CN113119934A

CN113119934A - Decoupling type electric brake booster

Info

Publication number: CN113119934A
Application number: CN201911405656.0A
Authority: CN
Inventors: 章秋生
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2019-12-31
Filing date: 2019-12-31
Publication date: 2021-07-16
Also published as: JP2021109649A

Abstract

An electric brake booster for use in a vehicle comprising: a pedal force transmission mechanism configured to be driven by the brake pedal to move in an axial direction; a detection device configured to detect an action of the pedal force transmission mechanism; a brake motor configured to perform a motor braking operation to generate a braking assist force; an assist force transmission mechanism configured to be driven by the brake motor (4) to move in an axial direction so as to transmit a brake assist force generated by the brake motor to a piston of the master cylinder; in the motor brake operating mode, the assistance force transmission mechanism is kinematically decoupled from the pedal force transmission mechanism, and the brake assistance force generated by the brake motor alone forms the input force of the electric brake booster.

Description

Decoupling type electric brake booster

Technical Field

The present application relates to an electric brake booster for use in a vehicle brake system, wherein the brake motor has an independent brake booster transmission line decoupled from the pedal force transmission line.

Background

Many vehicles add a brake booster to their hydraulic braking system. Brake boosters typically include a vacuum booster and an electric booster. The vacuum booster uses the vacuum of the air inlet pipe of the engine as a braking boosting source, and the electric booster uses the motor as the braking boosting source.

In the case of known electric boosters, the brake booster and the pedal force usually have a coupled transmission line, i.e. via a common force transmission element, to the brake master cylinder piston. In such an electric booster, when a driver performs a braking operation, the driver depresses a brake pedal to apply a pedal force, a motor of the electric booster generates a braking assist force, and the pedal force and the braking assist force are transmitted to a master cylinder piston in combination. For vehicles having an automatic braking function (e.g., an automatic driving or active braking module, etc.), the electric motor of the electric booster actively generates braking assistance without driver intervention (i.e., no pedal force input) when the vehicle automatically applies braking. However, when the brake assist force generated by the motor is transmitted to the master cylinder piston, the pedal force transmitting member, and even the brake pedal, is pulled. This may affect the transmission efficiency of the brake assist force and may also give a driver a bad feeling of putting his foot on the brake pedal.

Disclosure of Invention

The present application is directed to an improved electric brake booster in which a brake boosting transmission line of a brake motor is decoupled with respect to a pedal force transmission element when a motor braking operation is performed, so that the pedal force transmission element and a brake pedal are not pulled.

To this end, according to one aspect of the present application, there is provided a decoupled electric brake booster for use in a vehicle braking system, comprising: a pedal force transmission mechanism configured to be driven by the brake pedal to move in an axial direction; a detection device configured to detect an action of the pedal force transmission mechanism; a brake motor configured to perform a motor braking operation to generate a braking assist force; and an assist force transmitting mechanism configured to be driven by the brake motor to move in an axial direction so as to transmit the brake assist force generated by the brake motor to the piston of the master cylinder; wherein, in the motor braking operation, the assistance force transmission mechanism is kinematically decoupled from the pedal force transmission mechanism, and the braking assistance force generated by the braking motor alone constitutes the input force of the electric brake booster.

According to the present application, the brake boosting transmission line of the brake motor of the electric brake booster is decoupled from the pedal force transmission line when the motor braking operation is performed. The action of the brake assist force transmitting element does not pull the pedal force transmitting element and the brake pedal, thus avoiding the driver from experiencing an unpleasant sensation from the brake pedal. Further, when the brake motor is operated, the input force of the electric brake booster is derived only from the brake motor without combining the pedal force.

The decoupling electric brake booster of the present application can replace conventional vacuum boosters.

