Abstract:
An electric brake booster for a vehicle brake system includes a push bar configured to be driven by a brake pedal to move forwards in an axial direction; a brake force outputting element configured to be pushed forwards by the push bar to output a brake force from the brake pedal; a booster force outputting element for outputting a booster force; a motor for driving the booster force outputting element to create the booster force; a poppet configured to be pushed forwards in the axial direction by the push bar with a spring force, wherein the poppet forms a boost switch, through which an electrical current can pass, with the brake force outputting element and forms a release switch, through which another electrical current can pass, with the booster force outputting element; and an electronic control unit for controlling the operation of the motor in responsive to the on-off states of the boost switch and the release switch. The electric brake booster is independent of the vacuum degree of the inlet pipe of the vehicle engine and can provide active and precise control to the actions of the booster.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to an electric brake booster for a vehicle brake system. 
       BACKGROUND ART 
       [0002]    A vehicle generally comprises a hydraulic brake system for reducing down the speed of the vehicle and/or stopping the vehicle. Manipulating the brake pedal by the manual force of the driver is laborious, thus many vehicles are equipped with boosters in addition to the hydraulic-force transmitting mechanisms. A booster generally uses the vacuum in the inlet pipe of the engine as boost force source to increase the brake force of the hydraulic brake system. 
         [0003]    The most common booster is the membrane type vacuum booster, which is mounted between the brake pedal and the primary brake cylinder of the hydraulic brake system and comprises a vacuum chamber and a working chamber which are separated in a housing by a membrane, the vacuum chamber being connected with the inlet pipe of the engine, the working chamber being in communication with the atmosphere via an air valve, and the vacuum chamber and the working chamber are connected with each other via a vacuum valve. In a rest state, the air valve is closed and the vacuum valve is opened so that the pressure in the vacuum chamber and the pressure in the working chamber are maintained as substantially the same. In a brake state, in response to a force applied to the brake pedal by the vehicle driver, the vacuum valve is closed and the air valve is opened, so that air is drawn into the working chamber. As a result, the pressure in the working chamber becomes higher than the pressure in the vacuum chamber. The membrane moves under the unbalance of pressure between the vacuum chamber and the working chamber, by means of which, a vacuum boost force which may be several times larger than the pedal force is applied to the primary brake cylinder. In this condition, the primary brake cylinder is subjected to the actions of both the pedal force and the vacuum boost force, so that the output pressure of the primary brake cylinder is increased, and the pedal force may be reduced. 
         [0004]    It can be seen that the vacuum booster generates a vacuum boost force using the vacuum formed in the inlet pipe of the engine. When the engine is turned off, the vacuum of the inlet air will not exist, and no enough vacuum boost force will be able to be generated then. 
       SUMMARY OF THE INVENTION 
       [0005]    An object of the invention is to solve some problems found in the conventional vacuum boosters, including complete depending on the vacuum in the inlet pipe of the engine, difficulties in the controlling of the actions of the brake booster, etc. 
         [0006]    For this end, according to one aspect of the invention, there is provided an electric brake booster for a vehicle brake system, comprising: a push bar configured to be driven by a brake pedal to move forwards in an axial direction; a brake force outputting element configured to be pushed forwards by the push bar to output a brake force from the brake pedal; a booster force outputting element for outputting a booster force; a motor for driving the booster force outputting element to create the booster force; a poppet configured to be pushed forwards in the axial direction by the push bar with a spring force, wherein the poppet forms a boost switch, through which an electrical current can pass, with the brake force outputting element and forms a release switch, through which another electrical current can pass, with the booster force outputting element; and an electronic control unit for controlling the operation of the motor in response to the on-off states of the boost switch and the release switch. 
         [0007]    According to a preferred embodiment of the invention, the booster force outputting element has an internal space extending in the axial direction, and the brake force outputting element is disposed in the internal space in a manner of being able to be moved relative to the booster force outputting element in the axial direction. 
         [0008]    According to a preferred embodiment of the invention, the booster force outputting element and the brake force outputting element each have a back end facing towards the poppet in the axial direction, the release switch and the boost switch being formed respectively between the poppet and each of the back ends of the booster force outputting element and the brake force outputting element , wherein conductive contact is established between each of the back ends of the booster force outputting element and the brake force outputting element and the poppet when the electric brake booster is in a rest position. 
