Abstract:
The present invention provides a brake booster that, although simple in construction, capable of adjusting the braking force derived by operation of a solenoid mechanism without worsening the braking response. In the brake booster according to the present invention, when a movable member ( 34 ) is shifted toward the rear side by electromagnetic force of a solenoid ( 31 ), the shifting movement is transmitted to a valve seat member ( 21 ) through pins ( 50 ) to open an atmospheric valve ( 27 ) through which air is introduced into a variable pressure chamber ( 9,10 ), whereby a pressure difference is created between the variable pressure chamber and a constant pressure chamber and generates booster output (thrust output force). In the present invention, pressure in the constant pressure chamber is introduced into a chamber ( 54 ) formed at one end of the movable member ( 34 ) through a passage ( 56 ), and atmosphere is introduced into a chamber ( 55 ) formed at the other end of the movable member ( 34 ) through a passage ( 57 ), thereby exerting on the movable member ( 34 ) a reaction force caused by the pressure difference between the variable pressure chamber and the constant pressure chamber. Thus, the booster output becomes proportional to the electric current supplied to the solenoid ( 31 ). Further in the present invention, the size of the passage ( 57 ) is determined properly so as to limit air introduced therethrough into the chamber ( 54 ) and keep the reaction force small until the booster output reaches an adequately level, thereby preventing the breaking response from deteriorating.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a control-type brake booster capable of automatically increasing and decreasing braking power by electromagnetically operating an atmospheric valve or vacuum valve. 
     An example of such a brake booster is disclosed in Japanese Patent Laid-open No. 7-503214 (1995), in which a valve body contains a solenoid mechanism with a member movable by applying electricity to the solenoid of the solenoid mechanism, and a movement of the movable member opens a valve mechanism, which may be an atmospheric valve or vacuum valve, independently of the valve operation by movements of the input rod connected to the brake pedal. 
     It has been desired that as is the braking power output from a brake booster when its valve mechanism is operated by movements of the input rod connected to the brake pedal, the braking power output from a brake booster having the above-mentioned solenoid mechanism can also be adjusted across a wide range when operated by the activation of the solenoid mechanism. 
     However, in the above-mentioned conventional brake booster, the solenoid mechanism merely functions to open the valve mechanism to the degree determined by the magnitude of electric current supplied to the solenoid, and no correlation (or proportional relationship) exists between the magnitude of the electric current supplied to the solenoid mechanism and the magnitude of the braking power output from the brake booster. 
     As a makeshift measure to obtain a desired magnitude of braking power from a brake booster when its solenoid mechanism is operated is to control the operation of the solenoid mechanism based on a feed-back signal indicating the braking power being output from the brake booster, for instance, on an output signal from a master cylinder pressure sensor provided for detecting pressure of braking liquid output from the master cylinder. 
     However, since an additional sensor is required that functions like the master cylinder pressure sensor to output a signal indicating the braking power being output from the brake booster, the entire system becomes more expensive. 
     SUMMARY OF THE INVENTION 
     The present invention aims to eliminate the drawbacks in the above-mentioned conventional brake booster. An object of the present invention is to provide a brake booster that, while being simple in construction, can fully adjust the braking power across a wide range when its solenoid mechanism is operated. 
     According to one aspect of the present invention, there is provided a brake booster wherein an interior of a shell is divided into a constant pressure chamber and a variable pressure chamber by a power piston, and a valve mechanism for controlling supply of operating fluid to the variable pressure chamber by a shifting movement of a plunger connected to an input rod cooperating with a brake pedal is provided within a valve body supported by the power piston, so that a thrust force generated on the power piston by pressure difference between the variable pressure chamber and the constant pressure chamber based on an operation of the valve mechanism is output as a booster output force (thrust output force). 
     The brake booster further comprises an electromagnetic biasing means having a movable member for operating the valve mechanism independently from an operation of the input rod, and a reaction force mechanism for applying reaction force corresponding to the pressure difference between the variable pressure chamber and the constant pressure chamber to the movable member, whereby the reaction force mechanism applies the reaction force to the movable member so that an increasing rate of the booster output force is in accord with an increasing rate of the reaction force applied to the movable member. 
     According to another aspect of the present invention, in a brake booster having the electromagnetic biasing means mentioned above and the reaction force mechanism mentioned above, the reaction force mechanism applies the reaction force to the movable member at the same time or with a delay after the booster outputs force. 