Drawings

FIG. 1 is a schematic cross-sectional view (through the booster central axis) of a decoupled electric brake booster according to one possible embodiment of the present application;

FIG. 2 is a schematic cross-sectional view of the electric brake booster in another direction (through the booster center axis in a direction perpendicular to FIG. 1);

FIG. 3 is a schematic view for explaining a pedal force transmission line in the electric brake booster;

FIG. 4 is a schematic diagram for explaining a brake assist transmission line in the electric brake booster;

FIGS. 5-7 are schematic views of a drive nut, plunger and pedal spring, respectively, in an electric brake booster;

FIGS. 8 and 9 are schematic views of the constituent elements of one possible embodiment of a pedal spring limit structure in an electric brake booster;

FIG. 10 is a schematic view of another possible embodiment of a pedal spring limit structure in an electric brake booster;

FIG. 11 is a schematic view of an initial stage of a conventional braking operation of the electric brake booster;

FIG. 12 is a schematic illustration of the pressurization phase of a conventional braking operation of the electric brake booster.

Detailed Description

Some possible embodiments of the present application are described below with reference to the drawings. It is to be noted that the drawings are designed solely to embody the principles of the present application and not to represent the actual configuration of the present application. Accordingly, the drawings are not to scale; also, some details are exaggerated and some details are omitted for clarity.

It is first of all pointed out that in the present application, "rear side" means the side which is kinematically close to the brake pedal of the vehicle, and "front side" means the side which is kinematically facing away from the brake pedal, i.e. close to the master cylinder.

As shown in fig. 1 and 2, a decoupling electric brake booster for use in a vehicle brake system according to one possible embodiment of the present application is used to transmit an output force to a piston 2 of a master cylinder 1 of a vehicle hydraulic brake system. Some structural details of the electric brake booster are shown in fig. 3-10. Fig. 1 and 2 show the home position (non-operating state) of the electric brake booster.

The piston 2 is axially movable relative to the cylinder body of the master cylinder 1. The master cylinder 1 and its piston 2 of a hydraulic vehicle brake system are common in the art and will not be described in detail. Note that the master cylinder 1 has a double piston, the piston 2 is shown as a rear chamber (first chamber) piston of the master cylinder 1, and the master cylinder 1 further includes a front chamber (second chamber) piston, not shown.

The electric brake booster of the present application is associated with the brake pedal side presser 3, and includes a brake motor 4 for generating brake assist. The electric brake booster further includes a control unit 5 and a pedal stroke sensor 6. The pedal stroke sensor 6 is for detecting the stroke of the pushing member 3, that is, the stroke of the brake pedal. The control unit 5 is capable of receiving a brake pedal stroke signal detected by the pedal stroke sensor 6, and of controlling the operation of the brake motor 4. The control unit 5 activates the brake motor 4 to generate the brake assist pressure upon receiving the brake signal. The braking signal may be a brake pedal travel signal or a braking signal from an automatic vehicle braking function.

In the case of performing braking with the brake motor 4, only the brake assist force generated by the brake motor 4 is used as the input force of the electric brake booster, which does not include the pedal force from the brake pedal. In the case where braking is performed without using the brake motor 4, for example, the brake motor 4 is not supplied with electric power, or the brake motor 4 is disabled, the pedal force input by the driver through the brake pedal via the pushing member 3 may be used as the input force of the electric brake booster.

The electric brake booster further comprises a transmission sleeve 11 and a drive nut 12, both arranged coaxially with the piston 2 and thus defining a central axis of the electric brake booster. The transmission sleeve 11 is arranged in the drive nut 12 with a threaded transmission fit between the two. The drive nut 12 is rotatably arranged in a booster housing (not shown) by means of a bearing 13 (and possibly other bearings). The brake motor 4 rotates the drive nut 12 via a corresponding transmission mechanism (e.g., a gear train). According to a possible embodiment, in the case of an electrically charged brake motor 4 (motor rotor rotating or in the static state), the brake motor 4 can lock the axial position of the drive nut 12 by means of a transmission (in particular a gear train) between it and the drive nut 12, so that the drive nut 12 cannot move axially. Under the condition that the brake motor 4 is not electrified, as the motor rotor can rotate freely, the axial movement locking function of the brake motor 4 on the drive nut 12 is released, and the drive nut 12 can move axially.

Alternatively, a separate locking structure may be provided. In the case where the brake motor 4 is electrically charged, the locking structure locks the drive nut 12 axially, and thus the drive nut 12 does not have axial displacement capability. And in the case where the brake motor 4 is not charged, the locking structure unlocks the axial movement of the drive nut 12 so that the drive nut 12 can move axially.

