Abstract:
A self contained, supplemental vacuum assist unit is provided for use with a vacuum brake booster and a source of vacuum in a motor vehicle. The unit includes, within a single housing, an electric motor, an air pump driven by the motor, a manifold defining air exhaust and assist vacuum chambers, a pair of check valves permitting air flow from the pump outlet and the assist vacuum chamber to the air exhaust chamber, an outlet from the air exhaust chamber with a fitting for connection to the vacuum source, an opening from the assist vacuum chamber with a fitting providing communication directly to the interior of the vacuum chamber of the booster, and a pressure sensor including a diaphragm, plunger, magnet with a Hall effect sensor and motor control circuitry on a circuit board within the assist vacuum chamber. The unit is compact, light weight and efficient and is designed for attachment directly to the booster with no intervening vacuum hose therebetween.

Description:
RELATED APPLICATIONS 
     This application is related to U.S. Ser. No. 09/374,664, filed Aug. 16, 1999 and assigned to the same assignee as this application. The relevant portion of the referenced application is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The technical field of this invention is vacuum brake booster apparatus for motor vehicles. 
     BACKGROUND OF THE INVENTION 
     Power brake assist using differential pressure actuated brake boosters is standard in the motor vehicle industry. These brake boosters typically have a diaphragm separating a vacuum chamber always open to a source of vacuum, such as created in an engine intake passage, and a working chamber normally provided with vacuum but controlled by valve apparatus responsive to input brake pedal actuation to allow atmospheric air therein and thus provide brake force assist through a differential pressure across the diaphragm. In some systems, it is also known to provide a vacuum pump, either in place of the engine vacuum source or as a backup therefor, to provide a normal supplied vacuum. 
     The braking assist force provided by such known vacuum brake boosters is only an assist force, which is added to the force provided by the vehicle operator through the brake pedal. The vehicle operator applied force is itself transmitted directly through the booster apparatus and applied to the master cylinder, whether or not the assist braking force is generated by the vacuum brake booster. Thus, vehicles provided with such vacuum brake boosters may be stopped even in the unlikely occurrence of loss of vacuum, producing reduced or non-existent braking assist force, although reduced braking force can result in longer stopping distance. Braking systems are designed for safe stopping within specified distances at predetermined vehicle speeds, even with no braking assist; but this task is difficult for heavier vehicles, and a source of back-up vacuum is desired in some cases to ensure that such vacuum based braking assist force is not lost. The object of this invention is to provide a self-contained, compact, light-weight, backup vacuum assist apparatus, and particularly such apparatus that is capable of mounting directly to a vacuum brake booster without necessity of additional vacuum hoses and with minimal additional external electrical wiring requirements. Such apparatus is capable of helping large, heavy vehicles to meet the standards of FMVSS 135. 
     SUMMARY OF THE INVENTION 
     A self contained, supplemental vacuum assist unit is provided for use with a vacuum brake booster and a source of vacuum in a motor vehicle, wherein the vacuum brake booster has a vacuum chamber and a working pressure chamber. The unit has a housing defining an assist vacuum chamber having an opening with a fitting adapted for connection to the vacuum chamber of the vacuum brake booster and further defining an air exhaust chamber having an opening with a fitting adapted for connection to the source of vacuum. The assist vacuum chamber and the air exhaust chamber are separated by a wall having a first opening with a first unidirectional flow valve permitting air flow only from the assist vacuum chamber to the air exhaust chamber. An electric motor driven air pump is further provided in the housing with an inlet from the assist vacuum chamber and an outlet to the air exhaust chamber. The air pump is associated with a second unidirectional flow valve permitting air flow through the air pump only from the assist vacuum chamber to the air exhaust chamber. An electric circuit board is provided within the assist vacuum chamber and has a Hall effect sensor thereon to sense a magnetic field of a magnet adjacent the sensor and further has a control circuit thereon responsive to the Hall effect sensor to control operation of the electric motor driven air pump. The housing also has an opening from the assist vacuum chamber to a source of air at atmospheric pressure, the opening being closed by a flexible diaphragm. A plunger is activated by the diaphragm for axial movement therewith in response to changes in pressure thereacross; and a permanent magnet is affixed to the plunger adjacent the Hall effect sensor. The permanent magnet generates a magnetic field sensed by the Hall effect sensor, and the Hall effect sensor is responsive thereto to generate a signal changing with the position of the permanent magnet and thus of the pressure across the diaphragm. 
