GB2525968A - Hydrostatic transmission method for controlling the hydrostatic transmission - Google Patents

Abstract

A hydrostatic transmission for regulating an output power from a natural energy source such as wind or wave power has at least two hydrostatic actuators (pumps) 8, 10 on the input side, driven by the input power, and at least two hydrostatic actuators (motors) 12, 14 on the output side, whose outputs are connected together e.g. to drive a generator. Each pump 8, 10 is connected in a separate hydraulic circuit with a respective motor 12, 14. At least one of the circuits has a capacity which can impose a working pressure on the circuit (i.e. an accumulator) 22. The circuit without the accumulator may be controlled to transmit the input power minus a predetermined offset. The circuit with an accumulator and hence a relatively constant pressure may be controlled to transmit the difference between the target output power and the power transmitted by the first circuit. Output power delivery is therefore smoothed at a very early stage in the power conversion process.

Description

Hydrostatic transmission Method for regulating the hydrostatic transmission

Description

The invention relates to a hydrostatic transmission for converting an in particular multichromatic input power into an adjustable output power according to the preamble of claim 1, and to a method for regulating the output power of the transmission according to the preamble of claim 13.

The significance of regenerative energies, in particular of wind power, is increasing. The focus is also directed is increasingly to the use of maritime wave energy in particular. However, the input power offered by such natural energy sources is often of multichromatic guality and is thus not constant, but can have an irregular amplitude and frequency. Since stringent requirements are imposed on the quality of the power when feeding the in particular electrical output power of the transmission into an in particular electrical grid, the conversion of the multichromatic input power into a precisely adjustable output power is a substantial challenge when using such energy sources.

Previous hydrostatic transmissions for the conversion of wind energy have a common feeder line, for example, which is supplied by one or more hydrostatic pumps and to which several hydraulic motors are connected. A drive train of this kind is shown in the printed publication DE 10 2008 021 111 Al.

The printed publication DE 10 2011 016 592 shows a transmission with a hydrostatic circuit, the hydraulic machine on the output side of which is coupled mechanically to another hydraulic machine, via which a hydraulic accumulator can be charged and discharged. The two last-named form a storage unit, the function of which is the storage and release of pressure medium energy and damping of load peaks. The actual hydraulic circuit for power conversion/transmission is hydraulically separated from the hydraulic accumulator, however. It is possible by use of the storage unit to regulate the pressure (torque) and the power of the drive train at the same time.

However, the additionally provided hydraulic machine of the storage unit in particular reduces the overall efficiency of the transmission.

A transmission with a hydrostatic circuit with good damping properties of torque surges, high flexibility of the circuitry, good redundancy against failure and a satisfactory level of efficiency is shown in the publication "Efficient hydrostatic heavy-duty drive train in renewable energy branch," Schmitz, J., Vatheuer, N., Murrenhoff, H., l2 Scandinavian International Conference and Fluid Power (SICPF), 2011. This solution facilitates exclusively a pressure and thus torque regulation on hydraulic pumps on the input side of the drive train, which leads, however, to a fluctuating output power and in connection with this to loading of downstream components and of the electrical grid.

In contrast, the object of the invention is to create a hydrostatic transmission for the conversion of a multichromatic input power into a regulated output power with improved efficiency, and a method for such regulation.

The first object is achieved by a hydrostatic transmission with the features of claim 1, the second by a method with the features of claim 13.

Advantageous developments of the drive train are described in claims 2 to 12, and those of the method in claims 13 to 15.

