Method for controlling a wheel rotational speed of at least one wheel of a drivable axle of a two-track vehicle having two drivable axles, and two-track vehicle having at least two drivable axles

Abstract

The invention relates to a method for controlling a wheel speed of a wheel of a drivable axle of a two-track vehicle with two drivable axles as well as to a corresponding vehicle, with the vehicle having a first drive motor for driving the first axle, a second drive motor for driving the second axle, a device for detecting driving state variables, and a control device. The method comprises the steps: Detecting an actual speed of the first axle, determining a target speed for the second axle as a function of the actual speed of the first axle, and controlling the second drive motor such that the determined target speed is achieved on the second axle. In order to determine the target speed, a synchronous target speed at which the same wheel circumferential speed is achieved on at least one wheel of the second axle as on the wheels of the first axle is determined as a function of the actual speed of the first axle. Subsequently, the target speed for the second axle is determined as a function of the determined synchronous target speed.

Claims

1. A method for controlling a wheel speed of at least one wheel of a drivable axle of a two-track vehicle with two drivable axles, wherein the vehicle comprises: a first, drivable axle with at least two wheels; a second, drivable axle with at least two wheels; a first drive motor; a second drive motor; a device for detecting driving state variables in order to determine a driving state of the vehicle; and a control device, wherein the first drive motor is provided to drive at least one wheel of the first axle, wherein the second drive motor is provided to drive at least one wheel of the second axle, wherein the device for detecting driving state variables is at least designed to detect an actual speed of the first axle, and wherein the control device is designed to determine, at least as a function of the detected actual speed of the first axle, a target speed for the at least one wheel of the second axle and to control at least the second drive motor such that the desired target speed is achieved on the at least one wheel of the second axle, with the steps: detecting an actual speed of the first axle; determining the target speed for the at least one wheel of the second axle a least as a function of the actual speed of the first axle; and controlling the second drive motor such that the determined target speed is achieved on the at least one wheel of the second axle, wherein, in order to determine the target speed, a synchronous target speed is determined as a function of the actual speed of the first axle, and the target speed is determined as a function of the determined synchronous target speed, with the synchronous target speed being the target speed at which the same wheel circumferential speed is achieved on the at least one wheel of the second axle as on the wheels of the first axle.

2. The method as set forth in claim 1, wherein the synchronous target speed is determined by applying a predefined synchronous correction factor to the actual speed of the first axle.

3. The method as set forth in claim 1, wherein the target speed is additionally determined as a function of a differential speed ratio, which defines the difference between the wheel circumferential speed of at least one wheel of the second axle and the wheel circumferential speed of the wheels of the first axle, with the differential speed ratio being determined by applying a predefined differential correction factor to the determined synchronous target speed.

4. The method as set forth in claim 1, wherein the device for detecting driving state variables is designed to detect not only the actual speed of the first axle, but also an additional driving state variable, wherein the additional driving state variable is detected, and a differential correction factor is determined as a function of at least one other detected driving state variable by means of a predefined characteristic map and/or through a predefined mathematical function that is dependent on the at least one other detected driving state variable.

5. The method as set forth in claim 1, wherein the target speed is additionally determined as a function of a predefined slip-speed ratio, with the slip-speed ratio being used to set a desired slip of at least one wheel of the second axle in relation to the driving surface with the slip-speed ratio being determined as a function of the actual speed of the first axle and/or as a function of at least one other driving state variable that is detected by the device for detecting a driving state variable.

6. The method as set forth in claim 1, wherein the target speed is determined as a function of a sum of the synchronous target speed, the differential speed ratio, and the slip-speed ratio.

7. The method as set forth in claim 1, wherein the vehicle is coupled with an implement and, together with the implement, forms a pairing, with the tractor forming a tractor vehicle of the pairing, wherein the implement has at least a drivable axle, and wherein the control device of the vehicle is designed to set a drive power that can be transferred to the drivable axle, wherein the drive power transferred to the drivable axle of the implement is set such that a desired slip is achieved on the wheels of the drivable axle of the implement in relation to a driving surface as a function of a detected driving state.

8. The method as set forth in claim 7, wherein in order to bring about a stretching of the pairing when starting while traveling downhill in a direction of forward travel, the drive power that is transferred to the drivable axle of the implement can be set such that a leading negative slip of the wheels of the drivable axle of the implement in relation to the driving surface is achieved relative to the slip of the wheels of the first axle in relation to the driving surface and relative to the slip of the wheels of the second axle in relation to the driving surface.

9. The method as set forth in claim 7, wherein in order to bring about a compressing of the pairing when starting while traveling uphill in a direction of forward travel, the drive power that is transferred to the drivable axle of the implement can be set such that a leading positive slip of the wheels of the drivable axle of the implement in relation to the driving surface is achieved relative to the slip of the wheels of the first axle in relation to the driving surface and relative to the slip of the wheels of the second axle in relation to the driving surface.

10. The method as set forth in claim 1, wherein the vehicle additionally has a third drive motor that is also provided for the purpose of driving at least one wheel of the second axle, with the second drive motor being provided to drive a left wheel of the second axle and the third drive motor being provided to drive a right wheel of the second axle, wherein the control device is designed to determine, at least as a function of the detected actual speed of the first axle, a target speed of the left wheel of the second axle and a target speed of the right wheel and to control at least the second and third drive motors that are provided to drive the second axle such that the desired target speed is achieved on the left wheel and on the right wheel, respectively, with the steps: detecting an actual speed of the first axle; determining the target speed for the left wheel of the second axle and the target speed for the right wheel of the second axle at least as a function of the actual speed of the first axle; and controlling the second drive motor and the third drive motor such that the determined target speed is achieved on the left wheel and on the right wheel of the second axle.

