Hydrostatic transmission and methods for braking with it

Abstract

Hydrostatic transmission for a drive system, comprising a drive shaft (4) of a primary unit (1) that can be coupled to an internal combustion engine of the drive system and a secondary unit (2) that can be coupled to an output of the drive system, wherein the two units (1, 2) are fluidically connected to each other via two working lines (10) of a closed circuit, and wherein the primary unit (1) has an adjustable swivel angle (angle_pump) and an adjustable displacement volume (Vg_pump) that can be controlled by an electrical control unit (8) when the hydrostatic transmission is braked, characterized by a controller (38;138), from which a correction value (Vg_pump_add_incr) for the swivel angle (angle_pump) or the displacement volume (Vg_pump) can be output, wherein the correction value (Vg_pump_add_incr) can be added to a swivel angle (angle_pump_brake) or displacement volume (Vg_pump-brake) pre-controlled by the control unit (8), wherein an input variable of the controller (38; 138) is a speed difference (Δn_eng) between an actual speed (n_eng_act) of the internal combustion engine or a quantity derived therefrom (n_pump_act) and a maximum speed (n_eng_max) of the internal combustion engine or a quantity derived therefrom (n_pump_max).;

Description

[0001] The invention relates to a hydrostatic transmission with which hydrostatic braking is possible, according to the preamble of claim 1, and a method for braking with such a hydrostatic transmission.

[0002] Hydrostatic transmissions for mobile machinery are known from the prior art, in which a hydrostatic pump (primary unit) and a hydrostatic motor (secondary unit) are fluidically connected to each other via a closed circuit. An internal combustion engine, e.g., a diesel engine of the mobile machinery, is coupled to the primary unit, and an output, e.g., an axle or a wheel of the mobile machinery, is coupled to the secondary unit in a rotationally fixed manner. Thus, the mobile machinery has a drive system that incorporates a hydrostatic transmission.

[0003] Such a hydrostatic transmission, which can also be used for braking, is disclosed in EP 1 960 699 B1. In this system, the power flows in the opposite direction to traction operation, from the output shaft via the secondary unit (acting as a pump) and the primary unit (acting as a motor) to the internal combustion engine, which is then driven in a passive towing mode. The high-pressure working line of the closed circuit is protected by a pressure relief valve, which also allows a portion of the braking power to be dissipated during braking.

[0004] The problem with such braking is that the internal combustion engine must only be driven at a maximum speed to avoid being damaged. Therefore, the primary unit, designed as an adjustable axial piston machine and functioning as the motor, is permanently set to a pivot angle during braking that generates a torque the internal combustion engine can still handle without over-revving. This is done taking into account the pressure in the working line, determined by the setting of the pressure relief valve.

[0005] A disadvantage of prior art hydrostatic transmissions is that the combustion engine is not brought up to its maximum speed during braking. Therefore, a significant portion of the braking power must be dissipated via the pressure relief valve of the affected high-pressure line, which is detrimental.

[0006] Documents DE 10 2014 211 393 A1 and DE 10 2014 211 394 A1 also disclose a hydrostatic transmission that can also be used for braking, thereby protecting the internal combustion engine from overspeeding. A first portion of the braking power is transferred to the internal combustion engine via the primary unit, while a second portion is converted into heat via the pressure relief valve of the affected high-pressure line. This process accepts an initial engine speed slightly above the permissible speed limit.

[0007] Documents DE 10 2014 206 123 A1 and DE 10 2012 221 944 A1 also disclose a hydrostatic drive system and its method.

[0008] In contrast, the invention is based on the objective of creating a hydrostatic transmission and a method in which the swivel angle and thus the stroke volume (in braking operation, this is a displacement volume) of the primary unit is optimally controlled or regulated, so that the speed of the internal combustion engine is brought up to its maximum speed as quickly as possible and maintained as close as possible to this maximum speed. The internal combustion engine should never exceed its maximum speed.

[0009] This problem is solved with respect to the hydrostatic transmission by the combination of features of claim 1 and with respect to the method by the combination of features of claim 11.