A hydrostatic transmission, in particular for use in a wind energy converter, a wind turbine, a marine current power station (river, sea) , a ship, a mobile application, a press or a roller, can be driven by an input power that is based in particular on a natural force and is in particular dynamic. The input power results in this case in particular from a mechanical input load, such as a torque, a force, a velocity or a rotational speed/stroke rate, for example, and is supplied to the hydrostatic transmission preferably via an output of a primary energy converter. The output can be a rotor shaft of a wind or wave energy converter, for example, a rod of a linear oscillating wave energy converter, a rotor shaft of a tidal current plant, a rotor shaft of a wind turbine or similar. The transmission has at least two hydrostatic actuators, in particular hydraulic motors, on the input side, to which the input power can be branched, in particular is branched. In addition, it has at least two hydrostatic actuators, in particular hydraulic machines, on the output side, the powers of which can be summed, in particular are summed, to an output power of the transmission. In this case a first actuator on the input side can be connected, in particular is connected, fluidicaiiy to a first actuator on the output side via a first working line of the transmission to form a first hydraulic circuit and a second actuator on the input side can be connected, in particular is connected, fluidically to a second actuator on the output side via a second working iine of the transmission to form a second hydraulic circuit. The actuators on the input side are preferabiy formed as pumps, the actuators on the output side as motors. In particular, the actuators can additionally be operated in the other operating mode motor or pump in each case. The transmission also has a control unit, via which the output power can be regulated.

According to the invention, the transmission has a loadable capacity that can be connected, in particular is connected fluidically, at least to the first working line and via which a working pressure can be imposed on the first working line. Filling/emptying of the capacity can preferably be controlled via the control unit. The capacity is to be understood as a chamber for receiving and emitting pressure medium to the working line connected to it. The capacity can be gas-, spring-or mass-loaded.

The power of the hydrostatic actuators is proportional to the product of their pressure medium volume flow and a pressure difference from their working pressure and low pressure. The capacity according to the invention causes the working pressure of the first actuator on the output side to be subjected only to slight to negligible varIations. The working pressure is consequently basically imposed. Due to this, the power of the first actuator on the output side is substantially adjustable via a change in the pressure medium volume flow of the first hydrostatic circuit without this inducing relevant pressure variations in the first working line, as would be the result without the capacity according to the invention. The capacity represents a simple, cost-effective solution in technical device terms for enabling the power regulation of the first circuit. The change in the pressure medium volume flow can be made by a change in a rotational speed or a displacement volume.

Furthermore, the capacity offers the possibility of intercepting load peaks on the input side by the take-up of pressure medium and of bridging performance losses or at least of damping this accordingly. Units, in particular electrical units, coupled to the transmission on the output side are thereby protected well and cost-effectively against overload.

This also has the conseguence that these units can be rated and operated more securely and in particular to tighter operating conditions. The capacity can be filled and emptied not least via the first hydraulic circuit that is present in any case, so that compared with the teaching of printed publication IDE 10 2011 016 592 the efficiency of the transmission is not reduced by an additional hydraulic machine.

In a particularly preferred development, the first hydrostatic circuit, in particular the first actuator on the output side, can be power-regulated, in particular is power-regulated via the control unit, largely decoupled from the working pressure of the first working line. The flatter a load characteristic of the capacity here -which is the case in particular the greater the volume selected -the narrower the limits In which the variations in working pressure are damped and/or limited via it and the smaller the effect cf the change in pressure medium volume flow on the working pressure. The degree of decoupling thus increases as the load characteristic becomes flatter or the volume increases.

A volume of the capacity is preferably rated so that variations in the working pressure of the first working line are limited to a required or tolerated interval with a view to the power regulation.

In particular, if the input power is based on a natural force, for example wind power or wave power, it normally is has an unforeseeable, multichromatic progression with uneven amplitude and/cr freguency. Without the knowledge of the input power available, however, it is scarcely possible to provide a sensible target value for the output power to be regulated. In a preferred development, the transmission therefore has at least one means for acquiring at least one input load or process variable of the transmission and/or its environment on the input side that influences the input power.

Additionally the control unit is preferably configured in such a way that the input power from the input load or loads and/or from the process variable or variables can be determined via it.

A preferred measure of the in particular multichromatic input power available for conversion into the output power is its temporally moving average. Other definitions are naturally also possible. In a preferred development, the control unit is therefore configured in such a way that the moving average of the input power can be determined from its temporal progression.