11. The method as set forth in claim 10, for a drive operation of the vehicle while traveling transversely across a slope, wherein the second axle forms a front axle of the vehicle, wherein the differential speed ratio for determining the target speed of a downhill-side front wheel is greater than the differential speed ratio for determining the target speed of an uphill-side front wheel in order to counteract a downward drift of the vehicle caused by a downgrade force.

12. The method as set forth in claim 11, for the autonomous drive operation of the vehicle while traveling transversely across a slope, wherein the vehicle is designed for autonomous drive operation and has a steering actuator for the autonomous adjustment of a wheel steering angle, wherein the wheel steering angle and the respective differential speed ratio of the two front wheels are set during an autonomously executed trip as a function of a detected slope gradient and/or as a function of detected steering forces such that a tractive resistance of the vehicle is reduced.

13. A two-track vehicle with at least two drivable axles wherein the vehicle comprises: a first, drivable axle with at least two wheels; a second, drivable axle with at least two wheels; a first drive motor; a second drive motor; a device for detecting driving state variables in order to determine a driving state of the vehicle; and a control device, wherein the first drive motor is provided to drive at least one wheel of the first axle, wherein the second drive motor is provided to drive at least one wheel of the second axle, wherein the device for detecting driving state variables is at least designed to detect an actual speed of the first axle, wherein the control device is designed to determine, at least as a function of the detected actual speed of the first axle, a target speed for the at least one wheel of the second axle and to control the second drive motor such that the desired target speed is achieved on the at least one wheel of the second axle, and wherein the vehicle is designed to execute a method as set forth in claim 1.

14. The two-track vehicle as set forth in claim 13, wherein the vehicle is a hybrid vehicle, with the first drive motor, which is provided for driving the first axle, being a combustion engine, and with the second drive motor, which is provided at least for driving a wheel of the second axle, being an electric machine that can be operated as an electric motor, wherein the vehicle also includes an electric machine that can be operated as a generator and an electrical energy store, wherein the electric machine that can be operated as the generator can be powered by means of the combustion engine and is designed to output electrical energy to the electrical energy store and/or to the electric machine that can be operated as the electric motor, and wherein the electric machine that can be operated as the electric motor can be powered by means of the electrical energy made available by the generator and/or by the electrical energy store.

15. The two-track vehicle as set forth in claim 13, wherein the vehicle has a gearbox in a power branch from the first drive motor to the first axle, with an electric machine that can be operated as a generator being arranged in this power branch between the first drive motor and the gearbox.

16. The two-track vehicle as set forth in claim 13, wherein the vehicle also has a third drive motor, with the third drive motor also being provided for the purpose of driving the second axle at least partially, with the second drive motor being provided to drive a left wheel of the second axle and the third drive motor being provided to drive a right wheel of the second axle, and with the control device being designed to determine, at least as a function of the detected actual speed of the first axle, a target speed of the left wheel of the second axle and a target speed of the right wheel and to control at least the second and third drive motors that are provided to drive the second axle such that the desired target speed is achieved on the left wheel and on the right wheel, respectively, of the second axle.

Description

(1) In the following, the invention is explained in further detail on the basis of several exemplary embodiments, with the invention being illustrated schematically for this purpose in the enclosed drawings.

(2) FIG. 1 shows a schematic representation of a first exemplary embodiment of a vehicle according to the invention;

(3) FIG. 2 shows a schematic representation of a second exemplary embodiment of a vehicle according to the invention;

(4) FIG. 3 shows a schematic representation of a third exemplary embodiment of a vehicle according to the invention;

(5) FIG. 4 shows a schematic representation of a fourth exemplary embodiment of a vehicle according to the invention;

(6) FIG. 5 shows a simplified block diagram illustrating a first exemplary embodiment of a method according to the invention for controlling the wheel speeds of the front wheels of the vehicle according to the invention from FIG. 1;

(7) FIG. 6 shows a simplified block diagram illustrating a second exemplary embodiment of a method according to the invention for controlling the wheel speeds of the front wheels of the vehicle according to the invention from FIG. 2;

(8) FIG. 7 shows a simplified block diagram illustrating a third exemplary embodiment of a method according to the invention for controlling the wheel speeds of the front wheels of the vehicle according to the invention from FIG. 3; and

(9) FIG. 8 shows a simplified block diagram illustrating a fourth exemplary embodiment of a method according to the invention for controlling the wheel speeds of the front wheels of the vehicle according to the invention from FIG. 4.

(10) The two-track vehicle 100 according to the invention shown schematically in FIG. 1 is a tractor in the form of a hybrid vehicle that is designed to execute a method according to the invention, with a first axle 110 that forms the rear axle and a second axle 120 that forms the front axle, with the rear axle 110 and the front axle 120 each having a left wheel 111.sub.l or 121.sub.l as well as a right wheel 111.sub.r or 121.sub.r.

