Abstract:
An actuator device for a selectable clutch having three or more clutch modes may include a dual rate piston with a stable middle position. A positive stop provides very accurate control when shifting from an end position to a middle position without overshoot. Precision position control of the actuator device facilitates consistent, stable control of the current mode of the selectable clutch.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This patent application claims priority under 35 USC §119(e) to U.S. Provisional Patent Application Ser. No. 62/302,110 filed on Mar. 1, 2016. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates generally to clutches, and in particular to clutches having multiple modes of engagement with a rotating element for selectively locking the element against rotation and allowing the element to rotate freely in one or both directions. 
       BACKGROUND 
       [0003]    An automotive vehicle typically includes an internal combustion engine containing a rotary crankshaft configured to transfer motive power from the engine through a driveshaft to turn the wheels. A transmission is interposed between engine and driveshaft components to selectively control torque and speed ratios between the crankshaft and driveshaft. In a manually operated transmission, a corresponding manually operated clutch may be interposed between the engine and transmission to selectively engage and disengage the crankshaft from the driveshaft to facilitate manual shifting among available transmission gear ratios. 
         [0004]    On the other hand, if the transmission is automatic, the transmission will normally include an internal plurality of automatically actuated clutch units adapted to dynamically shift among variously available gear ratios without requiring driver intervention. Pluralities of such clutch units, also called clutch modules, are incorporated within such transmissions to facilitate the automatic gear ratio changes. 
         [0005]    In an automatic transmission for an automobile, anywhere from three to ten forward gear ratios may be available, not including a reverse gear. The various gears may be structurally comprised of inner gears, intermediate gears such as planet or pinion gears supported by carriers, and outer ring gears. Specific transmission clutches may be associated with specific sets of the selectable gears within the transmission to facilitate the desired ratio changes. 
         [0006]    Because automatic transmissions include pluralities of gear sets to accommodate multiple gear ratios, the reliability of actuators used for automatically switching clutch modules between and/or among various available operating modes is a consistent design concern. It is also desirable to provide smooth transitions between the operating modes when the clutch modules engage and disengage from the gears. These considerations are also important in other operating environments where selectable clutch modules may be implemented to selectively allow and restrict the rotation of rotating components such as gears, shafts, torque converter components and the like. Therefore, much effort has been directed to finding ways to assure actuator reliability and seamless performance at competitive costs. 
       SUMMARY OF THE DISCLOSURE 
       [0007]    In one aspect of the present disclosure, an actuator device for a selectable clutch having a plurality of mode positions for controlling relative rotation between two components connected by the selectable clutch is disclosed. The actuator device may include a piston housing having an exterior surface, a piston housing longitudinal bore extending longitudinally there through, a first fluid passage extending inwardly from the exterior surface and intersecting the piston housing longitudinal bore proximate a first bore end, and a second fluid passage extending inwardly from the exterior surface and intersecting the piston housing longitudinal bore proximate a second bore end. The actuator device may further include a main piston disposed within the piston housing longitudinal bore for longitudinal motion therein. The main piston may have a main piston main portion having a main piston main portion outer diameter and being disposed between the first fluid passage and the second fluid passage, and a main piston secondary portion having a main piston secondary portion outer diameter that is less than the main piston main portion outer diameter and extending longitudinally from the main piston main portion toward the second fluid passage. The actuator device may also include a secondary piston having a secondary piston longitudinal bore such that the secondary piston is disposed and slidable on the main piston secondary portion. The secondary piston may include a secondary piston main portion having a secondary piston main portion outer diameter that is less than the main piston main portion outer diameter, and a secondary piston secondary portion having a secondary piston secondary portion outer diameter that is less than the secondary piston main portion outer diameter and being disposed between the main piston main portion and the secondary piston main portion. The actuator device may still further include a stop snap ring having an annular shape and a stop snap ring inner diameter that is less than the secondary piston main portion outer diameter and greater than the secondary piston secondary portion outer diameter, wherein the stop snap ring is fixed within the piston housing longitudinal bore between the main piston main portion and the secondary piston main portion and with the secondary piston secondary portion extending there through. A first pressure force acting on the main piston toward the second bore end may be equal to a first pressure supplied at the first fluid passage multiplied by a first area equal to a main piston cross-sectional area of the main piston main portion. A second pressure force acting on the main piston toward the first bore end when the secondary piston main portion is not engaged by the stop snap ring may be equal to a second pressure supplied at the second fluid passage multiplied by a second area equal to a combined cross-sectional area of a main piston secondary portion cross-sectional area and a secondary piston main portion cross-sectional area. A third pressure force acting on the main piston toward the first bore end when the secondary piston main portion is engaged by the stop snap ring may be equal to the second pressure supplied at the second fluid passage multiplied by a third area equal to the main piston secondary portion cross-sectional area. 
