Abstract:
A brake booster for a vehicle brake system includes a brake force transmitting element configured to be moveable in an axial direction for transmitting a pedal brake force from a brake pedal to a primary brake cylinder of the vehicle, a booster force transmitting element configured to be moveable in the axial direction for transmitting a booster brake force to the primary brake cylinder, a first rotation sensor kinematically coupled with the brake force transmitting element via a motion converting mechanism, which is configured for converting a translational motion into a rotational motion, for measuring the axial position of the brake force transmitting element, and a second rotation sensor kinematically coupled with the booster force transmitting element for measuring the axial position of the booster force transmitting element; wherein the measurements of the first and second rotation sensors are used for controlling the generation of the booster brake force. According to the disclosure, the motions of the brake force transmitting element and the booster force transmitting element can be measured precisely so that the actions of the brake booster can be controlled with high accuracy.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure relates to a brake booster, in particular an electric brake booster, to be used in a vehicle brake system. 
       BACKGROUND ART 
       [0002]    A vehicle generally comprises a hydraulic brake system for reducing the speed of the vehicle and/or for stopping the vehicle. It needs great effort for manipulating a brake pedal by the driver, so in many vehicles, a brake booster is added in addition to the hydraulic brake device of the hydraulic brake system. 
         [0003]    Traditional brake boosters mainly comprise vacuum brake boosters, in which the vacuum in an inlet pipe of an engine is used as a source of booster brake force which generates a vacuum booster force, in the level of several times higher than the pedal brake force, to be applied to a primary brake cylinder. In this way, the primary brake cylinder receives both the pedal brake force and the vacuum booster force, so that the output pressure of the primary brake cylinder is increased, and thus the pedal brake force needed for vehicle braking can be decreased. 
         [0004]    The operation of the engine is affected by the operation of the vacuum brake booster since the booster brake force is generated by the vacuum brake booster using the vacuum in the inlet pipe of the engine. Further, after the engine is shut off, no inlet vacuum exists, and thus no booster brake force can be generated. 
         [0005]    As a substitution of the vacuum brake booster, an electric brake booster is developed, which drives a booster force transmitting element by an electric motor to generate a booster brake force, independent of the vacuum in the inlet pipe of the engine. 
         [0006]    In an electric brake booster, the action of the electric motor shall be coordinated with the action of the brake pedal. For this purpose, sensors are used for sensing the motions of the electric motor and the brake pedal. In general, the electric motor has its own rotor-rotation sensor for monitoring the movement of the electric motor. In addition, a stroke sensor is used for monitoring the displacement of the brake pedal or the displacement of a brake element driven by the brake pedal. 
         [0007]    In such an electric brake booster, it is not easy to provide a measurement with high precision by the stroke sensor. Meanwhile, the stroke sensor has low robustness and is easy to be affected by environment factors, like magnetic field, contamination and icing. Further, the stroke sensor is relative expensive. 
       SUMMARY OF THE DISCLOSURE 
       [0008]    The disclosure is aimed at solving one or more problems found in the brake booster according to prior art. 
         [0009]    For this end, according to one aspect of the disclosure, there provides a brake booster to be used in a vehicle brake system, the brake booster comprising a brake force transmitting element configured to be moveable in an axial direction for transmitting a pedal brake force from a brake pedal to a primary brake cylinder of the vehicle, a booster force transmitting element configured to be moveable in the axial direction for transmitting a booster brake force to the primary brake cylinder, a first rotation sensor kinematically coupled with the brake force transmitting element via a motion converting mechanism, which is configured for converting a translational motion into a rotational motion, for measuring the axial position of the brake force transmitting element, and a second rotation sensor kinematically coupled with the booster force transmitting element for measuring the axial position of the booster force transmitting element; wherein the measurements of the first and second rotation sensors are used for controlling the generation of the booster brake force. 
         [0010]    According to a preferred embodiment of the disclosure, the brake booster further comprises an electric motor for generating the booster brake force. 
         [0011]    According to a preferred embodiment of the disclosure, the electric motor drives the booster force transmitting element via a transmission mechanism which is configured for converting a rotational motion into a translational motion. 
