Abstract:
An improved electro-hydraulic brake system having an electrically powered normal source of pressurized hydraulic brake fluid, and a manually powered backup source of pressurized hydraulic brake fluid to all four of the vehicle brakes in the event of failure of the normal source. During normal braking, fluid from the backup source is redirected from the vehicle brakes to a pedal simulator. The brake system of the invention further diverse arrangements for providing braking in the event of a failure of the normal source of pressurized fluid. The brake circuit for the front wheels are provided with relatively low cost fluid separator units, while the rear brake circuit is not.

Description:
This application claims the benefit of Provisional Application No. 60,382,746, filed May 23, 2002. 

   This invention relates in general to brake systems for ground vehicles, and in particular to electro-hydraulic brake systems with normal braking pressure supplied by an electrically driven pump. 
   Electro-hydraulic braking systems with manually powered backup systems have been shown in some publications. For example, German Patent Application DE 4413579A1 (the disclosure of which is incorporated herein by reference) illustrates a system having a manually powered master cylinder connected through isolation valves to brakes at a vehicle&#39;s wheels. When the isolation valves are shut, pressurized brake fluid from the master cylinder is delivered to a pedal simulator. Pressure transducers are used to develop a signal representative of a desired braking effort, which is fed to an electronic control unit. The electronic control unit controls the operation of motor operated braking pressure generators (pumps) to correspondingly deliver pressurized hydraulic brake fluid to the vehicle brakes. 
   SUMMARY OF THE INVENTION 
   This invention relates to a brake system for a vehicle having a normal source of pressurized hydraulic brake fluid, such as a electric motor-driven pump, and a backup source of pressurized hydraulic brake fluid, such as a manually operated brake pedal unit (including a master cylinder). The vehicle is provided with a front vehicle brake that is operated by application of pressurized hydraulic brake fluid thereto. A front fluid conduit connects the front vehicle brake with the backup source. A front isolation valve is disposed in the front fluid conduit for selectively preventing the flow of hydraulic brake fluid between the backup source and the front vehicle brake. A fluid separator unit prevents intermixing of the hydraulic brake fluid of the normal source and the hydraulic brake fluid of the backup source. The fluid separator unit has a movable pressure boundary which enables, through movement thereof, the normal source of pressurized hydraulic brake fluid to selectively act upon the vehicle brake via a portion of the front fluid conduit when the front isolation valve is shut. A front valve arrangement is also provided for selectively supplying pressurized fluid from the normal source to the fluid separator unit and for selectively venting fluid from the fluid separator unit to selectively move the movable pressure boundary. 
   The vehicle is also provided with a rear vehicle brake that is operated by application of pressurized hydraulic brake fluid thereto. A rear fluid conduit connects the rear vehicle brake with the backup source. A rear isolation valve is disposed in the rear fluid conduit for selectively preventing the flow of hydraulic brake fluid between the backup source and the rear vehicle brake. A rear valve arrangement is also provided for selectively supplying pressurized fluid from the normal source to the rear vehicle brake and for selectively venting fluid from the rear vehicle brake. 
   Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of an embodiment of a vehicle brake system according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Preliminarily, it should be noted that as used in this application, the term “normally open” when used to describe a valve describes the operation of the valve with respect to the energized/deenergized state of the valve, and does not implicate the valve&#39;s position during normal or abnormal braking operations. A normally open valve is one which is open when electrically deenergized. In a similar vein, a normally closed valve is one which is closed when the valve is electrically deenergized. 
   Referring now to the drawings, there is illustrated in  FIG. 1  a brake system for a vehicle, indicated generally at  10 . The brake system  10  includes a normal source of pressurized hydraulic brake fluid, indicated generally at  12 , and a backup source of pressurized hydraulic brake fluid, indicated generally at  14 . The vehicle is provided with a plurality of wheels; in the illustrated embodiment, the brake system  10  includes front brakes for a pair of front wheels, namely the front right brake  16 , and the front left brake  18 , and rear brakes for a pair of rear wheels, namely the rear right brake  20  and the rear left brake  22 . 
