Abstract:
A braking assembly for providing a braking force to a wheel brake of a vehicle based upon receipt of an electrical brake signal is provided. The brake assembly comprises an actuator including a motor that applies and releases a piston within an actuator body. The actuator is fluidly connected to a wheel brake whereby the wheel brake can be applied and released. A solenoid valve is interposed between the actuator and the wheel brake on a primary fluid path for opening and closing the fluid connection. A step piston is placed within a secondary fluid path between the actuator and the wheel brake and operates to momentarily increase fluid displacement applied to the wheel brake when the solenoid valve closed.

Description:
INCORPORATION BY REFERENCE 
     Baumgartner U.S. Pat. No. 5,749,633 is incorporated by reference herein in so that electric brake by wire systems (BBW) need not be described in detail hereinafter. 
    
    
     The above referenced invention relates to an improvement in brake actuator design and specifically improves initial brake system response time. 
     BACKGROUND OF THE INVENTION 
     Hydraulic braking systems have typically been the basis for vehicle braking systems, especially automotive braking systems. Hydraulic systems are used to convert fluid pressure into linear and/or mechanical motion. Such systems allow the source of the hydraulic pressure to be positioned remotely from the cylinders which affect the braking action. These systems comprise an actuator, such as a brake pedal, reservoir fluid which is responsive to pressure applied by the actuator, (such as a master cylinder) and means for converting the hydraulic pressure to a braking force, generally fluid cylinders. Mechanical braking pressure is achieved by utilizing the force of the depression of the brake pedal by the driver to increase the pressure on the master cylinder. Such systems are typically accompanied by a vacuum boost which multiplies the force supplied to the brake pedal, throughout the braking operation. The increased pressure in the master cylinder is then transmitted through fluid lines to the fluid cylinders. The fluid cylinders operate the calipers thereby forcing the calipers and brake pads against the rotors and/or drums which slows the vehicle by frictional force. 
     Hydraulic systems of the above described type have many disadvantages. These include the large amount of volume and mass that the master cylinder vacuum booster, ABS modulator and hydraulic line add to the completed vehicle. Installation of standard hydraulic braking systems is also complicated and labor intensive. Additionally, the large number of parts and installation also adds to repair and maintenance issues as individual parts reach the end of their useful life. Standard hydraulic braking systems have also become dependent on the vacuum boost to assist in braking operations. However, vehicles such as electric or hybrid vehicles do not produce vacuum as a by-product of the vehicle operation. Thus vacuum boost is not an option on such vehicles. 
     In order to overcome some of the hydraulic system disadvantages, electric brake systems are known. While there are many variant forms, including electrical hydraulic systems, the use of electric in the variant forms is also referred to as a brake by wire brake system (BBW). BBW describes the ability to activate vehicle wheel brakes via an electric signal generated by an onboard processor/controller as a result of input signals thereto. Brake torque is applied to the wheels without direct mechanical interaction between the vehicle&#39;s brake pedal and the wheel brake. 
     A particular type of BBW systems is known as a “dry interface comer” system (DIC). The typical DIC system operates when a driver inputs a force to the brake pedal. A force sensor and/or travel sensor attached to the pedal transmits an electronic signal to an electronic controller, which in turn sends the signal to the self contained braking device typically located at each wheel of the vehicle. The DIC system is known as a hybrid system in that electric signals are used to generate the type and amount of braking force required at each wheel of the vehicle with electrical wires rather than standard hydraulic brake lines. Located at each comer of the vehicle is a self-contained module which takes the electrical signal and mechanically brakes the vehicle. The self contained module utilizes an individual motor that drives a ball screw piston assembly which pressurizes hydraulic brake fluid to ultimately apply the brake caliper to a rotor at that comer of the vehicle. Such a DIC system significantly reduces assembly cost. The individual modules can be separately assembled and fluid filled prior to the manufacture of the vehicle. DIC modules then only need to be bolted to the automobile during the assembly process and plugged in using standard electrical connections. Finally, the elimination of hydraulic lines stretching throughout the vehicle as well as the elimination of the master cylinder booster, and ABS modulator reduces space requirements within the engine compartment. 
