Torque transfer device having an electric motor/brake actuator and friction clutch

A torque transfer mechanism is provided for controlling the magnitude of a clutch engagement force exerted on a multi-plate clutch assembly that is operably disposed between a first rotary and a second rotary member. The torque transfer mechanism includes a clutch actuator assembly for generating and applying a clutch engagement force on the clutch assembly. The clutch actuator assembly includes an electric motor/brake unit, a torque/force conversion mechanism, and a force amplification mechanism. The motor/brake unit can be operated in either of a motor mode or a brake mode to cause bi-directional linear movement of an output member of the torque/force conversion mechanism. The thrust force generated by the torque/force conversion mechanism is increased by the force amplification mechanism with the resultant clutch engagement force applied to the clutch assembly. The dual mode feature of the electric motor/brake unit significantly reduces the power requirements.

FIELD OF THE INVENTION

The present invention relates generally to power transfer systems for controlling the distribution of drive torque between the front and rear drivelines of a four-wheel drive vehicle and/or the left and right wheels of an axle assembly. More particularly, the present invention is directed to a power transmission device for use in motor vehicle driveline applications having a torque transfer mechanism equipped with a power-operated clutch actuator that is operable for controlling actuation of a multi-plate friction clutch.

BACKGROUND OF THE INVENTION

In view of increased demand for four-wheel drive vehicles, a plethora of power transfer systems are currently being incorporated into vehicular driveline applications for transferring drive torque to the wheels. In many vehicles, a power transmission device is operably installed between the primary and secondary drivelines. Such power transmission devices are typically equipped with a torque transfer mechanism for selectively and/or automatically transferring drive torque from the primary driveline to the secondary driveline to establish a four-wheel drive mode of operation. For example, the torque transfer mechanism can include a dog-type lock-up clutch that can be selectively engaged for rigidly coupling the secondary driveline to the primary driveline to establish a “part-time” four-wheel drive mode. When the lock-up clutch is released, drive torque is only delivered to the primary driveline for establishing a two-wheel drive mode.

A modern trend in four-wheel drive motor vehicles is to equip the power transmission device with an adaptively controlled transfer clutch in place of the lock-up clutch. The transfer clutch is operable for automatically directing drive torque to the secondary wheels, without any input or action on the part of the vehicle operator, when traction is lost at the primary wheels for establishing an “on-demand” four-wheel drive mode. Typically, the transfer clutch includes a multi-plate clutch assembly that is installed between the primary and secondary drivelines and a clutch actuator for generating a clutch engagement force that is applied to the clutch assembly. The clutch actuator can be a power-operated device that is actuated in response to electric control signals sent from an electronic controller unit (ECU). Variable control of the electric control signal is typically based on changes in current operating characteristics of the vehicle (i.e., vehicle speed, interaxle speed difference, acceleration, steering angle, etc.) as detected by various sensors. Thus, such “on-demand” transfer clutch can utilize adaptive control schemes for automatically controlling torque distribution during all types of driving and road conditions.

A large number of on-demand transfer clutches have been developed with an electrically-controlled clutch actuator that can regulate the amount of drive torque transferred to the secondary output shaft as a function of the value of the electrical control signal applied thereto. In some applications, the transfer clutch employs an electromagnetic clutch as the power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024 discloses an electromagnetic coil that is incrementally activated to control movement of a ball-ramp drive assembly for applying a clutch engagement force to the multi-plate clutch assembly. Likewise, Japanese Laid-open Patent Application No. 62-18117 discloses a transfer clutch equipped with an electromagnetic actuator for directly controlling actuation of the multi-plate clutch pack assembly.

As an alternative, the transfer clutch can employ an electric motor and a drive assembly as the power-operated clutch actuator. For example, U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having a transfer clutch equipped with an electric motor that controls rotation of a sector plate which, in turn, controls pivotal movement of a lever arm that is operable for applying the clutch engagement force to the multi-plate clutch assembly. In addition, Japanese Laid-open Patent Application No. 63-66927 discloses a transfer clutch which uses an electric motor to rotate one cam plate of a ball-ramp operator for engaging the multi-plate clutch assembly. Finally, U.S. Pat. Nos. 4,895,236 and 5,423,235 respectively disclose a transfer case equipped with a transfer clutch having an electric motor driving a reduction gearset for controlling movement of a ball screw operator and a ball-ramp operator which, in turn, apply the clutch engagement force to the clutch assembly.

While many on-demand clutch control systems similar to those described above are currently used in four-wheel drive vehicles, a need exists to advance the technology and address recognized system limitations. For example, the size and weight of the friction clutch components and the electrical power requirements of the clutch actuator needed to provide the large clutch engagement loads may make such system cost prohibitive in some four-wheel drive vehicle applications. In an effort to address these concerns, new technologies are being considered for use in power-operated clutch actuator applications.

