Abstract:
A control for a differential including magnetic powder torque transfer clutch for controlling slip across the differential mechanism. The clutch includes a first element driveably connected to a first control shaft, a second element driveably connected to the second output shaft, chamber bounded by the first and second elements, magnetic powder located in the chamber; and an elecromagnetic coil producing a magnetic field of variable strength passing through the chamber and magnetic powder and changing a capacity of the clutch to transmit torque between the first and second control shafts as the field strength changes.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to the control of torque transfer in a differential mechanism having a magnetic powder differential lock providing limited and controllable slip.  
         [0003]     2. Background of the Invention  
         [0004]     In automotive applications, two or more wheels typically drive the vehicle. Since each of the driven wheels takes a different path length, especially around turns or over uneven road surfaces, the drive wheels must not be forced to drive at exactly the same rotational speed. The automotive differential solves the problem of wheel contact and path length by sharing torque equally on both sides of a gear set, with the driven speed determined only by the torque provided. The negative consequence of torque sharing is that under conditions of limited traction, no wheel is given any more torque than the wheel with the least traction. The generally accepted solution for improved traction is differential braking or lockup, or a limited slip differential. There are a number of problems with these techniques. This invention provides an improved embodiment of a controllable differential lock with controllable slip.  
         [0005]     A differential brake or lock allows the operator of the vehicle to intentionally lock the rotational speed of the wheels of the vehicle together by closing a friction brake tied to the axle shaft of each wheel. When the lock is engaged, the differential is no longer operative and the wheels all turn at the same speed. Control is typically up to the vehicle operator, and a significant torque pulse usually accompanies engagement of the differential lock. This type of device is common on agricultural and industrial off road equipment.  
         [0006]     A limited slip differential automates control of the lock up of a differential lock. Typically wheel spin is sensed mechanically, by shearing a fluid, or electronically, by wheel or shaft speed sensors. The brake is applied mechanically, by pressure, or electrically, by locking up the axles of the vehicle utilizing a differential lock. Operator intervention is not required. A significant torque pulse still usually accompanies the engagement. In the case of a fluid lock, significant energy must also be dissipated as heat. Individual wheel braking, to maintain traction control, also generates significant heat because it is applied with respect to the chassis.  
         [0007]     In all these cases, when a wheel spins, the axle wind-up, i.e., energy stored in compliance of the member, is released suddenly. Finally, locking clutches are not typically used in front-drive vehicles, and never for torque bias ratios greater than 2.5:1 because of torque-steer problems.  
         [0008]     Magnetorheological (MR) clutches and brakes enable electrical control of torque transfer and rotational slip. These clutches and brakes typically use MR fluids, which are slurries of 2 to 5 micron particles suspended in oil. As magnetic fields are applied to the fluid, the particles tend to form chains, which are capable of carrying torque proportional to the magnetic field. Magnetic powder clutches and brakes are fluid-less analogs of MR clutches and brakes. The magnetic portion of their design is nearly identical to MR fluid devices, but since air is their fluid medium, inexpensive baffles can be used in place of seals. The magnetic particles are typically magnetic, martensitic stainless steel for corrosion resistance, with 10-100 micron diameters. Other advantages include engagement can be made at any speed, the powder is relatively tolerant of slip, and it can operate at very high temperature, roughly 500° C.  
         [0009]     The use of a MR fluid clutch to control the locking of a differential is disclosed in U.S. Pat. No. 5,845,753. Its implementation is daunting, because the entire wheel torque of the vehicle (on the order of 1000 N-m) is carried across a MR fluid coupling. In order to transfer that magnitude of torque, the device would have to be large and very heavy. At high differential speeds between axles, the particles would tend to disperse away from the center of the multi-disc clutch due to centrifugal force. This tendency to centrifugate would severely and irreversibly reduce the torque carrying capacity of the clutch.  
         [0010]     U.S. Pat. No. 5,915,513 attempts to overcome packaging problems by using a multi-disc, MR clutch to control a ball-ramp clutch, which performs the locking. Control is by dedicated sensors. Again, engagement would not be possible at high differential shaft speeds. Similarly, U.S. Pat. No. 6,428,441 controls a ball-ramp based on targeted differential speed. U.S. Pat. No. 6,412,618 affects a similar fix with the enhancement of a magnetic powder brake.  
