Abstract:
A vehicle braking system includes a master cylinder coupled to a brake pedal and establishing fluid communication to wheel cylinder(s) of primary and secondary circuits via respective outputs. A pressure sensor detects actual, non-boosted demand pressure. Each circuit includes a motor-driven pump defining pressure and suction sides, inlet and outlet valves, a normally-open isolation valve, and a normally-closed apply pressure control valve. A pedal feel simulator receives hydraulic fluid from the master cylinder when the isolation valves are closed. A controller receives a signal from the pressure sensor and generates corresponding control signals for the motor-driven pump and the apply pressure control valve of each circuit to pressurize the pressure side of each circuit equal to the sensed demand pressure plus a predetermined boost factor, and the actual pressure at each wheel cylinder is configured for individual manipulation by selective opening and closing of the inlet and outlet valves.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/825,737 filed May 21, 2013, the entire contents of which are incorporated by reference herein. 
    
    
     BACKGROUND 
     The present invention relates to vehicle brake systems. Generally, brake systems require a dedicated booster device to provide brake force amplification so that the brake force exceeds the input force from a driver&#39;s foot on a brake pedal under normal operation. In conventional vehicles with internal combustion engines, the booster device is most typically a vacuum booster. However, in alternative vehicles (e.g., electric vehicles) vacuum from an internal combustion engine may not be available. Thus, brake boost must be provided by other means, such as an electro-mechanical device or a high pressure accumulator. For example, in a full-power brake system (also referred to as a “decoupled” or “brake-by-wire” system) in which driver applied force is not used whatsoever to produce the actual braking force to the brake devices, a high pressure accumulator is provided to store a quantity of hydraulic fluid pumped to a high pressure for on-demand application to the brakes. Although satisfactory for the intended purpose, the accumulator adds complexity to the system and inherent component and assembly cost. Furthermore, many vehicles with internal combustion engines have recently adopted direct injection technology, in which vacuum levels may not be sufficient for a conventional vacuum brake booster. This can necessitate the addition of a vacuum pump to generate sufficient vacuum for a brake booster. 
     SUMMARY 
     In one aspect, the invention provides a vehicle braking system including a master cylinder configured to receive an input from a brake pedal. The master cylinder has an outlet for a primary circuit including at least one wheel cylinder operable to provide a braking force on a wheel when supplied with pressurized hydraulic fluid, and an outlet for a secondary circuit including at least one wheel cylinder operable to provide a braking force on a wheel when supplied with pressurized hydraulic fluid. A pressure sensor is operable to detect an actual, non-boosted driver demand pressure generated at one of the master cylinder outputs from depression of the brake pedal. At least one pump is provided in each of the primary circuit and the secondary circuit to define a pressure side and a suction side of each of the respective primary and secondary circuits. An inlet valve is positioned adjacent each wheel cylinder on the pressure side of each of the primary circuit and the secondary circuit, and an outlet valve is positioned adjacent each wheel cylinder on the suction side of each of the primary circuit and the secondary circuit. A normally open isolation valve is positioned between each master cylinder output and the pressure side of the respective primary circuit and the secondary circuit. Each isolation valve is movable between an open position that allows fluid communication and a closed position that blocks fluid communication. A pedal feel simulator is configured to receive hydraulic fluid from the master cylinder to provide a reaction force to the brake pedal when the isolation valves of the primary and secondary circuits are closed. At least one motor is operable to drive the pumps of the primary and secondary circuits to generate hydraulic fluid pressure in the pressure side for supplying to the respective wheel cylinders through the respective inlet valves. A normally closed apply pressure control valve is positioned in each of the primary circuit and the secondary circuit between the pressure side and the suction side of the respective circuit, each apply pressure control valve being movable between a closed position in which the full hydraulic fluid pressure at the pressure side is provided to the respective inlet valve and an open position in which the pressure provided to the respective inlet valve is reduced in comparison to that at the pressure side. A controller is operable to receive a signal from the pressure sensor and to generate corresponding control signals for the at least one motor and the apply pressure control valves of each of the primary and secondary circuits to produce a hydraulic fluid pressure in the pressure side of each of the primary and secondary circuits equal to the sensed driver demand pressure plus a predetermined boost factor, and the actual pressure at each wheel cylinder is configured for individual manipulation by selective opening and closing of the associated inlet and outlet valves. 