SUMMARY OF THE INVENTION

Thus, its is an object of the present invention to provide a power transmission device for use in a motor vehicle having a torque transfer mechanism equipped with a power-operated clutch actuator that is operable to control engagement of a multi-plate clutch assembly.

As a related object, the torque transfer mechanism of the present invention is well-suited for use in motor vehicle driveline applications to control the transfer of drive torque between a first rotary member and a second rotary member.

According to a preferred embodiment of the present invention, a torque transfer mechanism and control system are disclosed for adaptively controlling transfer of drive torque from a first rotary member to a second rotary member in a power transmission device of the type used in motor vehicle driveline applications. The torque transfer mechanism includes a multi-plate friction clutch assembly operably disposed between the first and second rotary members, and a clutch actuator assembly for generating a clutch engagement force to be exerted on the clutch assembly. The clutch actuator assembly includes an electric motor/brake unit, a torque/force conversion mechanism and a force amplification mechanism. The electric motor/brake unit can be switched by the control system between a motor mode and a brake mode for generating an output torque that is converted by the torque/force conversion mechanism into an axially-directed thrust force. Thereafter, thrust force is amplified by the force amplification mechanism to define the clutch engagement force.

According to the present invention, the control system operates the motor/brake unit in its motor mode when the speed of one of the rotary members is less than a predetermined threshold speed value so as to drive a rotor of the motor/brake unit which causes axial movement of an output member of the torque/force conversion mechanism. The control system switches the motor/brake unit into its brake mode when the rotary speed exceeds the threshold speed value so as to apply a dynamic brake torque to the rotor for controlling axial movement of the output member of the torque/force conversion mechanism. The present invention provides a clutch actuator assembly utilizing a low torque motor which acts as a generator during the brake mode so as to significantly reduce the electrical power requirement needed to adaptively control torque transfer through the clutch assembly.

The torque transfer mechanism of the present invention is adapted for use in a power transmission device for adaptively controlling the drive torque transferred between a primary driveline and a secondary driveline. According to a preferred application, the power transmission device of the present invention is a transfer case with the torque transfer mechanism arranged as a torque transfer coupling for providing on-demand torque transfer from the primary driveline to the secondary driveline. In a related application, the torque transfer mechanism is arranged as a torque bias coupling for varying the torque distribution and limiting interaxle slip between the primary and secondary driveline. According to another preferred application, the power transmission device is a drive axle assembly with the torque transfer mechanism arranged as a torque bias coupling to control speed differentiation and torque distribution across a differential unit.

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 from a first rotary member to a second rotary member. The torque transfer mechanism finds particular application in power transmission devices for use in motor vehicle drivelines such as, for example, an on-demand transfer clutch in a transfer case or an in-line torque coupling, a biasing clutch associated with a differential assembly in a transfer case or a drive axle assembly, or as a shift clutch in a multi-speed automatic transmission. Thus, while the present invention is hereinafter described in association with particular power transmission devices for use in specific driveline applications, it will be understood that the arrangements shown and described are merely intended to illustrate embodiments of the present invention.

With particular reference toFIG. 1of the drawings, a drivetrain10for a four-wheel drive vehicle is shown. Drivetrain10includes a primary driveline12, a secondary driveline14, and a powertrain16for delivering rotary tractive power (i.e., drive torque) to the drivelines. In the particular arrangement shown, primary driveline12is the rear driveline while secondary driveline14is the front driveline. Powertrain16includes an engine18, a multi-speed transmission20, and a power transmission device hereinafter referred to as transfer case22. Rear driveline12includes a pair of rear wheels24connected at opposite ends of a rear axle assembly26having a rear differential28coupled to one end of a rear prop shaft30, the opposite end of which is coupled to a rear output shaft32of transfer case22. Likewise, front driveline14includes a pair of front wheels34connected at opposite ends of a front axle assembly36having a front differential38coupled to one end of a front prop shaft40, the opposite end of which is coupled to a front output shaft42of transfer case22.

With continued reference to the drawings, drivetrain10is shown to further include an electronically-controlled power transfer system for permitting a vehicle operator to select between 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, transfer case22is equipped with a transfer clutch50that can be selectively actuated for transferring drive torque from rear output shaft32to front output shaft42for establishing both of the part-time and on-demand four-wheel drive modes. The power transfer system further includes a power-operated mode actuator52for actuating transfer clutch50, vehicle sensors54for detecting certain dynamic and operational characteristics of the motor vehicle, a mode select mechanism56for permitting the vehicle operator to select one of the available drive modes, and a controller58for controlling actuation of mode actuator52in response to input signals from vehicle sensors54and mode selector56.

