Low power modulating clutch control system

A power transmission device includes a rotary input member adapted to receive drive torque from a source of torque, a rotary output member adapted to provide drive torque to an output device and a torque transfer mechanism operable to transferring drive torque from the input member to 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 electric motor drivingly coupled to a first piston. The first piston is slidably positioned within the housing for supplying pressurized fluid to an accumulator. The pressurized fluid within the accumulator is in communication with a second piston to provide the clutch engagement force.

FIELD OF THE INVENTION

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

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.

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.

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.

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

A power transmission device includes a rotary input member adapted to receive drive torque from a source of torque, a rotary output member adapted to provide drive torque to an output device and a torque transfer mechanism operable to transferring drive torque from the input member to 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 electric motor drivingly coupled to a first piston. The first piston is slidably positioned within the housing for supplying pressurized fluid to an accumulator. The pressurized fluid within the accumulator is in communication with a second piston to provide the clutch engagement force.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

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.

Referring toFIG. 1of the drawings, a drivetrain for a four-wheel drive vehicle is schematically shown interactively associated with a power transfer system10. The motor vehicle drivetrain has a pair of front wheels12and rear wheels14both drivable from a source of power, such as an engine16, through a transmission18which 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 case20operable to receive drive torque from engine16and transmission18for normally driving rear wheels14(i.e., the “driven” wheels) in a two-wheel drive mode of operation. Front wheels12and rear wheels14are shown connected at opposite ends of front and rear axle assemblies22and24, respectively. As is known, a rear differential26is interconnected between rear axle assembly24and one end of a rear drive shaft28, the opposite end of which is interconnected to a first output shaft30of transfer case20. Similarly, front axle assembly22includes a front differential32that is coupled to one end of a front drive shaft34, the opposite end of which is coupled to a second output shaft36of transfer case20. 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 wheels12.

Transfer case20is equipped with a torque transfer clutch38for selectively delivering drive torque to front wheels12(i.e., the non-driven wheels) to establish a four-wheel drive mode of operation. The operating mode of transfer clutch38is generally controlled in response to a mode signal generated by a mode selector40and which is sent to a controller42. Controller42also receives input signals from one or more vehicle sensors44that are indicative of various operational characteristic of the vehicle.

When the two-wheel drive mode is selected, all drive torque is delivered from first output shaft30to rear wheels14and transfer clutch38is maintained in a “non-actuated” condition. When the part-time four-wheel drive mode is selected, transfer clutch38is fully actuated and maintained in a “lock-up” condition such that second output shaft36is, in effect, rigidly coupled for driven rotation with first output shaft30. When the “on-demand” drive mode is selected, controller42communicates with a clutch control system200to control the degree of actuation of transfer clutch38for varying the amount of drive torque directed to front wheels12through transfer clutch38as a function of the sensor input signals for providing improved tractive performance when needed. In addition, controller42is adapted to controllably modulate the actuated state of transfer clutch38as 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 clutch38during on-demand operation.

FIGS. 2–7depict 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'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.

The first exemplary embodiment clutch control system200is depicted inFIG. 2. Clutch control system200includes an accumulator202as the energy storage device. Accumulator202may be of the gas or spring type. Clutch control system200also includes an electric motor204, a piston206, a gear reduction unit208and a lead screw210. Electric motor204is drivingly coupled to gear reduction unit208. The output from gear reduction unit208is engaged with lead screw210. Operation of motor204causes lead screw210to rotate. Lead screw210is coupled to piston206such that rotation of lead screw210causes piston206to axially translate within a cavity212formed within a cylinder housing213. An optional vent211extends from housing213to interconnect cavity212with a reservoir (not shown) containing additional fluid.

Clutch control system200also includes a first pressure sensor214in communication with accumulator202. First pressure sensor214is operable to provide a signal indicative of the fluid pressure within accumulator202to a controller215. It should be appreciated that controller215may be a stand alone unit or may be incorporated as part of controller42. A non-returning check valve216is plumbed between cavity212and accumulator202to allow pressurized fluid to enter the accumulator but restrict flow from the accumulator toward the pressurized fluid source. A first control valve218is operable to selectively supply pressurized fluid within accumulator202to a clutch actuator assembly220. Depending on system requirements, first control valve218may be a variable force solenoid, a pulse width modulation control valve, a proportional flow control valve or a proportional pressure control valve. Clutch actuator assembly220includes a plurality of slave pistons222substantially circumferentially spaced apart from one another and in communication with an apply plate224.

