Abstract:
A clutch actuation system for engaging and disengaging a clutch includes a hydraulically-actuated piston movable in response to hydraulic pressure to an apply position in which the piston is operable to engage the clutch. An electro mechanical actuator is movable in response to electric power to contact the piston at the apply position such that the clutch is maintained in an engaged state by the actuator, thereby permitting hydraulic pressure to be released. A method of controlling engagement of the clutch is also provided.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to an actuation system for a clutch that combines hydraulic and electro-mechanical actuation to allow the clutch to be mechanically locked in an engaged state to reduce hydraulic pressure requirements. 
       BACKGROUND OF THE INVENTION 
       [0002]    In automatic transmissions, torque-transmitting mechanisms such as friction clutches and brakes are selectively engaged using hydraulic pressure provided by a transmission pump. Transmission efficiency can be improved by reducing the required pump size and minimizing hydraulic pressure requirements in the transmission. Improved transmission efficiency leads to fuel economy improvement. 
       SUMMARY OF THE INVENTION 
       [0003]    A clutch actuation system for engaging and disengaging a clutch is provided that utilizes both hydraulic actuation and electro-mechanical actuation in order to minimize hydraulic pressure requirements. Specifically, the clutch actuation system includes a hydraulically-actuated piston movable in response to hydraulic pressure to an apply position in which the piston causes to engagement of the clutch. Within the scope of the invention, a “clutch” may be a brake-type clutch or a rotating-type clutch. The clutch actuation system also includes an electro-mechanical actuator movable in response to electric power to contact the piston at the apply position and thereby maintain the clutch in the engaged state so that hydraulic pressure is not required to maintain the engaged state, and may be released. Preferably, a direct current (DC) motor is used to provide electrical power to the actuator, causing movement of the actuator to contact the piston, and the actuator has a self-locking feature that prevents movement of the actuator in the absence of electrical power thereto. A battery powers the DC motor via a controller which operatively connects the battery to the electric motor to control the flow of power from the battery to the motor. For example, in one embodiment, the actuator includes a self-locking lead screw and a nut threadingly engaged thereon. The nut travels axially in response to rotation of the lead screw caused by the electric power thereto. Preferably, an axial thrust bearing is positioned concentrically about the lead screw and absorbs axial force acting the lead screw. The self-locking feature of the lead screw prevents the nut from traveling axially in the absence of electrical power to the lead screw. The nut includes axial leads that contact the piston when the nut is moved toward the piston. When the nut contacts the piston with sufficient pressure, the nut will cease to rotate and the DC motor will stall. This causes current drawn by the DC motor to increase rapidly, and the DC motor to be switched off. 
         [0004]    The actuator will remain in the contact position, contacting the clutch to keep it engaged, and hydraulic pressure on the piston may be released. When the clutch is not engaged, a first clutch member is relatively rotatable with respect to a second clutch member. When the clutch is engaged, the first clutch member is not rotatable relative to the second clutch member. 
         [0005]    Hydraulic pressure is provided by a hydraulic pressure source such as a transmission pump. A valve such as in a transmission valve body controls the flow of pressurized hydraulic fluid to a hydraulic chamber formed by a housing, such as the transmission housing. The hydraulic chamber is in fluid communication with the pressure source and with the clutch apply piston. The valve of the hydraulic chamber is controlled so that pressurized fluid is provided to the chamber to initially engage the clutch and to empty from the hydraulic chamber when the actuator locks the clutch in the engaged state. 
         [0006]    The clutch actuation system permits a method of controlling engagement of the transmission clutch. The method includes directing pressurized fluid to an apply piston to thereby move the piston into contact with the clutch to engage the clutch. After directing the pressurized fluid, the method includes providing electrical power to an actuator to thereby move the actuator into contact with the piston. Electrical power is ceased when the actuator contacts the piston. Preferably, this occurs automatically due to a surge in current in the motor when the actuator contacts the piston. After providing electrical power to the actuator, the method includes releasing the pressurized fluid so that the clutch remains in the engaged state via the actuator without electrical power or fluid pressure. 
