Abstract:
A method for measuring the volume of hydraulic fluid in the secondary circuit of a braking system, especially in an accumulator, through the use of a Hall-effect magneto resistive device. A secondary hydraulic circuit includes an accumulator ( 11, 137 ), an electric motor ( 71, 101 ) driven hydraulic pump ( 73, 143 ), and appropriate valving ( 133 ) for supplying braking force hydraulic pressure from a master cylinder to one or more wheel cylinders. There is a secondary hydraulic circuit supply path for each wheel brake cylinder and each path includes a first normally open solenoid actuable valve ( 41, 43, 57, 59, 97, 149 ) providing a brake fluid path from the master cylinder to the wheel brake cylinder and a first normally closed solenoid actuable valve ( 61, 65, 67, 69, 139, 141 ) selectively opening during an anti-lock event to provide a path to bleed fluid from the associated wheel cylinder to the accumulator ( 11, 137 ). One or more accumulator volume sensors ( 37, 39, 145, 147 ) provide an indication of the volume of brake fluid in the accumulators and an electronic control unit ( 127 ) responds to sensed accumulator volume for selectively opening a first normally closed valve during normal vehicle braking to maintain a preferred volume of brake fluid in the accumulator ( 11, 137 ) such that either an optimum anti-lock event or a traction control event may immediately occur.

Description:
The present invention relates to improvements to the performance and operation of anti-lock braking systems by employing non-contact position monitoring of an accumulator piston to control the volume of fluid retained within the secondary circuit of the braking system hydraulic unit. 
     BACKGROUND OF THE INVENTION 
     Many known anti-lock devices operate by cyclically increasing and decreasing the braking force exerted on the wheels so that a wheel having a tendency to lock is permitted to re-accelerate back toward 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 the brake cylinder to first lower and then raise the brake pressure in the brake system. Some anti-lock systems employ a pump-back scheme where fluid is dumped from the wheel cylinder to a local accumulator and the same hydraulic fluid is re-supplied from the local accumulator to the brake pad actuators. Simple ABS designs do not have the ability to generate wheel pressures in excess of master cylinder pressure, thus making them incapable of performing any active pressure events. 
     Most of such anti-lock braking systems are further capable of operating in a traction control mode. Traction control and anti-lock operation are both responses to aberrant vehicle wheel behavior. 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 the vehicle, a braking force is applied to the powered wheel rotating at a higher speed to effectively transfer driving torque back to the other wheel that has better traction. Many anti-lock systems having such a traction control feature employ a motor and hydraulic pump or pumps which operate independent of the service braking system to supply fluid from a local accumulator to brake the wheel which has lost traction. The same local accumulator may be utilized during anti-lock operation and during a traction control event. In this case, it is highly desirable that the accumulator be sufficiently empty to receive braking fluid it is highly desirable that the accumulator be sufficiently empty to receive braking fluid dumped from a wheel cylinder during the anti-lock event and be sufficiently full to supply the needed fluid to the pump during a traction control event. 
     U.S. Pat. No. 5,971,502 is illustrative of such anti-lock braking systems with traction control (sometimes “dynamic rear proportioning”) capability. This patent also discloses a process for maintaining an optimum level of pressurized hydraulic fluid in a hydraulic accumulator. This optimum level allows the accumulator to immediately respond to a sensed aberrant wheel condition to receive fluid during an anti-lock braking event and to dispense fluid at the beginning of either a traction control event or back-up braking situations. The process includes a step of monitoring a plurality of vehicle operating parameters such as speed and recent braking activity (hydraulic fluid pressure) and utilizing those monitored parameters to periodically determining the likelihood of the occurrence of an anti-lock braking event. The amount of fluid in the accumulator is increased when the likelihood of an anti-lock braking event is low and is decreased when the likelihood of an anti-lock braking event is high. The likelihood of a traction control event may also be monitored and the amount of fluid in the accumulator increased when the likelihood of an anti-lock braking event is low and the likelihood of a traction control event is high. 
     The volume of fluid in an accumulator was also addressed by the patentee of U.S. Pat. No. 5,522,650, who, in an attempt to minimize motor “on-time” and reduce vehicle noise and vibration, devised an algorithm for modeling the amount of fluid in the accumulator and enabled a motor to remove fluid from that local accumulator only as necessary. This patent is typical of all current designs in that software must make an estimation of the accumulator volume. This leads to increased motor running times, and reduced accuracy of pressure control algorithms. 
