Abstract:
An adaptation to a vehicle antilock braking system modulator to increase the initial displacement of the master cylinder, provide quicker system response and improved pedal feel. A solenoid assembly is added to the non fluid side of each of the low pressure accumulators in the ABS modulator. Upon the operator initiation of a brake apply, the solenoids are energized to immediately push upon the sump pistons and inject fluid from the low pressure accumulators into the brake system. This extra “shot” of fluid decreases the amount of fluid that the master cylinder delivers to the brake system. This improves pedal feel as it reduces the required initial pedal travel for a given brake system displacement. The low pressure accumulator pistons are spring balanced to ensure retention of fluid between brake applies and to allow normal ABS operation. During ABS operation, the low pressure accumulator displacer solenoids are de-energized to allow effective operation. The holding force (electrical current) of the displacer solenoids can be reduced by using a toggle linkage apply mechanism.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to a power assisted braking system for a vehicle and more particularly to methods and apparatus for providing optimal pedal feel and improved time response for a braking system having, for example, an antilock feature. At the beginning of a brake apply, the fluid displaced out of the master cylinder is used to bring the brake friction material in contact with the brake rotors or drums. This take-up of running clearances is basically lost travel from a pedal feel standpoint. The goal of this invention is to reduce this initial pedal travel and thus to improve pedal feel. 
   It is desirable to decrease brake response time from a safety viewpoint and desirable to improve the feel of the brake pedal to the vehicle operator from an ergonomic viewpoint. Many known antilock devices operate by cyclically increasing and decreasing a braking force exerted on the wheels so that a slipping wheel having a tendency to lock is permitted to re-accelerate back to a speed corresponding to the speed of the vehicle. This is typically achieved by control valves alternately allowing fluid to flow out of and then into a brake cylinder to first lower and then raise the brake pressure in the brake system. Typically antilock braking systems utilize either a so-called pump-back scheme or a replenish scheme during a reapply or build operational sequence to maintain a desired level of hydraulic fluid in a brake system. In a pump-back scheme, the same hydraulic fluid is re-supplied from a local low pressure accumulator to the brake pad actuators while in a replenish scheme hydraulic fluid comes from a separate source such as either a hydraulic accumulator or a separate pump and motor. Most of such antilock braking systems are further capable of operating in a traction control function. A traction control function is established by detecting conditions where the rotational speed of a first powered wheel substantially exceeds that of a second powered wheel. To provide a power balance in the operation of a vehicle, a braking force is applied to the powered wheel rotating at a higher speed effectively transferring driving torque back to that wheel with better traction. Many antilock systems having such a traction control feature employ a motor and hydraulic pump or pumps along with fluid accumulators which operate somewhat independently of the service braking system. 
   SUMMARY OF THE INVENTION 
   The present invention provides the desirable pedal feel and brake response time by using the existing low pressure accumulator bore and piston in the antilock braking system (ABS) in an expanded role of providing a means to fast fill the brake system upon the initiation of a brake apply. To accomplish this additional function of fast fill, a solenoid is added to push the existing low pressure accumulator piston down the bore and displace fluid into the brake system. The solenoid is enabled or energized upon the initiation of a brake apply. Three specific constructions that fulfill the desired goals while maintaining the original ABS functionality of the low pressure accumulator are disclosed. The function of the low pressure accumulator is to accept and temporarily store and return to the system the prescribed decay fluid. 
   The invention comprises, in one form thereof, a brake fluid accumulator for a vehicle braking system which is operable in a passive mode to receive fluid from and return fluid to the vehicle braking system, and operable in an active mode to supply an initial shot of pressurized fluid to the vehicle braking system upon initial operator actuation of the braking system. The accumulator has a housing with a cylindrical bore and a piston assembly reciprocably disposed therein to define a variable volume chamber. The chamber has an inlet for receiving pressure fluid from the vehicle braking system and for expelling pressure fluid from the chamber to the vehicle braking system along with a resilient spring which biases the piston assembly in a direction to diminish the chamber volume. A solenoid has an armature reciprocable along a solenoid axis disposed generally orthogonally to the bore axis in response to solenoid energization and there is a mechanical coupling in the form of a toggle linkage mechanism between the solenoid armature and the piston assembly operable to transmit armature motion induced by solenoid energization to the piston assembly expelling pressure fluid from the chamber to the vehicle braking system. 
   An advantage of the present invention is reduced brake response time. 
   Another advantage is the reduced initial brake pedal travel and resulting more responsive feel of the brake pedal to a vehicle operator. 