(11) To drive the tractor 100, a first drive motor 130 and a second drive motor 140 are provided, with the first drive motor 130 being a combustion engine and the second drive motor 140 being an electric machine that can be operated as an electric motor. The combustion engine 130 is provided to drive the rear axle 110, while the electric motor 140 is provided to drive the front axle 120. The rear axle 110 and front axle 120 can be operated mechanically independently of one anotherthat is, the drivetrain of the rear axle 110, with which the power is transferred from the combustion engine 130 to the rear axle 110, is not coupled mechanically with the drivetrain of the front axle 120, with which the power is transferred from the electric motor 140 to the front axle 120.

(12) In order to enable the wheel speeds of the wheels 121.sub.l and 121.sub.r of the front axle 120 to be controlled according to a method according to the invention, the tractor also has a device (not shown in FIG. 1) for detecting driving state variables in order to determine a driving state, as well as a control device (also not shown in FIG. 1) by means of which, as a function of an actual speed of the rear wheels 111.sub.l and 111.sub.r, a desired target speed for the wheels 121.sub.l and 121.sub.r of the front axle 120 can be determined and set, with the control device being designed to determine, as a function of the determined target speed, a corresponding control variable for controlling the electric motor 140 and to control power electronics 170 associated with the electric motor 140 such that the desired target speed is achieved on the front wheels 121.sub.l and 121.sub.r; cf. FIG. 5.

(13) The drivetrain of the rear axle 110 has a clutch 181, a gearbox 180, and a rear axle differential 112 as well as corresponding shafts, with it being possible for the power output by the combustion engine 130 to be conducted through the clutch 181 and the gearbox 180 to the rear axle differential 112, by means of which the drive power is distributed to the two rear wheels 111.sub.l and 111.sub.r. The power transferred from the combustion engine 130 to the gearbox 180 can be diverted not only to the rear axle 110, but also through the so-called power take-off 190, which is referred to in the following as a so-called PTO shaft and represents an additional gearbox output that is designed for coupling with a mechanically drivable implement (not shown here), such as a wood splitter or the like.

(14) The power output from the electric motor 140 is conducted to a front axle differential 122 and distributed to the two front wheels 121.sub.l and 121.sub.r. The electric motor 140 can draw the energy required to drive the front axle 120 from an electrical energy store 160, provided that the latter is appropriately charged, and/or directly from an electric machine 150 that can be operated as a generator, with it being possible for the generator 150 to be driven by the combustion engine 130 in order to generate electrical energy.

(15) The electrical energy generated by the generator 150 can be output appropriately to the electrical energy store 160 and to the electric motor 140. Furthermore, the electric machine 150 that is embodied as a generator can also output electrical energy to other loads of the vehicle 100 and/or to an electrically drivable implement (not shown here) that can be coupled with the tractor 100, for example to a trailer with an electrically drivable axle or another electrically drivable implement.

(16) The generator 150 that is provided in order to make the electrical energy available to the electric motor 140 and to charge the electrical energy store 160 is advantageously arranged in the power branch from the combustion engine 130 to the rear axle 110, with respect to an output flow direction, between the combustion engine 130 and the gearbox 180. This arrangement offers the advantage that the power output from the combustion engine 130 can be branched off before the gearbox 180, and a portion of the power for generating the electrical energy for the electric machine that can be operated as an electric motor 140 can be diverted before the gearbox 180 through the generator 150. As a result, the gearbox 180 is not loaded with all of the power output from the combustion engine 130, but rather only by the power that is provided for driving the rear axle 110. Consequently, the gearbox 180 need not be designed for the maximum possible power output from the combustion engine 130, but can be given smaller dimensions, or the gearbox 180 can be used in conjunction with a combustion engine with a greater power output.

(17) In this exemplary embodiment of a tractor 100 according to the invention, a portion of the power that is diverted through the generator 150 can also be adjusted in a targeted manner and thus adapted to the situation. This makes an especially advantageous gearbox design possible, since a portion of the power can be diverted through the generator 150 in load-critical situations.

(18) Furthermore, it is possible in the depicted exemplary embodiment to utilize all of the power output from the combustion engine 130 to generate electrical energy for driving the front axle 120 and/or for charging the electrical energy store 160. On trips without excessive load requirements and without the need for all-wheel drive, such as on trips on paved roads and streets, for example, the tractor 100 can be moved by driving just the front axle 120 by means of the electric motor 140, while the drive for the rear axle 110 is deactivated. In that case, the power that is required to drive the front axle 120 is preferably provided via the generator 150, which is driven by the combustion engine 130. One advantage of this mode is that the combustion engine 130 can be operated in a speed range that is optimal for consumption and thus for efficiency, thus enabling the vehicle to be operated in a fuel-saving and thus efficient manner.

(19) By opening the clutch 181, the drivetrain of the rear axle 110 can be separated, so that the rear axle 110 and the gearbox 180 are decoupled from the combustion engine 130. Friction losses can thus be reduced.

(20) When the combustion engine 130 is shut off, the tractor 100 can also be driven only electrically, that is, only using an electrical drive. For this purpose, the electric motor 140 can be supplied with electrical energy by the electrical energy store 160.

(21) Moreover, the electric machine 150 of the tractor 100 that can be operated as a generator can also be operated as an electric motor when the combustion engine 130 is shut off, for example as a starter motor for the combustion engine 130 or as a drive for the power take-off 190, i.e., as a drive for the PTO shaft. For this purpose, the electric machine 150 can also be supplied with electrical energy by the energy store 160. This type of drive, which is also referred to as power take-off, is advantageous particularly in vehicles such as the tractor 100 according to the invention shown here in order to drive a mechanically drivable implement, such as a wood splitter or the like, that is coupled with the tractor 100 while the tractor 100 is stationary.