[0010] The claimed hydrostatic transmission is intended for a traction drive system comprising an internal combustion engine, e.g., a diesel engine, and an output, e.g., a wheel or axle. The hydrostatic transmission has a primary unit's drive shaft, which can be coupled to the traction drive's internal combustion engine and functions as a pump during traction operation, and a secondary unit, or several secondary units connected in parallel hydraulically, which can be coupled to the traction drive's output and function as motor(s) during traction operation. Both units are fluidically connected to each other via two working lines in a closed circuit. The hydrostatic transmission also includes an electrical control unit for controlling or regulating braking, whereby a braking torque from the secondary unit, acting as a pump, is supported at its drive shaft by the primary unit, acting as a motor.With regard to the entire drive system, the braking torque of the output is thus supported by the two units at the internal combustion engine. According to the invention, a pre-controlled swivel angle or a pre-controlled displacement volume for the primary unit is specified by the control unit, which is below the maximum speed of the internal combustion engine, so that it is certain that the engine does not over-rev at the beginning of braking. Then, a correction value is calculated and output by a controller, which is added to the pre-controlled swivel angle or the pre-controlled displacement volume. This increases the swivel angle and the displacement volume from a subcritical range for the internal combustion engine in such a way that the internal combustion engine is brought from this subcritical range to its maximum speed, thus maximizing the braking power flowing to the internal combustion engine via the primary unit without causing the internal combustion engine to over-rev.

[0011] With regard to the hydrostatic transmission according to the invention, which does not include the internal combustion engine, the speed of the crankshaft of the internal combustion engine to be limited can be transmitted to the control unit via a signal input or taken directly from the drive shaft of the primary unit if the crankshaft and the drive shaft are one piece, or can also be calculated from a speed of the drive shaft of the primary unit if a mechanical transmission stage is provided between the crankshaft and the drive shaft.

[0012] Further advantageous embodiments of the invention are described in the dependent patent claims.

[0013] According to the invention, the controller has as its input a speed difference between, on the one hand, an actual speed of the internal combustion engine or a quantity derived therefrom, in particular the actual speed of the primary unit, and, on the other hand, a maximum speed of the internal combustion engine or a quantity derived therefrom.

[0014] Preferably, the controller is a P or PI controller. Preferably, the control unit includes the controller.

[0015] In order to be able to use the hydrostatic transmission according to the invention in various cases in which the internal combustion engine could at least temporarily overspeed according to the prior art, the control unit can be further developed in such a way that braking is initiated either via a control element, e.g. brake pedal, in particular via an input for a signal line of the control element, or on the basis of automatic monitoring of the travel speed of the mobile working machine, in particular an input for a signal from a speedometer, or a quantity derived therefrom, such as the speed of the secondary unit, or on the basis of automatic monitoring of the speed of the internal combustion engine or the quantity derived therefrom.

[0016] Preferably, the secondary unit is also designed with an adjustable stroke volume. This can be adjusted, for example, depending on the control element.

[0017] To enable high-performance braking, it is preferred to have a pressure relief valve on each of the two working lines. A first portion of the braking force can be dissipated via the respective pressure relief valve, while a second portion can be dissipated via the primary unit and the internal combustion engine. The achievable braking force is particularly high when the first portion is greater than the second.

[0018] During high-performance braking, if the flow rate through the primary unit increases, the flow rate through the affected pressure relief valve decreases. This can cause the pressure in the high-pressure working line to drop. To minimize this pressure reduction or to keep the pressure nearly constant, pressure relief valves with a flat characteristic curve regarding their pressure differential as a function of the flow rate are preferred.

[0019] In a preferred embodiment of the hydrostatic transmission according to the invention, the swivel angle and the stroke volume of the primary unit are adjustable on both sides of a zero position. This allows the affected drive system to be used in both directions of travel of the mobile work machine in towing operation, while maintaining the same direction of rotation of the combustion engine, and to be braked accordingly in both directions according to the invention.

[0020] In a first embodiment of the hydrostatic transmission according to the invention, a load-sensing axial piston machine is used as the primary unit. During pump operation, forces act on the drive mechanism of this machine in the direction of a reduction in its swivel angle. These forces depend on a pressure difference between the two working lines, the rotational speed of the drive shaft, and the swivel angle of the primary unit. A characteristic map of the primary unit is stored in the control unit, in which a corresponding swivel angle or stroke volume is assigned to different pressure differences and rotational speeds. Because the characteristic map is taken into account, no feedback of the swivel angle of the primary unit is required. Thus, instead of position-controlled primary units, such as those found in agricultural machinery, load-sensing primary units, which are usually somewhat less expensive, are used.