         [0008]    In another aspect of the present disclosure, a selectable clutch is disclosed. The selectable clutch may include an outer race, an inner race rotatable relative to the outer race, a selective locking mechanism having a plurality of locking modes for controlling relative rotation between two components connected by the selectable clutch, an actuator cam that is rotatable between a plurality of mode positions each causing the selective locking mechanism to engage one of the plurality of locking modes, and an actuator device such as that described in the preceding paragraph operatively connected to the actuator cam to move the selective locking mechanism between the plurality of mode positions as the main piston moves longitudinally within the piston housing longitudinal bore. 
         [0009]    In a further aspect of the present disclosure, an actuator device for a selectable clutch having a plurality of mode positions for controlling relative rotation between two components connected by the selectable clutch is disclosed. The actuator device may include a piston housing having an exterior surface, a piston housing longitudinal bore extending longitudinally there through, a first fluid passage extending inwardly from the exterior surface and intersecting the piston housing longitudinal bore proximate a first bore end, and a second fluid passage extending inwardly from the exterior surface and intersecting the piston housing longitudinal bore proximate a second bore end, and a main piston disposed within the piston housing longitudinal bore for longitudinal motion therein. The main piston may include a main piston main portion having a main piston main portion outer diameter and being disposed between the first fluid passage and the second fluid passage, and a main piston secondary portion having a main piston secondary portion outer diameter that is less than the main piston main portion outer diameter and extending longitudinally from the main piston main portion toward the second fluid passage. The actuator device may further include a secondary piston having a secondary piston outer diameter and a secondary piston longitudinal bore such that the secondary piston is disposed and slidable on the main piston secondary portion within the piston housing longitudinal bore, and a stop snap ring fixed within the piston housing longitudinal bore between the main piston main portion and the second bore end and having an annular shape and a stop snap ring inner diameter that allow at least a portion of the secondary piston to pass through the stop snap ring. A first pressure force acting on the main piston toward the second bore end is equal to a first pressure supplied at the first fluid passage multiplied by a first area equal to a main piston cross-sectional area of the main piston main portion, a second pressure force acting on the main piston toward the first bore end when the secondary piston is not engaged by the stop snap ring is equal to a second pressure supplied at the second fluid passage multiplied by a second area equal to a combined cross-sectional area of a main piston secondary portion cross-sectional area and a secondary piston main portion cross-sectional area, and the second pressure force acting on the main piston when the secondary piston is engaged by the stop snap ring is equal to the second pressure supplied at the second fluid passage multiplied by a third area equal to the main piston secondary portion cross-sectional area. 
         [0010]    Additional aspects are defined by the claims of this patent. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is both a perspective and a cross-sectional view of a portion of one possible embodiment of a selectable in the form of a multimode clutch module that may be implemented in vehicles; 
           [0012]      FIG. 2  is an enlarged side view of a portion of one possible embodiment of the multimode clutch module of  FIG. 1  with the near inner race plate removed to reveal the internal components, and with an actuator cam in a one-way locked, one-way unlocked position; 
           [0013]      FIG. 3  is the enlarge view of one possible embodiment of the multimode clutch module of  FIG. 1  with the actuator cam in a two-way unlocked position; 
           [0014]      FIG. 4  is the enlarge view of the multimode clutch module of  FIG. 1  with the actuator cam in a two-way locked position; 
           [0015]      FIG. 5  is a cross-sectional view taken through line  5 - 5  of  FIG. 2  of an embodiment of an actuator device in accordance with the present disclosure in position to place the actuator cam in the one-way locked, one-way unlocked position; 
           [0016]      FIG. 6  is a cross-sectional view taken through line  6 - 6  of  FIG. 3  of the embodiment of the actuator device in position to place the actuator cam in the two-way unlocked position; 
           [0017]      FIG. 7  is a cross-sectional view taken through line  7 - 7  of  FIG. 4  of the embodiment of the actuator device in position to place the actuator cam in the one-way locked, one-way unlocked position; and 
           [0018]      FIG. 8  is a graph of a first pressure that is controlled versus a displacement of a main piston of the actuator device of  FIGS. 5-7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    In accordance with the present disclosure, a selectable clutch, such as a multimode clutch module, may be implemented at various locations of a vehicle (not shown) to provide multiple modes for connecting and disconnecting rotatable components to prevent or allow, respectively, relative rotation between two components. Referring to  FIG. 1 , a multimode clutch  10  of a vehicle may be of the type illustrated and described in Intl. Publ. No. WO 2014/120595 A1, published on Aug. 7, 2014, by Papania, entitled “Multi-Mode Clutch Module,” which is expressly incorporated by reference herein. While the multimode clutch  10  is illustrated and described herein, those skilled in art will understand that actuator devices in accordance with the present disclosure may be implemented with other types of selectable clutches providing multiple modes for connecting and disconnecting rotatable components to prevent or allow, respectively, relative rotation between two components, and the use of the actuator device with such selectable clutches is contemplated by the inventors. In the illustrated embodiment, the multimode clutch  10  may incorporate an interior driven hub  50  and an outer housing  52  that may be locked for rotation together in some modes of the multimode clutch  10  and may be unlocked for independent rotation with respect to each other in other modes of the multimode clutch  10  as will be described more fully below. The driven hub  50  may contain an array of circumferentially spaced cogs  54  adapted to secure an inner race  56  to the driven hub  50  for rotation therewith. As disclosed, the inner race  56  is comprised of first and second spaced plates  56 A and  56 B. An outer race  58  sandwiched between the pair of inner race plates  56 A,  56 B, is situated so as to allow for relative rotation between inner race  56  and the outer race  58 , and with the outer race  58  being operatively coupled to the outer housing  52  for rotation therewith. 