     In another aspect, the invention provides a method of operating a vehicle braking system. Primary and secondary circuits are provided, each including at least one wheel cylinder operable to provide a braking force on a wheel when supplied with pressurized hydraulic fluid, the primary and secondary circuits being in selective fluid communication with respective outputs of a master cylinder. An inlet valve is provided positioned adjacent each wheel cylinder on a pressure side of each of the primary circuit and the secondary circuit, and an outlet valve is provided positioned adjacent each wheel cylinder on a suction side of each of the primary circuit and the secondary circuit. An input from a brake pedal is received at the master cylinder. The pressure side of both the primary and secondary circuits is isolated from the master cylinder outputs and fluid communication is established between at least one of the master cylinder outputs and a pedal feel simulator. An actual, non-boosted driver demand pressure generated at one of the master cylinder outputs from depression of the brake pedal is detected. Both a pump(s) provided in each of the primary circuit and the secondary circuit is operated and a normally closed apply pressure control valve positioned in each of the primary circuit and the secondary circuit are modulated with respective signals corresponding to the detected driver demand pressure to produce a hydraulic fluid pressure in the pressure side of each of the primary and secondary circuits equal to the sensed driver demand pressure plus a predetermined boost factor. The actual pressure at at least one individual wheel cylinder is modulated by selective opening and closing of the associated inlet and outlet valves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a braking system according to one aspect of the present invention. A normal braking mode is illustrated. 
         FIG. 2  is a schematic drawing of the braking system of  FIG. 1 . A braking mode for a failed primary circuit motor is illustrated. 
         FIG. 3  is a schematic drawing of the braking system of  FIG. 1 . A braking mode for a failed secondary circuit motor is illustrated. 
         FIG. 4  is a schematic drawing of the braking system of  FIG. 1 . A braking mode for a full power failure is illustrated. 
         FIG. 5  is a schematic drawing of a braking system according to one aspect of the present invention, including a circuit isolation shuttle valve. A normal braking mode is illustrated. 
         FIG. 6  is a schematic drawing of the braking system of  FIG. 5 . The circuit isolation shuttle valve is stroked during failure of a primary circuit motor. 
         FIG. 7  is a schematic drawing of the braking system of  FIG. 5 . The circuit isolation shuttle valve is re-centered during failure of a primary circuit motor. 
         FIG. 8  is a schematic drawing of the braking system of  FIG. 5 . The circuit isolation shuttle valve is re-stroked during failure of a primary circuit motor. 
         FIG. 9  is a schematic drawing of the braking system of  FIG. 5 . A braking mode for a full power failure is illustrated. 
         FIG. 10  is a schematic drawing of a braking system according to one aspect of the present invention, including a circuit isolation shuttle valve, a shared motor for the pumps of the primary and secondary circuits, and a shared return line. A normal braking mode is illustrated. 
         FIG. 11  is a schematic view illustrating how anti-lock braking (ABS) sensors are monitored by an electronic control unit to control the system components and ultimately the operational state of the braking system. 
         FIG. 12  illustrates the various intervention states for ABS and a graphical representation of an exemplary ABS deceleration event. 
         FIG. 13  illustrates the basic oversteer and understeer conditions addressed by electronic stability control (ESP) intervention. 
         FIG. 14  illustrates a control strategy for brake blending between hydraulic braking and alternative braking for a brake system according to the present invention. 
