Abstract:
A clutch assembly is provided including a controller, a plurality of vibration sensors, a clutch housing containing a lubricated clutch pack having a friction interface, a clutch piston responsive to a current command from the controller and operable for applying a compression force on the clutch pack, and a high-frequency (HF) oscillation source configured to generate at least one HF oscillation, and to direct the HF oscillation to the friction interface, wherein the controller is operable to detect clutch shudder and activate the source in response thereto to minimize the clutch shudder. The source includes HF hardware, and generates different HF oscillations applied directly to the clutch housing or to the clutch-apply current command. A method of reducing clutch shudder includes setting a threshold clutch shudder amplitude, detecting clutch shudder, and applying a HF oscillation to a friction interface to minimize clutch shudder when the detected shudder exceeds the threshold.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an anti-lock clutch system having a wet clutch pack with at least one pair of mating clutch plates forming a friction interface therebetween, the anti-lock clutch system being configured to introduce a high-frequency (HF) oscillation to the friction interface in order to minimize clutch vibration or shudder. 
       BACKGROUND OF THE INVENTION 
       [0002]    In an automotive transmission, clutch assemblies or clutches are commonly used to transmit rotational motion or torque between two disparately rotating members, such as an engine crankshaft and a transmission driveshaft. Standard friction-type clutches generally include a series of alternating friction and reaction plates that together make up a clutch pack, with the clutch pack being disposed within a clutch drum contained within an outer clutch housing. A friction plate typically has a layer or surface coating of rough friction material which is bonded or otherwise attached to the primary contact surfaces of the friction plate, while the reaction plate typically has a relatively smooth contact surface configured to oppose the friction plate whenever the friction clutch is engaged. A friction-type clutch is engaged by applying an actuation force, such as a controllable hydraulic force supplied by a transmission pump. This clutch-apply force actuates an apply mechanism, such as a clutch-apply piston, in order to compress or force together the various friction and reaction plates of the clutch pack. Once compressed, the alternating clutch plates become interlocked due to the substantial friction forces imparted by the combined effect of the clutch-apply force and the friction material, thereby allowing the clutch plates to rotate in unison. 
         [0003]    Friction clutches may be of the dry-plate or wet-plate variety, with wet-plate or fluid lubricated friction clutches providing enhanced thermal performance due to the cooling qualities of the pressurized lubricating fluid. Within a wet-plate clutch, which may take the form of, for example, a shift clutch, torque converter clutch, limited slip differential, or other such lubricated clutching device, enhanced thermal performance is accomplished by passing or directing the pressurized fluid, such as transmission fluid or oil, through and around the mating clutch surfaces to dissipate the heat generated by the friction forces in proximity to the friction interface. At high temperatures, or under high apply pressures and/or low relative velocities or slip speed between the opposing surfaces forming a friction interface, there may be little or no remaining fluid film separating the opposing surfaces. This temporary absence of lubrication at the friction interface may lead to strong local adhesive bonds between opposing surfaces or friction elements, and thus may cause a spike in the coefficient of friction at the friction interface. When this change in friction is related to a change in slip speed, the effect can be approximated mechanically as negative damping, which can combine with existing powertrain resonance to create regenerative and often noticeable and objectionable clutch “shudder” or “chatter” under certain vehicle operating conditions. 
         [0004]    In order to reduce or minimize clutch shudder, friction modifiers or boundary lubrication additives are often added to the lubricant. However, these friction modifiers may be expensive, and they are depleted over time, requiring frequent replenishment. Also, enlarging the clutch or adding a larger clutch damper may also help to alleviate clutch shudder, although such solutions generally are less than optimal due to the added cost, size, and/or weight of such larger devices. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly, a clutch assembly is provided having a pair of clutch plates forming a friction interface therebetween, and including a controller, at least one sensor configured to detect clutch vibration, and a controllable source of high-frequency oscillation, wherein the controller is configured to activate the source of high-frequency oscillation in response to the sensor to thereby apply a high-frequency oscillation to the friction interface to minimize clutch vibration. 
         [0006]    In one aspect of the invention, the source includes high-frequency hardware, and the high-frequency oscillation includes a plurality of different high-frequency oscillations each having a different amplitude and frequency. 
         [0007]    In another aspect of the invention, the high-frequency hardware is configured to deliver a plurality of different high-frequency oscillations to the clutch housing. 
         [0008]    In another aspect of the invention, a controllable clutch actuation device is responsive to a current command from the controller, wherein the source of high-frequency oscillation is configured to apply the at least one high-frequency oscillation to the controllable clutch actuation device. 
         [0009]    In another aspect of the invention, the high-frequency oscillation is an AC component that is added to the current command for the clutch actuation device. 