         [0009]    According to a preferred embodiment of the invention, conductive contact is established between each of the back ends of the booster force outputting element and the brake force outputting element and the poppet when the electric brake booster is in a braking position. 
         [0010]    According to a preferred embodiment of the invention, one of the back ends of the booster force outputting element and the brake force outputting element is out of contact with the poppet when the electric brake booster is in an intermediate position between the rest position and the braking position. 
         [0011]    According to a preferred embodiment of the invention, the back ends of the booster force outputting element and the brake force outputting element are each provided with a conductive bump protruded towards the poppet; alternatively, the poppet is provided with conductive bumps protruded respectively towards the back ends of the booster force outputting element and the brake force outputting element. 
         [0012]    According to a preferred embodiment of the invention, the electric brake booster further comprises a reaction disk disposed between the front ends of the booster force outputting element and the brake force outputting element and a main piston of a primary brake cylinder of the vehicle brake system, the boost force outputted from the booster force outputting element and the brake force outputted from the brake force outputting element are transmitted to the main piston through the reaction disk. 
         [0013]    According to a preferred embodiment of the invention, the electric brake booster further comprises a returning spring which applies a returning force to the booster force outputting element, the returning force having a tendency of moving the booster force outputting element backwards in the axial direction. 
         [0014]    According to a preferred embodiment of the invention, the motor is coupled to the booster force outputting element via speed reducing means. The motor may be a rotary DC motor or a linear DC motor. 
         [0015]    According to the invention, a motor, in stead of the vacuum in the inlet pipe of the engine, is used as the boost force source, so that an advantageous option or substitution is provided to the boost force solutions of brake boosters. 
         [0016]    Further, according to the invention, the structure of the brake booster itself forms a boost switch and a release switch which operate in combination to provide functions similar to the air valve and the vacuum valve in the traditional vacuum booster. The motor, the boost switch and the release switch are all coupled with an ECU (Electronic Control Unit) which determines the purpose of the braking action of the driver on the basis of electric signals from the boost switch and the release switch and then precisely controls the operation of the motor, so that the actions of the brake booster can be actively controlled precisely in real time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a sectional view of an electric brake booster for vehicle according to a preferred embodiment of the invention; 
           [0018]      FIGS. 2 and 3  are enlarged schematic views of a valve body and a plunger used in the brake booster of the invention respectively; 
           [0019]      FIG. 4  is a schematic view of a boost switch and a release switch used in the brake booster of the invention; 
           [0020]      FIGS. 5 to 7  are schematic views of the boost switch and the release switch of the electric brake booster for vehicle according to the invention in a braking mode respectively; and 
           [0021]      FIGS. 8-10  are schematic views of the boost switch and the release switch of the electric brake booster for vehicle according to the invention in a brake releasing mode respectively. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    Some preferred embodiments of the invention will be described now with reference to the drawings. 
         [0023]    As shown in  FIG. 1 , an electric brake booster used in a vehicle brake system according to a preferred embodiment of the invention comprises a housing  1  which may be made of any suitable material, for example, formed by punching a sheet metal, like that in some traditional vacuum boosters. 
         [0024]    The housing  1  may be fixed to the vehicle body by any suitable fastening elements, for example, fastening bars  2  shown in  FIG. 1 . Of course, other suitable fastening elements or structures may also be used for fixing the housing  1  to the vehicle body. 
         [0025]    A carrier  4 , which may be made of a metal or a high strength plastic material, is fixedly mounted in the housing  1 . The carrier  4  mainly comprises three portions, i.e., a middle carrier portion  4   a , a proximal carrier portion  4   b  extending towards the proximal side in the axial direction from a substantially center part of the middle carrier portion, and a distal carrier portion  4   c  extending towards the distal side in the axial direction from the substantially center part of the middle carrier portion. Said three portions may be formed integrally, or be formed separately and then assembled together. 
         [0026]    The proximal carrier portion  4   b  and the distal carrier portion  4   c  are preferably in the form of cylinders disposed co axially, and the proximal carrier portion  4   b  and the distal carrier portion  4   c  comprise internal cavities which are configured to receive a valve body  6  therein, at least one of the internal cavities being provided with an element or structure, such as a guide pin, a guide slot or the like, for preventing the valve body  6  from relative rotating therein. 