     In the above two aspects of the present invention in which the reaction force corresponding to the pressure difference between the variable pressure chamber and the constant pressure chamber is exerted on the movable member, the booster output force becomes proportional to the magnitude of the electric current supplied to the electromagnetic biasing mechanism. Further, since the reaction force is exerted on the movable member so that the booster output force increasing rate is in accord with the reaction force increasing rate acting on the movable member, or, so that the reaction force is created at the same time or after the booster output force is generated, the booster output force generated is not prevented by the reaction force. 
     In the present invention, the reaction force mechanism is provided with at least one passage providing communication between a pressure chamber formed at one end of the movable member and the variable pressure chamber. The at least one passage has a limited flow area for limiting air flowing into the pressure chamber. Preferably, the total flow area of the passage is designed 0.5-10 square millimeters. 
     Limiting the flow area of the passage as above, the timing of the reaction force exerted on the movable member is adjusted properly, and thus a better response of automatic braking can be established. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially enlarged sectional view showing a brake booster of one embodiment according to the present invention (a partial enlarged view of FIG.  2 ); 
     FIG. 2 is a partial sectional view showing the brake booster of the same embodiment according to the present invention (a partially enlarged view of FIG.  3 ); 
     FIG. 3 is a sectional view showing the brake booster of the same embodiment of the present invention; and 
     FIG. 4 is an enlarged cross-sectional view of X—X line in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, a brake booster according to an embodiment of the present invention will be explained with reference to FIGS. 1 through 4. 
     A brake booster shown on FIGS. 1 through 4 is a tandem type and has a shell  1  which is divided into a front chamber and a rear chamber by a center shell  2 . These two respective chambers are further divided into constant pressure chambers  7 , 8  and variable pressure chambers  9 , 10  by power pistons  5 , 6  having diaphragms  3 , 4 . The power pistons  5 , 6  support a valve body  11  that comprises a large diameter cup portion  11   a  and a small diameter cylindrical portion  11   b  connected thereto. The valve body  11  is disposed such that the cup portion  11   a  thereof sealingly and slidably passes through the center shell  2 , and that the cylindrical portion  11   b  thereof sealingly and slidably passes through a cylindrical support portion  1   a  arranged on the rear side of the shell  1  and extends rearwardly of the support portion. 
     The valve body  11  is formed with a constant pressure passage (negative pressure passage)  12  that provides communication between the two constant pressure chambers  7  and  8  and between the constant pressure chambers  7 , 8  and the inside of the cylindrical portion  11   b  of the valve body  11 . The valve body  11  is also formed with an atmosphere passage  13  that provides communication between the two variable pressure chambers  9  and  10  and between the two variable pressure chambers  9 , 10  and the inside of the cylindrical portion  11   b  of the valve body  11 . Negative pressure, e.g. negative pressure from the engine, is introduced into the constant pressure chamber  7  shown on the left side (front side) of the FIGURES. The negative pressure is also introduced into the constant pressure chamber  8  shown on the right side (rear side) of the Figures via the negative pressure passage  12 . On the other hand, the atmosphere is introduced into the cylindrical portion  11   b  of the valve body  11  through a filter unit  14  having a silencing function. The atmosphere is supplied into the two variable pressure chambers  9  and  10  through the atmosphere passage  13  by operation of valve mechanism  15  (as described later). 
     The valve mechanism  15  is connected to an input rod  16  cooperating with a brake pedal (not shown), which comprises: a plunger  18  slidable along a hollow guide  17  (as described later) disposed in the valve body  11 ; an annular atmospheric valve seat  19  formed at rear end of the plunger  18 ; a cylindrical valve seat member  21  slidably inserted into the valve body  11  via a seal member  20 ; an annular negative pressure valve seat  22  formed at rear end of the valve seat member  21 , a poppet valve  26  whose proximal end portion is fixed to the cylindrical portion  11   b  of the valve body  11  by means of a ring member  23  and a pressing member  24  and which is usually biased to sit on the atmospheric valve seat  19  and the negative pressure valve seat  22  by means of a valve spring  25  held at one end thereof onto the input rod  16 . The atmospheric valve seat  19  and the annular inner portion formed at the distal end of the poppet valve  26  resting thereon constitute an atmospheric valve  27 . The negative pressure valve seat  22  and the annular outer portion formed at the distal end of the poppet valve  26  resting thereon constitute a vacuum valve  28 . The atmosphere or the negative pressure is selectably supplied into the two variable pressure chambers  9 , 10  when the atmospheric valve  27  or the vacuum valve  28  is opened, respectively. Meanwhile, a return spring  29  is placed between the pressing member  24  and the input rod  16  (see FIG.  2 ). The plunger  18  is forced so as to abut the atmospheric valve seat  19  onto the poppet valve  26  by a biasing force of the return spring  29  and the valve spring  25  during a rest state of the brake booster. 
