Abstract:
A system for actuating a clutch that alternately driveably connects and disconnects components, includes a clutch having a piston that includes a first apply area and a second apply area, a fluid pressure source, a source of variable control pressure, and a control coupled with the fluid pressure source and operative in response to the control pressure to engage the clutch initially by increasing pressure steadily at the first apply area up to a first magnitude followed by a rapid increase in pressure at the first apply area and the second apply area above the first magnitude to a second magnitude.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a friction control element, such as a hydraulically or pneumatically actuated clutch, of the type used to control operation of an automatic transmission. In particular, the invention pertains to a control for producing staged engagement and disengagement of such a clutch having at least two sealed areas on its actuating piston. 
   Automatic transmissions are typically designed to transmit full engine torque and the engine torque as amplified by a torque converter at stall torque under static. i.e., non-shifting conditions. The control system of an automatic transmission includes a low/reverse clutch, which is applied or engaged to produce the lowest forward speed ratio and the reverse drive speed ratio. Such engagement produces a drive connection between components of the planetary gearing, which when selectively combined with the engagement of other control elements, results in the transmission operating in low gear or reverse gear. When the clutch is disengaged, another of the several forward speed ratios can be produced upon engagement of another combination of friction control elements. Therefore, gearshifts into and out of low gear, 1–2 upshifts and 2–1 downshifts, are produced at least in part by engaging and disengaging, respectively, the low/reverse clutch. Throughout this discussion, the term “friction control element” refers to a hydraulically actuated friction clutch or brake of a control system. 
   In order for the transmission to have the static torque capacity required to hold full stall torque, the low/reverse clutch is typically designed with a high gain to provide the required torque capacity to the low/reverse clutch. This high gain requirement, however, can affect good shift quality. 
   In a fully synchronous automatic transmission, all the gear ratio changes occur by coordinating the simultaneous disengagement and engagement of two friction control elements. In a fully synchronous automatic transmission, the low/reverse clutch controls 2–1 downshift events using a low gain clutch. In order to meet the shift quality requirements for all 2–1 events as well as to provide the static capacity required to hold stall torque, a low/reverse clutch must have at least two magnitudes of gain. A clutch having only a single gain will not suffice. 
   A clutch can produce multiple gains by providing multiple pressure areas on the hydraulic piston that actuates the clutch, primary and secondary pressurized areas. Production automatic transmissions have used this design technique in combination with control of the secondary pressure area on the actuating piston through operation of the transmission manual valve. This approach merely pressurizes both piston areas based on manual valve position with some degree of hydraulic control. 
   There is a need to provide direct control of the secondary area, preferably under control of an electronic control module and a pressure control device. This need is especially acute for a synchronous transmissions. 
   This invention provides direct control when the secondary area applies and is controlled via the electronic control module and a pressure control device. This allows the dual area clutch design to be used for shift events in synchronous transmissions. 
   SUMMARY OF THE INVENTION 
   The invention relates to a single low/reverse clutch piston with two distinct areas, which create distinct static and dynamic clutch gains. This invention provides direct control of the pressurized state of the secondary piston area, the application and control of pressure in the clutch via an electronic control module and a pressure control device. This allows the dual area clutch design to be used for shift events in synchronous transmissions. 
   The clutch and control system of this invention produce very fast response times, low dynamic gain for excellent shift quality and high static capacity for high torque applications. 
   A control according to this invention uses dual valve trains to control application of each element and allows tuning of the response of each portion of the piston. The sequential nature of the operation of the clutch also reduces any excessive load on the hydraulic system of the transmission, thereby eliminating any capacity drops, and the resulting clutch slip, during application of pressure to the static area of the clutch piston. 
