Controllable torque transfer differential mechanism using magnetorheological fluid

The present invention relates generally to torque transfer differential systems and more particularly to controllable torque transfer differential mechanisms using magnetorheological fluid.

A differential delivering torque to the wheels of a vehicle allows for wheel slippage and over-spin. Essentially through the gearing within a differential assembly, a balance of torque is achieved for both wheels semi-independent of wheel slippage.

Control of torque and wheel spin in a differential mechanism usually takes the form of friction surfaces and clutch engagement packages. However, one drawback of these systems is that they are usually subject to wear out and fatigue.

Another method used is to use an electrically controllable limited slip differential employing a “ball/ramp” torque multiplier device actuated by a solenoid to provide an electromechanical method to achieve desired friction levels. However, these systems can exhibit nonlinear torque transfer upon actuation.

Another method contemplated is to use magnetorheological (“MR”) fluid in a differential mechanism or coupler to control torque transfer. These devices focus on the fluidic shear action of the MR fluid in a disc-to-disc coupling device to transfer torque. One problem with these devices is that the abrasive structure of the MR fluid when actuated causes the friction surfaces to wear out and fatigue.

Linear control of torque transfer is highly desirable in a differential assembly to optimize vehicle performance over traction and stability events.

An object of the present invention is to provide a controllable torque transfer differential mechanism for use in a vehicle.

The above object is accomplished by coupling a closed fluid pump system of magnetorheological fluid to various points on a differential assembly. The pump routes the magnetorheological fluid in a manner for straightforward actuation via a magnetic circuit allowing clear separation of rotating and nonrotating members.

The present invention offers several advantages over previous systems. First, the present invention offers linear control over actuation and corresponding torque transfer. Second, the present invention offers simple differential assembly adaption and a simple electrical actuation method. Third, the present invention requires low power to the actuator. Finally, the present invention offers increased durability over previous MR coupled devices.

Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.

FIG. 1 depicts a differential assembly according to the present art;

FIG. 2A depicts a differential assembly according having a closed fluid pump system coupled to a differential pinion gear according to one preferred embodiment of the present invention;

FIG. 2B depicts a sectional view a portion of FIG. 2A; and

FIG. 3 depicts a differential assembly according having a closed fluid pump system coupled to one of the side gears according to another preferred embodiment of the present invention.

Referring now to FIG. 1, a differential assembly is illustrated generally as 10 . The differential assembly 10 is a gear system that transfers power from an input source to the wheels. The input source includes a drive shaft 11 coupled to an engine crankshaft (not shown) that is also coupled to a drive pinion gear 15 . The drive pinion gear 15 is also coupled with a ring gear 18 . The ring gear 18 is typically coupled to the differential casing 13 .

A pair of driving axle shafts 17 are coupled between one of a pair of differential side gears 12 , 14 and the wheel assembly (not shown). The driving axle shafts 17 are splined to a pair of differential side gears 12 , 14 at right angles to the line of drive. The differential 10 uses a differential pinion gear 16 coupled to the ring gear 18 to redirect the transfer of power to the side gears 12 , 14 , which in turn directs the power to the driving axle shafts 17 and wheels to control a vehicle.

In operation, as power is requested to drive the driving axle shafts 17 of a vehicle, power is transferred from the drive shaft 11 to the drive pinion gear 15 , which in turn causes the ring gear 18 and the differential casing 13 attached to it to rotate. The differential casing 13 encloses the differential pinion gears 16 and side gears 12 , 14 .

In straight-ahead operations, the ring gear 18 and the differential casing 13 rotate as a unit. The differential pinion gears 16 do not turn about their own axes, but apply equal effort to each of the differential side gears 12 , 14 and axle shafts.

On turns, the resistance against rotation of one axle shaft 17 increases as the wheels turn at different speeds. This causes the differential pinion gears 16 to turn on their own axes and roll around the differential side gears 12 , 14 of the reluctant one of the two axle shafts 17 . This allows the reluctant axle shaft 17 to slow down or stand still, causing a corresponding increase in speed of the rotation of the other axle shaft 17 . If one axle shaft 17 does not turn at all, the other axle shaft 17 will turn at almost twice the normal speed. It is possible for the drive wheels to turn at different speeds while the same amount of power is applied to them.