   The backup source  14  may be embodied as any suitable source of pressurized hydraulic fluid, including electrically driven pumps with sources of electrical power separate from that supplying the normal source  12 , or supplied with pneumatic or mechanical reservoirs of energy, and so forth. However in the preferred embodiment illustrated in  FIG. 1 , the backup source  14  is embodied as a brake pedal unit and is powered by the operator of the vehicle through a brake pedal  24  to drive a master cylinder  26 . The master cylinder  26  includes a primary chamber  26   a  which provides pressurized brake fluid for the rear brakes  20 ,  22 , and a secondary chamber  26   b , which provides pressurized brake fluid for the front brakes  16 ,  18 . A front fluid conduit  28  connects the front vehicle brakes  16 ,  18  with the secondary chamber  26   b . Similarly, a rear fluid conduit  30  connects the rear vehicle brakes  20 ,  22  with the primary chamber  26   a . The master cylinder  26  also includes an intermediate chamber  26   c . The primary chamber  26   a  is defined between a primary piston  26   d  and an intermediate piston  26   e  disposed in the master cylinder  26 . The intermediate chamber  26   c  is defined between the intermediate piston  26   e  and a secondary piston  26   f  disposed in the master cylinder  26 . The primary chamber  26   a  and secondary chamber  26   b  of the master cylinder  26  are conventionally connected to a fluid reservoir  32 , when the primary piston  26   d  and secondary piston  26   f , are in their respective rest (unactuated) positions. The fluid reservoir  32  has a pair of baffles  33  extending along the bottom of the reservoir  32  to divide the volume of the reservoir  32 , below the level of the tops of the baffles, into three separate volumes: one for each of the primary and secondary chambers  26   a ,  26   b , and one connected to the normal source  12  as will be described below. The baffles  33  can have any suitable dimensions to divide the reservoir  32  into separate volumes. 
   The intermediate piston  26   e  is sealed to the walls of the master cylinder by dynamic seals  34 ,  36 , and  38  at respective axial locations along the piston  26   e . Dynamic seals move with the piston on which they are mounted. An annular space  40  is defined about the intermediate piston  26   e  between the seals  36  and  38 . When the intermediate piston  26   e  is in its rest position (i.e., the position that the intermediate piston  26   e  is in when the brake pedal  24  is not depressed) the annular space  40  is axially aligned with an opening in the master cylinder  26 , through which the annular space can communicate with a pedal simulator  42 . The annular space  40  is also continuously (regardless of the axial position of the intermediate piston  26   e ) in fluid communication with the primary chamber  26   a  through one or more fluid passages in the intermediate piston  26   e , which fluid passages may be provided with orifices and check valves to cause the resistance to fluid flow through the intermediate piston  26   e  to be different in one direction than the other. If fluid flow out of the secondary chamber  26   b  is prevented, in a manner which will be discussed below, the volume of hydraulic fluid in the intermediate chamber  26   c  prevents movement of the intermediate piston  26   e  when the brake pedal  24  is depressed, thus increasing pressure in the primary chamber  26   a  forces fluid through the passages in the intermediate piston  26   e  into the pedal simulator  42 . 
   The pedal simulator  42  may be any suitable type of pedal simulator. In the illustrated embodiment, the pedal simulator  42  is a spring loaded piston, which gives progressively increasing resistance to piston movement, and accepts fluid displaced from the primary chamber  26   a  of the master cylinder  26 , thereby giving a suitable feel to the brake pedal  24  during operation when flow of fluid out of the master cylinder through the front fluid conduit  28  and the rear fluid conduit  30  is prevented, as will be discussed below. Generally, the use of pedal simulators for pedal feel in brake-by-wire and other electro-hydraulic braking systems is known (taught by, for example, U.S. Pat. No. 5,941,608 to Campau et al., the disclosure of which is hereby incorporated by reference) and will not be discussed in further detail. 
   The front fluid conduit  28  extends from the secondary chamber  26   b  to the front right brake  16 . A normally open right front isolation valve  28   a  is disposed in the front fluid conduit  28  for selectively preventing the flow of hydraulic brake fluid between the backup source  14  (more specifically, the secondary chamber  26   b ) and the front right vehicle brake  16 . A branch of the front fluid conduit  28  connects die front left brake  18  to the secondary chamber  26   b  regardless of the position of the front right isolation valve  28   a . A normally open left front isolation valve  28   b  is disposed in this branch of the front fluid conduit  28  for selectively preventing the flow of hydraulic brake fluid between the secondary chamber  26   b  and the front left vehicle brake  16 . 
