Abstract:
A method for controlling a locking differential includes determining a temperature-dependent reference voltage at which a coil locks the differentia, determining an electric potential of a battery, using the battery to energize the coil and lock the differential, if the electric potential is equal to or greater than the reference voltage for a current temperature, and maintaining the differential unlocked, if the electric potential is less than the reference voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to a differential mechanism, which transmits rotating power to the wheels of a motor vehicle and locks to prevent the wheels from rotating at different speeds. 
     2. Description of the Prior Art 
     It is conventional to use an open or limited slip differential mechanism to permit limited wheel slip at a vehicle axle, i.e., to produce a rotational speed difference between the driven wheels. A purpose of a locking differential is to prevent relative rotation of one driven wheel with respect to another driven wheel. This is usually accomplished by locking one differential side gear to a differential case, thereby preventing rotation of the side gear with respect to the differential case, and preventing relative wheel speed differentiation across any one axle. 
     A locking differential can also be used as an inter-wheel differential or as a center differential in 4×4 and AWD vehicles. In this case, the axis of the differential assembly is parallel to the longitudinal axis of the vehicle. The center differential allows drive shaft speed differences between the front and rear axles. But there are some cases where it is desired to lock the front and rear axle drive shafts together such that a single rotation speed is re attained. This condition is known as a locked center differential. 
     When activated, an electronically locking differential uses a voltage source to produce a magnetic force that overcomes a reactionary spring force applied to a locking ring (mechanical engagement mechanism), thereby mechanically coupling a side gear to a differential case through the lock ring. When activated, the electronic locking differential prevents relative speed differences between the controlled wheels. 
     If the electronic locking differential is required to function at low voltage and high coil temperature without a temperature compensation strategy, then the electromagnetic hardware, including wire and the coil winding, must have a relatively large size in order to produce a magnetic actuating force that is able to overcome the spring force. The spring force must be set high enough to prevent partial engagement of the lock ring to the side gear during all dynamic vehicle operating conditions. 
     A need exists in the industry for a control strategy that enables functionality of the electronic locking differential on all road and off-road surfaces, axle fluid temperatures and coil temperature while minimizing the size of the electronic actuating locker hardware. 
     SUMMARY OF THE INVENTION 
     A method for controlling a locking differential includes determining a temperature-dependent reference voltage at which a coil locks the differential and determining an electric potential of a voltage source, usually a battery. The battery is used to energize the coil, creates a magnetic force that actuates the lock ring that overcomes the spring reactionary force and locks the differential. If the electric potential of the voltage source is equal to or greater than the reference voltage at a specified coil temperature then the control system is calibrated to allow actuation of the electronic locking differential. 
     The control system avoids the possibility attempting to lock the differential using an actuating voltage that is too low to fully overcome the return spring or reaction member. The temperature compensation ensures that the magnetic force produced by an electric coil is sufficient to cause clutch teeth on a locking plate to fully engage clutch teeth on a side gear and to competently lock the differential. The temperature compensation strategy reduces the cost and weight of the electromagnetic hardware, i.e., the wiring, coil, air gap, windings, etc., and minimizes the size of these components. 
     The temperature compensation strategy prevents potential hardware damage due to partial engagement of the clutch teeth on the locking ring with those on the side gear. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a perspective cross section of an electronic locking differential mechanism; 
         FIG. 2  is an isometric view of a locking ring; 
         FIG. 3  is an isometric view of the case and clutch; 
         FIG. 4  is a side view of a field core coil assembly; 
         FIG. 5  is schematic diagram of a temperature compensation control system for the differential; and 
         FIG. 6  shows a lockup table relating a minimum voltage for locking the differential and coil temperature. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIGS. 1-4 , an electronic locking differential  8  includes a differential case  10 , preferably of cast iron or steel, supported on a stationary housing (not shown) for rotation about a lateral axis  12 . A bevel ring gear, secured to the case at the attachment bole holes on the flange  11 , drives the case  10  in rotation about axis  12  from an output of a transmission or transfer case. 
