FIELD OF THE INVENTION 
     The invention relates to a drive system for a vehicle, particularly for an agricultural or industrial utility vehicle. The present invention further relates to a vehicle with such a drive system, as well as to a drive module and/or a converter module and/or a controller for such a drive system. 
     BACKGROUND OF THE INVENTION 
     Electric motors are increasingly being used to drive vehicles, which draw their energy from internal combustion engine driven generators, batteries, or fuel cells. To achieve a higher spread, in many cases shiftable gear stages are arranged after the electric motors, but usually the power transfer is realized without shifting stages. The term “spread” is understood to be the speed range over which nominal power can be reached at the power take off. 
     For road and rail vehicles, the state-of-the-art procedure currently described usually has been sufficient for achieving the desired driving power. Here, the spread lies on the order of 5-10. The lowest speed at which nominal power is achieved frequently lies over 20 km/h. For agricultural utility vehicles and especially for tractors, this spread is not sufficient. Values over 15 are necessary to cover the driving tasks of a tractor. The lowest speed at which nominal power is achieved lies in the vicinity of 3 km/h. Due to the low absolute speed and high traction force of tractors, shifting processes, with which it must be shifted into a different speed range for a similar traction force, are very uncomfortable due to the transmission jump in the drive system. The continuously variable power shift gears frequently used in tractors typically have two branches, by means of which the torque of the drive module or the energy source is selectively transmitted to the traction drive. When shifting to a different speed range, torque is also transmitted from the drive module to the traction drive during the shifting process (power shift gear). If a shifting process is performed, one branch of the gear is separated or decoupled from the traction drive, while the other branch of the gear is coupled and thus connected to the traction drive. A shifting process is subject to constraining conditions, because the rotational speeds of the two branches must essentially match at the time of the shifting process. 
     Moreover, in contrast to road and rail vehicles, tractors, in addition to the traction drive, are usually equipped with one or more additional mechanical power take off devices for attachments, a so called power take off (PTO). 
     SUMMARY OF THE INVENTION 
     In the following, the term “branch” designates a part of a drive system or a gear, which can transmit mechanical torque, or, very generally, energy. Thus, it can involve a shaft and rotating transmission elements connected to this shaft and/or shifting stages. 
     Therefore, the present invention is based on the task of specifying and improving a drive system of the type named above, through which the previously mentioned problems are overcome. In particular, a drive system shall be specified, which features a spread that is expanded relative to the state of the art and which at least essentially prevents uncomfortable shifting processes. 
     According to the invention, the drive system comprises a first and a second drive module, a first and a second branch, at least one controller, and at least one output interface. The first drive module can be connected to the first branch. The second drive module can be connected to the second branch. The first branch and/or the second branch can be connected reversibly to the output interface. The drive modules can be controlled with at least one controller, so that the drive modules can output a given power or energy continuously and independently of each other. 
     One output interface could be, for example, a shaft, which can be coupled to a traction drive of a vehicle and by means of which, for example, mechanical torque can be transmitted to the drive of the vehicle. 
     The drive system according to the invention has two branches—especially for continuous shifting processes under loading—between which the system can be shifted back and forth, but which can also be connected simultaneously to the output shaft. The drive modules can be controlled by the controller independently of each other for the output of a given power; for example, in the form of a mechanical torque or very generally in the form of energy. Thus, a state can be created in the drive system, in which the mechanical rotational speeds of the two branches are each adapted to that of the output interface, so that a synchronous shifting process is possible, if, for example, the connection of the first branch to the output interface is broken and the connection of the second branch to the output interface is established. With the drive system according to the invention, the state is also provided that both the first and also the second branch are connected to the output interface, whereby the preferably mechanical power output by the two drive modules can be added. Each of the two branches could have fixed speed and transmission ranges in which they are used. 
     Due to the independent control of the two drive modules, it is possible to design the speed ranges of the two branches, such that these ranges overlap. Thus, in the overlapping ranges, advantageously one or both branches can be arbitrarily connected to or separated from the output interface. Even if both branches are connected to the output interface, the transmission between the first branch and the output interface and the second branch and the output interface can be adjusted; thus, in contrast to conventional multi step or multi range transmissions, there is not a constraining condition for the existing mechanical transmission. Accordingly, a shifting process of the drive system according to the invention requires only the coupling or decoupling (or connection or disconnection) of one branch to the output interface, for example, such that a corresponding shift coupling between the branch and the output interface is activated. Finally, two shifting processes—for example, the connection of one branch to the output interface and the disconnection of the other branch from the output interface—can lie arbitrarily far apart from each other in time, whereby overall a nearly jerk free shifting of the speed ranges of the drive system is possible. 