(22) FIG. 2 shows an alternative embodiment of a tractor 200 according to the invention, with the tractor 200 differing from the tractor 100 described with reference to FIG. 1 in that, in order to drive the front axle 120, not only one electric motor 140 is provided (that is, only one second drive motor), but rather two electric machines 240.sub.l and 240.sub.r, each of which can be operated as an electric motor, with the electric motor 240.sub.l being provided to drive the left front wheel 121.sub.l and the electric motor 240.sub.r being provided to drive the right front wheel 121.sub.r. The two electric motors 240.sub.l and 240.sub.r are each embodied as wheel hub motors and enable the wheel speeds on the left front wheel 121.sub.l and on the right front wheel 121.sub.r to be adjusted separately and independently of one another.

(23) FIG. 3 shows another alternative embodiment of a vehicle according to the invention in the form of a tractor 300, with the tractor 300 differing from the tractor 100 described with reference to FIG. 1 in that it is not the combustion engine 130 that is provided to drive the rear axle 110, but rather an electric machine 340 that can be operated as an electric motor. That is, the first drive motor is formed by the electric motor 340 in this vehicle according to the invention.

(24) Furthermore, the combustion engine 130 no longer requires any mechanical connection to the rear axle 110; in particular, no gearbox and no clutch is provided between the combustion engine 130 and the rear axle 110. In this case, the combustion engine 130 is primarily intended to drive the electric machine 150 that can be operated as a generator.

(25) Like the drive motor 140 that is provided to drive the front axle 120, the electric motor 340 can be controlled by means of a control device (not shown here) and the power electronics 170 such that a desired target speed is also achieved on the rear axle 110, with the control device being designed to determine, as a function of the determined target speed, an appropriate control variable for controlling the electric motor 340 and to control the power electronics 170, which is also coupled with the electric motor 340, such that the desired target speed is achieved on the rear wheels 111.sub.l and 111.sub.r of the rear axle 110; cf. FIG. 7.

(26) The power output from the electric motor 340 is conducted to a rear axle differential 112 and distributed to the two rear wheels 111.sub.l and 111.sub.r. The electrical energy required to drive the rear axle 110 can also, like the electric motor 140, be drawn from the electrical energy store 160, provided that the latter is appropriately charged, and/or directly from the electric machine 150 that can be operated as a generator.

(27) In comparison to the tractor 100 according to the invention described with reference to FIG. 1, the tractor 300 according to the invention described with reference to FIG. 3 offers the advantage that, by virtue of the electric motor 340, which is provided to drive the rear axle 110 and has a different power characteristic than a combustion engine, a very precise and particularly dynamic speed control or speed regulation of the rear wheels 111.sub.l and 111.sub.r of the rear axle 110 is possible, whereby the drivability can be improved even further, particularly the traction.

(28) FIG. 4 shows another alternative embodiment of a tractor 400 according to the invention, with the tractor 400 differing from the tractor 300 described with reference to FIG. 3 in that, in order to drive the rear axle 110, not only one electric motor 140 is provided as the first drive motor, but rather two electric machines 440.sub.l and 440.sub.r, each of which can be operated as an electric motor, with the electric motor 440.sub.l being provided to drive the left rear wheel 111.sub.l and the electric motor 440.sub.r being provided to drive the right rear wheel 111.sub.r. Moreover, no rear axle differential is provided. Like the electric motors 240.sub.l and 240.sub.r on the front axle 120, the two electric motors 440.sub.l and 440.sub.r are also each embodied as wheel hub motors and enable the wheel speeds on the left rear wheel 111.sub.l and on the right rear wheel 111.sub.r to be set separately and independently of one another. Furthermore, like the tractor 300 described with reference to FIG. 3, this tractor 400 according to the invention has two electric machines 240.sub.l and 240.sub.r, each of which can be operated as an electric motor, to drive the front axle 120.

(29) FIG. 5 shows a simplified block diagram illustrating a method according to the invention for controlling the wheel speeds of the front axle 120 of the tractor 100 from FIG. 1, wherein the tractor 100 has a device for detecting driving state variables in order to determine a driving state of the vehicle with a corresponding wheel speed sensor system 10.sub.R for detecting the actual speed n.sub.R,act of the rear axle 110 as well as a corresponding wheel speed sensor system 10.sub.F for detecting the actual speed n.sub.F,act of the front axle 120.

(30) As a function of the detected actual speed n.sub.R,act of the rear axle 110, a target speed n.sub.F,Ref for the wheels 121.sub.l, 121.sub.r of the front axle 120 can be determined by means of the control device 20, and the electric motor 140 can be controlled such that the desired target speed n.sub.F,Ref is achieved on at least one wheel 121.sub.l, 121.sub.r of the front axle 120.

(31) For this purpose, the current actual speed n.sub.R,act of the wheels 111.sub.l and 111.sub.r of the rear axle 110 is detected and first multiplied by a synchronous correction factor k.sub.F/R that is stored in the control device 20. The product of the actual speed n.sub.R,act and the synchronous correction factor k.sub.F/R is the so-called synchronous target speed n.sub.F,syn, which represents the target speed n.sub.F,Ref at which the wheel circumferential speed of the wheels 121.sub.l and 121.sub.r of the front axle 120 corresponds to the wheel circumferential speed of the wheels 111.sub.l and 111.sub.r of the rear axle 110.