         [0020]    In the present design of the multimode clutch  10 , an actuator cam  60  is interposed between one of the race plates  56 A,  56 B and the outer race  58  for rotation over a predetermined angle about a common axis of the driven hub  50  and the outer housing  52  to control movements of pairs of opposed pawls  62 ,  64  as will be described further hereinafter. The sets of pawls  62 ,  64  are trapped, and hence retained, between the inner race plates  56 A,  56 B to allow limited angular movements of the pawls  62 ,  64  held within bowtie shaped apertures  66 ,  68 , respectively, subject to the control of the actuator cam  60 . In each set, the combined pawl  62  and corresponding aperture  66  is similar to but oppositely oriented to the combined pawl  64  and corresponding aperture  68 . The elements of the multimode clutch  10  are contained within the outer housing  52 . A plurality of spaced apertures  70  are adapted to accommodate rivets (not shown) for providing fixed and rigid securement of each of the two inner race plates  56 A and  56 B relative to the other. 
         [0021]    The operational components of the multimode clutch  10  are illustrated in  FIGS. 2-4  that illustrate the various operational modes of the multimode clutch  10  for controlling the relative rotation between the components attached to the driven hub  50  and the outer housing  52 . Referring first to  FIG. 2 , the outer race  58  is configured to accommodate interactions with the pawls  62 ,  64  by providing the inner circumference of the outer race  58  with circumferentially spaced notches  72 , each defined by and positioned between pairs of radially inwardly projecting cogs  74 . The notches  72  and cogs  74  are configured so that, in the absence of the actuator cam  60 , a toe end  76  of each pawl  62  enters one of the notches  72  and is engaged by the corresponding cog  74  when the driven hub  50  and the inner race  56  rotate in a clockwise direction as viewed in  FIG. 2  relative to the outer housing  52  and the outer race  58  to cause the connected components to rotate together. Similarly, a toe end  78  of each pawl  64  enters one of the notches  72  and is engaged by the corresponding cog  74  when the driven hub  50  and the inner race  56  rotate in a counterclockwise direction relative to the outer housing  52  and the outer race  58  to cause the connected components to rotate together. 
         [0022]    Within its interior periphery, the actuator cam  60  incorporates a strategically situated array of circumferentially spaced recesses, herein called slots  80 , defined by and situated between projections, herein called cam teeth  82 . The slots  80  and cam teeth  82  are adapted to interact with the pawls  62 ,  64  to control their movement within the apertures  66 ,  68 , respectively, and disposition within the notches  72  and engagement by the cogs  74  as will be described. The actuator cam  60  may further include an actuator tab  84  or other appropriate member or surface that may be engaged by an actuator device  100  that is capable of causing the actuator cam  60  to move through its rotational range to the positions shown in  FIGS. 2-4 . The actuator device  100  may be any appropriate actuation mechanism capable of moving the actuator cam  60 , such as a hydraulic actuator as illustrated and described below operatively coupled to the actuator cam  60  and capable of rotating the actuator cam  60  to multiple positions. The actuator tab  84  may include a radially extending slot  85  that receives a cam actuator bar  102  extending from a longitudinally extending slot  104  of the actuator device  100 . The cam actuator bar  102  may transmit forces from the actuator device  100  to rotate the actuator cam  60  in the clockwise and counterclockwise directions. The interconnection between the actuator cam  60  and the actuator device  100  is illustrative, and alternative arrangements and linkages facilitating conversion of translational motion of the actuator device  100  into rotational motion of the actuator cam  60  to shift between a plurality of available clutch modes are contemplated and will be apparent to those skilled in the art. In the illustrated embodiment, the actuator tab  84  may be disposed within a slot  86  through the outer race and the rotation of the actuator cam  60  may be limited by a first limit surface  88  engaging the actuator tab  84  at the position shown in  FIG. 2  and a second limit surface  90  engaging the actuator tab  84  at the position shown in  FIG. 4 . 