         [0027]    It is noted that, in the present disclosure, “proximal”, “backwards” or similar expressions indicate the side near the brake pedal, while “distal” or “forwards” or similar expressions indicate the side away from the brake pedal or near the primary brake cylinder of the vehicle brake system. 
         [0028]    The valve body  6 , which functions as a booster force outputting element, is disposed in and carried by the proximal carrier portion  4   b  and the distal carrier portion  4   c  in a way of axially movable. The detailed structure of the valve body  6  is more clearly shown in  FIG. 2 . 
         [0029]    In general, the valve body  6  comprises three portions in the axial direction, i.e., a middle driven portion  6   a , a proximal sleeve portion  6   b  extending towards the proximal side from the proximal end of the middle driven portion in the axial direction, and a distal sleeve portion  6   c  extending towards the distal side from the distal end of the middle driven portion in the axial direction. Said three portions are formed therein with corresponding accommodating spaces  6 - 1 ,  6 - 2  and  6 - 3  which are in communication with one another in the axial direction. Further, said three portions may be formed integrally, or be formed separately and then assembled together. 
         [0030]    The accommodating space  6 - 1  of the middle driven portion  6   a  has a substantially constant inner diameter. 
         [0031]    The accommodating space  6 - 2  of the proximal sleeve portion  6   b  comprises a small diameter portion  6 - 2   a , an intermediate portion  6 - 2   b  and a large diameter portion  6 - 2   c  from the distal side to the proximal side, having diameters increased in sequence. A shoulder  6 - 2   d  facing towards the proximal side is formed between the intermediate portion  6 - 2   b  and the large diameter portion  6 - 2   c.    
         [0032]    The accommodating space  6 - 3  of the distal sleeve portion  6   c  comprises a proximal small diameter portion  6 - 3   a  and a distal large diameter portion  6 - 3   b . A shoulder  6 - 3   c  facing towards the distal side is formed between the proximal small diameter portion  6 - 3   a  and the distal large diameter portion  6 - 3   b . The small diameter portion  6 - 3   a  is defined by an end surface  6 - 3   d  facing towards the distal side and a periphery wall extending towards the distal side from the end surface  6 - 3   d . The large diameter portion  6 - 3   b  is defined by a plane in which the shoulder  6 - 3   c  lies and a periphery wall extending towards the distal side from this plane. 
         [0033]    The proximal sleeve portion  6   b  is fitted in the proximal carrier portion  4   b  and is slidable in the proximal carrier portion  4   b  in the axial direction. The distal sleeve portion  6   c  is fitted in the distal carrier portion  4   c  and is slidable in the distal carrier portion  4   c  in the axial direction. In this way, the valve body  6  as a whole is able to be moved in the carrier  4  in the axial direction. 
         [0034]    Further, the valve body  6  is not able to be rotated around its central axis in the carrier  4 . For this end, a guiding element (not shown), such as a key, a guiding pin and the like may be provided between the proximal sleeve portion  6   b  and the proximal carrier portion  4   b  and/or be provided between the distal sleeve portion  6   c  and the distal carrier portion  4   c.    
         [0035]    The middle driven portion  6   a  is the part where the valve body  6  is driven to move axially. In the example shown in  FIG. 1 , the middle driven portion  6   a  is in the form of a ball screw, that is, the outer surface of the middle driven portion  6   a  is formed with a helical raceway. A gear  8  is carried by the middle driven portion  6   a  and has a radially inner sleeve part  8   a  and a radially outer gear part  8   b . The central axis of the gear  8  is collinear with the central axis of the middle driven portion  6   a.    
         [0036]    The gear  8  is mounted in the middle carrier portion  4   a  by axially opposite bearings  10  (for example, thrust bearings) so that the gear  8  is rotatable around its central axis but is not movable in the axial direction. 
         [0037]    Bearing balls  11  are carried by the sleeve part  8   a  on its inner periphery and are received in the raceway of the middle driven portion  6   a . In this way, the gear  8 , when driven to rotate, is able to drive the middle driven portion  6   a  to move axially, and thus the valve body  6  is moved in the axial direction relative to the carrier  4 . 