In the case of an axially locked drive nut 12, rotation of the drive nut 12 may force the transmission sleeve 11 to move axially in the drive nut 12.

Referring to fig. 5, the drive nut 12 is generally cylindrical with an axial section being a thread section 12a provided with inwardly facing threads for engagement with the external threads of the transmission sleeve 11 (the transmission sleeve 11 may be provided with external threads over its entire length). In addition, the thread section 12a divides the inner bore of the drive nut 12 into a front section 12b and a rear section 12 c. The axial length of the thread segments 12a is about 1/3 or less of the axial length of the drive nut 12. The forward section 12b preferably has a smaller axial length than the rearward section 12 c.

Returning to fig. 1, 2, between the transmission sleeve 11 and the piston 2, a plunger 14 is arranged. The driving force generated by the brake motor 4, i.e. the braking assistance force, can be transmitted to the transmission sleeve 11 via the drive nut 12 and then from the transmission sleeve 11 to the piston 3 via the plunger 14. The plunger 14 and the piston 2 can be directly pushed against each other, so that the plunger 14 can directly transmit the braking assistance to the piston 2. Alternatively, a force transmission element (e.g. a ram) may be arranged between the plunger 14 and the piston 2, such that the plunger 14 transmits the braking assistance force to the piston 2 via the force transmission element. The arrangement of such force transmitting elements may facilitate the distribution of the braking assistance over the piston 2, etc.

Referring to fig. 6, the plunger 14 has a generally cylindrical body 14a, the front end of the body 14a forming a pair of diametrically opposed projecting flanges 14b, and the rear section of the body 14a being reduced in diameter to form a reduced diameter section 14 c. Further, in the body 14a, a radial through hole (cavity) 14d penetrating the body 14a is formed in a radial direction perpendicular to a radial direction in which the flange 14b extends. The front end of the radial through hole 14d terminates near the front end face of the body 14a, and the rear end of the radial through hole 14d communicates with the rear end face of the body 14a through the axial through hole 14 e. A front end wall 14f is present between the front end of the radial through hole 14d and the front end face of the body 14 a.

The front end of the push rod 15 is connected with a pedal spring 16. The pedal spring 16 may include a combination spring in which spring pieces are stacked such that the spring rate of the pedal spring 16 in the axial direction gradually increases from the longitudinal (radially) outer end toward the radial center. For example, referring to fig. 7, the pedal spring 16 includes

spring pieces

16a, 16b, 16c stacked in order in the axial direction. The radial dimension of each spring piece is gradually reduced from the axial front side to the axial rear side.

Returning to fig. 1 and 2, the front end of the plunger 14 is configured and adapted to push against the piston 2 (directly or indirectly). The small diameter section 14c of the plunger 14 is inserted into the front section 12b of the drive nut 12 such that the plunger 14 is axially movable relative to the drive nut 12.

A push rod 15 is arranged in the axial through hole defined by the transmission sleeve 11. The front end of the pushing component 3 is connected with a push rod 15. The front portion of the push rod 15 protrudes from the front end of the transmission sleeve 11, and is inserted into the radial through hole 14d through the axial through hole 14e of the plunger 14. The forward end of the push rod 15 is also connected to a reaction plate 17 located forward of the pedal spring 16. The pedal spring 16 and the reaction plate 17 are exposed from both sides in the radial direction of the plunger 14 through the radial through hole 14d of the plunger 14. In the home position, the reaction plate 17 is located axially rearward of the front end wall 14f of the plunger 14 and is spaced an axial distance from the front end wall 14 f.