         [0012]    According to a preferred embodiment of the disclosure, the electric motor comprises a rotation motor or a linear motor. 
         [0013]    According to a preferred embodiment of the disclosure, the second rotation sensor comprises a rotor-rotation sensor of the electric motor. 
         [0014]    According to a preferred embodiment of the disclosure, the brake booster further comprises an electronic control unit which controls the generation of the booster brake force based on the measurements of the first and second rotation sensors, in particular, by controlling the operation of the electric motor. 
         [0015]    According to a preferred embodiment of the disclosure, the motion converting mechanism comprises a toothed rack coupled with the brake force transmitting element in a way of being able to move in the axial direction together with the brake force transmitting element, and a sensor gear adapted to be driven by the toothed rack to generate a rotational motion which can be sensed by the first rotation sensor. 
         [0016]    According to a preferred embodiment of the disclosure, the toothed rack is carried by the booster force transmitting element and is able to move in the axial direction with respect to the booster force transmitting element. 
         [0017]    According to a preferred embodiment of the disclosure, the first rotation sensor comprises a sensor magnet carried by the sensor gear and a sensing element which is able to sense the change in the magnetic field of the sensor magnet. 
         [0018]    According to a preferred embodiment of the disclosure, the booster force transmitting element comprises a hollow valve body extending in the axial direction, and the brake force transmitting element comprises a plunger to be driven forwards by the brake pedal via a push bar, the plunger being configured to be axially movable in the valve body. 
         [0019]    According to a preferred embodiment of the disclosure, the brake force transmitting element is coupled with the motion converting mechanism by a locating pin. 
         [0020]    According to a preferred embodiment of the disclosure, the locating pin is arranged to be able to move a limited distance in the axial direction with respect to the booster force transmitting element. 
         [0021]    According to a preferred embodiment of the disclosure, the brake booster further comprises a brake force outputting element configured to transmit the pedal brake force and the booster brake force to a primary piston of the primary brake cylinder. 
         [0022]    According to a preferred embodiment of the disclosure, the brake booster further comprises a returning element, such as a returning spring, for applying a returning force to the booster force transmitting element, under the action of which the booster force transmitting element tends to move backwards in the axial direction. 
         [0023]    According to the disclosure, rotation sensors are used for sensing the positions and movements of the brake force transmitting element and the booster force transmitting element, so that sensing of axial position with high precision can be achieved. 
         [0024]    Further, the rotation sensors can provide higher signal stability since the rotation sensors are less affected by the direction and intensity of a magnetic field. 
         [0025]    Further more, the performance of the rotation sensors does not become degenerated severely when they are contaminated with outside materials, like dust, oil and ice, and thus the rotation sensors have high robustness. 
         [0026]    Still Further, the rotation sensors are not expensive, which helps to reduce the overall cost of the brake booster. 
         [0027]    According to the disclosure, the electronic control unit is able to accurately control the operation of the electric motor on the basis of the sensed signals from the rotation sensors, so that the action of the brake booster can be actively controlled with high precision in real time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a sectional front view of a vehicle brake booster according to a possible embodiment of the disclosure in an unbraking state; 
           [0029]      FIG. 2  is a sectional top view of the brake booster shown in  FIG. 1  in the unbraking state; 
           [0030]      FIG. 3  is a sectional side view of a transmission mechanism of the brake booster shown in  FIG. 1 ; 
           [0031]      FIG. 4  is an enlarged sectional view of a plunger used in the brake booster shown in  FIG. 1 ; 
           [0032]      FIG. 5  is an enlarged sectional view of a valve body used in the brake booster shown in  FIG. 1 ; and 
           [0033]      FIG. 6  is an enlarged sectional view of a toothed rack used in the brake booster shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0034]    Now some preferred embodiments of the disclosure will be described with reference to the drawings. 
         [0035]    It is noted that the drawings are provided only for schematically illustrating some possible embodiments of the brake booster of the disclosure, and thus the components of the brake booster are not drawn to scale. 
