Abstract:
A power transmission device includes a rotary input member receiving drive torque from a source of torque, a rotary output member for providing drive torque to an output device and a torque transfer mechanism for transferring drive torque between the input member and the output member. The torque transfer mechanism includes a friction clutch assembly operably disposed between the input member and the output member and a hydraulic clutch actuation system operable for applying a clutch engagement force to the friction clutch assembly. The hydraulic clutch actuation system includes an electromagnet and a piston operable to supply pressurized fluid and provide the clutch engagement force.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application ser. No. 10/931,590 filed on Sep. 1, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to power transfer systems operable for controlling the distribution of drive torque between a pair of rotary shafts and, more particularly, to clutch control systems operable to efficiently convert electrical energy to mechanical potential energy for subsequent actuation of a clutch.  
       BACKGROUND OF THE INVENTION  
       [0003]     In view of increased consumer demand for four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing power (i.e., drive torque) to the non-driven wheels of the vehicle. In many power transfer systems, a part-time transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. A mechanical mode shift mechanism can be selectively actuated by the vehicle operator for rigidly coupling the non-driven wheel to the driven wheels in order to establish a part-time four-wheel drive mode. As will be appreciated, a motor vehicle equipped with a part-time transfer case offers the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the part-time four-wheel drive mode for operation under adverse road conditions.  
         [0004]     Alternatively, it is known to use “on-demand ” power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. Modernly, it is known to incorporate the on-demand feature into a transfer case by replacing the mechanically-actuated mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated condition such that the drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque “on-demand ” to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the normally non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement.  
         [0005]     Conventional clutch assemblies typically include a clutch pack operably connected between a drive member and a driven member. A power-operated actuator controls engagement of the clutch pack. Specifically, torque is transferred from the drive member to the driven member by actuating the power-operated actuator. The power-operated actuator displaces an apply plate which acts on the clutch pack and increases the frictional engagement between the interleaved plates.  
         [0006]     A variety of power-operated actuators have been used in the art. Exemplary embodiments include those disclosed in U.S. Pat. No. 5,407,024 wherein a ball-ramp arrangement is used to displace the apply plate when a current is provided to an induction motor. Another example disclosed in U.S. Pat. No. 5,332,060, assigned to the assignee of the present application, includes a linear actuator that pivots a lever arm to regulate the frictional forces applied to the clutch pack. These types of systems are often equipped with motors that may require peak electrical currents greater than optimally desired to operate the clutch actuators. While the above actuator devices may perform adequately for their intended purpose, a need exists for an improved clutch actuation system that requires a relatively low, minimally fluctuating supply of electrical power for operation.  
       SUMMARY OF THE INVENTION  
       [0007]     A power transmission device includes a rotary input member receiving drive torque from a source of torque, a rotary output member for providing drive torque to an output device and a torque transfer mechanism for transferring drive torque between the input member and the output member. The torque transfer mechanism includes a friction clutch assembly operably disposed between the input member and the output member and a hydraulic clutch actuation system operable for applying a clutch engagement force to the friction clutch assembly. The hydraulic clutch actuation system includes an electromagnet drivingly coupled to the hydraulic actuator. The hydraulic actuator includes a first piston biasedly engaged by a spring wherein the first piston is slidably positioned within a housing and operable to supply pressurized fluid to a second piston. Supply of pressurized fluid to the second piston provides the clutch engagement force.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will become more fully understood from the detailed description given below, the appended claims, and the accompanying drawings in which:  
         [0009]      FIG. 1  is a schematic representation of an exemplary four-wheel drive vehicle having the clutch control systems of the present invention incorporated therein;  
         [0010]      FIG. 2  is a schematic representation of a first embodiment clutch actuation system;  
         [0011]      FIG. 3  is a schematic representation of an alternate embodiment clutch actuation system; and  
         [0012]      FIG. 4  is a schematic representation of an alternate embodiment clutch actuation system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     In general, the present invention is directed to a power transfer system which is operably installed between the driven and non-driven wheels of a four-wheel drive vehicle. In operation, the amount of drive torque transferred to the non-driven wheels is controllably regulated in accordance with various system and driver-initiated inputs for optimizing the tractive characteristics of the vehicle. In addition, the power transfer system may also include a mode select mechanism for permitting a vehicle operator to select between a two-drive wheel mode, a part-time four-wheel drive mode, and an “on-demand ” drive mode. The power transfer system of the present invention includes a clutch control system for converting electrical energy to mechanical potential energy to alleviate exceedingly high peak electrical current requirements that may occur during vehicle operation.  