In a further preferred development, a target value of the output power can be determined via the control unit as a function of the input power, in particular as a function of its moving average, and of an efficiency rating of the transmission. The efficiency rating is preferably filed for this purpose in the form of a characteristic diagram or a function in the control unit. The target value thils determined is then available for the regulation of the output power. The target value can have a constant time value or a variable time curve. It can be smaller than or equal to a maximum possible target value resulting from the input power or its average and the efficiency rating.

In a preferred development, the power of the second actuator on the output side can be regulated via the control unit to a predetermined, in particular substantially constant, offset to the input power, wherein it is preferred that the second hydrostatic circuit can be regulated in pressure via the control unit, in that the working pressure in the second working line can be

regulated by it by specification of the offset.

It is especially preferred if the predetermined offset is fixed in time, at least in sections. Then the power of the second actuator on the output side, its working pressure and the input power have substantially the same time curves. The last-named working pressure or a pressure that can be derived from this is also present at the second actuator on the input side due to the fluidic connection via the second working line. In the case of an oscillating construction of the second actuator on the input side, this pressure gives rise to a force counteracting the input power, and in the case of a rotating construction, to a torgue counteracting the input power. The varying input power is then counteracted by a likewise varying force or a likewise varying torgue via the fixed-time offset, due to which an oscillatory input stroke rate or rotational input speed can be kept substantially constant.

To regulate the output power, the control unit is configured in a preferred development in such a way that the power of the first actuator on the output side can be regulated by it to a difference from the target value of the output power and the power of the second actuator on the output side. In this way the output power can be regulated to be arbitrarily "smooth" or constant, in order to load an in particular electrical grid as little as possible and to meet its quality criteria.

To optimise a power yield in particular or as a technical regulation measure or for purposes of maintaining the primary energy converter (the rotor, for example) , in a further development of the transmission its input can be driven, in that pressure medium energy stored in the capacity is used for this. The load on the grid is relieved by this. Another advantage is that the input can then also be driven via the capacity if a voltage loss occurs in the (electrical) grid.

In a preferred development, subsystems such as a pitch adjustment of the rotor, for example, can be supplied via the capacity according to the invention. This can be achieved directly with the hydraulic energy of the capacity or indirectly, following a conversion to electrical energy.

In general, the power regulation facilitated via the capacity, which regulation leads to smoothing of the output power, has a positive effect on the efficiency and the service life of electrical and power electronics components, which are connected in series to the output of the transmission, in particular to a generator shaft. In addition, the capacity damps pulsations, which also has a positive effect on components affected by these.

In a preferred development, the transmission is configured such that it facilitates a reversal of the power flow from the output to the input. In this way uniform loading is possible of a power-regulated battery or of a power-regulated combustion engine, via which the output of the transmission can be driven, for example, with simultaneous load regulation of a consumer connected to the input (for example a hydraulic motor or hydraulic cylinder) In a particularly preferred development, the power of the first actuator on the output side can be regulated via the control unit via an adjustment of its pressure medium volume flow.

The adjustability of the pressure medium volume flow of the first actuator on the output side that is required for regulation is realised particularly simply in technical device terms in a preferred development if at least one of the first actuators has a displacement volume that is adjustable via the control unit.

In this case a development is particularly preferred in which at least the first actuator on the output side has the displacement volume adjustable via the control unit, so that the first hydrostatic circuit can be subjected to secondary regulation by the adjustment of the displacement volume.

In a particularly preferred development, both actuators on the output side have a displacement volume that is adjustable via the control unit, so that both hydrostatic circuits can be subjected to secondary regulation via the adjustment of the displacement volumes.

It is particularly preferred that the hydraulic circuits can be regulated separately via the control unit, and wherein the power of the first hydraulic circuit can be regulated and the pressure of the second hydraulic circuit can be regulated.

