Abstract:
An apparatus for transferring power includes a sensor circuit for identifying a position of an axially-movable locking element. The axially-movable element is movable between a first position, which inhibits transmission of rotary power through the apparatus, and a second position that permits transmission of rotary power through the apparatus. The sensor circuit includes two Hall-effect sensors that are employed to sense a position of the axially-movable locking element and produce sensor signals that are employed to control a magnitude of the current that is output from the sensor circuit. A method of detecting an operating mode of a power transfer device that is configured to transmit rotary power is also provided.

Description:
[0001]    This application is a continuation of U.S. patent application Ser. No. 11/359,907 filed Feb. 22, 2006 (issued as U.S. Pat. No. 7,507,176 on Mar. 24, 2009), which is a continuation-in-part of U.S. patent application Ser. No. 11/137,997 filed May 26, 2005 (issued as U.S. Pat. No. 7,211,020), the disclosures of which are hereby incorporated by reference as if fully set forth in detail herein in their entirety. 
     
    
     BACKGROUND 
       [0002]    The present disclosure generally relates to differentials for motor vehicles and, more particularly, to a power transfer device having a sensor circuit with dual sensors for identifying a locking state of the power transfer device. 
         [0003]    As is known, many motor vehicles are equipped with driveline systems including differentials which function to drivingly interconnect an input shaft and a pair of output shafts. The differential functions to transmit drive torque to the output shafts while permitting speed differentiation between the output shafts. 
         [0004]    Conventional differentials include a pair of side gears fixed for rotation with the output shafts and two or more sets of meshed pinion gears mounted within a differential case. However, the conventional differential mechanism has a deficiency when a vehicle is operated on a slippery surface. When one wheel of the vehicle is on a surface having a low coefficient of friction, most or all of the torque will be delivered to the slipping wheel. As a result, the vehicle often becomes immobilized. 
         [0005]    To overcome this problem, it is known to provide a mechanical differential having an additional mechanism that limits or selectively prevents differentiation of the speed between the output shafts. Typically, the mechanical device used to provide the limited-slip or non-slip function is a friction clutch. The friction clutch is a passive device which limits the differential speed between the output shafts only after a certain differential speed has been met. Additionally, such mechanical devices may not be selectively disengaged during operation of anti-lock braking systems or vehicle traction control systems. For example, four-wheel anti-lock braking systems may attempt to measure and control the rotational speed of each wheel independently. If a mechanical type limited slip differential is present, independent control of the speed of each wheel coupled to a differential is no longer possible. Accordingly, it would be desirable to provide an improved differential which may be actively controlled in conjunction with other control systems present on the vehicle. A detection system operable to determine the present state of operation of the differential may also be desirable. 
       SUMMARY 
       [0006]    This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
         [0007]    In one form, the present teachings provide an apparatus for transferring power. The apparatus includes a first power transmitting element, a second power transmitting element, a housing, a first locking element received in the housing and coupled to the first power transmitting element, a second locking element and a sensor circuit. The second locking element is received in the housing and coupled to the second power transmitting element. The second locking element is axially movable along a rotational axis about which the first and second locking elements are rotatably disposed such that the second locking element is movable between a first position, in which rotary power is not transmitted between the first and second locking elements, and a second position in which rotary power is transmitted between the first and second locking elements. The sensor circuit is disposed in the housing and includes a first Hall-effect sensor and a second Hall-effect sensor that are configured to sense an axial position of the second locking element. The sensor circuit produces a single analog output current that is indicative of a sensed axial position of the second locking element. 
         [0008]    In another form, the present teachings provide a method of detecting an operating mode of a power transfer device that is configured to transmit rotary power. The power transmitting device has a first power transmitting element, a second power transmitting element, a housing, a first locking element received in the housing and coupled to the first power transmitting element, a second locking element received in the housing and coupled to the second power transmitting element, and a sensor circuit having first and second Hall-effect sensors and first and second switches. The second locking element is axially movable along a rotational axis about which the first and second locking elements are rotatably disposed such that the second locking element is movable between a first position, in which rotary power is not transmitted between the first and second locking elements, and a second position in which rotary power is transmitted between the first and second locking elements. The method includes: fixedly coupling the first and second Hall-effect sensors to one of the second locking element and the housing; fixedly coupling a magnet to the other one of the second locking element and the housing; sensing a position of the second locking element with the first Hall-effect sensor and responsively producing a first sensor signal in response thereto; sensing the position of the second locking element with the second Hall-effect sensor and responsively producing a second sensor signal in response thereto; and operating the first and second switches in response to the first and second sensor signals, respectively, to control power distribution through the sensor circuit so as to affect a magnitude of an output current produced by the sensor circuit. 
