Control system for an all-wheel clutch

Abstract

A method and a corresponding controller for the four-wheel drive of a motor vehicle are described, consisting of a clutch (AK) disposed in the drive train between a primary axle and a secondary axle (VA, HA) of the four-wheel drive, by means of which the drive torque of a primary axle (KHA, HA) driven by an engine (VKM, SG) can be distributed to a secondary axle (KVA, VA), wherein the controller (S) has a signal connection to the clutch (AK) and to sensors recording the revolution rates of at least one wheel of the primary axle (HA) and one wheel of the secondary axle (VA) and the clutch (AK) is controlled by means of the controller (S) below a specified torque such that a significantly reduced transfer of torque to the secondary axle (VA) is carried out.

Claims

1. A method for controlling a clutch in a drive train between a primary axle and a secondary axle of a four-wheel drive vehicle, the method including: distributing drive torque from an engine directly to the primary axle; distributing drive torque from the engine to the secondary axle with the clutch or a distribution gearbox; determining a speed of the vehicle and revolution rates of wheels of the primary and secondary axles of the vehicle, and determining a specified drive torque based on the speed of the vehicle and the revolution rates of the wheels of the vehicle; setting a second axle torque to a provided value, wherein the second axle torque is transmitted through the variable clutch or the distribution gearbox to the secondary axle, and wherein the provided value is approximately zero when the specified drive torque is below a predetermined value.

2. The method as claimed in claim 1, wherein the provided value is set such that no torque transfer to the wheels of the secondary axle takes place.

3. The method as claimed in claim 1, wherein the specified drive torque is determined from an effective dynamic tire radii of the wheels of the primary and the secondary axles.

4. The method as claimed in claim 3, wherein wheel revolution rate signals from revolution rate sensors associated with the wheels of the primary and the secondary axles are analyzed for determining the specified drive torque.

5. The method as claimed in claim 3, wherein the second axle torque transmitted through the clutch or gearbox is selected such that revolution rates of the primary and the secondary axles are identical for given effective tire radii of the wheels of the primary and secondary axles and a determined tire stiffness characteristic value.

6. The method as claimed in claim 5, wherein transmission ratios between the primary and the secondary axles are taken into account during the determination of the specified drive torque.

7. The method as claimed in claim 1, wherein tires with a higher effective dynamic radius are fitted on the secondary axle than the primary axle.

8. A controlling system for a four-wheel drive system of a motor vehicle, the controlling system including a clutch disposed in a drive train of the vehicle between a primary axle and a secondary axle of the four-wheel drive system, by means of which a drive torque of the primary axle driven by an engine can be distributed to the secondary axle, wherein a controller has a signal connection to the clutch as well as to sensors recording revolution rates of at least one wheel of the primary axle and at least one wheel of the secondary axle and the clutch is controlled by means of the controller to determine a specified drive torque based on the sensors, and to set a second axle torque to a provided value, wherein the second axle torque is transmitted through the clutch to the second axle, and wherein the provided value is approximately zero when the specified drive torque is below a predetermined value.

9. The controlling system as claimed in claim 8, wherein the clutch is switched by the controller below the specified drive torque for maximum disconnection of the flow of second axle torque from the primary axle to the secondary axle.

10. The controlling system as claimed in claim 8, wherein the specified drive torque is determined from an effective dynamic tire radii of the at least one wheel of the primary axle and the at least one wheel of the secondary axle.

11. The controlling system as claimed in claim 10, wherein wheel revolution rate signals from the sensors associated with the at least one wheel of the primary axle and the at least one wheel of the secondary axle are analyzed for determining the specified drive torque.

12. The controlling system as claimed in claim 10, wherein the second axle torque is calculated as the specified drive torque for which revolution rates of the primary and the secondary axles are identical for given effective tire radii of the at least one wheel of the primary axle and the at least one wheel of the secondary axle and a specific tire stiffness characteristic value.

13. The controlling system as claimed in claim 12, wherein transmission ratios between the primary and the secondary axles are taken into account during the determination of the specified drive torque.

14. A four wheel drive system of a vehicle including: an engine; a primary axle including a pair of primary wheels; the primary axle directly connected to the engine for distributing a primary drive torque from the engine directly to the primary axle; a secondary axle including a pair of secondary wheels; a variable clutch interconnecting the secondary axle to the engine for selectively distributing a provided value of secondary drive torque to the secondary axle; at least one primary sensor configured to record a revolution rate of at least one of the primary wheels; at least one secondary sensor configured to record a revolution rate of at least one of the secondary wheels; and a controller electrically connected to the variable clutch and the primary and secondary sensors and configured to determine a specified drive torque based on the primary and secondary sensors and to establish a provided value of the secondary drive torque to be transmitted to the secondary axle, wherein the provided value is approximately zero when the specified drive torque is below a predetermined value.

