximation using a polynomial as illustrated in  FIG. 10 , or may be used as a processed constant value, such as a calculated maximum value, minimum value, and average value. Further, the electric booster  30  has been described referring to the example in which the power piston position Xpp increases in the first embodiment, but a similar method can be applied even in a case where the power piston position Xpp reduces. 
     Further, in the first embodiment, the operation for calculating the correction amount ΔXcor (the operation illustrated in  FIG. 8 ) should be performed without the brake pedal  6  pressed by the driver. Further, the hydraulic pressure is generated in the master cylinder  21  as a result of the operation, and therefore the operation should be performed when receiving a braking instruction independent of the drivers&#39; brake pedal operation (for example, the autonomous brake instruction) from the other ECU using the vehicle data bus  12 , which is the communication net between vehicle ECUs, and thrusting only the power piston  45  forward based thereon to provide the braking force. Further, the operation should be performed when the driver does not operate the brake pedal  6  while the vehicle is stopped. 
     In any case, in the first embodiment, the electric actuator  36  (the electric motor  37 ) is subjected to a mechanical limitation on the displacement of the input member  32  relative to the power piston  45 . More specifically, the relative displacement between the input member  32  and the power piston  45  is mechanically limited due to the abutment between the one-side step X 3  of the power piston  45  (the side surface of the cylindrical portion  45 C 2  of the annular member  45 C) and the one end edge of the piston main body  34 A of the input member  32  when the electric actuator  36  is driven. On the other hand, the ECU  51  advances/retracts the power piston  45  independently of the movement of the input member  32 , and determines the abutment state between the input member  32  and the power piston  45  under the mechanical limitation based on the detected relative position. Then, the ECU  51  corrects the relative position between the input member  32  and the power piston  45  based on this determination to control the electric actuator  36  (the electric motor  37 ). In this case, the ECU  51  determines that the input member  32  and the power piston  45  are brought into abutment with each other under the mechanical limitation and the input member  32  is moved based on the detected relative position when thrusting the power piston  45  forward independently of the movement of the input member  32 , and corrects the relative position based on the detected value at this time to control the electric actuator  36  (the electric motor  37 ). 
     In this manner, according to the first embodiment, the ECU  51  advances/retracts the power piston  45  independently of the movement of the input member  32 , and determines the abutment state between the input member  32  and the power piston  45  under the mechanical limitation based on the detected relative position. Due to this determination, the ECU  51  can use, for example, the state in which the input member  32  and the power piston  45  are in abutment with each other under the mechanical limitation as a reference (a reference for estimating the error). Then, the ECU  51  corrects the relative position between the input member  32  and the power piston  45  to control the electric actuator  36  (the electric motor  37 ) based on this reference and thus the estimated error. Therefore, the electric booster  30  can prevent the change in the brake characteristic regardless of the error due to the sensor error or the mechanical tolerance. In other words, the electric booster  30  can prevent the brake characteristic (for example, the jump-in characteristic) from deviating from the desired brake characteristic and acquire the desired brake characteristic regardless of the error due to the sensor error or the mechanical tolerance. 
     In addition, according to the first embodiment, the electric booster  30  can detect that the input member  32  and the power piston  45  are brought into abutment with each other and the input member  32  is moved under the mechanical limitation, and also use this detected value as the reference of the relative position (the reference for estimating the error). Therefore, the electric booster  30  can prevent the change in the brake characteristic by correcting the relative position based on this reference (detected value) to control the electric actuator  36  (the electric motor  37 ). 
     Next,  FIGS. 11 and 12  illustrate a second embodiment. The second embodiment is characterized by being configured to determine the separation and the connection due to the abutment between the assist member and the input member according to a change in the electric current and also correct the relative position based on the relative position at this time. The second embodiment will be described, indicating similar components to the first embodiment by the same reference numerals and omitting descriptions thereof. 
