Abstract:
A pressure assisted park servo assembly for an automatic transmission includes a servo or spool valve which receives pressurized hydraulic fluid from various sources including two solenoid valves and transmission ports. The servo valve controls two flows of pressurized hydraulic fluid to a servo assembly to place the transmission in or release it from park. The improved park servo assembly exhibits enhanced operating speed.

Description:
CROSS-REFFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/892,683 filed on Mar. 2, 2007. The disclosure of the above application is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to a park servo for an automatic transmission and more particularly to a park servo for an automatic transmission having a pressure assisted return to park piston. 
       BACKGROUND 
       [0003]    In previous automatic transmissions having an electronic transmission range shift (ETRS) feature, electro-hydraulic solenoid/valves (solenoids) have been used to provide pressurized hydraulic fluid to a hydraulic servo that rotates the manual shaft to release and place the vehicle transmission in park. Particularly in cold environments, the time necessary for the hydraulic servo to return the transmission to park after such a shift command, though not long, may be viewed as less than optimal. Accordingly, it has been determined that improvements in return to park actuators for automatic transmissions are desirable. 
       SUMMARY 
       [0004]    A pressure assisted park servo assembly for an automatic transmission typically having an electronic transmission range shift (ETRS) configuration includes a multiple port servo or spool valve which receives pressurized hydraulic fluid from various sources including two solenoids and transmission ports. The spool valve controls two flows of pressurized hydraulic fluid to a park servo to quickly place the transmission in or release it from park. The park servo includes a compression spring which also urges the park servo toward its park position. The fluid sources within the transmission maintain or latch the spool valve and park servo on the out of park position. 
         [0005]    Thus it is an object of the present invention to provide a pressure assisted park servo for an automatic transmission having electronic transmission range shift. 
         [0006]    It is a further object of the present invention to provide a pressure assisted park servo assembly for an automatic transmission having a servo or spool valve which receives pressurized hydraulic fluid from solenoids and the transmission. 
         [0007]    It is a still further object of the present invention to provide a servo valve and pressure assisted park servo that provides enhanced operating speed. 
         [0008]    It is a still further object of the present invention to provide a pressure assisted park servo having a compression spring which urges the servo piston toward the park position. 
         [0009]    Further objects and advantages of the present invention will become apparent by reference to the following description and appended drawings wherein like reference numbers refer to the same component, element or feature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a pictorial view of an improved pressure assisted park servo assembly according to the present invention installed on an automatic transmission; 
           [0011]      FIG. 2  is a diagrammatic view of a servo valve and pressure assisted park servo according to the present invention in the park position; 
           [0012]      FIG. 3  is a diagrammatic view of a servo valve and pressure assisted park servo according to the present invention moving out of the park position; 
           [0013]      FIG. 4  is a diagrammatic view of a servo valve and pressure assisted park servo according to the present invention in the out of park position; 
           [0014]      FIG. 5  is a diagrammatic view of a servo valve and pressure assisted park servo according to the present invention illustrating operation of a back up fluid circuit which maintains the pressure assisted park servo in the out of park position. 
           [0015]      FIG. 6  is a diagrammatic view of a servo valve and pressure assisted park servo according to the present invention illustrating operation of a hydraulic latch which maintains the pressure assisted park servo in the out of park position; 
           [0016]      FIG. 7  is a diagrammatic view of a servo valve and pressure assisted park servo according to the present invention commencing movement back to the park position; 
           [0017]      FIG. 8  is a diagrammatic view of a servo valve and pressure assisted park servo according to the present invention moving back to the park position; and 
           [0018]      FIG. 9  is a diagrammatic view of a servo valve and pressure assisted park servo according to the present invention in the park position. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    Referring now to  FIG. 1 , an automatic transmission is illustrated and generally designated by the reference number  10 . Typically, the automatic transmission  10  will include an electronic transmission range shift (ETRS) feature. The automatic transmission  10  includes a housing  11  which is preferably a metal casting and includes numerous openings, shoulders, flanges and other features (not illustrated) which receive, locate and support various components of the automatic transmission  10 . Extending through one such opening in the housing  11  is a manual shaft  12  which is coupled to and translates park components between Park and Out of Park ranges within the automatic transmission  10 . The manual shaft  12  is coupled though a suitable linkage  14  to the bi-directionally translating output shaft or piston rod  16  of a pressure assisted park servo  20  according to the present invention. 