Transfer case22is shown inFIG. 2to include a multi-piece housing60from which rear output shaft32is rotatably supported by a pair of laterally-spaced bearing assemblies62. Rear output shaft32includes an internally-splined first end segment64adapted for connection to the output shaft of transmission20and a yoke assembly66secured to its second end segment68that is adapted for connection to rear propshaft30. Front output shaft42is likewise rotatably supported from housing60by a pair of laterally-spaced bearing assemblies70and72and includes an internally-splined end segment74that is adapted for connection to front propshaft40.

Transfer clutch50is a multi-plate friction clutch assembly80and mode actuator52is a power-operated clutch actuator assembly82which together define a torque transfer mechanism according to a preferred embodiment of the present invention. Friction clutch assembly80includes a hub84fixed via a spline connection86to rear output shaft32, a drum88, and a multi-plate clutch pack90that is operably disposed between hub84and drum88. Clutch pack90includes a set of outer clutch plates92splined for rotation with drum88and which are interleaved with a set of inner clutch plates94splined for rotation with hub84. Clutch assembly80further includes a pressure plate96that is splined for rotation with drum88and which has an annular rim flange98formed thereon. Pressure plate96is operably arranged to rotate with, and move axially relative to, drum88for exerting a compressive clutch engagement force on clutch pack90. Such engagement of clutch pack90causes rotary power (“drive torque”) to be transferred from rear output shaft32to front output shaft42via a transfer assembly100. Transfer assembly100includes a first sprocket102fixed via a spline connection104for rotation with drum88, a second sprocket106fixed for rotation with front output shaft42, and a power chain108encircling sprockets102and106. First sprocket102is shown fixed to a tubular stub shaft segment89of drum88which is rotatably supported on rear output shaft32via a suitable bearing assembly such as sleeve bushing109.

As will be detailed, clutch actuator assembly82is operable for controlling axial movement of pressure plate96and thus, the magnitude of the clutch engagement force applied to clutch pack90. In particular, pressure plate96is axially moveable relative to clutch pack90between a first or “released” position and a second or “locked” position. With pressure plate96in its released position, a minimum clutch engagement force is exerted on clutch pack90such that virtually no drive torque is transferred from rear output shaft32through clutch assembly80and transfer assembly100to front output shaft42, thereby establishing the two-wheel drive mode. In contrast, movement of pressure plate96to its locked position causes a maximum clutch engagement force to be applied to clutch pack90such that front output shaft42is, in effect, coupled for common rotation with rear output shaft32, thereby establishing the part-time four-wheel drive mode. Accordingly, control of the position of pressure plate96between its released and locked positions permits adaptive regulation of the amount of drive torque transferred from rear output shaft32to front output shaft42, thereby establishing the on-demand four-wheel drive mode.

To provide means for moving pressure plate96between its released and locked positions, clutch actuator assembly82is shown to generally include an electric motor/brake unit110, a torque/force conversion mechanism112, and force amplification mechanism114. Motor/brake unit110is an annular assembly which includes a stator116and a rotor120. Stator116is shown to be non-rotationally secured to housing60and includes sets of windings, referred to as coil118, which has its electrical lead wires122extending out of housing60via a sealed plug hole124. Rotor120includes a plate segment126and an annular rim segment128. As will be detailed, plate segment126of rotor120is fixed for rotation with a first component of torque/force conversion mechanism112. As seen, rim segment128of rotor120has a plurality of permanent magnets130secured thereto which are arranged in close proximity to the field windings of coil118. The annular configuration of motor/brake unit110permits simple assembly in concentric relation to rear output shaft32within housing60. In addition, the packaging of motor/brake unit110inside housing60is advantageous in comparison to externally-mounted electric motor-type clutch actuators that are exposed to the hostile road and weather conditions associated with power transmission devices in motor vehicles.

Torque/force conversion mechanism112is shown inFIGS. 2 and 3as a ball screw operator132having an externally-threaded screw134, an internally-threaded nut136, and balls138disposed in the aligned threads therebetween. Screw134is rotatably supported on rear output shaft32via a pair of needle bearing assemblies140. Screw134is located and axially restrained between hub84and a thrust bearing assembly142via a snap ring144. As seen, plate segment126of rotor120is fixed (i.e., welded, splined, etc.) for rotation with screw134. Ball screw operator132is operable to cause axial movement of nut136relative to screw134in response to relative rotation therebetween. In this manner, the torque outputted from motor/brake unit110is converted into an axially-directed thrust force. This axially-directed thrust force is amplified and subsequently transferred to pressure plate96via force amplification mechanism114. In some clutch applications, it may be possible to eliminate force amplification mechanism114and apply the thrust force outputted from ball screw operator132to pressure plate96.

Force amplification mechanism114is shown to include a disk-type spring plate, such as a belleville spring148, having a first end restrained against an annular retainer150fixed to nut136and a second end restrained in a circumferential groove152formed in drum88. Preferably, belleville spring148has lugs at its outer peripheral edge that are coupled to drum88and lugs at its inner peripheral edge that are coupled to retainer150. As such, belleville spring148couples nut136of ball screw operator132for common rotation with drum88. In operation, when no torque is applied to rotor120, screw134and nut136rotate together in response to rotation of drum88.