     In the second embodiment, the relative displacement correction amount calculation processing portion  64  (refer to  FIG. 7 ) also corrects the relative displacement correction amount ΔXcor, similarly to the first embodiment. The relative displacement correction amount ΔXcor according to the second embodiment will be described with reference to operation diagrams of  FIG. 11  and chronological characteristic line diagrams of  FIG. 12 . In this case, the operation diagrams of  FIG. 11  illustrate states in which the power piston  45  is thrust toward the master cylinder  21  side by driving the electric motor  37  without operating the brake pedal  6  as three stages in order from the top, similarly to  FIG. 8  according to the above-described first embodiment. Further, in the second embodiment, the assist member is formed by the power piston  45  and the linear motion member  44 . 
     The second embodiment is constructed assuming that the electric booster  30  operates as illustrated in  FIG. 11 . More specifically, as illustrated in the top view among  FIG. 11 , when the electric motor  37  is driven in an opposite direction (a retraction direction opposite from the advance direction) to linearly move the power piston  45  in the retraction direction, the power piston  45  is brought into abutment with the input member  32  before the linear motion member  44  is prohibited from being retracted by abutting against the stopper member  31 D of the booster housing  31 . In this case, the power piston  45  is brought into abutment with the input member  32  prohibited from being retracted in abutment with the stopper member  31 D (the stopper pieces  31 D 1  thereof). This prohibits the power piston  45  from being retracted. Further, the electric booster  30  is assumed to be configured in such a manner that the linear motion member  44  is separated from the power piston  45  (the flange portion  44 A of the linear motion member  44  is separated from the flange portion  45 C 1  of the annular member  45 C of the power piston  45 ) and is slidably moved by continuously driving the electric motor  37  in the opposite direction from the state in which the power piston  45  is prohibited from being retracted. Further, at this time, the spring force of the second return spring  46  biased between the booster housing  31  and the power piston  45  is greater than the spring force of the first return spring  35  biased between the power piston  45  and the input member  32 . Therefore, the second embodiment is constructed assuming that the power piston  45  is pressed against the booster housing  31  via the input member  32  in the retraction direction. 
     When the electric motor  37  is driven in the advance direction (the forward direction) to linearly move the linear motion member  44  from “(A) RETRACTED STATE (MAXIMUMLY RETRACTED POSITION OR SEPARATED STATE) illustrated at the top among  FIG. 11 , the linear motion member  44  and the power piston  45  are brought into abutment with each other as illustrated in “(B) CONNECTED STATE” at the middle among  FIG. 11 . In other words, the flange portion  44 A of the linear motion member  44  is brought into abutment with the annular member  45 C (the flange portion  45 C 1 ) of the power piston  45 . After that, when the electric motor  37  is further driven in the advance direction, the power piston  45  is linearly moved (thrust forward) integrally with the linear motion member  44  as illustrate in “(C) FURTHER THRUST FORWARD” at the bottom among  FIG. 11 . 
     In the second embodiment, the relative displacement correction amount calculation processing portion  64  calculates the relative displacement correction amount ΔXcor with use of the power piston position Xpp and a motor electric current Im detected at the time of the operation illustrated in  FIG. 11 . In other words,  FIG. 12  illustrate temporal changes in the power piston position Xpp detected by the angle sensor  39  and the motor electric current Im detected by the electric current sensor  52 A when the electric booster  30  operates from the state illustrated at the top to the state illustrated at the bottom among  FIG. 11 . When the electric motor  37  is driven, first, an electric current for linearly moving only the linear motion member  44  is generated, and this electric current is detected by the electric current sensor  52 A. After that, when the linear motion member  44  and the power piston  45  are brought into abutment with each other, the electric motor  37  requires an electric current sufficient to cause both the linear motion member  44  and the power piston  45  to be linearly moved, and compress the second return spring  46  at the same time, and therefore the detected electric current increases. 
