Abstract:
A hydrostatic power train includes a hydraulic pump, which is connected, via a first main pipe and a second main pipe, to a first hydromotor unit driving a front axle and to a second hydromotor unit driving a rear axle. The first and second hydromotor units can be adjusted in terms of their displacement via a first and second variation device. A direction of travel is defined as forward movement (F) or reverse movement (R) by a position of an operating lever. The first and the second variation devices are controlled by a control valve which takes up a first control position upon a forward movement (F) determined by the position of the operating ever and which takes up a second control position upon reverse movement (R) determined by the position of the operating lever and which takes up a second control position upon reverse movement (R) determined by the operating lever. In the first control position, the first variation device is controlled such that the first motor unit is adjusted for a smaller displacement and, in the second control position; the second variation device is controlled such that the second motor unit is adjusted for a smaller displacement.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a controller for a hydrostatic traversing mechanism. 
     Hydrostatic traversing mechanisms, in which two hydraulic motors are connected to a hydraulic pump and the hydraulic pump is driven by a driving motor, are known as traversing mechanisms of working machines such as road rollers. In such a system, in which one hydraulic motor each is assigned to the front and rear axles, to adapt the torque distribution of the individual motors to the conditions of use, e.g. uphill and downhill, various systems for traction control are known. 
     2. Discussion of the Prior Art 
     In DE 101 01 748 A1, for instance, a system in which signals from pressure sensors, which are arranged in the pressure-side and induction-side main duct, are fed to an electronic control unit, is proposed. On the basis of changes of the pressure signals and changes of the position of a driving lever, the electronic control unit determines the present driving situation of the vehicle. For this purpose, a threshold value which is corrected depending on changes of the position of the driving lever is used, so that different driving situations can be clearly distinguished from each other. If one of the pressure sensors determines a pressure value which exceeds the corrected threshold value, the driving situation which the electronic control unit determines is used to send an electrical signal for adjustment towards a lower absorption volume to an adjustment device of the up-side hydraulic motor. 
     The system which is proposed in DE 101 01 748 A1 has the disadvantage that the cost of construction is relatively great. Thus not only are pressure sensors required in both main ducts, but also an electronic controller, which is capable of storing changes of the position of the driving lever at least for a short time, so that it can not only evaluate an instantaneous actual value, but also include the change of the driving lever position in the determination of the driving situation. 
     Also, from DE 196 38 421 A1 a traction controller, in which a gradient sensor is used to prevent an unwanted adjustment of a down-side hydraulic motor in the direction of smaller pivoting angles, is known. The system is based on a traction controller, which detects differing wheel circumference speeds on the front and rear axles on the basis of r.p.m. sensors, and by adjusting the absorption volume adjusts the r.p.m. of the corresponding hydraulic motor. In this case the situation which can occur is that the wheel circumference speed of the down-side axle during downhill motion is less than the wheel circumference speed of the up-side axle, because of the slip which occurs. Because of the regulating intervention of the traction controller, this would result in an adjustment of the down-side hydraulic motor in the direction of a smaller pivoting angle, to raise the r.p.m. of the hydraulic motor, which however causes a reduction of the moment. To maintain the braking moment during downhill motion, the information of a gradient sensor is used, to suppress the control commands which the traction controller gives to the adjustment device of the hydraulic motor and thus to prevent the adjustment of the hydraulic motor in the direction of a smaller pivoting angle. The gradient sensor therefore prevents an adjustment of the pivoting angle of the hydraulic motor. 
     The system which is known from DE 196 38 421 A1 has the disadvantage that the traction controller intervenes only when a slip has already occurred. On the other hand, reduction of the transmitted torque to one of the axles before a slip occurs on one of the axles is not provided. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of creating a controller for a hydrostatic traversing mechanism. With this controller, a preventive adjustment of hydraulic motor units takes place in a simple way, to act against the inclination to form a slip. 
     In the case of the controller according to the invention for the hydrostatic traversing mechanism, it is advantageous that drive torques are differentially distributed to the front and rear axles of the vehicle on the basis of a simple signal. According to claims  1  and  2 , this simple signal is either the direction of motion which is given via the driving lever or the direction of inclination of the vehicle, which is determined by an inclination sensor. Corresponding to the direction of motion or the determined inclination of the vehicle, a control valve, through which, by means of a variation device which is provided on the appropriate hydraulic motor unit, a hydraulic motor unit is adjusted in the direction of smaller absorption volume, is operated. Through the adjustment of the hydraulic motor unit in the direction of smaller absorption volume, the torque which can be transmitted to the corresponding axle is reduced, thus acting against the occurrence of slip. 