(32) The value of the synchronous correction factor k.sub.F/R that is stored in the control device is preferably determined by detecting a current actual speed n.sub.R,act of the rear axle 110 and a current actual speed n.sub.F,act of the front axle 120 in regular intervals during a trip on a solid driving surface, preferably at the same time, and then calculating the quotient from the detected actual speed n.sub.F,act of the front axle 120 and the detected actual speed n.sub.R,act of the rear axle 110, with the result of this quotient being the value of the synchronous correction factor k.sub.F/R.

(33) In this described exemplary embodiment of a method according to the invention, in order to determine the target speed n.sub.F,Ref, not only the synchronous target speed n.sub.F,syn, but also a differential speed ratio n.sub.F,Forerun is added, with the differential speed ratio n.sub.F,Forerun being calculated in this case by multiplying the synchronous target speed n.sub.F,syn by a differential correction factor S.sub.F,Forerun, which is determined as a function of the driving state with the aid of a predefined characteristic map 11 that is stored in the control device 20. In this case, the differential correction factor S.sub.F,Forerun is determined as a function of the driving state variables vehicle speed v, gas pedal position a, braking pressure p, and steering angle . By means of the differential correction factor S.sub.F,Forerun, which is preferably indicated in percentage points, the percentage deviation of the wheel circumferential speed of the front wheels 121.sub.l and 121.sub.r of the front axle 120 from the wheel circumferential speed of the wheels 111.sub.l and 111.sub.r of the rear axle 110 can be predefined that is, a desired forerun, synchronization, or lag can be set in a targeted manner.

(34) In order to optimize traction, a slip-speed ratio n.sub.F,slip is also added to the synchronous target speed n.sub.F,syn and the differential speed ratio n.sub.F,Forerun, with the slip-speed ratio n.sub.F,slip also being determined as a function of the driving state by means of a characteristic map that is also stored in the control device 20, particularly in a driving state control device 12 that forms a portion of the control device 20. In the described exemplary embodiment, the slip-speed ratio n.sub.F,slip is also determined as a function of the driving state variables vehicle speed v, gas pedal position a, braking pressure p, and steering angle . As will readily be understood, other driving state variables or other parameters such as a terrain condition predefined by the driver, a slope gradient, or the like can also be taken into account alternatively or in addition. In this way, a desired slip can be set, thereby optimizing the road grip and, consequently, the traction of the tractor 100.

(35) In order to set the desired target speed n.sub.F,Ref on the wheels 121.sub.l and 121.sub.r of the front axle 120, the detected actual speed n.sub.F,act of the front axle 120 is compared with the desired target speed n.sub.F,Ref and the current control deviation e.sub.n,F determined. Based on the control deviation e.sub.n,F, a suitable speed controller 13 is used to determine a required control variable T.sub.F, which is a target torque in this case, for controlling the electric motor 140.

(36) In order to prevent the electric motor 140 from reaching a critical operating state during the setting of the desired target speed n.sub.F,Ref on the front axle 120 and from having an excessive current consumption that damages the power electronics 170 of the electric motor 140, and in order to prevent the traction of the front axle 120 from worsening as a result of the setting of the desired target speed n.sub.F,Ref, a power limiting device 14 is provided that limits the determined control variable T.sub.F for controlling the electric motor 140 to a maximum control variable T.sub.F,Ref when a control variable threshold value is exceeded. For example, the determined control variable T.sub.F for controlling the electric motor 140 is limited if the setting of the target speed n.sub.F,Ref requires an increase in the actual speed n.sub.F,act but one of the wheels 121.sub.l or 121.sub.r of the front axle 120 is already at the traction limit, so that an increase in the wheel circumferential speed of the respective wheel 121.sub.l or 121.sub.r would result in the spinning of the wheel.

(37) The output of the control variable T.sub.F is also limited in this exemplary embodiment as a function of the driving state. For this purpose, the driving state control device 12, also as a function of the driving state variables vehicle speed v, gas pedal position a, braking pressure p, and steering angle , determines on the basis of a characteristic map that is stored in the control device 20 and/or in the driving state control device 12 a maximum permissible drive power P.sub.F,limit of the electric motor 140 for the respective driving situation and forwards it to the power limiting device 14.

(38) In order to prevent the speed controller 13 from surging, a suitable feedback of the limited control variable T.sub.F,Ref is also provided in the controller 13 in order to implement an anti-windup function.

(39) On the basis of the control variable T.sub.F,Ref of the electric motor 140 output by the control device 20, the desired drive power can be set on the electric motor 140 by means of the power electronics 170 with which the desired target speed n.sub.F,Ref is achieved on the wheels 121.sub.l and 121.sub.r of the front axle 120.

(40) As described in connection with FIG. 1, the tractor 100 can be coupled with an electrically drivable implement (not shown here), particularly with an implement with electrically drivable axle, in which case the generator 150 is also designed so as to output electrical energy to an electrically drivable implement that is coupled with the tractor 100, such as a trailer with an electrically drivable axle or another electrically drivable implement.

(41) The drive power that is transferred to the driven axle of the implement can be set in a targeted manner by means of the control device 20 if the tractor 100 is coupled with a commensurately suitable implement that is compatible with the tractor 100 and has a drivable axle, particularly if the tractor 100 is coupled with a compatible implement with an electrically drivable axle and forms a pairing, with the tractor 100 being the towing vehicle of the pairing.