     In principle, it could be provided that a drive module has an internal combustion engine. Especially preferred, a diesel engine would be used in this case. Diesel engines are regularly used, especially for agricultural utility vehicles, due to their multiple control possibilities and their high efficiency. It would be conceivable, for example, that one drive system has two diesel engines. 
     Alternatively or additionally, a drive module could have an energy source, which generates electric current, for example, a fuel cell. This drive module could further have a mechanical conversion stage, with which the electric current is converted into mechanical torque. 
     The provision of two energy sources, which can be controlled independently of each other and which can each be connected to the first or to the second branch, is associated in an especially advantageous way with great flexibility in terms of the control possibilities and the operating states of the drive system, but can result in increased production costs. The basic concept of the drive system according to the invention can also be used when only one energy source is provided for a vehicle. In such a case, the drive system also has an input interface and at least one converter module. The input interface can be connected to an energy source, preferably embodied in the form of an internal combustion engine of a vehicle. Energy generated by the energy source or delivered power can be distributed to the first and to the second branch via the input interface. The converter module is connected to the drive modules. The converter module can be connected to the input interface. In the present sense, an input interface is understood to be an interface, which receives energy generated by the energy source and supplies it to the drive system. The input interface could be constructed in the form of a shaft, if the energy source is embodied in the form of an internal combustion engine and provides mechanical torque. 
     So that the drive modules can output a given power independently of each other or can be operated with a given torque, energy or power can be distributed or transported arbitrarily between the converter module and the drive modules. Such an energy/power transport is preferably controlled by the controller. If the converter module has an electric generator and the two drive modules each has an electric machine, such an energy transport could be realized with the aid of a power electronics controller. 
     Preferably, an energy source is used, which generates mechanical and/or electrical energy. For controlling the energy source, a separate controller could be provided. In this way, the energy generated by the energy source is variable; thus, it can be adapted to the corresponding operating state of the drive system or a vehicle. 
     The energy source could have an internal combustion engine, especially a diesel engine. Furthermore, the energy source could include a generator driven by an internal combustion engine and/or a fuel cell and/or an electric storage device, for example, an accumulator, a capacitor, or a battery. 
     In an especially preferred embodiment, another output interface is provided. This additional output interface could be connected, in principle, reversibly to the first or to the second branch; preferably, the drive system is embodied such that the additional output interface can be connected reversibly to the second branch. The additional output interface could involve a power take off (PTO) for transmitting mechanical torque, which is typically provided on tractors for mechanically driving work equipment. 
     Very generally, mechanical and/or electrical energy can be transmitted via the input interface, the output interface, and/or the additional output interface. If mechanical energy, for example, in the form of torque, is to be transmitted via an interface, a shaft could be provided for this purpose. Electrical energy could be transmitted inductively or with the aid of sliding contacts by means of correspondingly designed electrical lines. 
     Preferably the first and/or the second branch and/or the output interface each has at least one mechanical gear stage. With such a gear stage, a rotational speed reduction and/or a rotational speed reversal could be achieved. Thus, the flexibility of a branch can be further increased and the spread of the drive system according to the invention can be increased still more. In detail, the mechanical gear stage could have at least one spur gear stage and/or one planetary gear unit. 
     Especially preferred, a reversible connection between an output interface and a branch is produced with the aid of a positive fit coupling. Synchronization, which is necessary for this purpose, between the rotational speeds of a shaft allocated to the output interface and a shaft allocated to a corresponding branch can be achieved with the aid of the two drive modules, which can be controlled independently of each other. The positive fit coupling could be shifted by means of an electrically controllable shifting element, but a mechanical or hydraulic activation of the shifting element could also be conceivable. Preferably, the shifting element for coupling or decoupling the reversible connection works against a spring force, so that only an actuator—namely, for example, the electrically activated shifting element—is to be provided. The positive fit coupling could work according to the principle of a claw coupling. 
     In principle, between the converter module and the two drive modules, an energy or power transport could be performed, which enables part or all of the energy/power provided by the energy source to be distributed arbitrarily from the converter module to the two drive modules. Here, it may be necessary to convert the energy before distributing it; examples for this purpose are given below. Therefore, it is to be provided, in particular, that a converter module receives mechanical and/or electrical energy. Additionally or alternatively, one drive module could output mechanical and/or electrical energy. 
     In an especially preferred embodiment, a conversion between electrical and mechanical energy is performed with the converter module and the drive modules. For this purpose, the converter module could have at least one electric machine that can be operated as a generator. Furthermore, the first and the second drive modules could each have an electric machine that could be operated as a motor. Thus, for example, the mechanical energy generated by the energy source is fed at least partially to the converter module, which converts this energy into electric current. The electric current is fed to the drive modules, which, on its side, converts this electrical energy back into mechanical torque. 