     In the case wherein instead of an electrical signal there is used the sign of the pressure different between a first and second main duct to apply pressure to the variation device of the relevant hydraulic motor unit by means of a control valve unit, in such a way that one of the two hydraulic motor units is adjusted in the direction of a smaller pivoting angle. Use of the sign of the pressure difference between the first and the second main duct has the advantage that for both forward motion and backward motion, the hydraulic motor unit of the axle which is oriented uphill is adjusted in the direction of smaller absorption volume. Because of the pressure reversal in the first and second main ducts during downhill motion, even for a downhill motion the hydraulic motor unit which is oriented uphill is pivoted in the direction of smaller toque, so that for downhill braking too, optimum distribution of moment on the axles is achieved. 
     Advantageous extensions of the controller according to the invention are presented in the subclaims. 
     It is particularly advantageous to provide, on the control valve or control valve unit, an additional switching position, in which both the hydraulic motor unit of the front axle and the hydraulic motor unit of the rear axle are adjusted in the direction of smaller absorption volume, to make a higher speed of motion possible, e.g. for transport. 
     A further advantage is, simultaneously with the adjustment of a hydraulic motor unit in the direction of smaller torque, to adjust the absorption volume of the other hydraulic motor unit oppositely, in the direction of greater absorption volume, so that the sum of the absorption volumes remains constant. For this purpose, a hydraulic motor unit consisting of several hydraulic motors, each of which has a constant absorption volume, can be used specially advantageously. To change the absorption volume of such a hydraulic motor unit, one of the hydraulic motors can then be switched on or off. 
     According to another advantageous extension, it is provided that the absorption volume of the hydraulic motor units is continuously adjustable. For this purpose, the control valve or control valve unit, as well as the control valves which are provided on each of the variation devices, are in the form of continuously adjustable valves. Thus, particularly if an inclination sensor, which not only determines the existence of a downhill or uphill gradient but also its steepness, is used, a continuous shift of the ratio between the torques on the front axle and rear axle can occur. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the controller according to the invention for a hydrostatic traversing mechanism are shown in the drawings, and are explained in more detail on the basis of the following description. 
         FIG. 1  shows a schematic representation of a hydrostatic traversing mechanism, with a first embodiment of a controller according to the invention; 
         FIG. 2  shows a schematic representation of a hydrostatic traversing mechanism, with a second embodiment of a controller according to the invention; 
         FIG. 3  shows a schematic representation of a hydrostatic traversing mechanism, with a third embodiment of a controller according to the invention; 
         FIG. 4  shows a schematic representation of a hydrostatic traversing mechanism, with a fourth embodiment of a controller according to the invention; 
         FIG. 5  shows a schematic representation of a hydrostatic traversing mechanism, with a fifth embodiment of a controller according to the invention; 
         FIG. 6  shows a schematic representation of a hydrostatic traversing mechanism, with a sixth embodiment of a controller according to the invention; and 
         FIG. 7  shows an example of a vehicle, with a hydrostatic traversing mechanism which is equipped with the controller according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of a controller according to the invention for a hydrostatic traversing mechanism is shown in  FIG. 1 . A hydrostatic traversing mechanism  1  includes a pump  2 , which is driven by a prime mover  3 . The pump  2  is connected via a drive shaft  4  to the prime mover  3 . The hydrostatic traversing mechanism also includes a first hydraulic motor unit  5  and a second hydraulic motor unit  6 . The first hydraulic motor unit  5  and the second hydraulic motor unit  6  are connected in parallel via a first main duct  7  and a second main duct  8  to the pump  2 . Depending on the delivery direction of the pump  2 , the hydraulic fluid is fed from the first main duct  7  via a first connecting duct  7 ′ to the first hydraulic motor unit  5 , and via a second connecting duct  7 ″ to the second hydraulic motor unit  6 , or, in the case of delivery in the opposite direction, from the second main duct  8  via a third connecting duct  8 ′ to the first hydraulic motor unit  5 , and via a fourth connecting duct  8 ″ to the second hydraulic motor unit  6 . 
     The pump  2  is adjustable for delivery volume, and suitable for delivery in two directions. The delivery volume is set by an adjustment device (not shown). The adjustment device can be adjusted by signals from an electronic control unit (also not shown). The electronic control unit preferably also acts on the prime mover  3  and adjusts it for power and r.p.m. An auxiliary pump  9  is driven by the prime mover  3  via a section  4 ′ of the drive shaft  4 . The auxiliary pump  9  is suitable for delivery in one direction, and its delivery volume is permanently set. On its induction side, the auxiliary pump  9  is connected via an induction duct  10  to a tank volume  11 . In a duct  12  which is connected on the pressure side to the auxiliary pump  9 , a pressure relief valve  13 , which relieves the duct  12  into the tank volume  11  when a maximum pressure which is determined by a spring is exceeded, is provided. The duct  12  is connected via a first feed duct  14  to the first main duct  7 , and via a second feed duct  15  to the second main duct  8 . 