Transfer clutch38is a multi-plate clutch assembly that is arranged to transfer torque between first output shaft30and second output shaft36. Transfer clutch.38includes a cylindrical drum226shown to be operably fixed for rotation with second output shaft36and having a plurality of first or outer clutch plates228mounted (i.e., splined) for rotation with drum226. A clutch hub230of transfer clutch38is fixed for rotation with first output shaft30. A second set of clutch plates232, referred to as inner clutch plates, are mounted (i.e., splined) for rotation with clutch hub230. Torque is transferred between first output shaft30and second output shaft36by frictionally engaging first clutch plates228with second clutch plates232with a compression force supplied by apply plate224.

Slave pistons222are slidably engageable with apply plate224and transmit a force proportional to the pressure acting on each of slave pistons222. A second pressure sensor234is plumbed in communication with slave pistons222. Second pressure sensor234is operable to output a signal indicative of the fluid pressure acting on slave pistons222. The signal is provided to controller215and used as a feedback signal to control the torque generated by transfer clutch38. A second non-returning check valve236acts as a pressure relief valve to allow fluid previously acting on slave pistons222to return to cavity212. One skilled in the art will appreciate that clutch control system200is a closed hydraulic system. Accordingly, fluid need not be continually supplied to clutch control system200once the system has been initially filled with hydraulic fluid. An account for fluid leakage may be made as will be described.

In operation, electric motor204is rotated in a first direction to cause lead screw210to rotate thereby causing piston206to translate in an advancing direction. Pressurized fluid passes by non-returning check valve216and charges accumulator202. Advancement of piston206continues until a desired pressure is reached as indicated by a signal output from first pressure sensor214. The charging of accumulator202occurs over time such that peak currents need not be drawn from motor204.

If a torque transfer between first output shaft30and second output shaft36is desired, first control valve218is operated to allow pressurized fluid from accumulator202to act on slave pistons222. Slave pistons222axially translate to cause apply plate224to actuate transfer clutch38by clamping first clutch plates228to second clutch plates232. If a reduction in torque is requested, motor204is operated in the reverse direction causing piston206to axially translate in a retracting direction. During retraction of piston206, a pressure differential occurs across second non-returning check valve236. To equalize the pressure on non-returning check valve236, pressurized fluid previously acting on slave pistons222returns to cavity212. At this time, a force transferred by apply plate224is reduced.

FIG. 3depicts an alternate embodiment clutch control system300. Clutch control system300is substantially similar to clutch control system200and like elements will retain their previously introduced reference numerals. For clarity, controller215is not shown but is included in clutch control system300. Clutch control system300includes a second control valve302operable to selectively supply pressurized fluid acting on slave pistons222to a second accumulator304. Second accumulator304contains fluid at a substantially lower pressure than accumulator202. Pressure acting on slave pistons222may be selectively released to second accumulator304by actuating second control valve302.

An optional third control valve306may be positioned between first control valve218and accumulator202if required. Use of third control valve306is contemplated for systems having a relatively high leakage rate between accumulator202and first control valve218. Third control valve306includes a ball seat type arrangement to more completely contain pressurized fluid within accumulator202. Third control valve306remains in the closed position until the accumulator has been charged to a desired pressure as indicated by first pressure sensor214. Third control valve306acts as an on/off valve for providing pressurized fluid to first control valve218.

In an alternate form, clutch control system300may 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 inFIG. 3. If second control valve302is a normally closed valve, leakage of fluid past first control valve218may cause transfer clutch38to 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.