         [0010]    In another aspect of the invention, a lubricated clutch assembly is provided including a controller, a plurality of vibration sensors, a clutch housing at least partially containing a lubricated clutch pack having at least one friction interface, a hydraulically-actuated clutch piston responsive to a current command from the controller and operable for applying a compression force on the clutch pack in response thereto, and an oscillation source configured to generate at least one high-frequency oscillation in response to the controller, and to direct the oscillation to the friction interface, wherein the controller is operable to detect shudder of the clutch assembly and activate the oscillation source in response thereto for minimizing clutch shudder. 
         [0011]    In another aspect of the invention, a method of reducing clutch shudder is provided for use in a clutch having a controller and a clutch pack disposed within a clutch housing, the clutch pack having at least one friction interface therein and the clutch being actuatable in response to a current command from the controller, the method including setting a threshold clutch shudder frequency and amplitude, detecting clutch shudder, and applying a high-frequency oscillation to the friction interface when the detected clutch shudder exceeds the threshold, thereby minimizing the clutch shudder. 
         [0012]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic graphical illustration of the relationship between the coefficient of friction (μ) and slip speed (ν) of the clutch assembly of the invention; 
           [0014]      FIG. 2A  is a schematic exploded perspective view of a representative clutch pack usable with the invention; 
           [0015]      FIG. 2B  is a schematic graphical illustration of clutch plate surface asperities; 
           [0016]      FIG. 3  is a fragmentary cross-sectional side view of a portion of a clutch assembly according to the invention; 
           [0017]      FIG. 4A  is a schematic graphical illustration showing the effect on the relationship between the coefficient of friction (μ) and slip speed (ν) of a high frequency (HF) oscillation applied to the friction interface, in accordance with the invention; 
           [0018]      FIG. 4B  is another schematic graphical illustration showing the effect on the relationship between the coefficient of friction (μ) and slip speed (ν) of an additional high frequency (HF) oscillation applied to the friction interface; and 
           [0019]      FIG. 5  is a flow chart describing a method or algorithm of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Referring to the drawings wherein like reference numbers correspond to like or similar components throughout the several figures, there is shown in  FIG. 1  a schematic graphical illustration or curve  10  describing the relative relationship between the coefficient of friction (μ) and slip speed (ν) occurring between two mating clutch plates at a friction interface formed therebetween. As used herein, the term “coefficient of friction” refers generally to the ratio of the force of friction between two bodies, i.e. the two opposing clutch plates in a wet clutch pack and the force pressing the bodies or clutch plates together. For example, a representative clutch pack  15  is shown in  FIG. 2A  having a friction plate  18  with friction material  19  bonded or otherwise attached thereto on both sides, and an opposing reaction plate  21 , with the friction interface  27  representing the mating surfaces of the respective plates  18 ,  21 . In the perspective view of  FIG. 2A , only one surface of friction plate  18  is visible, however as stated above the reverse or opposite surface (not shown) is preferably identically configured with friction material  19 . The clutch pack  15  also may take the form of alternating unitary clutch plates (not shown) each having friction material  19  bonded to both sides, or any other combination of clutch plates forming a friction interface  27  having opposing surfaces with a coefficient of friction (μ) therebetween. 
         [0021]    Turning back to  FIG. 1 , point A on curve  10  generally represents a condition of relatively high slip speed (ν), i.e. the difference in rotational speed between mating clutch plates, and the coefficient of friction (μ). Such a condition generally occurs during a predominantly hydrodynamic lubrication regime, or the lubrication regime in which a comparatively thick layer or wedge of lubricating fluid is formed between the rotating bodies, such as the clutch plates  18 ,  21  of a clutch pack  15  (see  FIG. 2A ). Moving from point A along curve  10 , the slip speed (ν) gradually decreases to point B, upon which the surface asperities  18 A and  21 A (see  FIG. 2B ), i.e. the roughness profile of mating clutch plate surfaces  18  and  21 , respectively, begin to emerge from or “poke through” the thinning oil wedge, and gradually coming into direct mutual contact. This reduction in film thickness may also occur due to elevated temperature, changes in viscosity, and/or increased or elevated apply pressure, as understood by those of ordinary skill in the art. 
         [0022]    Turning to  FIG. 2B , which depicts representative surface asperities  18 A and  21 A, with the height of the surface asperities  18 A and  21 A shown along the y-axis, and the width of the surface asperities  18 A and  21 A shown along the x-axis. As the surface asperities  18 A and  21 A come into direct mutual contact, strong adhesive bonds  23  are formed therebetween, which can result in a sharp increase or spike in the coefficient of friction (μ) as relative velocity or slip speed (ν) continues to slow. This sharp increase or spike is represented on curve  10  of  FIG. 1  as the shaded area  14  having a maximum amplitude  12  at point C, i.e. at zero slip speed (ν). Reduction of amplitude  12  of area  14  effectively reduces the amount or degree of perceived clutch vibration or shudder. Therefore, breaking the adhesive bonds  23  that form between the surface asperities  18 A,  21 A during boundary lubrication conditions effectively flattens or reduces the amplitude  12 , and therefore is an object of this invention, as will now be explained. 