     In each of the first and second feed duct  14  and  15 , a first non-return valve  16  and second non-return valve  17  which open in the direction of the first main duct  7  and second main duct  8  respectively are arranged. Parallel to the first and second non-return valve  16  and  17  respectively, a maximum pressure relief valve  18  and  19 , which relieves the first main duct  7  and second main duct  8  respectively via the duct  12  and pressure relief valve  13  into the tank volume  11  when a maximum permitted pressure for the main ducts  7  and  8  is exceeded, is provided. 
     The first hydraulic motor unit  5  drives, for instance, a roller which is arranged on the front axle of a road roller, via a first driven shaft  20 . A second roller, which is arranged on the rear axle of a road roller, is driven via a second driven shaft  21  by the second hydraulic motor unit  6 . The first hydraulic motor unit  5  and second hydraulic motor unit  6  are preferably constructed identically, and each include a first hydraulic motor  22  and  23  respectively and a second hydraulic motor  22 ′ and  23 ′ respectively which is assigned to the first hydraulic motor  22  and  23 . The first and second hydraulic motor  22  and  22 ′ of the first hydraulic motor unit  5  and the first and second hydraulic motor  23  and  23 ′ of the second hydraulic motor unit  6  are designed for driving in both directions, and each have a constant absorption volume. The absorption volume of the two first hydraulic motors  22  and  23  is preferably identical. The absorption volume of the two second hydraulic motors  22 ′ and  23 ′ is also preferably identical. On the other hand, the absorption volume of the first hydraulic motor  22  and second hydraulic motor  22 ′, and of the first hydraulic motor  23  and second hydraulic motor  23 ′, can be different. 
     The second hydraulic motor  22 ′ of the first hydraulic motor unit  5  and the second hydraulic motor  23 ′ of the second hydraulic motor unit  6  are implemented so that they can be switched on and off. To switch the second hydraulic motor  22 ′ and  23 ′ on and off, in each case a control valve  24  and  25  respectively is provided in the first hydraulic motor unit  5  and second hydraulic motor unit  6  respectively. The control valves  24  and  25  are held in an initial position by a control valve spring  26  and  27  respectively. On the control valves  24  and  25 , a control surface  28  and  29  respectively, to which a control pressure which steers the control valve  24  and  25  respectively out of its initial position against the force of the control valve spring  26  and  27  respectively is applied, is provided. 
     If the control valve  24  and  25  is in its initial position, the pressure means which is fed out of the first main duct  7  and second main duct  8  of the respective hydraulic motor unit  5  or  6  is also fed to the second hydraulic motor  22 ′ and  23 ′. For this purpose, a first linking duct  41 , which is connected via the control valve  24  in its initial position to a first joining duct  45  and thus to an input of the second hydraulic motor  22 ′, branches off from the first connecting duct  7 ′. A second terminal of the first hydraulic motor  22 ′ is connected via a second joining duct  46 , the control valve  24  in its initial position, and a second linking duct  42  to the third connecting duct  8 ′. The second hydraulic motor  23 ′ of the second hydraulic motor unit  6  is similarly connected via a third and fourth joining duct  47  and  48 , which in the initial position of the control valve  25  are connected to a third and fourth linking duct  43  and  44  respectively, to the second connecting duct  7 ″ and fourth connecting duct  8 ″ respectively. 
     On the other hand, if the control valve  24  is in the opposite final position, the first and second joining duct  45  and  46  of the second hydraulic motor  22 ′ of the first hydraulic motor unit  5  are connected to each other. In this switch position of the control valve  24 , the second hydraulic motor  22 ′ is short-circuited, and does not contribute to the generation of a torque on the first driven shaft  20 . Correspondingly, the third joining duct  47  and fourth joining duct  48  are connected to each other if the control valve  25  is in the opposite final position. The control valve  24  acts with the hydraulic motor  22 ′ of the first hydraulic motor unit  5  to form a first variation device  30 . The control valve  25  acts with the hydraulic motor  23 ′ of the second hydraulic motor unit  6  to form a second variation device  31 . 
     To control the first variation device  30  and the second variation device  31 , a control valve  32  is provided. The control valve  32  is held by a control valve spring  33  in a first switch position. Against the force of the control valve spring  33 , a force can be applied to the control valve  32  by an electromagnet  34 . The electromagnet  34  is connected via a signal line  35  to an electronic device  36 , which depending on the position of a driving lever  37  applies a switching current to the electromagnet  34 . Via another signal line  38 , the electronic device  36  receives from the driving lever  37  a signal for whether the position of the driving lever is pointing in the direction of forward driving or backward driving. 