         [0038]    The gear  8  is provided with rotational power by a rotary motor  12 , which is mounted in the housing  1  and has an output shaft carrying a pinion  14 . The pinion  14  is disposed in the middle carrier portion  4   a  and engages with the outer gear part  8   b  of the gear  8 . 
         [0039]    The motor  12  may be a brushless DC motor widely used in vehicles. The motor  12  may be powered by the vehicle battery. The motor  12  is connected with and controlled by an ECU (not shown). 
         [0040]    The transmission ratio of the pinion  14  to the gear  8  and that of the gear  8  to the middle driven portion  6   a  are selected so that the middle driven portion  6   a  may obtain a proper axial speed when the motor  12  is activated. In the condition that the transmission ratio of the pinion  14  to the gear  8  engaged directly therewith as shown in  FIG. 1  is not high enough, one or more additional transmission gears may be provided between the pinion  14  and the gear  8 . 
         [0041]    It is appreciated that the configuration for driving the middle driven portion  6   a  to move axially as shown in  FIG. 1  is only illustrative, not restrictive. Various suitable manners for driving the middle driven portion  6   a  to move axially by the motor are conceivable to those skilled in the art based on the spirits and concepts disclosed by the invention. For example, the driving mechanism between the gear  8  and the middle driven portion  6   a  may be substituted by a worm-gear type, a gear-rack type and the like. 
         [0042]    Further, the rotary motor  12  illustrated here may be substituted by a linear motor, which may be kinematically coupled to the middle driven portion  6   a  by a lever type transmission and thus drives the middle driven portion  6   a  to move axially. 
         [0043]    Furthermore, it is appreciated that the motor  12  may drive the valve body  6  at any portion thereof, rather than limited to the middle driven portion  6   a . For example, it is possible that the motor  12  drives the proximal sleeve portion  6   b  or the distal sleeve portion  6   c  to move axially. 
         [0044]    Thus, it is appreciated that any manners of driving the valve body  6  to move axially relative to the carrier  4  by the motor fall within the scope of protection of the invention. 
         [0045]    A plunger  16 , which functions as a brake force outputting element, is disposed in the valve body  6 . The detailed structure of the plunger  16  is more clearly shown in  FIG. 3 . 
         [0046]    The plunger  16  comprises a shaft portion  16   a  extending in the axial direction and an enlarged portion  16   b  extending towards proximal side from the proximal end of the shaft portion. The proximal end of the enlarged portion  16   b  is formed with a flange portion  16   e  protruding outwards radially. The flange portion  16   e  defines an end surface  16   f  of the plunger  16  which faces the proximal side. 
         [0047]    The shaft portion  16   a  is inserted into the axial accommodating space  6 - 1  of the middle driven portion  6   a . A radial locating hole  20  is formed through the middle driven portion  6   a , and a corresponding radial through hole  22  is formed through the shaft portion  16   a . A locating pin  18  is inserted through the through hole  22  and is fixed into the locating hole  20 . The through hole  22  has a dimension in the axial direction slightly larger than the locating hole  20 , so that the shaft portion  16   a  is able to move a small axial distance in the axial direction relative to the middle driven portion  6   a . This axial distance is substantially equal to the difference between the axial dimensions of the through hole  22  and the locating hole  20 . 
         [0048]    By providing the locating pin  18  and said difference between the axial dimensions, the plunger  16  is allowed to move axially in synchronization with the valve body  6  after the plunger  16  has been moved by a small axial distance in the axial direction. 
         [0049]    An accommodating space  16 - 1  is formed in the enlarged portion  16   b . The accommodating space  16 - 1  is defined by an end wall  16   c  facing towards the proximal side and a periphery wall  16   d  extending towards the proximal side from the end wall. 
         [0050]    A portion of a push bar  24  is received in the accommodating space  16 - 1 , the push bar  24  being driven by the brake pedal (not shown). The push bar  24  extends in the axial direction and has a central axis collinear with the central axis of the plunger  16 . The push bar  24  comprises a middle bar portion, a proximal end  24   a  having an enlarged diameter and a distal end  24   b  in the form of a ball. The proximal end  24   a  defines a shoulder  24   c  facing towards the distal side. 