     With the converter module and the drive modules, a conversion between hydraulic and mechanical energy could also take place. For this purpose, the converter module could have at least one mechanical drive hydropump. The hydropump is preferably adjustable, so that the amount and thus the pressure of the hydraulic fluid generated by the hydropump is variable. With the hydropump, the mechanical energy is converted into hydrostatic energy. The hydrostatic energy can then be converted back into mechanical energy by a drive module, if the first and/or the second drive module each has a hydromotor. Preferably, such a hydromotor is also adjustable, i.e., the hydromotor can be operated at different rotational speeds for a constant pressure of the hydraulic fluid. 
     As another example, a purely mechanical conversion between the converter module and the drive modules is conceivable. For this purpose, the converter module could have an input shaft of a gear, for example, a loop gear, a friction gear, or a chain converter. The first and the second drive module could each have at least one output shaft of the corresponding gear. 
     According to the first embodiment, the input interface is mechanically coupled to the first and to the second branch. The converter module is either allocated to the energy source or the converter module has an electric machine driven by the energy source and operating as a generator. Accordingly, the energy source directly provides electrical energy or is embodied, for example, in the form of an internal combustion engine, which drives the converter module mechanically, so that the converter module—the electric machine operating as a generator—generates electric current. The first and the second drive module each have an electric machine operating as a motor. Here, one of the two drive modules is always connected to its allocated branch. 
     According to a second embodiment, the input interface is mechanically coupled to one of the two branches. If this branch is connected to the output interface, preferably only mechanical energy is transmitted via this branch. Connection to the drive module allocated to this branch would also be conceivable. The input interface is electrically or hydraulically coupled to the other branch. Here, the converter module has, e.g., an electric machine driven mechanically by the energy source and operating as a generator. The electric machine operating as a generator is then preferably always driven mechanically by the energy source. 
     The first branch can be driven mechanically by the first drive module. The second drive module can be connected to the second branch or power diverted to this branch that could be realized with the aid of a planetary gear. In the second branch, a brake is provided, with which at least part of the second branch can be stopped relative to a housing of the drive system. The brake could involve a friction brake. Thus, for example, the mechanical torque transmitted by the input interface could be fed to a sun wheel of a planetary gear. The second drive module could be connected to the planet carrier of the planetary gear. The part of the second branch that can be brought into connection with the output interface could be connected to the ring gear of the planetary gear. This part of the second branch could be stopped with the brake, so that for a stopped brake, and thus a stopped ring gear of the planetary gear, the second drive module is driven directly. The electrical energy generated by the converter module and the second drive module can be fed to the first drive module, which can be connected, on its side, to the output interface of the drive system. In this operating state, only the transmission of electrical power to the output interface is performed. 
     A compact construction can be achieved especially in this embodiment when the converter module and/or the first drive module are/is arranged essentially coaxial to the input interface. The same applies for the case that the second drive module is arranged essentially coaxial to the output interface. All three modules—i.e., the converter module and the two drive modules—are especially preferably arranged coaxial to the input interface. 
     In structural terms, it is advantageous if the first drive module is arranged spatially downstream of the converter module relative to the input interface. Optionally, it is possible to combine the converter module and the first drive module into one housing. Likewise, the first drive module could be downstream of the second drive module relative to the input interface. Overall, a sequence of components is preferred, which, viewed outwards from the input interface, is arranged as follows: converter module, second drive module, first drive module. Thus, of the converter module and/or the drive modules, preferably at least two modules are arranged essentially coaxial to each other. 
     Preferably, the first branch and the second branch are each connected reversibly to the output interface via a shiftable multi step transmission. The second branch can be connected reversibly to the other output interface via a shiftable multi step transmission. The shiftable multi step transmission could be embodied such that at least two different transmission ratios can be realized. Here, the torque applied to one branch can be transmitted with different rotational speeds and/or rotational directions to the output interface. 
     The output interface of the drive system according to the invention could be connected in a vehicle to a traction drive or can be connected in this way. The other output interface could be connected to a power take off (PTO), so that mechanical torque can also be transmitted—for example, to work equipment coupled to a tractor—via the second output interface. 
     Especially preferred, the drive system is designed such that shifting between the two branches is possible under loading, that is, mechanical torque is always transmitted to the traction drive. Here, the drive system according to the invention is suitable especially for tractors, because, especially when culling during an accelerating phase, a shifting process of the drive system must be performed under loading. 