     The unit is self-contained, with the motor, pump, fluid flow passages, check valves, pressure sensor and electronics all within the housing for compact packaging within a crowded vehicle engine compartment, light weight for minimal fuel penalty, and protection of the parts from environmental dirt and humidity. In the unit, the pressure sensor incorporates a Hall effect sensor for significant cost savings and mounts the Hall effect sensor on a circuit board within the assist vacuum chamber, together with the diaphragm activated plunger and magnet, for maximum protection and convenience of design. In a preferred embodiment, the magnet is axially adjustable with respect to the plunger and adjusted in calibration for minimal temperature variation of motor pump motor switching point. In a preferred embodiment, the plunger is guided by the circuit board to maintain a specified normal distance from the Hall effect sensor in axial motion. In a preferred embodiment, separate adjustment may be made for preferred spring bias on the plunger. In a preferred embodiment, the unit is provided with a fitting for direct connection to the vacuum chamber of the booster to eliminate a connecting hose therebetween. In a preferred embodiment, the unidirectional valve associated with the air pump acts on the air outlet of the pump to create a pressure drop that increases pump efficiency. In a preferred embodiment, in which the unit is attached directly to the booster, the unit is provided with two levels of sound isolation from the booster, with isolated mounting of the motor within the housing and isolation pads on the booster attachment brackets. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a vehicle brake system including a self-contained, supplemental vacuum assist unit according to the invention. 
     FIG. 2 is a schematic diagram of the vacuum supply system of the vehicle brake system of FIG.  1 . 
     FIG. 3 is an elevation view of a preferred embodiment of a self-contained, supplemental vacuum assist unit according to this invention mounted on a brake booster. 
     FIG. 4 is a longitudinal section view of the vacuum assist unit of FIG.  3 . 
     FIG. 5 is an enlarged section view, from the reverse direction, of a manifold housing of the vacuum assist unit of FIGS. 3 and 4. 
     FIG. 6 is a section view along lines  6 — 6  in FIG.  5 . 
     FIG. 7 is a perspective view of the vacuum assist unit of FIGS. 3-6. 
     FIG. 8 is a section view of the pressure sensor apparatus in the vacuum assist unit of FIGS. 3-7. 
     FIG. 9 is a view along lines  9 — 9  in FIG.  8 . 
     FIG,  10  is a graph showing output voltage as a function of magnet position at the temperature independent axis of the Hall effect sensor at various temperatures for the sensor of FIG.  9 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a motor vehicle, generally indicated as  10 , includes a primary source of vacuum  12 , which is typically an air induction passage of a vehicle engine but could alternatively be a motor driven vacuum pump. A vehicle power brake system includes a brake booster  20  that receives an input brake activation force from a brake pedal  22  and communicates the activation force to a master cylinder  24 , and thus to one or more vehicle brake units  21 , through a valve and power piston apparatus  25  of known construction in the normal manner. A portion of the body of booster  20  is divided by a diaphragm  26  into a vacuum chamber  27  and a working chamber  28  to create a differential pressure actuator. Vacuum chamber  27  is connected to vacuum source  12  through a conduit  14  and vacuum assist unit  30 . Unit  30  contains an electric motor driven pump and check valve apparatus, to be described below, to ensure a vacuum supply for vacuum chamber  27 ; and a substantially constant vacuum level, relative to atmospheric pressure, is maintained in vacuum chamber  27 , normally by vacuum source  12  but, if required, by assist unit  30 . Working chamber  28  is controlled by a multiple valve apparatus, not shown, to normally maintain the supplied vacuum when brake pedal  22  is not activated but to admit atmospheric air in response to activation of brake pedal  22  so as to provide pressure on diaphragm  26  to the left in FIG.  1 . This pressure is communicated by diaphragm  26 , through apparatus not shown, as an output brake assist force to master cylinder  24 . The parts of brake booster  20  not shown, are standard in construction and operation, the details of which are well known in the art and unimportant to this invention. Examples may be found in U.S. Pat. No. 3,249,021 to Wuellner and U.S. Pat. No. 4,069,742 to Gephart et al, as well as many others. 
     Vacuum assist unit  30  is shown in its preferred position, attached directly to booster  20 , in FIG.  3 . The unit is attached by means of one or more mounting brackets  32  assembled on threaded studs  34  and held tightly thereon with nuts  36 . Rubber isolators  38  may be used between brackets  32  and the body of unit  30  to reduce the transmission of motor and pump noise from unit  30  to booster  20 . Studs  34  may be welded directly to the case of booster  20 ; and two such studs will provide sufficient support, with a third support point provided for a fitting  40  providing fluid communication from unit  30  directly into booster  20  without the need for a vacuum hose therebetween. Another fitting  42  accepts conduit  14  and communicates unit  30  to vacuum source  12 . An electrical connector  44  is provided for connection of electrical devices and circuits within unit  30  to a vehicle wiring harness. 