         [0051]    The proximal end  24   a  of the push bar  24  is configured to be driven by the brake pedal, so that the push bar  24  moves towards the distal side in the axial direction. The ball shaped distal end  24   b  of the push bar  24  is received in the accommodating space  16 - 1  of the enlarged portion  16   b  and pushes against the end wall  16   c , which defines the accommodating space  16 - 1 , in a direction towards the distal side. 
         [0052]    The distal end of the shaft portion  16   a extends to an axial position near the end surface  6 - 3   d  of the distal sleeve portion  6   c . A reaction disk  28  is disposed in the accommodating space  6 - 3  of the distal sleeve portion  6   c , and a plunger plate  26  is disposed in the small diameter portion  6 - 3   a . The plunger plate  26  is clamped in the axial direction between an inner portion of the proximal side surface of the reaction disk  28  and the distal end of the shaft portion  16   a . An outer periphery portion of the proximal side surface of the reaction disk  28  is pushed against by the shoulder  6 - 3   c  in the direction towards the distal side. 
         [0053]    The distal side surface of the reaction disk  28  biases against a push rod  30  in the direction towards the distal side. An axially distal end of the push rod  30  is coupled with a main piston  32  of the primary brake cylinder of the vehicle brake system. 
         [0054]    The reaction disk  28  is preferably elastic, for example, made of rubber having elasticity. 
         [0055]    The brake booster further comprises a returning spring  34  having a proximal end biasing against the distal end surface of the distal sleeve portion  6   c  in a direction towards the proximal side, and a distal end fixed in the housing  1 . In a rest position (non-braking position) of the brake booster as shown in  FIG. 1 , the returning spring  34  pushes the distal sleeve portion  6   c  in the direction towards the proximal side, so that the proximal end surface of the distal sleeve portion  6   c  biases against a stop portion  4   d  formed or arranged in the carrier  4 , where the home position (the right most position in  FIG. 1 ) of the valve body  6  is defined. Now the plunger  16  is pushed by the main piston  32  via the push rod  30 , the reaction disk  28  and the plunger plate  26  in the direction towards the proximal side, so that the plunger  16  is also in its home position. 
         [0056]    It is appreciated that, as an substitution or addition to the returning spring  34  which biases the valve body  6  so that it has a tendency of returning back to and being kept in its home position, the outer periphery portion of the reaction disk  28  may push in the axial direction against the shoulder  6 - 3   c  of the distal sleeve portion  6   c  so that the valve body  6  tends to be returned back to and then be kept in its home position. 
         [0057]    A poppet  36  is disposed in the proximal sleeve portion  6   b  of the valve body  6 . The poppet  36  is in the form of a substantially circular disk and has a central through hole which is inserted through by the middle bar portion of the push bar  24 . In the rest position of the brake booster shown in  FIG. 1 , the proximal end surface  16   f  of the plunger  16  and the shoulder  6 - 2   d  in the proximal sleeve portion  6   b  of the valve body  6  have substantially the same axial position, and the poppet  36  is biased in the direction towards the distal side against at least one of the proximal end surface  16   f  of the plunger  16  and the shoulder  6 - 2   d  in the proximal sleeve portion  6   b  of the valve body  6  by biasing means arranged between the poppet  36  and the push bar  24 . 
         [0058]    In the example shown in  FIG. 1 , the biasing means arranged between the poppet  36  and the push bar  24  comprises an inserted tube  38  slidably disposed in the proximal sleeve portion  6   b  at an axial position which is more proximal than that of the poppet  36 , the inserted tube  38  comprising a distal end inward flange  38   a  and a proximal end outward flange  38   b . When the proximal end outward flange  38   b  biases against the proximal end of the proximal sleeve portion  6   b , the inserted tube  38  is prevented from further moving in the direction towards the distal side relative to the proximal sleeve portion  6   b . The biasing means further comprises a sleeve  44  mounted to the middle bar portion of the push bar  24  and being configured to bias against the shoulder  24   c  of the push bar  24 , the proximal end of the sleeve  44  being provided with an outward flange, a first compression spring  46  compressed between the poppet  36  and the distal end surface of the inserted tube  38 , and a second compression spring  48  compressed between the distal end inward flange  38   a  of the inserted tube  38  and the proximal end outward flange of the sleeve  44 . 