         FIG. 15  is a cross-section view of a radial piston pump. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
     The braking system  100  of  FIG. 1  includes a master cylinder  104  coupled to a brake pedal  106  to pressurize hydraulic fluid therein. The master cylinder  104  includes dual outputs to provide fluid to a primary circuit and a secondary circuit. In the illustrated construction, each of the circuits includes a pair of brake devices or wheel cylinders  110 . The wheel cylinders  110  of a particular circuit can be associated with a set of front wheels, a set of rear wheels, or a set of diagonal wheels. A pedal travel sensor  112  is coupled to the brake pedal  106  and is operable to detect an amount of travel of the brake pedal  106 , so that a corresponding signal can be sent to a controller (not shown). Each circuit includes an inlet valve EV and an outlet valve AV associated with each wheel cylinder  110 . A check valve RVR adjacent each outlet valve AV ensures one-way flow of fluid through the outlet valve AV, which is away from the wheel cylinder  110 . One or more pumps  116  are provided in each circuit, each pump  116  having a suction side adjacent the outlet valve AV and the check valve RVR associated with each wheel cylinder  110  of the circuit. A pressure side of each pump  116  is provided in fluid communication with the inlet valves EV to provide pressurized fluid to the inlet valves EV, and through the inlet valves EV (when open) to the wheel cylinders  110  when the pump(s)  116  are operated by a motor. In the illustrated construction a first electric motor M 1  is provided to drive two pumps  116  in the primary circuit and a second electric motor M 2  is provided to drive two pumps  116  in the secondary circuit. Alternate arrangements may be provided, some of which are discussed below. Each of the pumps  116  can be a variable speed pump, operable to generate a variable output corresponding to a variable drive speed from the associated motor M 1 , M 2 . 
     A first apply pressure control valve  120  is provided in parallel with the pumps  116  of the primary circuit and a second apply pressure control valve  120  is provided in parallel with the pumps  116  of the secondary circuit. The apply pressure control valves  120  can be modulated solenoid valves (e.g., having a range of open positions, or receiving a pulse-width modulation signal to achieve a similar effect). A driver isolation valve  124  is positioned in each circuit between the master cylinder output and the inlet valves EV such that movement of hydraulic fluid from the master cylinder  104  can be selectively enabled and selectively blocked. The driver isolation valves  124  can be normally-open switched solenoid valves. During normal (powered) operation, the driver isolation valves  124  are closed so that the depression of the brake pedal  106  does not directly apply braking force to the wheel cylinders  110  through the hydraulic fluid. Rather, the fluid displaced from the master cylinder  104  is directed to a pedal feel simulator PFS to mimic the feel and travel present during braking of a conventional braking system. The pedal feel simulator PFS is coupled to one of the master cylinder outputs via a simulator isolation valve (e.g., a normally-closed switched solenoid valve)  128  in parallel with one of the driver isolation valves  124 . The simulator isolation valve  128  is open during normal (powered) operation. A pressure sensor  130  is provided in the shared supply line to the master cylinder side (or “upstream” side) of the simulator isolation valve  128  and the driver isolation valve  124  in fluid communication with one of the master cylinder outputs. The pressure sensor  130  is operable to detect a driver demand pressure and output a corresponding signal. 