To provide the desired force amplification characteristic, belleville spring148acts as a lever arm with an intermediate portion engaging rim flange98on pressure plate96. A resilient ring154is retained in groove152between the outer end of belleville spring148and a reaction flange156that extends from drum88. As is known, forward travel (i.e., to the left inFIG. 3) of nut136causes spring148to amplify the magnitude of the longitudinally-directed thrust force generated by ball screw operator132and apply the resultant clutch engagement force on pressure plate96. The use of ball screw operator132in combination with disk spring148permits use of a low torque motor/brake unit110. In operation, motor/brake unit110will be controlled in either of a first (“motor”) mode or a second (“brake”) mode for controlling the torque applied to rotor120so as to control relative rotation between screw134and nut136, thereby controlling the magnitude of the clutch engagement force applied by pressure plate96on clutch pack90.

Compared to conventional electrically-operated clutch actuator systems, the present invention provides significant operational advantages. For instance, clutch actuator assembly82requires only minimal electric power from the vehicle's host electrical supply system since, throughout most of its typical duty cycle, motor/brake unit110functions in its brake mode and acts as an absorber/generator for generating electrical power that can be dissipated or used to power one or more auxiliary electric devices such as, for example, an electric lube pump. Specifically, when the rotary speed of rear output shaft32is below a predefined threshold value, motor/brake unit110operates in its motor mode wherein coil118must be energized via an electrical control signal from controller58to drive rotor120in the appropriate rotary direction and through a desired amount of angular travel. Such controlled rotation of rotor120causes nut136of ball screw operator132to move axially relative to screw134in a corresponding direction and through a desired length of travel, thereby varying the magnitude of the clutch engagement force applied to clutch pack90. The predefined threshold rotary speed value is preferably, but not limited to, about 150 rpm which equates to a vehicle rolling speed of about 5 mph. Thus, the torque transfer mechanism of the present invention only uses motor/brake unit110in its motor mode to control torque transfer requirements during low speed situations. For example, motor/brake unit110operates in its motor mode to control the transfer of drive torque to front output shaft42during a quick start or acceleration situation to avoid traction loss of rear wheels24.

Once the rotary speed of rear output shaft32exceeds the predefined threshold value, the control system switches functions such that motor/brake unit110operates in its brake mode as an electric brake (absorber/generator) for creating (regenerating) electric power. In particular, when the rotary speed of rear output shaft32is above the threshold value, rotation of rotor120(caused by rotation of ball screw operator132) causes magnets130to generate a voltage in the field windings of coil118. However, since coil118is not energized, no torque is applied to rotor120. As such, ball screw operator132continues to rotate as a unit and nut136does not move axially in either direction. Upon energization of coil118, a brake torque is generated which acts to slow rotation of rotor120and thus slow rotation of screw134relative to nut136, thereby causing axial travel of nut136relative to clutch pack90. With motor/brake unit110operating in the brake mode, the control system functions to maintain a predetermined torque on ball screw operator132which, in turn, acts to control engagement of clutch pack90so as to generate the desired amount of torque transfer to front output shaft42. Preferably, motor/brake unit110is a dc pemanetic magnetic device since it will not require a commutator or brushes.

In operation, when mode selector56indicates selection of the two-wheel drive mode, controller58signals electric motor/brake unit110to rotate screw134until nut136is located in a rearward or “retracted” position. Such action permits pressure plate96to move to its released position. If mode selector56thereafter indicates selection of the part-time four-wheel drive mode, coil118of electric motor/brake unit110is signaled by controller58to rotate screw134for axially advancing nut136until it is located in a forward or “extended” position. Such movement of nut136to its extended position acts to cause corresponding movement of pressure plate96to its locked position, thereby coupling front output shaft42to rear output shaft32through clutch assembly80and transfer assembly100.

When mode selector56indicates selection of the on-demand four-wheel drive mode, controller58signals motor/brake unit110to rotate screw134until nut136is located in a “stand-by” position. This stand-by position may be its retracted position or, in the alternative, an intermediate position. In either case, a predetermined minimum amount of drive torque is delivered to front output shaft42through clutch assembly80which is considered to be in its “ready” condition. Thereafter, controller58determines when and how much drive torque needs to be transferred to front output shaft42based on the current tractive conditions and/or operating characteristics of the motor vehicle, as detected by sensors54. Many control schemes are known in the art for determining a desired torque level to be transferred through a transfer clutch and adaptively controlling such actuation of the transfer clutch. In this regard, commonly owned U.S. Pat. No. 5,323,871 discloses a non-limiting example of a clutch control scheme and the various sensors used therewith, the entire disclosure of which is incorporated by reference.