         [0059]    The second compression spring  48  has a pushing force higher than that of the first compression spring  46 . By means of the biasing means comprising the two compression springs, and in combination of the action of the locating pin  18 , positional relation between the valve body  6  and the plunger  16  is set. 
         [0060]    The brake booster may comprise a sealing shield sleeve (not shown), for example, a rubber shield sleeve, for protecting the functional components of the brake booster. 
         [0061]    According to an important aspect of the invention, electric circuit paths L 1  and L 2  are formed between the poppet  36  and the two of the valve body  6  and the plunger  16  respectively, with reference to  FIG. 4 . The ECU determines the braking purpose of the driver according to the on-off states of the two electric circuit paths. For this reason, the poppet  36  is made of a conductive material, for example, metal, and the valve body  6  and the plunger  16  are made of a conductive material, for example, metal at least in the portions of them to be in contact with the poppet  36  to form the electric circuit paths. 
         [0062]    As describe above, in the rest position of the brake booster shown in  FIG. 1 , the proximal end surface  16   f  of the plunger  16  and the shoulder  6 - 2   d  in the proximal sleeve portion  6   b  of the valve body  6  are both in contact with the poppet  36 . Now, as shown in  FIG. 4 , the poppet  36  is pushed against by the biasing means (here illustrated schematically and represented by “P”) in the direction towards the distal side, and the electric circuit path L 1  between the valve body  6  and the poppet  36  and the electric circuit path L 2  between the plunger  16  and the poppet  36  are all in “on” state. 
         [0063]    For increasing the reliability of the contact between the poppet  36  and the two of the valve body  6  and the plunger  16 , a protrusion  6 A (with reference to  FIG. 2 , etc.) protruding towards the poppet  36  is formed on the shoulder  6 - 2   d  in the proximal sleeve portion  6   b  of the valve body  6 , and a protrusion  16 A (with reference to  FIG. 3 , etc.) protruding towards the poppet  36  is formed on the proximal end surface  16   f  of the plunger  16 . The protrusions  6 A and  16 A may each be in the form of a ring or be formed by discrete points of arc segments. 
         [0064]    Alternatively, the distal side surface of the poppet  36  may be formed with protrusions protruding towards the valve body  6  and the plunger  16  respectively, by means of which, the reliability of the contact between the poppet  36  and the two of the valve body  6  and the plunger  16  can also be increased. 
         [0065]    As shown in  FIG. 4 , the electric circuit path L 1  flows through the protrusion  6 A from the valve body  6  to the outer periphery of the poppet  36  in the directions shown by corresponding arrows, and the electric circuit path L 2  flows through the protrusion  16 A from the plunger  16  to the inner periphery of the poppet  36  in the directions shown by corresponding arrows. The inner and outer peripheries of the poppet  36  may be electrically separated from each other by an insulative material. 
         [0066]    Thus, it can be considered that an electric circuit switch (referred to here as release switch S 1 ) is formed between the valve body  6  and the poppet  36 , and another electric circuit switch (referred to here as boost switch S 2 ) is formed between the plunger  16  and the poppet  36 . The ECU can determine the on-off states of the electric circuit paths L 1  and L 2 , or the on-off states of the two electric circuit switches, by measuring whether there is an electric current in each of the electric circuit paths L 1  and L 2 . Then, the ECU can determine the braking purpose of the driver. 
         [0067]      FIGS. 5 to 7  show the actions and on-off states of the boost switch and the release switch of the brake booster according to the invention in a braking mode, and  FIGS. 8 to 10  show the actions and on-off states of the boost switch and the release switch of the brake booster according to the invention in a brake releasing (un-braking) mode. 
         [0068]    Now the braking operations of the brake booster according to the invention will be described with reference to  FIG. 1  and  FIGS. 5 to 7 . 
         [0069]    As shown in  FIG. 1 , the brake booster is in the rest position, i.e., the driver does not press down the brake pedal. Now, as shown in  FIG. 5 , the valve body  6  and the plunger  16  are both in contact with the poppet  36 , and the release switch S 1  and the boost switch S 2  in the electric circuit paths L 1  and L 2  are both in an “on” state. The ECU determines the states of the two switches by detecting the electric currents flowing through the electric circuit paths L 1  and L 2 . 