         [0011]     U.S. Pat. No. 6,334,832 overcomes the torque carrying capacity requirements by gearing up each side of the differential significantly to directly drive an MR clutch. Shaft speed is higher, but torque is lower, so package size, weight, and cost are substantially reduced. This patent describes a disc-geometry clutch, with all of the relative speed engagement issues raised above, but multiplied by the higher rotational speed. It is unlikely that engagement at automotive axle speeds would be possible.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention provides a magnetic powder clutch, which controllably adjusts the magnitude of torque transmitted by a clutch between two rotating shafts. Alternative embodiments may use magnetorheological or electrorheological fluids in concentric-cylinder clutches or brakes.  
         [0013]     An advantage of the invention is high long-term durability of the clutch because aggregate engagement time of the clutch is short and there is little wear of clutch surfaces. Torque reduction in the mechanism can be accomplished using gears, chains, belts, higher speed gearing. The mechanism is simple and employs an inexpensive control that relies on the appropriate speed ratio of axle half shafts.  
         [0014]     The torque required after gearing-up both front and rear axles is readily realizable at low electrical power expenditure, size and weight. In addition, the clutch uses concentric cylinders, which allows engagement at essentially any automotive axle speed. It is well known that centrifugation is not a problem with this geometry.  
         [0015]     Because the energy dissipated by the clutch is the product of the relative rotational rate of the two clutch members and the torque produced, heat buildup is low and completely manageable at the desired low slip rates. Heat dissipation due to frictional contact and slip across the clutch is low.  
         [0016]     In realizing these and other advantages, a method according to the present invention controls wheel slip across a differential mechanism of a motor vehicle having a vehicle speed, a steering angle and a steering direction, the differential mechanism including two rotating output shafts driveably connected to the wheels and connectable by a clutch that is activated by a magnetic field of variable strength to change a torque capacity of the clutch. The method includes comprising the steps of determining the current driveshaft speed, the magnitude and the direction of the current steering angle; determining a target speed of the output shafts that corresponds to the current driveshaft speed, steering angle and steering direction; determining the speeds of the output shafts; comparing the speeds of the output shafts and the target speeds to determine whether wheel slip is present; and adjusting the magnitude of the field strength to increase the torque capacity of the clutch and control the speed of the rotating shafts such that wheel slip is reduced.  
         [0017]     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a schematic illustration of a rear wheel drive limited slip differential gearset controlled by a magnetic powder clutch;  
         [0019]      FIG. 2  schematically shows the clutch in more detail; and  
         [0020]      FIG. 3  is a control strategy for monitoring and controlling the relative wheel speeds of the powertrain of  FIG. 1   
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]     Referring now to the drawings, there is illustrated in  FIG. 1 a  schematic diagram of a rear wheel drive, limited slip differential mechanism  10  and a gearset controlled by clutch  12 , which is actuated by selective creation of a magnetic field operating on magnetic powder contained in the clutch. The clutch and its associated control determine the relative magnitudes of torque supplied to right-hand and left-hand axle shafts  14 ,  16 . All shafts in this design are supported rotatably by appropriately sized bearings.  
         [0022]     An engine or another power source (not shown) is driveably connected to a transmission, which produces a range of gear ratios some of which increase the speed and reduce the torque at the transmission output relative to those of the engine output. Some of the gear ratios produced by the transmission decrease the speed and increase the torque at the transmission output relative to those of the engine output.  
         [0023]     The transmission output is driveably connected to and transmitted by a driveshaft  18  through a bevel pinion gear  20 , which meshes with a ring gear  22 , creating a 90-degree change in the direction of angular momentum. The ring gear  22  is secured to a pinion carrier  24  surrounding a differential gearset. Bevel pinions  26 , supported on pinion carrier  24  for rotation about the axis of the axle shafts  14 ,  16 , are in continuous meshing engagement with bevel side gears  28 ,  29 , secured to axle  14  and  16 , respectively. The differential gearing operates such that when the car is moving in a straight line, either forward or rearward, the left axle  16  and right axle  14  have the same speed. In this condition, there is no wheel or tire slip, and the speeds of the side gear  29 , bevel pinions  26 , and right side gear  28  are equal.  
         [0024]     When the vehicle turns at a constant angle from the straight direction, the wheel at the outside of the turn increases in speed, and the inner wheel decreases in speed, such that the average speed of the vehicle does not change from the straight-line speed, otherwise the wheels will experience slip. For a given angular velocity of the pinion carrier  24 , if the rotational speed of the left axle  16  increases, then the rotational speed of the right axle  14  decreases a commensurate amount.  