         [0036]    It is also noted that the term “proximal” or “backward” refer to the side proximal to the brake pedal of the vehicle, while the term “distal” or “forward” refer to the side distal from the brake pedal and proximal to the primary brake cylinder of the wheels. 
         [0037]    As shown in  FIGS. 1 and 2 , according to a preferred embodiment of the disclosure, a brake booster to be used in a vehicle brake system comprises a housing  1  which can be formed of any suitable materials, such as sheet metal or plastic. 
         [0038]    The housing  1  can be mounted to the vehicle body by means of any suitable fastening device, for example, using tie bars  2  shown in  FIG. 1 . Of course, other suitable fastening devices or structures can alternatively be used for fixing the housing  1  to the vehicle body. 
         [0039]    The housing  1  fixedly carries a valve body guider  4  which can be formed of metal, rubber or plastic material. The valve body guider  4  defines therein an internal guiding space which extends through the valve body guider  4  in an axial direction. The valve body guider  4  is mounted to a side of the housing  1  which faces towards the brake pedal, and is partly within the housing  1  and partly outside the housing  1 . 
         [0040]    A valve body  6 , which forms a booster force transmitting element of the brake booster, comprises a substantially cylindrical body portion  6   a  extending in the axial direction and an urging portion  6   b  formed on the distal end of the body portion  6   a . The two portions may be formed integrally, or be formed separately and then assembled together. 
         [0041]    The body portion  6   a  extends through the internal guiding space of the valve body guider  4  and is guided by the valve body guider  4  to be able to move in the axial direction. A proximal end portion of the body portion  6   a  is exposed outside the housing  1 , and a distal end portion of the body portion  6   a  and the urging portion  6   b  are located in the housing  1 . 
         [0042]    Further, the body portion  6   a  is prevented from rotating around its central axis in the valve body guider  4 . For this purpose, a locating element (not shown), such as a key or a guiding pin, can be arranged between the body portion  6   a  and the valve body guider  4 . 
         [0043]    The valve body  6  defines therein a plurality of accommodating spaces in the axial direction, as more clearly shown in  FIG. 5 . These accommodating spaces comprise a middle through hole  60  extending in the axial direction, a first accommodating socket  61  extending backwards from the proximal end of the middle through hole  60  and having a diametric dimension (for example, diameter) larger than that of the middle through hole  60  so that a first step  61   a  facing towards the proximal side is formed therebetween, a second accommodating socket  62  extending backwards from the proximal end of the first accommodating socket  61  and having a diametric dimension larger than that of the first accommodating socket  61  so that a second step  62   a  facing towards the proximal side is formed therebetween, a third accommodating socket  63  extending forwards from the distal end of the middle through hole  60  and having a diametric dimension larger than that of the middle through hole  60  so that a third step  63   a  facing towards the distal side is formed therebetween, and a fourth accommodating socket  64  extending forwards from the distal end of the third accommodating socket  63  and having a diametric dimension larger than that of the third accommodating socket  63  so that a fourth step  64   a  facing towards the distal side is formed therebetween. The middle through hole  60 , the first accommodating socket  61 , the second accommodating socket  62  and the third accommodating socket  63  are formed in the body portion  6   a , and the fourth accommodating socket  64  is formed in the urging portion  6   b . The urging portion  6   b  has a diametric dimension preferably larger than that of the body portion  6   a  so that the fourth accommodating socket  64  has a diametric dimension which is sufficiently large, even larger than the diametric dimension of the outer periphery of the body portion  6   a.    
         [0044]    Further, a circular flange  65  is formed around the outer periphery of the urging portion  6   b , a pair of diametrically opposed through holes  66  are formed in the body portion  6   a , extending from the outer periphery of the body portion  6   a  to the first accommodating socket  61  in a diametric direction, and a rack receiving socket  67  is formed in one side of the outer periphery of the body portion  6   a , extending in the axial direction. The functions of these portions will be described later. 
         [0045]    A plunger rod  8  is disposed in the middle through hole  60  of the valve body  6  in an axially movable manner, the plunger rod  8  being biased at its proximal end by the plunger  10  towards the distal side. 