     FIG. 2 shows a schematic diagram of unit  30  and its vacuum connections to source  12  and booster  20 . Booster  20  is connected through fitting  40  of unit  30  to a vacuum assist chamber  80 , which includes an integral pressure sensor  81  for vacuum within the chamber relative to atmosphere. Vacuum assist chamber  80  is connected via a check valve  93  to an air exhaust chamber  70  so that air flows only from chamber  80  to chamber  70 . Chamber  70  is connected via fitting  42  of unit  30  and conduit  14  to vacuum source  12 . Vacuum assist chamber  80  is also connected to the air inlet of a pump  62 ; and the air outlet of pump  62  is connected via a check valve  99  to air exhaust chamber  70 . Pump  62  may thus pump air only from chamber  80  to chamber  70 . Motor  60  is electrically connected in series with a switch  79  across a voltage +V and mechanically drives pump  62  when the switch is activated to a closed position. Switch  79  is responsive to sensor  81  to close when the sensed vacuum within assist chamber  80 , and thus within vacuum chamber  27  of booster  20 , falls below a predetermined minimum desired level. Switch  79  is preferably a semiconductor switch in a circuit in a switch control circuit on circuit board  74 , the circuit being responsive to the output of sensor  81  and most probably containing a programmed microprocessor. 
     Vacuum assist unit  30  is shown in cross section in FIG.  4  and in perspective in FIG. 7. A housing for unit  30  comprises three parts: a motor/pump housing  50 , a manifold housing  52  and an end cap  54 . Motor/pump housing  50 , preferably made of aluminum or steel but possibly thermo-plastic, contains an electric motor  60  and an air pump  62 , in this embodiment shown as a vane pump, adapted to be driven by motor  60 . An open axial end  64  of motor/pump housing  50  is affixed to an axial side  65  of manifold housing  52 , with an O-ring seal and isolation member  66 . Member  66  slightly separates housings  50  and  52  for noise isolation of motor  60 , which is supported at its opposite end in housing  50  by sound isolating member  56 , typically made of rubber or a similar vibration absorbing material. End cap  54 , made of a thermo-plastic material, is affixed to the opposite axial side  67  of manifold housing  52 , in a connection that is also sealed. An internal divider member  70  internally affixed to manifold housing helps create a wall that divides the interior of manifold housing  52  and end cap  54  into an air exhaust chamber  72  and an assist vacuum chamber  80 . Air exhaust chamber  72  is open through an opening  73  to fitting  42 . End cap  54  includes a circuit board  74  affixed thereto on a plurality of studs  75  so as to be contained within chamber assist vacuum chamber  80 . End cap further includes an integral connector fitting  76 , through which project electrical connecting terminals  77  having one end connected to circuitry on circuit board  74  within chamber  80  and another end projecting out of unit  30  for connection to a vehicle wiring harness. End cap  54  further includes a pressure sensor housing portion  78  housing a differential pressure diaphragm and magnet tipped plunger for use with a Hall effect sensor  110  mounted on circuit board  74 . The pressure sensing arrangement is described in greater detail with respect to FIG.  8 . 
     Manifold housing  52  is shown in an enlarged section viewed from the opposite side in FIG. 5 and, in a section normal to that of FIG. 5, in FIG.  6 . Manifold housing  52 , made of a thermo-plastic material, comprises a main, cup-shaped, outer portion  82  with a wall  83  at one axial end and open at the opposite axial end. Another wall  84  extends generally axially from wall  83  toward the open end and defines a receptacle  85  for internal divider member  70 , which is fixed in a sealing manner therein to create air exhaust chamber  72 . An axial projection  90  projects axially from wall  83  toward the open end of housing  52  to engage an axial projection  91  projecting in the opposite direction from divider member  70 . A valve seat  92  is formed on the surface of divider member  70  surrounding projection  91 ; and a movable valve element  93  is retained adjacent valve seat  92  to close one or more openings  94  when the pressure in air exhaust chamber  72  exceeds that in assist vacuum chamber  80 . Opening  73  is provided from air exhaust chamber  72  to fitting  42 , and thus through conduit  14  to vacuum source  12 . Elements  90 - 94  define a check valve that automatically applies the vacuum of vacuum source  12  to assist vacuum chamber  80 . 
     A similar arrangement, but on the opposite side of air exhaust chamber  72 , provides engaging projections  96  from divider member  70  and  97  from wall  83 , with a valve seat  98  in the surface of wall  83  surrounding projection  97  and a movable valve element  99  retained adjacent valve seat  98  to close one or more openings  100  through wall  83  when the pressure in air exhaust chamber exceeds that on the other side of wall  83 . A short circular wall  101  projects from the side of wall  83  opposite air exhaust chamber  72  and is provided with a circular groove  102  to receive sealing O-ring  66 . Openings  100  are sealingly connected to the air outlet of pump  62 , with the air inlet of pump  62  being connected to assist vacuum chamber  80 . Elements  96 - 100  thus define a check valve that automatically applies the vacuum created by pump  62  to assist vacuum chamber  80 , which is thus maintained at the greater of the vacuum from vacuum source  12  and the vacuum created by pump  62 . Assist vacuum chamber  80  is open through an opening  105  in outer portion  82  of manifold housing  52  and fitting  40  to vacuum chamber  27  of booster  20 , which is thus also maintained at the same vacuum level. In addition, the placement of the check valve including element  99  in the outlet path of pump  62  provides a small pressure drop that improves pump efficiency and thus contributes to the compactness of unit  30 . 
