Abstract:
A clutch actuator for controlling engagement of a friction clutch and having a first actuator plate rotatable about an axis, a second actuator plate adjacent to the first actuator plate, and a ballramp unit disposed between the first and second actuator plates. A piston assembly acts to induce rotation of the first actuator plate relative to the second actuator plate. Relative rotation between the first actuator plate and the second actuator plate induces linear movement of one of the first and second actuator plates along the axis to regulate engagement of the friction clutch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/968,763, filed Oct. 19, 2004, now U.S. Pat. No. 7,104,379. 

   FIELD OF THE INVENTION 
   The present invention relates generally to power transfer systems, and more particularly, to a clutch actuator for actuating a clutch assembly in a power transfer system. 
   BACKGROUND OF THE INVENTION 
   Power transfer systems of the type used in motor vehicles including, but not limited to, four-wheel drive transfer cases, all-wheel drive power take-off units (PTU), limited slip drive axles and torque vectoring drive modules are commonly equipped with a torque transfer mechanism. In general, the torque transfer mechanism functions to regulate the transfer of drive torque between a rotary input component and a rotary output component. More specifically, a multi-plate friction clutch is typically disposed between the rotary input and output components and its engagement is varied to regulate the amount of drive torque transferred from the input component to the output component. 
   Engagement of the friction clutch is varied by adaptively controlling the magnitude of a clutch engagement force that is applied to the multi-plate friction clutch via a clutch actuator system. Traditional clutch actuator systems include a power-operated drive mechanism and an operator mechanism. The operator mechanism typically converts the force or torque generated by the power-operated drive mechanism into the engagement force which, in turn, can be further amplified prior to being applied to the friction clutch. Actuation of the power-operated drive mechanism is controlled based on control signals generated by a control system. 
   The quality and accuracy of the drive torque transferred across the friction clutch is largely based on the frictional interface between the interleaved clutch plates of the clutch pack. When partially engaged, the clutch plates slip relative to one another, thereby generating heat. As such, lubricating fluid must be directed through and around the clutch pack to cool the clutch plates. Excessive heat generation, however, can degrade the lubricating fluid and damage the friction clutch components. Additionally, electronic traction control systems require the clutch control system to respond to torque commands in a quick and accurate manner. The accuracy required to such a torque request is largely dependent on the coefficient of friction of the clutch pack. It has been demonstrated that the coefficient can change quite rapidly under various loading and/or slip conditions. The coefficient tends to fade due to significant temperature rise in the clutch pack, resulting from insufficient heat removal. The heat removal rate is dependent upon lubricating fluid flow rate and condition of the lubricating fluid. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed toward a clutch actuator that is operable to adaptively regulate engagement of a friction clutch assembly. The clutch actuator includes a power-operated drive mechanism and an operator mechanism. The operator mechanism generally includes a first actuator plate, a second actuator plate, a ballramp unit operably disposed between the first and second actuator plates, and a linear operator for controlling rotary angular movement between the first and second actuator plates. Such angular movement causes the ballramp unit to move one of the first and second actuator plates axially for generating a clutch engagement force that is applied to the friction clutch assembly. 
   Pursuant to a preferred construction, the ballramp unit is integrated into the first and second actuator plates to provide a compact operator mechanism. In addition, the linear operator is disposed between first and second arm segments provided on the corresponding first and second actuator plates. Preferably, the linear operator is a dual piston assembly having first and second pistons disposed in a common pressure chamber. The first piston has a first roller engaging a first cam surface formed on the first arm segment of the first actuator plate while the second piston has a second roller engaging a second cam surface formed on the second arm segment of the second actuator plate. 
   In accordance with another feature, the operator mechanism associated with the clutch actuator of the present invention further includes an apply plate that is disposed adjacent to the second actuator plate and which is axially moveable therewith to apply the clutch engagement force to the friction clutch assembly. In yet another feature, the operator mechanism of the clutch actuator further includes a stop plate that is disposed adjacent to the first actuator plate and which inhibits axial movement of the first actuator plate. 