The pedal spring 16 is connected at its radial ends to respective limit rods 18. The stopper rod 18 extends parallel to the booster center axis and is restrained by a stopper plate 19 arranged around the small diameter section 14c of the plunger 14, so that the stopper rod 18 can move back and forth over a limited axial distance with respect to the stopper plate 19. In the example shown in fig. 8, the front end of the stopper rod 18 is fixed to the longitudinally outer end of the pedal spring 16, and the rear portion of the stopper rod 18 is formed with a thin rod portion 18a, which thin rod portion 18a is inserted into a through hole 19a in the corresponding radial end portion of the stopper plate 19 shown in fig. 9 and is axially movable in the through hole 19 a. When the pedal spring 16 is axially elastically deformed by the push rod 15, the axial movement range of the stopper rod 18, that is, the axial movement range of both ends in the radial direction of the pedal spring 16, is limited by the abutment between the large-diameter portions of both ends in the axial direction of the thin rod portion 18a and the end surfaces of both sides in the axial direction of the stopper plate 19. Therefore, the stopper rod 18 and the stopper plate 19 constitute a stopper structure of the pedal spring 16.

It will be appreciated that other forms of pedal spring limiting elements may be employed. For example, fig. 10 shows another stopper structure of the pedal spring 16, in which a front end of a stopper rod 18 is hinged to a longitudinally outer end of the pedal spring 16, and a rear end of the stopper rod 18 is provided with a hinge shaft inserted into a guide groove 19a extending parallel to an axial direction in a stopper plate 19 so that the hinge shaft can move back and forth in the guide groove 19 a. In situ, the radial distance between the front ends of the two restraint rods 18 and the radial distance between the rear ends of the two restraint rods 18 may be different or the same. When the push rod 15 pushes the pedal spring 16 forward, the rear hinge shaft of the stopper rod 18 slides forward in the guide groove 19 a. When the rear end hinge shaft of the stopper rod 18 reaches the front groove bottom of the guide groove 19a, the rear end of the stopper rod 18 is restrained from further advancing, and therefore, the both ends in the radial direction of the pedal spring 16 cannot further advance. As the push rod 15 pushes the middle portion of the pedal spring 16 forward, the pedal spring 16 starts to elastically deform in the axial direction.

The retainer plate 19 is non-rotatable and remains in a constant axial position with the drive nut 12. In other words, the limit plate 19 is axially movable with the drive nut 12, but is not rotatable. The above-described capability of the stopper plate 19 can be achieved by an appropriate holding structure. For example, in the illustrated example, the stopper plate 19 is fixed to the outer ring of the bearing 13.

Spring pressing pieces 20 for pressing first return springs 21 are installed on the front sides of both ends in the radial direction of the reaction plate 17. The first return spring 21 serves to urge the push rod 15 axially rearward via the spring pressure piece 20, the reaction plate 17, and the pedal spring 16. The first return spring 21 may be installed between the cylinder body of the master cylinder 1 and the spring pressure block 20.

Further, a second return spring 22 is interposed between the cylinder body of the brake master cylinder 1 and the flange 14b of the plunger 14 for urging the plunger 14 axially rearward. The second return spring 22 has a spring rate greater than that of the first return spring 21.

It is understood that the first return spring 21 and the second return spring 22 may be disposed at other positions according to the internal structure of the booster. In the illustrated example, both are in the form of helical compression springs, but other types of springs may be employed as long as the above-described pressing functions of both can be achieved. For example, in the example shown in fig. 10, the first return spring 21 is a 3D wire spring, one end of which is hooked to the longitudinally outer end of the pedal spring 16, and the other end of which is fixed to the stopper plate 19. It will be appreciated that the second return spring 22 may also take the form of a 3D wire spring.

Fig. 3 schematically depicts a pedal force transmission mechanism (line) in the electric brake booster, in which pedal force can be transmitted to the pedal spring 16, the reaction plate 17 via the push piece 3, the push rod 15, so that the pedal spring 16, the reaction plate 17 can move forward against the first return spring 21. A radially extending section 17a is mounted or formed at one end of the reaction plate 17 in the radial direction, the radially extending section 17a facing the pedal stroke sensor 6, so that the axial movement of the radially extending section 17a can be detected by the pedal stroke sensor 6 (e.g. by a varying magnetic field), whereby the control unit 5 can determine the pedal stroke, and thus the braking intention of the driver.

Fig. 4 schematically depicts a brake assist transmission mechanism (line) in the electric brake booster, in which the driving force (brake assist) of the brake motor 4 is transmitted to the plunger 14 through the drive nut 12, the transmission sleeve 11, so that the plunger 14 is moved forward against the urging force of the second return spring 22, and the plunger 14 transmits the brake assist generated by the brake motor 4 to the piston 2 as the output force of the electric brake booster.