         [0009]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0011]      FIG. 1  is a schematic view of an exemplary motor vehicle drivetrain including a differential assembly constructed in accordance with the teachings of the present disclosure; 
           [0012]      FIG. 2  is a fragmentary perspective view of a front driving axle of  FIG. 1 ; 
           [0013]      FIG. 3  is a fragmentary perspective view of the front driving axle of  FIG. 1 ; 
           [0014]      FIG. 4  is an exploded perspective view of a differential assembly of  FIG. 1 ; 
           [0015]      FIG. 5  is an end view of the differential assembly of  FIG. 1 ; 
           [0016]      FIG. 6  is a cross-sectional side view of the differential assembly taken along line  6 - 6  of  FIG. 5 ; 
           [0017]      FIG. 7  is a fragmentary side view of the differential assembly of  FIG. 1  showing the actuating ring in a position disengaged from the side gear; 
           [0018]      FIG. 8  is a fragmentary side view of the differential assembly of  FIG. 1  showing the actuating ring in a position drivingly engaged with the side gear; 
           [0019]      FIG. 9  is a fragmentary perspective of a second embodiment differential assembly constructed in accordance with the teachings of the present disclosure; 
           [0020]      FIG. 10  is a schematic depicting a circuit including a second embodiment sensor assembly in accordance with the teachings of the present disclosure; 
           [0021]      FIG. 11  is a plot showing magnetic field density as a function of distance for a first embodiment sensor assembly; 
           [0022]      FIG. 12  is a plot showing magnetic field density as a function of distance for a second embodiment sensor assembly; 
           [0023]      FIG. 13  is a schematic illustration depicting a circuit constructed in accordance with the teachings of the present disclosure, the circuit having a two wire, dual sensor arrangement that is identified as Sensor Configuration  1 ; 
           [0024]      FIG. 14  is a schematic diagram depicting another power transmission device constructed in accordance with the teachings of the present disclosure; 
           [0025]      FIG. 15  is a plot showing magnetic field density as a function of distance for a dual Hall sensor arrangement operating in a single magnetic field per Sensor Configuration  1 ; 
           [0026]      FIG. 16  is a table depicting the output of the circuit of  FIG. 13  based on the operational state of the sensors; 
           [0027]      FIG. 17  is a schematic depicting an alternate circuit constructed in accordance with the teachings of the present disclosure and identified as Sensor Configuration  2 ; 
           [0028]      FIG. 18  is a plot showing magnetic field density as a function of distance for Sensor Configuration  2 ; 
           [0029]      FIG. 19  is a table depicting the output of the circuit of  FIG. 17 ; 
           [0030]      FIG. 20  is a schematic depicting an alternate circuit constructed in accordance with the teachings of the present disclosure and identified as Sensor Configuration  3 ; 
           [0031]      FIG. 21  is a plot showing magnetic field density as a function of distance for Sensor Configuration  3 ; 
           [0032]      FIG. 22  is a table depicting the output of the circuit of  FIG. 20 ; 
           [0033]      FIG. 23  is a schematic depicting an alternate embodiment circuit identified as Sensor Configuration  4 ; 
           [0034]      FIG. 24  is a plot showing magnetic field density as a function of distance for Sensor Configuration  4 ; 
           [0035]      FIG. 25  is a table depicting the output of the circuit of  FIG. 23 ; 
           [0036]      FIG. 26  is a schematic depicting an alternate circuit constructed in accordance with the teachings of the present disclosure and identified as Sensor Configuration  5 ; 
           [0037]      FIG. 27  is a plot showing magnetic field density as a function of distance for Sensor Configuration  5 ; 
           [0038]      FIG. 28  is a table depicting the output of the circuit of  FIG. 26 ; 
           [0039]      FIG. 29  is a schematic depicting an alternate circuit constructed in accordance with the teachings of the present disclosure and identified as Sensor Configuration  6 ; 
           [0040]      FIG. 30  is a plot showing magnetic field density as a function of distance for Sensor Configuration  6 ; 
           [0041]      FIG. 31  is a table depicting the output of the circuit of  FIG. 29 ; 
           [0042]      FIG. 32  is a plot showing magnetic field density as a function of distance for Sensor Configuration  6  where the Hall elements are programmed to have overlapping operational magnetic field ranges; 
           [0043]      FIG. 33  is a table depicting the output of Sensor Configuration  6  having operational switch points as defined in  FIG. 32 ; 
           [0044]      FIG. 34  is a plot showing magnetic field density as a function of distance for a circuit constructed according to Sensor Configuration  6  where the Hall effect sensors operate within dual magnetic fields; 
           [0045]      FIG. 35  is a table depicting the output of the circuit according to Sensor Configuration  6  and  FIG. 34 ; 
           [0046]      FIG. 36  is a plot showing magnetic field density as a function of distance for a circuit constructed according to Sensor Configuration  6  operating in dual magnetic fields where the operational magnetic field density ranges of the Hall effect sensors overlap; and 
           [0047]      FIG. 37  is a table depicting the output of the circuit according to Sensor Configuration  6  operating under the parameters defined by  FIG. 36 . 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    The present disclosure is directed to an improved differential with a locking state detection system for a drivetrain of a motor vehicle. The differential of the present disclosure includes an actuator operable to place the differential in an “open” or “locked” condition. The detection system provides a signal indicating whether the differential is in the “open” or “locked” condition. It should be appreciated that the differential of the present disclosure may be utilized with a wide variety of driveline components and is not intended to be specifically limited to the particular application described herein. In addition, the actuator of the differential of the present disclosure may be used in conjunction with many types of differentials such as those having a bevel gear design or a parallel-axis helical design which may be of an open or limited-slip variety. 
