Abstract:
A brake-by-wire braking system including a braking unit, an electro-hydraulic actuator in fluid communication with the braking unit by way of a first fluid path, a master cylinder in fluid communication with the braking unit by way of a second fluid path, a normally open solenoid valve operatively associated with the second fluid path, and a control unit adapted to actuate the normally open solenoid valve, wherein actuation of the normally open solenoid valve generally fluidly isolates the master cylinder from the braking unit.

Description:
[0001]    This application claims priority from U.S. Provisional Ser. No. 60/773,003 filed on Feb. 14, 2006, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present application is directed to brake-by-wire braking systems and, more particularly, to full brake-by-wire braking systems with a hydraulic fail-safe. 
         [0003]    Modern vehicles, including electric vehicles, hybrid vehicles and traditional petroleum-powered vehicles, may include numerous electrical and computerized systems, such as brake-by-wire braking systems. Brake-by-wire braking systems typically replace the traditional mechanical and hydraulic fluid connection between the brake pedal and the braking units (e.g., disk or drum brakes) with an electrical connection (i.e., brake-by-wire). The electrical connection typically communicates user input signals from the brake pedal to a control unit and the control unit in turn controls the operation of the brake units to apply or release a braking force. 
         [0004]    A disadvantage associated with brake-by-wire braking systems is the risk that a single failure loss of electrical power will disable the brake system, leaving the vehicle operator with no means for stopping and/or controlling the vehicle. Attempts have been made to reduce the risks associated with an electrical failure by providing redundant or multiple independent electrical systems complete with separate battery reserves to serve as back-up systems in the event of an electrical system failure. However, such redundant electrical systems substantially increase manufacturing and system costs and typically increase the overall complexity of the electrical system. 
         [0005]    Accordingly, there is a need for a brake-by-wire braking system having a hydraulic fail-safe feature that can reduce manufacturing and system costs and electrical system complexity, as well as provide a simple, robust and proven source of braking energy in the event of electrical system malfunction. 
       SUMMARY 
       [0006]    In one aspect, the disclosed brake-by-wire braking system may include a braking unit, an electro-hydraulic actuator in fluid communication with the braking unit by way of a first fluid path, a master cylinder in fluid communication with the braking unit by way of a second fluid path, a normally open solenoid valve operatively associated with the second fluid path, and a control unit adapted to actuate the normally open solenoid valve, wherein actuation of the normally open solenoid valve generally fluidly isolates the master cylinder from the braking unit. 
         [0007]    In another aspect, the disclosed brake-by-wire braking system may include a control unit, an electro-mechanical brake caliper associated with a first vehicle wheel, the electro-mechanical brake caliper being in communication with and actuateable by the control unit, a hydraulically actuated braking unit associated with a second vehicle wheel, an electro-hydraulic actuator in fluid communication with the hydraulically actuated braking unit by way of a first fluid path, the electro-hydraulic actuator being in communication with and actuateable by the control unit, a master cylinder in fluid communication with the hydraulically actuated braking unit by way of a second fluid path, and a normally open solenoid valve operatively associated with the second fluid path, the normally open solenoid valve being in communication with and actuateable by the control unit, wherein, when the electro-hydraulic actuator is actuated, the normally open solenoid valve is actuated. 
         [0008]    Other aspects of the disclosed braking system will become apparent from the following description, the accompanying drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic view of one aspect of the disclosed brake-by-wire braking system having a hydraulic fail-safe; 
           [0010]      FIG. 2  is a partial schematic and partial sectional view of an electro-hydraulic brake actuator of the system of  FIG. 1 ; 
           [0011]      FIG. 3  is a front elevational view, in section, of an electro-mechanical brake caliper of the system of  FIG. 1 ; 
           [0012]      FIG. 4  is a front elevational view, in section, of a pedal feel emulator of the system of  FIG. 1 ; 
           [0013]      FIG. 5  is a perspective view of a master cylinder assembly of the system of  FIG. 1  including the pedal feel emulator of  FIG. 4 ; and 
           [0014]      FIG. 6  is a front elevational view, in section of the master cylinder and pedal feel emulator assembly of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    As shown in  FIG. 1 , a first aspect of a brake-by-wire braking system having a hydraulic fail-safe, generally designated  10 , may include a brake pedal  12 , a master cylinder  14 , a pedal feel emulator  16 , a hydraulically actuated bypass valve  18 , a main control unit  20 , a first electro-hydraulic actuator  22 , a second electro-hydraulic actuator  24 , a first electro-mechanical brake caliper  26 , a second electro-mechanical brake caliper  28 , a first normally open solenoid valve  30  and a second normally open solenoid valve  32 . Additionally, the system  10  may include a brake pedal switch  34 , a pedal travel sensor  36 , a hydraulic fluid reservoir  38 , a master cylinder pressure sensor  40 , a front axle control unit  42 , a first wheel pressure sensor  44  and a second wheel pressure sensor  46 . 