     Ideally, the power piston position where the electric current increases as described above is located at a value Cgap 2  determined based on the component dimensions of the linear motion member  44 , the power piston  45 , the input member  32 , and the like, but, actually, is not located at Cgap 2  due to, for example, a variation in the tolerance of the component dimension. To address this inconvenience, the detection error can be calculated from the following equation, an equation 6, assuming that Xpp 2  represents a power piston position detected when the electric current increases as described above, and a detection error Xerr 2  refers to a difference between this power piston position Xpp 2  and Cgap 2 . In this equation, a value calculated with use of the component design value out of consideration of the tolerance can be used as Cgap 2 . 
         Xerr 2= Xpp 2 −C gap2  [Equation 6]
 
     As illustrated in  FIG. 12 , it is considered that, when the calculated detection error Xerr 2  has a positive value, the “detected power piston position Xpp 2  has a smaller value than the actual power piston position” or the “distal end of the power piston  45  is actually located ahead of the design value due to the variation in the tolerance”. In either case, it is considered that the relative displacement mount ΔX calculated with use of this detected value has a larger value than the actual relative displacement amount. Therefore, a result of inverting the sign of this calculated detection error Xerr 2  is set as ΔXcor, and is calculated from the following equation, an equation 7. 
       Δ Xcor=−Xerr 2  [Equation 7]
 
     The relative displacement correction amount calculation processing portion  64  according to the second embodiment outputs ΔXcor calculated from this equation 7 to the addition portion  65  as the relative displacement correction amount ΔXcor. More specifically, the relative displacement amount calculation processing portion  55  adds the relative displacement correction amount ΔXcor to the base relative displacement amount ΔXcom.base, and sets this added value as the relative displacement amount ΔXcom. This method allows the actual relative displacement amount to approach the base relative displacement amount ΔXcom.base. 
     The calculated detection error Xerr 2  is directly used as the correction amount ΔXcor in the second embodiment, but an average of results of measuring it a plurality of times may be used as the correction amount. Alternatively, only a part of them may be employed according to the tendency of the variation calculated based on the result of the actual dimension of the manufactured component. For example, a maximum value may be imposed on the detection error Xerr 2  (a maximum value of the correction amount may be set). Further, the electric booster  30  has been described referring to the example in which the power piston position Xpp increases in the second embodiment, but a similar method can be applied even in the case where the power piston position Xpp reduces. 
     Further, in the embodiment, it is desirable that, basically, the operation for calculating the correction amount ΔXcor (the operation illustrated in  FIG. 11 ) is performed without the brake pedal  6  pressed by the driver. However, this shall not apply to when there is a situation that the power piston  45  does not have to be linearly moved by the driving of the motor even with the brake pedal  6  pressed by the driver, such as immediately after the electric booster  30  is actuated. 
     The detection error Xerr 2  when the driver presses the brake pedal  6  can be calculated from the following equation, an equation 8 with use of a power piston position Xpp 2 ′ and an input rod position Xir 2 ′ when the electric current exceeds a threshold value. 
         Xerr 2= Xpp 2′− Xir 2′− C gap2  [Equation 8]
 
     Further, the increase in the motor electric current may be detected by a method of preparing an electric current threshold value Im 2  and determining the increase in the motor electric current based on whether the motor electric current exceeds this threshold value as illustrated in the characteristic line diagram on the lower side of  FIG. 12 . Alternatively, this determination may be made based on an amount of the increase in the electric current per unit time or an amount of the increase in the electric current per unit power piston position. 
     In any case, in the second embodiment, the electric actuator  36  (the electric motor  37 ) is subjected to the mechanical limitation on the displacement of the input member  32  relative to the power piston  45  and the linear motion member  44 . For example, the relative displacement between the input member  32  and the power piston  45  is mechanically limited due to the abutment between the other-side step X 2  of the power piston  45  (one side surface of the flange portion  45 B 1 ) and the other end edge of the piston main body  34 A of the input member  32 . Further, the relative displacement between the linear motion member  44  and the power piston  45  is mechanically limited due to the abutment between the flange portion  45 C 1  of the power piston  45  and the flange portion  44 A of the linear motion member  44 . On the other hand, the ECU  51  advances/retracts the power piston  45  together with the linear motion member  44  independently of the movement of the input member  32 , and determines the abutment state between the input member  32  and the power piston  45  and the linear motion member  44  under the mechanical limitation based on the detected relative position. Then, the ECU  51  corrects the relative position between the input member  32  and the power piston  45  based on this determination to control the electric actuator  36  (the electric motor  37 ). 