(42) The drive power transferred to the drive axle of the implement can be set such that, in particular, a desired slip is achieved on the wheels of the drive axle of the implement in relation to the driving surface as a function of a detected driving state, particularly a slip with which maximally optimal traction of the pairing and thus maximum tractive power can be achieved.

(43) In order to adjust the drive power that is transferred to the drive axle of the implement, the driving state control device 12, also as a function of the driving state variables vehicle speed v, gas pedal position a, braking pressure p, steering angle , and a detected actual speed n.sub.implement,act of the driven axle of the implement and as a function of a terrain condition (not shown here) that is input by the driver, particularly as a function of the slope gradient, a target speed n.sub.implement,Ref for the driven axle of the implement is determined as a function of which the drive power of the driven axle of the implement can be appropriately set. Likewise, an appropriate maximum permissible drive power P.sub.implement,limit can be determined for the drive axle of the implement; that is, a power limitation is provided in this case as well.

(44) By virtue of the fact that the terrain condition, particularly the slope gradient, can be taken into account, it is possible when starting while traveling downhill in the forward direction, for example, to adjust the drive power that is transferred to the drive axle of the implement such that a leading negative slip of the wheels of the drivable axle of the implement in relation to the driving surface is achieved relative to the slip of the wheels 111.sub.l and 111.sub.r of the rear axle 110 in relation to the driving surface and relative to the slip of the wheels 121.sub.l and 121.sub.r of the front axle 120 in relation to the driving surfacethat is, a lag is produced in relation to the front axle 120 and the rear axle 110. A stretching of the pairing can be achieved in this way, thus improving the controllability of the pairing.

(45) In order to achieve especially good traction with the tractor 100, the control device 20 further comprises an engine control device 131, with which the power output from the combustion engine 130 to the rear axle 110 can be adjusted in a targeted manner, with the engine control device 131 being designed to adjust the power output from the combustion engine 130 as a function of a target speed n.sub.CE,Ref detected by the driving state control device and a limited control variable T.sub.CE,limit in the form of a target torque.

(46) FIG. 6 shows the corresponding simplified block diagram for controlling the wheel speeds of a vehicle 200 according to the invention as described with reference to FIG. 2, in which, in order to drive the left front wheel 121.sub.l and in order to drive the right front wheel 121.sub.r of the front axle 100, a respective separate electric motor 240.sub.l and 240.sub.r is provided, with the two electric motors 240.sub.l and 240.sub.r each being embodied as wheel hub motors that can be controlled separately.

(47) A respective separate target torque T.sub.F,Ref for controlling is determined for each electric machine 240.sub.l and 240.sub.r that can be operated as an electric motor. However, as in the method according to the invention that was described with reference to FIG. 5, the control variable T.sub.F,Ref is calculated separately for the left front wheel 121.sub.l and the right front wheel 121.sub.r, respectively. The corresponding variables associated with the left front wheel 121.sub.l are indicated accordingly with .sub.l, variables associated with the right front wheel 121.sub.r are indicated accordingly with .sub.r, with the corresponding target speed n.sub.Fl,Ref and n.sub.Fr,Ref of the left front wheel 121.sub.l and of the right front wheel 121.sub.r, respectively, being determined as a function of an actual speed n.sub.Fl,act or n.sub.Fr,act detected for the respective wheel.

(48) With a tractor 200 according to the invention that is designed to execute a method according to the invention as described with reference to FIG. 6, it is possible when traveling obliquely across a slope or transversely across a slope in the direction of forward travel, for example, for the differential speed ratio S.sub.Fr,Forerun for determining the target speed n.sub.Fr,Ref of the downhill-side front wheel, for example of the right front wheel 121.sub.r, to be selected so as to be greater than the differential speed ratio S.sub.Fl,Forerun for determining the target speed n.sub.Fl,Ref of the left, downhill-side front wheel 121.sub.l, whereby the downward drifting of the tractor 200 as a result of the downgrade force can be counteracted

(49) FIG. 7 shows the corresponding simplified block diagram for controlling the wheel speeds of a vehicle 300 according to the invention as described with reference to FIG. 3, in which the first drive motor for driving the rear axle 110 is an electric machine 340 that can be operated as an electric motor, for an additional exemplary embodiment of a method according to the invention, although, unlike the methods described with reference to FIGS. 5 and 6, a target speed n.sub.R,Ref for the wheels 111.sub.l and 111.sub.r of the rear axle 110 can also be determined and set in this exemplary embodiment.

(50) The target speed n.sub.R,Ref of the rear axle 110 is determined by means of the control device 320, particularly by means of the driving state control device 312, in accordance with the driving state as a function of the driving state variables vehicle speed v, gas pedal position a, braking pressure p, and steering angle . As will readily be understood, other driving state variables or other parameters such as a terrain condition predefined by the driver, a slope gradient, or the like can also be taken into account alternatively or in addition.

(51) In order to set the desired target speed n.sub.R,Ref on the wheels 111.sub.l and 111.sub.r of the rear axle, a required control variable for controlling the electric motor 340, which is also a target torque in this case, is also first determined subsequently from the determined target speed n.sub.R,Ref for the rear axle 110 and the detected actual speed n.sub.R,act of the rear axle 110 by means of a suitable speed controller 313.