         [0023]    The pawls  62 ,  64  are asymmetrically shaped, and reversely identical. Each of the opposed pawls  62 ,  64  is movably retained within its own bowtie-shaped pawl aperture  66 ,  68 , respectively, of the inner race plates  56 A and  56 B. The toe end  76 ,  78  of each individual pawl  62 ,  64 , respectively, is urged radially outwardly via a spring  92 . Each spring  92  has a base  94 , and a pair of spring arms  96  and  98 . The spring arms  96  bear against the bottoms of the pawls  62 , while the spring arms  98  bear against the bottoms of the pawls  64 , each to urge respective toe ends  76 ,  78  into engagement with the cogs  74  of the outer race  58  when not obstructed by the cam teeth  82  of the actuator cam  60 . It will be appreciated from  FIG. 2  that axially extending rivets  99  are used to secure the inner race plates  56 A,  56 B together. The rivets  99  extend through the apertures  70  in each of the plates  56 A,  56 B to hold the two plates  56 A,  56 B rigidly together, and to thus assure against any relative rotation with respect to the plates  56 A,  56 B. In lieu of the rivets  99 , other structural fasteners may be employed within the scope of this disclosure to secure the inner race plates  56 A,  56 B. 
         [0024]    It will be appreciated that the actuator device  100  ultimately controls the actuator tab  84  which, in turn, moves the actuator cam  60  between multiple distinct angular positions. Thus, the positioning of the pawls  62 ,  64  as axially retained between the riveted inner race plates  56 A,  56 B is directly controlled by the actuator cam  60  against forces of springs  92 . In  FIG. 2 , the actuator tab  84  is shown positioned by the actuator device  100  in a first, angularly rightward selectable position, representative of a first, one-way locked, one-way unlocked or open mode. In this position, the slots  80  and cam teeth  82  of the actuator cam  60  are positioned so that the toe ends  76  of the pawls  62  are blocked by cam teeth  82  from engagement with notches  72 , and hence with the cogs  74  on the interior of the outer race  58 . As such, the inner race  56  is enabled to freewheel relative to the outer race  58 , and to thus provide for an overrunning condition when the inner race  56  and the driven hub  50  are rotating clockwise relative to the outer race  58  and the outer housing  52 . Conversely, however, the position of the actuator cam  60  allows of the toe ends  78  of the pawls  64  to enter the slots  80  of the actuator cam  60  due to the biasing force of the spring arms  98 , and to thereby directly engage the cogs  74  of the outer race  58  to lock the inner race  56  and the outer race  58  together whenever the inner race  56  and the driven hub  50  undergo a driving, or counterclockwise rotational movement, thereby causing the driven hub  50  and the outer housing  52  to rotate together. 
         [0025]      FIG. 3  illustrates the actuator tab  84  placed by the actuator device  100  in a second, intermediate selectable position, representative of a two-way unlocked or open mode of the multimode clutch  10 . In this position, the slots  80  and the cam teeth  82  of the actuator cam  60  are positioned to prevent the toe ends  76 ,  78  of both pawls  62 ,  64  from entering the slots  80  of the actuator cam  60 , and to maintain disengagement from the cogs  74  of the outer race  58 . With the pawls  62 ,  64  blocked from engagement with the cogs  74 , the inner race  56  and the driven hub  50  are enabled to freewheel relative to the outer race  58  and the outer housing  52  during relative rotation in either the clockwise or the counterclockwise direction. 
         [0026]    In  FIG. 4 , the actuator tab  84  is shown in a third, angularly leftward selectable position, representative of a two-way locked mode of the multimode clutch  10 . In this configuration, the actuator cam  60  is positioned so that the toe ends  76 ,  78  of both pawls  62 ,  64  enter the slots  80  of the actuator cam  60  under the biasing forces of the spring arms  96 ,  98 , respectively, and are engaged by the cogs  74  of the outer race  58  as described above to lock the inner race  56  and the driven hub  50  to the outer race  58  and the outer housing  52  for rotation therewith, irrespective of the rotational direction of the inner race  56  and the driven hub  50 . 
