A torque-biasing system (10) includes a torque-biasing device (12) and a control unit (14) for use in a motor vehicle. The torque-biasing system is preferably installed in a vehicle having a first front wheel (18) with a first rotational speed and second wheel (20) with a rear rotational speed and an engine (22) with a torque output. The control unit (14) functions to determined when and how to bias the torque output of the front and rear wheels, and to control the torque-biasing device (12) based on this determination.

TECHNICAL FIELD

This invention relates generally to the field of torque-biasing systems and, more particularly, to the field of torque-biasing systems for a vehicle with a first front wheel and a rear wheel.

BACKGROUND

Some conventional vehicles include all-wheel-drive capabilities. These vehicles power two wheels during high-traction situations to enhance fuel-economy, and power all four wheels during reduced-traction situations to enhance road traction and stability. Torque-biasing devices are conventionally used to transfer the torque output from the engine source away from a first wheel and towards a second wheel during the reduced-traction situation.

During a reduced-traction situation, one of the wheels of the vehicle often has a much faster rotational speed than another wheel. Torque-biasing devices are conventionally controlled based upon the difference between the rotational speeds of a first wheel and a second wheel. Once the torque-biasing device is activated, the vehicle powers all four wheels and the effect of the reduced-traction situation is hopefully minimized.

A problem arises, however, during a tight turning maneuver of the vehicle with a conventional torque-biasing system. During such maneuver, each of the four wheels of the vehicle track a different turning radius and, consequently, are turning at a different rotational speed. As an example, the number of tire tracks of a typical vehicle change from two tire tracks to four tire tracks as the vehicle turns into a snowy driveway. In this situation, the torque-biasing system may improperly interpret the different rotational speeds of the wheels as a reduced-traction situation and may improperly power all four wheels. In a tight turning situation, the powering of all four wheels causes a disturbing “crow hop” situation as the torque-biasing device attempts to reduce the rotational speed difference between the wheels of the vehicle, which must turn at a different rotational speed. Thus, there is a need in the field of torque-biasing systems to provide a torque-biasing system with an improved control algorithm and control unit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description of the preferred embodiment of the invention is not intended to limit the invention to this preferred embodiment, but rather to enable any person skilled in the art of torque-biasing systems to make and use an embodiment of the invention.

As shown inFIG. 1, the torque-biasing system10of the preferred embodiment includes a torque-biasing device12and a control unit14. The torque-biasing system10is preferably installed and used in a vehicle16having a first front wheel18with a first front rotational speed, a rear wheel20with a rear rotational speed, and an engine22with a torque output. More preferably, the torque-biasing system10is installed and used in an all-wheel-drive (“AWD”) vehicle, such as a truck or so-called sport-utility-vehicle, having two front wheels, two rear wheels, and a longitudinally-mounted engine. With this arrangement, the torque-biasing device12functions as a “transaxle-type” torque-biasing device to transfer the torque output away from the rear wheels and towards the front wheels during a reduced-traction condition. The torque-biasing system10, however, may be installed and used in any suitable environment. For example, the torque-biasing system10may be installed and used in an AWD vehicle, such as a minivan or sedan, having a transversely-mounted engine. With this arrangement, the torque-biasing device12functions to transfer the torque output away from the front wheels and towards the rear wheels during a reduced-traction condition.

The torque-biasing device12of the preferred embodiment includes a conventional electrically-actuated multi-plate clutch, as typically sold under the trade-name TORQUE ON DEMAND by the BorgWarner company. The torque-biasing device12may, however, include other suitable devices, such as a hydraulic or viscous-actuated multi-plate clutch or a gear, dog, or cone-type clutch without a multi-plate. The particular choice for the torque-biasing device12may vary based on several factors, including the size and layout of the vehicle16and the torque output of the engine22.

The control unit14of the preferred embodiment functions to determine when and how to bias the torque output to the front wheels and to the rear wheels20, and to control the torque-biasing device12based on this determination. The first function, determining when and how to bias the torque output, is preferably accomplished in two steps. In the first step101inFIG. 2, the control unit14estimates a difference between the first front rotational speed and the rear rotational speed caused by the turning maneuver. In the second step102inFIG. 2, the control unit14derives a control signal based upon the estimated difference caused by the turning maneuver subtracted from a measured difference between the first front rotational speed and the rear rotational speed. Once a signal has been derived, the signal is sent to the torque biasing device, as indicated at103of FIG.2.

