Abstract:
A supply valve ( 40 ), a relief valve ( 42 ) and a microprocessor ( 46 ) control the supply of pressurized hydraulic fluid from a dedicated pressure source ( 12 ) to a brake boost piston ( 18, 60, 108 ) interposed between an operator brake pedal ( 22 ) input member ( 20, 106 ) and a master cylinder. Transducers ( 48, 50, 102 ) sense a current relationship between the input member ( 20, 106 ) and the boost piston ( 18, 60, 108 ) and supply the microprocessor ( 46 ) with a input to accordingly operate the supply valve ( 40 ) and relief valve ( 42 ). In one embodiment, a single transducer ( 102 ) senses relative motion between the input member and the piston while in a second embodiment, a single transducer ( 48 ) senses the force exerted on the piston by the input member and a third embodiment first and second transducers ( 48, 50 ) to sense relative motion between the input member and the piston.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a brake booster system and more particularly to an improved technique for controlling hydraulic boost in response to vehicle operator pedal input. 
     2. Description of the Related Art 
     Vehicle braking systems have evolved from simple mechanical brakes to hydraulic brakes and then to power assisted braking systems to reduce a driver&#39;s brake pedal effort. Many power assisted braking systems rely on the manifold vacuum created by the engine pistons as they draw air into the engine. A common power brake mechanism such as disclosed on U.S. Pat. No. 5,943,863 employs a housing that is intermediate an operator brake actuating pedal mechanism and a master cylinder. When this brake mechanism is enabled, fluid pressure is supplied to individual wheel brake cylinders or actuators. The housing includes a piston or diaphragm normally exposed on both sides to vehicle manifold vacuum. When the operator actuates the brake pedal, atmospheric pressure is admitted to one side of the piston supplying additional force to the master cylinder piston and enhanced brake line pressure to the individual wheel cylinders. Vehicle braking is still possible in the event of vacuum source failure since operator applied pedal force (without boost) is transmitted to the master cylinder through the boost mechanism. 
     Hydraulic rather than pneumatic brake boosters have also been proposed. For example, U.S. Pat. No. 4,311,085 utilizes a power steering pump as a source of pressurized hydraulic fluid to provide a power assist during a brake application. This hydraulic brake booster includes a housing which communicates with the pressure source. A control valve within the housing is operable to control the communication of fluid pressure through the housing. In order to operate the control valve, an input member coupled to an operator actuable brake pedal extends into the housing and is movable during a brake application to impart movement to the control valve. Movement of the control valve communicates fluid pressure to a pressure chamber wherein an output member is movable in response to the fluid pressure to effect a power-assisted brake application. The output member is coupled to a conventional brake system master cylinder of which U.S. Pat. No. 4,341,076 may be considered to be typical system. 
     More recently, electronic control of the valve which applies pressure fluid to the booster has been suggested. U.S. Pat. No. 6,007,160, for example, teaches a method of controlling the operation of an electrohydraulic brake booster to achieve a desired pedal feel. This patent also suggests a power steering pump as a boost pressure source. Brake pedal applied force or the distance the brake pedal travels is monitored by an electronic controller which, in turn, opens or closes a pressure control valve to increase or decrease boost. The method includes sensing brake pedal movement from a fully retracted rest position before a significant resistance to travel of the brake pedal is developed and generating a command pulse that results in application of pressurized hydraulic fluid to the boost piston assembly sufficient to overcome preloaded spring forces and seal friction in the boost piston assembly that would otherwise tend to resist further brake pedal travel. A variable resistance or linear variable displacement transducer are suggested as sensing devices. The method further includes providing a control signal override when a brake pedal “bounce” condition is detected to avoid undesired vehicle braking. A brake pedal bounce condition may occur if the brake pedal is released suddenly so that the brake pedal returns to the fully retracted rest position rapidly enough to bounce off of a mechanical stop at that position and move in the brake apply direction. The patent acknowledges earlier similar systems. Combined hydraulic and pneumatic systems have also been suggested in the prior as disclosed in U.S. Pat. Nos. 4,199,948 and 7,008,024. 
     SUMMARY OF THE INVENTION 
     The present invention takes advantage of a lost motion coupling between an operator brake input and a hydraulic boost piston and provides the advantages of a conventional vacuum boost system without the disadvantages thereof. 