     The case  10  provides an internal chamber  14 , which contains bevel pinions  16 , a right side gear  18  meshing with the pinions and driveably connected to a right output shaft  20 , which extends from the case  10  to a driven wheel of a motor vehicle, and a left side gear  22  meshing with the pinions and driveably connected to a left output shaft (not shown), which extends from the case to a driven wheel at the left side. The pinions  16  are each secured by pins  24  to the rotating case  10 , such that the pinions  16  rotate about the axis of pins  24  perpendicular to axis  12 , and the pinions and pins  24  rotate about axis  12 . 
     Also located in the case  10  is a locking ring  26 , secured to the case such that it rotates about axis  12  and moves axially relative to the case along the axis. The ring  26  is formed with three posts  28 , each post extending axially through a hole in web  30 , which is formed in the case  10 ; a planar surface  32  facing the web  30 ; and a series of clutch teeth  34  and spaces  36  angularly arranged alternately about axis  12  on the axially opposite side of the locking ring from surface  32 . The clutch teeth and spaces are adjacent and face the side gear  22 . 
     The side gear  22  is formed with a series of clutch teeth  38  and spaces  40  the teeth  40  angularly arranged alternately about axis  12  on its axial outer face adjacent the clutch teeth  34  and spaces  36  of the locking ring  26 . The clutch teeth and spaces of the side gear  22  and locking ring  26  are mutually complementary such that they can engage and disengage as the locking ring moves toward and away from the side gear. The locking ring  26  is normally not engaged with the side gear  22  and permits the side gear to rotate with respect to the differential case  10  and the locking ring, thereby producing an unlocked or disengaged state. 
     When the locking ring  26  is actuated to engage the side gear  22 , their clutch teeth and spaces mesh, thereby driveably connecting the side gear to the locking ring and case  10 , preventing the side gear from rotating relative to the case and locking ring, and producing a locked or engaged state. 
       FIGS. 1 and 4  show a field core coil assembly  42  supported on the case  10  outside the chamber  14 . The field assembly  42  includes an electromagnetic coil  44 , fitted into an annular recess  46 , formed in a ring  48 . The coil  44  produces a magnetic field when energized with electric current through the leads  50 . The field assembly is secured to the housing by brackets  52 , which prevent the coil assembly  42  and coil  44  from rotating. The magnetic field produces an axial force on the coil assembly  42 , whose magnitude varies with the width of an air gap  52  between the coil assembly and the case  10 . 
     When the coil  44  is energized, it is attracted to the differential case due to the magnetic field generated by the coil. The coil assembly  42  is fixed against rotation with respect to the differential case  10 , but it can translate axially toward and away from the differential case. Axial translation of the coil assembly  42  is transmitted to a sliding collar  54 , which is secured to the coil assembly  42  by a press fit and an overlapping rim  58 . A bushing  60 , which is press fit onto the inside diameter of the sliding collar  54 , allows rotation of the case with respect to the sliding collar  54  and coil assembly  42 . The bushing  60  also provides a linear guide for the sliding collar  60  and coil assembly  42 , allowing them to translate axially. 
     When the coil  42  is energized, the sliding collar  54  applies an axial force directed rightward to a roller thrust bearing  62  and an annular thrust plate  64 . Bearing  62  and thrust plate  64  are located in an annular recess formed in the case. Thrust plate  64  applies axial force to the lock ring  26  through the posts  28  on the locking ring. The posts  28  extend through the axial holes  29  in web  30 , causing the locking ring  26  to rotate with the case  10  and allowing the locking ring to move axially relative to the case. The post surfaces  70  are located at the left side of the web  30  adjacent the thrust plate lugs  68 . 
     The locking ring  26  moves into mechanical engagement with the side gear  22  to prevent rotation of the side gear. Springs  80  and  82  are located adjacent to the locking ring  26  and are arranged in series such that spring  80  contacts and applies resilient force to the locking ring, and spring  82  is secured to the case  10  by a snap ring  84  and applies resilient force to spring  80 . Preferably springs  80 ,  82  are wave springs having a corrugations directed radially from axis  12  to their radial outer peripheries, the corrugations being formed with alternating radial ridges and grooves. The springs  80 ,  82  are separated by a flat plate  86 , located axially between the springs, such that the ridges of each spring corrugation contact the plate, thereby preventing mutual contact of the springs. The springs continually apply resilient axial force directed leftward to the locking ring  26  to oppose movement of the locking ring toward the locked position with the side gear  22  in response to the magnetic force produced by the coil  44 . 