     So that the drive system according to the invention can be embodied reliably in terms of operating safety, at least one sensor is provided, with which the operating state of at least one component of the drive system can be detected and fed to the controller. Thus, for example, rotational speed sensors could be provided, which each detect the rotational speed of a shaft of a branch. Furthermore, pressure sensors could detect the pressure of the hydraulic fluid of the hydraulically operating drive modules. The operating state of electrically operating components—the converter module and/or a drive module—could be detected with current/voltage sensors. Also, redundancy is provided in terms of the possible shift or operating states of the drive system. 
     Because the two branches of the drive system according to the invention can be driven with the aid of the drive modules continuously and completely independently of each other, in contrast to conventional gears, the drive system according to the invention does not necessarily have a well defined shift state or operating states. Instead, the drive system according to the invention can have a plurality of different shift states, of which a few preferred shift states are discussed below merely as examples. 
     Thus, for example, in a first shift state, the first branch is connected to the output interface and the first drive module is coupled to the first branch. Therefore, in this shift state, the energy delivered by the energy source—for example, in the form of mechanical torque—is output via the first branch to the output interface. Here, for example, a tractor equipped with the drive system according to the invention could perform a slow forward or reverse travel. In principle, the second branch in this shift state has no function. However, in this shift state the second branch could be connected to the other output interface and the second drive module could be coupled to the second branch. Thus, in the tractor equipped with the drive system according to the invention, in the second shift state the power take off could be connected and adjustable independently in its rotational speed. 
     In a second shift state, the first and the second branch are connected to the output interface. Here, the power transmitted from the first and the second branches to the output interface are added, so that in this shift state, a tractor equipped with the drive system according to the invention can perform a slow forward or reverse travel, especially for large traction force requirements. Here, the rotational speeds of the two drive modules could be set to the rotational speed of the output interface or synchronized with this speed. The other output interface—in the example of a tractor the power take off—is in this shift state dependent on the traveling speed and therefore can be used as a so called “motion power take off.” 
     Thus, preferably a third shift state is further provided, in which the second branch is connected to the output interface. In a corresponding design or construction of the drive system according to the invention for a tractor, in this shift state, the traveling speed could be significantly higher and the traction force could be significantly lower than, for example, in the first shift state. In the third shift state, it is preferably provided that the second branch is connected to the other output interface. For the example of a tractor equipped with the drive system according to the invention—here the rotational speed of the output interface; i.e., the traction drive—would be coupled with the rotational speed of the other output interface; i.e., the power take off, so that in this shift state, a “motion power take off” can also be realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , is a first embodiment according to the invention, 
         FIG. 2 , is a second embodiment according to the invention, 
         FIG. 3 , is a third embodiment according to the invention, 
         FIG. 4 , is a fourth embodiment according to the invention, and 
         FIG. 5 , is a fifth embodiment, which is similar to the embodiment from  FIG. 3  and shows more details. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The same or similar assemblies are characterized in the figures with the same reference symbols. 
       FIG. 1  shows a first embodiment of a drive system  10  according to the invention. 
     The drive system  10  comprises a first drive module  12  and a second drive module  14 , which can be controlled by the controller  16  via the lines  18 ,  20 . The two drive modules  12 ,  14  can be controlled by a controller  16  independently of each other and can continuously output power set by the controller  16 . The drive system  10  comprises a first branch  22  and a second branch  24 , wherein the two branches in  FIG. 1  are shown merely schematically in the form of a shaft. 
     The drive module  12  can be connected to the first branch  22  via the schematically shown gear connection  26 . The drive module  14  can be connected to the second branch  24  via the schematically shown gear connection  28 . Preferably, the gear connections  26 ,  28  are always connected; for example, an output shaft of the drive modules  12 ,  14  is locked in rotation via a corresponding gear connection  26 ,  28  to the first and to the second branch  22 ,  24 , respectively. 
     The drive system further comprises the output interface  30 , which can be connected reversibly to the first and/or to the second branch  22 ,  24 . Each connection is realized schematically with a module  32 ,  34 . 
     For example, the drive modules  12 ,  14  can each involve an internal combustion engine embodied in the form of a diesel engine. However, at least one of the two drive modules  12 ,  14  could also be embodied in the form of a fuel cell, which initially generates electric current, which is converted by a conversion stage (not shown in  FIG. 1 ) into mechanical torque. 
     Assuming that the two drive modules  12 ,  14  are always locked in rotation with the two branches  22 ,  24 , in principle, three shift states of the drive system  10  are conceivable. In a first shift state, only the drive module  12  is controlled by the controller  16 , so that the torque generated by the drive module  12  is transmitted to the branch  22  via the gear connection  26 . The first branch  22  is connected in this shift state to the output interface  30  via the module  32 , so that the branch  22  transmits the torque to the output interface  30 . A vehicle equipped with such a drive system  10  could be driven forwards in this shift state, for example, in a first speed range. 