     Due to the modularity of the DIC system, each of the individual components is preferably kept relatively small while still meeting a baseline brake response. Such a system keeps the DIC module a manageable size and does not overextend the existing electrical system on a vehicle. Such a system works well in most brake system applications. However, in certain instances the vehicle operator desires to decrease the initial brake system response time and shorten the vehicle stopping distance. However, the amount of fluid displacement to achieve pressure is limited by the size and speed of the motor and the gear ratio of the ball screw assembly. While increasing the size of the motor or the piston assembly and/or a combination thereof can decrease the initial brake system response time and result in a shorter vehicle stopping distance, such systems are unnecessary or impractical for most braking applications. Furthermore, such a combination would also result in greater size requirements for the DIC module and could have larger electrical load requirements from the vehicle power system. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a feature of the present invention to provide a vehicle braking system which overcomes the disadvantages of prior art vehicle braking systems by temporarily applying a greater amount of fluid to the brake caliper for the same amount of input motor turns. This feature is utilized in those situations in which the vehicle operator has indicated, through the brake pedal, that a shorter vehicle stopping distance is desired. The preferred embodiment of the invention is accomplished with a step piston which is added to the actuator and is preferably activated by closing a normally open solenoid valve. The step piston will result in a higher reaction load on the motor and thus limits the maximum pressure that can be developed while the step piston is in operation. Upon receipt of a signal from the vehicle operator that an improved vehicle stopping distance is desired, the normally open solenoid valve is closed for a very short period of time, after which it is re-opened at a pre-determined pressure during the brake apply, allowing the brake actuator to develop the desired maximum system pressure. The invention allows more initial fluid displacement to the brake caliper, which is not limited by the motor or actuator. The invention improves initial brake system response time and shortens vehicle stopping distance without increasing either the size of the DIC brake module or the electrical system requirements of the vehicle. 
     This feature, along with other features of the present invention, is achieved in an assembly for providing a braking force to a wheel of the vehicle based upon receipt of an electrical signal. The braking assembly comprises an actuator including a motor that applies and releases a piston within an actuator body. The actuator is fluidly connected to a wheel brake whereby the wheel brake can be applied and released. A valve is interposed between the actuator and the wheel brake on a primary fluid path for opening and closing the fluid connection on the primary fluid path. A secondary fluid path between the actuator and the wheel brake is also provided. A mechanism for increasing the fluid displacement or flow downstream of the secondary fluid path is provided and operates when the valve on the primary fluid path is closed. 
     In accordance with a more specific feature of the invention, the mechanism for increasing the fluid displacement on the secondary fluid path is preferably a step piston which includes first and second surfaces. The surface area of the second surface is generally greater than the surface area of the first surface and is also located downstream of the first surface. A second valve assembly is also disposed on the secondary fluid path between the actuator and the step piston. The second valve assembly preferably provides an orifice which controls the rate at which fluid flows upstream in the secondary path after the first valve in the primary path has been opened. As such, the gain in fluid pressure initially achieved by use of the step piston at the wheel brake is not lost when the first valve in the primary fluid path is opened. 
     In accordance with another aspect of the invention, a braking system to provide a braking force to a plurality of wheels of a vehicle is provided. This braking system comprises a plurality of actuators for braking the plurality of wheels, each of the actuators fluidly connected to at least one wheel brake on a primary fluid path. A sensor, for sensing a brake force and/or travel applied to a brake pedal is also provided. The sensed force and/or travel is sent to a controller which electrically generates a brake signal to the plurality of actuators based upon the force and/or travel applied to the brake pedal. A valve is provided on the primary path. This valve is normally opened below a preset value of the rate of change of sensed force or travel applied to the brake pedal. The primary path is closed when the brake signal exceeds that preset value. A secondary fluid path between the actuator and the wheel brake is also provided. The secondary fluid path includes a mechanism for multiplying the fluid displacement in the second fluid path between the mechanism and the wheel brake. A pressure sensor measures the fluid pressure generated in the second fluid path. Once the pressure sensor determines that fluid pressure exceeds a preset value, a signal is sent from the pressure sensor to the controller and thereupon from the controller to the mechanism for opening and closing the primary path, which then reopens the primary path. 
     In yet another aspect of the invention, a method is provided for initially boosting brake pressure. It includes providing a brake actuator assembly having a fluid displacement rate and providing a wheel brake. The wheel brake actuator assembly is connected to the wheel brake by both a primary fluid path and via a secondary fluid path. The operator of the vehicle generates an electrical brake signal to activate the brake actuator. The electrical brake signal is measured and compared against a preset value. If it is determined that the signal exceeds a preset value the normally open primary path is closed and fluid pressure is diverted to the secondary path. The fluid displacement rate in the secondary path is multiplied and the multiplied fluid displacement is provided to the wheel brake via the secondary path. On determining the fluid pressure in the secondary path exceeds a second preset value, the first primary path is reopened. The method further includes controllably reducing the fluid pressure and restoring displaced fluid in the secondary path after reopening the primary path. 
     Thus, it is a principal object of the present invention to provide an improved electrical brake assembly which provides a better initial brake system response time. 
     It is yet another object of the present invention to provide an improved electric brake system which results in shorter vehicle stopping distances. 
     Still another object of the present invention is to provide an electric brake assembly which provides a better initial brake system response time while not limiting the maximum pressure that can be developed by the wheel brake. 