   The clutch design is combined with a control system that uses the smaller dynamic piston area to stroke the clutch and conduct the shift event. After the dynamic event is complete, the control system seats or closes a check ball located behind the secondary piston area and then pressurizes the secondary piston area to provide the added capacity required for static events. The clutch and control system use a single pressure control device and a valve train for each portion of the dual area piston to control activation of the clutch. The dynamic low gain portion of the piston has an optimized small volume to react quickly, a check ball to prevent creating a vacuum in the secondary piston volume when stroking the piston using the secondary area and to provide a low overall gain for excellent gearshift quality. The dynamic low gain portion of the piston has an optimized small volume to react quickly and provide a low overall gain for excellent gearshift quality. The secondary area is controlled via the same pressure control device as the first area, but uses its own valve train to determine when to apply. Once in static capacity mode with both piston areas applied, the clutch gain is high for static capacity purposes. The release of the clutch is also coordinated. The larger, static piston area is dumped quickly, while the shift event takes place on the smaller low gain portion of the piston. 
   A system according to this invention for actuating a clutch that alternately driveably connects and disconnects components, includes a clutch having a piston that includes a first apply area and a second apply area, a fluid pressure source, a source of variable control pressure, and a control coupled with the fluid pressure source and operative in response to the control pressure to engage the clutch initially by increasing pressure steadily at the first apply area up to a first magnitude followed by a rapid increase in pressure at the first apply area and the second apply area above the first magnitude to a second magnitude. 
   Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross section taken at a diametric plane through a hydraulically actuated friction clutch of an automatic transmission whose piston has a static area and a dynamic area; 
       FIG. 2  is a schematic diagram of a system for controlling engagement and disengagement of the clutch by sequentially pressurizing the control areas of the clutch piston; 
       FIG. 3  is a graph showing the variation of D 1  (primary area) clutch apply pressure and D 2  (secondary area) clutch release pressure vs. the magnitude of commanded current applied to the variable force solenoid of  FIG. 2  by a transmission control unit; and 
       FIG. 4  is a cross section taken at a diametric plane through a friction clutch having nested pistons for actuating the clutch. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings, there is illustrated in  FIG. 1  a hydraulically actuated friction clutch  10 , preferably the low/reverse clutch of an automatic transmission, which is located in a transmission housing. A connecting member  12 , secured to and rotating with a component of a planetary gear set, having an inner surface on which spline teeth  14 , directed parallel to an axis  16 , are formed. The clutch is arranged substantially symmetrically about axis  16 . Pressure plates  18 , spaced mutually along the axis  16 , have teeth  20  located at a radially outer periphery and engaging the spline teeth  14 . Located between each pressure plate  18  is a clutch disc  22  having teeth  24  located at a radially inner periphery and engaging axially directed spline teeth  26  formed on a member connecting  28 , which is secured to and rotates with another component of a planetary gearset. A backing plate  30 , similarly splined to the internal splines  14 , is secure to the housing against displacement. As is conventional, each discs  22  carries friction material, which contacts and frictionally engages the adjacent pressure plate when the clutch  10  is applied. In this way, the clutch alternately driveably connects and releases the components secured to connecting members  12  and  28 . 
   A piston  34  is supported on a hydraulic cylinder  36  for axial displacement relative to the discs  22  and pressure plates  18 . The piston is sealed on the cylinder preferably by O-rings  38 ,  40 ,  42  or another type of dynamic seal, against the passage of hydraulic fluid. The seals, divide the piston into two hydraulically separated zones. A primary, dynamic piston surface area  44  is located in one zone between seals  38  and  40 ; a secondary static piston surface area  46  is located in the other zone between seals  40  and  42 . A check ball  41 , located behind the piston area  46 , opens to admit air into the cylinder space adjacent the secondary, static piston area  46  when piston  34  is displaced by pressure applied to the primary, dynamic piston area  44  area. This opening through the check valve  41  prevents a vacuum from forming in that portion of the cylinder as the piston moves in response to DI pressure. The check valve seats and closes when hydraulic pressure is applied to piston area  46 . As an alternative to the check valve  41 , any suitable device, such as a dynamic seal that responds to a pressure differential, can be used for this purpose. 
   A return spring  50 , preferably a Belleville spring, is resiliently preloaded in contact with a snap ring  52 , which is secured in a groove  54  on the cylinder  36 , and with the piston  34 . A force developed in the spring  50 , as the piston moves rightward from the position of  FIG. 1 , opposes such displacement and tends to return the piston to the disengaged position of  FIG. 1 . 