         [0046]    In the illustrated embodiment, the plunger rod  8  and the plunger  10  are formed separately. As an alternative, the plunger rod  8  may be formed as an integrated portion of the plunger  10 . 
         [0047]    The plunger rod  8  and the plunger  10  (or the plunger  10  with the integrated plunger rod  8 ) form a brake force transmitting element of the brake booster. 
         [0048]    The structure of the plunger  10  is shown in more details in  FIG. 4 . As illustrated, the plunger  10  comprises a distal portion  101  and a proximal portion  102  integral therewith. The shape and the diametric dimension of the distal portion  101  are set so that it can be inserted into the middle through hole  60  of the valve body  6 . The proximal portion  102  has a diametric dimension (for example, diameter) larger than that of the distal portion  101 , and defines therein an accommodating space  103 . The accommodating space  103  is opened at the proximal end surface of the plunger  10 . 
         [0049]    The proximal ends of the distal portion  101  and the proximal portion  102  are formed with outer periphery flanges  104  and  105  respectively. The shapes and the diametric dimensions of the outer periphery flanges  104  and  105  are set so that they can be disposed in the first accommodating socket  61  and the second accommodating socket  62  of the valve body  6  respectively in an axially movable manner. 
         [0050]    A radial through hole  106  is formed through the outer periphery flange  104  for receiving the locating pin  108  (see  FIG. 2 ) and fixing it therein. The locating pin  108  has diametrically opposite ends which are inserted into the through holes  66  of the valve body  6 . Each through hole  66  has a certain axial length so that the opposite ends of the locating pin  108  can slide in the axial direction in the through holes  66 . 
         [0051]    A push bar  12  which is driven by the brake pedal (not shown) is inserted in the accommodating space  103 . Specifically, the push bar  12  extends in the axial direction, having a central axis coincidence with the central axis of the plunger  10 . The push bar  12  comprises a bar portion  12   a  and a front portion  12   b  in the form of a spherical head connected to the front end of the bar portion  12   a . Further, a retention flange  12   c  is formed around or mounted to the outer periphery of the bar portion  12   a.    
         [0052]    The bar portion  12   a  of the push bar  12  is configured to be driven by the brake pedal so that the push bar  12  moves forwards in the axial direction. The spherical-head shaped front portion  12   b  of the push bar  12  biases forwards against an end wall formed in the accommodating space  103 . 
         [0053]    An insertion sleeve  14  is inserted in the back end of the valve body  6 . The insertion sleeve  14  comprises a substantially cylindrical main body  14   a  configured to be inserted in the accommodating space  62  of the plunger  6 , a front end flange  14   b  formed in the front end of the main body  14   a  and extending radially inwards, and a back end flange  14   c  formed at the back end of the main body  14   a  and extending radially outwards. The back end flange  14   c  is fixed to the back end of the valve body  6 . The front end flange  14   b  is configured to be abutted against by the back end of the plunger  10 . 
         [0054]    Further, a returning spring  16  is disposed between the front end flange  14   b  and the retention flange  12   c  of the push bar  12 . In this way, when the push bar  12  receives a pushing force from the brake pedal, the push bar  12  can move forwards against the spring force of the returning spring  16 . When the pushing force from the brake pedal disappears, the push bar  12  moves backwards to its original position by the biasing effect of the spring force of the returning spring  16 . 
         [0055]    The distal end of the plunger rod  8  extends to a location proximal to the third step  63   a  in the valve body  6 . A plunger plate  18  and a reaction plate  20  are disposed in the third accommodating socket  63  and the fourth accommodating socket  64  of the valve body  6  respectively. 
         [0056]    The plunger plate  18  comprises a front body portion and a back pushing portion continuous therewith. The shape and the diametric dimension of the front body portion are set so that it is axially slidable in the third accommodating socket  63 . The back pushing portion has a diametric dimension preferably smaller than that of the front body portion. 
         [0057]    The shape and the diametric dimension the reaction plate  20  are set so that it is axially slidable in the fourth accommodating socket  64 . 