     The pressure sensor arrangement is shown in FIG. 8 and 9. A cylindrical recess  120  is provided in sensor fitting  78  of end cap  54 . Recess  120  is open to assist vacuum chamber  80  at its inner end and, through an opening  122 , to outside atmosphere at its opposite end. A flexible diaphragm  124  adjacent opening  122  prevents fluid communication between cylindrical recess  120  and the atmosphere and is subject to a differential pressure equal to the vacuum level, relative to atmosphere, in assist vacuum chamber  80 . A plunger  130  is disposed in cylindrical recess  120  for axial movement therein. Plunger  130  has a diaphragm contacting head  132  at the end adjacent diaphragm  124  and an opposite axial end  134  projecting into vacuum assist chamber  80  and through a guide opening  126  in circuit board  74 . Diaphragm  124  is preferably a rolling diaphragm so as to eliminate or reduce tension effects in the clamped portion of the diaphragm. A permanent magnet  140  is mounted on end  134  of plunger  130  in an axially adjustable manner, such as by mounting on a flat head screw  136  threadably inserted into an axial opening  138  on end  134  of plunger  130 . Of course, plunger  130 , screw  136  and other parts near magnet  140  are made of appropriate non-magnetic materials. Magnet  140  is magnetized axially, with north and south poles at opposing axial ends thereof. Plunger  130  is biased outwardly from chamber  80 , into engagement with diaphragm  124 , by a coil spring  142  reacting against a spring base member  144  which may be threadably adjustable within cylindrical recess  120  and has an opening  146  therethrough for plunger  130 . Spring base member  144  may be positionally adjusted during assembly of unit  30  to provide a desired spring preload; and openings  148  may be provided for the insertion of a tool, before positioning of circuit board  74 , to rotate spring base member  144  for such adjustment. The preload should be adjusted as sufficient to prevent movement of the plunger by forces other than a pressure differential across diaphragm  124  but low enough to allow response of the plunger to the minimum desired sensed pressure across diaphragm  124 , as is known to those of skill in the art. The diaphragm  124 , plunger head  132  and magnet  140  are shown in one extreme position in solid lines and, in another extreme position, in dashed lines. 
     An important consideration of the pressure sensing system is its temperature sensitivity. The pressure sensing system is essentially be used as a switch, to activate motor  60  and thus drive pump  62  when sensed vacuum in assist vacuum chamber  80 , which is essentially the same as that in vacuum chamber  27  of booster  20 , falls below a predetermined level, in order to maintain that level as a minimum, even if there is a failure of vacuum source  12 . Hall effect sensor  110  is a temperature sensitive element that produces a voltage output, for a given relative position of magnet  140 . Thus, the output voltage of Hall effect sensor  110  will vary with temperature as well as with the axial position of magnet  140 . However, Hall effect sensor  110  defines a particular position, which may be called the temperature invariant axis  115 , at which the voltage is independent of temperature. In the graph of FIG. 10, which shows the relationship between magnet axial position and the output voltage of Hall effect sensor  110  at various temperatures, this temperature independent axis is represented by point  117  on the horizontal axis of the graph. In production, after the spring preload is set (if it is), the desired minimum supply vacuum is applied across the diaphragm; and screw  136  is turned to axially adjust the magnet until the until the desired temperature independent voltage is output. This voltage is essentially the mid-point output voltage of the Hall effect sensor, at which the sensed magnetic polarity reverses, and is represented by point  118  of the vertical axis of the graph. The two points define an operating point  116  through which several operating lines pass. The lines have slightly different slopes because they correspond to different operating temperatures; but they all pass through point  116 . Once this calibration is made, the temperature offset is essentially removed; and a minimum desired supply vacuum will be more consistently provided, regardless of temperature. 
     FIG. 6 shows an opening  112  in wall  83  of manifold housing  52  that communicates assist vacuum chamber  80  with the interior of motor/pump housing  50 , so that electrical connections, not shown, may be provided between motor  60  and circuit board  74  within chamber  80 . Thus, all electrical apparatus in unit  30 , including motor  60 , sensor  81  and circuit board  74 , are maintained in a vacuum environment, which is repeatedly and continuously pumped free of contaminants and corrosion causing moisture whenever the vehicle is being operated, whether the vacuum is provided by pump  62  or vacuum source  12 .