     In this case, in the second embodiment, the power piston  45  (i.e., the power piston  45  thrust forward by the electric actuator  36 ) of the electric actuator  36  (the electric motor  37 ) is biased in the retraction direction by the second return spring  46  as the spring. In this case, the power piston  45  and the linear motion member  44  are separated when they are retracted and the power piston  45  is brought into abutment with the input member  32 , and the linear motion member  44  is permitted to be farther retracted than the power piston  45 . In this case, the second return spring  46  is placed between the housing (the motor case  31 A of the booster housing  31 ) of the electric actuator  36  (the electric motor  37 ) and the power piston  45 . 
     On the other hand, the ECU  51  includes the electric current sensor  52 A as a detection portion that detects the electric current increasing in proportion to the torque or the force generated by the electric actuator  36  (the electric motor  37 ). Then, the ECU  51  determines the separation and the connection between the power piston  45  and the linear motion member  44  due to the abutment with the input member  32  based on the detected electric current. The ECU  51  corrects the relative position based on the relative position detected at this time to control the electric actuator  36  (the electric motor  37 ). 
     The second embodiment is configured to determine the separation and the connection between the power piston  45  and the linear motion member  44  based on the motor electric current as described above, and a basic operation thereof is not especially different from the operation performed by the first embodiment. Especially, in the second embodiment, the electric booster  30  can determine the separation and the connection between the power piston  45  and the linear motion member  44  due to the abutment with the input member  32  based on the detected electric current, and also use the relative position detected at this time as a reference (a reference for estimating the error). Therefore, the electric booster  30  can prevent the change in the brake characteristic by correcting the relative position based on this reference (the relative position of the separation and the connection) and thus the estimated error to control the electric actuator  36  (the electric motor  37 ). 
     In the first embodiment, the electric booster  30  has been described referring to the example in which the electric motor  30  is configured to be able to drive the electric motor  37  of the electric booster  30  based on the autonomous brake instruction, i.e., include the autonomous brake function therein. However, the electric booster  30  is not limited thereto, and, for example, the autonomous brake function may be omitted. The same also applies to the second embodiment. 
     In the first embodiment, the electric booster  30  has been described referring to the example in which the rotational motor is employed as the electric motor  37  forming the electric actuator  36 . However, the electric booster  30  is not limited thereto, and may employ, for example, a linearly movable motor (a linear motor) as the electric motor. In other words, various kinds of electric actuators can be employed as the electric actuator (the electric motor) that thrusts the assist member (the power piston or the linear motion member) forward. The same also applies to the second embodiment. 
     Further, each of the embodiments is only an example, and it is apparent that the configurations indicated in the different embodiments can be partially replaced or combined. For example, the electric booster  30  may determine the abutment state by the operation according to the first embodiment and make the correction based on the relative position thereof while being configured according to the second embodiment. In other words, the electric booster  30  may be configured to make both the corrections according to the first embodiment and the second embodiment. 
     Possible configurations as the electric booster based on the above-described embodiments include the following examples. 
     (1) According to a first configuration, an electric booster includes an input member configured to receive transmission of a part of a reaction force from a piston of a master cylinder coupled with a brake pedal, an assist member advanceable and retractable relative to this input member, an electric actuator configured to thrust the assist member forward by the movement of the input member, a reaction force distribution member configured to combine thrust forces of the input member and the assist member to transmit them to the piston of the master cylinder, and distribute the reaction force from the piston to the input member and the assist member, and a control device configured to detect a relative position between the input member and the assist member, and drive and control the electric actuator. The electric actuator is subjected to a mechanical limitation on a displacement of the input member relative to the assist member. The control device moves forward/backward the assist member independently of the movement of the input member and determines an abutment state between the input member and the assist member under the mechanical limitation based on the detected relative position, and corrects the relative position between the input member and the assist member to control the electric actuator. 