FIG. 4depicts an alternate embodiment clutch control system400. Clutch control system400is substantially similar to clutch control system200with the exception that lead screw210is replaced by a ball screw402. Rotation of motor204causes ball screw402to rotate and translate piston206. Because the piston to ball screw interconnection is a very low friction overrunning interface, a brake404is coupled to motor204to selectively restrict rotation of ball screw402. Brake404is operable to maintain a desired pressure acting on slave pistons222by selectively restricting or allowing pressurized fluid to pass by a second non-returning check valve. Specifically, if brake404is applied, piston206will not be allowed to move in the retracting direction and additional fluid will not be allowed to enter cavity212. When brake404is released, a pressure differential across second non-returning check valve236will result in piston206being driven in the retracting direction until the pressure differential is minimized. During this fluid transfer, the torque generated by transfer clutch38will be reduced.

FIG. 5depicts another alternate embodiment clutch control system identified at reference numeral500. Clutch control system500combines the features of clutch control system400and clutch control system300. For clarity, previously introduced like elements will retain their reference numerals. Specifically, clutch control system500is substantially identical to clutch control system300except lead screw210has been replaced by ball screw402. Brake404has been added to perform the functions previously described.

FIG. 6depicts an alternate embodiment clutch control system600. Clutch control system600includes many elements substantially similar to those previously described in relation to clutch control system200. Like elements will retain their previously introduced reference numerals. Clutch control system600includes a first control valve602in communication with accumulator202, a second accumulator604, and a second control valve606. First control valve602is a three position valve. In the first position, cavity212and housing213are in fluid communication with accumulator202. When first control valve602is in a second position, cavity212is blocked and pressurized fluid from accumulator202is in communication with second control valve606. At a third position of first control valve602, pressurized fluid is trapped within accumulator202and the pathway interconnecting cavity212, second accumulator604and second control valve606is opened. A third pressure sensor608outputs a signal indicative of the pressure within second accumulator604.

In operation, lead screw210and piston206will act as a reciprocating piston pump under power of motor204. Fluid is drawn into cavity212during retraction of piston206when first control valve602is in the third position. First control valve602is moved to the first position and motor204drives piston206in the advancing direction to push fluid through first control valve602and pressurize accumulator202. This procedure is continued until a desired pressure is measured by first pressure sensor214. Once accumulator202is charged, pressurized fluid may be released to second control valve606by positioning first control valve602in the second position. Depending on the system requirements, second control valve606may be a variable force solenoid, a pulse width modulation control valve, proportional flow control valve or a proportional pressure control valve. Second control valve606is selectively operable to release pressurized fluid to act on slave pistons222. Second pressure sensor234provides a signal indicative of the fluid pressure acting on the slave pistons. To release pressure acting on slave pistons222and reduce the torque generated by transfer clutch38, second control valve606is opened and first control valve is placed in its third position to allow fluid to return to second accumulator604and/or cavity212.

FIG. 7shows another alternate embodiment clutch control system identified by reference numeral700. Clutch control system700includes many elements substantially similar to those previously described in relation to clutch control system300. As such, like elements will retain their previously introduced reference numerals. Clutch control system700includes a hydraulic actuator702selectively operable to provide pressurized fluid to high pressure accumulator202. Hydraulic actuator702includes a master piston704slidably positioned within a cavity706of a housing708. A seal710sealingly engages master piston704and housing708to maintain a closed hydraulic system. A roller712is rotatably coupled to master piston704.

A sector gear714includes a splined aperture716, a cam surface718and a range slot720. Cam surface718is positioned relative to the axis of rotation of sector gear714such that rotation of the sector gear causes master piston704to translate within housing708. Preferably, cam surface718is shaped to translate master piston704from a retracted position shown inFIG. 7to an advanced position (not shown) during oscillation of sector gear714.

Range slot720is configured to accept a member721for shifting the present gear range on a torque transfer mechanism. Range slot720includes at least one dwell portion722where oscillation of sector gear714may occur without radially translating the member disposed within range slot720. This configuration allows operation of hydraulic actuator702without causing a range shift. In the preferred embodiment, sector gear714is a component of a two-speed transfer case. The member disposed within range slot720is operable to cause a change in the gear reduction from low to high or vice versa during operation of the vehicle.

A third control valve724is plumbed in place of non-returning valve216between accumulator202and cavity706. Third control valve724includes a non-returning check valve position and a flow through position. During normal operation, the third control valve724is biased toward the check valve position and operates as previously described. However, third control valve724may selectively be shifted to allow highly pressurized fluid contained within accumulator202to act upon piston704. At this time, roller712applies a force to sector gear714to assist in a range shift operation if so desirable. Specifically, rotation of sector gear714will cause member721to radially translate and change the gear position within a torque transfer mechanism to which it is connected.