To be able to switch pressure medium volume flows of the two actuators on the input side to just one of the actuators on the output side, the two working lines can be connected fluidically to one another, in particular via a valve device, in particular via a 2/2-directional valve, in a preferred development.

The capacity can be formed simply in technical device terms by a hydraulic accumulator or in a comparatively more complex way in technical device terms by several hydraulic accumulators.

II

In a preferred development, the transmission has a valve device for the fluidic connection or separation of the capacity. The valve device can be configured in such a way that only the first working line, both working lines individually or both working lines at the same time can be or is/are connected via this to one hydraulic accumulator or to the hydraulic accumulators or to a partial quantity of the hydraulic accumulators. Thus to vary the volume of the capacity, for example as a function of power amplitudes to be damped, a suitable number of hydraulic accumulators can be connected or disconnected.

Alternatively or in addition It is thereby possible to provide hydraulic accumulators with different pressure levels, which can then be connected or disconnected as a function of the level of the input power.

The actuators on the input side are preferably mechanically coupled to one another via a first coupling element and/or the actuators on the output side are preferably mechanically coupled to one another via a second coupling element. In the case of actuators formed by way of rotational hydraulic machines, the respective coupling element is preferably a drive shaft, whereby the two linked hydraulic machines have a so-called tandem arrangement.

Alternatively the actuators on the input side are connected parallel to one another and/or the actuators on the output side are connected parallel to one another. In this case the transmission preferably has a branching section on the input side for branching of the input power and/or a summing section on the output side for summation of the powers.

In a preferred development the aotuators on the output side are jointly connected to an electric generator.

Alternatively they can each be connected individually to a generator, so that if the input or output power is low, one of the generators can be switched off and drag losses can be minimised.

According to the invention, a method for regulating the output power of the hydrostatic transmission described previously has at least one step "Imposing the first working pressure in the first working line via the loadable capacity connectable fluidically to at least the first working line".

In a further development, the method has a step "Determining a target value of the output power via the control unit, at least as a function of the input power and an efficiency rating of the transmission".

Io be able to regulate an input rotational speed or stroke rate, in particular to keep it constant, a preferred development of the method has a step "Regulating a torque of the second actuator on the input side by regulating a second working pressure in the second working line via the control unit in such a way that said torque has a predetermined offset to an input torque of the input power".

Regulation of the output power is preferably carried out via a step "Regulation of a power of the first actuator on the output side via the control unit to a difference from the target value of the output power and a power of the second actuator on the output side".

Two embodiments of a hydrostatic transmission according to the invention are explained in greater detail below in four drawings.

Figure 1 shows a hydraulic circuit diagram of a first embodiment of a hydrostatic transmission, Figure 2 shows a time diagram of a torgue on the input side of the hydrostatic transmission according to figure Figure 3 shows a time diagram of powers of the hydrostatic transmission on the input and output side according to figure 1 and 2, and Figure 4 shows a second embodiment of a hydrostatic transmission.

According to figure 1, a hydrostatic transmission 1 has an input shaft 2, which is driven by an input power TRot. The latter is transmitted by a primary energy converter, in this case a wind turbine, to the input shaft 2. According to the relationship Rot = MRQt w, an input torgue MRQt and an input angular velocity Wp.0L, and an input speed nL are present there. Furthermore, the hydrostatic transmission 1 has an output shaft 4, to which a generator 6 is connected, which delivers electrical power e1.

On the input side the hydrostatic transmission 1 has a first and second hydrostatic actuator 8 and 10 respectively. Both are configured as reversible radial-piston machines, which operate preferably in pump mode.

They are therefore described below as radial-piston pumps 8. 10. On the output side, a first hydrostatio actuator 12 and a second hydrostatic actuator 14 are connected to the output shaft 4 and are configured as reversible, adjustable axial-piston machines of swash plate design and operate preferably in motor mode. They are therefore described below as axial-piston motors 8, 10.