     A solenoid mechanism (electromagnetic biasing means)  30  is disposed in the cup portion  11   a  of the valve body  11 . The solenoid mechanism  30  substantially comprises: a solenoid  31 ; a housing  32  containing the solenoid  31  therein; and an annular movable member (an armature)  34  having a seal member  33  in the housing  32  (see FIG. 1) and being slidably disposed in the housing  32 . The housing  32  comprises: a double cylindrical shaped main body portion  35  containing the solenoid  31  therein; an extension cylindrical portion  36  extending rearward from the rear end of the main body portion  36 ; and a separating portion  37  separating the main body portion  35  thereof from the extension cylindrical portion  36 . While the extension cylindrical portion  36  of the housing  32  is inserted into the inner surface of the valve body  11  via a seal member  38 , and a flange portion  39  formed on the outer circumference surface of the main body portion  35  rests on a step portion  40  formed on the inner surface of the cup portion  11   a  of the valve body  11 , the housing  32  is restrained in its movement against the valve body  11  by biasing force of a return spring  41  disposed in the constant pressure chamber  7  provided on the front side. An inner flange  5   b  is provided on a cylindrical portion  5   a  of the power piston  5  provided on the front side (see FIG. 1) and is interposed between the flange portion  39  and the step portion  40 . Thus, the valve body  11  and the housing  32  move together in the axial direction. In FIG. 3, reference mark “W” is a wire for supplying electricity to the solenoid  31 . The wire “W” usually extends outside of the brake booster via a connector (not shown) disposed in the front face of the shell  1 . 
     The hollow guide  17  is disposed inside of the main body  35  of the housing  32 . The hollow guide  17  comprises a large end portion  42  engaged with a step portion  35   a  of the main body  35  and a small diameter axis portion  43  extending from the large end portion  42  rearwardly through the separating portion  37  of the housing  32  (see FIG.  2 ). The plunger  18  is slidably inserted into the small diameter axis portion  43 . A stop key  44  extending the radial direction is inserted into the valve body  11  through the atmosphere passage  13 , and the distal end portion of the stop key  44  is connected to the plunger  18 . The proximal end portion of the stop key  44  is arranged to abut against a stopper plate  45  (see FIG. 1) fixed on a cylindrical support portion  1   a  of the shell  1 . The maximum setback position of the plunger  18  is the position where the stop key  44  abuts against the stopper plate  45 . The stop key  44  is also inserted through a opening (long face hole)  21   a . The valve seat member  21  is allowed to move relatively to the plunger  18  within the range where the stop key  44  is movable within the opening  21   a.    
     A reaction disk  46  and a large diameter proximal end portion  47   a  of an output rod  47  are disposed on the forward side of the step portion  35   a  of the main body  35  of the housing  32 . The distal end portion of the output rod  47  is extended forwardly through the front side of the shell  1  air-tightly. The distal end portion of the output rod  47  is connected to and is simultaneously moved with a piston of a master cylinder (not shown). Further, the proximal end large diameter portion  47   a  of the output rod  47  and the reaction disk  46  are restrained from coming out of the housing  32  by means of a retainer  48  press-fixed on the front end of the valve body  11  by means of the return spring  41 . A recess  42   a  is formed in the front face of the large end portion  42  of the hollow guide  17 . A pressure receiving plate  49  is disposed in the recess  42   a . A depth of the recess  42   a  is designed slightly more than the thickness of the pressure receiving plate  49 . The front end of the plunger  18  is located slightly forwardly from the bottom of the recess  42   a  when the brake booster is in its rest state. A little gap “S” (see FIG. 1) is formed between the pressure receiving plate  49  and the reaction disk  46  in such a state. 
     There are pins  50  one ends of which are inserted and fixed into the movable member  34  of the solenoid mechanism  30  and the other ends of which are inserted into through-holes  51  formed in the separating portion  37  in the housing  32 . The valve seat member  21  having the negative pressure valve seat  22  at its rear end is urged forwardly by means of a spring  52  fixed to the plunger  18  at its one end. The front end of the valve seat member  21  is normally placed in abutment against the separating portion  37  in the housing  32 . The through-holes  51  of the separating portion  37  of the housing  32  are formed in alignment with the front end of the valve seat member  21 . The other ends of the pins  50  inserted into the through-holes  51  face the valve seat member  21  with a small gap therebetween. Further, the movable member  34  of the solenoid mechanism  30  is moved rearwardly by energizing the solenoid  31 , whereby the pins  50  fixed to the movable member  34  move the valve seat member  21  rearwardly against the urging force from the spring  52  and the valve spring  25 . Consequently, the poppet valve  26  is separated from the atmospheric valve seat  19  formed at the rear end of the plunger  18 . Thus, the atmospheric valve  27  opens independently of the movement of the plunger  18 , that is, the movement of the input rod  16 . 