Description

DRAWINGS

(1) FIG. 1 illustrates an all-wheel drive vehicle and a controller in accordance with the present disclosure;

(2) FIG. 2 diagramatically illustrates a possible torque distribution between the primary axle and the secondary axle of the all-wheel drive vehicle; and

(3) FIG. 3 diagramatically illustrates another possible torque distribution between the primary axle and the secondary axle of the all-wheel drive vehicle.

DETAILED DESCRIPTION

(4) FIG. 1 shows the basic components of an all-wheel drive vehicle, consisting of an internal combustion engine VKM, a downstream automatically or manually shifted gearbox SG, which drives the wheels of the rear axle HA, in this case the primary axle PA, by means of a propeller shaft KHA. In the exemplary embodiment described here, the rear axle is to be understood to be the primary drive axle, which should not limit the general idea of the invention however.

(5) The output of the gearbox SG acts on the rear wheels HA of a rear axlei.e. on the wheels of the primary axleby means of a propeller shaft KHA. The output of the gearbox acts in parallel by means of an all-wheel drive clutch AK, which for its part transfers a variable drive torque by means of a propeller shaft KVA to the wheels of the front axlei.e., the secondary axle in the exemplary embodiment shown. Losses occur with the all-wheel drive clutch AK engagedfor example through radius differences of the wheels. The axles run at the same revolution rate, i.e. there is a higher loss of power compared to the power loss with a revolution rate difference that is set according to the tire radii. Because of the occurrence of stresses in the drive train, increased torques occur in the axle gears and therefore increased losses.

(6) Furthermore, FIG. 1 shows a controller S with a signal connection to wheel revolution rate sensors associated with the wheels of the front and rear axles VA, HA, which is indicated by the arrows. The clutch AK is controlled by the controller S such that a defined proportion of the torque of the propeller shaft KHA (in the exemplary embodiment the torque fed to the primary axle) is taken off by means of the propeller shaft KVA to drive the front wheels (in this case the secondary axle).

(7) With a vehicle the variables stated below, i.e. measurement variables, are available or are recorded: engine torque, wheel revolution rates (front and rear axles), steering angle and the current overall transmission ratio. Furthermore, the speed of the vehicle also results from the revolution rates of the wheels in combination with the tire radii.

(8) The ratio between primary and secondary axle tire radii r.sub.PA/r.sub.SA,i.e. the ratio of the tire radii of the rear and the front axles HA, VA, can be calculated from the wheel revolution rates. Furthermore, it is assumed that the slip in the relevant region is a linear function (approximately or completely linear) of the torque.

(9) In order to calculate the speed from which the wheel revolution rates (front axle VA, rear axle HA) are compensated, the following driving situation is considered: constant speedall-wheel drive clutch disengagedthe drive is purely by means of the primary or main drive axle.

(10) The following applies or results: n=n.sub.PAn.sub.SA difference between primary and secondary axis revolution rate
n.sub.PA=(v.sub.Fzg./r.sub.PA)+(v.sub.Schlupf.sub._.sub.PA/r.sub.PA)
Where: v.sub.Schlupf.sub._.sub.PA=v.sub.Fzg. *M/k slip speed at given speed of the vehicle and axle torque M and k is a linear assumed tire stiffness characteristic value
n=V.sub.Fzg./r.sub.PA*(1+M.sub.krit/k)v.sub.Fzg./r.sub.SA
Setting non-slip level to n=0 gives:
r.sub.PA/r.sub.SA=(1+M.sub.krit/k)

(11) From the recordable ratio between the primary and secondary axle tire radii r.sub.PA/r.sub.SA, the critical drive torque and hence the speed of the vehicle can be determined at which n=0. i.e.:
M.sub.krit=k*(r.sub.PA/r.sub.SA1)

(12) This calculation can be expanded further by taking into account a possible transmission ration difference of the axles: z number of teeth
i.sub.SA=Z.sub.SA output/Z.sub.SA input
i.sub.PA=z.sub.PA output/Z.sub.PA input
M.sub.krit=k*((r.sub.PA*i.sub.SA)/(r.sub.SA*i.sub.PA)1)

(13) FIG. 2 shows possible torque distributions between the primary axle PA and the secondary axle SA in a diagram. The abscissa is graduated in the torque of the axle input drivethe primary and the secondary axle. The ordinate represents the sum of the torques of the axle inputs. The line M.sub.50/.sub.50 corresponds to an equal torque distribution, i.e. the all-wheel clutch AK or a suitable differential (open) apportions 50% of the drive power to the primary axle, 50% to the secondary axle.