         [0058]    The plunger plate  18  is clamped in the axial direction between an inner portion of a proximal surface of the reaction plate  20  and the distal end of the plunger rod  8 . An outer portion of the proximal surface of the reaction plate  20  is biased forwards by the fourth step  64   a  of the valve body  6 . 
         [0059]    A brake force outputting bar  22  is disposed in front of the reaction plate  20 . The brake force outputting bar  22  comprises a back base portion and a front bar portion continuous therewith. The shape and the diametric dimension of the back base portion of the brake force outputting bar  22  correspond substantially to that of the reaction plate  20 . The front bar portion of the brake force outputting bar  22  has a diametric dimension preferably smaller than that of the back base portion. 
         [0060]    The front end surface of the reaction plate  20  biases forwards against the back base portion of the brake force outputting bar  22 , and the front bar portion of the brake force outputting bar  22  is coupled to a primary piston  26  of a primary brake cylinder  24  of the vehicle brake system. The primary piston  26  has a central axis which is preferably coincidence with the central axis of the valve body  6 , and the central axis of the whole brake booster is defined at least by the above mentioned two central axes. 
         [0061]    The reaction plate  20  is preferably elastic, for example, formed by elastic rubber. 
         [0062]    The brake force outputting bar  22  is clamped to the front end of the valve body  6  by a disc-like clamper  28 . The clamper  28  comprises a front plate portion  28   a , a sleeve portion  28   b  extending backwards in the axial direction from the outer periphery of the front plate portion  28   a , and a back flange portion  28   c  extending outwards from the back end of the sleeve portion  28   b . A central hole is formed through the front plate portion  28 , and the front bar portion of the brake force outputting bar  22  passes through this central hole. The shape and the diametric dimension of the back flange portion  28   c  correspond substantially to that of the circular flange  65  of the valve body  6 . Under the action of the returning spring  30  which is disposed between the front end surface of the housing  1  and the back flange portion  28   c , the back flange portion  28   c  is biased against the front surface of the circular flange  65 , and the front plate portion  28   a  is biased against the front surface of the back base portion of the brake force outputting bar  22 . In this way, the brake force outputting bar  22  is clamped to the front end of the valve body  6 , and the brake force outputting bar  22  in turn keeps the reaction plate  20  and the plunger plate  18  in the fourth accommodating socket  64  and the third accommodating socket  63  respectively. 
         [0063]    The valve body  6  is axially movable with respect to the housing  1  under the guiding of the valve body guider  4 . Specifically, the valve body  6  is able to move forwards when driven by the electric motor  32  (see  FIGS. 2 and 3 ), and will move towards the proximal side under the action of the returning spring  30  when the driving action of the electric motor  32  disappears. 
         [0064]    The electric motor  32  used in the disclosure can be selected from various rotation motors, for example, a DC rotation motor. Various possible transmission mechanisms can be used for converting the output rotational motion of the electric motor  32  into an axially forward displacement or a translational motion of the valve body  6 . As an example, in the illustrated embodiment, the electric motor  32  is in the form of a rotation motor, and the transmission mechanism is in the form comprising a gear set  34  and a screw device  36 . In this manner, the output rotational motion of the electric motor  32  is converted into an output forward linear motion by the gear set  34  and the screw device  36 . As an alternative, other devices, like a worm and worm gear device, for converting a rotational motion into a linear motion can be used here. 
         [0065]    Alternatively, the electric motor  32  may be a linear motor, such as a DC linear motor. In the case that a linear motor is used, a corresponding transmission mechanism can be used to convert the output linear motion of the electric motor into an axially forward displacement of the valve body  6 . 
         [0066]    The output forward displacement of the transmission mechanism is transmitted to the valve body  6  via a transmitting tube  50  so that the valve body  6  moves forwards in the axial direction against the spring force of the returning spring  30 . 
         [0067]    The forward travel of the valve body  6  is limited, for example, not larger than the axial distance between the clamper  28  and the primary brake cylinder  24 . 