   The drive mechanism associated with the clutch actuator of the present invention is operable to control the fluid pressure within the pressure chamber, thereby controlling the position of the first and second pistons and the relative angular position of the first actuator plate relative to the second actuator plate. The drive mechanism includes an electric motor, a ballscrew unit, a gearset interconnecting a rotary output of the motor to a rotary component of the ballscrew unit, and a control piston disposed in a control chamber. The control piston is fixed to an axially moveable component of the ballscrew unit while a fluid delivery system provides fluid communication between the control chamber and the pressure chamber. In operation, the location of the axially moveable ballscrew component within the control chamber controls the fluid pressure within the pressure chamber. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects, features and advantages of the present invention will become apparent to those skilled in the art from analysis of the following written description, the appended claims, and accompanying drawings in which: 
       FIG. 1  illustrates an exemplary drivetrain in a four-wheel drive vehicle equipped with a power transfer system; 
       FIG. 2  is a plan view of a friction clutch assembly and a clutch actuator according to the present invention integrated in the power transfer system; and 
       FIG. 3  is another view of the clutch actuator of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is directed to a torque transfer mechanism that can be adaptively controlled for modulating the torque transferred between a first rotary member and a second rotary member. The torque transfer mechanism finds particular application in power transfer systems of the type used in motor vehicle drivelines and which include, for example, transfer cases, power take-off units, limited slip drive axles and torque vectoring drive modules. Thus, while the present invention is hereinafter described in association with a particular arrangement for a specific driveline application, it will be understood that the arrangement shown and described is merely intended to illustrate an embodiment of the present invention. 
   With particular reference to  FIG. 1 , a schematic layout of a vehicle drivetrain  10  is shown to include a powertrain  12 , a first or primary driveline  14  driven by powertrain  12 , and a second or secondary driveline  16 . Powertrain  12  includes an engine  18  and a multi-speed transaxle  20  arranged to normally provide motive power (i.e., drive torque) to a pair of first wheels  22  associated with primary driveline  14 . Primary driveline  14  further includes a pair of axle shafts  24  connecting wheels  22  to a front differential unit  25  associated with transaxle  20 . 
   Secondary driveline  16  includes a power take-off unit (PTU)  26  driven by the output of transaxle  20 , a propshaft  28  driven by PTU  26 , a pair of axle shafts  30  connected to a pair of second wheels  32 , a rear differential unit  34  driving axle shafts  30 , and a power transfer device  36  that is operable to selectively transfer drive torque from propshaft  28  to rear differential unit  34 . Power transfer device  36  is shown integrated into a drive axle assembly and includes a torque transfer mechanism  38 . Torque transfer mechanism  38  functions to selectively transfer drive torque from propshaft  28  to differential unit  34 . More specifically, torque transfer mechanism  38  includes an input shaft  42  driven by propshaft  28  and a pinion shaft  44  that drives differential unit  34 . 
   Vehicle drivetrain  10  further includes a control system for regulating actuation of torque transfer mechanism  38 . The control system includes a clutch actuator  50 , vehicle sensors  52 , a mode select mechanism  54  and an electronic control unit (ECU)  56 . Vehicle sensors  52  are provided to detect specific dynamic and operational characteristics of drivetrain  10  while mode select mechanism  54  enables the vehicle operator to select one of a plurality of available drive modes. The drive modes may include a two-wheel drive mode, a locked (“part-time”) four-wheel drive mode, and an adaptive (“on-demand”) four-wheel drive mode. In this regard, torque transfer mechanism  38  can be selectively engaged for transferring drive torque from input shaft  42  to pinion shaft  44  to establish both of the part-time and on-demand four-wheel drive modes. ECU  56  controls actuation of clutch actuator  50  which, in turn, controls the drive torque transferred through torque transfer mechanism  38 . 
   Referring now to  FIGS. 2 and 3 , a cross-section of torque transfer mechanism  38  is shown. Torque transfer mechanism  38  generally includes a friction clutch assembly  60  having a multi-plate clutch pack  62 . Clutch actuator  50  is operable to generate and apply a clutch engagement force on clutch pack  62  so as to regulate engagement and thus, the amount of drive torque transfer through clutch pack  62 . Friction clutch assembly  60  also includes a clutch hub  64  and a drum  66 . Hub  64  is adapted to be coupled for rotation with input shaft  42  while drum  66  is adapted to be coupled for rotation with pinion shaft  44 . As seen, a set of first or inner clutch plates  68  associated with clutch pack  62  are fixed for rotation with hub  64 . Likewise, a set of second clutch plates  70  are interleaved with first clutch plates  68  and are fixed for rotation with drum  66 . 
