Abstract:
A vehicle park-brake system includes a park brake, a solenoid; a first pressure source; a servo for disengaging the park-brake using the first pressure source, and for disengaging the park-brake using a force produced by the solenoid that actuates first and second pistons; and a second pressure source applied to the servo causing hydraulic pressure to vent from the servo, the pistons to detach mutually, and the servo to engage the park brake.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an apparatus for engaging and disengaging a vehicle&#39;s park brake through hydraulic and electrical actuation rather than by mechanical actuation by the vehicle operator. 
     2. Description of the Prior Art 
     In an automatic transmission that employs a shift-by-wire (SBW) control Park, Reverse, Neutral and Drive ranges of the transmission are engaged and disengaged under electrical control. SWB systems have begun to replace the conventional shifter cable, which is used in automatic transmissions to control the park pawl and a hydraulic manual valve that feeds pressure to clutches that distinguishes the Neutral range from the Reverse and Drive ranges. 
     SBW systems generally are of two types: (i) electromechanical having redundant hardware and supplemental power, and (ii) electro-hydraulic having a redundant electro-mechanical path. Electro-hydraulic SBW systems allow for reduced cost, but generally do not cover unintended loss of the Park function with single point failures. 
     SUMMARY OF THE INVENTION 
     A vehicle park-brake system includes a park brake, a solenoid; a first pressure source; a servo for disengaging the park-brake using the first pressure source, and for disengaging the park-brake using a force produced by the solenoid that actuates first and second pistons; and a second pressure source applied to the servo causing hydraulic pressure to vent from the servo, the pistons to detach mutually, and the servo to engage the park brake. 
     The system has secondary control of park, which allows for park re-engagement, if the primary control fails to function. 
     The system does not permit loss of park-engagement due to a single point failure. Using a latch valve from one of the transmission clutches, allows park-engagement, if an error state occurs. The system has a decoupling feature that allows for park-engagement, if the electro-mechanical solenoid fails in the latch position. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a block diagram showing the components of a shift-by-wire system that control the selection of drive ranges of an automatic transmission; and 
         FIG. 2  is schematic diagram of the SBW system of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The SBW system  10  of  FIG. 1  includes a latch valve  12  for a low-reverse clutch, i.e., a clutch that must be engaged hydraulically for a motor vehicle&#39;s automatic transmission to produce first gear or reverse gear; park control valve  14 ; park servo  16 ; solenoid  17 ; electro-hydraulic on-off solenoid  18 ; electro-mechanical on-off solenoid  20 ; and electro-hydraulic on-off solenoid  22 . 
     Solenoid  17  opens a connection between a source of line pressure  28  and line  32  when valve  12  is latched by control pressure in line  19 , i.e., when the low-reverse clutch is engaged, and closes that connection when valve  12  is unlatched. Solenoid  18  shuttles the park control valve  14  using pressure in hydraulic line  26 . A source of line pressure  28  is connected through hydraulic line  30  to the latch valve  12 . Latch valve  12  communicates hydraulically with park control valve  14  through hydraulic line  32 . 
     Solenoid  20  holds the park servo  16  out of its Park position. Displacement of park servo  16  disengages a park assembly  34 , against resistance force produced by a spring  36 , which force urges the park assembly  34  into its park position. Park control valve  14  communicates hydraulically with park servo  16  through hydraulic line  38 . 
     Solenoid  22  is supplied with fluid through line  40  from the outlet of an e-pump  24 , which is driven by an electric motor with electric energy from the vehicle&#39;s battery  25  and supplied with fluid from a sump  41 . Solenoid  22  opens and closes a hydraulic connection through hydraulic line  42  between the e-pump  24  and park servo  16 . 
       FIG. 2  shows that latch valve  12  includes a valve spool  50 , located in a cylinder bore  52 ; a line pressure port connected by line  30  to the source of line pressure  28 ; an exhaust port EX communicating with the bore; and a port connected to line  32 . 
     Park control valve  14  includes a valve spool  54 , located in a cylinder bore  56 ; a spring  57  urging spool  54  leftward; a port connected by line  32  to latch valve  12 ; an exhaust port EX communicating with cylinder bore  56 ; and a port connected to line  38 . Solenoid  18  opens and closes control pressure supplied to cylinder bore  56  through line  26 . 
     Park servo  16  includes a cylinder  58 , a first piston  60  located in cylinder  58  and connected mechanically to a crank arm  62 , whose angular position about an axis  64  is affected by torsion spring  36 ; an park rod  66  mechanically connected to park pawl  68 , which pivots about axis  69  into and out of engagement with a parking gear (not shown); a second piston  70  located in cylinder  58  and releaseably connected by a detent  72  to the first piston  60 ; a port communicating line  38  to cylinder  58 ; and a port communicating cylinder  58  to line  42 . Solenoid  20  uses an actuator  74  to engage second piston  70  and leftward away from piston  70 . Engagement of pawl  68  with the parking gear locks the driven wheels and prevents movement of the vehicle. 
     Under normal operating conditions, when line pressure is produced, either by an engine driving a transmission pump or the battery  25  powering e-pump  24 , hydraulic fluid at line pressure passes through latch valve  12  and line  32  to park control valve  14 . Solenoid  18  moves park control valve  14  to a position wherein line pressure is carried in line  38  to the park servo  16 . Line pressure in cylinder  58  of the park servo moves piston  60  leftward. Clockwise pivoting of crank arm  62  against the counterclockwise torque produced by torsion spring  36  causes park pawl  68  to pivot clockwise about axis  69  out of engagement with the parking gear, producing Park-disengagement. 
     While Park is disengaged, solenoid  20  and detent  72  hold the park servo  16  in the disengaged position. Under normal operating conditions, solenoid  20  is closed, i.e., in the Park-disengaged position, such that no power is consumed. With solenoid  20  in the closed position, the vehicle can be towed with four wheels contacting the road surface. Line pressure in cylinder  58  applies a secondary force to maintain the park servo  16  in the Park-disengaged position. 
     Under normal operating conditions, when solenoid  20  is electrically energized, actuator  74  releases the pistons  60 ,  70  allowing rightward movement, pivoting crank arm  62  counterclockwise, displacing park rod  66  rightward, and causing park pawl  68  to pivot counterclockwise into engagement with the parking gear and producing Park-engagement. 
     Under normal operating conditions, while the system  10  produces Park-engagement, fluid in cylinder  58  is forced through line  38  to the exhaust port of park control valve  14 , as pistons  70 ,  60  move rightward in cylinder  58 . 
     If solenoid  20  becomes inoperative, such as due to loss of electric power supply to solenoid  20  or failure of a component of the solenoid, the system returns to Park-engagement as hydraulic pressure, produced by e-pump  24 , pressurizes cylinder  58  through solenoid  22  and line  42 . That pressure is present also in the space  80  between the pistons  60 ,  70  due to radial passage  82 . Space  80  is retained by dent balls  84  and spring  72 . Pressure in space  80  detaches piston  60  from piston  70 . Pressure in cylinder  58  is vented though line  38  and the exhaust port EX of park control valve  14 . Spring  36  pivots crank arm  62  clockwise and park pawl clockwise into engagement with the parking gear, thereby engaging Park. 
     Solenoid  20  may be electrically energized by a charged capacitor through a FET at  86 . 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.