FIGS. 8 through 10depict circuit options that may be used with any of the clutch control systems previously described. These circuit options provide another degree of flexibility for controlling additional clutch packs, range sleeves, range forks or any other hydraulically actuated device on the vehicle.

FIG. 8shows a hydraulic circuit branch800having an end802that is tapped into a clutch control system immediately adjacent the high pressure accumulator. As such, highly pressurized fluid is present in a line804. An optional on/off solenoid806is plumbed in series with a control valve808. On/off solenoid806and control valve808are selectively operable to allow pressurized fluid to enter a cavity810containing a piston812. A pressure sensor814provides a signal indicative of the pressure acting on piston812. Piston812may provide actuation force to any number of devices as previously described. Depending on the system to be energized, control valve808may be the only valve between the high pressure accumulator and piston812. Alternatively, if concerns arise regarding leakage of highly pressurized fluid into cavity810, optional on/off solenoid806includes a non-returning check valve position816to limit ingress of fluid.

FIG. 9depicts a circuit branch900substantially similar to circuit branch800. Accordingly, like numerals will be used to identify previously introduced elements. Circuit branch900includes an additional control valve902operable to selectively supply pressurized fluid to piston812or a second piston904. A second pressure sensor906provides a signal indicative of the pressure acting on piston904to the controller (not shown).

FIG. 10depicts another optional circuit branch1000. Circuit branch1000is a variant of circuit branches800and900. As such, like elements will retain their previously introduced reference numerals. Circuit branch1000includes a third piston1002in selective communication with highly pressurized fluid from the high pressure accumulator. An additional control valve1004is operable to direct pressurized fluid to piston904or piston1002as is required. One skilled in the art will appreciate that any number of variations of circuit branches800,900and1000may be constructed to provide application force to additional mechanisms.

FIGS. 11 and 12show combined accumulators each having a low pressure side and high pressure side. A combined accumulator may replace the accumulators previously described.FIG. 11shows a combined accumulator1100including a housing1102having a low pressure piston1104and a high pressure piston1106slidably positioned therein. Housing1102and high pressure piston1106define a high pressure cavity1108. Housing1102and low pressure piston1104define a low pressure cavity1110. A first port1112is in communication with low pressure cavity1110. A second port1114is in communication with high pressure cavity1108. Low pressure piston1104includes a body portion1116and a push rod1118. Push rod1118extends through an aperture1120extending through high pressure piston1106. A seal1122engages push rod1118and high pressure piston1106. A plate1124and snap ring1126are arranged within housing1102between high pressure piston1106and body portion1116of low pressure piston1104. A spring1128biases high pressure piston1106away from plate1124.

Combined accumulator1100provides storage of highly pressurized fluid in high pressure cavity1108and storage capacity for low pressure fluid in low pressure cavity1110. It should be appreciated that only one spring is required within combined accumulator1100. Furthermore, a small increase of pressure will occur within high pressure cavity1108when pressure is added to low pressure cavity1110. This occurs due to movement of push rod1118within high pressure cavity1108.

FIG. 12depicts an alternate embodiment combined accumulator1200that includes a housing1202having a low pressure piston1204and a high pressure piston1206slidably disposed within housing1202. Housing1202and high pressure piston1206define a high pressure cavity1208. Housing1202and low pressure piston1204define a low pressure cavity1210. A first port1212is in communication with low pressure cavity1210. A second port1214is in communication with high pressure cavity1208. A plate1216is positioned within housing1202between high pressure piston1206and low pressure piston1204. A snap ring1218restricts plate1216from moving toward low pressure piston1204. A first spring1220is positioned between plate1216and high pressure piston1206. First spring1220biases high pressure piston1206away from plate1216. A second spring1222interconnects high pressure piston1206and low pressure piston1204. Combined accumulator1200is a space saving accumulator because only one housing is required for both a high pressure and a low pressure accumulator. Furthermore, combined accumulator1200functions such that an increase in pressure within low pressure cavity1210causes an increase in pressure within high pressure cavity1208due to second spring1222.

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.