     In a second shift state of the drive system  10 , only the drive modules  14  are controlled by the controller  16 . The second branch  24  is connected to the output interface  30  via the module  34 , so that the torque generated by the drive module  14  is transmitted to the output interface  30  via the second branch  24 . In this shift state, the vehicle could be driven forwards at a greater speed, for example, in a second traveling range. 
     In a third shift state of the drive system, both drive modules  12 ,  14  are controlled by the controller  16 , wherein both the first branch  22  and also the second branch  24  are connected to the output interface  30  via the modules  32 ,  34 . In this shift state, the torque generated by the two drive modules  12 ,  14  is transmitted as a sum to the output interface  30 . Here, the first and the second branch  22 ,  24  have a fixed rotational speed ratio, which is adapted to the rotational speed of the output interface  30 . In this shift state, the vehicle could be operated under increased load. 
       FIG. 2  also shows in a schematic view a second embodiment of a drive system  10  according to the invention. In terms of the components shown in  FIG. 1 , the drive system  10  shown in  FIG. 2  is comparably constructed. In this embodiment, an energy source  36  is provided, which can be connected at the input interface  38  (shown with dashed lines) to the drive system  10 . In detail, the energy source  36  comprises a diesel engine, which is coupled to the input shaft  40  of the drive system  10 . The diesel engine transmits mechanical torque to the two branches  22 ,  24 , which is realized with the aid of the schematically shown component  42 . The energy source  36  also comprises a converter module  44 , which is also driven mechanically by the diesel engine or the energy source  36 . The converter module  44  comprises an electric machine, which operates as a generator and which generates electric alternating current when the diesel engine is operating. The generated alternating current is first converted into direct current by a rectifier unit allocated to the converter module  44  (and not shown in  FIG. 2 ) and fed to the two drive modules  12 ,  14  via the input interface  38  with the aid of the connection line  46  via the controller  16 . The two drive modules  12 ,  14  are electric machines, which are constructed in the form of motors and which each has a rectifier unit (not shown in  FIG. 2 ) with which the direct current is converted into alternating current. The controller  16  comprises power electronics, which enables it to feed electric current merely to the first and/or to the second drive module  12 ,  14 , so that also in this embodiment, the two drive modules  12 ,  14  can each drive the first and/or the second branch  22 ,  24  continuously and independently of each other and thus—analogous to the functioning in the embodiment from FIG.  1 —also the drive interface  30 . 
       FIG. 3  shows a third embodiment of the present invention, wherein the drive system  10  according to the invention can be connected at its input interface  38  to a energy source  36  constructed in the form of a diesel engine. In this embodiment, the energy source  36  is mechanically coupled to the second branch  24 . The first branch  22  is non mechanically coupled to the energy source  36 . The converter module  44 , which is constructed in the form of an electric machine embodied as a generator, is always coupled to and driven with the second branch  24 . The electric current generated by the converter module  44  is fed via the connection line  46  to the controller  16 , which on its side can temporarily store the electric current in a buffer (not shown in  FIG. 3 ); for example, in a capacitor or accumulator. On the one hand, the second drive module  14  can be driven via connection line  20  and, on the other hand, the first drive module  12  can be driven via connection line  18 . 
     The first branch  22  can be driven only by the first drive module  12  via the gear connection  26 . The first branch  22  can be connected to the output interface  30  via the two meshing spur gears  48 ,  50 , as long as the shift point  52  creates a rotationally locked connection between the output interface  30  constructed in the form of a shaft and the spur gear  50 . The second drive module  14  can be connected to the second branch  24  via a shaft  53  via a gear stage  54  constructed in the form of a planetary gear. In this respect, it is conceivable that the mechanical torque generated by the energy source  36  is transmitted to the output interface  30  via the shafts  56 ,  58 , and also via the two spur gears  60 ,  62 , as long as the shift point  64  creates a rotationally locked connection between the spur gear  62  and the output interface  30 . 
     However, it would also be conceivable for the shaft  56  to be decoupled from the shaft  58  and the drive module  14  to be connected to the shaft  58 . In this case, the electric current generated by the converter module  44  is led by means of the controller  16  to the drive module  14 , which drives the shaft  53 , so that the mechanical torque generated by the drive module  14  is transmitted to the output interface  30  via the gear stage  54 , the shaft  58 , the two spur gears  60 ,  62 , and the closed shift point  64 . Finally, a shift state for the second branch  24  is also conceivable, in which, via gear stage  54 , mechanical torque is transmitted as a sum to the shaft  58  both from the shaft  56  and also from the shaft  53 , so that, on the one hand, the torque generated by the energy source  36  can be transmitted and, on the other hand with the mechanical torque generated by the second drive module  14  can be ultimately transmitted to the output interface  30 . In this shift state, the shafts  56 ,  53 , and  58  are coupled to each other via the gear stage  54 . 