         [0068]    When the electric motor  32  rotates in a reverse direction, the valve body  6  can move backwards mainly under the action of the spring force of the returning spring  30  and an reaction force of the primary piston  26 . 
         [0069]    The plunger  10  is axially movable with respect to the valve body  6 . Specifically, in an unbraking state of the brake booster as shown in  FIGS. 1 and 2 , the returning spring  30  biases the valve body  6  backwards via the back flange portion  28   c  of the clamper  28  so that the valve body  6  is kept in its original position. Meanwhile, the returning spring  30  biases the brake force outputting bar  22  backwards via the front plate portion  28   a  of the clamper  28  so that the plunger  10  is pushed to its most proximal position by the reaction plate  20 , the plunger plate  18  and the plunger rod  8  in serial. In this position, the locating pin  108  is biased against the portions of the valve body  6  which define the proximal ends of the through holes  66 , and the outer periphery flanges  104  and  105  of the plunger  10  are separate from the first and second steps  61   a  and  62   a  of the valve body  6  respectively. In this state, when the push bar  12  receives a pushing force from the brake pedal, the push bar  12  moves the plunger  10  forwards, and then the outer periphery flange  104  engages the first step  61   a  and/or the outer periphery flange  105  engages the second step  62   a  and/or the locating pin  108  engages the portions of the valve body  6  which define the distal ends of the through holes  66 . Then, the push bar  12  pushes the valve body  6  forwards via the plunger  10 . When the pushing force from the brake pedal disappears, the plunger  10  moves backwards under the actions of the returning spring  30  and the primary piston  26  to return to its original position. 
         [0070]    The brake booster may comprise a sealing sleeve  52 , for example, a rubble sealing sleeve, for protecting the functional components of the brake booster which are exposed outside the housing  1 , in particular, the valve body  6 . 
         [0071]    The brake booster further comprises an electronic control unit for detecting the positions of the plunger  10  and the valve body  6  and then controlling the operation of the electric motor  32  based on the detection. 
         [0072]    Specifically, the brake booster is in an unbraking position in  FIGS. 1 and 2 , that is to say, the driver does not step down the brake pedal. Now there is no brake force applied to the primary piston  26  of the primary brake cylinder  24 . 
         [0073]    Then, when the driver steps down the brake pedal to perform vehicle braking, the downward motion of the brake pedal results in axially forward moving of the push bar  12  against the pushing force of the returning spring  16 . The front portion  12   b  of the push bar  12  pushes the plunger  10  and the plunger rod  8  to move forwards, and the distal end of the plunger rod  8  pushes the inner portion of the reaction plate  20  forwards via the plunger plate  18  so that the inner portion of the reaction plate  20  urges the primary piston  26  of the primary brake cylinder via the brake force outputting bar  22 . In this way, the pedal brake force (the driver&#39;s brake force) applied by the driver is transmitted to the primary piston  26 . 
         [0074]    In an initial stage of the forward displacement of the plunger  10 , the valve body  6  keeps stationary in the axial direction under the backward pushing action of the returning spring  30 . In this stage, the electronic control unit detects that the plunger  10  moves axially forwards while the axial position of the valve body  6  is unchanged. In this condition, the electronic control unit judges out the braking intention of the driver on the basis of respective positions of the valve body  6  and the plunger  10  and the positional difference between them. Then, the electronic control unit instructs the electric motor  32  to run in a forward direction, and the output forward rotation of the electric motor  32  is transmitted to the valve body  6  by the transmission mechanism so that the valve body  6  is driven to move forwards against the pushing force of the returning spring  30 . The fourth step  64   a  of the valve body  6  urges the outer portion of the reaction plate  20  forwards so that the outer portion of the reaction plate  20  pushes the primary piston  26  of the primary brake cylinder forwards via the brake force outputting bar  22 . In this way, a booster brake force provided by the electric motor  32  is transmitted to the primary piston  26 . 
         [0075]    It can be seen that the brake force outputting bar  22  forms a brake force outputting element for transmitting the pedal brake force and the booster brake force to the primary piston of the primary brake cylinder. It is appreciated that other forms of the brake force outputting element can also be used here. 