   Similarly, the rear fluid conduit  30  extends from the primary chamber  26   a  to the rear right brake  20  (and via a cross-tie valve  58   e  described below, to the rear left brake  22 ). A normally open rear isolation valve  30   a  is disposed in the rear fluid conduit  30  for selectively preventing the flow of hydraulic brake fluid between the backup source  14  (more specifically, the primary chamber  26   a ) and the rear vehicle brakes  20 ,  22 . 
   When the front right isolation valve  28   a  or the front left isolation valve  28   b  is deenergized (and thus open), flow is permitted out of the secondary chamber  26   b  through the front fluid conduit  28  when the brake pedal  24  is depressed. When the pedal  24  is depressed, the intermediate piston will be displaced from the rest position, and the communication between the annular space  40  and the pedal simulator  42  will be prevented by movement of the seal  38  past the opening in the master cylinder connected to the pedal simulator  42 . The seal  34  prevents fluid communication through the annular space surrounding the intermediate piston  26   e  between the opening connected to the pedal simulator  42  and the primary chamber  26   a  in all operating positions of the intermediate piston  26   e.    
   The normal source  12  is preferably partially or completely contained within a hydraulic control unit (HCU)  44 . The normal source  12  is preferably embodied as a positive displacement pump  46  driven by an electric motor  48 . The normal source  12  can be embodied in a number of ways, some of which are illustrated and described in the previously mentioned U.S. Pat. No. 5,941,608 to Campau et al. However, in the illustrated embodiment of  FIG. 1 , a slightly different arrangement is presented. The pump  46  draws fluid from the fluid reservoir  32  (from the third volume below the baffles described above) through a low pressure supply line  50 . A filter  52  is disposed in the low pressure supply line  50  at the inlet of the pump  46 . The pump  46  discharges into a high pressure supply line  54 . A high pressure accumulator  54   a  and a pressure relief valve  54   b  are connected via the high pressure supply line  54  to the discharge of the pump  46 . The high pressure supply line  54  is connected to the front fluid conduit  28  via a front valve arrangement, indicated generally at  56 , and to the rear fluid conduit  30  via a rear valve arrangement, indicated generally at  58 . 
   The front valve arrangement  56  consists, in the illustrated embodiment, of a normally closed front right apply valve  56   a , a normally closed front left apply valve  56   b , a normally open front right release valve  56   c , a normally open front left release valve  56   d , and a normally open front cross-tie valve  56   e . However, the front valve arrangement could include any suitable arrangement of valves for controlling the flow of fluid to the front brakes  16 ,  18 . Other specific valve arrangements which could be made include the substitution of a three-way valve for a pair of apply and release valves, such as was done in U.S. Pat. No. 5,941,608 to Campau et al. 
   More specifically, the high pressure supply line  54  selectively connects the discharge of the pump  46 , via the front right apply valve  56   a , to the front fluid conduit  28  between the front right brake  16  and the front right isolation valve  28   a  in order to raise the front right brake pressure  16 . The high pressure supply line  54  also selectively connects the discharge of the pump  46 , via the front left apply valve  56   b , to the front fluid conduit  28  between the front left brake  18  and the front left isolation valve  28   b  in order to raise the front left brake pressure  16 . Actually, the normally open cross-tie valve  56   e  will normally be kept open during normal braking, and only one of the apply valves  56   a  and  56   b  will be opened, thus ensuring relatively even braking pressures on the two front brakes. Of course, the front release valves  56   c ,  56   d  are energized closed when the associated front apply valve  56   a ,  56   b  is energized to open, thus directing the high pressure brake fluid to the brake rather than back to the fluid reservoir  32 . The front release valves  56   c ,  56   d  are connected to the fluid reservoir  32  via a low pressure return line  59 . 
   Of course, if individual control of the front brake pressures is desired, such as during a Vehicle Stability Control event, the front cross-tie valve  56   e  can be closed, and the front apply valves  56   a  and  56   b  operated as necessary to achieve desired individual brake pressures at the front brakes  16 ,  18 . Pressure at the front right brake  16  can be reduced by opening the front right release valve  56   c  (actually, if the front cross-tie valve  56   e  is open, pressure will be reduced at both front brakes  16 ,  18 ). In a similar manner, pressure at the front left brake  18  (or both front brakes  16 ,  18  if the front cross-tie valve  56   e  is open) can be reduced by opening the front left release valve  56   d.    