     When the coil current is removed, the springs  80 ,  82  return the locking ring  26  to the disengaged position. The force applied by the springs is sufficient to prevent inadvertent locking of the differential during normal driving conditions when the coil is deenergized. Furthermore, spring  80  has a much lower spring rate than that of spring  82 , such that a nonlinear spring force curve is generated. The spring arrangement ensures that the spring force is always lower than the force applied to the locking ring  26  by coil assembly  42  when the coil  44  is energized. Since the force produced by the coil assembly  42  when coil  44  is energized is nonlinear, springs  80 ,  82  are selected so that the magnitude of the spring force applied to the locking ring  76  is less than the force applied by the coil assembly when energized. 
     The locking of differential  8  is controlled through a voltage source, coil  44  and locking ring  26 . Electric current from a voltage supply, such as a battery, applied to coil  44  creates a magnetic force that actuates the locking ring  26 , causing the side gear  22  to engage the differential case  10 . When the electronic locker is engaged, coil  44  is cooled by the axle fluid, which directs heat, generated by the constant flow of current, away from the coil. 
     When the vehicle is stationary, the axle fluid flows back to a sump and the coil  44  is partially submerged in the fluid. The portion of the coil  44  that is not submerged transfers much less heat to the surrounding air than is transferred to the axle fluid, causing the coil to have a higher temperature on the portion of its surface that is not submerged in axle fluid. 
     When a voltage is applied across the ends of coil  44  and the coil temperature increases, the electrical resistance of the coil increases and electric current in the coil decreases, i.e., current in coil  44  is inversely proportional to its temperature. Consequently, the magnetic force necessary to overcome the force produced by the locking ring return springs  80 ,  82  also decreases. 
       FIG. 5  illustrates a system  100  that includes an electronic controller  102  for actuating the locking ring  26  of differential  8 . The terminals of an electric storage battery  104 , such as that used to start an engine  106 , are connected to a meter  108 , that produces a signal  110 , supplied as input to controller  102 , representing the current battery voltage. Differential  8  includes a temperature sensor  112 , which produces a signal  114 , supplied as input to controller  102 , representing the current temperature of coil  44  or a temperature representative of the current coil temperature, such as the temperature of the axle fluid in the differential housing  10 . 
     To ensure that the differential locker operates without functional degradation over a temperature range from −40° F. to 300° F., a lookup table  116 , stored in electronic memory  118  and accessible to controller  102  through a communications bus  120 , relates a minimum coil voltage to temperature of the coil  44 . 
     If the battery voltage is greater than the minimum voltage that corresponds to the current coil temperature set forth in lookup table  116 , controller  102  closes a switch  122 , completing a circuit that connects the battery terminals to the ends of coil  44 . When coil  44  is energized, locking ring  26  secures side gear  22  to case  10 , locking the differential  8 . 
     But if the battery voltage is less than the minimum voltage of table  116  corresponding to the current coil temperature, controller  102  opens switch  122 , thereby electrically disconnecting the battery terminal from the ends of coil  44 , deenergizing coil  44 , and allowing the springs  80 ,  82  to unlock the differential mechanism  8 . 
     Preferably while the available battery voltage is lower than the minimum voltage, the vehicle operator is alerted by a sensible indicator  124 , such as a warning lamp or a buzzer or chime on the instrument panel, that the differential  8  cannot be currently locked. Controller  102  actuates the indicator  124  while the available battery voltage is lower than the minimum voltage. 
     If the locker control system  100  for differential  8  is required to function at low voltages and high temperature without a temperature compensation strategy, then the cost and weight of electromagnetic hardware, i.e., the wiring, coil, air gap, windings, etc., would increase significantly and their size would not meet the packaging requirements. 
     The temperature compensation prevents potential hardware damage due to partial engagement of the clutch teeth  34  on the locking ring  26  with the clutch teeth on the side gear  22 , locking of the locking ring  26  and unnecessary warranty claims due to battery and or alternator failure. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.