   The piston is displaced rightward to engage the clutch when hydraulic pressure is applied to one or both of the spaces between the cylinder piston areas  44  and  46 . Before fully engaging the clutch, the clutch is first stroked by applying regulated pressure to the primary area  44 , thereby taking up clearances between clutch components principally spaces between the clutch discs and pressure plates. Preferably, the stroke displacement of the clutch is performed with close control so that it is completed without excess displacement or pressure. After the clutch is stroked, the clutch becomes fully engaged by applying pressure to the secondary piston area  46 . The clutch must have torque capacity sufficient to produce and hold a force between the pressure plates  18  and discs  22  such that the clutch can transmit between the connecting members  12  and  14  the magnitude of torque required in the oncoming gear ratio. 
     FIG. 2  illustrates a system  60  for controlling the staged application of hydraulic pressures and fluid flow, which first stroke and then fully engage clutch  10 . The system  60  includes a valve controlled by a variable force solenoid (VFS)  62  that responds to a command signal produced by an electronic transmission control unit  63  (TCU), which controls operation of the transmission and its gear ratio changes. The VFS  62  controls a hydraulic valve, whose output pressure varies inversely with the magnitude of electric current supplied to the VFS  62 . In the non-limiting example discussed here, the current control signal applied to VFS  62  varies in the range 850–50 mA. In response to the control current, the VFS-controlled valve produces pressure, which is applied to the end surface of a land on each of a D 1  regulator valve  64 , a D 1  latch valve  66 , and a D 2  latch valve  68 .  FIG. 3  illustrates the variation of D 1  clutch apply pressure and D 2  clutch apply pressure produced by system  60  as the magnitude of the VFS current changes. 
   When VFS current is in the range of about 250–675 mA, the forces on the spool regulator valve  64  include the force of VFS pressure on land  70 , the force of spring  72  on land  76 , and the force of D 1  feedback pressure on land  76 . These forces regulate D 1  pressure at clutch area  44  causing it to increase linearly and inversely with VFS current while VFS current is between about 250 mA and 675 mA, as illustrated in  FIG. 3 . Subject to these forces, regulator valve  64  alternately increases the magnitude of D 1  pressure by opening a connection between line pressure feed  78  and line  80  and closing exhaust port  82  to line  80  when the spool of the valve moves upward, and decreases the magnitude of D 1  pressure by closing a connection between line pressure feed  78  and line  80  and opening exhaust port  82  to line  80  when the spool of valve  64  moves downward. 
   D 1  latch valve  66  has potential both to control D 1  feedback pressure and to have no control over feedback pressure in line  74 , depending on the magnitude of VFS current and VFS pressure. When VFS current is greater than about 250 nA and VFS pressure is relatively low, land  84  opens a connection between D 1  feedback line  74  and line  86 , which communicates with D 1  area  44 . When VFS current is equal to or less than about 250 mA, VFS pressure forces spool  88  of the D 1  latch valve  66  rightward against the force of control spring  90 , thereby closing line  86  and opening a connection between feedback line  74  and exhaust port  92 . This eliminates feedback regulation of D 1  regulating valve  64  and fully opens line pressure feed  78  to D 1  area  44 .  FIG. 3  illustrates the step increase in D 1  clutch apply pressure carried to area  44  through line  80  when VFS current reaches its latching pressure current. 
   The D 2  latch valve  68  is continually connected to VFS pressure, which is applied to land  94 . An orificed line pressure feed line  96  connects line pressure to D 2  latch valve  68  through an orifice  98 , which is sized to produce a desired flow rate of hydraulic fluid to D 2  area  46 . When pressure is applied to the D 2  area  46 , that pressure seats the check ball  41  located behind piston  34 , thereby sealing the area  46  and allowing pressure to build in the D 2  volume. That flow rate is preferably established such that the relatively large volume of fluid required to fill area  46  does not exceed the capacity of the transmission pump required to supply adequately other portions of the transmission hydraulic circuit. 
   When VFS current is about 250 mA, pressure on land  94  forces the spool  98  of D 2  latch valve  68  upward against the force of spring  100 , thereby allowing land  102  to open a connection between orificed line pressure feed line  96  and line  104 , through which D 2  clutch area  46  is filled with fluid and pressurized at a rate determined by the size of orifice  98 . The VFS current and the corresponding VFS pressure at which D 1  and D 2  are latched may be substantially equal. The clutch torque capacity continues to increase until the commanded VFS current reaches about 70 mA and pressure at D 1  area  44  and D 2  area  46  are about 15.5 bar. 