         [0025]     In normal operation of the vehicle when both wheels have reasonably high coefficient of friction with the ground, there will be no slippage of either tire. Torque at the axles  14 ,  16  will be converted almost entirely to vehicle movement. However, if the left axle  16  is on a surface having a high coefficient of friction such as pavement, and the right axle  14  is on surface having a low coefficient of friction such as ice, then the left axle  16  will have no angular velocity and the right axle will spin at twice the speed of the pinion carrier  24 . This occurs because the torque at the left axle  16  and the right axle  7  must always be equal. Since the left shaft  16  is not rotating, it has no torque other than that caused by friction losses due to gear meshing. Therefore, the right axle shaft  14  also has virtually no torque, and the vehicle will not move.  
         [0026]     In the condition described with the vehicle is not moving, a device to allow the differential to transfer driveshaft angular velocity to both the left and right axles in appropriate proportions, depending on steering angle, is desired. A gear train is used to transfer high torque and low speed at the axles  14 , 16  to high speed and low torque in the magnetic powder clutch  12 . A left axle gear  28  is driveably connected to an outer shell  30 , which surrounds the magnetic powder clutch  12 . This drive connection is produced by a set of meshing gears  32 ,  34  and a shaft  36 , to which gear  34  is secured. Alternatively a sprocket wheel or pulley, secured to axle  16 , and a sprocket wheel or pulley, secured to shell  30 , are engaged by a drive belt or a drive chain so that the speed of shell  30  is appropriate increased. The magnitude of the required speed increase will depend on the operating characteristics of the vehicle and of the magnetic powder clutch  12 .  
         [0027]     A right axle gear  38  is driveably connected to the inner shell  40 , which surrounds the magnetic powder clutch  12 . This drive connection is produced by a set of meshing gears  42 ,  44  and a shaft  46 , to which gear  44  is secured. Alternatively a sprocket wheel or pulley, secured to axle  14  and a sprocket wheel or pulley, secured to shell  38 , are engaged by a drive belt or a drive chain. In either case, the gear ratio of the right side matches that of the left side. When clutch  12  is fully engaged, the right and left axle shafts  14 ,  16  are geared together, or otherwise driveably connected, through a fixed connection, and they are forced to turn at the same speed. By varying the degree of engagement of clutch  12 , the magnitude of slip, or differential speed across the clutch, the speed of axle shafts  14 ,  16  can be controlled so that they rotate at an appropriate speed based on input from a steering indicator.  
         [0028]      FIG. 2  illustrates clutch  12  in more detail. The outer shell  30  of the clutch is connected by shaft  36  to, and rotates synchronously with the left axle  16 . Shell  30  is constructed of low-carbon steel or some other material having a high saturation magnetization and low residual magnetization. The inner shell  40  of the clutch is connected through shaft  46  to, and rotates synchronously with the right axle  14 . Shell  40  is also constructed of steel or some other material having a high saturation magnetization.  
         [0029]     In order for the two axles  14 ,  16  to rotate at equal speed or with an appropriate differential speed, the two shells  30 ,  40  are coupled together using magnetic powder  48 , which is contained in a narrow, confined gap between the two shells. The magnetic powder has the property of being sensitive to a magnetic field. When the powder is not magnetized, the powder is urged by centrifugal force into contact with the inner surface of the outer shell  30  and out of contact with the inner shell  40 . In this condition, the powder fills a fraction of the thickness of the gap. A coil of electrically conductive wire  50  is wound around a core  52  of steel or other ferromagnetic material, supported on a bearing  54  and secured to the inner shell  40 . In this way, the bearing  54  acts as a centering device to keep the gap between the inner and outer shells  40 ,  30  constant. Core  52  is fixed to a mechanical ground such as the differential case. When a magnetic field is produced, magnetic flux lines  56  are generated that pass perpendicularly through the gap between the two shells  30 ,  40 .  
         [0030]     The magnetic field is strong enough to overcome the effect of the centrifugal force on the powder, and the field creates chains of magnetic powder along the flux lines between the inner surface of the outer shell  30  and outer surface of the inner shell  40 . The chains have a yield strength that is proportional to the strength of the magnetic field. A mechanical force greater than the magnetic force is required to pull the chains of magnetic powder apart and allow the inner and outer shells to rotate at different speeds. Those who are skilled in the art can easily derive the equations that relate the variables necessary to perform this function.  