(52) In order to prevent the electric motor 340 from reaching a critical operating state during the setting of the desired target speed n.sub.R,Ref on the rear axle 110 and from having an excessive current consumption that damages the power electronics 170 of the electric motor 340, and in order to prevent the traction of the rear axle 110 from worsening as a result of the setting of the desired target wheel speed n.sub.R,Ref, a power limiting device 314 is likewise additionally provided that limits the determined control variable for controlling the electric motor 340 to a maximum control variable T.sub.R,Ref when a control variable threshold value is exceeded. For example, the determined control variable for controlling the electric motor 340 is limited if the setting of the target speed n.sub.R,Ref requires an increase in the actual speed n.sub.R,act but one of the wheels 111.sub.l or 111.sub.r of the rear axle 110 is already at the traction limit, so that an increase in the wheel circumferential speed of the respective wheel 111.sub.l or 111.sub.r would result in the spinning of the wheel.

(53) The output of the control variable is also limited in this exemplary embodiment as a function of the driving state. For this purpose, the driving state control device 312, also as a function of the driving state variables vehicle speed v, gas pedal position a, braking pressure p, and steering angle , determines a maximum permissible drive power P.sub.R,limit of the electric motor 340 for the respective driving situation and forwards it to the power limiting device 314.

(54) On the basis of the control variable T.sub.R,Ref output by the control device 320 for controlling the electric motor 340, the desired drive power can be set on the electric motor 340 by means of the power electronics 170 with which the desired target speed n.sub.R,Ref is achieved on the wheels 111.sub.l and 111.sub.r of the rear axle 110.

(55) FIG. 8 shows the corresponding simplified block diagram for controlling the wheel speeds of a vehicle 400 according to the invention as described with reference to FIG. 4, in which, in order to drive the left rear wheel 111.sub.l and in order to drive the right rear wheel 111.sub.r of the rear axle 110, a separate electric motor 440.sub.l or 440.sub.r is provided in each case, with the two electric motors 440.sub.l and 440.sub.r each being embodied as wheel hub motors that can be controlled separately.

(56) A respective separate target torque T.sub.R,Ref or T.sub.Rr,Ref for controlling is determined by means of the control device 420 for each electric machine 440.sub.l and 440.sub.r that can be operated as an electric motor. However, as in the method according to the invention that was described with reference to FIG. 7, the control variable T.sub.Rl,Ref or T.sub.Rr,Ref is calculated separately for the left rear wheel 111.sub.l and the right rear wheel 111.sub.r, respectively.

(57) The corresponding variables associated with the left rear wheel 111.sub.l are indicated accordingly with l, variables associated with the right rear wheel 111.sub.r are indicated accordingly with r, with the corresponding target speeds n.sub.Rl,Ref and n.sub.Rr,Ref of the left rear wheel 111.sub.l and of the right rear wheel 111.sub.r, respectively, being determined as a function of an actual speed n.sub.Rl,act or n.sub.Rr,act detected for the respective wheel.

(58) Furthermore, through the wheel-by-wheel detection of the actual speeds n.sub.Rl,act and n.sub.Rr,act of the rear wheels 111.sub.l and 111.sub.r of the rear axle 110, the synchronous target speeds n.sub.Fl,syn and n.sub.Fr,syn of the left front wheel 121.sub.l and of the right front wheel 121.sub.r can each be determined as a function of the detected actual speeds n.sub.Rl,act and n.sub.Rr,act of the left rear wheel 111.sub.l and of the right rear wheel 111.sub.r, respectively.

(59) The synchronous target speed n.sub.Fl,syn of the left front wheel 121.sub.l is determined by multiplying the current actual speed n.sub.Rl,act of the left rear wheel 111.sub.l by the synchronous correction factor k.sub.Fl/Rl for the left front wheel 121.sub.l stored in the control device 420, while the synchronous target speed n.sub.Fr,syn of the right front wheel 121.sub.r is determined by multiplying the current actual speed n.sub.Rr,act of the right rear wheel 111.sub.r by the synchronous correction factor k.sub.Fr/Rr for the right front wheel 121.sub.r.

(60) The synchronous correction factors k.sub.Fr/Rr and k.sub.Fl/Rl are each determined analogously to the method described with reference to FIG. 5, by simultaneously detecting the current actual speeds n.sub.Rl,act and n.sub.Rr,act of the of the left rear wheel 111.sub.l and of the right rear wheel 111.sub.r and the current actual speeds n.sub.Fl,act and n.sub.Fr,act of the left front wheel 121.sub.l and of the right front wheel 121.sub.r in regular intervals during a trip on a solid driving surface, and then calculating the respective quotient from the detected actual speed n.sub.Fl,act and n.sub.Fr,act of the left front wheel 121.sub.l and of the right front wheel 121.sub.r, respectively, and of the detected actual speed n.sub.Rl,act and n.sub.Rr,act of the left rear wheel 111.sub.l and of the right rear wheel 111.sub.r, respectively, with the result of this quotient being the value of the synchronous correction factor k.sub.Fl/Rl and k.sub.Fr/Rr, respectively, that can be stored as a parameter in the control device 420.