     In the shown embodiment, the control valve  32  is in the form of a switch valve, which depending on the signal at the electromagnet  34  switches between a first and a second switch position. In the first switch position, in which in the idle state the control valve  32  is, because of the control valve spring  33 , the control surface  28  of the control valve  24  of the first hydraulic motor unit  5  is connected via a first control pressure duct  39  to a removal duct  40 . The removal duct  40 , at the end facing away from the control valve  32 , is connected to the duct  12 , so that in the first switch position of the control valve  32  the pressure in the duct  12  is applied to the control surface  28  of the control valve  24  of the first hydraulic motor unit  5 . The pressure which is generated by the auxiliary pump  9  in the duct  12  is sufficient to overcome the counter-force of the control valve spring  26 , so that the control valve  24  is displaced in the direction of the final position opposite the initial position. By displacing the control valve  24  into its final position opposite the initial position, the first and second joining duct  45  and  46  of the second hydraulic motor  22 ′ are short-circuited, so that, as explained above, the second hydraulic motor  22 ′ supplies no contribution to the torque on the first driven shaft  20 . 
     Simultaneously, in the first switch position of the control valve  32 , the control surface  29  of the control valve  25  of the second hydraulic motor unit  6  is connected via a second control pressure duct  49  to a relief duct  50 , which opens into the tank volume  11  on its side facing away from the control valve  32 . The control valve  25  of the second hydraulic motor unit  6  is therefore, because of the force of the control valve spring  27 , in its initial position. The control valve  25  in its initial position connects a third linking duct  43  to the third joining duct  47  and a fourth linking duct  44  to the fourth joining duct  48 . The third and fourth linking duct  43  and  44  are connected via the second connecting duct  7 ″ and fourth connecting duct  8 ″ to the first main duct  7  and second main duct  8  respectively, so that the pressure means which the pump  2  conveys into one of the main ducts  7  or  8  is also conveyed to the second hydraulic motor  23 ′ of the second hydraulic motor unit  6 . A torque which is generated by the hydraulic motor  23  and the second hydraulic motor  23 ′ therefore acts on the driven shaft  21 . 
     If the vehicle, for instance a road roller, moves forwards, the operator has steered the driving lever  37  in the direction of a forward motion F. The steering of the driving lever  37  in the direction of a forward motion F is transmitted via the further signal line  38  of the electronic device  36 . In the electronic device  36 , a suitable control signal for the electromagnet  34  is generated, or a switched current, which is transmitted via the signal line  35  to the electromagnet  34 , is generated directly. In the shown embodiment, because of this signal, no current flows through the electromagnet  34 , so that the control valve  32  is held in its first switch position because of the control valve spring  33 . 
     The auxiliary pump  9  is driven by the prime mover  3  via the section  4 ′ of the drive shaft  4 , and generates a pressure which is fed via the removal duct  40  of the control surface  28  of the control valve  24  of the first hydraulic motor unit  5 . In the embodiment shown in  FIG. 1 , the first hydraulic motor unit  5  is assigned to the front axle of the road roller. Depending on the desired driving speed, the pump  2  generates a volume stream which is delivered into the second main duct  8 , and is fed via both the third connecting duct  8 ′ and the fourth connecting duct  8 ″ to both the first hydraulic motor unit  5  and the second hydraulic motor unit  6 . Because of the control pressure which is present on the control surface  28  of the control valve  24  of the first hydraulic motor unit  5 , the first joining duct  45  and second joining duct  46  of the second hydraulic motor  22 ′ of the first hydraulic motor unit  5  are short-circuited, so that on the front axle, via the first driven shaft  20 , only the torque which is generated by the first hydraulic motor  22  of the first hydraulic motor unit  5  is present. 
     On the other hand, in the case of the second hydraulic motor unit  6  which is assigned to the rear axle, a higher torque, which is generated because of the connected second hydraulic motor  23 ′ and the hydraulic motor  23 , acts on the second driven shaft  21 . For this purpose, the second hydraulic motor  23 ′ of the second hydraulic motor unit  6  is connected via the control valve  25 , by being connected via the third and fourth linking duct  43  and  44  to the first main duct  7  and second main duct  8  respectively. 