         [0023]    The introduction of a high-frequency (HF) vibration or oscillation directly or indirectly to the friction interface  27  (also see  FIG. 2A ) before the onset of or during a clutch shudder event facilitates the breaking of the adhesive bonds  23 . While some degree of hydrodynamic lubrication still exists at the friction interface  27 , that is, some level of film thickness remains within the friction interface  27 , a properly selected HF oscillation component superimposed on the nominal velocity profile or curve  10  of  FIG. 1 , will effectively further flatten, “smear”, or otherwise filter curve  10  in the ν-direction. This result can be best seen in  FIG. 4A , with shaded area  114  replacing shaded area  14  of  FIG. 1 , with the “smearing” effect due to the relative motion of surface asperities  18 A and  21 A, represented by arrow  22  in  FIG. 2B , generating a film thickness therebetween. 
         [0024]    As the film layer or oil wedge continues to thin, the surface asperities  18 A and  21 A (see  FIG. 2B ) come into direct, non-lubricated contact, and a boundary lubrication condition commences. While operating under a boundary lubrication regime, the introduction of a properly selected HF-component or oscillation forces or causes a greater number of surface asperities  18 A,  21 A to be bypassed or “skipped over” during the high-slip portion of the speed cycle, that is, the portion of curve  10  to the left of point B. This “skip effect” is more pronounced as the slip speed (ν) approaches zero. The result of the properly applied HF-component is shown in  FIG. 4B , as the shaded area  214  formed between points C′ and B′. 
         [0025]    Turning now to  FIG. 3 , a representative clutch assembly  20  is shown in a cutaway side view having an axis of rotation  17  and a clutch housing  28  containing a hydraulically-actuated clutch apply piston  30  separating a clutch-apply cavity  34  from a main cavity  35 . For simplicity, only one half of the symmetrical clutch assembly  20  is shown relative to the axis of rotation  17 . The clutch-apply piston  30  is preferably biased by a return spring  37  disposed or positioned between the clutch-apply piston  30  and a substantially stationary balance piston  38 , the return spring  37  having a suitable return force, as represented by arrow F R . Pressurized fluid  11  is fed into the clutch-apply cavity  34  from a controllable source or pump  13 , such as a positive displacement pump, through a fluid passage  16 . The pump  13  is variably and selectively controllable as required by a controller  32  having memory  39 . The clutch-apply piston  30  is engageable with a clutch pack  15  having at least one reaction plate  21  and at least one friction plate  18 , as previously described hereinabove, with either or both of plates  18  and  21  having friction material or surface  19  (also see  FIG. 2A ). As pressurized fluid  11  is fed or directed into the clutch-apply cavity  34 , the clutch-apply piston  30  slides or moves into engagement with the clutch pack  15 , pressing the respective plates  18  and  21  together. The friction material  19  then slows or stops the disparately moving plates  18  and  21  to enable full engagement of the clutch pack  15 , allowing for example a gear shifting event. 
         [0026]    In one embodiment, the reduction of clutch shudder may be achieved by carefully selecting an alternating current (AC) component, represented by arrow HF A , and adding this AC component HF A  to the current command (i) which controls the clutch-apply pressure, represented in  FIG. 3  by arrow F A . Controller  32  is therefore preferably configured to execute an method or algorithm  105  (see  FIG. 5 ) contained or programmed in one or more software and/or firmware programs (not shown) to rapidly detect and/or determine the presence or absence of an impending or current clutch shudder condition, preferably using one or more vibration sensors  41  operatively connected at selected portions of the transmission and clutch assembly  20 , and then apply the AC component HF A  via the clutch-apply piston  30  so that the clutch-apply piston  30  vibrates or resonates at a predetermined frequency. Alternately, the clutch shudder condition is detected and quantified prior to vehicle production, such as during modeling, research, development, and/or pre-production testing, and a predetermined AC-component HF A  is continuously applied via clutch-apply piston  30  while the vehicle is in operation. 