     The invention comprises, in one form thereof, a hydraulically boosted vehicle brake system having an operator controlled input member and a master cylinder operable by the input member. A power boost piston is interposed between the input member and master cylinder for enhancing operator applied force to the master cylinder. A valve assembly selectively supplies pressure fluid from a pressure source to the power boost piston. A sensor arrangement determines relative motion between the input member and the boost piston and an electronic control unit responds to the sensor arrangement to control the valve assembly. The sensor arrangement may include two piezoelectric sensors to respectively identify an operator request for additional braking torque and an operator request for reduced braking torque. In another form, the sensor arrangement includes a linear travel sensor to respectively identify motion of an input member motion toward the boost piston indicative of an operator request for additional braking torque and input member motion away from the boost piston indicative of an operator request for reduced braking torque. 
     The invention discloses a method of supplying hydraulic boost pressure to a vehicle brake system by monitoring the relative axial relationship between the input member and power boost piston by increasing boost pressure to the power boost piston when the input member is urged toward the power boost piston and decreasing boost pressure to the power boost piston when the input member is urged away from the power boost piston, and maintaining the boost pressure constant when the relative axial relationship remains constant. The monitoring may include sensing for variations in the relative linear displacement between the input member and the power boost piston, or sensing for variations in the force exerted by the input member on the power boost piston. 
     An advantage of the present invention is that many features of conventional braking systems are retained. The reaction force and gain principle are essentially the same as in a conventional actuation system. There is the same failed boost performance as conventional actuation system. An active boost function is available to support pressure build during an ESP event and the system can be used as an actuator for regenerative brake systems (RBS). A high flow master cylinder is not required. In one form, the system is interchangeable with standard booster to cover a wide range of vehicle platform variations. The system may use known ABS valves, pump and controller technology. Input force from the pedal is transferred to the master cylinder to build pressure, the same as on conventional brake systems. 
     Other advantages include short packaging and ease of assembly due to a minimum number of component parts. There are no dynamic seals under permanent high pressure and as a result a relatively lower booster pressure is needed than in the prior art boosters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a brake system according to the invention with a partial cross-section of a piezoelectric hydraulic booster unit; 
         FIG. 2  is an enlarged view of a sensor region of the booster unit of  FIG. 1  in a rest condition; 
         FIG. 3  shows the enlarged view of the sensor region of the booster unit of  FIG. 1  in a braking condition; 
         FIG. 4  is a schematic illustration of a brake system according to the invention with a partial cross-section of a piezoelectric combined hydraulic booster and a tandem master cylinder unit; 
         FIG. 5  is a schematic illustration of a brake system with a partial cross-section of the piezoelectric hydraulic booster and master cylinder unit of  FIG. 2  for use to control an antilock braking arrangement; 
         FIG. 6  is a schematic illustration of a brake system according to the present invention with a partial in cross-section of a travel sensing hydraulic booster unit; and 
         FIG. 7  is a schematic illustration, of a brake system according to the present invention with a partial cross-section, of a travel sensing combined hydraulic booster and master cylinder unit for use to control an antilock braking system. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several drawing views of the brake system according to the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and particularly to  FIG. 1 , there is shown a hydraulic power boost unit  10  and a boost fluid pressure source such as energy unit  12 . The boost unit or booster  10  includes a housing  14  having a cylindrical bore  16  with a boost piston  18  reciprocably disposed therein. An input member or rod  20  is coupled to an operator brake pedal  22  by a linkage  24  and is responsive to an operator pedal input to supply pressure fluid to individual wheel brake actuators such as illustrated in  FIGS. 4 and 5 . In an event that the boost unit  12  is inoperative, input member  20  moves piston  18  within bore  16  to provide an output member  56  with a force that is transmitted to a conventional master cylinder in much the same way as a vacuum failure mode in a brake system having a conventional vacuum booster. The cylindrical bore  16  retains a boost piston return spring  26 , a cut in spring  28 , and in conjunction with piston  18 . Piston  18  is positioned within bore  16  to define a variable volume boost pressure chamber  30 . The energy unit  12  functions as a fluid pressure source and includes an electric motor  32  that is driven by a hydraulic pump  34  to build and maintain a desired pressure in accumulator  36  and a reservoir  44 . Line pressure is monitored by sensor  38  and a microprocessor defined by electronic control unit (ecu)  46  to control motor  32  and obtain and maintain a desired fluid pressure for energy unit  12 . The electronic control unit  46  also monitors sensors  48  and  50  of the boost unit  10 , as best shown in  FIGS. 2 and 3 , to selectively enables inlet valve  40  and outlet valve  42  accordingly. The inlet valve  40  and outlet valve  42  form an assembly that selectively supplies fluid pressure from the source or energy unit  12  to the boost pressure chamber  30  and vents fluid pressure from the boost pressure chamber  30  to reservoir  44 . The input member  20  extends into the housing  14  and is coupled to the boost piston  18 . The coupling includes the piezoelectric sensor arrangement  48  and  50  for detecting variations in the coupling between the input member  20  and the boost piston  18  as shown in greater detail in  FIGS. 2 and 3 . 