         [0049]    With reference to  FIGS. 1-3 , a drivetrain  6  for an exemplary motor vehicle is shown to include an engine  8 , a transmission  10 , a transfer case  12 , a forward propeller shaft  14  and a rearward propeller shaft  16 . Rearward propeller shaft  16  provides torque to a rear axle assembly  18 . Forward propeller shaft  14  provides torque from engine  8  to a pinion shaft  20  of a front axle assembly  22 . Front axle assembly  22  includes an axle housing  24 , a differential assembly  26  supported in axle housing  24  and a pair of axle shafts  28  and  30  respectively interconnected to left and right front wheels  32  and  34 . 
         [0050]    Pinion shaft  20  has a pinion gear  36  fixed thereto which drives a ring gear  38  that is fixed to a differential case  40  of differential assembly  26 . Differential case  40  is rotatably supported in axle housing  24  by a pair of laterally spaced bearings  41 . Bearings  41  are retained by bearing caps  42  coupled to axle housing  24 . A gearset  43  ( FIG. 4 ) supported within differential case  40  transfers rotary power from differential case  40  to axle shafts  28  and  30 , and facilitates relative rotation (i.e., differentiation) therebetween. Thus, rotary power from engine  8  is transmitted to axle shafts  28  and  30  for driving front wheels  32  and  34  via transmission  10 , transfer case  12 , forward propeller shaft  14 , pinion shaft  20 , differential case  40  and gearset  43 . While differential assembly  26  is depicted in a front-wheel drive application, the present disclosure is contemplated for use in differential assemblies installed in trailing axles, rear axles, transfer cases for use in four-wheel drive vehicles and/or any other known vehicular driveline application. 
         [0051]      FIGS. 4-8  depict differential assembly  26  to include differential case  40  and gearset  43 . Gearset  43  includes a pair of pinion gears  44  rotatably supported on a cross shaft  45 . First and second side gears  46  and  47  are drivingly interconnected to pinion gears  44  and axle shafts  28  and  30 . Differential assembly  26  also includes an actuator and sensor assembly  48  operable to selectively couple first side gear  46  to differential case  40 , thereby placing differential assembly  26  in a fully locked condition. 
         [0052]    A cap  49  is coupled to differential case  40  to define a pocket  50  for receipt of actuator and sensor assembly  48 . Actuator and sensor assembly  48  includes a solenoid assembly  52 , an actuating ring  54 , a draw plate  56 , a retainer  58  and a sensor assembly  59 . Cap  49  includes a flange  60  coupled to a flange  62  of case  40 . Flange  60  of cap  49  includes a recess  64  sized to receive a portion of solenoid assembly  52  during actuation. Cap  49  includes a pair of stepped bores  66  and  68  which define pocket  50 . Specifically, first bore  66  includes an annular surface  70  while second bore  68  includes an annular surface  72 . First bore  66  includes an end face  74  radially inwardly extending from annular surface  70 . An aperture  76  extends through the cap  49  and is in communication with second bore  68  where aperture  76  and second bore  68  are sized to receive a portion of the axle shaft. 
         [0053]    Actuating ring  54  includes a generally hollow cylindrical body  78  having an annular recess  80  formed at one end. Side gear  46  includes a similarly sized annular recess  82  formed on an outboard face  84 . A compression spring  85  is positioned between actuating ring  54  and side gear  46  within annular recesses  80  and  82 . A plurality of axially extending dogs  86  protrude from an end face  88  of actuating ring  54 . A corresponding plurality of dogs  90  axially extend from outboard face  84  of side gear  46 . Actuating ring  54  is moveable from a disengaged position as shown in  FIGS. 6 and 7  to an engaged position shown in  FIG. 8 . In the disengaged position, dogs  86  of actuating ring  54  are released from engagement with dogs  90  of side gear  46 . In contrast, when actuating ring  54  is moved to its engaged position, dogs  86  engage dogs  90  to rotatably fix side gear  46  to differential case  40 . 