   Yet another advantage of the present invention is the capability of the solenoid to maintain the displacement of the fast fill fluid in the brake system under high system pressure without significant force and resulting electrical current demand upon the solenoid. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a portion of an antilock braking hydraulic circuit incorporating the invention in one form; 
       FIG. 2  is a cross-sectional view of an illustrative solenoid powered accumulator suitable for use in the circuit of  FIG. 1  in its quiescent state; 
       FIG. 3  is a cross-sectional view of the solenoid powered accumulator of  FIG. 2  shown in the full stroke position; 
       FIG. 4  is a cross-sectional view of the solenoid powered accumulator of  FIG. 2  in the fast fill position; 
       FIG. 5  is a cross-sectional view of a toggle with pin lock variation on a solenoid powered accumulator suitable for use in the circuit of  FIG. 1  in its quiescent state; 
       FIG. 6  is a cross-sectional view of a toggle with pin and cross lock-out variation on a solenoid powered accumulator suitable for use in the circuit of  FIG. 1  in its quiescent state; 
       FIG. 7  is a cross-sectional view of the solenoid powered accumulator of  FIG. 6  along the lines  7 - 7  of  FIG. 6 ; 
       FIG. 8   a  is a cross-sectional view of the solenoid powered accumulator of  FIG. 6  along the lines  8 - 8  of  FIG. 6 ; and 
       FIG. 8   b  is also a cross-sectional view of the solenoid powered accumulator of  FIG. 6  along the lines  8 - 8  of  FIG. 6 , but showing the effect of solenoid energization. 
   

   Corresponding reference characters indicate corresponding parts throughout the several drawing views. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings and particularly to  FIG. 1 , there is shown an antilock hydraulic braking system  111  for use in a vehicle. The braking system includes solenoid actuated antilock valves  13  and  15  located between an operator controlled pressure source or master cylinder  17  and a hydraulic actuator for a rear wheel brake  19 . Valve  13  functions as a build and hold valve while valve  15  functions as a decay valve. Similar antilock valves, e.g.,  27  and  45 , are provided for the other wheel brakes. Typically, the pressure source  17  is a conventional master cylinder having two separate circuits, one for the front vehicle wheel brakes and the other for the rear wheel brakes, or one for a left front and right rear and the other for a left rear and right front wheel brakes as illustrated in  FIG. 1 . The vehicle wheels also typically have rotational speed sensors for providing electrical indications of the angular velocities of individual wheels to a conventional antilock electronic control unit. When the driver wishes to slow the vehicle, the pedal  21  is depressed and hydraulic fluid pressure is transmitted from the master cylinder  17  by way of conduits (brake lines)  23  and  25  to the respective brake actuators by way of four individual solenoid actuated antilock valves  13 ,  27 ,  29  and  31 . The individual wheel antilock valves such as  13  are normally open to selectively supply braking fluid pressure from the source  17  by way of line  23  and  25  to the individual brake actuators. Valves such as  13  and  15  function as build and hold valves supplying braking fluid pressure from either line  23  during normal braking or from the accumulator  33  during antilock or traction control operation. 
   In particular,  FIG. 1  shows two substantially identical fluid circuits each having an accumulator such as  33 , a pump  37 , two normally closed outlet valves,  15  and  45 , for example, for venting fluid from the wheel cylinders during antilock events and two normally open inlet valves such as  13  and  27  providing a brake fluid path to their corresponding wheel cylinders. The circuits may share a pump drive motor  41 . The normally open solenoid actuated inlet valves  13  and  27  are located between an operator controlled pressure source such as the master cylinder  17  for supplying pressurized fluid to line  23  and hydraulic brake actuators which receive that pressurized fluid by way of lines  47  and  49 . 
   If, during a braking event, a wheel skid is detected, say the right rear wheel associated with line  49 , the solenoid of valve  13  is energized closing that valve and the outlet valve  15  is enabled to open the valve and vent fluid pressure from the slipping wheel cylinder by way of line  51  to the accumulator  33  and/or to a low pressure reservoir. Inlet valves  27 ,  29  and  31  function similarly. The inlet and outlet valves associated with the slipping wheel may be pulsed or otherwise controlled as is common in antilock braking technology. For example, periodically during the time hydraulic fluid is being bled from the brake actuator  19 , valve  13  is opened to supply rebuild pressure. The primary function of the low pressure accumulators  33  and  43  is to absorb excess fluid during an ABS event. This excess fluid typically occurs for only brief periods and helps prevent wheel locking. The modification to the accumulators shown in detail in  FIGS. 2-7  provide an additional fast fill benefit during normal braking. 