         [0076]    Under the action of the pedal brake force applied by the driver and the booster brake force provided by the electric motor, the primary piston  26  force the brake fluid accommodated in the primary brake cylinder to be supplied to the brake devices for the vehicle wheels to conduct braking to the vehicle. Now the brake booster is in the braking position. The electronic control unit judges out the continuation of the braking action based on respective positions of the valve body  6  and the plunger  10 , and keeps the electric motor  32  to run in the forward direction; alternatively, the electric motor  32  may stop after is has run in the forward direction for a period of time to wait the ending of the braking action. 
         [0077]    When the driver decides to end the braking action, he/she will release the brake pedal. A hydraulic force applied to the primary piston  26  from the primary brake cylinder acts on the plunger  10  in the backward direction via the brake force outputting bar  22 , the inner portion of the reaction plate  20 , the plunger plate  18  and the plunger rod  8  so that the plunger  10  moves backwards to its original position shown in  FIGS. 1 and 2 . When the brake booster is in an intermediate position in the backward transitional travel from its braking position towards its unbraking position, the electronic control unit detects the state that the plunger  10  is moving axially backwards while the axial position of the valve body  6  remains unchanged or the valve body  6  moves backwards slower than the plunger  10 . Now the electronic control unit judges out the brake releasing intention of the driver on the basis of respective positions of the valve body  6  and the plunger  10  and the positional difference between them. Then, the electronic control unit instructs the electric motor  32  to run reversely, and the valve body  6  is forced to move backwards under the pushing forces of the returning spring  30  and the primary piston  2 . Ultimately, the valve body  6  returns to its original position in the unbraking state. The electronic control unit judges out the ending of the brake releasing (or removing) action, and then stops the reverse running of the electric motor  32 . 
         [0078]    No matter in the braking operation or in the brake releasing operation, the plunger  10  always moves a small distance in the axial direction with respect to the valve body  6  first, then the electronic control unit controls the electric motor  32  to rotate forwardly or reversely. For controlling the activating of the electric motor  32 , it needs to detect respective axial positions of the plunger  10  and the valve body  6  (and the difference in the axial positions of them). Further, during the forward or reverse rotation of the electric motor  32 , it may also need to detect the axial positions of the plunger  10  and the valve body  6  for determining the rotational speed of the electric motor  32 . Thus, for accurately controlling the activating and running of the electric motor  32 , the axial positions and speeds of the plunger  10  and the valve body  6  shall be detected with high precision. 
         [0079]    For this purpose, according to the disclosure, rotation sensors are provided for detecting the axial positions of the plunger  10  and the valve body  6 . 
         [0080]    The electric motor  32  comprises therein a rotor rotation sensor for monitoring the motion of the electric motor  32 . Thus, the rotor rotation sensor in the electric motor  32  can be used for detecting the axial position of the valve body  6 . 
         [0081]    For detecting the axial position of the plunger  10 , a specific rotation sensor  80 , as shown clearly in  FIG. 3 , is provided in the disclosure. The rotation sensor  80  mainly comprises a sensor magnet  82  and a sensing element  84  for sensing the change in the magnetic field of the magnet  82 . The sensor magnet  82  is mounted to a sensor gear  86 , the sensor gear  86  meshes with a free gear wheel  88 , and the free gear wheel  88  in turn meshes with a toothed rack  90 . 
         [0082]    As shown in  FIG. 6 , the toothed rack  90  comprises a main portion  92  and a handle portion  96  continuous with the back end of the main portion  92 . A fixing hole  96  is formed in the handle portion  96  and is configured to be inserted through by an end of the locating pin  108 . The main portion  92  is formed with teeth  98  to be engaged with corresponding teeth of the free gear wheel  88 . 