     Because of this unequal distribution of the torques to the front axle and rear axle of the driven vehicle, during uphill motion the axle which is relieved of load by the inclination of the plane is driven with less moment. If a vehicle moves forwards up a hill, the front axle first tends to slip. This tendency for slip to occur is counteracted by the reduction of the moment on the front axle. The described distribution of the torques to the front axle and rear axle is carried out preventively irrespective of an actually occurring uphill motion, so that even in the case of a forward motion on the level, the torque on the front axle is reduced compared with the torque on the rear axle. The danger of errors in recognising the driving situation is reduced by simply using the direction of motion as the basis. The distribution of the torque in favour of the drive on the rear axle during forward motion also corresponds to the requirements which occur during motion on the level, since during acceleration the rear axle can transmit a higher torque than the front axle. 
     In a change of direction of motion, the operator moves the driving lever  37  out of the direction for forward motion F into the direction for backward motion R. The change of direction of motion signal is in turn converted by the electronic device  36  into a suitable signal for the electromagnet and transmitted via the signal line  35  to the electromagnet  34 . Corresponding to the signal which is now present on the electromagnet  34 , current flows through the electromagnet  34  and switches the control valve  32  into its second switch position. In the second switch position, the removal duct  40  is connected to the second control pressure duct  49 , so that the pressure which the auxiliary pump  9  generates is present on the control surface  29  of the control valve  25  of the second hydraulic motor unit  6 . Correspondingly, the first control pressure duct  39  is now connected to the load relief duct  50 . In this way, the control surface  28  of the control valve  24  of the first hydraulic motor unit  5  is relieved of pressure by the connection to the tank volume  11 , so that the control valve  24  returns to its initial position because of the control valve spring  26 . 
     In the second switch position of the control valve  32 , the second hydraulic motor  23 ′ is consequently switched off, by the third joining duct  47  being short-circuited to the fourth joining duct  48  by the control valve  25 . On the other hand, the second hydraulic motor  22 ′ of the first hydraulic motor unit  5  is connected via the first linking duct  41  and second linking duct  42  to the first main duct  7  and second main duct  8  respectively, and thus supplies a contribution to the torque which is available on the first driven shaft  20 . Therefore, if the driving lever  37  is steered in the direction of a backward motion, a smaller torque is generated on the rear axle than on the front axle. In this way, in each case a smaller torque is generated on the front axle seen in the direction of motion. 
     The total absorption volume remains constant in each case, because when the second hydraulic motor  22 ′ of the first hydraulic motor unit  5  is switched on, correspondingly the second hydraulic motor  23 ′ of the second hydraulic motor unit  6  is switched off, and vice versa. By this compensation of the change of the absorption volume on the first or second hydraulic motor unit  5  or  6 , control of the delivery volume of the pump  2  is simplified. 
     In  FIG. 2 , a second embodiment of a controller according to the invention is shown. Identical components are given identical reference symbols. To avoid repetitions, reference is made to the description of  FIG. 1 . 
     In contrast to the first embodiment of  FIG. 1 , in the hydrostatic traversing mechanism according to  FIG. 2  a first hydraulic motor unit  5 ′ and a second hydraulic motor unit  6 ′, which have a first hydraulic motor  55  and a second hydraulic motor  56  respectively, are provided. The absorption volume of the first and second hydraulic motor  55  and  56  is adjustable. For instance, machines in swash plate construction, in which the absorption volume is adjusted by adjusting the pivoting angle of the swash plate, can be used. 
     To adjust the absorption volume of the first hydraulic motor  55  or second hydraulic motor  56 , a control valve  32 ′, which can take a first and a second switch position which are identical to the first and second switch position of the control valve  32  from  FIG. 1 , is provided. Additionally, the control valve  32 ′ can take a third switch position, and is continuously adjustable. The control valve  32 ′ is controlled via the electromagnet  34 , and in the opposite direction by a further electromagnet  34 ′. The further electromagnet  34 ′ is controlled by an additional signal line  35 ′ from the electronic device  36 . If, because the driving lever  37  is steered in the direction of forward motion F, a corresponding signal is transmitted via the further signal line  38  to the electronic device  36 , in the shown embodiment current is supplied to the further electromagnet  34 ′ by means of the additional signal line  35 ′, and the control valve  32 ′ is in its first switch position. 
     In the first switch position, the removal duct  40  is connected to the first control pressure duct  39 . On the other hand, the second control pressure duct  49  is relieved via the relief duct  50  into the tank volume  11 . 