The first radial-piston pump 8 is connected fluidically to the first axial-piston motor 12 via a first working line 16, the second radial-piston pump 10 is connected fluidically to the second axial-piston motor 14 via a second working line 18. In normal operation of the hydrostatic transmission 1 the working lines 16, 18 thus connect the high-pressure connections of the radial-piston pumps 8, 10 to those of the axial-piston motors 12, 14. On the low-pressure side, the hydraulic machines 8 and 12 and the hydraulic machines 10 and 14 are fluidically connected respectively via a low-pressure line 20 and 21. The two separate hydraulic circuits (10, 14, 18, 21 and 8, 12, 16, 20) each facilitate operation in all four quadrants. This can then be necessary, for example, if the hydraulic machine 14 has to drive the hydraulic machine 10, if for example the input torque MRQU is smaller than a torque MLR,S of the hydraulic machine 8. The closed circuit construction, as shown for both embodiments, proves the most flexible in this case.

Furthermore, the hydrostatic transmission 1 has a capacity 22 formed as a gas-loaded hydraulic accumulator, which can be connected fluidically via a valve device 24 formed as a 3/3-directional valve individually to the first working line 16 and the second working line 18.

The hydrostatic transmission 1 has a control unit 26 for regulating the output power P..j. To this end it processes at least the actual value of the electrical output power eJ its target value e1,sf a target torque Mp.3L, of the input shaft 2 that counteracts the input torque a working pressure of the first working line 16 and a working pressure P1) of the second working line 18. For the regulation the control unit 26 acts on swash angles or absorption volumes of the two axial-piston machines 12, 14 via their respective setting device 28 and 30. In addition, the 3/3-directional valve 24 and a 2/2-directional valve 32, via which the two working lines 16, 18 can be connected fluidically, can be actuated via the control unit 26.

According to figure 2, the torque MOL, the torque MtR,e of the first radial-piston pump 8 and a torgue M,10 of the second radial-piston pump 10 act on the input shaft 2. The speed of the input shaft 2 is set as a function of the loads (torgues) acting on the rotor shaft. Speed regulation is conceivable at this point. The torques and MDR,1o would then be regulated via the control unit 26 in such a way that the desired, in particular constant, speed np0L results.

The torques MLR,s and MD,1o result from the respective pressure difference of the working line 16 and 18 to the low-pressure line 20 and 21 and the respective conveying volume of the radial-piston pump 8 and 10. If both conveying volumes and the speed of the common input shaft 2 are constant, the torques ML,B and Mp10 are basically a function of the working pressures LR and at a given low pressure END.

On the output side, the eleotrical power is oomposed of the powers 2LR and 2D of the two axial-piston mocors 12, 14, disregarding the efficiency of the generator 6. The power P or P is proportional in each case here to the pressure difference between or P2 and the low pressure 2ND as well as to the pressure medium volume flow of the axial-piston motor 12 or 14. The respective pressure medium volume flow is a function in turn of the swash angle of the axial-piston motor 12 or 14.

According to the figures 2 and 3, a method for regulating the electrical output power Per that is valid according to the invention for both embodiments is filed in the control unit 26 for execution. A target value of the electrical output power Pa-, to be generated is stipulated for this via the regulation. This cannot normally be selected arbitrarily, but must naturally be geared to the offered input power F50-, which varies according to figure 3. A moving average PRoj of the input power F is therefore determined in a first step via the control unit 26. The control unit 26 then determines a possible target value of the electrical output power e1,3 via an efficiency rm of the hydrostatic transmission 1; 101 that is stored in the control unit 26, in particular in the form of a characteristic diagram or a function.

The control unit 26 first has the job of keeping the speed at the input shaft 2 constant, as already described, which is achieved via regulation of the torque eguilibrium described above. Regulation of the torgue M,10 is carried out here via regulation of the pressure F95 in that the swash angle of the second axial-piston pump 14 is adjusted by the control unit 26 in such a way that the torque MDR,10 has a predetermined offset to the input torque MliQt. The control unit 26 thus readjusts a temporal profile of the last-named torque MR0L.