     FIGS. 2-8  illustrate three illustrative ways in which a multiple function accumulator may be realized. The implementation of  FIGS. 2-4  has a piston assembly comprising a single piston  53  reciprocable within the bore  52  and there is a mechanical coupling comprising a toggle linkage mechanism  65 ,  67 ,  69  interconnecting the piston and a solenoid armature  63  with the toggle arm  65  coupled to the piston. A piston spring  57  urges the piston in a direction to increase chamber  56  volume and an armature bias spring  61  urges the armature in a direction to oppose an increase in chamber volume so that a fluid ingress induced increase in chamber volume and piston translation is transmitted by the linkage to compress the armature bias spring, while a solenoid induced armature motion is transmitted by the linkage to the piston compressing the piston spring and expelling fluid from the chamber. In  FIG. 5 , the piston assembly comprises a generally cylindrical sleeve  75  disposed in the bore  74  and a reciprocable piston  73  coaxially received in the sleeve. The piston moves under urging of the armature while the sleeve remains stationary to expel pressure fluid from the chamber to the vehicle braking system, while only the sleeve moves when receiving pressure fluid from the system. In  FIGS. 6-8 , the piston assembly is reciprocable within the bore  98  along a bore axis and comprises a single piston  99  while the mechanical coupling comprises a toggle linkage mechanism  105 ,  107 ,  117  and a piston actuator  103  reciprocably disposed within the bore axially adjacent to the piston. The actuator and piston move together in response to armature movement, however, only the piston moves axially toward the actuator in response to a fluid ingress induced increase in chamber volume. 
   More specifically, in  FIG. 2 , the ABS and fast fill fluidic functions are accomplished by a piston  53  which is reciprocable in a bore  52  with a seal  54  there between. The piston and bore together define a variable volume chamber  56 . Piston  53  is coupled to a movable solenoid armature  63  by the pivotable linkages  65 ,  67  and  69 . Piston  53  is resiliently biased toward the right as viewed by a helical spring  57  and the armature  63  is biased upwardly by another helical spring  61 . 
   The low pressure accumulator function is accomplished as fluid enters the chamber  56  from the left end at  55  and the spring  57  loaded piston  53  moves to the right toward the position shown in  FIG. 3 . This fluid acts against the toggle  65 ,  67 ,  69  and solenoid armature  63  in their normal at rest position, causing the solenoid armature  63  to be pushed back/down from its normal at rest position compressing spring  61  as seen in  FIG. 3 . This over retraction or stroke of the armature is provided for in the design of the solenoid assembly and is biased back to the at rest position by the spring  61  that represents the nominal force found in an ABS low pressure accumulator. The fast fill function is accomplished by energization of the solenoid  59  which causes the solenoid armature  63  to move upward, further causing, the toggle arms  65  and  67  to expand away from one another thus causing the piston  53  to move to the left fast displacing fluid out of the chamber  56  into the brake system (at rest position shown in  FIG. 2 ). In the fast fill apply position ( FIG. 4 ) of the toggle, the toggle angularity is geometrically favorable that high pressure acting upon the piston will not cause high forces on the solenoid armature. If the angle between the link  65 , and the horizontal axis of the piston  53  and cylindrical bore  52  is á, then the solenoid force exerted directly upwardly Fs is related to the horizontal force Fp applied to the piston through link  65  by: Fs=2Fp tan á 
   As depicted, the angle between the link  65  and the horizontal axis of the piston  53  and cylindrical bore  52  is about ten degrees. Under that assumption, the holding force required of the solenoid  59  in  FIG. 4  is only about 3.5% of the force applied to the hydraulic piston. For modestly small angles, the mechanical advantage (Fp/Fs) is substantially greater than one. 
   In  FIG. 5 , a piston  73  is reciprocably disposed within a sleeve  75  and that sleeve in turn is reciprocably disposed within a cylindrical bore  74 . The piston  73  and sleeve  75  comprise a piston assembly, and the assembly and bore  74  together define a variable volume chamber  76 . The piston is spring biased toward the right as viewed by a helical spring  71  and the piston and sleeve are spring biased away from one another by another helical spring  83 . The sleeve carries one or more pins  77  which are movable radially inwardly into an annular piston groove  81  or radially outwardly into side wall detent notches such as  79 . A pivotable linkage arrangement  85 ,  87 ,  89  couples the piston  73  to a solenoid  93  armature  91 . Armature  91  is biased upwardly as viewed by a helical (coil) spring  95 . When the solenoid  93  is unenergized, the springs  71 ,  83  and  95  balance the piston  73  and sleeve  75  in the positions illustrated in  FIG. 5 , but when that solenoid is enabled or energized, the armature  91  moves upwardly spreading the linkage arms  85  and  87  away from one another and urging the piston  73  toward the left. Piston motion displaces the groove  81  urging the pins such as  77  radially outwardly into the notches  79  locking the sleeve  75  in the position shown. 