     A seal member  53  held on the inner circumference of the movable member  34  provides sealing between the movable member  34  and the hollow guide  17 . The outer circumference of the movable member  34  is sealed by the seal member  33  against the housing  32 . Thus, two pressure chambers  54  and  55  are formed at both ends of the movable member  34  respectively. The pressure chamber  54  formed on the front side of the movable member  34  communicates with the constant pressure chamber  7  provided on the front side through a passage  56  formed in the housing  32 . The pressure chamber  55  formed on the rear side of the movable member  34  communicates with the variable pressure chambers  10 , 9  through a passage  57  formed in the separating portion  37  of the housing  32 . 
     Now, operation of the brake booster having the above-mentioned construction will be explained. 
     The brake booster is installed in a vehicle (not shown) by means of plural stud bolts  58  provided on the backside face of the shell  1  (see FIG.  3 ), and a brake pedal (not shown) is connected to the input rod  16  in this state. Further, a master cylinder (not shown) is attached to the brake booster by means of a stud bolt  59  provided on the front side face of the shell  1 . The output rod  47  is connected to a piston in the master cylinder in this state. 
     In the brake booster placed in the initial condition as shown in the FIGURES (FIGS.  1  through  3 ), the constant pressure chambers  7 , 8  and the variable pressure chambers  9 , 10  are both subjected to the given (negative) pressure, and the atmospheric valve  27  and the vacuum valve  28  are closed. In this condition, when the brake pedal is depressed to urge the input rod  16 , the plunger  18  moves forward and separates the atmospheric valve seat  19  formed at the rear end thereof from the poppet valve  26 , thereby opening the atmospheric valve  27 . As a result, air flows into the variable pressure chambers  10 , 9  through the atmosphere passage  13 , thereby creating a pressure difference between the variable pressure chambers  10 , 9  and the constant pressure chambers  7 , 8 . Consequently, the power pistons  5 , 6  advance to generate thrust force which is transmitted to the output rod  47  through the valve body  11  and starts the braking action. In this initial braking condition, while the gap “S” between the pressure receiving plate  49  and the reaction disk  46  is being closed, so-called “jump-in” occurs, and the booster output force increases irrespective of the input force, thereby generating an adequate initial braking force. 
     After the jump-in ends, part of the output reaction force from the output rod  47  is transmitted back to the input rod  16  through the reaction disk  46 , the pressure receiving plate  49  and the plunger  18 . When the output reaction force (from the output rod  47 ) generated by the advancement of the valve body  11 , becomes equal to the brake pedal pressing force, the atmospheric valve  27  is closed, thereby maintaining the booster output force. Further from this condition, when the brake pedal pressing force is increased or decreased to cause imbalance between the reaction force based on the pressure difference and the brake pedal pressing force, either the atmospheric valve  27  or the vacuum valve  28  is opened again, so that the reaction force based on the pressure difference becomes equal to the brake pedal pressing force, thereby adjusting the pressure difference caused between the variable pressure chambers  10 , 9  and the constant pressure chambers  7 , 8 . Accordingly, when the solenoid  31  is not energized, the brake pedal pressing force is multiplied by a predetermined boost ratio, resulting in that the brake booster is operated as a normal brake booster. 
     In the automatic braking mode in which the solenoid  31  is energized by the electric current supplied to the solenoid  31 , the electromagnetic force acts on the movable member  34  and shifts the movable member  34  towards the rear side. At that time, the pressure chamber  54  formed at the one end of the movable member  34  becomes equal in pressure to the constant pressure chamber  7  through the passage  56 , and the pressure chamber  55  formed at the other end of the movable member  34  becomes equal in pressure to the variable pressure chamber  10  through the passage  57 , gaps formed around the pins  50  in the through-holes  51  and the atmosphere passage  13 . If the electric current supplied to the solenoid  31  increases, the electromagnetic force acting on the movable member  34  exceeds the total spring force of the spring  52  and the valve spring  25 , and the movable member  34  is shifted toward the rear side, whereby the pin  50  fixed thereto moves the valve seat member  21  towards the rear side. As a result of this, the poppet valve  26  is separated from the atmospheric valve seat  19  formed at the rear end of the plunger  18  to open the atmospheric valve  27 , thereby introducing air into the variable pressure chambers  10 , 9  and creating a pressure difference between the variable pressure chambers  10 , 9  and the constant pressure chambers  7 , 8 . Consequently, the power pistons  5 , 6  advance and generate thrust force which is transmitted to the output rod  47  through the valve body  11 , thereby starting the braking action. 