     Current designs require that the secondary circuit volumes be minimized in order to accurately estimate the suction path and maximize the through flow of this line in an ESP or TCS (traction control) event. Sophisticated algorithms are used to minimize the noise while maximizing the performance. Several ways currently exist to improve the performance of the ESP system, most including the addition of some volume in the suction path near the pump element inlet. Software algorithm development and implementation is limited to the information available through the electronic control unit (ECU) and its sensor array. Without an accurate measurement of the brake circuit volume, an estimation of this volume is utilized. If the secondary circuit volume could be measured, many improvements to the control algorithm could be implemented. The presence of an optimum amount of fluid in a secondary brake circuit residual reservoir can significantly improve pressure build times during traction control events. 
     There are many methods available for position monitoring. For example, International Application No. PCT/EP 01/12269 published as WO 02/36400 Ai teaches a Hall sensing element integrated into the master cylinder of a hydraulic brake system to sense the field of a magnetic region associated with the master cylinder piston for relatively rapid determinations of brake pedal position. Many other position sensing methods, such as linear variable differential transformers (LVDT&#39;s) or various other mechanically actuated measuring techniques are also known, however they typically involve significant modifications to the existing hydraulic unit design box volume. Additionally, they are cost prohibitive, and do not provide a simple way to measure volume in the sealed designs already in production. 
     SUMMARY OF THE INVENTION 
     The present invention employs accumulator chamber management that is a unique and superior method to these designs. It provides a magnetic field sensor such as a Hall-effect solid-state sensing device or magneto resistive sensor to monitor the accumulator piston position to determine the amount of brake fluid contained in the accumulator chambers of an ABS hydraulic unit. 
     The accumulator chamber is a component of what is collectively referred to herein as the secondary braking circuit. The secondary circuit is that portion of the anti-lock braking modulator system that is employed during an anti-lock or traction event to make possible the necessary modulation of brake fluid pressure. During normal braking events such as moderate deceleration on a good road surface, the secondary circuit remains isolated from the primary hydraulic braking circuit that is used to transfer pressure from the master cylinder to the wheel cylinder. 
     The invention comprises, in one form thereof, a process of controlling the secondary hydraulic circuit of a vehicle braking system during aberrant wheel behavior, e.g., one wheel slipping excessively as in a traction control event or one wheel tending to lock during braking as in an anti-lock braking event. The process includes creating a magnetic field adjacent an accumulator in the secondary braking circuit and modifying the magnetic field in accordance with changes in the fluid volume within the accumulator. Changes in the magnetic field are monitored and those changes translated into an indication of accumulator fluid volume. The indication is then utilized in controlling operation of the secondary circuit, for example, by increasing the volume of fluid in the accumulator during anti-lock operation or supplying fluid to the pump from the accumulator to brake a slipping wheel as in a traction control event. 
     In another form, a process of braking a slipping one of a pair of differentially driven wheels to transfer driving torque therefrom to the other of the differentially driven pair of wheels includes maintaining a preferred volume of brake fluid in an accumulator, monitoring the rotational speed of each of the driven wheels, identifying one wheel speed as deviant if the monitored speed thereof becomes too much larger than that of the other driven wheel, and closing a normally open valve which provides a path for the application of normal braking fluid pressure to a brake actuating mechanism associated with the non-deviant wheel to isolate the non-deviant wheel from fluid braking pressure. A pump is then energized to supply brake fluid from the accumulator to a brake actuating mechanism associated with the deviant wheel to slow the deviant wheel, which allows the transfer of driving torque to the non-deviant wheel. Monitoring of the deviant wheel speed continues and the pump is periodically energized and de-energized to reduce the supply of brake fluid to the brake actuating mechanism associated with the deviant wheel. The pump is disabled and the previously closed valve is allowed to open upon an indication that the deviant wheel has achieved a target speed. To accommodate extreme cases, the volume of fluid remaining in the accumulator continues to be monitored and a further valve is opened to supply brake fluid from a master cylinder to the pump inlet when the monitored accumulator volume reaches a minimum value. 