         [0016]    As shown in  FIG. 1 , the first electro-hydraulic actuator  22  may be in communication with a brake caliper  48  associated with the right front wheel  50  of a vehicle (not shown) by way of fluid line  52 . Fluid line  52  may be in fluid communication with the master cylinder  14  by way of fluid line  54  and the first normally open solenoid valve  30 . Pressure sensor  44  may be positioned on fluid line  52  to monitor the hydraulic fluid pressure and communicate the pressure to the front axle control unit  42  and ultimately to the master control unit  20 . 
         [0017]    As shown in  FIG. 1 , the second electro-hydraulic actuator  24  may be in communication with a brake caliper  56  associated with the left front wheel  58  of the vehicle by way of fluid line  60 . Fluid line  60  may be in fluid communication with the master cylinder  14  by way of fluid line  54  and the second normally open solenoid valve  32 . Pressure sensor  46  may be positioned on fluid line  60  to monitor the hydraulic fluid pressure and communicate the pressure to the front axle control unit  42  and ultimately to the master control unit  20 . 
         [0018]    Referring to  FIG. 2 , the electro-hydraulic actuators  22 ,  24  (only unit  22  is shown in  FIG. 2 ) may include a motor  62  (e.g., an electric motor) having a shaft  64  extending therefrom, an actuator housing  66  having a central bore  68  extending therethrough and a piston  70  closely and slidably received within the bore  68  to define a hydraulic fluid chamber  72  within the housing  66 . A gear assembly  74  and a high efficiency screw  76  may be provided to translate rotational torque from the shaft  64  of the motor  62  into axial movement (see arrow A) of the piston  70  within in the bore  68 . In addition, the motor  62  of the electro-hydraulic actuators  22 ,  24  may contain a mechanical or electro-mechanical brake mechanism (not shown) that can lock the shaft of the motor and thereby prevent unwanted back-driving of the gear and ball screw mechanisms even when the electrical signal is removed. 
         [0019]    Accordingly, in response to a command from the control unit  42  (or the master control unit  20 ) the piston  70  may advance distally through the hydraulic fluid chamber  72  to urge hydraulic fluid out of the hydraulic fluid chamber  72  and into the fluid line  52 , thereby increasing the fluid pressure in the fluid line  52  and actuating the brake calipers  48  ( FIG. 1 ) to apply a braking force. Similarly, electro-hydraulic actuator  24  may independently actuate brake caliper  56  by increasing fluid pressure in corresponding fluid line  60 . In one aspect, the control unit  42  may actuate (i.e., close) the normally open solenoid valves  30 ,  32  when the electro-hydraulic actuators  22 ,  24  are actuated to prevent hydraulic fluid from passing to fluid line  54 . 
         [0020]    Once the desired pressure in the fluid lines  52 ,  60  is reached, as determined by the wheel pressure sensors  44 ,  46 , the control unit  42  may stop each of the motors  62  and corresponding advancement of the pistons  70 . The braking force may be released by retracting the separate pistons  70  and/or opening the solenoid valves  30 ,  32  to depressurize the fluid lines  52 ,  60 . 
         [0021]    While the two normally open solenoid valves  30 ,  32  are actuated (i.e., closed), hydraulic fluid may not pass from the master cylinder  14  to the fluid line  54  (i.e., there is no hydraulic connection between the master cylinder  14  and the brake units at the wheels  50 ,  58 ,  78 ,  80 ). Therefore, to permit rod  13  displacement through the master cylinder  14  (i.e., to simulate a traditional brake pedal movement), the hydraulic fluid may be urged out of the master cylinder, through the normally open hydraulically actuated bypass valve  18  and into the pedal feel emulator  16 , as shown in  FIGS. 1 ,  5  and  6 . 
         [0022]    Referring to  FIG. 4 , the pedal feel emulator  16  may include a channel  96 , a fluid accumulating chamber  98  in communication with the channel  96 , a piston  100  closely and slidably received within the chamber  98  and a spring  102 . The channel  96  may be in fluid communication with the master cylinder  14  by way of the normally open hydraulically actuated bypass valve  18  ( FIG. 1 ). The spring  102  may be coaxially received over the piston  100  to urge the piston in the direction shown by arrow C and resist the introduction of hydraulic fluid into the chamber  98 . 