   The clutch disengages in response to VFS pressure increasing to 250 mA, which delatches the latch valves  66 ,  68  allowing the D 2  volume to drain through line  104  and exhaust port  106 , and the check ball  41  then opens to atmospheric pressure. As VFS pressure declines, D 1  latch valve  66  again controls feedback pressure in line  74 , thereby linearly reducing D 1  pressure until VFS current increases to about 850 mA. 
   In this way the clutch is engaged and disengaged in stages. First during an early, dynamic phase of clutch engagement, the clutch is quickly stroked with low gain control producing linearly increasing D 1  pressure that is applied to the relatively small D 1  area  44  and the corresponding clutch cylinder volume. After the dynamic phase, the area D 1   44  is rapidly pressurized to line pressure. The full torque capacity of the clutch is developed upon filling and pressurizing the relatively large D 2  area  46  and its corresponding clutch cylinder volume with fluid from a source of line pressure through orifice  98 . Both D 1  area  44  and D 2  area  46  are pressurized at relatively high pressure, during the static phase of clutch engagement. 
     FIG. 4  is a cross section of a clutch  110  for use with a system according to this invention, the clutch including nested actuating pistons  112 ,  114 , displaceable in a hydraulic cylinder  116 , rather than a single piston. The first piston  112  is sealed at the cylinder surface by O-rings  118 ,  120 , or another type of dynamic seal, against the passage of hydraulic fluid, the seals  118 ,  120  providing a boundary for a primary, dynamic pressure area  122  on the face of the piston  112  between the seals. The second piston  114  is sealed at the cylinder surface by O-rings  124 ,  126 , against the passage of hydraulic fluid, the seals  124 ,  126  providing a boundary for a secondary, static pressure area  128  on the face of the piston  114  between those seals. 
   The pistons  112 ,  114  are actuated by hydraulic pressure supplied through lines (not shown) connected to the outputs of the system of  FIG. 2 , i.e., clutch areas D 1  and D 2 . Piston  112  moves rightward to engage the clutch in response to hydraulic pressure applied to the clutch area  122  (D 1 ). Before the clutch  110  is fully engaged, the clutch is first stroked by applying pressure to the primary area  122 , thereby taking up clearances between clutch components, principally spaces between the clutch discs and pressure plates  20 ,  24 . Preferably, the stroke displacement of the clutch is performed with close control so that it is completed without excess displacement or pressure. After the clutch is stroked, the clutch becomes fully engaged by applying pressure to the secondary piston area  46 . The clutch must have torque capacity sufficient to produce and hold a force between the pressure plates  18  and discs  22  such that the clutch can transmit between the connecting members  12  and  14  the magnitude of torque required in the oncoming gear ratio. 
   A check ball  41 , located behind the piston area  46 , opens to admit air into the cylinder space adjacent the secondary, static piston area  46  when piston  34  is displaced by pressure applied to the primary, dynamic piston area  44  area. This opening through the check valve  41  prevents a vacuum from forming in that portion of the cylinder as the piston moves in response to DI pressure. The check valve seats and closes when hydraulic pressure is applied to piston area  46 . 
   The piston is actuated for rightward displacement to engage the clutch when hydraulic pressure is applied to one or both of the spaces between the cylinder piston areas  44  and  46 . Before fully engaging the clutch  110 , the clutch is first stroked by applying regulated pressure to the primary area  122 , the D 1  area, thereby taking up clearances between clutch components, principally spaces between the clutch discs and pressure plates  18 ,  22 . After the clutch  110  is stroked, the clutch becomes fully engaged by applying pressure to the secondary piston area  128 , the D 2  area. The force applied by hydraulic pressure to secondary piston  114  adds to the force applied to primary piston  112  because the pistons are in mutual contact at both extremities of their travel in the cylinder  116 . Therefore when both pressure areas, both when the clutch is disengaged as shown in  FIG. 4 , and by the 
   In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.