         [0020]    Also associated and in fluid communication with the pressure assisted park servo  20  is a return to park solenoid  22 , an out of park solenoid  24  which are both connected to a source of pressurized hydraulic fluid (not illustrated) and a hydraulic, multiple port ETRS, servo or spool valve  30 . 
         [0021]    Referring now to  FIG. 2 , it will be appreciated that the servo or spool valve  30  includes a housing  32  which defines a cylindrical bore  34  of diverse diameters and a plurality of preferably radial passageways or ports which communicate with the cylindrical bore  34 . A first control port  40  is disposed at one end of the bore  34  and communicates with the outlet or control side of the out of park solenoid  24 . A second control port  42  is disposed adjacent the first port  40  and communicates between the bore  34  and the outlet or control side of a drive and braking solenoid  44 . A first exhaust port  46  is adjacent the second control port  42  and communicates between the bore  34  and a hydraulic sump or reservoir (not illustrated). A third control port  48  communicates through a first hydraulic line  52  to a first chamber  54  in the pressure assisted park servo  20 . Pressurized hydraulic fluid in the first chamber  54  provides force against one face  55  of a piston  56  and extends the shaft or piston rod  16 . Such extension is assisted by a first compression spring  58  disposed in the first chamber  54  about the piston rod  16 . A fluid port  57  communicates with the first chamber  54  and a valve  59 . The valve  59  is illustrated as a ball/check valve, however, other kinds of valves may be employed without departing from the scope of the present invention. The valve  59  allows air or fluid to enter the first chamber  54  in order to prevent a vacuum from forming in the first chamber  54  when the piston  56  is returning to the Park position and when there is no pressurized fluid provided by a pressurized hydraulic fluid source, such as when the engine of the motor vehicle is off. Accordingly, the valve  59  increases the rate of return to the Park position. 
         [0022]    The housing  32  of the servo or spool valve  30  also defines a first inlet port  62  which communicates between a source of pressurized hydraulic fluid (not illustrated) and the cylindrical bore  34  adjacent the third control port  48 . A fourth control port  64  communicates through a second hydraulic line  66  with a second chamber  68  in the pressure assisted park servo  20 . Pressurized hydraulic fluid in the second chamber  68  provides force against an opposite face  69  of the piston  56  and retracts the shaft or piston rod  16 . A second exhaust port  72  adjacent the fourth control port  64  communicates between the bore  34  and the hydraulic sump or reservoir. A fifth control port  74  communicates with the bore  34  and receives pressurized hydraulic fluid from components within the transmission indicating that the transmission is in a forward gear. Finally, a sixth control port  76  is disposed at the second end of the bore  34  and communicates with the outlet or control side of the return to park solenoid  22 . 
         [0023]    Axially, slidably disposed within the cylindrical bore  34  of the housing  32  of the servo or spool valve  30  is a valve spool  80  having various diameters and shoulders which cooperate with the cylindrical bore  34  and the ports  40 ,  42 ,  46 ,  48 ,  62 ,  64 ,  72 ,  74  and  76  to control the direction and flow of hydraulic fluid to the pressure assisted park servo  20 . 
         [0024]    The valve spool  80 , from left to right, includes a first shoulder  82  operatively associated with the first inlet port  40 , a second shoulder  84  operatively associated with the second inlet port  42 , a first control disc  86  operatively associated with the first exhaust port  46  and the third control port  48 , a second control disc  88  operatively associated with the fourth control port  64  and the second exhaust port  72 , a third shoulder  92  operatively associated with the fifth control port  74  and a fourth shoulder  94  operatively associated with the sixth control port  76 . 