   A fluid separator unit  60  is provided between each of the right and the left apply/release valve pairs ( 56   a ,  56   c / 56   b ,  56   d ) to prevent intermixing of the hydraulic brake fluid of the normal source  12  and the hydraulic brake fluid of the backup source  14 . The fluid separator units  60  have a movable pressure boundary which enables, through movement thereof, the normal source of pressurized hydraulic brake fluid  12  to selectively act upon the associated vehicle front brake via a portion of the front fluid conduit  28  when associated front isolation valve  28   a ,  28   b  is shut. The fluid separator unit  60  prevents any gas bubbles which might be released into the high pressure supply line  54  in the event of a failure of the accumulator  54   a  (which is preferably embodied as a gas-charged volume with a moveable membrane boundary) from entering the front fluid conduit, and adversely affecting the ability of the backup source  14  to adequately apply pressure to the front brakes  16 ,  18 . It should be also noted that during operation of the normal source  12  to supply pressure to the front brakes  16 ,  18 , the associated isolation valve  28   a ,  28   b  should be shut to prevent fluid displaced from the piston of a fluid separator unit  60  from being sent to the master cylinder  26  instead of being directed to the associated front brake  16 ,  18 . 
   The rear valve arrangement  58  is similar to the front valve arrangement  56 , with certain notable exceptions. First, the rear valve arrangement does not have any fluid separators. Applicants have found that for various reasons, such as the normal weight transfer during braking of forward vehicle movement, the rear of the vehicle normally will be more lightly laden than the front of the vehicle, and thus the rear brakes  20 ,  22  will start to lose traction during braking at a lower brake pressure than the front brakes  16 ,  18 . Accordingly, if some gas bubbles were to migrate from the high pressure supply line  54  to the rear fluid conduit  30 , the backup source  14  could still achieve the desired braking pressure at the rear brakes  20 ,  22 . 
   Because there are no fluid separators, the rear valve arrangement  58  has release valves with different normal positions than the front valve arrangement  56 . More specifically, the rear valve arrangement  58  includes a normally closed rear right apply valve  58   a , a normally closed rear left apply valve  58   b , a normally closed rear right release valve  58   c , a normally closed rear left release valve  58   d , and a normally open rear cross-tie valve  58   e . As with the front valve arrangement  56 , the rear valve arrangement  58  could include any suitable arrangement of valves for controlling the flow of fluid to the rear brakes  20 ,  22 . Other specific valve arrangements which could be made include the substitution of a three-way valve for a pair of apply and release valves, such as was done in U.S. Pat. No. 5,941,608 to Campau et al. 
   The rear vehicle brakes  20 ,  22  are normally operated by application of pressurized hydraulic brake fluid thereto from the normal source  12  via the high pressure line  54  and the rear valve arrangement  58 , and selectively releasing pressure from the rear vehicle brakes  20 ,  22  via the release valves  58   c ,  58   d , which are connected via the low pressure return line  59  to the fluid reservoir  32 . 
   The rear fluid conduit  30  connects the rear vehicle brakes  20 ,  22  with the backup source  14 . The rear isolation valve  30   a  is disposed in the rear fluid conduit  30  for selectively preventing the flow of hydraulic brake fluid between the backup source  14  and the rear vehicle brakes  20 , 22 . 
   Since all of the rear apply valves  58   a ,  58   b  and the rear release valves  58   c ,  58   d  are normally closed, in the event of a loss of electrical power to drive the motor  48 , the rear apply valves  58   a ,  58   b  and the rear release valves  58   c ,  58   d  will be closed, and the normally open cross-tie valve  58   e  and the normally open rear isolation valve  30   a  will be open, allowing pressurized fluid to be sent from the backup source  14  (specifically the primary chamber  26   a  of the master cylinder  26 ) to the right rear brake  20  and, via the rear cross-tie valve  58   e , to the rear left brake  22 . 
   In closing, it should be noted that all the valves described herein preferably are provided with filter screens on both sides of the valve (inlet and outlet). Also, the operation of the various electrically operated valves, and pump motor  48 , is preferably under the control of an Electrical Control Unit (ECU)  62 , which may be embodied in any suitable type of electronic component or components. Preferably the ECU  62  receives a wide variety of appropriate inputs including, but not limited to, various system pressures and temperatures, position indications from the brake pedal  24 , yaw rate and acceleration sensors, steering angle, and wheel speeds. 
   The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.