         [0054]    Solenoid assembly  52  includes a metallic cup  94  and a wire coil  96 . Wire coil  96  is positioned within cup  94  and secured thereto by an epoxy  98 . Cup  94  includes an inner annular wall  100 , an outer annular wall  102  and an end wall  104  interconnecting annular walls  100  and  102 . Retainer  58  is a substantially disc-shaped member having an outer edge  106  mounted to end wall  104  of cup  94 . A portion of retainer  58  is spaced apart from end wall  104  to define a slot  108 . 
         [0055]    Retainer  58  includes a pair of axially extending tabs  109  positioned proximate to bearing cap  42 . Tabs  109  restrict rotation of retainer  58  relative to axle housing  24 . Sensor assembly  59  is mounted to retainer  58 . Sensor assembly  59  includes a Hall element  110  having a substantially rectangular body. Hall element  110  includes a first face  112  extending substantially perpendicularly to the axis of rotation of axle shafts  28  and  30 . Sensor assembly  59  also includes a pair of wires  114  extending from Hall element  110  that end at terminals  116  mounted within a connector  118 . Connector  118  includes a body  120  extending through an aperture  122  formed in axle housing  24 . The ends of the wire on wire coil  96  terminate at terminals  124  mounted within connector  118 . In this manner, electrical connection to solenoid assembly  52  and sensor assembly  59  may be made from outside of axle housing  24 . 
         [0056]    A target  126  includes a bracket  128 , a magnet  130  and a fastener  132 . Bracket  128  includes a first leg  134  having an aperture  136  extending therethrough. Fastener  132  extends through aperture  136  and is used to mount target  126  to bearing cap  42 . Bracket  128  includes a second leg  138  positioned at a right angle to first leg  134 . Second leg  138  is substantially planar and positioned substantially parallel to first face  112  of Hall element  110 . Magnet  130  is a substantially cylindrical disk-shaped member mounted to second leg  138 . Accordingly, magnet  130  includes an outer surface  139  (shown in  FIG. 7 ) positioned substantially parallel to first face  112 . One skilled in the art will appreciate that the sensor and magnet may be re-oriented 90 degrees to the orientation shown in the Figures. As such, the orientation of sensor and magnet shown in the drawings is merely exemplary and should not limit the scope of the disclosure. 
         [0057]    Draw plate  56  is positioned within slot  108  defined by retainer  58  and is coupled to actuating ring  54  via a plurality of fasteners  140 . A washer  142  is positioned between cap  49  and actuating ring  54 . Preferably, washer  142  is constructed from a non-ferromagnetic material so as to reduce any tendency for actuating ring  54  to move toward end face  74  of metallic cap  49  instead of differential case  40  during energization of solenoid assembly  52 . A bearing  144  supports cup  94  on an outer journal  146  of cap  49 . 
         [0058]    Coil  96  is coupled to a controller  148  ( FIG. 1 ) that operates to selectively energize and de-energize coil  96 . During coil energization, a magnetic field is generated by current passing through coil  96 . The magnetic field causes actuator and sensor assembly  48  to be drawn toward flange  60  of cap  49 . As solenoid assembly  52  enters recess  64 , dogs  86  of actuating ring  54  engage dogs  90  of side gear  46 . Once the dogs are engaged, actuating ring  54  is in its engaged position and differential assembly  26  is in a fully locked condition as shown in  FIG. 8 . In the fully locked position, the Hall element  110  encompassed in sensor assembly  59  is spaced apart from outer surface  139  of magnet  130  by a distance “X.” At distance “X,” magnet  130  generates a predetermined magnetic field density. Sensor assembly  59  outputs a signal indicative of the axial position of actuating ring  54 . This signal is used by controller  148  as verification that differential assembly  26  is in a fully locked position. 
         [0059]    One skilled in the art will appreciate that the axially moveable electromagnet of the present disclosure provides a simplified design having a reduced number of components. Additionally, the present disclosure utilizes the entire differential case as the armature for the electromagnet. This allows a more efficient use of the available magnetic force. These features allow a designer to reduce the size of the electromagnet because the armature more efficiently utilizes the electromotive force supplied by the electromagnet. Such a compact design allows for minor modification of previously used components and packaging with a standard sized axle housing. 