Referring now toFIGS. 4A and 4B, a modified version of transfer case22is identified by reference numeral22A which includes a multi-plate clutch assembly180and a power-operated clutch actuator assembly182which together define a torque transfer mechanism according to another preferred embodiment of the present invention. Clutch assembly180includes a hub184fixed via a spline connection186to first sprocket102, a drum188fixed via a spline connection189to rear output shaft32, and a multi-plate clutch pack190. Clutch pack190includes a set of outer clutch plates192splined for rotation with drum188which are alternatively interleaved with a set of inner clutch plates194that are splined for rotation with hub184. Clutch assembly180further includes a pressure plate196that is splined for rotation with drum188and having an annular rim flange198formed thereon. A reaction plate200is splined to drum188and axially restrained thereon via a snap ring202.

To provide means for moving pressure plate196between its released and locked positions, clutch actuator assembly182is generally shown to include an electric motor/brake unit210, a torque/force conversion mechanism212, and a force amplification mechanism214. Motor/brake unit210includes an annular stator216that is secured to housing60and has a coil218, and a rotor220having a plurality of permanent magnets230secured thereto in close proximity to coil218.

Torque/force conversion mechanism212is a ball screw operator232having an internally-threaded nut234, an externally threaded screw236, and balls238disposed in the aligned threads therebetween. Screw236is supported on an annular hub segment240of drum188. A drive plate242is secured to one end of screw236and has a series of circumferentially aligned axially-extending pins244. Pins244pass through a series of commonly aligned throughbores246formed in a plate segment248of drum188. Nut234is shown to be formed integrally with rotor220and axially restrained between a pair of thrust washer assemblies250. One of the thrust washer assemblies250is disposed between a first end of nut234and a support plate252that is rotatably supported from housing via a bearing assembly254. The other thrust washer assembly250is disposed between a second end of nut234and a cup-shaped retainer256that is secured to plate segment248of drum188. Since drum188is driven by rear output shaft32, the location of pins244within throughbores246causes screw236to likewise rotate in common with rear output shaft32. As before, when no energy is applied/absorbed to drive/brake rotation of rotor220, nut234rotates in unison with screw236.

Ball screw operator232is operable to cause axial movement of screw236relative to nut234between its retracted and extended positions in response to relative rotation therebetween. The axially-directed thrust force generated by such axial movement of screw234is transferred from pins244to pressure plate196via force amplification mechanism214. Force amplification mechanism214includes a series of disk levers260and having an outer end fixed via a spline connection to drum188and an inner end in engagement with the free end of pins244. Levers260each have an intermediate portion engaging rim flange198on pressure plate196. A return spring assembly262is disposed between hub184and disk levers260and includes a spring retainer264and a plurality of wave springs266disposed between a flange on spring retainer264and the inner end of disk levers260opposite pins244. As seen, retainer264is located on rear output shaft32between an end of hub segment268of sprocket102by a thrust washer270and snap ring272. Wave springs266are provided to bias disk levers260to a released position which, in turn, functions to bias screw234toward its retracted position.

The function and operation of motor/brake unit210is generally similar to that of motor/brake unit110in that energization of coil218in either of the motor or brake modes controls axial travel of screw236relative to nut234. Screw236is moveable between retracted and extended positions relative to nut234for causing pins244to pivot levers260so as to move pressure plate196between its corresponding released and locked positions. By way of example, screw236is shown inFIG. 4Ain its retracted position and inFIG. 4Bin its extended position. Spring assembly262is arranged to normally bias screw236toward its retracted position. Again, only minimal electric power is required to precisely control engagement of clutch assembly180and thus, the drive torque transferred from rear output shaft32to front output shaft42.

To illustrate an alternative power transmission device to which the present invention is applicable,FIG. 5schematically depicts a front-wheel based four-wheel drivetrain layout10′ for a motor vehicle. In particular, engine18drives a multi-speed transmission20′ having an integrated front differential unit38′ for driving front wheels34via axle shafts33. A transfer unit35is also driven by transmission20′ for delivering drive torque to the input member of a torque transfer mechanism, such as an in-line torque coupling280, via a drive shaft30′. In particular, the input member of torque coupling280is coupled to drive shaft30′ while its output member is coupled to a drive component of rear differential28which, in turn, drives rear wheels24via axleshafts25. Accordingly, when sensors54indicate the occurrence of a front wheel slip condition, controller58adaptively controls actuation of torque coupling280such that drive torque is delivered “on-demand” to rear wheels24. It is contemplated that torque transfer coupling280would include a multi-plate clutch assembly and a clutch actuator assembly that are similar in structure and function to either of the torque transfer mechanisms previously described herein.

