Abstract:
A transmission park-brake system includes a transmission park brake, a solenoid, first, second and third pressure sources, and a servo valve for disengaging the park-brake using the first and second pressure sources, holding the park-brake disengaged using a force produced by the solenoid and one of the first and second pressure sources, and engaging the park-brake using the third pressure source.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an apparatus for engaging and disengaging a transmission 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 transmission park-brake system includes a transmission park brake, a solenoid, first, second and third pressure sources, and a servo valve for disengaging the park-brake using the first and second pressure sources, holding the park-brake disengaged using a force produced by the solenoid and one of the first and second pressure sources, and engaging the park-brake using the third pressure source. 
     A method for operating the park-brake system includes disengaging the park-brake using first and second pressure sources, holding the park-brake disengaged using a force produced by a solenoid and one of the first and second pressure sources, and engaging the park-brake using a third pressure source. 
     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: 
       The Figure is a cross sectional side view of a park servo for a shift-by-wire system that controls the selection of drive ranges of an automatic transmission. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a schematic diagram of a park servo valve  90 . Valve  90  includes a body  92  formed with a cylinder  94  containing a valve spool  96 , helically coiled compression spring  98  and the actuator  100  of an on/off, electro-mechanical latch solenoid  102 . Spool  96  is formed with a first surface  104  having an area A1, a second surface  106  having an area A2, and a third surface  108  having an area A3. Spool  96  is connected by a rod  60 , which is secured to crank arm  62 . A park rod  66  is connected to crank arm  62  and to the bullet  110 , which contacts pawl  68 , which pivots about axis  69  into and out of engagement with a park gear, which is secured to the driven output of a vehicle transmission or is driveably connected to the vehicle wheels. 
     Crank arm  62  pivots about axis  64 . A torsional spring  36  applies torque to the crank arm  62 . 
     The actuator  100  of solenoid  102  is releaseably latched to spool  96  at a detent recess  112  and can be delatched when the solenoid is activated to move leftward to the position shown in  FIG. 1 , or can be delatched if force generated on area A1 in conjunction with spring  98  overcomes the holding force of detent  112  and areas A2 and A3 have low pressure acting on them. 
     Hydraulic line  78  supplies hydraulic fluid at line pressure LP to area A1 of surface  104  of spool  96 . 
     Actuating pressure ACL is supplied by the transmission&#39;s hydraulic system to a servo of a first clutch and to area A2. The first clutch is actuated to engage when ACL pressure is relative high and one of the lower forward gears or reverse gear is to be produced. For example, in a transmission that produces ten forward gears, the first clutch is engaged during operation in reverse gear and the first, second, third, fourth, fifth and sixth forward gears. 
     Similarly, actuating pressure FCL is supplied by the transmission&#39;s hydraulic system to a servo of a second clutch and to area A3. The second clutch is actuated to engage when FCL pressure is relative high and one of the higher gears or reverse gear is to be produced. For example, in a transmission that produces ten forward gears, the second clutch is engaged during operation in reverse gear and the fourth, fifth, sixth, seventh, eighth, ninth and tenth forward gears. 
     In operation, spool  96  is displaced leftward in cylinder  94  causing the bullet  110  to be moved away from the Park-engaged position and the actuator  100  of solenoid  102  to latch onto detent recess  112  of spool  96  due to the forces produced by pressures ACL and FCL on areas A2 and A3, respectively. Counteracting the forces produced by pressures ACL and FCL on areas A2 and A3 are the force due to line pressure acting on area A1, the force of spring  98  and the force required to move bullet  110  out of engagement with pawl  68 . 
     When the first clutch is engaged and the second clutch is disengaged, spool  96  and bullet  110  are held leftward in the Park-disengaged positions due to the forces produced by pressure ACL on area A2 and the holding force produced by solenoid  102 . Counteracting these forces are the force of spring  98  and the force produced by line pressure on area A1. Alternatively, when the second clutch is engaged and the first clutch is disengaged, spool  96  and bullet  110  are held leftward in their Park-disengaged positions due to the forces produced by pressure FCL on area A3 and the holding force produced by solenoid  102 . Counteracting these forces are the force of spring  98  and the force produced by line pressure on area A1. 
     The park system  34  is held in the leftward, Park-disengaged position during an automatically produced engine stop-engine restart cycle by the holding force produced by solenoid  102  acting against the rightward force of spring  98 . 
     The system  34  is returned to the Park-engaged position by displacing spool  96  rightward causing bullet  110  to engage the park pawl  68  and to pivot the pawl to the Park-engaged position and into engagement with the park gear. Spool  96  and bullet  110  move rightward in response to the rightward force produced by line pressure LP acting on area Al and the rightward force of spring  98  acting against the leftward force of resistance required to reengage bullet  110  with park pawl  68  and the holding force produced by solenoid  102 , if solenoid  102  is not functioning or power is lost to control solenoid  102 . With normal function of the solenoid  102 , the solenoid will open, thereby having zero holding force. 
     In the event of a loss of vehicle electric power onboard the vehicle, system  34  is returned to the Park-engaged position by displacing spool  96  rightward causing bullet  110  to engage the park pawl  68  and to pivot the pawl to the Park-engaged position and into engagement with the park gear in response to the rightward force produced by line pressure LP acting on area A1 acting against the leftward force of resistance required to reengage bullet  110  with park pawl  68  and overcoming the holding force produced by solenoid  102 . 
     In the event of an loss of hydraulic pressure, system  34  his returned to the Park-engaged position by displacing spool  96  rightward causing bullet  110  to engage the park pawl  68  and to pivot the pawl to the Park-engaged position and into engagement with the park gear in response to the rightward force of spring  98  acting against the leftward force of resistance required to reengage bullet  110  with park pawl  68 . 
     Alternatively, system  34  and valve  90  may be rearranged such that a compression force pushes bullet  110  away from engagement with pawl  68  causing the system to be pushed out of Park rather than being pulled out of Park by a tension force. 
     Alternatively a Park-holdout pressure area may be added to valve  90  and a Park-holdout pressure may be applied to that area such that more than two pressures or line pressure LP hold system  34  out of Park than just the two pressures ACL and FCL that pull out of Park. 
     Park holdout pressure may be routed through the park actuator valve  90 , thereby eliminating need for additional hardware to holdout of Park using more than only two pressures ACL and FCL. 
     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.