         [0060]    To place differential assembly  26  in the open, unlocked condition, current is discontinued to coil  96 . The magnetic field ceases to exist once current to coil  96  is stopped. At this time, compression in spring  85  causes actuator and sensor assembly  48  to axially translate and disengage dogs  86  from dogs  90 . Accordingly, side gear  46  is no longer drivingly coupled to differential case  40 , thereby placing differential assembly  26  in the open condition shown in  FIG. 7 . When differential assembly is in the open, unlocked condition, Hall element  110  is positioned substantially closer to target  126  than when differential assembly  26  was in the locked position. Specifically, first face  112  is spaced apart from outer surface  139  of magnet  130  a distance “Y” when coil  96  is not energized. At distance “Y,” the magnetic field density generated by magnet  130  is significantly greater than the field density at distance “X.” Sensor assembly  59  is configured to output a signal to controller  148  indicating that actuating ring  54  is at a position where dogs  86  are disengaged from dogs  90  and the differential is in an open condition. It should also be appreciated that actuation and deactuation times are very short due to the small number of moving components involved. Specifically, no relative ramping or actuation of other components is required to cause engagement or disengagement of dogs  86  and dogs  90 . 
         [0061]    Electronic controller  148  controls the operation of actuator and sensor assembly  48 . Electronic controller  148  is in receipt of data collected by a first speed sensor  150  and a second speed sensor  152  as shown in  FIG. 1 . First speed sensor  150  provides data corresponding to the rotational speed of axle shaft  28 . Similarly, second speed sensor  152  measures the rotational speed of axle shaft  30  and outputs a signal to controller  148  indicative thereof. Depending on the data collected at any number of vehicle sensors such as a gear position sensor  154 , a vehicle speed sensor  156 , a transfer case range position sensor or a brake sensor  158  as shown in  FIG. 1 , controller  148  will determine if an electrical signal is sent to coil  96 . Controller  148  compares the measured or calculated parameters to predetermined values and outputs an electrical signal to place differential assembly  26  in the locked position only when specific conditions are met. As such, controller  148  assures that an “open” condition is maintained when events such as anti-lock braking occur. The “open” condition is verified by the signal output from sensor assembly  59 . Limiting axle differentiation during anti-lock braking would possibly counteract the anti-lock braking system. Other such situations may be programmed within controller  148 . 
         [0062]      FIG. 9  depicts a second embodiment differential assembly  160 . Differential assembly  160  is substantially similar to differential assembly  26 . For clarity, like elements have been identified with previously introduced reference numerals. Differential assembly  160  differs from differential assembly  26  in that a coil  162  is rotatably mounted on differential case  40  in a fixed axial position. An anti-rotation bracket  164  interconnects a cup  166  with the axle housing  24  ( FIG. 3 ) to restrict coil  162  from rotation. A bearing  167  rotatably supports cup  166  to allow the differential case  40  to rotate relative to the coil  162  during operation of the differential assembly. 
         [0063]    Through the use of a stationary coil  162 , power supply and sensor wire routing complexities may be reduced because the wires no longer need to account for axial movement of the coil. As such, coil  162  does not axially translate nor rotate during any mode of operation of differential assembly  160 . An axially moveable armature  168  is coupled to actuating ring  54 . Armature  168  is shaped as an annular flat ring positioned proximate coil  162 . Armature  168  and actuating ring  54  are drivingly coupled to differential case  40  and axially moveable relative to coil  162  and differential case  40 . Armature  168  and actuating ring  54  are biased toward a disengaged, open differential, position shown in  FIG. 9  by a compression spring as previously described in relation to differential assembly  26 . 
         [0064]    To place differential assembly  160  in a locked condition, coil  162  is energized to generate a magnetic field. Armature  168  is constructed from a ferromagnetic material. Accordingly, armature  168  and actuating ring  54  are axially displaced to drivingly engage actuating ring  54  with side gear  46  to place differential assembly  160  in a locked condition. 
         [0065]    While a front drive axle assembly has been described in detail, it should be appreciated that the power transmitting device of the present disclosure is not limited to such an application. Specifically, the present disclosure may be used in rear drive axles, transaxles for front-wheel drive vehicles, transfer cases for use in four-drive vehicles and/or a number of other vehicular driveline applications. 
         [0066]      FIG. 10  depicts a circuit  198  having a second embodiment sensor assembly  200 . Sensor assembly  200  includes a first Hall element  202 , a second Hall element  204  and a body  206  encompassing both of the Hall elements. Sensor assembly  200  is shaped substantially similarly to sensor assembly  59 . Sensor assembly  200  is positioned in communication with a differential assembly in a substantially similar manner to sensor assembly  59 . Accordingly, the description relating to the mounting of sensor assembly  200  within the axle assembly will not be reiterated. 
         [0067]    Due to the nature of Hall effect devices, permanent magnets and the general environment in which sensor assembly  200  is required to function, a very large mechanical hysteresis is inherent in the system. Mechanical hysteresis in this instance is best described as the absolute distance the sensor assembly must travel in relation to the target magnet in order to change its output state. The Hall effect device switches state, or outputs a different signal, based on the Hall element being exposed to a changing magnetic field density. The Hall effect device may be configured to start switching at a predetermined magnetic field density described as its operating point (Bop) and the field density must change an amount equal to the inherent hysteresis (Bhys) of the Hall effect device in order to switch. 