     The drive system  10  from  FIG. 3  comprises a second output interface  66 , which is locked in rotation via the shift point  70  via a spur gear  68  meshing with the spur gear  60 . The second output interface  66  could be, for example, a power take off of a tractor, which is equipped with a drive system  10 . A brake  71 , with which the shaft  58  and the corresponding part of the gear stage  54  can be stopped relative to a housing of the drive system  10  (not shown in  FIG. 3 ), is provided for the shaft  58 . If the brake  71  is engaged, not only the converter module  44 , but also the second drive module  14  is driven by the energy source  36 . In this mode, the second drive module  14  is operated as a generator, so that both the converter module  44  and also the second drive module  14  can each generate electric current and the first drive module  12  or an electrical on board distribution system (not shown in  FIG. 3 ) can be made available. 
       FIG. 4  shows a fourth embodiment of the present invention. Here, the drive system  10  can be connected to an electrical energy source  36  via the input interface  38 , wherein the energy source  36  could be a generator driven by an internal combustion engine or a fuel cell. The electrical energy generated by the energy source  36  is fed to the controllers with rectifier units  74  or  76  of the two drive modules  12 ,  14  via the connection lines  72 . 
     The electrical energy could involve direct current. However, if the energy source  36  has a generator driven by an internal combustion engine, this engine typically delivers alternating or rotating current at a frequency dependent on its rotational speed. Because the drive modules  12 ,  14  were to be operated at a constantly changing frequency, the drive modules  12 ,  14  could output an arbitrary given power, although not unlimited. Therefore, the alternating or rotating current is first converted into direct current with the aid of a rectifier unit not shown in the figures, before it is fed to the controllers  74 ,  76 . The electrical energy converted into direct current is converted back into alternating current at a given frequency, in this case with the aid of another rectifier unit allocated to each controller  74 ,  76 , in order to finally drive the drive modules  12 ,  14  constructed in the form of electric motors. The drive modules  12 ,  14  each drive the first or second branch  22 ,  24 . The first branch  22  can be connected to the output interface  30 , on the one hand with the aid of the two spur gears  48 ,  50  via the shift point  52 . On the other hand, the first branch  22  can be connected to the output interface  30  via the spur gears  78 ,  80  and the shift point  82 . The second branch  24  can be connected to the output interface  30  via the spur gears  60 ,  62  and the shift point  64 . Furthermore, the second branch  24  can be connected to the output interface  30  via the spur gears  84 ,  86  and the shift point  88 . Depending on the shift points  64 ,  88 ,  52 , and  82 , the first branch  22  and/or the second branch  24  can be reversibly connected to the output interface  30 . The second branch  24  can be reversibly connected to the second output interface  66  via the shift point  90  and the spur gears  92 ,  94 . 
     The drive system  10  shown in  FIG. 4  is provided in an especially preferred way for a tractor and designed or configured such that it is distinguished by at least four travel ranges. In a first travel range, the shift point  52  is coupled so that the drive module  12  is locked in rotation with the output interface  30  via the spur gears  48 ,  50 , wherein the output interface  30  is connected to a traction drive in a tractor. By changing the rotational speed and reversing the direction of rotation of the drive module  12 , the traveling speed of the tractor can be changed or the direction of travel can be reversed. Operating the other output interface  66  in this travel range is possible via the second branch  24 , wherein the output interface  66  is connected to a power take off in a tractor. 
     In a second travel range, the shift points  52  and  64  are coupled or closed simultaneously so that the drive module  12  is locked in rotation via the spur gears  48 ,  50  and the drive module  14  is locked in rotation via the control line  60 ,  62  to the output interface  30  and thus to the traction drive of the tractor. Here, the drive power of the two drive modules  12 ,  14  combine, so that for the same traveling speed of the tractor, a higher traction force and a higher power are made available. In this shift state, operation of the other output interface  66  is not possible or possible only to a limited extent. The traveling speed in the first and in the second traveling range is limited by the highest permissible rotational speed of the drive module  12 . 
     In a third traveling range of the tractor, the shift point  64  is closed, so that the second drive module  14  is connected via the spur gears  60 ,  62  to the output interface  30 . In a useful design of the drive system  10  according to the invention, the traveling speed is significantly higher in this third traveling range and the traction forces are significantly lower than in the first traveling range, so that an expansion of the spread is achieved. 