         [0027]    Even though one specific embodiment of the multimode clutch  10  is illustrated and described herein, those skilled in the art will understand that alternative configurations of multimode clutches and other selectable clutches are possible that provide operational modes or positions as alternatives or in addition to two-way unlocked and two-way locked modes ( FIGS. 3 and 4 ), and the one-way locked, one-way unlocked mode ( FIG. 2 ). For example, an additional one-way locked, one-way unlocked mode that may provide for an overrunning condition when the inner race  56  and the driven hub  50  are rotating counter clockwise relative to the outer race  58  and the outer housing  52 , and to lock the inner race  56  and the outer race  58  together whenever the inner race  56  and the driven hub  50  undergo a clockwise rotational movement so the driven hub  50  and the outer housing  52  rotate together. Moreover, alternate structures providing some or all of the modes discuss herein for selectable clutches may be implemented in a similar manner in vehicles, such as that illustrated and described in U.S. Pat. No. 8,079,453, published on Dec. 20, 2011, by Kimes, entitled “Controllable Overrunning Coupling Assembly.” The implementation of such alternative selectable clutches in vehicles and controlling the mode switching using such clutches with actuator devices in accordance with the present disclosure would be within the capabilities of those skilled in the art and is contemplated by the inventors. 
         [0028]      FIG. 5  illustrates one embodiment of the actuator device  100  shown in a cross-sectional view taken through line  5 - 5  of  FIG. 2 . The actuator device  100  may include a piston housing  110  having a longitudinal bore  112  extending inwardly into the piston housing  110  from an open end  114  to a closed end  116  disposed opposite the open end  114 . The longitudinal bore  112  may have several transitions of an inner diameter as the longitudinal bore  112  extends inwardly to accommodate various internal components of the actuator device  100 . The longitudinal bore  112  may include a cap bore portion  118  proximate the open end  114  that transitions to a main bore portion  120  having a smaller inner diameter at a first bore shoulder  122 , and the main bore portion  120  may transition to a secondary bore portion  124  having a smaller inner diameter at a second bore shoulder  126 . The secondary bore portion  124  may transition to a spring retention portion  128  having a smaller inner diameter at a third bore shoulder  130  and a bore countersunk surface  132 , and the spring retention portion  128  may terminate at a bore end wall  134 . The longitudinal bore  112  may further define a cap snap ring annular groove  136  in the cap bore portion  118  having a larger inner diameter than the cap bore portion  118 , and a stop snap ring annular groove  138  in the secondary bore portion  124  having a larger inner diameter than the secondary bore portion  124 . The functions of the annular grooves  136 ,  138  are explained further below. 
         [0029]    Additional passages may be defined in the piston housing  110 . The longitudinal slot  104  may extend inwardly from an exterior surface  140  of the piston housing  110  and intersect the longitudinal bore  112  at the main bore portion  120  proximate the second bore shoulder  126  and the secondary bore portion  124 . A first fluid passage  142  may extend inwardly from the exterior surface  140  and intersect the main bore portion  120  proximate the first bore shoulder  122 . A second fluid passage  144  may extend inwardly from the exterior surface  140  and intersect the spring retention portion  128 . The first fluid passage  142  and the second fluid passage  144  may be configured for connection to conduits (not shown) from fluid sources (not shown) of the vehicle for provision hydraulic fluid to the main bore portion  120  and the spring retention portion  128 , respectively. As discussed further below, one or both of the fluid passages  142 ,  144  may be connected to pressurized fluid sources providing hydraulic fluid with varying pressures to control the operation of the actuator device  100  and, correspondingly, the multimode clutch  10 . 
         [0030]    The actuator device  100  may include a main piston  150  disposed within the longitudinal bore  112  and slidable back and forth in the longitudinal direction within the longitudinal bore  112 . The main piston  150  may include a main piston main portion  152  disposed within the main bore portion  120 . The main piston main portion  152  may have an outer diameter that is less than the inner diameter of the main bore portion  120  so that the main piston main portion  152  may slide therein without leakage of hydraulic fluid there between. If necessary, appropriate seals (not shown) may be provided at the interface between the main bore portion  120  and the main piston main portion  152  to further prevent leakage of hydraulic fluid. The main piston main portion  152  may have an actuator bar bore  154  extending radially inwardly into the main piston main portion  152  and aligned with the longitudinal slot  104  to receive and retain an end of the cam actuator bar  102 . The main piston  150  may transition from the main piston main portion  152  to a main piston secondary portion  156  having a smaller outer diameter at a first main piston shoulder  158 , and then transitioning to a main piston spring portion  160  having a still smaller outer diameter at a second main piston shoulder  162 . 