     An advantage of the present invention is that the ability to calculate accumulator volume facilitates both the software development and production control algorithms by providing an accurate accounting of the amount of brake fluid in the secondary circuit of the braking system. 
     Accumulator chamber management allows for the use of proven components that are significantly less expensive than the current ESP designs. Thus, another advantage of the present invention is that modifications to the hardware of existing hydraulic braking systems to incorporate the present invention are minimal. 
     A further advantage is through the use of secondary circuit brake fluid management with pulse width modulation motor drive and accumulator position monitoring, significant performance enhancements can be achieved using less complex hydraulic components. 
     An advantage of one form of the present invention is based on the use of controlled accumulator piston stroke to manage the secondary circuit volume for ABS (anti-lock) hydraulic units thus allowing for PBC (panic braking) and partially active ESP (electronic stability programs) to be performed without the need of isolation and suction valves. 
     As another advantage, the volume of brake fluid in the accumulator provides the additional benefit of acting as a spring loaded pre-charge for the pump element, optimizing the pump performance throughout the temperature envelope and eliminating the need for a supplemental pressurized brake fluid accumulator. This allows the use of an inexpensive pump for all designs. 
     A further advantage of the present invention is that the cost of providing direct accumulator volume information is quite modest. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a cross-sectional view of an illustrative accumulator having non-invasive volume monitoring according to one form of the invention; 
     FIG. 2 is a schematic illustration of a portion of an anti-lock braking hydraulic circuit incorporating the invention in one form; 
     FIG. 3 is a schematic illustration of a portion of an anti-lock braking hydraulic circuit incorporating the invention in a modified form; 
     FIG. 4 is a flow chart showing a pulse width modulation algorithm for the electric motor to properly control the secondary hydraulic circuit volumes and resultant wheel pressures that occur during modulation; and 
     FIG. 5 is a general block diagram of the anti-lock system architecture. 
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and particularly to FIG. 1, there is shown a cross-section view of an accumulator  11  having a cylindrical bore  13  within a housing  15 . A piston  17  is reciprocally contained therein and defines in conjunction therewith a chamber  19 . Chamber  19  has a variable volume that is a function of the position of the piston  17  along axis  27 . The piston  17  is resiliently urged in a direction (rightward as viewed) to diminish the chamber  19  volume by a bias spring  21 . Hydraulic fluid may enter or exit the chamber  19  by way of two conduits  23  and  25 . Imbedded in the piston  17  face, or otherwise affixed to and moving with the piston, is a radially magnetized annular magnet  29  having a radially inner south pole  31  and a radially outer north pole  33 . A series of magnetic sensors  37  and  39  such as Hall effect sensors or magneto resistive sensors are located closely adjacent the housing exterior to be influenced by the field of magnet  29 . Thus, the condition or status of the sensors  37  and  39  reflects the piston  17  position along axis  27  and, therefore, the volume of chamber  19 . The outputs of the Hall sensor or other sensors are coupled to an electronic control unit (ECU) shown in FIG.  5  and the ECU supplies control signals to the braking system. The sensors may provide either analog or digital outputs. In one preferred embodiment, the sensors  37  and  39  employed were Honeywell 2SS52M omnipolar magneto resistive sensors providing a digital output. Of course, the housing  15  and piston  17  should be of a relatively low permeability material such as aluminum or a plastic so as to not shunt the magnetic field away from the sensors. The magnetic material is affixed to the piston  17  of accumulator  11  in the secondary circuit of the hydraulic unit as shown in greater detail in FIGS. 2 and 3. A commercially available Hall effect or other suitable sensor  37  or  39  is positioned in a fixed location near the accumulator bore  13  and is used to detect the field generated by the magnetic material. As the accumulator is filled with brake fluid, the piston  17  moves and as a result the magnetic field density changes with respect to the sensor. Using two or more of these sensors provides a method for determining with varying accuracy, the position of the accumulator piston at any time. The sensor array&#39;s detection of piston movement is calibrated with software to calculate specific brake fluid volume. 