It is to be noted that in the example shown, the braking assistance is transmitted mainly through the drive nut 12 and the transmission sleeve 11; however, it is understood that other mechanisms for converting rotational motion into linear motion, such as rack and pinion mechanisms and the like, may be used herein to transmit braking assistance.

Furthermore, it should be noted that the plunger 14, the push rod 15, the pedal spring 16, the reaction plate 17 are only allowed to move axially, but not to rotate, and their rotation prevention can be achieved by suitable constraint structures, for example by a limit plate 19 (non-rotatable) through the rotation prevention constraint of the pedal spring 16 by a limit rod 18.

The operation of the electric brake booster is described below.

The normal operation mode of the electric brake booster is first described. The so-called normal operation is that the electric brake booster is started based on the action of the driver depressing the brake pedal, and the brake assist is provided by the brake motor 4. As shown in fig. 11, after the driver depresses the brake pedal, the pushing member 3 pushes the pedal spring 16 and the reaction plate 17 axially forward by a lost motion via the push rod 15. The distance of the idle stroke depends on the limit of the limit plate 19 to the limit rod 18. For example, the idle stroke is about 5mm or less. The control unit 5 detects the stroke through the pedal stroke sensor 6, thereby confirming that the driver depresses the brake pedal, thereby activating the brake motor 4 to rotate in the forward direction to drive the plunger 14 to move forward by a determined distance. This section of the plunger 14 moves forward, causing the piston 2 to move forward into the cylinder of the master cylinder 1, thereby initially building up pressure in the cylinder of the master cylinder 1.

After the idle stroke is finished, if the driver further steps on the brake pedal, the stopper rod 18 is restricted by the stopper rod 19 and cannot move any further, both ends of the pedal spring 16 are restricted by the stopper rod 18 and cannot move further axially forward, the pedal spring 16 starts to deform axially, and as shown in fig. 12, the pedal spring 16 generates a gradually increasing reaction force transmitted to the brake pedal, and the reaction plate 17 moves further forward. The control unit 5 detects the further forward movement by the pedal stroke sensor 6 and thus determines the driver's braking intention, thereby controlling the brake motor 4 to rotate in the forward direction to drive the plunger 14 to move further forward by a determined distance. This section of the plunger 14 moves forward, causing the piston 2 to move further forward into the cylinder of the master cylinder 1, so that the brake fluid pressure of the master cylinder 1 increases abruptly, i.e., the pressure jumps. The master cylinder 1 outputs brake fluid of increasing pressure to a braking element of a brake system, thereby braking the vehicle. Thereafter, a booster phase is entered, in which the pressure-force curve between the output pressure of the master cylinder 1 and the booster has a high slope to provide a rapid increase in the non-output pressure.

At the end of braking, the driver releases the brake pedal, the reaction plate 17 moves backwards under the hydraulic pressure in the brake master cylinder 1 and the urging of the first return spring 21, the control unit 5 thus judges the intention of the driver to end braking, and drives the brake motor 4 to rotate in reverse, so that the transmission sleeve 11 returns to the home position, the reaction plate 17, the pedal spring 16 and the push rod 15 return to the home position under the action of the first return spring 21, and the plunger 14 also returns to the home position under the action of the second return spring 22.

In the normal operation mode of the electric brake booster of the present application, on the one hand, the booster provides a booster curve similar to a conventional vacuum booster, and therefore, the vacuum booster can be replaced and a similar boosting function provided. On the other hand, the pedal reaction force and the stroke that the driver feels on the brake pedal are similar to those in the conventional brake pedal operation.

The electric-only operating mode of the electric brake booster is described next. The so-called electric-only operating mode is when the driver does not depress the brake pedal, the control unit 5 activates the electric brake booster on the basis of the brake signal from the automatic vehicle braking function, or the automatic vehicle braking function takes over the electric brake booster directly. In this case, the push rod 15, the pedal spring 16, and the reaction plate 17 are kept stationary, and the brake motor 4 is rotated in the forward direction to drive the plunger 14 to move forward through the drive nut 12 and the transmission sleeve 11 to push the piston 2 forward, thereby achieving vehicle braking. When it is determined that braking is finished, the brake motor 4 is controlled to rotate reversely so that the transmission sleeve 11 is returned to the home position, and the plunger 14 is also returned to the home position by the second return spring 22.