FIGS. 2A and 2B show one preferred embodiment of the present invention, in which a magnetorheological-based torque controlling system is coupled within the differential assembly.

Referring now to FIG. 2A, a vane-type fluid pump 22 of a closed magnetorheological fluid pump system 20 is connected with the differential pinion gear 16 to control the torque transfer from the drive shaft 11 to the driving axle shafts 17 . The pump system 20 also has a fluid capillary tube 24 in fluid communication with the pump 22 , a magnetic circuit 26 coupled to the capillary tube 24 , and an electronic control unit 28 coupled to the coil 27 by a pair of connections 30 , 32 . The fluid capillary tube 24 is made of a non-ferromagnetic material such as a hardened plastic, carbon fiber material, or aluminum.

The magnetic circuit 26 consists of a coil 27 wrapped around a ferromagnetic material (steel) to focus the magnetic flux. Actuation power for the coil 27 is low (in the order of Amperes) and the magnetic flux can be easily increased via more coil turns or wrappings (e.g. Ampere's Circuital Law). The electronic current through the coil 27 is controlled by the electronic control unit 28 .

The vane-type fluid pump 22 consists of an inner housing 34 having a plurality of vanes 36 affixed to the differential pinion gear 16 . The pump 22 also has a fluid inlet 40 and fluid outlet 42 contained on the differential casing 13 that is affixed to a non-rotating portion. In this respect, the inner housing 34 and vanes 36 rotate in response to the rotation of the differential pinion gear 16 , while the differential casing 13 rotates at a speed as a function of the drive pinion gear 15 .

Contained within the fluid pump system 20 is a magnetorheological (“MR”) fluid 44 . The MR fluid 44 is a controllable fluid medium that changes from a free flowing liquid to a semi-solid state when a magnetic field is applied by aligning magnetically polarized particles contained within the MR fluid 44 to form particle chains. This effectively increases the viscosity of the MR fluid 44 . When the magnetic field is removed, the MR fluid 44 returns to its original liquid state. Advantageously, the response time for MR fluid 44 to change between a steady-state semi-solid phase to a steady-state fluid (liquid) phase is in the range of a millisecond. Therefore, torque transfer control changes can be performed quickly.

Furthermore, MR fluid 44 can be operated at specific intermediate viscosities between the fluid state and the high-viscosity state by varying the magnetic field applied to the MR fluid 44 . Preferably, MR fluid 44 is a mineral-oil based fluid or a silicon-oil based fluid.

Since the inner housing 34 having the vane-type pump 22 and vanes 36 is coupled to the differential pinion gear 16 , these components rotate as well, causing MR fluid 44 to flow out of the fluid outlet 42 , through the capillary tube 24 , and return through the fluid inlet 40 in a closed loop.

During spin-out or turning conditions, the electronic control unit 28 will direct that current be sent through the coil 27 . This movement of current through the coil 27 induces a magnetic field within a portion of the capillary tube 24 . This magnetic field induces the MR fluid 44 flowing through the portion 25 of the capillary tube 24 to increase viscosity as described above. The capillary tube 24 typically is narrowed within this portion 25 . The larger the current flowing through the coil 27 , the higher the viscosity of the MR fluid 44 up to an upper limit. This increased viscosity limits the flow rate through the pump 22 , thereby decreasing the rotational speed of the pump 22 and the coupled differential pinion gear 16 . Essentially, this creates a braking effect that decreases the amount of torque transmitted to the driving axle shafts 17 and to the wheels.