The first step101, estimating a difference caused by the turning maneuver, is preferably accomplished by receiving data for the first front rotational speed and the rear rotational speed and by using a series of mathematical algorithms based on a simplified geometry of the vehicle16. The data is preferably sent and received in any suitable format and by any suitable means, such as a communication's cable.

The rotational speed difference of the first front wheel18and the rear wheel20caused by the reduced-traction situation, shown as Δω, is calculated with the following equation: Δω=Δωm−Δωtt. In this equation, the Δωmvalue is the rotational speed difference measured by a suitable sensor and communicated to the control unit14, and the Δωttvalue is the rotational speed difference caused by the tight turning maneuver and estimated with the followings equation:Δωtt=vfrwf-vrrwr=(Rfrwf-Rrrwr)⁢vR.
In this equation, the νf, νr, and ν values are the velocities of the first front wheel18, the rear wheel20, and the vehicle16, respectively, and are measured by a suitable sensor and communicated to the control unit14. The constants rwƒand rwrof the equation are the tire radii of the first front wheel18and the rear wheel20. The Rƒ, Rr, and R values are the turning radii of the first front wheel18, the rear wheel20, and the vehicle16, respectively, and are estimated with the following equations:Rf=R2+lf2-2⁢Rlf⁢cos⁡(π2+β)⁢⁢Rr=R2+lr2-2⁢Rlr⁢cos⁡(π2-β).
In these equations, the lƒand lrconstants are the longitudinal distance from the center of gravity of the vehicle16to the first front wheel18and the rear wheel20, respectively, while the β value is the body slip angle.

The β value is preferably obtained with the following equations:β=lrR-αr=lrR-Mr⁢V22⁢Cr⁢R=2⁢Cr⁢lr-mr⁢v22⁢Cr⁢R.
In these equations, the αrvalue is a wheel slip angle of the rear wheel20, the Crconstant is the spring constant of the rear wheel20, and the Mrvalue is the cornering force on the rear wheel20measured by a suitable sensor and communicated to the control unit14. By using these constants and measured variables in the above algorithms, the R value—representing the turning radius of the vehicle16—is the only other variable needed to calculate Δωtt.

The turning radius of the vehicle16may be derived in two ways. The first method uses the rotational speed of the first front wheel18and the rotational speed of a second front wheel24in the following equation:R=t⁡(Vout+Vi⁢⁢n)(Vout-Vi⁢⁢n).
In this equation, the t constant is the track of the vehicle16(the measurement between the first front wheel18and the second front wheel24) and the Voutand Vinvalues are the rotational speeds of the “outer turning wheel” of the first and second front wheels18and24and the “inner turning wheel” of the first and second front wheels18and24, respectively. The Voutand Vinvalues are measured by the suitable sensors, as discussed above.

The drawback of this algorithm, however, is the high dependency on the rotational speed of the wheels of the vehicle16. Because of this dependency, this algorithm may sometimes improperly handle a reduced-traction situation where one or two wheels are spinning at a relatively fast rotational speed. This problem can be overcome, however, by assuming a steady state handling response and by using the steering wheel angle, as shown in the following equation:R=(lf+lr+Kvx)⁢SSRδSWA.
In this equation, the K constant is a stability factor, the Vxvalue is a measured longitudinal velocity of the vehicle16, the SSR constant is a total steering system ratio of the vehicle16, and the δswavalue is the steering wheel angle. The data for the steering wheel angle is measured by a suitable sensor and communicated to the control unit14.

After the first step, estimating a difference between the first front rotational speed and the rear rotational speed caused by the turning maneuver, the control unit14proceeds to the second step102and derives a control signal. The control signal is preferably based upon the estimated difference caused by the turning maneuver subtracted from the measured difference between the first front rotational speed and the rear rotational speed.

The second function of the control unit14, controlling the torque-biasing device12, is preferably accomplished by sending the control signal to the torque-biasing device12. The control signal is preferably sent and received in any suitable format and by any suitable means, such as a communications cable.