Referring toFIG. 6, torque coupling280is schematically illustrated operably disposed between drive shaft30′ and rear differential28. Rear differential28includes a pair of side gears282that are connected to rear wheels24via rear axle shafts25. Differential28also includes pinions284that are rotatably supported on pinion shafts fixed to a carrier286and which mesh with side gears282. A right-angled drive mechanism is associated with differential28and includes a ring gear288that is fixed for rotation with carrier286and meshed with a pinion gear290that is fixed for rotation with a pinion shaft292.

Torque coupling280includes a mutli-plate clutch assembly294operably disposed between driveshaft30′ and pinion shaft292and which includes a hub296fixed for rotation with driveshaft30′, a drum298fixed for rotation with pinion shaft282, and a clutch pack300. Torque coupling280also includes a clutch actuator assembly302for controlling the magnitude of the clutch engagement force applied to clutch assembly294and thus the amount of drive torque transferred from drive shaft30′ to rear differential28. According to the present invention, clutch actuator assembly302is contemplated to be similar to either of clutch actuator assemblies82,182in that an electric motor/brake unit controls translation of a ball screw operator which, in turn, controls engagement of the clutch pack300.

Torque coupling280permits operation in any of the drive modes previously disclosed. For example, if the on-demand 4WD mode is selected, controller58regulates activation of clutch actuator302in response to the operating conditions detected by sensors54by controllably varying the electric control signal sent to the motor/brake unit. Selection of the part-time 4WD mode results in complete engagement of clutch pack300such that pinion shaft292is, in effect, rigidly coupled to driveshaft30′. Finally, in the two-wheel drive mode, clutch pack300is released such that pinion shaft292is free to rotate relative to driveshaft30′. Alternatively, elimination of mode select mechanism56would provide automatic on-demand operation of torque coupling280in a manner completely transparent to the vehicle operator.

Referring now toFIG. 7, torque coupling280A is schematically illustrated in association with a power transmission device adapted for use with an on-demand four-wheel drive system based on a front-wheel drive vehicle similar to that shown in FIG.5. Specifically, torque coupling280is shown operably associated with transfer unit35for transferring drive torque from transaxle20′ to drive shaft30′. In this regard, an output shaft303of transaxle20′ is shown to drive an output gear304which, in turn, drives an input gear306that is fixed to a carrier308associated with front differential unit38′. To provide drive torque to front wheels34, front differential unit38′ includes a pair of side gears310that are connected to front wheels34via axleshafts33. Differential unit38′ also includes a pair of pinions312that are rotatably supported on pinion shafts fixed to carrier308and which are meshed with side gears310. A transfer shaft314is provided to transfer drive torque from carrier308to a clutch hub316associated with a multi-pate clutch assembly318. Clutch assembly318further includes a drum320and a clutch pack322having interleaved inner and outer clutch plates respectively connected between hub316and drum320.

Transfer unit35is a right-angled drive mechanism including a ring gear324fixed for rotation with drum320of clutch assembly318and which is meshed with a pinion gear326fixed for rotation with drive shaft30. As seen, a clutch actuator assembly328is schematically illustrated for controlling actuation of clutch assembly318. According to the present invention, clutch actuator assembly328is similar to one of clutch actuator assemblies82,182previously described in that an electric motor/brake unit controls translational movement of a ball screw operator which, in turn, controls engagement of clutch pack322. In operation, drive torque is transferred from the primary (i.e., front) driveline to the secondary (i.e., rear) driveline in accordance with the particular mode selected by the vehicle operator via mode selector56. For example, if the on-demand 4WD mode is selected, controller58regulates actuation of clutch actuator328in response to the vehicle operating conditions detected by sensors54by varying the electric signal sent to the electric motor/brake unit. In this manner, the level of clutch engagement and the amount of drive torque that is transferred through clutch pack322to the rear driveline through transfer unit35and drive shaft30is adaptively controlled. Selection of a locked or part-time 4WD mode results in full engagement of clutch assembly318for rigidly coupling the front driveline to the rear driveline. In some applications, the mode selector56may be eliminated such that only the on-demand 4WD mode is available so as to continuously provide adaptive traction control without input from the vehicle operator.

FIG. 8illustrates a modified version ofFIG. 7wherein an on-demand four-wheel drive system is shown based on a rear-wheel drive motor vehicle that is arranged to normally deliver drive torque to rear wheels24while selectively transmitting drive torque to front wheels34through a torque coupling280B. In this arrangement, drive torque is transmitted directly from transmission output shaft303to transfer unit35via an intermediate shaft330interconnecting input gear306to ring gear324. Since ring gear324is driven by the output of transaxle20′, transfer unit35supplies drive torque to rear axle assembly26via driveshaft30. To provide drive torque to front wheels34, torque coupling280B is shown operably disposed between intermediate shaft330and transfer shaft314. In particular, clutch assembly318is arranged such that drum320is driven with ring gear324by intermediate shaft330. As such, actuation of clutch actuator328functions to transfer drive torque from drum320through clutch pack322to hub316which, in turn, drives carrier308of front differential unit38′ via transfer shaft314. Again, the vehicle could be equipped with mode selector56to permit selection by the vehicle operator of either the adaptively controlled on-demand 4WD mode or the locked part-time 4WD mode. In vehicles without mode selector56, the on-demand 4WD mode is the only drive mode available and provides continuous adaptive traction control without input from the vehicle operator.