     When brake pedal actuation is detected by the pedal travel sensor  112 , the driver isolation valves  124  are actuated to close and the simulator isolation valve  128  is actuated to open. As pedal feedback is provided by the pedal feel simulator PFS, a braking force (i.e., hydraulic fluid pressure) corresponding to the driver&#39;s demand, as sensed by the pressure sensor  130 , is generated by the pumps  116  in each circuit and applied to the corresponding wheel cylinders  110  through the apply pressure control valves  120 . The output of the pumps  116  is controlled by controlling the running speed of the motors M 1 , M 2  and modulating the apply pressure control valves  120  to achieve a braking force at the wheel cylinders  110  that equals the driver demand pressure as sensed by the sensor  130 , plus a predetermined boost amount. Feedback of the actual pressure generated at the outlet of the pumps  116  is provided by a pressure sensor  134  in each circuit. Thus, no dedicated booster device is required to achieve boosted braking, and the system responds to modulate braking force at the wheel cylinders  110  without a high pressure accumulator, since the pump output can simply be raised in real time in response to a sudden demand increase (from driver demand or vehicle demand in the case of autonomous braking). This can be accomplished by modulating the apply pressure control valve(s)  120 , and in some cases also increasing the running speed of the motor(s) M 1 , M 2 . In some constructions, the running speed of the motor(s) M 1 , M 2  may be set in relation to the initial brake pedal travel detected by the sensor  112 , and may remain substantially constant for the braking event, with precision pressure control carried out by the apply pressure control valves  120 . Of course, a dramatic increase in pedal travel beyond a threshold during a braking event may result in increasing motor speed. It should also be noted that the motor(s) and the apply pressure control valves  120  can be controlled in response to the signal from the pedal travel sensor  112  (e.g., if the pressure sensor  130  fails). 
     When both wheel cylinders  110  of a given circuit require the same pressure level, setting of the pressure can be accomplished with the apply pressure control valve  120 , while each inlet valve EV remains in its open, non-actuated state and each outlet valve AV remains in its closed, non-actuated state. The inlet valves EV and outlet valves AV are able to provide ABS and ESP function according to their normal operation in a conventional system. The operation is described in some detail in  FIGS. 11-13 .  FIG. 11  generally illustrates how information from various ABS sensors is communicated to an ECU, which in turn, controls the motor-driven pumps  116  and the valves within one or more braking circuits to effect a particular brake system state to achieve a desired braking action.  FIG. 12  illustrates an ABS control strategy which prevents wheel lock-up by modulating the wheel caliper pressure using the inlet and outlet valves EV, AV. The table in  FIG. 12  shows various ABS intervention states and the corresponding valve positions for the inlet and outlet valves EV, AV. The graph in  FIG. 12  includes plots of vehicle reference speed, actual wheel speed, and caliper pressure versus time for a deceleration event. Initially, caliper pressure builds with the inlet valve EV open and the outlet valve AV closed. When wheel speed is sensed to deviate from vehicle reference speed (indicative of impending lock-up), the caliper pressure is reduced by closing the inlet valve EV to prevent further pressure generated from the driver from entering the wheel caliper, and by opening the outlet valve AV. The system then enters a pressure hold state where both the inlet and outlet valves EV, AV are closed and a particular caliper pressure is maintained. Although the wheel speed returns to match the vehicle reference speed in this state, the wheel speed may exceed the desired brake deceleration estimated by the system. To keep the vehicle at the desired brake level, pressure must be increased, which is accomplished by opening the inlet valve EV, while the outlet valve AV remains closed. If the wheel speed again deviates significantly below vehicle reference speed, pressure at the wheel caliper is relieved by closing the inlet valve EV and opening the outlet valve AV, and the cycle of decreasing, holding, and increasing caliper pressure is repeated as necessary to decelerate the vehicle. Of course, if the wheel speed does not deviate below the vehicle reference speed, ABS intervention not needed, as the desired deceleration is being met without impending wheel lock-up in those circumstances. 
       FIG. 13  illustrates both an oversteer scenario and an understeer scenario, and the corrective yaw moment from the ESP function of the braking system that is required to correct each condition. Upon oversteer (upper portion of  FIG. 13 ), the vehicle is spinning (i.e., the rear axle of the vehicle is sliding toward the outside of the curve) because the yaw velocity is too large. Stabilization is imparted by controlled brake intervention at the front wheel at the outside of the curve. Upon understeer (lower portion of  FIG. 13 ), the front axle of the vehicle is sliding toward the outside of the curve because the yaw velocity is too small. Stabilization is imparted by controlled brake intervention at the rear wheel at the inside of the curve. 