         [0068]      FIG. 11  is a graph showing magnetic field density versus distance for the first embodiment sensor assembly  59  shown in  FIGS. 4-8 . As shown in  FIG. 11 , permanent magnet  130  generates an exponentially decaying field density, measured in gauss versus the distance traveled in millimeters. For example, if Hall element  110  was programmed to switch at a Bop of 80 gauss and had a Bhys of 10 gauss, Hall element  110  would initiate a switch at 80 gauss and change its state at 70 gauss. Because a magnetic field is generated when coil  96  is energized, two distinct gauss curves are created. The upper curve depicts the field density present when the electromagnet of solenoid assembly  52  is energized. The lower curve represents the magnetic field density generated by the permanent magnet alone when the coil  96  is not energized. As shown, a relatively large hysteresis is introduced into the system by operation of solenoid assembly  52 . The magnitude of hysteresis introduced is by choice. It should be appreciated that the coil may be wired in the opposite polarity to reduce the relative gap between the gauss curves. 
         [0069]    In the embodiment depicted in  FIG. 11 , sensor assembly  59  moves from a location where distance “Y” equals 4 mm and distance “X” equals 8 mm. Sensor assembly  59  does not output a signal indicating that the differential assembly is in the locked condition until sensor assembly  59  reaches a distance of 7.8 mm of spacing between first face  112  and outer surface  139 . During coil  96  deenergization, sensor assembly  59  does not output a signal indicating that the differential assembly is unlocked until the spacing between the Hall element and the permanent magnet is 4.8 mm. As such, a total mechanical hysteresis of approximately 3 mm exists with the single sensor embodiment. Depending on the operational characteristics of the mechanical system including sensor assembly  59 , this magnitude of hysteresis may or not be acceptable. 
         [0070]      FIG. 12  is a graph showing magnetic field density versus distance for the second embodiment sensor assembly  200  shown in  FIG. 10 . To reduce the magnitude of mechanical hysteresis, Hall elements  202  and  204  of sensor assembly  200  are configured in accordance with  FIGS. 10 and 12 . First Hall element  202  is set to have an operating point of 60 gauss while second Hall element  204  is set to have an operating point of 100 gauss. During operation, second Hall element  204  outputs a signal indicating that the differential assembly is in the locked condition once the magnetic field density reduces from 100 gauss to 90 gauss. This condition occurs when the spacing between second Hall element  204  and outer surface  139  of magnet  130  is approximately 6.3 mm. At electromagnet deenergization, first Hall element  202  outputs a signal indicative of an open differential condition once the magnetic field density changes from 50 to 60 gauss. This condition exists when first Hall element  202  is spaced from outer surface  139  a distance of approximately 5.6 mm. One skilled in the art will appreciate that the total mechanical hysteresis is now approximately 0.75 mm when using two Hall elements with different operating points. 
         [0071]    The circuit  198  depicted in  FIG. 10  includes first Hall effect sensor  202  and second Hall effect sensor  204 . First Hall effect sensor  202  is coupled in series with a differential gain amplifier  232 . Differential gain amplifier  232  is coupled to the base of a current gain transistor  234 . A constant current source  236  is supplied to the collector leg of current gain transistor  234 . The emitter leg of current gain transistor  234  provides an output signal labeled as I OUT1 . 
         [0072]    In similar fashion, second Hall effect sensor  204  is connected in series with a differential gain amplifier  240 . Differential gain amplifier  240  is coupled to the base of a current gain transistor  242 . Constant current source  236  is supplied to the collector leg of current gain transistor  242 . The emitter leg of current gain transistor  242  provides an output signal labeled as I OUT2 . Controller  148  analyzes I OUT1  and I OUT2  to determine the operating mode of differentiation as being locked or unlocked. When both I OUT1  and I OUT2  are low or zero, controller  148  determines that the differential is operating in the locked mode. When I OUT1  and I OUT2  are both high or one, controller  148  determines that the differential is operating in the unlocked mode. 
         [0073]      FIG. 13  depicts an alternate embodiment dual Hall sensor circuit  300  operable to output a signal indicative of the position of a moveable member within a power transmission device. Circuit  300  may be implemented in conjunction with the lockable differential assembly previously described. Furthermore, it is contemplated that circuit  300  may be used in conjunction with any number of power transmission subsystems that include an axially moveable member. 