In addition to the on-demand 4WD systems shown previously, the power transmission technology of the present invention can likewise be used in full-time 4WD systems to adaptively bias the torque distribution transmitted by a center or “interaxle” differential unit to the front and rear drivelines. For example,FIG. 9schematically illustrates a full-time four-wheel drive system which is generally similar to the on-demand four-wheel drive system shown inFIG. 8with the exception that an interaxle differential unit340is now operably installed between front differential unit38′ and transfer unit35. In particular, output gear306is fixed for rotation with a carrier342of interaxle differential340from which pinion gears344are rotatably supported. A first side gear346is meshed with pinion gears344and is fixed for rotation with intermediate shaft330so as to be drivingly interconnected to the rear driveline through transfer unit35. Likewise, a second side gear348is meshed with pinion gears344and is fixed for rotation with transfer shaft314and carrier308of front differential unit38′ so as to be drivingly interconnected to the front driveline.

A torque transfer mechanism, referred to as torque bias coupling280C, is shown to be operably disposed between side gears346and348. Torque bias coupling280C is similar to torque transfer coupling280B except that it is now operably arranged between the driven outputs of interaxle differential340for providing a torque biasing and slip limiting function. Torque bias coupling280C is shown to include multi-plate clutch assembly318and clutch actuator328. Clutch assembly318is operably arranged between transfer shaft314and intermediate shaft330. In operation, when sensor54detects a vehicle operating condition, such as excessive interaxle slip, which requires adaptive traction control, controller58controls the electric motor/brake unit associated with clutch actuator328for controlling engagement of clutch assembly318and thus the torque biasing between the front and rear driveline.

Referring now toFIG. 10, a full-time 4WD system is shown to include a transfer case22C equipped with an interaxle differential350between an input shaft351and output shafts32′ and42′. Differential350includes a rotary input member defined as a planet carrier352, a first rotary output member defined as a first sun gear354, a second rotary output member defined as a second sun gear356, and a gearset for accommodating speed differentiation between first and second sun gears354and356. The gearset includes meshed pairs of first planet gears358and second pinions360which are rotatably supported by carrier352. First planet gears358are shown to mesh with first sun gear354while second planet gears350are meshed with second sun gear356. First sun gear354is fixed for rotation with rear output shaft32′ so as to transmit drive torque to rear driveline12. To transmit drive torque to front driveline14, second sun gear356is coupled to transfer assembly100which includes a first sprocket78rotatably supported on rear output shaft32′, a second sprocket82fixed to front output shaft42′, and a power chain84. Transfer case22C further includes a torque biasing clutch50having a multi-plate clutch assembly86and a mode actuator52having a clutch actuator assembly88. Clutch assembly86includes a drum94fixed for rotation with first sprocket78, a hub90fixed for rotation with rear output shaft32′, and a multi-plate clutch pack98operably disposed therebetween. Clutch actuator assembly88is structurally and functionally similar to the clutch actuators previously described. If a mode select mechanism is available, transfer case22C would permit operation in either of an adaptive full-time four-wheel drive mode or a locked four-wheel drive mode.

Referring now toFIG. 11, a drive axle assembly370is shown which is generally a modified version of rear axle assembly26and which incorporates a torque transfer mechanism in association with rear differential28so as to permit adaptive control of the torque biasing and intra-axle speed differentiation between rear wheels24. The torque transfer mechanism is a torque bias coupling368shown to include a multi-plate clutch assembly372that is operably disposed between carrier286and one of axleshafts25, and a clutch actuator assembly374. Clutch assembly372includes a drum376fixed for rotation with carrier286, a hub378fixed for rotation with one of axleshafts25, and a clutch pack380disposed therebetween. Clutch actuator assembly374is operable for controlling the magnitude of a clutch engagement force applied to clutch pack380and thus, the torque biasing between the left and right wheels24. Clutch actuator assembly374is similar to clutch actuators82,182and includes a motor/brake unit, a torque/force conversion mechanism and a force amplification mechanism.