         [0070]    Next, the driver presses down the brake pedal to perform braking of the vehicle. Pressing down the brake pedal results in that the push bar  24  moves towards the distal side in the axial direction against the pushing force of the second compression spring  48 . The distal end  24   b  of the push bar  24  pushes the plunger  16  to move towards the distal side, and the distal end of the plunger  16  pushes against the inner portion of the reaction disk  28  in the direction towards the distal side via the plunger plate  26 , so that the inner portion of the reaction disk  28  pushes the main piston  32  of the primary brake cylinder towards the distal side via the push rod  30 . In this way, the manual brake force applied by the driver is transmitted to the main piston  32 . 
         [0071]    In the initial stage when the plunger  16  moves towards the distal side, the valve body  6  remains stationary in the axial direction under the biasing action of the returning spring  34  towards the proximal side. In this stage, i.e., when the brake booster is in an intermediate position in a transition stage from its rest position to its braking position, as shown in  FIG. 6 , the valve body  6  keeps in contact with the poppet  36 , so that the release switch S 1  keeps its “on” state, thus there is an electric current flowing through the electric circuit path L 1 . On the other hand, the plunger  16  moves towards the distal side in the direction of arrow R 1 , and is thus away from and out of contact with the poppet  36 . In this condition, the boost switch S 2  is turned off, and thus the electric current flowing through the electric circuit path L 2  disappears. The ECU detects the change in the switch states, that is, the condition where the release switch S 1  and the boost switch S 2  are both in “on” state is changed to a condition where the release switch S 1  is in “on” state while the boost switch S 2  is in “off” state, so that the ECU determines the braking action of the driver. 
         [0072]    Then, the ECU activates the motor  12  to run in a forward direction, and the forward rotation of the motor  12  is transmitted to the screw type middle driven portion  6   a  of the valve body  6  via the pinion  14  and the gear  8 , so that the valve body  6  is moved towards the distal side against the pushing force of the returning spring  34 . The shoulder  6 - 3   c  in the distal sleeve portion  6   c  of the valve body  6  pushes against the outer periphery portion of the reaction disk  28  in the direction towards the distal side, so that the outer periphery portion of the reaction disk  28  pushes the main piston  32  of the primary brake cylinder towards the distal side via the push rod  30 . In this manner, the boost force provided by the motor  12  is transmitted to the main piston  32 . 
         [0073]    Under the action of both the manual brake force provided by the driver and the boost force provided by the motor, the main piston  32  forces the brake liquid in the primary brake cylinder towards the brake devices equipped to corresponding wheels of the vehicle to perform braking to the vehicle. Now, the brake booster is in the braking position. 
         [0074]    When the valve body  6  moves towards the distal side, the biasing means between the poppet  36  and the push bar  24  forces the poppet  36  to move towards the distal side in the direction of arrow R 1  along with the valve body  6 , as shown in  FIG. 7 , and ultimately the poppet  36  comes into contact with both the valve body  6  and the plunger  16  again. In this way, the release switch S 1  and the boost switch S 2  reach the states shown in  FIG. 7 , i.e., they are both in “on” state. The ECU determines the continuation of the braking action, and thus controls the motor  12  to rotate forwards; alternatively, the ECU controls the motor  12  to rotate forwards in a period of time and then stops the motor  12 , to wait the end of the braking action. 
         [0075]    Then the brake releasing operation of the brake booster according to the invention will be describe with reference to  FIG. 1  and  FIGS. 8 to 10 . 
         [0076]    In the continuation state of the braking action, as shown in  FIG. 8 , the release switch S 1  and the boost switch S 2  are both in “on” state. 