         [0031]     The electromagnet  50  is supplied with electric current from a source of d. c. electric power through a line  58 . A variable magnetic field  56  can be produced alternatively by a moveable permanent magnet  50  or by a combination of a permanent magnet and an electromagnet, which biases the magnetic field  56  to control its strength. The current is supplied in the appropriate magnitude by a controller  60 , which monitors the angular disposition of the steering wheel, or that the steered vehicle wheels, or the position or displacement of another component such as a tie rod, which correctly represents the steering angle through which the vehicle is turning with respect to a reference angle, preferably zero degrees, the straight direction. The controller  60  is supplied with output produced by various sensors  62 ,  64 ,  66 , which, respectively, produce signals representing the steering angle SA, speed of the left axle LAS, and speed of the right axle RAS. Wheel speed sensing can be obtained from anti-lock brake system (ABS) wheel speed sensors located at the wheels, or from dedicated sensors.  
         [0032]     Controller  60  is preferably a microprocessor-based controller, which provides integrated control of the engine  14 , transmission, and vehicle dynamics. Controller  60  includes a microprocessor  70  in communication with input ports  72 , output ports  74 , and computer readable media  76  via a data/control bus  78 . Computer readable media  76  may include various types of volatile and nonvolatile memory, such as random access memory (RAM)  80 , read-only memory (ROM)  82 , and keep-alive memory (KAM)  84 . These functional descriptions of the various types of volatile and nonvolatile storage may be implemented by any of a number of known physical devices including, but not limited to EPROMs, EEPROMs, PROMS, flash memory, and the like. Computer readable media  76  include stored data representing instructions executable by microprocessor  70  to implement the method for controlling the differential  10  according to the present invention.  
         [0033]      FIG. 3  illustrates a control strategy for monitoring and controlling the relative wheel speeds by controlling the degree of engagement of clutch  12  and the resultant slip across clutch  12  required to control wheel slip. The structure and operation of clutch  12  and principle of the control according to this invention are applicable also to AWD systems. The energized coil  50  creates a magnetic field  56 , which causes the magnetic powder  48  to become magnetized and to act as either a slipping brake or lock-up clutch, by means of which the relative angular velocity of the axles  14 ,  16  is controlled. The controller  70  provides limited relative slip or complete engagement, called lock-up, of clutch  12 , depending on the design requirements. The magnetic field  56  can be produced by either a constant field, or by pulsing the electromagnet coil  50  with a signal having a PWM duty cycle.  
         [0034]     At step  90 , controller  60  receives and stores in memory the current magnitude of the speeds produced by sensors  64  and  66 . At  92 , an inquiry is made to determine whether those speeds are equal. If that test is true, indicating that the vehicle is not turning, control passes to  98 , where the current steering angle SA is sampled and the steering angle corresponding to the magnitudes of LAS and RAS is determined.  
         [0035]     If the test at  92  is false, another inquiry is made at  96  to determine whether LAS exceeds RAS. If the result of test  96  is true, indicating a right turn is in progress, control passes to  98  where the current steering angle SA is sampled and the steering angle corresponding to the magnitudes of LAS and RAS is determined. If the result of test  96  is false, a test is made at  100  to determine if RAS is less than LAS. If the result of test  100  is true, indicating a left turn is in progress, control passes to  98  where the current steering angle SA is sampled and the steering angle corresponding to the magnitudes of LAS and RAS is determined. If test  100  is false, then continued execution of the control loop is terminated, and control passes to  90 , where the loop is initialized and execution begins again.  
         [0036]     If at  102  the controller  60  determines that, based on the magnitudes of LAS, RAS and SA, there is some wheel slippage occurring, the controller determines at  104  the required PWM duty cycle magnitude, or the magnitude of current to be supplied to coil  50 , or the strength of the magnetic field, obtained by another method such as a permanent magnet or an electromagnet biased by a permanent magnet  50  to correct for wheel slippage and supplies the required duty cycle or magnitude of electrical current through leads  18  to coil  50  of the electromagnet. This duty cycle actuates clutch  12  by increasing the field strength, increasing the strength of the chains of thereby increasing the torque transmitting capacity of the clutch.  
         [0037]     A preferred magnetic powder for use in clutch  12  is commercially available from Carpenter Powder Products of Bridgeville, Pa., U.S.A, through its business unit Anval Powder of Torshalla, Sweden. A preferred magnetic powder is in the form of gas-atomized, spherical particles of 410 low carbon stainless steel, the particles having a size in the range 1-250 microns.  
         [0038]     Other ferromagnetic particles having particle sizes, metallurgical compositions, and morphologies outside these ranges are also possible candidates for changing the torque capacity of the clutch  12  and encompass other embodiments of this design.  
         [0039]     Alternative materials for changing the torque capacity of the clutch  12  include magnetorheological and electrorheological fluids located in concentric-cylinder clutches or brakes.  
         [0040]     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.