         [0031]    The outer diameter of the main piston secondary portion  156  may be smaller than the inner diameter of the secondary bore portion  124  so that a secondary piston  170  may be disposed there between. The secondary piston  170  may have a secondary piston longitudinal bore  172  there through having an inner diameter that is greater than the outer diameter of the main piston secondary portion  156  so that the main piston  150  and the secondary piston  170  may slide longitudinally relative to each other without leakage of hydraulic fluid there between. If necessary, appropriate seals (not shown) may be provided at the interface between the main piston secondary portion  156  and the secondary piston longitudinal bore  172  to further prevent leakage of hydraulic fluid. The secondary piston  170  may include a secondary piston main portion  174  having an outer diameter that is less than the inner diameter of the secondary bore portion  124  so that the secondary piston main portion  174  may slide therein without leakage of hydraulic fluid there between. If necessary, appropriate seals (not shown) may be provided at the interface between the secondary bore portion  124  and the secondary piston main portion  174  to further prevent leakage of hydraulic fluid. The secondary piston  170  may transition from the secondary piston main portion  174  to a secondary piston secondary portion  176  having a smaller outer diameter at a secondary piston shoulder  178 . The secondary piston secondary portion  176  may be disposed between the secondary piston main portion  174  and the main piston main portion  152  to function as described more fully below. 
         [0032]    The outer diameter of the main piston spring portion  160  may be smaller than the inner diameter of the spring retention portion  128  so that a piston spring  180  may be disposed there between. The piston spring  180  may be compressed between the bore end wall  134  and the second main piston shoulder  162  to provide a force biasing the main piston  150  toward the open end  114  of the longitudinal bore  112 . The main piston  150  may be retained within the longitudinal bore  112  by a cap  182  inserted through the open end  114  of the longitudinal bore  112  and engaged by the first bore shoulder  122 . The cap  182  may be held in place by a cap snap ring  184 . The cap snap ring  184  may be annular and have an outer diameter that is greater than the inner diameter of the cap bore portion  118 , and may be pressed into the cap snap ring annular groove  136  to lock the cap  182  in place. The actuator device  100  may further include a stop snap ring  186  that may be annular and may have an outer diameter that is greater than the inner diameter of the secondary bore portion  124  so that the stop snap ring  186  may be pressed into the stop snap ring annular groove  138 . The stop snap ring  186  may have an inner surface with an inner diameter that is greater than the outer diameter of the secondary piston secondary portion  176  so that the stop snap ring  186  may be disposed over the secondary piston secondary portion  176  without engaging the secondary piston secondary portion  176  and restricting longitudinal movement of the main piston  150  and the secondary piston  170 . 
         [0033]    In the illustrated embodiment, the position of the main piston  150 , the cam actuator bar  102  and, correspondingly, the actuator cam  60  will be dictated by a first pressure P 1  at the first fluid passage  142 , a second pressure P 1  at the second fluid passage  144 , and the amount of compression of the piston spring  180 . The first pressure P 1  acts on the main piston main portion  152  to exert a first pressure force F 1  to the right in as seen in  FIG. 5 , and has a magnitude equal to P 1 ×A 1 , where A 1  is the cross-sectional area of the main piston main portion  152 . The second pressure P 2  acts on the main piston secondary portion  156  and the main piston spring portion  160 , as well as the secondary piston main portion  174 , to exert a second pressure force F 2  on the main piston  150  to the left. The second pressure force F 2  has a magnitude equal to P 2 ×A 2 , where A 2  is the combined cross-sectional area of the main piston secondary portion  156  and the secondary piston main portion  174 . Finally, the piston spring  180  exerts a spring force FS on the main piston  150  to the left having a magnitude equal to kX, where k is the spring constant for the piston spring  180  and X is the amount of compression of the piston spring  180 . It is contemplated that the spring constant k will have a constant value over the operating range of the actuator device  100 . 
         [0034]    In the present example, the second pressure P 2  has a value that is approximately constant and equal to a system pressure of the vehicle that is known to the control system causing changes in the position of the actuator device  100  and the mode of the multimode clutch  10 . The first pressure P 1  is a control pressure that may be varied by controlling an output pressure of a pressurized hydraulic fluid source (not shown) in fluid communication with the first fluid passage  142 . As a result, the first pressure P 1  is controlled and varied to move the main piston  150  and the cam actuator bar  102 . 