[0021] It is particularly preferred if the control unit can, via an electric actuating pressure valve and an actuating cylinder of an adjusting device of the primary unit, control a control pressure in the direction of increasing its swivel angle and thus its stroke volume. The characteristic map specifies the dependence of this control pressure or a control pressure deviation on the pressure difference between the two working lines, the rotational speed of the drive shaft, and on the swivel angle or stroke volume of the primary unit.

[0022] In a device-technically simple further development, the actuating cylinder is single-acting against a spring, so that the actuating pressure only acts in the direction of increasing the swivel angle and the stroke volume, while the spring acts in the direction of decreasing the swivel angle and the stroke volume.

[0023] In a double-acting actuator cylinder, whose two actuating pressure chambers can be pressurized with the actuating pressure or the actuating pressure deviation via the common or a separate electrically adjustable actuating pressure valve, the affected drive system can be used in both directions of travel of the mobile working machine in towing mode while maintaining the same direction of rotation of the internal combustion engine, and can be braked accordingly in both directions according to the invention. A double-sided spring arrangement is provided, which acts in the direction of a central position in which the pivot angle and the stroke volume are zero.

[0024] The pressure differences of the two working lines and the rotational speeds of the drive shaft for the swivel angles and stroke volumes on both sides of the zero position are then stored accordingly in the characteristic map.

[0025] Preferably, a safety correction value for the displacement volume or the swashplate angle is assigned to or superimposed on the characteristic map, which reduces the swashplate angle or displacement volume, or which is subtracted from the swashplate angle or displacement volume. In particular, the safety correction value can be included in the characteristic map. This serves to compensate for inaccuracies in the characteristic map, especially due to variations (manufacturing inaccuracies) of the primary unit. Because of the safety correction value, the pre-controlled swashplate angle or the pre-controlled displacement volume is so small that the actual speed of the internal combustion engine always remains below its maximum speed, so that the internal combustion engine does not over-rev when braking begins. Immediately afterwards, the controller according to the invention rapidly and accurately approximates the actual speed to the maximum speed of the internal combustion engine.

[0026] In the first embodiment of the hydrostatic transmission according to the invention with the load-sensing primary unit, for example an electrically directly controlled adjusting device, in which a control pressure in an actuating cylinder is specified by an electrically adjustable pressure reducing valve (EV adjustment with a single pressure reducing valve and a directional control valve or ET adjustment with two electrically adjustable pressure reducing valves), or a hydraulic speed-dependent adjusting device, in which a control pressure in an actuating cylinder is specified by a speed-dependent adjustable control valve (DA adjustment), can be provided.

[0027] In a second embodiment of the hydrostatic transmission according to the invention, the swivel angle of the primary unit is adjustable via an adjusting device that provides feedback of the swivel angle. The adjusting device can be an electroproportional (EP) adjusting device in which the stroke volume is proportional to a current flowing through a proportional electromagnet that actuates a control valve.

[0028] The inventive method for controlling or regulating a braking system with a previously described hydrostatic transmission comprises the following steps: - Swiveling the primary unit back to a small swivel angle or a small stroke volume, e.g. to about 10% of the maximum swivel angle or maximum stroke volume, - Pre-controlling the swivel angle or the stroke volume, and - Control of the swivel angle or stroke volume, whereby a correction value is added to a pre-controlled swivel angle or stroke volume.

[0029] The first two steps can be performed in the order mentioned, thus preventing the combustion engine from over-revving with the highest possible reliability. Alternatively, the first two steps can be performed simultaneously, saving time and allowing braking to be initiated quickly.

[0030] In a particularly preferred embodiment of the method, the control is carried out by determining a corresponding correction value as a function of a speed difference between, on the one hand, an actual speed of the internal combustion engine or a quantity derived therefrom, in particular the actual speed of the primary unit, and, on the other hand, a maximum speed of the internal combustion engine or a quantity derived therefrom.

[0031] In a preferred embodiment of the inventive method, braking is initiated in advance by a driver request or due to an exceedance of the rotational speed of the internal combustion engine or a quantity derived therefrom (actual rotational speed exceeds a maximum permissible rotational speed) or due to an exceedance of a driving speed or a quantity derived therefrom, in particular the rotational speed of the secondary unit (driving speed exceeds a desired or a maximum permissible driving speed).

[0032] In the braking system according to the invention with the hydrostatic transmission, the pre-control of the swivel angle or the stroke volume can be carried out on the basis of the characteristic map according to the first embodiment.

[0033] Two embodiments of the hydrostatic transmission and drive according to the invention are shown in the drawings. The invention will now be explained in more detail with reference to the figures in these drawings.

[0034] They show Fig. 1 a circuit diagram of the hydrostatic transmission according to the invention in a first embodiment, Fig. 2 a schematic overview of the feedforward control with inventive control of the stroke volume of the primary unit during braking according to a second embodiment, Fig. 3 a diagram of the stroke volumes of the primary unit and the secondary unit of the hydrostatic transmission during braking according to both embodiments, Fig. 4 a characteristic map of the primary unit of the hydrostatic transmission according to the first embodiment, Fig.5 a schematic overview of the map-based feedforward control with inventive control of the stroke volume during braking according to the first embodiment and Fig. 6 Two diagrams of the map-based feedforward control with control according to the invention during braking according to the first embodiment.

[0035] Fig. Figure 1 shows a circuit diagram of the hydrostatic transmission according to the invention. It comprises a primary unit 1 and a secondary unit 2, both of which are designed with adjustable displacement. The primary unit 1 is an axial piston machine to whose drive shaft 4 a crankshaft of a diesel engine (not shown) is rotationally fixed. The rotational speed of the drive shaft 4 is monitored via a speed sensor 6 and an electrical control unit 8. This indirectly monitors the rotational speed of the crankshaft of the diesel engine as well.

[0036] The primary unit 1 is fluidically connected to the secondary unit 2 via a closed hydraulic circuit comprising two working lines 10. A (not shown) output shaft is rotationally fixed to a drive shaft 12 of the secondary unit 2. The output shaft is, for example, a differential gear of a driven axle of the mobile working machine.

[0037] The hydrostatic transmission according to the invention Fig. Thus, unit 1, together with the diesel engine and the output shaft, forms a drive system for a mobile work machine. During operation, the drive shaft 4 of primary unit 1 serves as the drive shaft and primary unit 1 functions as a pump, while secondary unit 2 operates as the engine, and the drive shaft 12 of secondary unit 2 is an output shaft.

[0038] In braking mode of the hydrostatic transmission according to the invention, the output is supported via the drive shaft 12 and via the secondary unit 2, which acts as a pump, and via one of the two working lines 10, and via the primary unit 1, which acts as a motor, and via the drive shaft 4 of the primary unit 1 on the diesel engine, which is then dragged along and, via its friction and acceleration forces of the pistons, dissipates at least part of the braking energy of the mobile working machine.

[0039] Each working line 10 is equipped with a pressure relief valve 14, through which the respective working line 10 can be relieved of pressure to a feed line 16. This feed line is filled with feed fluid from a tank T by a feed pump 18, which is rotationally fixed to the drive shaft 4 of the primary unit 1. Furthermore, the feed line 16 can be relieved of pressure to a tank T via a pressure relief valve 20.

[0040] The feed line 16 is connected to the two working lines 10 via a spring-loaded check valve 22, so that the low-pressure working line 10 can be supplied with supplementary pressure medium from the feed line 10 if necessary. For this purpose, the opening directions of the two check valves 22 are directed from the feed line 16 towards the respective working line 10.

[0041] The adjustment of the stroke volume of the primary unit 1 is effected by an adjusting device 24. This has a double-acting actuating cylinder 26, whose two pressure chambers act against each other on an actuating piston, which is coupled to a swashplate of the axial piston machine 1.

[0042] In the first embodiment, each of the two pressure chambers can be filled with pressure medium from the feed line 16 via a separate actuating pressure valve 28. Both actuating pressure valves 28 are electrically adjusted by the control unit 8. Furthermore, the adjusting device 24 has a spring arrangement (not shown) by which the piston of the actuating cylinder 26 and the swashplate of the primary unit 1 are biased to a central position. From there, the primary unit 1 can be adjusted in both directions.

[0043] Since the primary unit 1 enables four-quadrant operation in both embodiments of the hydrostatic transmission according to the invention, it is possible to drive forwards and brake forwards, and to drive backwards and brake backwards.

[0044] In both embodiments, as already mentioned, the secondary unit 2 is also adjustable. This is achieved by an adjustment device 30, which includes a valve with an electric actuator that is also controlled by the control unit 8. The adjustment device 30 has a feedback spring 32, enabling control of the swivel angle of the secondary unit 2.

[0045] Fig.Figure 2 shows a schematic overview of the feedforward control with control of the primary unit's displacement volume during braking according to the second embodiment. A speed difference Δn_eng between, on the one hand, the actual speed n_eng_act of the internal combustion engine or the proportional speed of the primary unit n_pump_act, and, on the other hand, a maximum speed n_eng_max of the internal combustion engine or the proportional speed of the primary unit n_pump_max, is an input variable of a PI controller 138. This controller outputs a correction value Vg_pump_add_incr for the displacement volume Vg_pump. More precisely, the correction value Vg_pump_add_incr is added to the feedforward displacement volume Vg_pump_brake, resulting in a control signal for the adjustment device of the primary unit. Thus, according to the invention, the diesel engine is brought from a subcritical range to its maximum speed n_eng_max without exceeding it.

[0046] The adjustment device for the swivel angle of the primary unit according to the second embodiment features feedback of the swivel angle and can be an electroproportional (EP) adjustment device. The adjustment device can be similar to the adjustment device of the secondary unit of the first embodiment. Fig. 1 corresponds.

[0047] Fig. Figure 3 shows a diagram illustrating the time course of the stroke volumes Vg_pump and Vg_mot of the primary unit 1 and the secondary unit 2 of both embodiments. First, a transition from driving mode to braking mode is shown, and then a transition from braking mode back to driving mode. Such braking via the hydrostatic transmission according to the invention can be initiated by: a) a signal transmission from a control element operated by a driver, for example a brake pedal, to the control unit 8, b) a borderline high speed of the diesel engine, which was detected by the speed sensor 6 and the control unit 8, c) a limiting travel speed v_veh of the mobile working machine, which was indirectly determined by a speed sensor 34 of the drive shaft 12 of the secondary unit 2 and transmitted to the control unit 8.

[0048] In both embodiments, braking is controlled by the control unit 8. To achieve this, the swivel angle ang-le_pump, and thus the displacement volume Vg_pump of the primary unit 1, is first reduced to a low value within a short time. This low value can, for example, be approximately 10% of the maximum swivel angle angle_pump_max or the maximum displacement volume Vg_pump_max of the primary unit 1. This initially prevents the diesel engine from exceeding its speed n_eng, as it does not reach the speed n_eng that would theoretically be required for the low swivel angle ang-le_pump of the primary unit within this short time. Consequently, the pressure in the now high-pressure working line 10 rises rapidly, and the corresponding high-pressure valve 14 opens a connection to the feed line 16.

[0049] In both embodiments, the control unit 8 then controls the adjusting device 30 of the secondary unit 2 such that its swivel angle and thus its displacement volume Vg_mot are increased. The swivel angle angle_pump of the primary unit 1 is also slightly increased again (not shown), thereby accelerating the diesel engine and dissipating some of the braking power via this engine.

[0050] The other abbreviations or formula letters in the diagram of Fig. The three have the following meanings: phasein_mot_ms Duration of the secondary unit's entry. phasein_mot_delay_ms Delay in the activation of the secondary unit if it reacts faster than the primary unit. phasein_pump_ms Duration of primary unit activation. phasein_pump_delay_ms Delay in the primary unit's activation if it reacts faster than the secondary unit. phaseout_mot_ms Duration of the secondary unit's ejection. phaseout_mot_delay_ms Delay in the output of the secondary unit if it reacts faster than the primary unit. phaseout_pump_ms Duration of the primary unit's ejection. phaseout_pump_delay_ms Delay in the primary unit's output if it reacts faster than the secondary unit

[0051] To maximize the speed of the diesel engine while not exceeding a permissible maximum speed, the first embodiment uses the control described with reference to the following figures, and the second embodiment uses the control described with reference to Fig. The second explained regulation is used.

[0052] Fig.Figure 4 shows a characteristic map 36 of the primary unit 1 according to the first embodiment, which is designed as a load-sensing unit. The characteristic map 36 represents the influence of the pressure difference Δp between the two working lines 10 and the actual rotational speed n_pump_act and, furthermore, the swivel angle angle_pump of the primary unit 1 on the actuator in the cylinder 24 (see Figure 4). Fig.1) prevailing control pressure p_st. More precisely, the relationship is represented such that the pressure difference Δp of the two working lines 10 is plotted along a first axis, where, for example, the positive pressure differences Δp represent the overrun operation and the negative pressure differences Δp represent the braking according to the invention. On a further axis, the actual rotational speed of the drive shaft 4 n_pump_act of the primary unit 1 is plotted. The different areas of the shown set of areas apply by way of example to three positive and three negative swivel angles angle_pump of the swashplate of the primary unit 1. On the vertical axis, a control pressure deviation p_st_comp is plotted, which is necessary to maintain the swivel angle angle_pump at the respective operating point. In the example mentioned above, the positive pressure values ​​represent overrun operation, while the negative pressure values ​​apply to the braking according to the invention.This means that when initiating braking in relation to the . Fig. As explained in section 1, adjusting device 24 requires a change of the pressure chamber pressurized with actuating pressure medium p_st.

[0053] In the first embodiment, the characteristic map 36 is located in the control unit 8 (see below). Fig. 1) such that, by means of a pressure sensor (not shown) provided on each of the working lines 10, the control unit 8 can control any desired swivel angle angle_pump by adjusting the corresponding control pressure deviation p_st_comp by appropriately actuating the control pressure valves 28. In contrast to prior art hydrostatic transmissions, no feedback of the swivel angle angle_pump of the primary unit 1 is required.

[0054] Due to variations in the manufacturing of the primary units 1 and other factors, such as wear and viscosity of the pressure medium used, small deviations of the various specific primary units 1 from the characteristic map 36 are possible, so that in the first embodiment the map-based control described above is used as feedforward control and additionally according to Fig. 5 is refined. In the middle area, the characteristic map 36 is from Fig.Figure 4 shows how this is used to assign a corresponding correction value p_st_comp to the control pressure p_st. In map 36, a safety correction value is also included to account for the aforementioned possible variations in the behavior of the different primary units 1 and to always set a displacement volume Vg_pump_comp through the device-technically simple feedforward control using map 36, ensuring that the diesel engine is not over-revved and that even in the event of a failure of the functions described below, a high but non-critical braking power is reduced.

[0055] According to the invention, a control circuit is superimposed on the feedforward control, which compares the actual rotational speed n_eng_act of the diesel engine with its maximum permissible rotational speed n_eng_max and generates a further correction value for the control pressure p_st. Alternatively, the rotational speed n_pump_act of the primary unit 1 can also be compared with a converted maximum permissible rotational speed n_pump_max of the primary unit 1, and the further correction value for the control pressure p_st can be generated.

[0056] By means of the feedforward control according to the characteristic map 36 (see Fig. 4 and Fig.5) The primary unit 1 is thus already pivoted close to the ideal value. However, according to the prior art, the primary unit 1 is still positioned at a slightly too large displacement volume Vg_pump (swallowing volume), which would lead to a slightly too high rotational speed n_eng_act of the diesel engine. In the case of the too small pivot angle angle_pump used according to the invention, the braking capacity of the diesel engine would only be partially utilized. While a reliable braking function is already fundamentally possible with the described load-sensitive primary unit 1, the portion of the braking power that flows through the primary unit is further improved.

[0057] The final deviation from the ideal value is ultimately balanced by the superimposed control with the one in Fig. 4 PID controller 38 of the first embodiment shown or with the one in Fig.The PI controller 138 of the second embodiment shown in Figure 2 uses the "last" actual speed n_eng_act of the diesel engine from its maximum permissible speed n_eng_max as an input and outputs an increased displacement volume Vg_pump_add_incr. This determines an optimal displacement volume Vg_pump_brake_corr for braking under the conditions of the specific primary units 1.

[0058] The impact is exemplified in Fig. Figure 6 illustrates this. This results in a consistent load on the diesel engine during braking without it reaching the impermissible speed range.

[0059] A hydrostatic transmission has been revealed that enables braking in which at least one traction motor, acting as a pump, is supported via a closed circuit by an adjustable axial piston pump, also acting as a motor, which in turn can be supported by an internal combustion engine. To prevent over-revving the internal combustion engine, a feedforward control is implemented at a subcritical speed, followed by regulation by a controller that outputs a correction value depending on the deviation of the current speed from the maximum, and therefore ideal, speed of the internal combustion engine. This ensures effective braking at a subcritical engine speed even in the event of a controller failure. Reference symbol list 1 primary unit 2 Secondary unit 4 Drive shaft 6 Speed ​​sensor 8 Control unit 10 Work Management 12 Drive shaft 14 Pressure relief valve 16 Feed line 18 Feed pump 20 Pressure relief valve 22 Check valve 24 Adjustment device 26 actuator cylinders 28 Control pressure valve 30 Adjustment device 32 Feedback spring 34 Speed ​​sensor 36 characteristic map 38 regulators 138 regulators angle_pump Primary unit swivel angle angle_pump_max maximum swivel angle of the primary unit n_eng_act Actual speed of the internal combustion engine n_eng_max maximum permissible speed of the internal combustion engine n_mot_act Actual rotational speed of the secondary unit n_pump_act Actual rotational speed of the primary unit n_pump_max Primary unit speed derived from the maxima- engine speed p_st Primary unit control pressure p_st_comp primary unit control pressure deviation phasein_mot_ms Duration of the secondary unit's entry phasein_mot_delay_ms Secondary unit start delay phasein_pump_ms Duration of primary unit jump-in phasein_pump_delay_ms Primary unit start-up delay phaseout_mot_ms Duration of the secondary unit's jump phaseout_mot_delay_ms Delay of secondary unit exit phaseout_pump_ms Duration of the primary unit's jump phaseout_pump_delay_ms Primary unit exit delay Vg_mot secondary unit displacement Vg_pump Primary unit displacement volume Vg_pump_add_incr increased stroke volume of the primary unit for braking Vg_pump_brake: Pre-controlled stroke volume of the primary unit for braking Vg_pump_brake_corr Target stroke volume of the primary unit for braking Vg_pump_comp Primary unit displacement deviation Vg_pump_max maximum stroke volume of the primary unit v_veh Driving speed Δn_eng Speed ​​difference between the target speed and the actual speed of the internal combustion engine Δp pressure difference between the two working lines T Tank

Claims

[1] Hydrostatic transmission for a drive system, comprising a drive shaft (4) of a primary unit (1) that can be coupled to an internal combustion engine of the drive system and a secondary unit (2) that can be coupled to an output of the drive system, wherein the two units (1, 2) are fluidically connected to each other via two working lines (10) of a closed circuit, and wherein the primary unit (1) has an adjustable swivel angle (angle_pump) and an adjustable displacement volume (Vg_pump) that can be controlled by an electrical control unit (8) when braking the hydrostatic transmission, characterized bya controller (38; 138) from which a correction value (Vg_pump_add_incr) for the swivel angle (angle_pump) or the displacement volume (Vg_pump) can be output, wherein the correction value (Vg_pump_add_incr) can be added to a swivel angle (angle_pump_brake) or displacement volume (Vg_pump-brake) pre-controlled by the control unit (8), wherein an input variable of the controller (38; 138) is a speed difference (Δn_eng) between an actual speed (n_eng_act) of the internal combustion engine or a quantity derived therefrom (n_pump_act) and a maximum speed (n_eng_max) of the internal combustion engine or a quantity derived therefrom (n_pump_max). [2] Hydrostatic transmission according to claim 1, wherein the control unit (8) is designed such that braking can be initiated via a control element or on the basis of automatic monitoring of the travel speed (v_veh) of the mobile working machine or a quantity derived therefrom (n_mot_act) or on the basis of automatic monitoring of the actual speed (n_eng_act) of the internal combustion engine or a quantity derived therefrom (n_pump_act). [3] Hydrostatic transmission according to one of the preceding claims, wherein a pressure relief valve (14) is arranged on each of the two working lines (10), and wherein the braking is a high-performance braking system in which a first part of the braking power can be reduced via one of the pressure relief valves (14), while a second part of the braking power can be reduced via the primary unit (1). [4] Hydrostatic transmission according to claim 3, wherein the pressure limiting valves (14) each have a flat characteristic curve with respect to their pressure difference as a function of their volume flow rate. [5] Hydrostatic transmission according to one of the preceding claims, wherein the swivel angle (angle_pump) and the stroke volume (Vg_pump) of the primary unit (1) are adjustable on both sides of a zero position. [6] Hydrostatic transmission according to one of the preceding claims, wherein the primary unit (1) is a load-sensing axial piston machine, in whose pumping operation forces act in the direction of a reduction of its swivel angle (angle_pump) which depend on a pressure difference (Δp) of the two working lines (10) and a rotational speed (n_pump_act) of the drive shaft (4) and the swivel angle (angle_pump), wherein a characteristic map (36) of the primary unit (1) is stored in the control unit (8) in which a respective swivel angle (angle_pump) is assigned to the pressure difference (Δp) and the rotational speed (n_pump_act). [7] Hydrostatic transmission according to claim 6, wherein the control unit (8) can control a control pressure (p_st) via an electric control pressure valve (28) and a control cylinder (26) of an adjusting device (24) of the primary unit (1), and wherein the control pressure (p_st) acts in the direction of an increase in its swivel angle (angle_pump), and wherein the dependence of the control pressure (p_st) or a control pressure deviation (p_st_comp) on the pressure difference (Δp) of the two working lines (10) and the rotational speed (n_pump_act) of the drive shaft (4) and the swivel angle (angle_pump) or the displacement volume (Vg_pump) is stored in a characteristic map (36). [8] Hydrostatic transmission according to claim 5 and according to claim 6 or 7, wherein the pressure differences (Δp) of the two working lines (10) and the rotational speeds (n_pump_act) of the drive shaft (4) for the swivel angles (angle_pump) or stroke volumes (Vg_pump) on both sides of the zero position are stored in the characteristic map (36). [9] Hydrostatic transmission according to claim 8, wherein the actuating cylinder (26) is double-acting and has two actuating pressure chambers, and wherein the actuating pressure (p_st) in both actuating pressure chambers can be controlled by the control unit (8) via a common or a respective electrically adjustable actuating pressure valve (28). [10] Hydrostatic transmission according to any one of claims 1 to 5, wherein the swivel angle (angle_pump) and the stroke volume (Vg_pump) of the primary unit (1) are adjustable via an adjusting device (24) which has feedback of the swivel angle (angle_pump), e.g. an electroproportional (EP) adjusting device [11] Method for controlling a braking system with a hydrostatic transmission according to one of the preceding claims comprising the steps - Swinging back the primary unit (1), - Pre-control of the swivel angle (angle_pump) or the stroke volume (Vg_pump) and - Control of the swivel angle (angle_pump) or the displacement volume (Vg_pump), in which a correction value (Vg_pump_add_incr) is added to a pre-controlled swivel angle (eng_pump_brake) or a pre-controlled displacement volume (Vg_pump_brake), wherein the control involves determining the correction value (Vg_pump_add_incr) as a function of a speed difference (Δn_eng) between an actual speed (n_eng_act) of the internal combustion engine or a quantity derived therefrom (n_pump_act) and a maximum speed (n_eng_max) of the internal combustion engine or a quantity derived therefrom (n_pump_max). [12] Method according to claim 11 with the preceding step: - Initiation of braking by a driver request or due to exceeding the actual rotational speed (n_eng_act) of the internal combustion engine or a derived quantity (n_pump_act) or due to exceeding a driving speed (v_veh) or a derived quantity (n_mot_act). [13] Method according to one of claims 11 to 12, wherein the pre-control of the swivel angle (angle_pump) or the stroke volume (Vg_pump) is based on a characteristic map (36) of the primary unit (1).

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