     Yet another object of the invention to provide an improved electric brake assembly which does not increase electrical current draw from the vehicle. 
     Still yet another object of the present invention is to provide an improved vehicle braking system in which fluid displacement can be momentarily multiplied resulting in shorter vehicle stopping distances. 
     These and other objects of the invention will become apparent to those skilled in the art upon reading and understanding the following detailed description of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail and illustrated in the accompanying drawings which form a part hereof and wherein: 
     FIG. 1 is a graphical illustration showing the performance of an electric braking system without the improvement of the present invention; 
     FIG. 2 is a schematic illustration of a braking system of the present invention; 
     FIG. 3 is an elevation view, partially in section, of the braking system of the present invention; 
     FIG. 4 is an elevation view, partially in section, of one feature of the present invention taken along line  4 — 4  of FIG. 3; 
     FIG. 5 an elevation view similar to FIG. 4 showing the step piston of the present invention in operation; and 
     FIG. 6 is a graphical illustration showing the boost assist accomplished by the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, where the showings are for the purpose of illustrating the preferred embodiment of the invention only, and not for the purpose of limiting same, FIG. 1 shows the performance of a prior electric braking system, and specifically a dry interface comer actuator which does not utilize the improvement of the present invention. As depicted therein, an actuator using a gearset with a ratio of 5 to 1 and a ballscrew with six turns per one inch of travel is capable of developing maximum pressure of 2100 psi with a 190 oz-in 7395 rpm motor for a specific caliper. This actuator assembly develops slightly less than a 100 psi in 50 ms and 336 psi in 60 ms. The present invention improves the initial response time as can be seen from the graph of FIG.  6  and as will be described hereafter. 
     FIG. 2 schematically illustrates a dry interface corner braking system of the present invention. As shown brake pedal  11  is acted upon by the operator of the vehicle. This force is applied to a force sensor  12  which in turn generates an electrical signal to a left front comer  14  and a right front comer  15  of a vehicle. While FIG. 2 only shows the left and right front comers  14 ,  15 , it will be appreciated that the brake signal is transmitted by sensor  12  to all four comers of the vehicle, or each of the individual braking systems of the vehicle as the case may be. The preferred embodiment shows the boost assist step actuators of the present invention used in the front wheel brake applications. It will be appreciated that others, upon experimentation with specific applications, may provide the invention at the rear brakes, both front and rear brakes, or some combination thereof. 
     A left front corner  14  of FIG. 2 will now be described in detail. Right front comer  15  is identical and is numerically identified in FIG. 2 with identical numerals. Force sensor  12  and/or travel sensor  86  transmits a brake signal to an electronic control module  16  which in turn is electrically connected to an actuator assembly  21  and specifically to a motor  22 . Motor  22  drives a gear mechanism and ball screw assembly  23  that applies and releases a hydraulic piston  24  within an actuator body  25 . Extending from actuator body  25  is a primary fluid line  71 . Located within primary line  71  is a normally open solenoid valve  26 . During a normal brake apply and release, displaced brake fluid flows within primary fluid line  71 , through normally open solenoid valve  26  and through outlet  27 . The fluid pressure therein applies and releases brake calipers  31  against rotor  32  which comprises a wheel brake  33 . As can be seen in FIG.  3  through FIG. 5, actuator assembly  21  includes a step piston  34 . The piston  34  includes two adjacent generally co-axial cylinders  35  and  36 . Cylinder  35  has a downstream surface area  37  and cylinder  36  has an upstream surface area  41  smaller than surface area  37 . In the preferred embodiment, surface area  37  is twice that of surface area  41 . O-rings  42  and  43  seal step piston  34  within the interior of piston wall  45 . The area between O-rings  42  and  43  is vented through a channel  46 . 
     Located downstream of hydraulic piston  24  is a flow valve assembly  51 . Flow valve assembly  51  is connected to hydraulic piston  24  via a channel  52 . Included within the body  53  of flow valve assembly  51  is a ball  54 , a spring  55 , and a retainer plate  56 . Flow valve assembly  51  is held in place by a retaining ring  57 . Ball  54  sits within a ball seat  61  which includes a small control orifice  62  fluidly connecting ball seat  61  with channel  52 . 
     As discussed above and as can be seen from FIG. 2, solenoid valve  26  is located along a primary fluid line  71  extending from a point upstream of channel  52  to brake caliper  31 . A secondary or bypass fluid line  72  extends from the outlet of flow valve assembly  51  to an intersection point  73  with primary fluid line  71 . As seen in FIG. 2, pressure transducer (or pressure sensor)  74  is located downstream of solenoid valve  26  and intersects primary line  71  via a port  75 . 