Drive axle assembly370can be used alone or in combination with other torque transfer mechanisms disclosed herein. In particular, drive axle assembly370can be associated with the primary axle in a rear wheel based on-demand 4WD drivetrain (FIGS.1and8), a front wheel based on-demand 4WD drivetrain (FIGS. 5 and 7) or in either (or both) axles in full-time 4WD drivetrains (FIGS.9and10). For example,FIG. 12is a schematic illustration of drivetrain10fromFIG. 1with drive axle assembly370used in substitution for rear axle assembly26. Electric power to clutch actuator assembly82of the torque transfer coupling in transfer case22is shown by power line390while regenerated electric power from clutch actuator assembly82is shown by dashed line392. Similarly, electric power flow to clutch actuator assembly374of torque bias coupling368in drive axle assembly370is shown by power line394while regenerated electric power from clutch actuator assembly374is shown by dashed power line396. Referring toFIG. 13, a block diagram is provided to better illustrate the electric power system associated with the drivetrain shown in FIG.12. Block400indicates the drive torque supplied to transfer case22by engine18and transmission20while block402indicates the electric power delivered to controller58from the vehicle's host system. As noted, a unique aspect of the present invention is that power from the vehicle's host system is only required during operation of the motor/brake unit in the motor mode to drive the rotor and in the brake mode to energize the coil windings. However, it should be understood that the electric power regenerated during operation of one clutch actuator in its brake mode can be used by controller58to provide electric power the other clutch actuator.

The drivetrain shown inFIGS. 12 and 13illustrate concurrent use and control of two distinct motor/brake units which are only minimally reliant on electric power from the vehicle's host electric system. A similar dual coupling arrangement using drive axle assembly370can be provided in association with the rear wheel based power transmission device shown in FIG.8. In contrast,FIG. 14is a modified version of the front wheel based power transmission device shown inFIG. 7which now further incorporates torque bias coupling368in association with front differential38′. In this arrangement, torque drive coupling368provides adaptive control of intra-axle differentiation between front wheels34while torque coupling280A provides adaptive control of the drive torque transferred on-demand to the rear driveline. The power sharing arrangement shown inFIG. 13would again be applicable for controlling the dual coupling powertrain of FIG.14.

A control system for controlling operation of the motor/brake unit(s) will now be detailed. In general, the control system, and its associated algorithms, is employed to control a brushless dc motor-based clutch actuator assembly. The actuator assembly, in turn, permits modulated control of the torque outputted from its associated clutch assembly. The control system can receive a torque output command from a powertrain control module via a communications link. This command is translated into an electric current level for the brushless motor by the algorithms. A desired current level is maintained in the motor by a feedback control loop, either by sensing the actual motor current or by sensing the actual torque outputted by the clutch assembly. Commutation of the brushless motor drive is also performed by the controller. The motor position is relayed to the controller by the output state of three hall effect sensors embedded in the coil windings. The controller energizes the correct winding pair based on the output from the hall sensors and the desired direction of rotor rotation.

Referring toFIG. 15, an exemplary circuit for the control system is shown. The torque command from controller58is delivered to a summing function410where the torque command value is compared to the actual torque output value measured by a torque sensor412on the output of the clutch assembly. A discrete control law function418has as its input the torque error (eT) value outputted from summing function410. Discrete control law function418transforms the torque error (eT) into a signal that commands the subsequent functions to compensate for the error. Specifically, the output signal of control law function418commands the magnitude and sign of the clutch engagement force. A PWM (pulse width modulation) generator420receives the output signal from discrete control law function418and outputs a directly proportional duty cycle pulse train that controls the magnitude of the electric current sent to the coil of motor/brake unit. A field switch422outputs binary signals that control the direction of rotation of the rotor of the motor/brake unit. These direction signals are dictated by the sign of the output signal from control law function418. Hence, if the current vector is negative, the motor will turn one way, and if the current vector is positive, the motor will turn in the opposite direction. One direction of rotation acts to increase output torque, while the other reduces pressure on the clutch and thereby reduces the output torque.

An H-bridge circuit424is configured from four controlled switches (i.e., relay, transistor) that allows control of both the direction and magnitude of electric current through a load (i.e., motor). Two of the four switches are activated to direct current in a given direction. In addition, one of the two remaining devices is modulated so as to control the amount (magnitude) of current.

Motor field block426represents the coils and pole pieces of the windings associated with motor/brake units' field. Motor armature428is the rotating member of the motor (i.e., the rotor) that also carries the magnet pole pairs. An encoder430is a sensor that outputs a signal which identifies the position of the motor armature with respect to the field coils, as well as the speed and direction of motor rotation. This block is necessary for realizations where the motor is electrically commutated (i.e., brushless motors). As is obvious, torque sensor412outputs an electrical signal that is proportional to the torque applied to the device to which the sensor is attached. A current sensor432outputs an electrical signal that is proportional to the electrical current acting thereon. In the absence of a torque sensor, a torque estimator434can be employed to estimate the clutch output torque. It does so by operating mathematically on the current sensor's signal to provide an estimate of the output torque. In practice, this may be a simple linear relationship or a more complex function.

A number of preferred embodiments have 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.