         [0025]    As illustrated, a first stub potion  96  of the valve spool  80  extends beyond the first shoulder  82  and a second stub portion  98  of the valve spool  80  extends beyond the fourth shoulder  94  to limit translation of the valve spool  80  in left and right directions, respectively. In a portion of the bore  34  communicating with the port  76  and concentrically located about a portion of the valve spool  80  and contacting the fourth shoulder  94  is disposed a second compression spring  102 . 
         [0026]    Operation of the pressure assisted park servo  20  will now be described with serial reference to the drawings, beginning with  FIG. 2 . In  FIG. 2 , the automatic transmission  10  is in park and the spool  80  of the ETRS or spool valve  30  is at its left limit of travel (as viewed in the drawings). In this condition, pressurized hydraulic fluid, provided to the first inlet port  62 , is present at the third control port  48  and pressurizes the first hydraulic line  52  and the first chamber  54  of the pressure assisted park servo  20 , driving or maintaining the piston  56  and shaft or piston rod  16  of the pressure assisted park servo  20  to or in its park position, to the right as illustrated in  FIG. 2 . 
         [0027]    In  FIGS. 3 and 4 , the vehicle operator has requested a transmission operating range other than park and the out of park solenoid  24  is activated, providing hydraulic fluid to the first control port  40 , applying pressure to the first shoulder  82  and causing translation of the valve spool  80  to the right. This action causes translation of the first control disc  86  which connects the first chamber  54  and the first hydraulic line  52  through the third control port  48  to the first exhaust port  46  which allows release of hydraulic fluid from the first chamber  54 . Additionally, the second control disc  88  translates to the right and pressurized hydraulic fluid, present at the first inlet port  62 , is provided to the fourth control port  64 , the second hydraulic line  66  and the second chamber  68  to retract the shaft or piston rod  16  and move the manual shaft  12  and automatic transmission  10  out of park. 
         [0028]    In  FIG. 5 , the drive and braking solenoid  44  is energized to supply pressurized hydraulic fluid to the second control port  42  and against the second shoulder  84 . This provides an additional force to the valve spool  80  to maintain it in its rightmost (out of park) position and provides a back up or redundant feature to the out of park solenoid  24  to ensure that the transmission remains out of park. 
         [0029]    In  FIG. 6 , the automatic transmission  10  is in a forward gear and pressurized hydraulic fluid from the transmission  10  is supplied to the fifth control port  74  and against the third shoulder  92  which also provides a force to the valve spool  80  to maintain it in its rightmost position. Given the redundancy, the out of park solenoid  24  may be turned off. While the drive and braking solenoid  44  may remain on, however, the transmission fluid provided to the fifth control port  74  provides a hydraulic latch which keeps the automatic transmission  10  out of park if the drive and braking solenoid  44  fails or the TEHCM controller stops working. 
         [0030]    In  FIG. 7 , the vehicle operator requests park. Both the out of park solenoid  24  and the drive and braking solenoid  44  are de-energized and the return to park solenoid  22  is energized. Pressurized hydraulic fluid is then supplied to the sixth control port  76  and the fourth shoulder  94  of the valve spool  80  adjacent the second compression spring  102 . The combination of hydraulic pressure and spring force moves the valve spool  80  back to the left, to the park position, faster than the compression spring  102  alone would be able to move the valve spool  80 . 
         [0031]    In  FIG. 8 , the valve spool  80  has translated to the left, to its park position, translating the second control disc  88  to the left and connecting the fourth control port  64 , the second hydraulic line  66  and the second chamber  68  to the second exhaust port  72  to allow the hydraulic fluid in the second chamber  68  to be released. At the same time, the first control disc  86  translates to the left, the first inlet port  62  is placed in fluid communication with the third control port  48  and the first hydraulic line  52  and pressurized hydraulic fluid begins to fill the first chamber  54  of the pressure assisted park servo  20 . 
         [0032]    In  FIG. 9 , the piston  56  and the shaft or piston rod  16  of the pressure assisted park servo  20  have fully returned to the right, to the park position. By utilizing pressurized hydraulic fluid from the first inlet port  62 , the motion of the piston  56  and the piston rod  16  of the pressure assisted park servo  20  is much faster than that achieved by utilizing the first compression spring  58  alone and the park position of the automatic transmission  10  is quickly achieved.