     A transition from the first to the second traveling range and back can take place by closing or opening the shift point  64  at a synchronized rotational speed between the drive interface  30  and the spur gear  62  and without changing the torque flow in the traction drive and thus it is also unnoticed by the driver. 
     A transition from the second to the third traveling range and back can take place by opening or closing the shift point  52  at a synchronized rotation speed between the drive interface  30  and the end wheel  50  and without changing the torque flow in the traction drive and thus it is also unnoticed by the driver as well. 
     In the first traveling range of the tractor, the second drive module  14  is not used for the traction drive. Therefore, it can be locked in rotation by means of the shift point  90  to the other output interface  66 , i.e., the power take off of the tractor. By changing the rotational speed of the second drive module  14 , the rotational speed of the other output interface  66  can change continuously. It then corresponds to its function of a modern “motor power take off.” 
     Also in the second and third traveling range, the shift point  90  can be closed and thus the other output interface  66  can be driven. The power take off rotational speed then changes in proportion to the traveling speed. The power take off thus corresponds to its function of a modern “motion power take off.” 
     In principle, it is possible to increase the number of traveling ranges through other transmission stages and shift elements arbitrarily and/or to increase the necessary spread of the drive system  10  according to the invention and/or to reduce the necessary spread of the two drive modules  12 ,  14 , without losing the advantages of synchronous, no load shifting. A fourth traveling range is produced by simultaneously closing of the shift points  64  and  82 , a fifth traveling range is produced by closing only the shift point  82 , a sixth traveling range is produced by closing the shift points  88  and  82 , and a seventh traveling range is produced by closing just the shift point  88 . In the traveling ranges two, four, and six, the traction drive power is transmitted from the two drive modules  12 ,  14 , so that a higher power is made available than in the ranges one, three, and five. For suitable selection of the transmission ratios, it is possible to cover the entire range of speeds of a tractor with the ranges with simultaneous power transfer, so that traveling can preferably be performed in these ranges. It is then also possible to design the two drive modules  12 ,  14  not for the entire drive power of the tractor. 
     By simultaneously closing two suitable shift points, the traction drive can be blocked, and thus a function of a parking brake device can be achieved. In the embodiment from  FIG. 4 , these could be, for example, the shift points  52  and  82  or  64  and  88 . Thus, the embodiment of a drive system  10  according to the invention sketched in  FIG. 4  is distinguished by a uniform design for the traveling and power take off mode. A good use of installation space can be achieved. The drive modules  12 ,  14  constructed in the form of electric machines can have a smaller size than the machines typically used from the state of the art. High traction forces can also be achieved for low traveling speeds without over dimensioning the two drive modules  12 ,  14  through parallel connection of the two drive modules  12 ,  14 . Especially advantageously, a shifting process can take place at a synchronized rotational speed via another range, wherein a torque free or jerk free shifting is possible. 
       FIG. 5  shows a fifth embodiment of a drive system  10  according to the invention, which is similar to the embodiment from  FIG. 3 , also in its functioning, and where equivalent or similar components are represented by the same reference symbols. Thus, the first branch  22  of the drive system  10  includes, in addition to the first drive module  12 , essentially a hollow shaft  96 , which is locked in rotation with the spur gear  48 . The spur gear  48  meshes with the spur gear  50 , which can be connected reversibly to the first output interface  30  via the shift point  52 . The second branch  24  includes, on the one hand, the shaft  56 , which is driven by the energy source  36  constructed in the form of a diesel engine. The second branch  24  further includes the converter module  44 , which is always driven by the diesel engine  36 . The second branch  24  further includes a hollow shaft  53 , which is locked in rotation with the rotor of the second drive module  14  constructed in the form of an electric machine. The shaft  56  and the hollow shaft  53  are connected to the planetary gear  54 , wherein the planetary gear is further connected to the shaft  58 . The second branch  24  further includes the brake  71 , with which the shaft  58  and a part of the planetary gear  54  can be stopped, as well as the spur gears  98 ,  100  meshing with this gear, and also the two spur gears  60 ,  62  meshing with each other. The shaft  102  rotationally locks the two spur gears  100 ,  60  to each other. The spur gear  62  of the second branch  24  can be connected to the shift point  64  reversibly to the output interface  30 . The second branch  24  can be further connected reversibly to the second output interface  66  via the spur gear  68  meshing with the spur gear  98  via the shift point  70 . 
     In this embodiment, the output interface  30  is also connected to the traction drive of a tractor not shown in  FIG. 5  and also the output interface  66  is connected to the power take off of a tractor similarly not shown in  FIG. 5 . The converter module  44  and also the first and the second drive module  12 ,  14  are connected via connection lines and each to a frequency converter  104 ,  106 , and  108 . The controller  16  is connected to each frequency converter  104 ,  106 ,  108 . Thus, the converter module  44  can be operated by the controller  16  and the frequency converter  104  in one direction of rotation—namely, that of the diesel engine  36 —and in two torque directions for braking or accelerating. The drive modules  12 ,  14  can be operated by the controller  16  and each frequency converter  106 ,  108  in two directions of rotation and in two torque directions for braking or accelerating. 