The pressure in the first working line, on the other hand, can scarcely be influenced by the first axial-piston motor 12 on the output side, since, as described above, it is imprinted by the hydraulic accumulator 22 in the context of the latter's positional curve/characteristic.

Accordingly the torque MLP.,, which is dependent solely on the pressure P and on the low pressure END, which is assumed to be constant, is basically constant according to figure 2. The pressure fUR, is set via the control unit 26 by filling or emptying of the hydraulic accumulator 22 with pressure medium via the first working line 16 or with gas via a gas connection (not shown) in such a way that the torgue MLR,g results with the magnitude of the offset and the torque equilibrium already discussed is set at the input shaft 2.

On the generator side, the electrical power P is composed at a constant generator speed of the products of the two pressure differences and swash-angle-dependent pressure medium volume flows of the axial-piston motors 12, 14 on the output side, according to figure 3. The target value e1,s of the output power e1 is stipulated by the control unit 26 as previously described. In figure 3 it is clearly recognisable that the power PD4 of the second axial-piston motor 14 on the output side follows the varying input power P with the offset D adjusted by the control unit. It is also clearly recognisable that to achieve the target value 2e1,s, the power 2L,12 of the first axial-piston motor 12 on the output side is adjusted by the control unit 26 in such a way that it corresponds to the difference d from the powers and This is aohieved simply in prooess engineering terms via the adjustment of the swash angle and thus of the absorption volume of the first axial-piston motor 12 on the output side via the control unit 26. Since this takes place at imposed, thus quasi constant, working pressure LR, this corresponds to a power regulation of the axial-piston motor 12, which is realised primarily via the modulation of the pressure medium volume flow via the swash angle adjustment.

The charging state of the hydraulic accumulator 22 can be used to adapt the output power P1 to be generated. This can be done, for example, by limitation via a two-point hysteresis at the hydraulic accumulator 22 or by an overlay of a constant portion as a function of the charging state.

The second embodiment of a hydrostatic transmission 101 according to figure 4 corresponds in device terms in large parts to the preceding first embodiment according to figures 1 to 3. Parts or components with the same function and execution in relation to the first embodiment are therefore provided with the same reference signs. The hydrostatic transmission 101 also has the control unit 26 according to the preceding description, but this is not shown for reasons of clarity. The regulation process as described for the first embodiment also applies to the hydrostatic transmission 101. Only differences from the first embodiment are looked at in detail below, therefore.

According to figure 4, the hydraulic circuit formed by the hydraulic machines 10 and 14, working line 18 and low-pressure line 21 has a hydraulic periphery. This has a feed pump 136 driven by a prime mover 134, which pump can aspirate pressure medium from a tank T and convey it into a low-pressure hydraulic accumulator 138. The latter can be connected fluidically via non-return valves 140 on the one hand to the second working line 18 and on the other hand to the low-pressure line 20, so that a leak can be compensated for in this hydraulic circuit. In addition, the second working line 18 and the low-pressure line 20 can be connected to the tank T via a 4/3-directional valve 142, via which a steady leakage flow is facilitated to the tank T from that line of the lines 18, 20 that is carrying high pressure at the moment. The leakage flow is routed here via a low-pressure pressure limiting valve 144, then taken through a filter 146 and conseguently recooled in a cooler 148 before entering the tank T. This hydraulic circuit is protected from overheating in this way.

The hydraulic circuit in this embodiment also, which circuit is formed by the hydraulic machines 8 and 12, the working line 16 and the low-pressure line 21, has a periphery such as described above. It is not represented, however, for reasons of clarity.

