Abstract:
A method for controlling speed of an input shaft to a hydraulically actuated clutch that includes a hydraulic balance dam volume includes determining a limit speed of the input shaft that varies as a function of time during a predetermined period after the clutch starts rotation and that causes a servo piston for actuating the clutch to move toward an engaged position, rotating the input shaft during the period, limiting a speed of the input shaft during said period to a determined limit speed corresponding to a current time after beginning the period, and allowing the speed of the input shaft to increase to a speed greater than the determined limit speed after expiration of the period.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to a hydraulically actuated control element of an automatic transmission, such as a clutch or brake, having a balance dam. 
   2. Description of the Prior Art 
   A balance dam is a device used in a hydraulically actuated clutch to reduce differential pressure across a piston of an actuating servo induced by centrifugal force. 
   Many balance dams include a vent near the inside diameter of the balance piston. This vent is intended to both allow oil to exit the balance dam when the clutch-apply piston strokes to engage a clutch and also to reduce the pressure in the balance dam cavity during normal operation. However, the balance cavity dam will at least partially empty through the vent when the clutch is stationary and when the engine is not running. 
   After the balance dam is partially drained and upon restarting the engine, the balance dam does not contain enough oil to function properly. If a vehicle operator immediately depresses the accelerator pedal and increases engine speed, this action may cause the clutch apply piston to inadvertently drift, thereby increasing torque capacity of the clutch. 
   In some transmissions while the engine rotates at idle speed, a clutch will rotate at a ratio of transmission input shaft speed, which will rotate at a ratio of engine speed. 
   This problem is further compounded by the frequent practice of applying other drive clutches when the transmission is in the neutral operating range. This allows the possibility of transferring torque to the output shaft, if engine speed is increased immediately after starting the engine and before the balance dam is refilled with fluid. 
   SUMMARY OF THE INVENTION 
   A method for controlling speed of an input shaft to a hydraulically actuated clutch that includes a hydraulic balance dam volume includes determining a limit speed of the input shaft that varies as a function of time during a predetermined period and while the input shaft rotates and that causes a servo piston for actuating the clutch to move toward an engaged position, rotating the input shaft during the period, limiting a speed of the input shaft during said period to a determined limit speed corresponding to a current time after beginning the period, and allowing the speed of the input shaft to increase to a speed greater than the determined limit speed after expiration of the period. 
   The control produces few design compromises and has virtually no sensible effect on vehicle operation or performance. Although the control briefly prevents immediate engine response in the event that a vehicle operator rapidly increases engine speed after starting the engine, the period of the delay is short and inadvertent movement of the vehicle is avoided. 
   The speed limiting control can be applied whenever a balance dam is not rotating and requires time to be refill with fluid when transitioning into an operating mode where it suddenly starts rotating during a change of operating gears of the transmission. 
   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 cross section of a portion of an automatic transmission showing a clutch, its servo and a balance dam; 
       FIG. 2  is graph showing a speed limit during a control period and curves representing various flow rates into the balance dam volume; and 
       FIG. 3  is a graph showing the speed of a clutch shaft and the time following initial rotation of the shaft at which a clutch piston begins to move toward engagement. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The clutch  10  shown in  FIG. 1  alternately driveably connects and disconnects a clutch input shaft  12  and a shaft  14 , which are supported for rotation about a central axis  16  within the casing of an automatic transmission. Shaft  12  is secured by a weld to a rotating drum  18 , and shaft  14  is secured by a weld  20  to a rotating drum  22 . Clutch input shaft  12  is driveably connected to an engine  23  through a torque converter  25 . The inner surface of drum  18  is formed with a longitudinal spline, which is engaged by external splines formed on a series of spacer plates  24 . Similarly, the external surface of drum  22  is formed with a longitudinal spline, which is engaged by internal splines formed on a series of friction discs  26 , each disc being interleaved between two consecutive spacer plates  24 . 
   Drum  18  defines the surface of a hydraulic cylinder  28 , within which are located an axially displaceable piston  30 , a balance dam  32 , and return spring  34 . Piston  30  carries a seal that contacts the outer surface of shaft  12  and a seal that contacts a surface of drum  18  against the passage of hydraulic fluid. Balance dam  32  is secured to shaft  12  by a snap ring  36  and carries a seal  38  at its radially outer surface that contacts an inner surface of piston  30 . Return spring  34  is preferably a Belleville spring, which continually applies an axial force leftward in opposition to the movement of piston  30 . 
   A snap ring  29 , located at the end of the stack of spacer plates  24  and friction discs  26 , provides a reaction to a force applied to the stack by the end  40  of piston  30  as it moves rightward in response to pressurizing the servo cylinder  28 . 
   A hydraulic passage  42  supplies pressurized hydraulic fluid to cylinder  28 , whereby a pressure force is produced on the end face of piston  30  that forces piston  30  rightward causing the spacer plates  24  and friction disks to engage frictionally and producing a drive connection between shaft  12  and shaft  14 . When actuating hydraulic pressure in cylinder  28  is vented, spring  34  forces piston  30  to move leftward out of engagement with the pressure plates  24  and friction disks  26 , thereby disengaging clutch  10  and releasing shaft  12  from shaft  14 . 