     The controller  16  is connected to sensors (not shown in  FIG. 5 ) and a data interface for vehicle relevant information of the operating state of the diesel engine  36 . It also receives the rotational speed of the other output interface  66 , wheel or axle rotational speeds, which are detected by corresponding sensors (not shown in  FIG. 5 ) and which are made available to the controller  16 . In this respect, the controller  16  shown in  FIG. 5  acts as a higher order controller of a vehicle equipped with the drive system  10  according to the invention and also takes over the energy management of the vehicle, as well as the power supply for other electric loads (also not shown). 
     The embodiment according to  FIG. 5  also includes a first traveling range in which the other output interface  66  is not activated. Here, the shift point  52  is coupled so that the spur gears  48 ,  50  are locked in rotation with the output interface  30 , and thus the first branch  22  is driven by the first drive module  12 . The brake  71  constructed in the form of a friction brake is here closed so that the converter module  44  and the second drive module  14  operating as a generator are driven by the diesel engine  36 , and the electrical power generated in this way is made available to the first drive module  12  operating as a motor. By changing the rotational speed and reversing the direction of rotation of the first drive module  12 , the traveling speed of the vehicle can be changed and the traveling direction reversed. 
     The brake  71  is opened in a second traveling range. The rotational speed of the diesel engine  36  and the rotational speed of the second drive module  14  combine in the planetary gear  54 . The interface  64  for a synchronized rotational speed can be connected to the spur gear  50  via the shaft  58 , the spur gears  98 ,  100 , the shaft  102 , the spur gears  60  and  62 . In the second traveling range, it is not possible to use the other output interface  66 . The shift point between the branches  22 ,  24  must not be realized at a discrete rotational speed. Depending on the design of the components, there is a certain overlapping range of rotational speeds of the direct and power diverted paths. Here, in an especially advantageous way, a comfortable and efficient shifting process is enabled. 
     Instead of shifting from the first to the second traveling range, the other output interface  66 —and thus for a tractor, the power take off—can also be activated. Here, the shift point  64  is opened and the shift point  70  is closed. The brake  71  is opened and power can be taken at the other output interface  66 . 
     The rotational speed of the diesel engine  36  can be freely selected within limits according to power requirements. The control of the diesel engine  36  as well as the control of the converter module  44  and the first and second drive module  12 ,  14  can be realized, such that an optimal objective stored in a higher-order controller—e.g., in the controller  16 —is rejected. The optimal objective can be, for example, lower fuel consumption or the lowest possible noise production. The design of this embodiment combines a continuous traction drive with a continuous power take off. Therefore, in a conventional tractor equipped with the drive system  10  according to the invention, the two installation spaces typically provided for these components is available for use. 
     The converter module  44  and the first and second drive modules  12 ,  14  form the electric drive part of the drive system  10  shown in  FIG. 5 . They are combined together compactly downstream of the diesel engine  36  and can thus be installed in an optimal environment for electric machines, for example, no oil in the gear and stators water cooled from the outside. For the installation of the drive system  10  according to the invention from  FIG. 5  in a tractor, the shift points  52 ,  64  and also the spur gears  48 ,  50 ,  60 ,  62  can also be located in front of the differential housing. The planetary gear  54 , the brake  71 , and also the spur gears  68 ,  98 ,  100  could be housed in the installation space of the power take off. 
     Other variations for this embodiment are conceivable. Thus, for example, for shifting to the second traveling range, a power shift coupling can be used. The second branch  24  and/or the second output interface  66  could also have a direct, instead of power diverted, configuration. The shift point  70  could have another transmission ratio. An electric front wheel drive can replace a mechanical front wheel drive. An electrically driven front axle with one or two electric machines can replace one conventional front wheel drive. 
     Finally, it should be specifically mentioned that the previously explained embodiments are used merely for describing the claimed teaching, but this is not limited to the embodiments. 
     Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.
A drive system for a vehicle is provided, in particular for an agricultural or industrial utility vehicle. The drive system comprises a first and a second drive assembly (, a first and a second branch, at least one control unit and at least one output interface. The first drive assembly is connectable to the first branch. The second drive assembly is connectable to the second branch. The first branch and/or the second branch is/are reversibly connectable to the output interface. The drive assemblies are controllable by the at least one control unit such that the drive assemblies output independently from each other infinitely variable power.