   In the embodiment of  FIG. 5 , the ABS and fast fill fluidic functions are accomplished by the piston  73  and sleeve  75 . The low pressure accumulator function is accomplished as fluid enters the chamber from the left end and the spring  83  loaded sleeve  75  moves to the right compressing spring  83 . The fast fill function is accomplished by energization of the solenoid  93  which causes the solenoid armature  91  to move upward, causing, the toggle arms  85  and  87  to expand away from one another toward a straight angle relationship and pushing the piston  73  to the left, thus displacing fluid out of the chamber to the brake system. The sleeve  75  must be kept from moving to the right during this fast fill action to ensure adequate fast fill displacement. This is accomplished by the angled annulus  81  on the piston which causes spring loaded pins  77  to move outward into the recesses  79  in the bore, thus preventing movement of the sleeve  75 . In the fast fill apply position of the toggle (at rest position shown in  FIG. 5 ), the toggle angularity is geometrically favorable that high pressure acting upon the piston will not cause high forces on the solenoid armature. This toggle arrangement/position is much the same as seen in  FIG. 4 . 
   In  FIG. 6 , a coil spring  97  biases a piston  99  rightwardly within a cylindrical bore  98  with sealing there between provided by a seal  109 . A piston assembly here as in  FIGS. 2-4  comprises a single piston. The piston  99  and bore  98  define a variable volume chamber  100 . A helical spring  101  resiliently biases the piston and an actuator  103  axially away from one another. Solenoid  111  includes a reciprocable armature  113  biased upwardly by coil spring  115 . The armature is mechanically coupled to the actuator  103  by a linkage arrangement  105 ,  107 ,  117 , however this toggle linkage mechanism functions somewhat differently than those shown in  FIGS. 2-5 . 
   The cross-section of  FIG. 7  shows the alignment groove  119  which extends axially along the surface of the piston  99 . This groove cooperates with a fixed boss or pin  125  to prevent rotation of the piston within the bore  98 , thereby maintaining the relative angular orientation of the horizontal piston slot  121 . In the quiescent condition depicted in  FIG. 6 , the slot  121  is aligned with a cross pin  123 . In this condition, an increase in fluid pressure in the chamber  100  can force the piston rightwardly compressing spring  101  and increasing the chamber  100  volume, i.e., the chamber provides its normal accumulator function. With this rightward piston motion, the slot  121  moves freely along the pin  123 . From the rest state shown in  FIG. 6 , energization of the solenoid  111  causes armature  113  to begin upward travel from the position shown in  FIG. 8   a , raising link  117  and spreading the toggle linkages  105  and  107  away from one another. Here the different behavior of this linkage arrangement surfaces. Spring  101  is sufficiently resistant to compression to prevent initial rightward motion of piston actuator  103  as well as preventing entry of the pin  123  into slot  121 . Instead, the off-center pivotal coupling of the link  105  to the actuator causes the actuator  103  to rotate clockwise as indicated by the arrow from the position shown in  FIG. 8   a  to that shown in  FIG. 8   b  misaligning the pin  123  and slot  121 . Now further upward armature motion causes the actuator  103  and piston  99  to move in unison leftwardly in the bore reducing chamber  100  volume and supplying pressure fluid to the braking system. 
   In  FIG. 6 , the ABS and fast fill fluidic functions are accomplished by piston  99  and piston actuator  103 . The low pressure accumulator function is accomplished as fluid enters the chamber from the left end and the spring  97  loaded piston  99  moves to the right. The fast fill function is accomplished by energization of the solenoid  111  which causes the solenoid armature  113  to move upward, further causing the toggle arms  105  and  107  to expand angularly away from one another and pushing upon the pivot attachment point of the piston actuator  103 . This causes the piston actuator  103  to rotate so that the piston actuator cross pin  123  does not align with the previously corresponding slot in the piston. Further expansion of the toggle arms causes the piston actuator to move to the left and push the piston to the left, thus fast displacing fluid out of the chamber into the brake system. In the fast fill apply position of the toggle (at rest position shown in  FIG. 6 ), the toggle angularity is geometrically favorable that high pressure acting, upon the piston will not cause high forces on the solenoid armature. This toggle arrangement/position is again very similar to that seen in  FIG. 4 .