     In a case where only one wheel of a given circuit requires braking pressure (e.g., in an autonomous stability control intervention), the demand pressure may be achieved by controlled operation of the pump(s)  116  and the apply pressure control valve  120 , and the wheel cylinder  110  not requiring the braking pressure can simply be blocked by closing the associated inlet valve EV. When two different brake pressures are required at the wheel cylinders  110  of a given circuit, the higher pressure may be achieved directly by the pump(s)  116  and the apply pressure control valve  120  of the circuit for applying directly to one of the wheel cylinders  110  (without actuation of the inlet and outlet valves EV, AV) and the lower pressure to be delivered to another wheel cylinder  110  of the circuit may be achieved by modulating at least one of the inlet and outlet valves EV, AV to apply a pressure to the wheel cylinder  110  that is less than the pressure from the apply pressure control valve  120 . For example, this may occur when a wheel cylinder for a front wheel and a wheel cylinder for a rear wheel are provided in the same circuit (X-split), in which the pressure to the wheel cylinder for the rear brake has a lower threshold for lock-up than the front. 
     In the event that one of the motors M 1 , M 2  fails or loses power, the opposite circuit is still able to apply boosted braking as normal with a set of wheel cylinders  110 . This situation is illustrated in  FIGS. 2 and 3 . With respect to a full system power failure,  FIG. 4  illustrates that a failsafe braking mode will be enacted, in which all valves assume their naturally biased state and the braking force generated by the driver&#39;s actuation of the brake pedal  106  is transferred from the master cylinder  104  to the wheel cylinders  110  of both circuits. In other words, the driver isolation valves  124  are biased open, the simulator isolation valve  128  is biased closed, and the apply pressure control valves  120  are biased closed. All inlet valves EV are biased open and all outlet valves AV are biased closed. Although no boost is present, the master cylinder  104  geometry is designed to provide adequate braking power to all wheel cylinders  110 . 
       FIGS. 5-9  illustrate a braking system  100 A according to another construction, which is similar in many respects to the braking system  100  of  FIGS. 1-4 . Components and functions of the braking system  100 A not specifically discussed below will be understood to correspond to the description of the braking system  100  provided above. The braking system  100 A includes a means for transferring braking force (hydraulic fluid pressure), without actually transferring fluid, from a functional circuit to a non-functional circuit (i.e., a circuit with a failed pump  116  and/or motor M 1 , M 2 ). In the illustrated construction, a circuit isolation shuttle valve  140  is provided in communication with the pressure side of the pumps  116  of the first circuit and in communication with the pressure side of the pumps  116  of the second circuit. Thus, the inlet valves EV for all wheel cylinders  110  are coupled to one side or the other of the circuit isolation shuttle valve  140 . A shuttle within the valve  140  is biased to a central position by two biasing members. A normally-closed pressure diversion valve  142  (e.g., a switched solenoid valve) is provided adjacent the circuit isolation shuttle valve  140  in fluid communication with one of the ports thereof so that the pressure diversion valve  142  normally prevents any movement of the shuttle, and thus any transfer of braking force between the two separate circuits through the circuit isolation shuttle valve  140 .  FIGS. 6-8  illustrate an exemplary operation method of the braking system  100 A, utilizing the circuit isolation shuttle valve  140  to provide boosted braking in a failed circuit. 
     As shown in  FIG. 6 , the motor M 1  and pumps  116  in the primary circuit are in a non-operational state. Upon identifying this failure, the pressure diversion valve  142  is actuated to an open state which establishes fluid communication between the circuit isolation shuttle valve  140  and the pressure side of the operational pumps  116  of the secondary circuit. Thus, boosted braking pressure in the secondary circuit can be “shared” with the primary circuit as the shuttle moves within the valve  140  against the bias of the biasing member on the primary circuit side. In some cases, the shuttle within the circuit isolation shuttle valve  140  may bottom out or run out of travel before the pressure in the failed primary circuit has reached the desired apply pressure (the pressure present in the fully operational secondary circuit), as illustrated in  FIG. 6 . However, the system  100 A can be configured to carry-out additional stroking of the circuit isolation shuttle valve  140  as shown in  FIGS. 7 and 8  to further increase the pressure in the failed primary circuit, up to the desired apply pressure that is present in the secondary circuit. 