     During normal braking operations, solenoid valve  26  is in a normally open state. Accordingly, the operator presses brake pedal  11  which is sensed by force sensor  12  and sends a signal to brake control module (or brake controller)  16 . Brake controller  16  in turn generates a signal to actuate motor  22  and drive ball screw assembly  23  and hydraulic piston  24  to pressurize primary fluid line  71 . As seal  87  covers a bypass hole  81 , a fluid reservoir  82  is isolated and braking of wheel brake  33  is accomplished by piston surface area  77  displacing fluid through the primary fluid line  71 . However, under certain conditions a vehicle operator may desire to stop the vehicle in a shorter stopping distance by applying force to brake pedal  11  at a faster rate. The rate of change of force from sensor  12  or the rate of change of travel from sensor  86  is compared to a preset value. If the sensed rate of change exceeds the preset value, (preferably equivalent to 3000 psi per second), electronic brake control module  16  sends a signal to solenoid valve  26  (via a line  83 ) to close. As solenoid valve  26  closes, brake fluid flows through channel  52  and unseats ball  54 . Brake fluid flows freely through flow valve assembly  51  and builds pressure against upstream surface area  41  of step piston  34 . In the preferred embodiment, the area of upstream surface area  41  is generally equal to piston surface area  77 . Therefore, when solenoid valve  26  is closed, the hydraulic piston  24  and step piston  34  effectively move the fluid column to the left as shown in FIG.  5 . As discussed previously the downstream surface area  37  is generally twice that of upstream surface area  41 . Therefore, the amount of fluid displaced to wheel brake  33  is twice the amount displaced (for the same number of motor turns) when the solenoid valve  26  is opened. This results in an improved initial brake system response time and consequently a shorter vehicle stopping distance, as can be seen graphically at FIG.  6 . 
     It will also be appreciated that the ratio of downstream surface area  37  to upstream surface area  41 causes the reaction load on motor  22  to be twice as much as if solenoid valve  26  were left open . Thus, the applied pressure acting against hydraulic piston  24  and specifically piston surface area  77  is twice that of when solenoid valve  26  is open and twice that of the downstream pressure in channels  72 ,  73 , and  75 . If solenoid valve  26  were kept closed throughout a maximum pressure brake pedal apply, motor  22  would stall at a lower than acceptable wheel brake apply pressure. In order to prevent motor  22  from stalling because of high apply pressure, pressure transducer  74  constantly senses the pressure downstream of solenoid valve  26  and sends such signal to brake controller  16  via line  84 . After solenoid valve  26  is closed, and once the downstream pressure sensed by pressure transducer  74  exceeds a predetermined (or preset value) wheel brake pressure (generally 400-500 psi), brake controller  16  sends a signal via line  83  to reopen solenoid valve  26 . 
     After the solenoid valve  26  is reopened at the predetermined wheel brake pressure, the reacting pressure force against motor  22  is shifted back to the piston surface area  77  of hydraulic piston  24  from surface  37  of step piston  34 . During the initial brake apply contemplated by the invention, flow valve assembly  51  has assumed the position shown in FIG. 5 in which fluid pressure within a cavity  76  exceeds that of piston surface area  77 . The reopening of solenoid valve  26  balances the pressure across flow valve assembly  51  and specifically between downstream surface area  37  and piston surface area  77 . Thus the reaction load pushes step piston  34  back to the right, as shown in FIG.  5 . This motion of step piston  34  results in a loss of fluid displacement in the system due to the area ratio of surfaces  37  and  41 . As seen in FIGS. 4 and 5, the rate at which step piston  34  can move depends on the rate at which the fluid trapped in cavity  76  can exit through control orifice  62  and ball seat  61 . The actual size of control orifice  62  is dependent upon the design of the actuator  21  and wheel brake  33  displacement. However, for actuator assembly  21  graphically represented in FIG. 6 a control orifice  62  of size equivalent to 0.38 mm diameter has been found to be useful. 
     In general, upon reopening solenoid valve  26 , the rate of release of trapped fluid in cavity  76  needs to be slow enough in order that the pressure build up to wheel brake  33  is approaching the maximum pressure level, as shown in FIG. 6 (in reaching maximum motor stall torque) prior to step piston  34  fully returning to the initial position. This allows additional fluid displacement to be made up when step piston  34  shuttles back to the right, as shown in FIG.  5 . The tuned time delay, in the return of step piston  34 , results in an optimum maximum brake apply response curve as shown in FIG.  6  . As can be seen at point A of FIG. 6, valve  26  is reopened but the upper portion of the curve does not significantly lose the initial pressure from step piston  34 . If the release rate of the trapped fluid in cavity  76  were too fast, step piston  34  would move too quickly to the right. Such a fast displacement loss would cause a lag in the response curve and negate the gains achieved by the momentary closing of solenoid  26  during a maximum brake apply. 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding this specification. It is intended to include all such modifications and alterations in so far as they come within the scope of the appended claims.