(14) The line M.sub.100/0 corresponds to a torque distribution between the primary and secondary axles of 100% to 0%, i.e. all the drive power goes to the primary axle (in this case the rear axle), giving pure two-wheel drive. The region BSA corresponds to the reactive torque region of the secondary axle, the region BPA to the reactive torque region of the primary axle. In the diagram according to FIG. 2, the torque distribution by means of the all-wheel drive clutch AK is within the area that is enclosed by lines characterized as the primary and secondary lines (dashed, dash-dotted).

(15) With a drive torque of less than M.sub.krit, it is not possible to set an all-wheel torque without generating a reactive torque. It is only possible to usefully apply a drive torque to the road with distribution to the primary and secondary axles from a drive torque greater than M.sub.krit.

(16) As long as M.sub.krit (sum of torques) is not reached, the torque on the all-wheel drive clutch AK is reduced as far as possible. Therefore an all-wheel system with an all-wheel drive clutch AK is used with which the residual torque on the clutch can be substantially reduced, i.e. with which as great a separation of torque as possible can be achieved.

(17) If M.sub.krit is approximately reached (depending on tolerances to be taken into account, the point cannot be exactly determined), the reduction of the torque demand is removed, the purely two-wheel drive being abandoned. Above the limit speed associated with M.sub.krit, the drive torque is distributed to the primary and the secondary axles according to the requirements of driving dynamics and traction.

(18) FIG. 3 shows possible torque setting ranges for a ratio of the dynamic tire radii (r.sub.PA*i.sub.SA)/(r.sub.SA*i.sub.PA)<1. If i.sub.SA and i.sub.PA are equal, then this is the state in which the dynamic radius r.sub.SA of the secondary axle SA (in this case the front axle) is greater than the dynamic radius r.sub.PA of the primary axle PA. Because of the geometric ratios here, the secondary axle SA rotates more slowly (because the radius r.sub.SA is larger) than the primary axle PA. With the all-wheel drive clutch AK, it is therefore possible to perform a useful torque distribution, even for a speed in the region below the critical drive torque M.sub.krit, i.e. to travel with true all-wheel drive. This state is illustrated in FIG. 3 and is also the aim.

(19) The profile of the torque distribution on the primary axle is shown with M.sub.PA and the profile of the torque distribution on the secondary axle is shown with M.sub.SA. It can be seen in such a situationthere is a detectable tire radius differencethat torque distribution is already performed before reaching the critical drive torque M.sub.krit or the corresponding speed, i.e. driving in the strict two-wheel mode, or according to the all-wheel drive clutch the maximum achievable two-wheel mode, is not carried out until M.sub.krit.

(20) In the described case, even for torques below M.sub.krit a low clutch torque is always set, i.e. part of the drive power can be distributed to the secondary axle. Complete separation of the all-wheel drive clutch is not carried out. In addition it is provided that with a small residual torque (distribution) the losses of the drive train elements to the secondary axle are compensated.

(21) Compensation of the loss torques arising in the angle drive of the secondary axle is advantageously carried out by means of the direct path (distribution gearbox, propeller shaft to the secondary axle), instead of by the indirect path via the primary axle (distribution gearbox, propeller shaft to the primary axle, wheels of primary axle, road, wheels of secondary axle, half shafts to secondary axle). The residual torque in the clutch should then correspond to that torque that is used to rotate the components of the secondary drive train. In general, efficiency advantages result from this.

(22) When using the invention it is therefore also useful, when selecting or fitting the wheels and tires, to ensure that the larger wheels are fitted to the secondary axle, i.e. the front axle in this case. Thus the tires can be delivered to a vehicle assembly plant classified by means of an IST size and classes with the larger rolling circumferences can be fitted to the secondary axle.

REFERENCE CHARACTER LIST

(23) VKM engine, internal combustion engine SG gearbox, manually or automatically shifted gearbox VA front axle SA secondary axle HA rear axle PA primary axle AK all-wheel drive clutch S controller KVA propeller shaft front axle KHA propeller shaft rear axle