         [0014]     Referring to  FIG. 1  of the drawings, a drivetrain for a four-wheel drive vehicle is schematically shown interactively associated with a power transfer system  10 . The motor vehicle drivetrain has a pair of front wheels  12  and rear wheels  14  both drivable from a source of power, such as an engine  16 , through a transmission  18  which may be of either the manual or automatic type. In the particular embodiment shown, the drivetrain is a rear wheel drive system which incorporates a transfer case  20  operable to receive drive torque from engine  16  and transmission  18  for normally driving rear wheels  14  (i.e., the “driven ” wheels) in a two-wheel drive mode of operation. Front wheels  12  and rear wheels  14  are shown connected at opposite ends of front and rear axle assemblies  22  and  24 , respectively. As is known, a rear differential  26  is interconnected between rear axle assembly  24  and one end of a rear drive shaft  28 , the opposite end of which is interconnected to a first output shaft  30  of transfer case  20 . Similarly, front axle assembly  22  includes a front differential  32  that is coupled to one end of a front drive shaft  34 , the opposite end of which is coupled to a second output shaft  36  of transfer case  20 . It is to be understood that the specific orientation of the drivetrain is merely exemplary in nature and that the drivetrain could be reversed for normally driving front wheels  12 .  
         [0015]     Transfer case  20  is equipped with a torque transfer clutch  38  for selectively delivering drive torque to front wheels  12  (i.e., the non-driven wheels) to establish a four-wheel drive mode of operation. The operating mode of transfer clutch  38  is generally controlled in response to a mode signal generated by a mode selector  40  and which is sent to a controller  42 . Controller  42  also receives input signals from one or more vehicle sensors  44  that are indicative of various operational characteristic of the vehicle.  
         [0016]     When the two-wheel drive mode is selected, all drive torque is delivered from first output shaft  30  to rear wheels  14  and transfer clutch  38  is maintained in a “non-actuated ” condition. When the part-time four-wheel drive mode is selected, transfer clutch  38  is fully actuated and maintained in a “lock-up “ condition such that second output shaft  36  is, in effect, rigidly coupled for driven rotation with first output shaft  30 . When the “on-demand ” drive mode is selected, controller  42  communicates with a clutch control system  200  to control the degree of actuation of transfer clutch  38  for varying the amount of drive torque directed to front wheels  12  through transfer clutch  38  as a function of the sensor input signals for providing improved tractive performance when needed. In addition, controller  42  is adapted to controllably modulate the actuated state of transfer clutch  38  as described in greater detail hereinafter. By way of example rather than limitation, the control scheme generally disclosed in U.S. Pat. No. 5,332,060 issued Jul. 26, 1994 to Sperduti et al. and assigned to the common assignee of the present invention (the disclosure of which is hereby incorporated by reference) can be used to control adaptive actuation of transfer clutch  38  during on-demand operation.  
         [0017]      FIGS. 2-4  depict various clutch control systems for storing mechanical energy and reducing the maximum required electrical current for clutch actuation. The clutch control systems discussed below are an improvement over prior systems due to their ability to reduce peak power draw and overall power consumption from the vehicle &#39;s electrical system while operating the modulating clutch. The decrease in power draw is primarily accomplished by using a relatively low amount of electrical energy over time to charge a mechanical energy storage device and releasing the energy rapidly when required. This control scheme makes it possible to reduce the size of vehicle electrical system including the wires and circuitry controlling the electrical system. Each of the clutch control systems described below provides for operating a modulating clutch or clutches. The controls for the modulating clutches utilize available vehicle information along with hydraulic system information to react to a vehicle command to provide the required torque and/or speed.  
         [0018]     The first exemplary embodiment clutch control system  200  is depicted in  FIG. 2 . Clutch control system  200  includes an accumulator  202  as an energy storage device. Accumulator  202  may be of the gas or spring type. Clutch control system  200  also includes a hydraulic actuator  204  in communication with accumulator  202 . Hydraulic actuator  204  is operable to provide intermittent pulses of highly pressurized fluid to accumulator  202  to store mechanical potential energy in the accumulator.  
         [0019]     Hydraulic actuator  204  includes a housing  206  defining a cavity  208 , an electromagnet  210  and a master piston  212  slidably positioned within cavity  208  of housing  206 . Electromagnet  210  is positioned within housing  206 . A spring  214  interconnects master piston  212  and housing  206  to biasedly urge master piston  212  toward a retracted position shown in  FIG. 2 .  