The input force of the electric brake booster comes only from the brake motor 4 without any pedal force component, regardless of the normal operation mode or the electric-only operation mode. Thus, these two braking modes may be collectively referred to as motor braking operation. Also, a similar boost ratio and boost profile to a conventional vacuum booster can be provided, whether in the normal or pure electric mode of operation. Furthermore, the pedal stroke applied by the driver is used by the control unit 5 for determining the driver's braking intention only in the normal operation mode. In the normal operation mode, the driver's braking experience is similar to a conventional braking system, including a conventional brake system with a vacuum booster, due to the deformation of the pedal spring 16, resulting in a gradually increasing reaction force being fed back to the driver's foot via the brake pedal.

Furthermore, the electric brake booster has a pure pedal operating mode. The pure pedal operation mode occurs when the brake motor 4 is not functioning properly, or is disabled, in which case the brake motor 4 is not being supplied with electrical energy. At this time, the axial movement locking function of the drive nut 12 is released, and thus the axial movement is enabled. When the driver depresses the brake pedal, the pushing element 3 pushes the pedal spring 16 axially forward via the push rod 15 until the reaction plate 17 pushes against the front end wall 14f of the plunger 14, and thus the plunger 14 is moved forward, thereby pushing the piston 2 forward to apply the vehicle brake. In the process, the pedal spring 16 pulls the limit plate 19 through the limit rod 18, and the limit plate 19 with the driving nut 12 and the transmission sleeve 11 also moves forwards axially. After braking is finished, the related elements return to the original positions under the action of the first return spring 21 and the second return spring 22.

In addition, electric brake boosters, when used in electric vehicles, also have the ability to participate in the recovery of braking energy. In the braking energy recovery operation, the control unit 5 detects depression of the brake pedal and controls the brake motor 4 to drive the plunger 14 to move forward by a set distance.

According to the present application, the brake boosting transmission line of the brake motor of the electric brake booster is decoupled from the pedal force transmission line when the motor braking operation is performed. The action of the brake boosting force transmission element does not drag the pedal force transmission element and the brake pedal, so that the transmission of the brake boosting force can be prevented from being blocked, and the driver can be prevented from having poor feeling from the brake pedal. Further, when the brake motor is operated, the input force of the electric brake booster is derived only from the brake motor without combining the pedal force.

In the case where the brake motor is normally operated, the pedal spring is used as a simulator for providing a tactile sensation for feedback of the braking state to the driver's foot and for judging the driver's intention to brake, and the pedal force is not converted into a part of the output force of the brake booster.

The decoupling electric brake booster can completely replace the traditional vacuum booster and can provide the brake boosting performance similar to the vacuum booster. In this regard, it should be noted that, since the vacuum brake booster utilizes the vacuum degree generated by the intake pipe of the engine to generate the brake boosting force, the operation of the vacuum brake booster has a certain influence on the operation of the engine itself. In addition, after the engine is shut down, there is no intake vacuum, and no brake assist can be generated. The electric brake booster of the application does not need to be assisted by the vacuum degree generated by an air inlet pipe of the engine, and therefore, the electric brake booster can provide a brake boosting function no matter whether the engine runs or not.

Furthermore, the decoupled electric brake booster according to the invention is particularly suitable for vehicles with an automatic braking function, i.e. the control of the electric brake booster and the vehicle brake system is taken over by the automatic braking function without the driver having to press down the brake pedal.

It should be noted that although the present application has been described and illustrated herein with reference to particular embodiments, the scope of the present application is not limited to the details shown. Various modifications may be made to these details without departing from the underlying principles of the application.