     The first hydraulic motor unit  5 ′ and the second hydraulic motor unit  6 ′ are constructed identically. The first hydraulic motor unit  5 ′ has, as well as the adjustable first hydraulic motor  55 , which drives the first driven shaft  20 , a variation device  30 ′. The variation device  30 ′ consists essentially of a set pressure regulating valve  51  as the control valve, and an adjustment device  53 . The adjustment device  53  acts on an adjustment mechanism of the first hydraulic motor  55 , and adjusts its absorption volume. The adjustment device  53  includes a pretensioning spring  57 , which applies a force to a set piston  59 , and adjusts the first hydraulic motor  55  in the direction of maximum absorption volume. Oppositely to the pretensioning spring  57 , in a set pressure chamber  61  a hydraulic force acts on the set piston  59 . The magnitude of the hydraulic force on the set piston  59  can be adjusted by changing the pressure in the set pressure chamber  61  via the set pressure regulating valve  51 . 
     For this purpose, the set pressure regulating valve  51  can be continuously adjusted between a first and a second final position. It is pressed by a set pressure regulating valve spring  26 ′ in the direction of its first final position. In this first final position of the set pressure regulating valve  51 , the set pressure chamber  61  is connected via a set pressure joining duct  65  to a tank duct  67 , which relieves the set pressure chamber  61  into the tank volume  11 . 
     Oppositely to the force of the set pressure regulating valve spring  26 ′, a hydraulic force acts on the set pressure regulating valve  51  on the control surface  28 ′ if a pressure is fed via the first control pressure duct  39 . 
     In the first switch position of the control valve  32 ′, the control surface  28 ′ is pressed by the auxiliary pump  9  via the first control pressure duct  39 , which is connected to the removal duct  40 . The set pressure regulating valve  51  is adjusted in the direction of its second final position, in which a set pressure duct  63  is connected to the set pressure joining duct  65 . The set pressure joining duct  63  is connected to the third connecting duct  8 ′, so that in this final position the set pressure chamber  61  is pressed with the pressure of the second main duct  8 . The increasing pressure in the set pressure chamber  61  moves the set piston  59  against the force of the pretensioning spring  57 , and thus adjusts the first hydraulic motor  55  in the direction of a smaller pivoting angle. 
     Simultaneously, the second control pressure duct  49  is connected to the relief duct  50  by the control valve  32 ′, so that the pressure which is applied to the control surface  29 ′ is relieved into the tank volume  11 . The set pressure regulating valve  52 , as the control valve of the second hydraulic motor unit  6 ′, is therefore, because of the set pressure regulating valve spring  27 ′, in its first final position, in which the set pressure chamber  62  of the adjustment device  54  of the variation device  31 ′ of the second hydraulic motor unit  6 ′ is connected to the tank volume  11  via a set pressure joining duct  66  and a tank duct  68 . The second hydraulic motor unit  6 ′ is thus adjusted in the direction of a large pivoting angle. 
     Similarly to the example from  FIG. 1 , in a first switch position of the control valve  32 ′ the hydraulic motor unit  5 ′ is adjusted so that a smaller torque acts on the first driven shaft  20 , which in turn corresponds to the front axle of the vehicle, than on the second drive shaft  21 . If the driving lever  37  is now moved from the position for forward motion F into a position for backward motion R, this new position is converted by the electronic device  36  into corresponding control signals for the electromagnets  34 ,  34 ′. Current no longer flows through the further electromagnet  34 ′, instead current flows via the signal line  35  through the electromagnet  34 , which adjusts the control valve  32 ′ in the direction of its second switch position. In the second switch position, the removal duct  40  is connected to the second control pressure duct  49 , and the first control pressure duct  39  is connected to the relief duct  50 . In this way, again similarly to the embodiment from  FIG. 1  and the adjustment described above of the hydraulic motor units  5 ′ and  6 ′, the second hydraulic motor unit  6 ′ is adjusted in the direction of smaller absorption volume, and the first hydraulic motor unit  5 ′ is adjusted in the direction of greater absorption volume. The change of the absorption volume of the first hydraulic motor unit  5 ′ and second hydraulic motor unit  6 ′ is again preferably equally great. 
     Additionally, the control valve  32 ′ can take a third switch position. In the third switch position, both the first control pressure duct  39  and the second control pressure duct  49  are connected to the removal duct  40 . To take the third switch position, both electromagnets  34 ,  34 ′ are controlled correspondingly by the electronic device  36 , if it is detected that the operator has specified a high driving speed. Specifying a high driving speed can be detected by the electronic device  36  because the driving lever  37  passes a specified position. The third switch position of the control valve  32 ′ corresponds to a fast gear. In this position, both the first hydraulic motor unit  5 ′ and the second hydraulic motor unit  6 ′ are adjusted in the direction of a smaller pivoting angle. In this way the transmissible torque is reduced, but simultaneously the r.p.m. of the first and second hydraulic motor  55  and  56  for a given delivery volume of the pump  2  is increased, resulting in a higher speed of the driven vehicle. 