         [0020]     Master piston  212  includes a stem portion  216  slidably positioned within a hydraulic cavity  218  formed within housing  206 . A seal  220  sealingly engages stem portion  216  and housing  206  to allow a relatively high pressure to be generated within hydraulic cavity  218 . Master piston  212  is preferably constructed from a magnetizable material such that energization of electromagnet  210  attracts a body portion  222  of master piston  212  toward electromagnet  210 .  
         [0021]     Hydraulic actuator  204  may be operated as a pump by alternately supplying and disconnecting power to electromagnet  210 . When the electromagnet is powered, body portion  222  of master piston  212  is attracted toward electromagnet  210 . When body portion  222  contacts electromagnet  210 , piston  212  is in the advanced position. During this operation, stem portion  216  axially translates in an advancing direction to supply pressurized fluid to accumulator  202 . Once body portion  222  contacts electromagnet  210 , power is discontinued to the electromagnet. Spring  214  forces master piston  212  toward the retracted position shown in  FIG. 2 .  
         [0022]     Clutch control system  200  also includes a first pressure sensor  224  in communication with accumulator  202 . First pressure sensor  224  is operable to provide a signal indicative of the fluid pressure within accumulator  202  to a controller  225 . It should be appreciated that controller  225  may be a stand alone unit or may be incorporated as part of controller  42 . A non-returning check valve  226  is plumbed between cavity  218  and accumulator  202  to allow pressurized fluid to enter the accumulator but restrict flow from the accumulator toward the pressurized fluid source. A first control valve  228  is operable to selectively supply pressurized fluid within accumulator  202  to a clutch actuator assembly  230 . Depending on system requirements, first control valve  228  may be a variable force solenoid, a pulse width modulation control valve, a proportional flow control valve or a proportional pressure control valve. Clutch actuator assembly  230  includes a plurality of slave pistons  232  substantially circumferentially spaced apart from one another and in communication with an apply plate  234 . Apply plate  234  is axially moveable and operable to transmit a clutch engagement force to transfer clutch  38 .  
         [0023]     Transfer clutch  38  is a multi-plate clutch assembly that is arranged to transfer torque between first output shaft  30  and second output shaft  36 . Transfer clutch  38  includes a cylindrical drum  236  shown to be operably fixed for rotation with second output shaft  36  and having a plurality of first or outer clutch plates  238  mounted (i.e., splined) for rotation with drum  236 . A clutch hub  240  of transfer clutch  38  is fixed for rotation with first output shaft  30 . A second set of clutch plates  242 , referred to as inner clutch plates, are mounted (i.e., splined) for rotation with clutch hub  240 . Torque is transferred between first output shaft  30  and second output shaft  36  by frictionally engaging first clutch plates  238  with second clutch plates  242  with a compression force supplied by apply plate  234 .  
         [0024]     Slave pistons  232  are slidably engageable with apply plate  234  and transmit a force proportional to the pressure acting on each of slave pistons  232 . A second pressure sensor  244  is plumbed in communication with slave pistons  232 . Second pressure sensor  244  is operable to output a signal indicative of the fluid pressure acting on slave pistons  232 . The signal is provided to controller  225  and used as a feedback signal to control the torque generated by transfer clutch  38 . A second control valve  245  is operable to selectively supply pressurized fluid acting on slave pistons  232  to a second accumulator  243 . Second accumulator  243  contains fluid at a substantially lower pressure than accumulator  202 . Pressure acting on slave pistons  232  may be selectively released to second accumulator  223  by actuating second control valve  245 .  
         [0025]     An optional second non-returning check valve  246  acts as a pressure relief valve to allow fluid previously acting on slave pistons  232  to return to cavity  218 . One skilled in the art will appreciate that clutch control system  200  is a closed hydraulic system. Accordingly, fluid need not be continually supplied to clutch control system  200  once the system has been initially filled with hydraulic fluid. An account for fluid leakage may be made as will be described.  
         [0026]     In operation, clutch control system  200  operates to charge accumulator  202  with fluid at a relatively high pressure by operating hydraulic actuator  204  and energizing electromagnet  210  to translate stem portion  216  in an advancing direction. Electromagnet  210  generates enough force to overcome the force generated by spring  214  and pressurize fluid within cavity  218 . Pressurized fluid passes through non-returning check valve  226  and enters accumulator  202 . During the accumulator charging cycle, first control valve  228  is closed. If clutch control system  200  is not equipped with a second non-returning check valve, second control valve  245  is also maintained in the closed position to charge accumulator  202 .  