         [0077]    When the braking action is to be ended, the driver releases the brake pedal. The hydraulic force in the primary brake cylinder applied to the main piston  32  is transmitted to the plunger  16  via the push rod  30 , the inner portion of the reaction disk  28  and the plunger plate  26  in the direction towards the proximal side, so that the plunger  16  is moved back to its home position shown in  FIG. 1  in the direction towards the proximal side. Meanwhile, the poppet  36  is moved back to its home position shown in  FIG. 1  in the direction towards the proximal side by biasing means disposed between the poppet  36  and the push bar  24 . In the initial stage of the brake releasing operation, i.e., when the brake booster is in a returning intermediate position in a transition stage from its braking position to its rest position, as shown in  FIG. 9 , the plunger  16  and the poppet  36  are both moved towards the proximal side in the direction of arrow R 2  as illustrated. Under the action of the first compression spring  46 , the plunger  16  and the poppet  36  keep in contact with each other, while the valve body  6  is stationary in the axial direction and is thus away from and out of contact with the poppet  36 . In this way, the release switch S 1  is turned off. The ECU detects the change in the switch states, that is, the condition where the release switch S 1  and the boost switch S 2  are both in “on” state is changed to a condition where the boost switch S 2  is in “on” state while the release switch S 1  is in “off” state, so that the ECU determines that the driver is ending the braking action. 
         [0078]    Then, the ECU activates the motor  12  to run in a backward direction, and the backward rotation of the motor  12  is transmitted to the screw type middle driven portion  6   a  of the valve body  6  via the pinion  14  and the gear  8 , so that the valve body  6  is moved towards the proximal side. The pushing forces of the returning spring  34  and the outer periphery portion of the reaction disk  28  force the valve body  6  to move towards the proximal side. As shown in  FIG. 10 , by the movement of the valve body  6  towards the proximal side in the direction of arrow R 2 , the valve body  6  ultimately comes into contact with the poppet  36  again, so that the condition of the switch states change to the condition in which both the release switch S 1  and the boost switch S 2  are in “on” state. The ECU determines the finish of the brake releasing (ending) action and then stops the backward rotation of the motor  12 . 
         [0079]    It is noted that, if the pushing force of the returning spring  34  is high enough, and in the mean time there is no self-lock between the bearing balls in the sleeve part  8   a  of the gear  8  and helical raceway of the middle driven portion  6   a , or in other words, the valve body  6  when moved axially can drive the gear  8  to rotate, the valve body  6  may return to its home position in the direction towards the proximal side only by means of the pushing force of the returning spring  34 , without needing the active backward rotation of the motor  12 . 
         [0080]    No matter it is in the braking operation or in the brake releasing operation, the plunger  16  is firstly moved by a small distance relative to the valve body  6  in the axial direction first. By means of adopting the locating pin  18  and the dimensional difference between the through hole  22  and the locating hole  20 , the first moving of the plunger  16  becomes possible. 
         [0081]    It can be seen that, according to the invention, the motor  12  is used as the boost force source as a substitution to the solution of the prior art where the vacuum in the inlet pipe of the engine is used as the boost force source. Thus, the brake booster of the invention provides a possibility that the brake booster can provide the boost force by the motor even after the engine has been turned off, which contributes to improvements in the driving comfortability and safety of the vehicle. 
         [0082]    At this point, it should be noted that, whether it needs to use the electric brake booster of the invention in assistant in the vehicle braking after the engine has been turned off depends on the power management of the whole vehicle. Anyway, the present disclosure provides the possibility of providing the boost force after the engine has been turned off. 
         [0083]    Further, according to the invention, the release switch S 1  and the boost switch S 2  are formed by the structure of the brake booster itself, i.e., the poppet  36  and the two of the valve body  6  and the plunger  16 , and the switch states of the two switches in combination are substantially corresponding to the air valve and the vacuum valve of the traditional vacuum booster. The motor  12 , the release switch S 1  and the boost switch S 2  are all connected with the ECU which determines the purpose of the braking action of the driver on the basis of the electric signals from the boost switch and the release switch and then actively and precisely controls the operation timing, direction of rotation, speed and other parameters of the motor, thus the action of the brake booster can be controlled precisely in real time. Further, no stroke sensor or rotation sensor is needed in the invention. Thus, the brake booster of the invention can be considered as an intelligent brake booster. 
         [0084]    While certain embodiments have been described here, the detailed structures of these embodiments are presented by way of example only and are not intended to limit the scope of the inventions. The attached claims and their equivalents are intended to cover all the modifications, substitutions and changes as would fall within the scope and spirit of the invention.