     For both the control valve  32 ′ and the control pressure regulating valve  51  and  52 , continuous adjustment between the final positions can be provided, so that the absorption volume of both the first hydraulic motor  55  and the second hydraulic motor  56  is continuously adjustable. 
     In  FIG. 3 , an embodiment of the controller according to the invention, in which instead of the driving lever  37  from  FIG. 1  an inclination sensor  70  is provided, is shown. The inclination sensor detects the inclination of the vehicle relative to the level. If the inclination sensor  70  detects that the level of the front axle is above the level of the rear axle, the electronic device  36  does not supply current to the electromagnet  34 , and the control valve  32  is in its first switch position, which is fixed by the control valve spring  33 . In this switch position of the control valve  32 , as was explained in detail about  FIG. 1 , the first hydraulic motor unit  5  is adjusted in the direction of smaller torque, and thus the torque on the front axle is preventively reduced. 
     On the other hand, if the vehicle is in a situation in which the level of the rear axle is higher than the level of the front axle, the electronic device  36  switches a current onto the electromagnet  34 , which then brings the control valve  32  into its second switch position. In the second switch position, the torque of the second drive shaft  21  is reduced by switching off the second hydraulic motor  23 ′ of the second hydraulic motor unit  6 . 
     The absorption volume of the hydraulic motor units  5  and  6  is adjusted solely on the basis of the detected inclination of the vehicle. In this way, corresponding to the procedure from  FIG. 1 , before slip occurs on the front or rear axle, the absorption volume of the first hydraulic motor unit  5  and second hydraulic motor unit  6  can be adjusted so that the tendency to form slip is reduced. Use of the inclination sensor  70  makes it possible to apply a smaller torque to the up-side axle. By preventive adjustment of the absorption volume of the first hydraulic motor unit  5  and second hydraulic motor unit  6 , because of a simple signal of the inclination sensor  70  a distribution, which is adapted to the driving situation, of the torque on the first driven shaft  20  and second driven shaft  21  is made possible. 
     In  FIG. 3 , an inclination sensor  70 , which has only two switch positions for the two different inclination directions, a so-called inclination sensor, is enough. However, use of the gradient sensor  70  is not restricted to the embodiment according to  FIG. 3 , but can also, in combination with the variable adjustment possibility of  FIG. 2 , make it possible to adapt the torque distribution to the relevant gradient. For this purpose, an inclination sensor  70  which outputs a continuous signal depending on the inclination is provided. 
     In  FIG. 4 , a further example of a controller according to the invention is shown. In this, instead of direction of motion or gradient signal, the ratio of the pressure in the first main duct  7  and second main duct  8  is used for control. The first hydraulic motor unit  5  and second hydraulic motor unit  6  are identical with those from  FIG. 1 , and were explained in detail there. In this embodiment, pressure is applied to the first control pressure duct  39  and second control pressure duct  49 , or they are connected to the tank volume  11 , by a control valve unit  80  instead of the control valve  32 . 
     The control valve unit  80  comprises a selection valve  81  and a relief valve  82 . On the input side, the selection valve  81  is connected via a first main duct branch  83  to the first main duct  7 , and via a second main duct branch  84  to the second main duct  8 . The first main duct branch  83  is also connected to a first measurement surface  85 , through which a force is applied to the selection valve  81 , counteracting a hydraulic force which acts on a second measurement surface  86 . To generate a hydraulic force on the second measurement surface  86 , the second measurement surface  86  is connected to the second main duct branch  84 . Additionally, on the selection valve  81 , selection valve springs  91  are provided. One selection valve spring in each case exerts a force on the selection valve  81  in the direction of the first and second measurement surface  85  and  86 . If the pump  2 , for instance, in the case of the forward motion described with reference to  FIG. 1 , delivers into the second main duct  8 , the pressure in the second main duct  8  exceeds the pressure in the first main duct  7 . Correspondingly, a greater pressure is delivered via the second main duct branch  84  to the second measurement surface  86  than to the first measurement surface  85 . With such a pressure gradient, the selection valve  81  is in its first switch position, shown in  FIG. 4 . In this first switch position of the selection valve  81 , a connection is made between the first main duct branch  83  and a first output  87  of the selection valve  81 . On the other hand, the connection between the second main duct branch  84  and the second output  88  of the selection valve  81  remains interrupted. 
     The relief valve  82  has a first input  89  and a second input  90 . The first input  89  is connected to the first output  87  of the selection valve  81 , and the second input  90  is connected to the second output  88  of the selection valve  81 . Also, the first control pressure duct  39  is connected to the first input  89  of the relief valve  82 . The second control pressure duct  49  is connected to the second input  90  of the relief valve  82 . 