         [0074]    For example,  FIG. 14  shows a power transmission device  306  operable to selectively transfer torque from a first rotatable shaft  308  to a second rotatable shaft  310 . The rotatable shafts are at least partially positioned within a housing  311  and are selectively drivingly interconnected by a clutch assembly  312 . Clutch assembly  312  includes a plurality of outer friction plates  314  slidably coupled to second shaft  310  and a plurality of inner friction plates  316  slidably coupled to shaft  308 . Outer plates  314  are interleaved with inner plates  316 . An actuator  318  is operable to axially displace an apply plate  320  such that a compressive force may be selectively applied to the clutch  312 . The output torque of clutch  312  may be varied according to the input force generated by actuator  318 . 
         [0075]    A sensor assembly  322  is mounted to housing  311 . A target  324  is mounted to axially moveable apply plate  320 . In operation, actuator  318  is operable to move apply plate  320  between at least three discrete positions. These positions are represented by target  324  being shown in solid line representation when apply plate  320  is at the first or returned position where no torque is transferred through clutch  312 , a second position as denoted by target  324 ′ in hidden line representation and a third position shown as target  324 ″ also in hidden lines. At position  324 ′, actuator  318  moves apply plate  320  to take up axial clearance between outer plates  314  and inner plates  316  to place the clutch in a ready mode. At this position, clutch  312  transmits minimal torque, if any, between shaft  308  and shaft  310 . However, very slight movement of apply plate  320  toward the clutch  312  will cause the clutch to generate a significant amount of torque in a relatively short period of time. In this manner, torque delivery will not be delayed due to the actuator having to travel large distances to account for the clearance between the actuator plate and the friction plates of the clutch. 
         [0076]    When the target is at position  324 ″, actuator  318  has driven apply plate  320  in full engagement with clutch  312  and torque is being transferred through the clutch. Accordingly, it may be beneficial to construct a sensor circuit operable to output signals indicating when an axially moveable member such as apply plate  320  is at one of three locations. Alternatively, only two locations may need to be determined if the sensor arrangement is used in a device such as differential assemblies  26  or  160  because the axially moveable actuating ring  54  is typically in one of two locations. Actuating ring  54  is either in the fully returned position when the differential is in an open condition or the fully advanced position when the differential is in the locked condition. Various circuit embodiments and sensor configurations will be described hereinafter. Depending on the sensor configuration, the circuit may output signals indicating that the target is in one of two different zones or that the target is located within one of three different zones of linear position. 
         [0077]    Referring again to  FIG. 13 , circuit  300  depicts a Sensor Configuration  1 .  FIGS. 15 and 16  also relate to Sensor Configuration  1 . Circuit  300  includes a first Hall sensor  302 , a second Hall sensor  304 , a number of resistors, R 1 , R 2  and R 3  as well as a diode D 1  electrically interconnected as shown. These resistors and the diode are located within the housing of the power transmission device. A first pin  350  and a second pin  352  exit the housing at a bulkhead connector  354 . First pin  350  is connected to a DC power source while second pin  352  is connected to a load resistor RL. Load resistor RL functions as a current sensing element and provides an output signal Iout. One skilled in the art will appreciate that minimizing the number of wires, terminals, pins or other electrical connectors passing through the wall of the housing is beneficial. For example, the impact on the housing structural integrity is minimized and the aperture extending through the housing may be more easily sealed if the size of the aperture is minimized. 
         [0078]      FIG. 16  represents a state diagram defining the output of circuit  300  based on the operational states of sensor  302  and sensor  304 . The table of  FIG. 16  identifies sensor  302  as sensor  1  and sensor  304  as sensor  2 . As is noted by reviewing the column labeled Sensor Iout, Configuration  1  outputs 5 mA when the distance between the Hall effect sensors and the target is within zone  1  or  2 . Both sensor  302  and sensor  304  are in the OFF state when the distance between the Hall effect sensors and the target is within zone  3 . When both sensors are in the OFF state, Iout equals 15 mA. Because the Hall effect sensors include inherent hysteresis, the distance at which the state of the sensor changes depends on whether the magnetic field density is increasing or decreasing. Accordingly, zones  1 ,  2  and  3  vary slightly depending on the direction of travel of the axially moveable member. For example, sensor  2  switches from the ON state to the OFF state after the magnetic field density changes from Bop to Bhys. This change represents the spacing between the Hall effect sensor and the target as increasing at the point of transition from zone  2  to zone  3  as shown at approximately 1.75 mm. If the Hall effect sensor is exposed to an increasing magnetic field density, sensor  2  is shown to switch from the OFF to the ON state only after the magnetic field density increases from Bhys to Bop. This condition is shown to occur at approximately a 1.3 mm spacing as zone  3 ′ is exited and zone  2 ′ is entered. As is illustrated by the graph, the beginning of zone  3  does not exactly correspond to the ending of zone  3 ′. This “tolerance” of the distance at which zone  2  ends and zone  3  starts should be accounted for in the logic of the controller utilizing the information output from circuit  300 . 