     In FIG. 2, a full function anti-lock and traction control hydraulic braking system is shown schematically. There are two substantially identical fluid circuits each having an accumulator such as  137 , a pump such as  143 , two normally closed outlet valves,  139  and  141  for example, for venting fluid from the wheel cylinders during anti-lock or traction control events and two normally open inlet valves such as  149  providing a brake fluid path to their corresponding wheel cylinders. The circuits may share a pump drive motor  101 . 
     More specifically, FIG. 2 shows normally open solenoid actuated inlet valves  149  and  97  located between an operator-controlled pressure source such as a master cylinder for supplying pressurized fluid to lines  151  and  153  and hydraulic brake actuators which receive that pressurized fluid by way of lines  155 ,  157 ,  159  and  161 . Lines  157  and  159  lead to brake actuators or cylinders associated with the power receiving or driven wheels, the front wheels for example, while lines  155  and  161  are coupled to the non-driven wheels, or rear wheel brake cylinders in this example. Two separate hydraulic circuits are shown, with two wheels on each circuit. The vehicle wheels also have rotational wheel speed sensors or WSS  125  (FIG. 5) for providing electrical indications of the angular velocities of individual wheels to an electronic control unit or ECU  127 . When the driver wishes to slow the vehicle, a pedal is depressed to develop an input force that is transformed by the vehicle&#39;s master cylinder (or other type of booster mechanism) to hydraulic fluid pressure. This hydraulic pressure is transmitted from the master cylinder by way of conduits (brake lines)  151  and  153  to the respective rear and front pairs of brake actuators by way of four individual solenoid actuated inlet valves  163 ,  149 ,  97  and  165 . All four of the individual inlet valves are normally open and provide the path for braking fluid pressure during normal braking. However, if during a braking event a wheel skid is detected, say the wheel associated with line  155 , the solenoid of valve  163  is energized closing that valve and the outlet valve  139  is enabled to vent fluid pressure from the slipping wheel cylinder by way of line  167  to the accumulator  137 . Inlet valves  149 ,  97  and  165  function similarly. The inlet and outlet valves associated with the slipping wheel may be pulsed or otherwise controlled as is common in anti-lock braking technology. The accumulator  137  is sized such that worst-case venting conditions during an anti-lock event do result in wheel lock. If loss of traction of a driven wheel the wheel associated with line  157  for example, is detected, valves  163 ,  97 ,  81 , and  165  are closed, motor  101  is energized driving pump  143  to supply braking fluid pressure by way of pressure damper  169 , flow restrictor  171  and the normally open valve  149  to restrict wheel rotation and transfer driving torque to the other wheel. Venting of fluid from the wheel cylinder is, as before, by way of valve  141  to the accumulator  137 . Valve  149  remains open and does not otherwise participate in a traction control event. 
     In operation, only one circuit will be described in further detail. The accumulator  137  is partially filled by switching the inlet valves  163  and  149 , and the outlet valves  139  and  141  briefly during standard braking. The brake fluid is retained in the accumulator by the pump element  143 , the suction valve  79 , and the outlet valves  139  and  141 . The amount of brake fluid held in the accumulator is determined by the position monitoring sensor array  37 ,  39 . The accumulator is designed such that there is sufficient reserve capacity to accommodate worst case surface transitions during ABS (anti-lock) events, but enough brake fluid is retained in the accumulator to pre-charge the pump  143  during the rapid pressure increases needed for PBC (Panic Brake Control) and ESP events. ESP or electronic stability programs typically function to smooth or appropriately modulate braking. When traction control is required, two hydraulic valves are actuated to close. The first valve such as  97  prevents braking of the non-deviant wheel while the second valve such as  81 , commonly known as an isolation valve, closes to permit the build up of hydraulic pressure by the pumps in the braking circuits. Initially, the threshold of check valve  87  is exceeded and additional fluid withdrawn from the accumulator  137  to apply a braking force to the deviant wheel. 
     The use of controlled accumulator piston stroke to manage the secondary circuit volume for ESP hydraulic units eliminates the need for a two-stage valve capable of switching against master cylinder pressure in the suction path during PBC and active pressure ESP events. Thus, the valve can be replaced with a simple single: stage valve such as  79  which opens after the accumulator empties during a traction control event in order to continue supplying brake fluid to the pump. 