         [0023]    Referring again to  FIG. 1 , the first electro-mechanical brake caliper  26  may be associated with the right rear wheel  78  of the vehicle and the second electro-mechanical brake caliper  28  may be associated with the left rear wheel  80  of the vehicle. Each electro-mechanical brake caliper  26 ,  28  may include an electronic control unit  82 ,  84 , which in turn may be in communication with the master control unit  20 . 
         [0024]    As shown in  FIG. 3 , the electro-mechanical brake calipers  26 ,  28  (only caliper  26  is shown in  FIG. 3 ) may include a motor  86 , a caliper housing  88 , a piston  90 , two brake pads  92 A,  92 B and a ball screw assembly  94 . The ball screw assembly  94  may be positioned between the motor  86  and the piston  90  to translate rotational torque of the motor  86  into distal advancement of the piston  90 . As the piston  90  advances distally (i.e., in the direction shown by arrow B), the piston may urge the brake pads  92 A,  92 B into engagement with an associated rotor (not shown), thereby clamping the rotor between the brake pads to apply a braking force. In addition, the electro-mechanical brake caliper may include a latching mechanism (not shown) that can be used to function as a parking brake device by prohibiting the motor and geartrain assembly from back-driving and thus maintain the full brake clamping force even when the electrical signal is removed. 
         [0025]    Accordingly, in response to a command from the control units  82 ,  84  (or the master control unit  20 ), the motor  86  may be actuated to drive the piston  90  into engagement with the brake pads  92 A,  92 B. The braking force may be released by reversing the rotation of the motor and retracting the piston  90 . 
         [0026]    Thus, the system  10  may allow a user to apply a braking force to the wheels  50 ,  58 ,  78 ,  80  of a vehicle (not shown) by depressing the brake pedal  12 . The movement of the pedal  12  may be detected by any combination of the brake pedal switch  34 , the pedal travel sensor  36 , and the master cylinder pressure sensor  40  and subsequently communicated to the master control unit. 
         [0027]    In normal operation (i.e., not in fail-safe mode), when pedal movement is detected, the master control unit  20  may signal the two normally open solenoid valves  30 ,  32  to close, thereby preventing fluid flow in line  54 . The locked fluid condition prevents movement of the master cylinder main piston ( FIG. 6 ), which in turn keeps the seal of the hydraulically actuated bypass valve  18  in its normally open position of the bore undercut and causes trapped fluid in the opposite chamber to flow into the pedal feel emulator  16  through passageway  96 . Then, based upon the inputs received by the master control unit  20 , such as pedal travel (sensor  36 ), master cylinder pressure (sensor  40 ), wheel pressures (sensors  44 ,  46 ), vehicle speed, yaw rate, steering angle, lateral acceleration, longitudinal acceleration or any other appropriate signal, the master control unit  20  may actuate one or more of the electro-hydraulic actuators  22 ,  24  and/or one or more of the electro-mechanical brake calipers  26 ,  28  to generate a desired braking force and/or control the vehicle dynamics. 
         [0028]    Alternatively, in the fail-safe mode (e.g., when an electrical failure has occurred), the two normally open solenoid valves  30 ,  32  remain open, thereby allowing hydraulic fluid displaced from the master cylinder  14  to pass directly to the brake calipers  48 ,  56  to apply a braking force (i.e., hydraulic braking) to the front wheels  50 ,  58 . At the same time, since the master cylinder outlet port is no longer blocked shut, the main master cylinder piston is permitted to displace forward in the bore, allowing the seal of the normally open hydraulically actuated bypass valve  18  to slide past the bore undercut groove. The internal passageway is closed and any additional brake fluid is prevented from entering emulator  16 . 
         [0029]    Accordingly, in normal operation, the system  10  may operate as a full brake-by-wire braking system (i.e., no hydraulic or other mechanical connection between the master cylinder  14  and the brake units). In the fail-safe mode, the system  10  may have a direct hydraulic connection between the master cylinder  14  and at least one brake unit. 
         [0030]    Those skilled in the art will appreciate that various arrangements of electro-hydraulic actuators and electro-mechanical brake calipers may be used. In one alternative aspect, the electro-hydraulic actuators may be positioned at the rear of the vehicle and the electro-mechanical brake calipers may be positioned at the front of the vehicle. In another alternative aspect, an electro-hydraulic actuator is associated with only one wheel of a vehicle. In another alternative aspect, electro-hydraulic actuators are associated with three or more wheels of a vehicle. 
         [0031]    Although various aspects of the disclosed braking system have been shown and described, modifications may occur to those skilled in the art upon reading the specification. The present application includes such modifications and is limited only by the scope of the claims.