   The degree of engagement of clutch pack  62 , and therefore the amount of drive torque transferred therethrough, is largely based on the frictional interaction of clutch plates  68  and  70 . More specifically, with friction clutch assembly  60  in a disengaged state, interleaved clutch plates  68  and  70  slip relative to one another and little or no torque is transferred through clutch pack  62 . However, when friction clutch assembly  60  is in a fully engaged state, there is no relative slip between clutch plates  68  and  70  and 100% of the drive torque is transferred from input shaft  42  to pinion shaft  44 . In a partially engaged state, the degree of relative slip between interleaved clutch plates  68  and  70  varies and a corresponding amount of drive torque is transferred through clutch pack  62 . 
   In general, clutch actuator  50  includes an operator mechanism  72  and a power-operated drive mechanism  73 . Operator mechanism  72  is shown to include a first actuator plate  74 , a second actuator plate  76 , a stop plate  78 , an apply plate  80 , a ballramp unit  82 , and a piston assembly  84 . First and second actuator plates  74  and  76  are rotatably supported on hub  64  by a bearing assembly  86  and include corresponding arm segments  74 A and  76 A, respectively, that extend tangentially. More specifically, arms  74 A and  76 A include respective edges  87  and  89  that are generally parallel to the axis A. 
   First and second actuator plates  74  and  76  also include first and second ballramp groove sets  90  and  92 , respectively. Balls  94  are disposed between first and second actuator plates  74  and  76  and ride within ballramp groove sets  90  and  92 . As best seen from  FIG. 3 , each set has three equally spaced grooves aligned circumferentially relative to the “A” axis. Thus, ballramp unit  82  is shown to be integrated into actuator plates  74  and  76  so as to provide a compact arrangement. Stop plate  78  is supported on hub  64  and is inhibited from axial movement by a lock ring  96 . More specifically, stop plate  78  is disposed between lock ring  96  and first actuator plate  74  and is separated from first actuator plate  74  by a thrust bearing assembly  98 . Apply plate  80  is disposed between clutch pack  62  and second actuator plate  76  and is separated from second actuator plate  76  by another thrust bearing assembly  100 . Apply plate  80  is adapted to move axially to regulate engagement of clutch pack  62 , as is explained in further detail below. 
   Piston assembly  84  is actuated by drive mechanism  73  to control relative rotation between first and second actuator plates  74  and  76 . More specifically, piston assembly  84  includes a first piston  104  and a second piston  106  that are disposed for sliding movement within a pressure chamber  108  formed in a cylinder housing  110 . As seen, first and second pistons  104  and  106  have first and second rollers  112  and  114 , respectively, attached thereto. First and second rollers  112  and  114  engage corresponding first and second cam surfaces  116  and  118  formed on first and second arms  74 A and  76 A, respectively. First and second rollers  112  and  114  are induced to ride against first and second cam surfaces  116  and  118  in response to movement of pistons  104  and  106  caused by actuation of drive mechanism  73 . Specifically, rolling movement of first and second rollers  112  and  114  against first and second cam surfaces  116  and  118  results in relative rotation between first and second actuator plates  74  and  76 . Pistons  104  and  106  are shown in  FIG. 3  in a first or “retracted” position within pressure chamber  108  such that first and second actuator plates  74  and  76  are located in a corresponding first angular position relative to each other. A return spring  120  is provided for normally biasing first and second actuator plates  74  and  76  toward this first angular position. With the actuator plates located in their first angular position, ballramp unit  82  functions to axially locate second actuator plate  76  in a corresponding first or “released” position whereat apply plate  80  is released from engagement with clutch pack  62 . In this position, a minimum clutch engagement force is applied to clutch pack  62  such that little or no drive torque is transmitted from input shaft  42  to pinion shaft  44 . 