         [0035]    As seen in  FIG. 5 , the main piston  150  is moved to the right with the first main piston shoulder  158  engaged by the second bore shoulder  126 . In this position, the cam actuator bar  102  has moved the actuator cam  60  to the first mode position shown in  FIG. 2 . The force equation for this position may be expressed as F 1 &gt;F 2 +FS, or P 1 *A 1 &gt;P 2 *A 2 +kX. Holding the first pressure P 1  constant, or increasing the first pressure P 1 , will maintain the main piston  150  at the right limit position and keep the multimode clutch  10  in the first mode. 
         [0036]    When a controller (not shown) of the vehicle detects that the multimode clutch  10  should move to a second mode such as that shown in  FIG. 3 , the controller may cause the pressurized hydraulic fluid source to reduce the first pressure P 1 . When the force equation changes to F 1 &lt;F 2 +FS, or P 1 *A 1 &lt;P 2 *A 2 +kX, the second pressure force F 2  and the spring force FS may overcome the first pressure force F 1  and cause the main piston  150  to begin to move to the left toward the second mode position shown in  FIG. 6 . At the position of  FIG. 6 , the secondary piston shoulder  178  has moved into engagement with the stop snap ring  186  and cannot move further to the left. As a consequence, the force applied to the secondary piston  170  is borne by the stop snap ring  186 , and is no longer transferred to the main piston  150 . Only the pressure force applied by the second pressure P 2  to the main piston secondary portion  156  and the main piston spring portion  160  acts on the main piston  150 . At this point, the magnitude of the second pressure force F 2  changes to P 2 ×A 3 , where A 3  is the cross-sectional area of the main piston secondary portion  156 . The area A 3  is smaller than the area A 2  and causes an instantaneous drop in the second pressure force F 2  so that the force equation converts to F 1 &gt;F 2 +FS, or P 1 *A 1 &gt;P 2 *A 3 +kX. 
         [0037]    Due to the transition to the smaller cross-sectional area A 3 , the main piston  150  will remain at the second mode position of  FIG. 6  until the first pressure P 1  decreases to a value where the force equation changes to F 1 &lt;F 2 +FS, or P 1 *A 1 &lt;P 2 *A 3 +kX. The magnitude of the change from the area A 2  to the area A 3  will dictate the necessary reduction of the first pressure P 1  necessary to shift the equation and cause the main piston  150  to again move to the left toward the third mode position shown in  FIG. 7  where the main piston  150  engages the cap  182  to define a hard stop at which the main piston  150  and the cam actuator bar  102  move the actuator cam  60  to the third mode position shown in  FIG. 4 . When the controller determines that the actuator device  100  should move the right form the third mode position to either the first or the second mode position, the controller may cause the pressurized hydraulic fluid source to increase the first pressure P 1  so that the first pressure force F 1  exceeds the sum of the second pressure force F 2  plus the spring force FS. 
       INDUSTRIAL APPLICABILITY 
       [0038]    The actuator device  100  in accordance with the present disclosure may eliminate the need for a position sensor to provide feedback to the selectable clutch control strategy of the position of the actuator cam  60 , the cam actuator bar  102  or the main piston  150 , while still allowing for precise precision control of the actuator device  100 .  FIG. 8  provides a graph  190  of the first pressure P 1  versus the displacement of the main piston  150 . A first point  192  on the graph  190  represents a first equilibrium point where the main piston  150  is disposed in the first mode position of  FIG. 5 . At the first point  192 , the force equation is P 1 *A 1 =P 2 *A 2 +kX such that any decrease in the first pressure P 1  will cause the main piston  150  to move to the left, and any increase in the first pressure P 1  will increase the force of the first main piston shoulder  158  against the second bore shoulder  126 , but will not result in further displacement to the right. 
         [0039]    At a second point  194  on the graph  190  represents a second equilibrium point where the main piston  150  arrives at the second mode position of  FIG. 6  from the first mode position. The force equation utilizing the area A 2  is also P 1 *A 1 =P 2 *A 2 +kX such that any increase in the first pressure P 1  will cause the main piston  150  to move to the right, and any decrease in the first pressure P 1  will reduce the first pressure force F 1  against the main piston  150 , but will not yet result in further displacement to the left because the force equation utilizing the area A 3  is also P 1 *A 1 &gt;P 2 *A 3 +kX. The main piston  150  will remain in place as the first pressure P 1  drops until reaching a third point  196  representing a third equilibrium point where P 1 *A 1 =P 2 *A 3 +kX. At pressures below the third point  196 , the main piston  150  may again move to the left until reaching the third mode position of  FIG. 7  at a fourth point  198 . 