     In the described first switch position of the selection valve  81 , the first input  89  of the relief valve  82  and thus also the first control pressure duct  39  are connected to the first main duct branch  83 . A third measurement surface  93 , through which a hydraulic force which is directed oppositely to a hydraulic force which acts on a fourth measurement surface  94  is applied to the relief valve  82 , is also connected to the first control pressure duct  39 . The fourth measurement surface  94  is connected to the second control pressure duct  49 . Because no pressure is applied to the second control pressure duct  49  in the described first switch position of the selection valve  81 , the pressure which acts on the third measurement surface  93  brings the relief valve  93  into a first position, in which the second input  90  of the relief valve  82  is connected via a further tank duct  95  to the tank volume  11 . 
     Accordingly, in the first position of the relief valve  82  the second control pressure duct  49  is connected to the tank volume  11 , and the first control pressure duct  39  is connected via the first main duct branch  83  to the first main duct  7 . 
     In the case of a pressure reversal in the first main duct  7  and second main duct  8 , the selection valve  81  is brought into a second switch position, in which the second main duct branch  84  is connected to the second output  88 , so that a control pressure is applied via the second control pressure duct  49  to the control surface  29  of the control valve  25  of the second hydraulic motor unit  6 . The control pressure is therefore the pressure of the main duct  7  or  8  which is lower, but is still clearly above the pressure level of the tank volume  11 . By connecting the second output  88  to the second main duct branch  84 , a hydraulic force is applied to the fourth measurement surface  94 , and brings the relief valve  82  into its second position. In the second position, the first input  89  is connected via the further tank duct  95  to the tank volume  11 , so that the control surface  28  of the control valve  24  of the first hydraulic motor unit  5  is relieved. 
     The adjustment of the hydraulic motors on the basis of the pressures on the control surfaces corresponds to the explanations of  FIG. 1 . 
     The selection valve  81 , and consequently the relief valve  82 , are switched solely on the basis of the sign of the pressure difference between the first main duct  7  and the second main duct  8 . This means that even without the occurrence of slip during motion on the level, the change of direction of motion, through a corresponding pressure change in the first and second main duct  7  and  8 , results in an adaptation of the distribution of the torques. Since the sign of the pressure difference is used exclusively, a pressure reversal is also detected if it is caused, for instance, by downhill motion. The control valve unit  80 , irrespective of the direction of motion of the vehicle, adjusts the hydraulic motor unit which is oriented to the up-side to a smaller absorption volume. 
     To reduce the length of the required joining ducts, it is also possible to integrate a control valve unit  80  in each of the two hydraulic motor units  5 ,  6 . The pressures, which are required for control, of the first main duct  7  and second main duct  8  are present there in any case. 
     In  FIG. 5 , a combination of the control valve unit  80  from  FIG. 4  with the adjustable hydraulic motor units  5 ′ and  6 ′ from  FIG. 2  is shown. In this embodiment, all the valves which are used can be adjusted continuously between their final positions. 
     In  FIG. 6 , an extension of the controller according to the invention from  FIG. 4  is shown. As an extension, between the selection valve  81  and the relief valve  82  an over-control valve  100 , which in an idle position defined by an over-control valve spring  101  connects the first output  87  of the selection valve  81  to the first input  89  and the second output  88  of the selection valve  81  to the second input  90  of the relief valve  82 , is shown. 
     The over-control valve  100  can be brought into an over-control position, in which additionally the first input  89  and the second input  90  of the relief valve  82  are connected to each other, by an over-control magnet  102 . In this way, the lower pressure of the main ducts  7  or  8  is applied to both the first control pressure duct  39  and the second control pressure duct  49 , and thus both the first hydraulic motor unit  5  and the second hydraulic motor unit  6  are adjusted for smaller absorption volume. With constant delivery volume of the pump  2 , the result is a lower torque with simultaneously increased driving speed. 
     Because of the pressure which is equally present on the third and fourth measurement surface, the relief valve takes a central, third position. The first input  89  and second input  90  are connected to the tank volume  11  via the tank duct  95 . The connection takes place choked. The connection enables a small quantity of oil from the low-pressure-side first or second main duct  7  or  8  to enter the tank volume  11 , thus achieving additional cooling. 
     In  FIG. 7 , an example of a vehicle which is equipped with a controller according to the invention is shown. The vehicle  105  is a road roller with a front roller  106  and a rear roller  107 . The front roller is driven by the first hydraulic motor unit  5  or  5 ′ and the rear roller  107  is driven by the second hydraulic motor unit  6  or  6 ′. To specify the direction and speed of motion, the driving lever  37  is provided. To simplify operation during work, a second driving lever  37 ′ is provided, so that a driving lever is available to the operator on both sides of a driver&#39;s seat  109 .