       FIG. 2  shows the input member  20  in a released position with a gap  52  between the front end  21  of the input member  20  and sensor  48  that is retained in boost piston  18 . The rear face  23  of the front end  21  of input member  20  is urged into engagement with a piezoelectric sensor  50  by a cut in spring  28  that acts on input member  20  when no input is applied to pedal  22 . A lost motion coupling or connection between input member  20  and piston  18  allows initial pedal depression to move input member leftward, and there is no corresponding motion of the boost piston  18  until gap  52  is spanned.  FIG. 3  shows the relationship between the input member  20  and boost piston  18  when the operator has depressed pedal  22  and gap  52  has been replaced by a gap  54 . Input member  20  is now urged into engagement with a piezoelectric sensor  48  by operator pedal input. The small chamber  31  wherein sensors  48  and  50  are located and where gaps  52  and  54  are created is isolated from the boost pressure chamber  30  by a seal  62 , chamber  31  is connected to the atmosphere by passage  64  in which leads from sensors  48  and  50  are located and connected to the controller (ecu)  46 . 
     In operation, at pedal apply, the cut in spring  28  will collapse and the input rod  20  rear face  23  of the front end  21  lifts off force sensor  50  and is urged toward and into engagement with force sensor  48 . On engagement of the front end  21  of input rod  20  with force sensor  48 , a corresponding force signal is sent to the controller (ecu)  46  which in turn sends an operational signal that closes the normally open valve  42  and opens the normally closed valve  40  to supply pressurized fluid from accumulator  36  to be presented and build up in the boost chamber  30 . This pressurized fluid acts on and pushes the piston  18  in bore  16  toward a master cylinder until the following force balance is achieved. Balance between the force from the pedal  22  on the input rod  20  and the reaction force from the boost pressure on the input rod plus the bias of spring  28  and the pressure force on boost piston  18  and a reaction force on the output member (master cylinder actuating rod)  56  from the master cylinder plus the bias of spring  26 . When a balance is achieved, the input rod  20  will be in a floating state between sensor  48  and sensor  50  and as a result no signals are transmitted from the sensors  48  and  50 . In a balanced state, the controller  46  closes valve  40  and fluid pressure is maintained in the boost chamber  30 . On release, the cut in spring  28  acts on input rod  20  to push the rear face  23  of the front end  21  against sensor  50  and a force signal is there after sent to the controller (ecu)  46  which in turn sends a signal to open the valve  42  and the fluid pressure in chamber  30  is released as fluid flows to reservoir  44 . As disclosed in  FIG. 1 , sensors  48  and  50  are designed to only operate as On/Off switches. 
     In  FIG. 4 , the master cylinder and the power boost unit are combined in a common housing  58  and the boost piston  60  also functions as a master cylinder piston. The master cylinder portion of the brake system of  FIG. 4  is illustrated as a split system with piston  60  providing braking fluid pressure from a variable volume chamber  74  to rear  66  and  68  wheel brake actuators. Pressure in chamber  74  urges piston  76  to reduce the volume of chamber  78  and this second braking circuit supplies braking fluid pressure to front  70  and  72  wheel brake actuators. Upon pedal release, two return springs  80  and  82  return the pistons to their rest positions. The boosted brake system of  FIG. 4  includes a energy unit  12 ′ that is similar to energy unit  12  of  FIG. 1  with the exception of a fluid reservoir  84  that is shared by both the boost system and the master cylinder. It will be recognized that the piston  76 , chamber  78  and spring  82  could be omitted and braking for all four wheels provided by a single master cylinder circuit. 
     Upon brake pedal operation, a force signal is sent from transducer  48  to the controller  46  which closes the valve  42  and opens the valve  40  to build up fluid pressure in the boost chamber  30  that pushes the primary piston  60  and with that the secondary piston  76 . The primary piston  60  moves until the following two force balances (ignoring the return springs) are achieved. A balance occurs between the force from the pedal  22  on the input rod  20  and the reaction force from the boost pressure on the input rod. Further, a balance appears between boost pressure force on primary piston  60  and the reaction force from primary circuit. When a balance occurs, the input rod is in a floating position between sensor  48  and sensor  50  and as a result no signals are sent from sensors  48  and  50  to the controller or ecu  46 . Without signals from sensors  48  and  50 , controller  46  closes the valve  40  and fluid pressure is held and maintained in the boost chamber  30 . At pedal apply, the cut in spring  28  collapse and the rear face  23  of the front end  21  of the input rod  20  lifts off force sensor  50  and pushes the front end  21  of input rod  20  into engagement with force sensor  48 . On release, the cut in spring acts on the pushes the rear face  23  of the front end  21  of input rod  20  against force sensor  50  such that a force signal is sent to the controller (ecu)  46  which in turn sends a signal to open valve  42  and release fluid pressure from chamber  30  and allow return springs  80  and  82  of the master cylinder push the pistons  60  and  76  back into their initial or rest position. As with the embodiment of  FIG. 1 , the force sensors  48  and  50  of  FIG. 4  operate only as On/Off switches. 