         [0083]    The toothed rack  90  is disposed in the rack receiving socket  67  of the valve body  6  in an axially movable manner. When the plunger  10  moves in the axial direction, the locating pin  108  on the plunger  10  drives the toothed rack  90  to move in the axial direction synchronously. The toothed rack  90  moving in the axial direction drives the sensor gear  86  via the free gear wheel  88 . As the sensor gear  86  rotates, the magnet  82  carried by the sensor gear  86  rotates therewith, and the change in the magnetic field of the magnet  82  is sensed by the sensing element  84 , and thus the rotational position of the sensor gear  86  is determined. Then, the axial position of the toothed rack  90 , also of the plunger  10 , can be calculated out based on the transmission ratio between the toothed rack  90  and the sensor gear  86 . 
         [0084]    It is appreciated that other forms of the motion converting mechanism which can convert the axial movement, i.e., the translational motion, of the plunger  10  into a rotational motion detectable by the rotation sensor can also be used here. 
         [0085]    It is noted that some embodiments of the disclosure in which the electric motor is used to provide the booster brake force are described here, so the brake booster described above can be referred to as an electric brake booster. However, it is appreciated that the basic concept of the disclosure is also applicable in embodiments in which the booster brake force is provided by other types of power sources. 
         [0086]    The rotation sensors used in the disclosure may make use of matured technology in the art of sensors (for example, rotation sensors in brushless motors), and they are not described here in more details. 
         [0087]    Comparing with the stroke sensor used in the prior art, a rotation sensor is used in the disclosure for detecting the position of the plunger  10 . The rotation sensor has a resolution ratio much higher than that of the stroke sensor, and thus the detection precision of the axial position of the plunger  10  can be increased. 
         [0088]    Further, comparing with the stroke sensor, the rotation sensor is less affected by the direction and intensity of a magnetic field, and thus the rotation sensors can provide higher signal stability. 
         [0089]    Further, the performance of a stroke sensor will be degenerated when it is contaminated with outside materials, like dust, oil and ice; as a comparison, when a rotation sensor is contaminated with outside materials, its performance is not degenerated as severely as the stroke sensor. Thus, the rotation sensor has high robustness. 
         [0090]    Further, a rotation sensor does not include a large, expensive magnet like that used in a stroke sensor, and thus has lower cost. In addition, all of the toothed rack  90 , the sensor gear  86  and the free gear wheel  88  may be formed of plastic, the cost of which is relative low, which contributes to reduction of the overall cost of the brake booster. Further, these plastic parts are light weighted and do not create large inertial forces, and thus they do not significant affect the operation of the brake booster. 
         [0091]    It shall be noted that, as an addition or alternative to the rotor-rotation sensor of the electric motor  32 , a rotation sensor similar to that described above can be used for detecting the axial position of the valve body  6 . 
         [0092]    Further, according to the disclosure, even if the electric motor  32  stops to operate due to malfunction of the electronic control unit, the axially forward movement of the valve body  6  is not obstructed in any sense by the transmission mechanism; thus, the brake force applied by the driver via the brake pedal can still be transmitted to the primary piston  26  of the primary brake cylinder  24 . Specifically, in this condition, the brake pedal pushes the plunger  10  forwards via the push bar  12  first. Then, the plunger  10  comes into contact with the valve body  6  and drives the valve body  6  to move forwards together in the axial direction. In this way, the brake force applied by the driver via the brake pedal is transmitted to the reaction plate  20  via the plunger  10  and the valve body  6 , and then transmitted therefrom to the primary piston  26 . Thus, even if the electronic control unit has some problems, vehicle braking can be performed by the action of the driver stepping down the brake pedal. 
         [0093]    It can be seen that, according to the disclosure, a rotation sensor is used to detect the axial positions of the plunger  10  and the valve body  6  so that the axial positions of them can be measured with high precision. As a result, the running time, running direction, running speed and the like of the electric motor can be actively controlled accurately, and thus the action of the brake booster can be controlled in real time with high precision. 
         [0094]    The brake booster of the disclosure is applicable in vehicles having various power sources, like fuel vehicles, gas vehicles, electric vehicle, hybrid vehicles, etc. 
         [0095]    Although the disclosure is illustrated and described here with reference to specific embodiments, the disclosure is not intended to be limited to the details shown. Rather, various modifications may be made to the details within the scope of the disclosure.