     According to this first configuration, the control device advances/retracts the assist member independently of the movement of the input member, and determines the abutment state between the input member and the assist member under the mechanical limitation based on the detected relative position. Due to this determination, the control device can use, for example, the state in which the input member and the assist member are in abutment with each other under the mechanical limitation as a reference. Then, the control device corrects the relative position between the input member and the assist member based on this reference to control the electric actuator. Therefore, the electric booster can prevent the change in the brake characteristic regardless of the error due to the sensor error or the mechanical tolerance. In other words, the electric booster can prevent the brake characteristic from deviating from the desired brake characteristic and acquire the desired brake characteristic regardless of the error due to the sensor error or the mechanical tolerance. 
     (2) According to a second figuration, in the first configuration, when thrusting the assist member forward independently of the movement of the input member, the control device determines that the input member and the assist member are brought into abutment with each other and the input member is moved under the mechanical limitation based on the detected relative position, and corrects the relative position based on the detected value at this time to control the electric actuator. 
     According to this second configuration, the electric booster can detect that the input member and the assist member are brought into abutment with each other and the input member is moved under the mechanical limitation, and also use this detected value as the reference of the relative position. Therefore, the electric booster can prevent the change in the brake characteristic by correcting the relative position based on this reference (detected value) to control the electric actuator. 
     (3) According to a third configuration, in the first configuration or the second configuration, the assist member of the electric actuator is biased in a retraction direction by a spring mounted between the assist member and a housing of the electric actuator, and is separated and permitted to be further retracted when being retracted and brought into abutment with the input member. The control device includes a detection portion configured to detect an electric current increasing in proportion to a torque or a force generated by the electric actuator, and determines the separation/connection of the assist member due to the abutment with the input member based on the detected electric current and corrects the relative position based on the detected value at this time to control the electric actuator. 
     According to this third configuration, the electric booster can determine the separation and the connection of the assist member due to the abutment with the input member based on the detected electric current, and also use the relative position detected at this time as a reference. Therefore, the electric booster can prevent the change in the brake characteristic by correcting the relative position based on this reference (the relative position of the separation and connection) to control the electric actuator. 
     The present invention is not limited to the above-described embodiments, and includes various modifications. For example, the above-described embodiments have been described in detail to facilitate better understanding of the present invention, and the present invention shall not necessarily be limited to the configuration including all of the described features. Further, a part of the configuration of some embodiment can be replaced with the configuration of another embodiment. Further, some embodiment can also be implemented with a configuration of another embodiment added to the configuration of this embodiment. Further, each of the embodiments can also be implemented with another configuration added, deleted, or replaced with respect to a part of the configuration of this embodiment. 
     The present application claims priority under the Paris Convention to Japanese Patent Application No. 2017-183535 filed on Sep. 25, 2017. The entire disclosure of Japanese Patent Application No. 2017-183535 filed on Sep. 25, 2017 including the specification, the claims, the drawings, and the abstract is incorporated herein by reference in its entirety. 
     REFERENCE SIGNS LIST 
     
         
         
           
               4 L,  4 R front wheel-side wheel cylinder (wheel cylinder) 
               5 L,  5 R rear wheel-side wheel cylinder (wheel cylinder) 
               6  brake pedal 
               7  brake operation sensor (operation amount detection device) 
               9  ESC 
               21  master cylinder 
               23  primary piston (piston) 
               30  electric booster 
               32  input member 
               33  input rod 
               34  input piston 
               36  electric actuator 
               37  electric motor 
               39  angle sensor (movement amount detection portion) 
               44  linear motion member (assist member) 
               45  power piston (assist member) 
               46  second return spring (spring) 
               47  reaction disk (reaction force distribution member) 
               48  output rod 
               51  electric booster ECU (control device) 
               52 A electric current sensor (detection portion that detects an electric current) 
               55  relative displacement amount calculation processing portion 
               63  base relative displacement amount calculation processing portion 
               64  relative displacement correction amount calculation processing portion