     In order to re-stroke the circuit isolation shuttle valve  140 , its shuttle must first be returned to the central or nominal position. This is done by closing all inlet valves EV to trap pressure at the wheel cylinders  110  and then reducing the apply pressure in the functional secondary circuit to zero (i.e., by opening the apply pressure control valve  120 ) as shown in  FIG. 7 , which allows the biasing member on the primary side of the circuit isolation shuttle valve  140  to restore the shuttle to the nominal position. With both circuits back to atmospheric pressure, pressure can again be built up in the secondary circuit and added to the pressure previously held at the wheel cylinders  110 , when the inlet valves EV are again opened. With the pressure diversion valve  142  open, pressure built in the secondary circuit is again shared with the failed primary circuit as the circuit isolation shuttle valve  140  is re-stroked ( FIG. 8 ). This process can be repeated as necessary to achieve a target apply pressure or pressure match between the operational circuit and the failed circuit. Of course, the same procedure can take place in mirrored fashion, when the secondary circuit fails and the primary circuit remains operational. Thus, boosted braking is not limited to one circuit, even when a complete failure of the boosting components (motor and pump(s)) occurs in the other circuit. In some constructions, the pressure diversion valve  142  can be a modulated valve having a range of open positions or being pulse-width modulated to achieve a similar effect, with the result that more precise control can be provided for the pressure from the operational circuit that is “shared” to the failed circuit.  FIG. 9  illustrates that the braking system  100 A has a failsafe condition similar to that of the system  100 . 
     It should also be noted that the “sharing” of fluid between circuits with the circuit isolation shuttle valve  140  is not just restricted to use in compensating for a failed circuit. Rather, fluid sharing via the circuit isolation shuttle valve  140  can be a strategy in the initial design, which could for example, enable down-sizing of the pumps  116  and/or motors M 1 , M 2  for each of the circuits. This may be advantageous to provide a lower cost without sacrificing pressure build dynamics in the system. 
     In one alternate configuration shown in  FIG. 10 , a braking system  100 B can be provided similar to the system  100 A, but having a single motor M which drives one or more pumps  116  of the primary circuit and one or more pumps  116  in the secondary circuit. The primary and secondary circuits also share a single return line coupling both apply pressure control valves  120  to the reservoir (at atmospheric pressure). Because the braking system  100 B is highly space-efficient, all valves, motor(s), pumps, and the pedal feel simulator PFS, and optionally even including the master cylinder  104  and its piston(s), may be provided in a single common housing H. Although only illustrated with the construction of  FIG. 10 , the use of the single common housing H may be applied to other constructions. It should also be noted that, unless specifically precluded, features illustrated in one construction may be applicable to other constructions, resulting in additional combinations that need not be particularly illustrated and discussed to be appreciated by one of ordinary skill in the art. 
     The pump elements utilized in the brake systems of the present invention can be of any number of designs, including gear pumps to reduce noise, vibration, and harshness. The pump-driving motor(s) can likewise be of the brushless type. In some constructions, the pumps are axial piston pumps having a construction as shown in  FIG. 15 . Each pump  116  includes a motor input shaft  150  coupled to an angled swash plate  154 . The swash plate  154  drives a plurality of pistons (i.e., pump elements)  158  to be reciprocated in a smooth, quiet manner. The pistons  158  are oriented parallel to the rotational axis of the motor input shaft  150 . A low pressure inlet  162  is in fluid communication with the swash plate  154  and a first side of the pistons  158 . A high pressure outlet  164  is in fluid communication with the second side of the pistons  158 . The brake systems described herein may be used in a variety of vehicles, including those with conventional internal combustion engines, direct injected internal combustion engines, hybrid drive trains having both an internal combustion engine and an alternative drive source (e.g., electric, hydraulic), and full electric drive trains, among others.