Furthermore, the hydrostatic transmission 101 has a 2/2-directional valve 150, which can be connected fluidically to the first working line 16 via a non-return valve 152 and to the low-pressure line 20 via a non-return valve 154. The non-return valves 152, 154 are arranged fluidically in parallel and prevent the overflow of pressure medium from the first working line 16 to the low-pressure line 20 or vice versa. In the embodiment shown, pressure medium to act on an adjustment mechanism of a pitch drive of the wind turbine rotor (not shown) can be removed from the first working line 16 or the low-pressure line 20, depending on which of the two has the higher pressure, via the 2/2-directional valve.

Deviating from the first embodiment, it is clearly recognisable according to figure 4 that the first radial-io piston pump 8 on the input side is configured smaller than the second radial-piston pump 10 on the input side. The hydrostatic transmission 101 can be operated in three power stages in this way, in that either both axial-piston motors 12, 14 on the output side or only one of the two have/has an absorption volume different from zero.

Deviating from the embodiments shown, it is possible, instead of the tandem arrangement shown of the radial-piston pumps 8, 10 and the axial-piston motors 12, 14 on the input shaft 2 and on the output shaft 4 respectively, to realise a parallel arrangement. In the case of the axial-piston motors 12, 14, this means that each is connected via its own drive shaft to a specifically assigned generator. The electrical output power e1 is adjustable or level-adjustable in this way and drag losses, for example if one of the axial-piston motors 12, 14 is also running in idle mode with an absorption volume of zero, can be reduced by simply turning off the correspondingly coupled generator. In the case of the radial-piston pumps on the input side, the parallel arrangement can be realised in such a way that a dedicated primary converter, in this case a wind turbine, is coupled via its own shaft to each of the radial-piston pumps 8, 10.

For both embodiments it is the case that the swash angle of the first axial-piston motor 12 arranged in the power-regulated circuit is set such that its torque MLR,12 and the corresponding power til,2 yields the difference from the powers 9e1, -Ds,i4* In the case of large loads, This can mean that the hydraulic accumulator 22 is charged in addition via the axial-piston motor 12. Vice versa, if it is desired to drive the input shaft 2, and thus the rotor (not shown) , the hydraulic accumulator 22 can be separated from at least the first working line 16 and thus be used to provide necessary drive power.

What is disclosed is a hydrostatic transmission with several pumps and several motors, wherein the pumps and the motors are arranged respectively in tandem or parallel arrangement. The pumps and motors can have a fixed or adjustable absorption volume depending on whether the power is summed mechanically or electrically. Each pump is connected to a motor in pairs via a working line. Here the working line of at least one pair is or can be connected to a loaded hydraulic accumulator. The power of the other hydraulic circuit separated from the hydraulic accumulator is regulated via a control unit of the transmission by means of a regulation of the working pressure in the assigned working line. Since it is possible via the hydraulic accumulator to keep the power of the hydraulic circuit connected to the hydraulic accumulator virtually independent of variations in the working pressure, the power of the hydraulic circuit connected to the hydraulic accumulator can be adjusted very simply by modulation of its pressure medium volume flow and thus the output power to be delivered can be smoothed at a very early stage in the power conversion process.

Reference symbol list 1; 101 Hydrostatic transmission 2 Input shaft 4 Output shaft 6 Generator 8 First hydrostatic actuator on the input side Second hydrostatic actuator on the input side 12 First hydrostatic actuator on the output side 14 Second hydrostatic actuator on the output side 16 First working line 18 Second working line 20, 21 Low-pressure line 22 Hydraulic accumulator is 24 3/3-directional valve 26 Control unit 28, 30 Adjusting device 32 2/2-directional valve 134 Prime mover 136 Feed pump 138 Low-pressure store Non-return valve 142 4/3-directional valve 144 Pressure limiting valve 146 Filter 148 Cooler 2/2-directional valve 152, 154 Non-return valve MliOt Torque, rotor Target value, torque, input shaft Torque power regulation MD,J Torque pressure regulation PpQt Input power eJ Output power * Target value, output power Power of first actuator on output side PD Power of second actuator on output side First working pressure Second working pressure Efficiency, transmission P Offset d Difference