Claims

1. An electric brake booster for use in a vehicle, comprising:

a pedal force transmission mechanism configured to be driven by the brake pedal to move in an axial direction;

a detection device configured to detect an action of the pedal force transmission mechanism;

a brake motor (4) configured to perform a motor braking operation to generate a brake assist force; and

an assist force transmission mechanism configured to be driven by the brake motor (4) to move in an axial direction so as to transmit a brake assist force generated by the brake motor to the piston (2) of the master cylinder (1);

wherein, in the motor braking operation, the assistance force transmission mechanism is kinematically decoupled from the pedal force transmission mechanism, and the braking assistance force generated by the braking motor alone constitutes the input force of the electric brake booster.

2. The electric brake booster of claim 1, wherein the motor braking operation includes:

a normal braking operation in which the brake motor (4) is started based on the action of the pedal force transmission mechanism detected by the detection means, so that the brake motor (4) is rotated in the forward direction to generate a braking assist force; and

and pure electric operation, wherein the brake motor (4) is started based on a brake signal of the automatic brake module of the vehicle, so that the brake motor (4) rotates forwards to generate brake assistance.

3. An electric brake booster according to claim 1 or 2, wherein the boost transfer mechanism comprises a drive nut (12) configured to be driven in rotation by the brake motor and a transmission sleeve (11) cooperating with the drive nut to convert the rotary motion of the drive nut into an axial linear motion;

the power transmission mechanism further comprises a plunger (14) arranged axially between the piston (2) of the master cylinder (1) and the transmission sleeve (11), the plunger being configured and adapted to be urged axially forward by the transmission sleeve (11) towards the cylinder of the master cylinder (1).

4. The electric brake booster of claim 3, wherein the pedal force transmission mechanism comprises:

a push rod (15) arranged in the transmission sleeve (11) and capable of sliding axially in the transmission sleeve, the rear part of the push rod is driven by a brake pedal, and the front part of the push rod extends into an inner cavity (14d) of the plunger (14); and

a pedal spring (16) connected to a front portion of the push rod (15) and radially protruding from the inner cavity (14d), the pedal spring (16) having an ability to elastically deform in the axial direction under an axial urging action of the push rod (15).

5. The electric brake booster according to claim 4, wherein the longitudinally outer end of the pedal spring (16) has a stroke limited in an axially forward direction;

optionally, the limited travel is achieved by a pedal spring limit structure comprising a limit rod (18) and a limit plate (19), the front end of the limit rod (18) being connected to the longitudinal outer end of the pedal spring (16), the rear end of the limit rod (18) being axially slidable within a limited distance with respect to the limit plate (19), the limit plate (19) being fixed during the motor braking operation.

6. The electric brake booster according to claim 4 or 5, wherein the pedal spring (16) comprises a combination spring in which spring pieces are stacked such that the spring rate of the pedal spring (16) in the axial direction gradually increases from the longitudinal outer end toward the radial center.

7. The electric brake booster according to any one of claims 4 to 6, wherein the pedal force transmission mechanism further includes a reaction plate (17) mounted in front of the push rod (15);

wherein, in the non-operating state of the electric brake booster, there is an axial distance between the reaction plate (17) and the front end wall (14f) of the plunger (14).

8. The electric brake booster according to claim 7, wherein a radial direction end of the reaction plate (17) is provided with a radially extending section (17a), and the detecting means determines the action of the pedal force transmission mechanism by detecting an axial position of the radially extending section (17 a).

9. The electric brake booster according to claim 7 or 8, further comprising a first return spring (21) configured to urge the reaction plate (17) in an axial direction away from the master cylinder (1).

10. Electric brake booster according to any one of claims 3 to 9, further comprising a second return spring (22) configured to urge the plunger (14) in an axial direction away from the master cylinder (1).

11. Electric brake booster according to any one of claims 3 to 9, in which, in the electrically charged state of the brake motor (4), the drive nut (12) is axially locked against axial movement but rotatable; in the uncharged state of the brake motor (4), the axial locking of the drive nut (12) is released.

12. The electric brake booster according to claim 11, wherein the electric brake booster is configured to be capable of performing a pure pedal braking operation in a state where the brake motor (4) is not charged, wherein the boosting force transmission mechanism transmits the braking force from the brake pedal to the piston (2) of the master cylinder (1) in accordance with a pedal force transmission mechanism.