LIST OF REFERENCE SYMBOLS

(61) The indices of the individual variables have the following respective meaning:

(62) l left r right R rear axle (rear) F front axle (front) act current quantity or actual quantity Ref target quantity syn pertaining to the synchronous target speed Forerun pertaining to the differential speed ratio slip pertaining to the slip-speed ratio implement pertaining to an implement coupled with the tractor limit limit value CE pertaining to the combustion engine
The following allocations apply to the reference symbols: 10F wheel speed sensor system for detecting the actual speed of the front axle 10Fl wheel speed sensor system for detecting the actual speed of the left front axle 10Fr wheel speed sensor system for detecting the actual speed of the right front axle 10R wheel speed sensor system for detecting the actual speed of the rear axle 10Rl wheel speed sensor system for detecting the actual speed of the left rear axle 10Rr wheel speed sensor system for detecting the actual speed of the right rear axle 11 characteristic map for determining the differential correction factor 12, 312 driving state control device 13, 13l, 13r speed controller 313, 313l, 313r 14, 14l, 14r, power limiting device 314, 314l, 314r 20, 220, 320, control device 420 100, 200, 300, tractor according to the invention 400 110 rear axle 111l left rear wheel 111r right rear wheel 112 rear axle differential 120 front axle 121l left front wheel 121r right front wheel 122 front axle differential 130 combustion engine 140 electric motor 150 electric machine that can be operated as a generator and electric motor 160 energy store 170 power electronics 180 gearbox 181 clutch 190 power take-off, so-called PTO shaft 240l electric wheel hub motor for driving the left front wheel 240r electric wheel hub motor for driving the right front wheel 340 electric motor 440l electric wheel hub motor for driving the left rear wheel 440r electric wheel hub motor for driving the right rear wheel a gas pedal position steering angle e.sub.n,F control deviation between actual and target speed of the front axle e.sub.n,Fl control deviation between actual and target speed of the left front wheel e.sub.n,Fr control deviation between actual and target speed of the right front wheel k.sub.F/R synchronous correction factor for the front axle k.sub.Fl/R, k.sub.Fl/R synchronous correction factor for the left front wheel k.sub.Fr/R, k.sub.Fr/Rr synchronous correction factor for the right front wheel n.sub.F,act actual speed of the front axle n.sub.Fl,act actual speed of the left front wheel n.sub.Fr,act actual speed of the right front wheel n.sub.R,act actual speed of the rear axle n.sub.Rl,act actual speed of the left rear wheel n.sub.Rr,act actual speed of the right rear wheel n.sub.F,syn synchronous target speed of the front axle n.sub.Fl,syn synchronous target speed of the left front wheel n.sub.Fr,syn synchronous target speed of the right front wheel n.sub.F,Forerun differential speed ratio of the front axle n.sub.Fl,Forerun differential speed ratio of the left front wheel n.sub.Fr,Forerun differential speed ratio of the right front wheel n.sub.F,slip slip-speed ratio of the front axle n.sub.Fl,slip slip-speed ratio of the left front wheel n.sub.Fr,slip slip-speed ratio of the right front wheel n.sub.F,Ref target speed of the front axle n.sub.Fl,Ref target speed of the left front wheel n.sub.Fr,Ref target speed of the right front wheel n.sub.implement,act actual speed of the driven axle of an implement coupled with the tractor n.sub.implement,Ref target speed of the driven axle of an implement coupled with the tractor n.sub.CE,Ref target speed of the combustion engine required to set a desired target speed on the rear axle n.sub.R,Ref target speed of the rear axle n.sub.Rl,Ref target speed of the left rear wheel n.sub.Rr,Ref target speed of the right rear wheel p braking pressure P.sub.F,limit maximum permissible output of the electric motor provided to drive the front axle for the current driving situation P.sub.Fl,limit maximum permissible output of the wheel hub motor provided to drive the left front wheel for the current driving situation P.sub.Fr,limit maximum permissible output of the wheel hub motor provided to drive the right front wheel for the current driving situation P.sub.R,limit maximum permissible output of the electric motor provided to drive the rear axle for the current driving situation P.sub.Rl,limit maximum permissible output of the wheel hub motor provided to drive the left rear wheel for the current driving situation P.sub.Rr,limit maximum permissible output of the wheel hub motor provided to drive the right rear wheel for the current driving situation P.sub.implement,limit maximum permissible output of the drive motor provided to drive the axle of an agricultural device coupled with the tractor for the current driving situation S.sub.F,Forerun differential correction factor for the front axle S.sub.Fl,Forerun differential correction factor for the left front wheel S.sub.Fr,Forerun differential correction factor for the right front wheel T.sub.F arithmetically determined control variable for target torque of the electric motor provided to drive the front axle T.sub.Fl arithmetically determined control variable for target torque of the wheel hub motor provided to drive the left front wheel T.sub.Fr arithmetically determined control variable for target torque of the wheel hub motor provided to drive the right front wheel T.sub.F,Ref limited control variable for target torque of the electric motor provided to drive the front axle for the current driving situation T.sub.Fl, Ref limited control variable for target torque of the wheel hub motor provided to drive the left front wheel for the current driving situation T.sub.Fr,Ref limited control variable for target torque of the wheel hub motor provided to drive the right front wheel for the current driving situation T.sub.R,Ref limited control variable for target torque of the electric motor provided to drive the rear axle for the current driving situation T.sub.Rl,Ref limited control variable for target torque of the wheel hub motor provided to drive the left rear wheel for the current driving situation T.sub.Rr,Ref limited control variable for target torque of the wheel hub motor provided to drive the right rear wheel for the current driving situation v vehicle speed