         [0027]     Once master piston  212  is in the advanced position, the power supply to electromagnet  210  is discontinued. Spring  214  forces master piston  212  to the retracted position. The pumping or charging sequence is continued until a desired pressure within accumulator  202  is reached as indicated by a signal output from first pressure sensor  224 . It should be appreciated that hydraulic actuator  204  may produce a maximum desired pressure while requiring minimal current from the vehicle power source.  
         [0028]     Once the desired pressure is stored in accumulator  202  the charging cycle is discontinued. At this time, clutch control system  200  awaits a torque demand signal. Upon receipt of a signal for torque from controller  225 , first control valve  228  is opened to supply pressurized fluid to slave pistons  232 . The signal output from second pressure sensor  244  indicates the pressure acting upon slave pistons  232  and may be correlated to torque generated by transfer clutch  38 . If a reduction in output torque from transfer clutch  38  is desired, second control valve  245  is allowed to shift to its normally open position thereby releasing pressurized fluid into low pressure accumulator  243  and reduce the pressure acting upon slave pistons  232 .  
         [0029]     In an alternate form, clutch control system  200  may be equipped with an alternate second control valve (not shown) that operates as a normally closed valve as opposed to the normally open configuration shown in  FIG. 2 . If second control valve  245  is a normally closed valve, leakage of fluid past first control valve  228  may cause transfer clutch  38  to be in an applied condition during vehicle inoperative times. Some Original Equipment Manufacturers may not wish this condition and specify the normally open second control valve. Furthermore, any number of the valves presently depicted may be plumbed as normally or normally closed valves to meet vehicle manufacturer requirements.  
         [0030]      FIG. 3  depicts an alternate embodiment clutch control system  300 . Clutch control system  300  is a closed system similar to the clutch control system previously discussed. Like elements will retain the reference numerals previously introduced. Clutch control system  300  includes an actuator  302  that functions substantially similarly to hydraulic actuator  204 . However, actuator  302  includes a first magnet  304  coupled to a master piston  305 . The first electromagnet and master piston subassembly is slidably positioned within a cavity  306  of a housing  308 . A second electromagnet  310  is also positioned within cavity  306 . Second electromagnet  310  is fixed to housing  308 . Master piston  305  includes a stem portion  312  operably acting on hydraulic fluid contained within a hydraulic cavity  314  of housing  308 . A seal  316  sealingly engages stem portion  312  and cavity  314  to maintain a closed hydraulic system.  
         [0031]     A gap  318  exists between first electromagnet  304  and second electromagnet  310  when master piston  305  is in the retracted position as shown in  FIG. 3 . As mentioned earlier, spring  214  urges master piston  305  toward the retracted position. When the first and second electromagnets are energized, the first electromagnet and master piston subassembly is attracted to second electromagnet  310 . During this operation, stem portion  312  forces pressurized hydraulic fluid past non-returning check valve  226  into accumulator  202 . When first electromagnet  304  contacts second electromagnet  310 , the first electromagnet master piston subassembly is in the advanced position. Power is discontinued to each of the electromagnets and spring  214  forces master piston  305  back to the retracted position once again. In this manner, actuator  302  may act as a pump to provide pressurized fluid to accumulator  202 . It should be appreciated that the remaining components of clutch control system  300  and their operation are substantially similar to the components previously described in relation to clutch control system  200 . Accordingly, a redundant description will not be provided.  
         [0032]      FIG. 4  depicts an alternate embodiment clutch control system  400  operable for selectively supplying an actuation force to transfer clutch  38 . Clutch control system  400  is substantially similar to clutch control system  300  previously described. For clarity, like elements will retain their previously introduced reference numerals.  
         [0033]     Clutch control system  400  includes an actuator  402  that functions substantially similarly to hydraulic actuators  204  and  302  previously described. However, actuator  402  includes a spring return solenoid  404  coupled to a master piston  406 . Master piston  406  is slidably positioned within a cavity  408  of a housing  410 . Solenoid  404  includes a return spring (not shown) operable to return master piston  406  to a retracted position as depicted in  FIG. 4 . Energization of solenoid  404  causes master piston  406  to translate in an advancing direction. As master piston  406  translates, pressurized fluid is supplied to accumulator  202 . At the end of the stroke of master piston  406 , power is no longer supplied to solenoid  404  and the internal return spring retracts master piston  406 . This sequence of events is repeated to pump highly pressurized fluid into accumulator  202 . Other components of clutch control system  400  function substantially similarly to those previously described.  
         [0034]     The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims.