Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Control of power distribution between vehicle axis or wheels
Reexamination Certificate
2001-08-14
2002-11-26
Camby, Richard M. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Control of power distribution between vehicle axis or wheels
C701S089000, C701S087000
Reexamination Certificate
active
06487486
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method of controlling a differential, and more specifically, to a method of automatically controlling the transfer of torque within a motor vehicle differential by monitoring the speeds of differential output shafts and comparing those speeds to a normalized target value and subsequently using specific combinations of proportional, integral and derivative (PID) control logic to transfer torque to the necessary output shaft of the differential.
BACKGROUND OF THE INVENTION
Four-wheel-drive systems for the automotive, pickup truck and sport utility vehicle markets utilize a variety of mechanical, electrical and electronic systems to engage and control the driving wheels when a four-wheel-drive system activates and applies torque to a desired wheel. The amount or quantity of torque that an equipped four-wheel-drive system is capable of supplying and the time within which the system is capable of supplying or transferring torque depends upon the design of a particular four-wheel-drive system and its method of control.
Several major categories of four-wheel-drive systems are employed in vehicles. One such system consists of a permanent or full-time four-wheel-drive system. This system has no two-wheel-drive mode. Vehicles with permanent four-wheel-drive typically have a locking center differential which may split torque to the front and rear drive shafts. Electronics may or may not be employed to lock the differential to transfer torque to the front or rear driveshaft and ultimately the front or rear axle. Driving with the center differential locked makes steering more difficult or strenuous because the vehicle will tend to travel directly forward, and resist turning, especially on dry or compacted surfaces. When the center differential is open (not locked), it does not make any torque adjustments between the front and rear driveshafts.
Another type of four-wheel-drive system is on-demand four-wheel-drive. The major difference between permanent four-wheel-drive and on-demand four-wheel drive is the lack of a center differential on an on-demand system. In on-demand systems, either the front or rear axle receives all of the torque for everyday driving. Torque is transferred to the non-slipping axle when necessary, if the vehicle is so equipped. However, in most on-demand systems there is generally no way to turn off the four-wheel-drive, and likewise, the driver does not have to activate the system.
Part-time four-wheel-drive is yet another type of, four-wheel-drive system. This type of system provides torque to all four wheels upon engagement of the system by the driver. A vehicle equipped with part-time four-wheel-drive generally has an axle that receives all of the torque when the system is disengaged. A transfer case is employed to provide simultaneous torque to the front and rear axles when the system is engaged, including the normally non-driven axle, typically the front axle in many vehicles. However, there is no center differential to regulate torque between the front and rear driveshafts. Upon engagement, the front and rear axles are generally synchronized and rotate at the same speed, which improves straight line traction but makes turning the vehicle more difficult than if the speeds of the front and rear driveshafts could be altered. Because of this design, there is no way for the two axles to rotate at different speeds, in a cornering situation, for instance. Therefore, vehicle operation on loose or forgiving surfaces is necessary to permit wheel slip which helps to prevent damage to driveline components which could occur if the system is engaged and operated on dry pavement. If the vehicle is driven on dry pavement with the four-wheel-drive system activated, vehicle occupants will likely feel an awkward, vibrating rumble as a corner is turned, or even when straight line driving is attempted due to wheel speed mismatch. This vibration is caused by binding within the driveline system and may indicate impending driveline damage.
Drawbacks of the part-time four-wheel-drive system are the omission of a center differential thereby committing the front and rear axles to a matched speed and provokes the possibility of damage when the system is operated on dry pavement. Additional drawbacks of this type of system are that vehicle operators may be required to stop the vehicle to engage or disengage the system and to make the initial decision of whether road conditions warrant engaging the four-wheel-drive system. Varying road conditions may present a particular problem to drivers when deciding to engage and disengage a system. Requiring the driver to make this decision is yet another drawback of the part-time four-wheel-drive system.
Ultimately, there are automatic four-wheel-drive systems. These systems automatically determine when four-wheel-drive is necessary and transfer, as requirements necessitate, torque to the wheel(s) with the most traction, or rather, to the wheels with the least amount of wheel slip. This system requires a limited slip center differential or similar type of viscous coupling or multi-plate clutch, in a transfer case, a front and rear driveshaft, and a limited slip differential in each axle. The benefit of an automatic four-wheel-drive system is that it senses its own traction needs as the vehicle travels over any terrain, and then continuously adjusts for wheel slippage or lack of traction. This permits the driver to concentrate on driving instead of having to concern himself or herself with shifting the vehicle into or out of four-wheel-drive. However, a drawback of the automatic four-wheel-drive system is that a driver's efforts may be hampered in severe off-road conditions. That is, because the system constantly monitors wheel slip according to the terrain, a driver may find the torque adjustment from one wheel or axle to another to be abrupt and unsettling while the vehicle negotiates such terrain. This may be especially true if the four-wheel-drive system senses a need for a torque adjustment and then lags in invoking that adjustment, causing an abrupt and unsettling shift in torque, regardless of the degree of that unsettling shift. As the speed of the torque adjustment upon wheel slip increases, the abruptness of the shift will decrease.
The above four-wheel-drive systems are all capable of providing four-wheel-drive capability in some capacity, whether it be full-time, on-demand, part-time or automatic. The automatic four-wheel-drive systems base the engagement of the system upon a change in a measurable variable. However, what all current four-wheel-drive systems lack is a method of controlling wheel slip through the utilization of a measurable variable that takes into consideration the output shaft speeds of a differential but that is also normalized with respect to vehicle speed.
What is needed then is a method of controlling the torque through a differential that does not suffer from the above four-wheel-drive system limitations. Furthermore, what is needed then is an automatic four-wheel-drive system that utilizes a method of controlling wheel slip through the invocation of a calculated target differential ratio that considers the difference between the output shaft speeds of a differential but that also normalizes the difference with respect to vehicle speed and which then continually bases torque requirements on the comparison of the actual differential ratio to the normalized target differential ratio.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, an automatic four-wheel-drive system is disclosed which utilizes automatic differential control logic. The invention provides a logic based system that will compare the actual difference (&Dgr;
A
) between a given differential's output shaft speeds to an acceptable range about a target difference (&Dgr;
T
) between the differential output shaft speeds and determine, for a given differential, whether a torque bias is needed in order to decrease or prevent wheel slip from occurring.
Calcaterra Mark P.
Camby Richard M.
DaimlerChrysler Corporation
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