   A similar hydraulic passage  44  carries hydraulic fluid to the volume  46  between balance dam  32  and the right hand face  48  of piston  30 . Volume  46 , piston  30  and cylinder  28  rotate about axis  16  as a unit with clutch input shaft  12 . When clutch input shaft  12  rotates and no actuating pressure is applied to cylinder  28 , piston  30  can move in response to differential pressure across the piston due to pressure induced by centrifugal force in cylinder  28  and balance dam volume  46 . 
   Hydraulic fluid in the balance dam volume  46  tends to drain from that volume through a vent  50  at the inner radius of balance dam  32  and past snap ring  36  when input shaft  12  is not rotating. 
   Hydraulic fluid is added to the balance dam through hole  44 , which communicates with the transmission&#39;s lube system. 
   The intended function of the balance dam vent  50  is to drain fluid from volume  46  so that the pressure in volume  46  acting against apply piston  30  is a function of rotational speed when the balance dam volume is filled with ATF, without any added pressure from the lube system. The vent  50  is sized so that ATF into the balance dam volume will exit through the vent without building up any backpressure. Vent  50  produces, however, the unintended function of allowing ATF in the balance dam volume  46  to drain out when piston  30  is not rotating, such as when the engine is off and the transmission pump is not operating. 
   The rate at which hydraulic fluid enters the balance dam volume  46  through passage  44  may vary.  FIG. 2  is a graph relating speed of shaft  12  and time during a period in which volume  46  is being filled. Graph  60  represents the maximum speed that can be achieved without clutch drift on when using a flow rate of 1.0 liters per minute into the balance dam volume  46  and indicates that the volume is filled in eight seconds. Graph  62  represents the maximum speed that can be achieved without clutch drift on when using a flow rate of 2.0 liters per minute into the balance dam volume  46  and indicates that the volume is filled in four seconds. Graph  64  represents the maximum rotational speed that can be achieved without clutch drift on when using a flow rate of 4.0 liters per minute into the balance dam volume  46  and indicates that the volume is filled in two seconds. Graph  66  shows an example of a maximum rotational speed allowed of shaft  12  during a period whose length is ten seconds. The intended design will result in a lower rotational speed limit than the speed where clutch drift on can occur. 
   If graph  66  is selected as the limit speed of shaft  12 , then the flow rate into volume  46  should be greater than 2 liters per minute. Otherwise, piston  12  begins to moves toward the clutch-engaged position due to an increase in pressure in sealed cylinder  28  caused by the effect of centrifugal force on the hydraulic fluid in cylinder  28  that is not sufficiently offset by the centrifugal pressure generated in the partially filled balance dam. If the flow rate into volume  46  is equal to or less than two liters per minute causing volume  46  to be only partially filled while shaft  12  is rotating, then the effect of centrifugal force on the hydraulic fluid in volume  46  will not be sufficient to cancel the effect of centrifugal force on the hydraulic fluid in cylinder  28 . Consequently, piston  30  begins prematurely to move to engage clutch  10  before the magnitude of pressure applied to cylinder  28  through passage  42  reaches the pressure at which the clutch is to be engaged. 
     FIG. 3  is a graph  68  showing the speed of clutch input shaft  12  and the time following initial rotation of shaft  12  at which piston  30  moves toward engagement of clutch  10  after the clutch has been stroked. Before this initiation of piston displacement, clutch  10  will have been stroked, i.e., cylinder  28  will have been filled with automatic transmission fluid (ATF) at relatively low stroking pressure, which pressure caused by centrifugal forces moves piston  30  rightward against the force of spring  34  from its position shown in  FIG. 1 , closing all the clearances among clutch plates  24  and discs  26 , but without engaging clutch  10 . 
     FIG. 3  also shows graph  70 , which represents the variation of a limit speed of clutch input shaft  12 , cylinder  28  and balance dam volume  46  during a predetermined period, whose length is ten seconds. At each point in time during the period, the limit speed  70  is lower than the reference speed at which piston  30  moves from its stroked position toward its engaged position. The maximum limit speed  72  is not exceeded during the period. 
   A control algorithm can be used to limit the speed of clutch input shaft  12  as a function of time which is related to the speed of shaft  12  by the speed ratio of the drive path that connects the engine crankshaft and shaft  12 . For example, engine speed may be limited to 3500 rpm until two seconds of operation after engine startup, then to 4000 rpm until four seconds after engine startup, then to 5000 rpm until six seconds after startup, then to 6000 rpm until eight seconds after startup, and then held to 6000 rpm after 8 seconds. 
   The algorithm prevents cylinder  28  and balance dam volume  46  from rotating fast enough to cause piston  30  to drift inadvertently toward application or engagement of clutch  10 . At some point in time, may be 5-10 seconds after rotation of cylinder  28  begins, the balance dam volume  46  will be full of ATF and will function as intended. After the period expires, ATF will have substantially filled the balance dam volume  46  such that a higher input shaft speed and cylinder speed do not move the piston without applying actuating pressure to cylinder  28  through passage  44 . 
   Although the control is described with reference to a clutch  10  that driveably connects and disconnects two rotating components  12 ,  14 , the control can be applied also to a brake that driveably connects and disconnects one rotating component and a non-rotating component, thereby braking or holding the rotating component against rotation. The terms “clutch” and “brake” are used interchangeably. 
   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.