     A PWM algorithm for the electric motor is also necessary to properly control the secondary circuit volumes and resultant wheel pressures that will occur during modulation and is shown in FIG.  4 . Comparing FIGS. 2 and 4, normal operation during modulation and is shown in FIG.  4 . Comparing FIGS. 2 and 4, normal operation with no wheel slip is indicated at  93 . Rectangle  95  indicates wheel speed sensors  125  are providing an indication of vehicle speed as it increases along line  117  of the adjacent graph. One wheel begins to slip (accelerate) and the speed of the slipping wheel (line  121 ) moves toward and perhaps beyond the threshold speed shown by dotted line  119 . The threshold speed  119  is not constant, but varies directly with vehicle speed. If the wheel slip threshold (excessive speed) is exceeded as shown at  95 , isolation valve  81  is activated to close and the stable driven wheel inlet valve  97  closes as indicated at  99 . Motor  101  is turned full on at a 100% duty cycle as at  103 . The slip of the deviant wheel continues to be monitored at  105  and the motor duty cycle is maintained at 100% so long as wheel slip does not diminish as shown by feedback line  107 . When the wheel speed decreases sufficiently (as shown by the change to a negative slope of line  121  at  123 ), the duty cycle of motor energization is reduced as indicated at  109 , for example, to about 50% and the motor receives power about one-half the time. Wheel slip continues to be monitored and, so long as a target slip has not yet been reached (undershoot), motor  101  continues at the illustrative 50% duty cycle. When the target slip is passed as shown by line  121  intersecting line  117  in the adjacent graph, the duty cycle of motor energization is reduced to zero as shown at  111  and outlet valve  141  is pulsed to slightly relieve braking fluid pressure. Slip is being monitored for achievement of a target slip as at  113  and so long as the target slip is not reached, motor energization followed by pulsing valve  141  is repeated as shown by line  115 . When that target is attained, valve  141  is opened relieving all braking force. 
     FIG. 3 illustrates two substantially identical hydraulic circuits each having an accumulator, a pump and various valves. The circuits may share a common pump drive motor  71 . The hydraulic circuit of FIG. 3 differs from that of FIG. 2 by the absence of a pair of normally closed suction valves  79  and  83  with their associated in-line filters such as  151 , a pair of isolation valves  81  and  85 , two check valves such as  87 , and a pair of integrated suction dampers (ISD&#39;s) such as  91  which are bladder-like structures that improve pump performance. The accumulators,  11 , for example, are partially filled by switching the inlet valves such as  41  and  57 , and the outlet valves such as  61  or  65  briefly during standard braking. The brake fluid is retained in the accumulator by the closing forces of the pump element  73  and the outlet valves. The amount of brake fluid held in the accumulators is determined by the position monitoring sensor array  37 ,  39  and controlled by the ECU. The accumulator is designed such that there is sufficient reserve volume to accommodate worst-case surface transitions during ABS events, but enough brake fluid is retained in the accumulator to supply the pump during limited PBC (Panic Brake Control) and active pressure ESP events. Note, the common isolation valves normally found in anti-lock systems (and in FIG. 2) are missing from FIG.  3 . The system of FIG. 3 uses master cylinder pressure in place of the isolation valve to hold the pressure in the circuit. Small to moderate pressure increases above the master cylinder pressure are possible for short periods such as are seen during braking transitions and lane change events. Thus, FIG. 3 illustrates an enhanced anti-lock braking system (ABS) with limited electronic stability program (ESP) and panic brake control (PBC) which is realized through the use of the ESP sensor array, pulse width modulated (PWM) motor management, and Non-Contact Accumulator Position monitoring devices. Master cylinder pressure is used in place of the isolation valve for pressure active events such as lane changes and panic braking situations. Fully active ESP and TCS (traction control) are not possible with this concept because there is no way to hold the pressure in the wheels in the absence of any master cylinder pressure. 
     FIG. 5 illustrates the overall architecture of a vehicle braking system incorporating the present invention. An electronic control (ECU)  127  receives input information on accumulator volume from sensors  129  and information on vehicle speed from the wheel speed sensors (WSS)  125  as well as other vehicle performance and/or additional inputs such as ESP sensor information as indicated generally at  131 . The ECU  127  issues commands to initiate any of several actions to the pump motor  101  and various solenoid valves  133 . Further information or actions may also be issued or initiated as indicated generally by  135 . 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.