         [0027]    In a second embodiment, HF vibration hardware  40  may be operatively connected to the clutch assembly  20 , preferably directly to the clutch housing  28 , to apply an HF-component HF B , with HF vibration hardware  40  being variably controllable via the controller  32 . HF vibration hardware preferably includes a plurality of simultaneously controllable vibration sources capable of generating and imparting an HF-oscillation or vibration to the clutch housing  28 , each having a different frequency so as to generate a noisy signal rather than a single tone, and attached to clutch housing  28 , such as an outer clutch housing or torque converter cover. Using such a device, clutch dampers (not shown) may be removed to offset any hardware costs and additional weight/space associated with the alternate HF vibration hardware  40 . Alternately, as with the first embodiment, the clutch shudder condition is detected and quantified prior to vehicle production, and a predetermined oscillation or vibration HF B  is continuously applied via HF vibration hardware  40  while the vehicle is in operation. 
         [0028]    A method of minimizing clutch shudder is also shown via the algorithm  105  of  FIG. 5 , which is preferably stored or otherwise programmed into memory  39  within controller  32  (see  FIG. 3 ). In step  110  of the algorithm  105 , the threshold shudder amplitude, noted for simplicity as [A] S THRESHOLD , is set or programmed into memory  39 . The shudder threshold amplitude is preferably selected by first determining the maximum amount or level of clutch shudder that is determined to be permissible or tolerable for a given vehicle design. Step  110  may be a factory-programmable variable, such as determined during pre-production vehicle testing and/or vehicle calibration, or optionally may be user-selectable for input into memory  39 . Once step  110  is complete, the algorithm  105  proceeds to step  112 . 
         [0029]    In step  112 , the controller  32 , using the vibration sensors  41 , detects the natural frequency of the clutch assembly  20  (see  FIG. 3 ) and its associated hydraulics, noted for simplicity as the variable [F] C . To simplify the design and/or programming complexity of the controller  32 , [F] C , which is effectively equivalent to the natural frequency of the powertrain (not shown), may be alternately determined a priori via modeling or simulation, by using a vehicle prototype, and/or by a calibration vehicle, and is stored in memory  39 . The algorithm  105  proceeds to step  114 . 
         [0030]    In step  114 , the controller  32 , using vibration sensors  41 , detects the amplitude of oscillation of any clutch vibration or shudder occurring during relatively low slip speed conditions (see  FIG. 1 ), noted hereinafter for simplicity as the variable [A] S . This quantity is then stored in memory  39 , and the algorithm  105  proceeds to step  116 . 
         [0031]    In step  116 , the controller  32  compares the stored shudder amplitude value [A] S  from the previous step to the stored threshold value, [A] S THRESHOLD  (see step  110 ). If [A] S  is greater than or equal to [A] S THRESHOLD , the algorithm  105  proceeds to step  118 . If, however, if [A] S  is less than the threshold value [A] S THRESHOLD , the algorithm  105  repeats step  114  and  116 . 
         [0032]    In step  118 , the controller  32  initiates the HF vibration or oscillation and applies it to or within the clutch assembly  20 , as previously discussed hereinabove. Preferably, the stored clutch assembly natural frequency value or [F] C  (see step  112 ) is used as an approximate lower boundary or limit of the applied frequency so as to generate a significant response in the slip speed (ν) at the friction interface  27  (see  FIGS. 2A ,  2 B, and  3 ). More specifically, the frequency region closely bounding [F] C  should be avoided so as to prevent exciting the resonant system into a regenerative response. The optimum lower boundary, as will be understood of those of ordinary skill in the art, may be determined for a given clutch assembly by testing and/or calibration, which may vary depending on the particular design of the clutch assembly and associated powertrain. However, other lower boundaries may also be used within the scope of the invention provided the applied HF oscillation is sufficient to break the adhesive bonds  23  (see  FIG. 2B ) as previously described hereinabove, but still having a low enough amplitude so as to not be detected by an occupant of the vehicle. Additionally, the upper boundary should be selected so as not to adversely affect the performance of the clutch-actuation device, such as clutch-apply piston  30  (see  FIG. 3 ), i.e. with attention to the bandwidth limitations of a given actuator. Therefore, the optimum waveform of an applied HF oscillation will ultimately depend on the specific design characteristics of a given vehicle and powertrain. 
         [0033]    Alternatively, under some circumstances initiating the HF vibration before shudder is detected and continuously applying an HF vibration to the clutch assembly  20  may be preferred in order to prevent the shudder from initiating in the first instance, and from subsequently building regeneratively upon itself. With such an alternative, steps  110 ,  112 , and  114  would be accomplished prior to vehicle production, with step  110  preferably setting [A] S THRESHOLD  at a low or near zero level to ensure continuous or constant application of the HF component upon vehicle start up. In this manner, step  114  would always immediately proceed to step  118 , i.e. application of the HF oscillation in a continuous or sustained manner upon vehicle start up, at a predetermined frequency and amplitude HF A  and/or HF B  suitable for minimizing the predetermined shudder condition. 
         [0034]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.