         [0007]    If it is determined that operating conditions warrant disengagement of the transmission clutch, the method includes directing pressurized fluid to the apply piston with the clutch in the engaged state, and then providing electrical power to the actuator to move the actuator out of contact with the piston. When the actuator is out of contact with the piston, the method includes releasing the pressurized fluid so that the apply piston moves out of contact with the clutch and the clutch is released to a disengaged state. 
         [0008]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic illustration of the clutch actuation system for a transmission with a clutch in a disengaged state; and 
           [0010]      FIG. 2  is a schematic illustration of the clutch actuation system of  FIG. 1  with the clutch in an engaged state. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0011]    Referring to the drawings, where like reference numbers refer to like components,  FIG. 1  shows a portion of a transmission  10  including a torque-transmitting mechanism or clutch  12  which is engageable to connect a first clutch member also referred to as an input member  14 , for common rotation with the second clutch member also referred to as an output member  16 . The first clutch member  14  is rotatable about a transmission centerline  18 . A first set of clutch plates  20  extend radially from the first clutch member  14 . In  FIG. 1 , the clutch  12  is shown in unengaged or disengaged states. In the disengaged state, a second set of clutch plates  22  that extend radially from the second clutch member  16  are not in contact with the first set of clutch plates  20 . In  FIG. 2 , the clutch  12  is shown in an engaged state, in which the clutch is referred to as  12 A, with the first set of clutch plates  20  in contact with the axially adjacent second set of clutch plates  22  and the transmission is referred to as  10 A. When the clutch is in the engaged state  12 A in  FIG. 2 , the first clutch member  14  rotates commonly with the second clutch member  16 . It should be appreciated that, within the scope of the invention, the clutch  12  may be a stationary type clutch in which a second clutch member is being a stationary housing, rather than a rotatable component like output member  16 , so that when the clutch  12  is engaged, the first clutch member  14  is grounded to the stationary housing. 
         [0012]    Referring again to  FIG. 1 , the clutch actuation system  24  is utilized to transition the clutch  12  between the disengaged state in  FIG. 1  and the engaged state  12 A of  FIG. 2 . The clutch actuation system  24  includes a hydraulically-actuated apply piston  26 . In  FIG. 1 , the apply piston  26  is in a release position corresponding with the disengaged state of the clutch  12 A. A transmission housing  29  forms a hydraulic chamber  31  juxtaposed with the apply piston  26 . A hydraulic pressure source, such as a pump  33  for transmission hydraulic and lubrication needs, is in selectively communication with the hydraulic chamber  31  through a valve mechanism  35  such as a transmission valve body or other valves to control the flow of fluid derived from a fluid source  37  and pressurized by the pump  33  and then supplied to the hydraulic chamber  31 . In  FIG. 1 , the hydraulic chamber  31  does not contain hydraulic fluid of sufficient pressure to move the apply piston  26  from the release position shown in  FIG. 1  to an apply position  26 A shown in  FIG. 2 . The hydraulic chamber  31  is depicted as being empty of fluid in  FIG. 1 ; however, the hydraulic chamber  31  need not be completely empty, and may contain fluid when the clutch  12  is in the disengaged state, as long as the fluid is at a pressure less than that required to move the apply piston  26  from the release position. 
         [0013]    In addition to the hydraulic pressure system established by the pump  33 , the valve mechanism  35 , the fluid source  37  and the hydraulic chamber  31 , the clutch actuation system  24  also includes an electro-mechanical actuator  28 . The electro-mechanical actuator  28  is electrically powered by a direct current (DC) motor  30 . An electrical power source  32 , such as a battery supplies power to the DC motor  30  selectively through the controller  34 . The electro-mechanical actuator  28  includes a self-locking lead screw  36  (also referred to as a first member) which is rotatable in a clockwise or a counter-clockwise direction, depending on the power supplied to the DC motor  30 . The electro-mechanical actuator  28  further includes an axial travel nut  38 , also referred to herein as a contacting member, which includes axial leads  40 . The axial travel nut  38  is threadingly engaged with the lead screw  36 . When the lead screw  36  rotates in a clockwise direction, the axial travel nut  38  moves axially toward the clutch  12  and apply piston  26 . When the lead screw  36  is rotated in a counter clockwise direction, the axial travel nut  38  moves axially away from the apply piston  26  and the clutch  12 . The lead screw  36  is a self-locking lead screw, which will be rotated by electrical power to drive the travel nut  38  axially, but will not rotate due to force applied to the travel nut  38 . That is, the travel nut  38  cannot drive the lead screw  36 . An axial thrust bearing  42  is concentrically arranged about the lead screw  36  to absorb axial forces on the lead screw  36 . Other mechanisms configured to provide a self-locking feature, such as a worm gear with a pinion thereon, may be utilized in place of a lead screw or a axial travel nut. Those skilled in the art will recognize various structures self-locking mechanisms such as this. 