     When the air is introduced into the variable pressure chambers  10 , 9 , the air is also introduced into the pressure chamber  55  provided at the other end of the movable member  34  through the passage  57  and the gaps around the pins  50  in the through-holes  51 . Thereby, a pressure difference force caused by a pressure difference between the pressure chamber  55  and the pressure chamber  54  which is maintained at a negative pressure is created and acts on the movable member  34 . This pressure difference force acts as the reaction force in the direction in which the movable member  34  is returned to the front side (i.e., the direction opposite to the direction of the electromagnetic force acting on the movable member  34 ). As a result, when constant electric current is supplied to the solenoid  31 , that is, when constant electromagnetic force acts on the movable member  34 , the movable member  34  is returned to the front side by means of the pressure difference force. At the moment when the electromagnetic force acting on the movable member  34  becomes equal to (i.e., balanced with) the pressure difference force, the shifting movement of the movable member  34  towards the front side stops, thereby closing the atmospheric valve  27 , and the booster output force is maintained thereafter. Thus, two pressure chambers  54 , 55  formed at both ends of the movable member  34  constitute a reaction force mechanism that exerts a reaction force on the movable member  34  in proportion to the pressure difference between the variable pressure chambers  7 , 8  and the constant pressure chambers  9 , 10 . 
     Further from this condition, if the electric current supplied to the solenoid  31  is increased or decreased to cause imbalance again between the electromagnetic force and the pressure difference force, both forces acting on the movable member  34 , the movable member  34  is shifted again to balance the pressure difference force and the electromagnetic force, resulting in that either the atmospheric valve  27  or the vacuum valve  28  is opened, thereby adjusting the pressure in the variable pressure chambers  9 , 10  in accordance with the magnitude of the electric current supplied to the solenoid  31 . 
     In this way, since the booster output force becomes proportional to the electric current supplied to the solenoid  31 , by changing the electric current supplied to the solenoid  31  across a wide range, the booster output force can be adjusted across a wide range as well. Thus, it is not required to provide a sensor like the master cylinder pressure sensor which was required in the conventional techniques for controlling the booster output force at desired levels. Therefore, the construction of the brake booster can be simplified and the brake booster can be made cheaper. 
     In the control type brake booster that functions as described above, if air is introduced into the pressure chamber  55  formed at the other end of the movable member  34  before air is introduced into the variable pressure chambers  9 , 10 , the pressure difference force acting on the movable member  34  i.e. the reaction force acting on the movable member  34 , increases before an adequate booster output force is generated. As a result, the atmospheric valve  27  will be prevented from fully opening, causing a delay in introducing an adequate amount of air into the variable pressure chambers  9 , 10  and thus a delay in generating an adequate booster output force. The delay will manifest itself as a poor response of the automatic braking. To avoid that, according to the brake booster of the present invention, the total passage area of the passages  57  formed in the separating portion  37  of the housing  32  and the gaps around the pins  50  in the through-holes  51  (see FIG. 4) is made small so that the booster output force will increase at the same rate as the reaction force acting on the movable member  34  increases, or so that an adequate reaction force will be created at the same time or after the booster output force is generated. Thereby, until a sufficient booster output force is generated in response to the electric current supplied to the solenoid  31 , the atmospheric valve  27  is kept open widely, improving the braking response. It is preferable that the total flow area of the passages  57  and the gaps around the pins  50  in the through-holes  51  is about 0.5-10 square millimeters. 
     In this way, since the brake booster of this invention is constructed so that the booster output force increases at the same rate as the reaction force acting on the movable member  34  increases, or so that an adequate reaction force is created at the same time or after the booster output force is generated, thereby improving the braking response in the automatic braking mode and increasing the reliability of the brake booster. 
     The above embodiment has been described, using a tandem type brake booster. It should however be appreciated that the present invention can also be embodied in a so-called single type brake booster having a single set of variable pressure chamber and constant pressure chamber. 
     Further, in the above embodiment, the passages  57  formed in the separating portion  37  of the housing  32  and the gaps around the pins  50  in the through-holes  51  function to introduce air into the pressure chamber  55 . It should however be appreciated that only one of the passages  57  formed in the separating portion  37  of the housing  32  or the gaps around the pins  50  in the through-holes  51  may function to introduce air into the pressure chamber  55 .