         [0079]      FIGS. 17-19  depict an electrical circuit  360  substantially similar to circuit  300  but having a different topology, identified as Sensor Configuration  2 . Circuit  360  includes first sensor  302  and second sensor  304  wired in communication with resistors R 1  and R 2  as well as diode D 1 . The resistance value for R 1  has been changed and R 3  has been removed. First pin  350  and second pin  352  exit the housing of the power transmission device as previously described. Pin  350  is coupled to a DC power source and pin  352  is coupled to a current sensing load resistor RL.  FIG. 19  includes a column labeled Sensor Iout which represents the output of the circuit  360  where sensor  1  has a Bop greater than the Bop of sensor  2 .  FIG. 19  includes another column entitled Alternate Sensor Iout which represents the output of circuit  360  if the operating points of sensors  1  and  2  were switched. One skilled in the art will appreciate that three different current levels are provided depending on the state of sensor  1  and sensor  2  according to the column labeled Sensor Iout. Specifically, Iout equals 5 mA when the spacing between the Hall effect sensors and the target is within zone  1 . Iout equals 15 mA when the spacing between the sensors and the target is within zone  2 . Iout equals 21 mA when the spacing between the sensors and the target is within zone  3 . The versatility of the use of two programmable Hall effect sensors is illustrated by reviewing the Alternate Sensor Iout column and noting that the same circuit may be used to provide an indication when the spacing between Hall sensors is within one of two areas. Different signals are output if the spacing lies within zone  1  or within zones  2  or  3 . Iout equals 5 mA only when sensor  1  and sensor  2  are both in the ON state. Otherwise, if one or both of the sensors are in the OFF state, 21 mA is output. Therefore, Sensor Configuration  2  is easily programmed to provide a two position sensing arrangement or a three position sensing arrangement. 
         [0080]    Another circuit configuration  370  is represented by  FIGS. 20-22 . Circuit  370  or Sensor Configuration  3  is substantially similar to Sensor Configurations  1  and  2  with minor changes to the circuit. The circuit modifications cause the magnitude of the output current levels to change. Furthermore, different sensor state combinations provide different outputs. The Sensor Iout column shows that 3 mA is output in zone  1  and zone  1 ′ while 21 mA will be output when the spacing between the sensor and the target is within zone  2 , zone  2 ′, zone  3  or zone  3 ′. 
         [0081]      FIGS. 23-25  relate to Sensor Configuration  4  having a circuit  380 .  FIGS. 26-28  correspond to Sensor Configuration  5  having a circuit  390 .  FIGS. 29-31  depict Sensor Configuration  6  having a circuit  395 . Each of these configurations is substantially similar to Sensor Configurations  1 - 3  previously described in detail. As such, like elements will retain their previously introduced reference numerals. Sensor Configurations  4 ,  5  and  6  further illustrate the versatility of the present disclosure by constructing simple circuits using two Hall elements to output signals indicative of the position of an axially moveable component within a power transmission device. 
         [0082]      FIGS. 32 and 33  depict a method of adjusting the width of certain detection zones by modifying the operating switch point of one sensor relative to the other. The embodiments previously described included a first sensor having an operating range of magnetic field density defined by its operating point and its hysteresis switch point. The operating range of sensor  1  is spaced apart from the operating range of magnetic field density of sensor  2  because sensor  2  is purposefully configured with different operating and hysteresis switch points. In the embodiment depicted in  FIG. 32 , Sensor Configuration  6  is shown to include the operating switch point of sensor  2  being programmed to lie within the operating range of magnetic field density defined by sensor  1 . Specifically, sensor  2  has an operating point (Bop) that is greater than the hysteresis switch point (Bhys) of sensor  1  but lower than the operating switch point (Bop) of sensor  1 . By setting the operating switch points of the two Hall effect sensors relatively closely together, the axial travel defined by zone  2  is greatly reduced. As depicted in  FIGS. 32 and 33 , the distance traveled to exit zone  1 , pass entirely through zone  2  and enter zone  3  is approximately 0.5 mm. Accordingly, the dual Hall sensor arrangement having overlapping operating ranges may be useful for an application where relatively small axial distances are traveled by the axially moveable member. 
         [0083]      FIGS. 34-37  illustrate that any one of the Sensor Configurations  1 - 6  may also be used in a dual field operation mode. These Figures also illustrate that the operating ranges of the Hall sensors may be overlapped or not overlapped in the dual field mode of operation as well as the single field mode of operation. The dual field operation mode was described in greater detail previously in reference to the lockable differential having an electromagnet with a coil operable to generate an electromagnetic field. However, in this embodiment the polarity of the permanent magnet and the electromagnet are positioned such that the magnetic field density at the sensors decreases when the electromagnet coil is on. 
         [0084]    Furthermore, the foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations may be made therein without department from the spirit and scope of the disclosure as defined in the following claims.