         [0040]    This arrangement of the actuator device  100  facilitates precise control of the position of the main piston  150  without requiring precise control of the first pressure P 1 . As shown in the graph  190  of  FIG. 8 , the positive stop provided by the stop snap ring  186  and corresponding transition between the area A 2  and the area A 3  allow the first pressure P 1  to have any value within a dead band range between the second point  194  and the third point  196  and maintain the main piston in the second mode position. The size of the dead band range may be varied as necessary by adjusting the areas A 2 , A 3  to yield a desired responsiveness of the actuator device  100  around the second mode position and the precision required in controlling the first pressure P 1  from the pressurize hydraulic fluid source. 
         [0041]    Those skilled in the art will understand that the configuration of the actuator device  100  and the control strategy described herein are exemplary, and modifications of the design are contemplated. For example, in alternative embodiments, the first pressure P 1  may be held constant and the second pressure P 2  may be controlled to move the main piston  150  to the right (decrease the second pressure P 2 ) and to the left (increase the second pressure P 2 . In further alternatives, both pressures P 1 , P 2  may be controlled so that a pressure differential is varied to move the main piston  150 . Also, while three clutch modes are illustrated and described herein, those skilled in the art will understand that the actuator device  100  may be configured with additional positive stops at which a transition between sizes of areas acted upon by pressures occur to create additional dead band ranges where the main piston  150  stops at a mode position for the multimode clutch  10 . Such variations are contemplated by the inventors as having use in actuator devices in accordance with the present disclosure. 
         [0042]    The design may also be varied in terms of the location and presence of the piston spring  180 . The piston spring  180  may be moved to other locations in and around the actuator device  100  while still having an effect on the graph  190  and the response and control of the main piston  150 . For example, the piston spring  180  could be moved to the opposite side of the main piston  150  and positioned between the main piston  150  and the cap  182 . In this position, the piston spring  180  would bias the main piston  150  toward the one-way locked, one-way unlocked position of  FIG. 5 . In these embodiments, the spring force FS would be subtracted from the second pressure force F 2  in the equations discussed above. With the spring force FS assisting the first pressure force F 1  in moving the main piston  150  to the right, the curve of the graph  190  will move downward and lower first pressures P 1  will need to be generated to move the main piston  150  between the locking positions. It is also contemplated that piston springs  150  may be installed on both sides of the main piston  150  so that the main piston  150  is biased to an intermediate locking positions such as that shown in  FIG. 6 . In such embodiments, factors representing the spring force FS will appear on both sides of the force balancing equations, and the first pressure F 1  will be adjusted accordingly in the control strategy for the actuator device  100 . 
         [0043]    In other embodiments, the piston spring  180  may be located external to the piston housing  110 , and still be operatively connected to the cam actuator bar  102  to provide the spring force FS to the main piston  150 . For example, the piston spring  180  may be coupled between a stationary portion of the vehicle, such as the vehicle frame, and the cam actuator bar  102 . Alternatively, the piston spring  180  may be connected between the stationary structure and the cam actuator  60  that will transfer the spring force FS of the piston spring  180  to the main piston  150  through the intervening connection provided by the cam actuator bar  102 . Such external arrangements of the piston spring  180  can function to apply the spring force FS in either direction to either work against or assist the first pressure force F 1  in moving the main piston  150  between the locking positions, or in both directions to bias the main piston  150  toward an intermediate locking position. 
         [0044]    In further alternative embodiments, the piston spring  180  may be omitted so that no spring force FS acts on the main piston  150 . In such embodiments, the controlled first pressure P 1  will be adjusted accordingly to reflect the absence of the spring force FS from the force balancing equations discussed above. With the spring force FS omitted, the curve of the graph  190  will move downward by an amount that is less than in the situation above where the spring force FS is shifted to assisting the first pressure force F 1 , but removal of the piston spring  180  will still lower first pressures P 1  required to move the main piston  150  between the locking positions. 
         [0045]    While the preceding text sets forth a detailed description of numerous different embodiments, it should be understood that the legal scope of protection is defined by the words of the claims set forth at the end of this patent. The detailed description is to be construed as exemplary only and does not describe every possible embodiment since describing every possible embodiment would be impractical, if not impossible. Numerous alternative embodiments could be implemented, using either current technology or technology developed after the filing date of this patent, which would still fall within the scope of the claims defining the scope of protection. 
         [0046]    It should also be understood that, unless a term was expressly defined herein, there is no intent to limit the meaning of that term, either expressly or by implication, beyond its plain or ordinary meaning, and such term should not be interpreted to be limited in scope based on any statement made in any section of this patent (other than the language of the claims). To the extent that any term recited in the claims at the end of this patent is referred to herein in a manner consistent with a single meaning, that is done for sake of clarity only so as to not confuse the reader, and it is not intended that such claim term be limited, by implication or otherwise, to that single meaning.