     The embodiment of  FIG. 5  differs from  FIG. 4  only in illustrating a hydraulically boosted brake system that could be used in conjunction with a conventional antilock braking system (ABS)  86  rather than the brake system of  FIG. 4 . Briefly, upon brake pedal actuation, fluid pressure is transmitted by way of a normally closed USV valve  88  and a normally closed EV valve  90  to the brake actuator of an illustrative front wheel  92 . If the speed of wheel  92  drops excessively, EV valve  90  is closed and AV valve  94  opened to bleed some pressure fluid to accumulator  96  while local rebuild pressure is maintained by a motor  98  driving pump  100 . 
     It is also possible to employ a single force sensor such as  48  to sense pedal apply and pedal release. Rather than operating as a simple On/Off switch, transducer  48  is adapted to provide a measure of the force applied to piston  18  or  60  by the input member. When the input rod  20  is fully released, the input force is decreasing, or the input member is urged away from the power boost piston, valve  42  is open and valve  40  is closed, both valves are in their normal or unenergized state. When the input rod force against the piston is steady, that is, their relative axial relationship remains constant, both valves  40  and  42  are closed and maintain the status quo. When the input member is urged toward the power boost piston and the input force increases, valve  42  is closed and valve  40  is open, that is, both are in their energized state and there is no need for a lost motion coupling between the input member  20  and the piston  60 . 
       FIG. 6  illustrates a variation on the brake system of  FIG. 1 . Basically the same energy unit  12  selectively supplies pressure fluid to the booster  210  by way of normally open valve  40  and relieves pressure by way of normally closed valve  42 . Certain ones of the signals received by the electronic control unit  46  are of a different nature originating from a travel sensor  102  which may be a linear variable displacement transducer or other suitable sensor for detecting relative axial movement between the input member  106  and the piston  108 . This travel sensor  102  measures the incremental travel between the input rod  106  and the boost piston  108 . Even though both of these components are moving generally together during a booster stroke, there is also relative movement between them depending on the forces applied on the input rod  106  and the reaction forces coming from the booster  210 . The structure of the housing  104 , the input member  106 , the boost piston  108  and the master cylinder actuating rod  110  have all been modified somewhat. Input member  106  is biased toward a retracted position relative to the piston  108  by cut in spring  112  and may be moved toward the left as viewed to directly engage a reaction disc or washer  114  allowing the same push through upon boost failure as discussed earlier. Boost pressure is selectively supplied to a variable volume chamber  116 . At pedal apply, the cut in spring  112  will collapse and the input rod  106  will move toward the reaction washer  114 . The travel signal from sensor  102  is sent to the controller  46  which closes the valve  42 . Upon further travel of the input rod the valve  40  opens. Pressure builds up in the boost chamber  116  to push the piston  108  and actuating rod  110  toward the master cylinder. The piston  108  moves until a force balance between output rod  110 , input rod  106  and piston  108  is achieved. The input rod will be in a floating position between open valve  40  (for further pressure increase) and open valve  42  (for pressure release). The controller  46  will close the valves  40  and  42 . The pressure will be held in the boost chamber  116 . On release, the cut in spring  112  will push the input rod into the released position. The travel signal is sent to the controller  46  which in turn sends a signal to open the valve  42  and release fluid pressure from chamber  116  of the booster  210 . When the input rod  106  is fully released, valve  42  is open and valve  40  is closed and both valves are in their normal and unenergized state. When the input rod is partially applied by an input force, both valves  40  and  42  are closed to maintain the status quo. When the input rod is fully applied, valve  42  is closed and valve  40  is open, that is, both are in their energized state. 
       FIG. 7  illustrates the sensing technique of  FIG. 6  that combined in a single housing  122  with a split circuit master cylinder similar to that of  FIGS. 4 and 5 , however, the boost piston  108  is now coupled by reaction washer  114  to a primary master cylinder piston  118  upon brake pedal operation. Pressure building in chamber  120  causes translation of piston  124  and pressurization of chamber  126  and the braking pressure in these two chambers is conveyed to an ABS system as in the embodiment of  FIG. 5 .