         [0014]    In  FIG. 1 , the axial travel nut  38  is shown out of contact with the apply piston  26 , consistent with the clutch  12  being in a disengaged state. In order to engage the clutch  12 , pressurized fluid  44  is directed from the pump  33  to the hydraulic chamber  31  filling the hydraulic chamber  31  in  FIG. 2 , as depicted by dashed lines. The hydraulic pressure acts on the apply piston to move it to the apply position  26 A of FIG.  2 , thus causing the first and second sets of clutch plates  20 ,  22  to contact one another, thereby engaging the clutch as shown as  12 A in  FIG. 2 . 
         [0015]    After the clutch is engaged as (shown as  12 A) using the hydraulic pressure, the electro-mechanical actuator  28  is electrically powered so that the DC motor  30  turns the lead screw  36  in a clockwise direction, thereby causing the axial travel nut  38  to move from the out of contact position shown in  FIG. 1  to a contact position in which the nut is referred to as  38 A in  FIG. 2 . In the contact position, the axial leads contact the apply piston  26  and are shown in the contacting position as  40 A in  FIG. 2 . When the axial leads are in the contact position  40 A, in physical contact with the apply piston in the apply position  26 A, the nut  38 A (shown in  FIG. 2 ) stops rotating and the DC motor  30  stalls. When the DC motor  30  stalls, current drawn by the DC motor  30  increases rapidly and the controller  34  switches off the motor  30 . Even in the absence of electrical power, the self-locking feature of the lead screw  36  will maintain the axial travel nut  38  in the contact position  40 A, thus holding the apply piston in the apply position  26 A, and the clutch  12 A in the engaged state. With the electro-mechanical actuator  28  effectively locking the clutch in the engaged state  12 A, the pressurized fluid  44  may be released or emptied from the hydraulic chamber  31 , preferably by the controller  34  opening the valve mechanism  35  to allow draining of the hydraulic chamber  31 . Thus, the clutch is maintained in the engaged state  12 A without hydraulic pressure or electrical power being required (that is the clutch is mechanically-locked). Such a mechanically-locked, engaged state of the clutch shown as  12 A ( FIG. 2 ) is desirable during steady state operating conditions, such as highway driving at high speeds in a gear ratio that requires the clutch  12  to be engaged for long periods of time. Because hydraulic pressure will not be required to maintain the clutch  12  during that period of time, pump requirements and hydraulic pressure requirements are minimized thereby potentially increasing fuel economy. 
         [0016]    When operating conditions warrant releasing the clutch to the disengaged state  12  shown in  FIG. 1 , the controller  34  will first direct pressurized fluid  44  to the empty or at least insufficiently pressurized hydraulic chamber  31 , and then provide electrical power from the battery  32  to the DC motor  30  to rotate the lead screw  36  in a counter clockwise direction, thus moving the axial travel nut  38  back to the out of contacting position  40 A of  FIG. 1 . The hydraulic pressure in the hydraulic chamber  31  can be slowly released so that the apply piston moves from the apply position  26 A of  FIG. 2  to the release position  26  of  FIG. 1 . A return spring  46 , shown in  FIG. 1  in a retracted position guides the apply piston from the contact position  26 A in which the return spring is shown in an extended state  46 A to the retracted position in which the apply piston  26  shown is in the release position. Thus, the spring  46  biases to the apply piston  26  to the release position. 
         [0017]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.