   As will be detailed, drive mechanism  73  is operable to cause pistons  104  and  106  to move toward a second or “expanded” position within pressure chamber  108  such that actuator plates  74  and  76  are caused by engagement with rollers  112  and  114  to circumferentially index to a second angular position. Such rotary indexing of actuator plates  74  and  76  causes ballramp unit  82  to axially displace second actuator plate  76  from its released position toward a second or “locked” position whereat apply plate  80  is fully engaged with clutch pack  62 . With second actuator plate  76  in its locked position, a maximum clutch engagement force is applied to clutch pack  62  such that pinion shaft  44  is, in effect, coupled for common rotation with input shaft  42 . 
   Drive mechanism  73  is shown in  FIG. 3  to include a piston housing  122 , a ballscrew and piston assembly  124 , a gearset  126 , and an electric motor  128 . Electric motor  128  rotatably drives gearset  126  which, in turn, rotatably drives a leadscrew  130  associated with piston assembly  124 . Such rotation of leadscrew  130  results in axial movement of a nut  131  mounted thereon which, in turn, causes corresponding axial movement of a piston plunger  132  within a fluid control chamber  134  formed in housing  122 . Control chamber  134  is in fluid communication with pressure chamber  108  via a closed hydraulic control system. Specifically, as piston plunger  132  translates along an axis “B”, it regulates the volume of fluid in control chamber  134 . As the volume of control chamber  134  decreases, fluid is supplied through a conduit  136  to pressure chamber  108  in piston assembly  84 , thereby causing pistons  104  and  106  to move in concert toward their expanded position. In contrast, as the volume of control chamber  134  increases, the fluid flows back through conduit  136  from piston chamber  108  to relieve the pressure exerted by first and second rollers  112  and  114  against first and second cam surfaces  116  and  118 . 
   Accordingly, rotation of leadscrew  130  in a first rotary direction results in axial movement of piston plunger  132  in a first direction (right in  FIG. 3 ), thereby causing pistons  104  and  106  to be forcibly moved toward their expanded position for angularly indexing first and second actuator plates  74  and  76  toward their second angular position in opposition to the biasing force exerted thereon by return spring  120 . In contrast, rotation of leadscrew  130  in a second rotary direction results in axial movement of piston plunger  132  in a second direction (left in  FIG. 3 ), thereby permitting the biasing force of return spring  120  to forcibly rotate actuator plates  74  and  76  toward their first angular position which causes pistons  104  and  106  to move back toward their retracted position. A pressure sensor  140  is responsive to the pressure within conduit  136  and generates a signal that is sent to ECU  56 . Preferably, ECU  56  is functional to correlate line pressure readings from pressure sensor  140  to the torque output of friction clutch assembly  60 . 
   In its neutral state (see  FIG. 3 ), clutch actuator  50  imparts no clutch engagement force on clutch pack  62  such that first and second clutch plates  68  and  70  are permitted to slip relative to one another. As first and second actuator plates  74  and  76  are caused to rotate relative to one another, balls  94  ride within ballramp grooves  90  and  92  to axially move second actuator plate  76 . Since stop plate  78  inhibits axial movement of first actuator plate  74 , as balls  94  ride up ballramp grooves  90  and  92 , second actuator plate  76  is separated from first actuator plate  74  and moves linearly to impart the clutch engagement force on apply plate  80  through thrust bearing assembly  100 . Apply plate  80 , in turn, imparts this linear clutch engagement force on clutch pack  62 , thereby regulating engagement of clutch pack  62 . 
   It is contemplated that alternative drive mechanisms can be used in place of the closed-circuit hydraulic system disclosed. For example, a motor-driven dual leadscrew system could be implemented to drive first and second pistons  104  and  106  of operator mechanism  72  in concert between their retracted and expanded positions. Likewise, it is to be understood that the particular drivetrain application shown is merely exemplary of but one application to which the clutch actuator of the present invention is well suited. 
   A preferred embodiment has been disclosed to provide those skilled in the art an understanding of the best mode currently contemplated for the operation and construction of the present invention. The invention being thus described, it will be obvious that various modifications can be made without departing from the true spirit and scope of the invention, and all such modifications as would be considered by those skilled in the art are intended to be included within the scope of the following claims.