FIG. 2B shows a closeup view of the pump system 20 of FIG. 2 A. The inner housing 34 of the pump 22 is affixed to the splined portion 75 of the shaft 38 of the differential pinion gear 16 and rotates to pump fluid through the capillary tubes 24 when the differential pinion gear 16 rotates. The capillary tube 24 is preferably helically wrapped in a screw like fashion around the differential casing 13 covering the splined portion 75 of one of the side gears 12 , 14 . This ensures proper exposure of the MR fluid 44 flowing through the capillary tube 24 to a magnetic field produced by the coil 27 of the magnetic circuit 26 . The magnetic circuit 26 encompasses a portion of the capillary tube 24 and is similarly affixed to the differential housing 73 such that magnetic circuit 26 does not rotate as the differential pinon gear 16 or side gears 12 , 14 rotate.

In another preferred embodiment, as depicted in FIG. 3, the vane-type pump 22 of the closed magnetorheological fluid pump system 20 is coupled to the differential casing 13 one of the side gears 12 , or side gear 14 (shown here connected to side gear 14 ). The mechanism for limiting the flow rate of the viscous magnetorheological fluid through the pump 22 is similar to that of FIG. 2 A. In these cases, the transmission of torque from the differential pinion gear 16 to the differential side gears 12 , 14 create flow of viscous magnetorheological fluid through the closed magnetorheological fluid pump system 20 . As current is directed through the coil 27 by the electronic control unit 28 , the viscosity of the magnetorheological fluid is increased by changing the phase of the magnetorheological fluid from a liquid phase to a semi-solid phase, which in turn limits the flow rate of the magnetorheological fluid through the pump 22 . This in turn limits the rotation of the coupled side gears 12 , 14 , thereby limiting the torque supplied to the driving axle shafts. As in FIG. 1, the amount of the braking effect is a function of the flow rate of magnetorheological fluid through the vane-type pump 22 , which is controlled by the amount of electrical current flowing through the coil 26 as directed by an electronic control unit 28 .

The embodiment depicted in FIG. 3 may be preferable to the embodiment depicted in FIGS. 2A and 2B since this embodiment also may help to eliminate potential rotational inertial effects.

Two other preferred embodiments combining the principles as described in FIGS. 2A and 3 are also contemplated within the scope of the present invention. First, it is specifically contemplated that an additional vane-type pump may be added to one of the side gears 12 or 14 in FIG. 2A to provide additional torque control within the closed magnetorheological fluid pump system 20 . Second, an additional vane-type pump could be added so that both of the side gears 12 , 14 have a coupled pump. These vane-type pumps may be coupled within a single closed loop system or within separate closed loop systems coupled to an electronic control unit 28 and still effectively control the torque transfer from the driving shaft to the driving axle shafts.

While the embodiments as depicted in FIGS. 2A and 3 show a vane-type fluid pump 22 , other types and sizes of pumps may be used and still fall within the spirit of this present invention. For example, the pump could be a gear pump such as a gerotor pump or multiple gear pump. Further, the size, number and location of the electrical coils 27 may be varied and still fall within the scope of the present invention.

The present invention offers many advantages over currently available torque limiting systems. First, the durability of the present invention is greater than that of a typical MR fluid-based clutch system. MR fluid abrasion, which affects the durability of a typical MR fluid-based clutch systems, is not a concern in the present invention because the fluid is not being sheared between friction surfaces and clutch engagement packages to create torque. This shearing process creates heat, which degrades the MR fluid, which affects clutch life. Further, the friction surfaces and clutch engagement packages are subject to wear out and fatigue.

Second, the present invention utilizes a linear actuation mechanism to control torque, as compared with typical differential torque limiting mechanisms which employ a “ball/ramp” torque multiplier device actuated by a solenoid to provide an electro-mechanical way to achieve the friction levels desired. Linear control of torque transfer is desirable in a differential assembly to optimize vehicle performance over traction and stability events.

Third, the present invention is easily adapted to differential assemblies. The rotating elements of the pump are simply splined to either the drive pinion gear, the side gears, or a combination of both, while the non-rotating elements are secured to the differential casing without creating packaging problems.

Finally, the present invention requires low input power to actuate the coils to create a magnetic field that is used to convert the MR fluid to a semi-solid state. The requirements for this type of actuation are typically a few Amperes.

While the invention has been described in terms of preferred embodiments, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings.