HYDROSTATIC DRIVE, METHOD FOR CONTROLLING THE HYDROSTATIC DRIVE

Description

[0001] The invention relates to a hydrostatic drive according to the preamble of claim 1, and a method for controlling it according to claim 11.

[0002] A drive system of this type (see JP S64 3033 A) has two hydraulic machines connected in an open hydraulic circuit, with displacement volume adjustable to zero. The first of the hydraulic machines can be coupled to a drive machine, for example, an internal combustion engine or an electric motor, and the second to an output, for example, a gearbox or an axle. The drive system is, for example, a drive system for a mobile work machine.

[0003] The control is carried out electrically / electronically via a control unit, preferably in such a way that the working pressure of the drive is regulated by adjusting the displacement volume of the first hydraulic machine and the output torque is regulated by adjusting the second hydraulic machine.

[0004] If an electrical fault occurs in the chain of action from the control unit to the respective electro-hydraulic adjustment unit, this can result in damage to the hydraulic machine, the hydraulic circuit, up to and including a failure of the drive and the machine driven by it.

[0005] In contrast, the invention is based on the objective of creating a hydrostatic drive that can be controlled to a safer operating state in the event of a fault. A further objective is to create a method for controlling the drive by which it can be controlled to a safer operating state in the event of a fault.

[0006] The first problem is solved by a hydrostatic drive having the features of claim 1, the second by a method having the features of claim 11.

[0007] Advantageous further developments of the inventions are described in the respective dependent claims.

[0008] A hydrostatic drive, in particular the drive system of a mobile machine, comprises hydraulic units, at least one of which can be coupled to a drive motor and at least one other to an output motor. These units are fluidically connected, particularly in an open hydraulic circuit, via a working line on one side and a pressure medium reservoir, in particular a tank, on the other. Both can operate in pump mode and motor mode, depending on the requirements. In particular, the respective pressure medium connections of the hydraulic units for the low-pressure and working lines are permanent; that is, the hydraulic units are specifically not designed for a change of pressure side. For an efficient, demand-based pressure medium supply, they are each designed with an adjustable displacement volume.To enable switching between motor and pump operation, or a change in the direction of torque, their displacement volume is adjustable from zero on both sides. Specifically, they are designed as axial piston machines with a swashplate design and a pivoting swashplate. For adjusting the displacement volume, each hydraulic machine has at least one electro-hydraulic adjustment unit. In the case of the swashplate design, the respective swashplate is articulated to this unit. The electro-hydraulic adjustment unit is effective in one adjustment direction and, for this purpose, has an electrically and / or electronically controlled valve assembly. This valve assembly allows the actuating pressure chamber of an actuating cylinder of this adjustment unit to be selectively connected to the working line and to a low-pressure line, and, in intermediate positions, to both.In particular, a control unit of the drive is configured such that, via the adjustment of the hydraulic machine coupled to the drive motor, the working pressure in the working line can be regulated, and, via the adjustment of the hydraulic machine coupled to the output, the torque at the output and the displacement volume of the hydraulic machine at the output can be regulated. Specifically, the actual values ​​of the working pressure and the aforementioned displacement volume are recorded and electronically fed back to the control unit. This concept of electronic feedback of the respective controlled variable, instead of hydraulic-mechanical feedback, is known from the applicant's product range as a so-called "Electronified Open Circuit" or "EOC" hydraulic machine and is described, for example, for a hydraulic pump in datasheet RD 30630 / 04.13.A target torque at the output can be transmitted to the control unit, particularly from a higher-level drive strategy, especially a driving strategy. Such a drive is known, in particular, as a secondary-controlled drive in an open hydraulic circuit. According to the invention, a control unit of the drive is configured such that faulty electrohydraulic adjustment units can be identified via it, and a non-faulty hydraulic machine, in particular its electrohydraulic adjustment unit, can be controlled such that the torque of the hydraulic machine with a faulty electrohydraulic adjustment unit tends towards zero.

[0009] In the event of a fault, provided at least one of the electro-hydraulic adjustment units is functioning correctly, the adjustment behavior of the faulty hydraulic unit can be influenced by controlling the functioning unit. This prevents critical conditions, particularly those caused by excessive or uncontrolled torque or speed. The result is a hydrostatic drive that can be controlled to a safer operating state, especially in the event of electrical and / or electronic faults. This prevents damage to the drive, for example, if chips were to enter the hydraulic circuit during a critical condition, which could lead to machine failure. A particular advantage is that this functionality is achieved solely through a feature of the control unit.Conventional drives can thus be adapted with minimal effort by modifying their control unit according to the invention or by simply replacing it, without the need to install additional hydraulic or mechanical components, such as auxiliary valves for safety. The adaptation is carried out solely by the software and / or hardware of the control unit.

[0010] In a further development, at least one of the hydraulic machines has an electrically independent adjustment unit that counteracts the electro-hydraulic adjustment unit in the opposite direction. In the simplest case, the electrically independent adjustment unit is implemented, for example, by means of a twisting of a control disk, from which drive forces result that cause the adjustment in the opposite direction to that of the hydraulic machine.

[0011] In addition to or as an alternative to this twisting, a mechanical adjustment unit, for example a spring, and / or a hydraulic adjustment unit, for example a counter or return cylinder, can be provided as an electrically independent adjustment unit.

[0012] If the respective hydraulic machine is designed as an axial piston machine with a swashplate construction, its swashplate is, in a preferred embodiment, articulated by the electro-hydraulic and the electrically independent adjustment unit. With respect to a pivot axis of the swashplate, this is articulated by the adjustment units, in particular diametrically or in a "boxer arrangement".

[0013] In a further development, the actuating pressure chamber of an actuating cylinder of the electro-independent hydraulic actuating unit, which acts in the opposite direction of adjustment, is permanently connected to the working line.

[0014] In a further development, the adjustment units of the respective hydraulic machine interact in such a way that, in a de-energized state, particularly if the power supply to the electrically and / or electronically controlled valve assembly is unintentionally interrupted / fails, the electro-hydraulic adjustment unit sets the displacement volume above a predetermined working pressure of the working line towards a zero displacement volume and beyond that towards a maximum displacement volume (e.g., -Vgmax). In a de-energized state and below the predetermined working pressure, the other, electrically independent adjustment unit sets the displacement volume towards the opposite maximum displacement volume (e.g., +Vgmax).

[0015] In a further training measure for the event of a fault, the control unit has a substitute control device that differs from a control device intended for normal and fault-free operation.

[0016] Three fault scenarios are of particular interest: a fault in the hydraulic machine coupled to the drive motor, a fault in the hydraulic machine coupled to the output motor, or a fault in both hydraulic machines simultaneously. Each fault scenario is assigned at least one of the backup control devices or a combination thereof.

[0017] In the event of a failure of the output-side hydraulic machine ("hydraulic motor"), the substitute control unit provides a substitute input value for the input-side hydraulic machine ("hydraulic pump") that corresponds to the actual displacement volume of the output-side hydraulic machine. This substitute input value is, for example, a detected swivel angle of the swashplate or a detected stroke of one of the adjustment units of the output-side hydraulic machine. The substitute control unit then uses the electro-hydraulic adjustment unit of the input-side hydraulic machine to control the working pressure of the power line in such a way that the substitute input value, in particular the swivel angle or stroke of the faulty output-side hydraulic machine, corresponds to a value that represents a zero displacement volume of this hydraulic machine, i.e., for example, a swivel angle of 0°.

[0018] In the event of a failure of the drive-side hydraulic machine ("hydraulic pump"), the substitute control unit uses a substitute setpoint for the requested displacement volume of the output-side hydraulic machine ("hydraulic motor") as its substitute input value. This substitute setpoint is then a constant zero, regardless of the requested torque. Thus, in the aforementioned fault scenario, the requested torque at the output is ignored or overwritten to zero.

[0019] The respective electro-hydraulic adjusting unit preferably has an electromagnetically actuated valve as a valve device, via which an actuating pressure chamber of an adjusting cylinder of the electro-hydraulic adjusting unit can be connected to the working line when unactuated and to a pressure medium sink when actuated.

[0020] The respective hydraulic adjustment unit, which counteracts the electro-hydraulic adjustment unit, has an adjustment cylinder with an actuating pressure chamber that is connected to the working line of the circuit with a throttled or unthrottled pressure medium.

[0021] In a preferred embodiment, the control unit is configured such that the non-defective hydraulic machine can be controlled in such a way that the displacement volume of the hydraulic machine with the defective electro-hydraulic adjustment unit tends towards zero. Since the torque at the output is a product of working pressure and displacement volume, the torque at the output therefore tends towards zero according to the invention.

[0022] To protect the hydraulic machinery and the drive against overload, the working line is connected to a pressure limiting device, in particular a pressure limiting valve, in a further training.

[0023] The pressure limiting device proves to be particularly variable in operation if it is designed to be controllable by the control unit and the control unit is configured so that a limit pressure of the pressure limiting device can be set as required, in particular based on fault-free operation and faulty operation.

[0024] In a further development, the control unit is configured to set the pressure limit of the pressure relief device to a specified value during normal operation and to a value below this value depending on the fault condition. In this way, the dynamic adjustment, which is dependent on the operating pressure, can be reduced in the event of a fault. Consequently, pressure fluctuations can be reduced in the event of a fault.

[0025] To enable suction with minimal pressure loss during pump operation of the respective hydraulic machine and to provide the necessary back pressure during engine operation, a Continuing educationIn a pressure medium flow path, at least one of the hydraulic machines is provided with a throttle device with an adjustable throttle cross-section that can be controlled by the control unit, leading to the pressure medium sink.

[0026] In a further development, the control unit is configured so that the throttle cross-section of the hydraulic machine can be reduced during motor operation and increased during pump operation. In the simplest case, the throttle device can be designed as an unlockable check valve that opens towards the hydraulic machine.

[0027] In a further training, the control unit is designed in such a way that a change of a respective hydraulic machine from motor to pump operation and / or a zero crossing of its displacement volume can be detected or even predicted via it.

[0028] When the suction side becomes the low-pressure side after a changeover, as occurs when switching from motor to pump operation, this transition or zero crossing represents a critical condition. If it occurs with excessive dynamics, cavitation threatens if the hydraulic fluid on the suction side is accelerated too strongly. Therefore, detecting or predicting the transition / zero crossing is an advantageous feature of the control unit, enabling countermeasures to be initiated and cavitation to be prevented.

[0029] In a further training, the control unit is specifically designed so that the dynamics of the change and / or the zero crossing, in particular the dynamics of the adjustment of the hydraulic machine that is in pump operation or switches to it, can be reduced and / or limited.

[0030] Since a critical dynamic that should not be exceeded is not constant, but depends on the rotational speed and the displacement volume, a further development proves advantageous in which the control unit is set up so that the dynamic can be reduced and / or limited as a function of the rotational speed and / or the displacement volume.

[0031] Advantageously, a characteristic map or similar is stored in the control unit, which represents this dependency.

[0032] Naturally, the drive can include at least one hydrostatic actuator, in particular a working consumer, which can be supplied with pressure medium fluid via the working line of the open circuit. In order to enable its load-pressure-independent control, it is preferably fluidically connected to the working line via a metering orifice, with which a pressure balance is connected in series, allowing a pressure differential across the metering orifice to be regulated, a method known as LS or LUDV control.

[0033] A method for controlling a drive, designed according to at least one aspect of the preceding description, has a step of "controlling the electro-hydraulic adjustment units of the hydraulic machines via the control unit." As explained above, this control is preferably carried out in the sense of the aforementioned secondary control. In order to control the drive to a safer operating state in the event of a fault, at least the following steps can be carried out via the control unit according to the invention: "Determining the faulty and / or the non-faulty electro-hydraulic adjustment units via the control unit" and "Controlling at least one non-faulty electro-hydraulic adjustment unit via the control unit such that the torque of the hydraulic machine with the faulty electro-hydraulic adjustment unit decreases, in particular to or at least close to zero."

[0034] In this way, a method is provided by which the hydrostatic drive can be controlled to a safer operating state with minimal effort in the event of a fault.

[0035] In order to influence the dynamics of the adjustment and, as explained above, in particular to dampen pressure fluctuations and / or prevent cavitation, the method includes in a further development the step "controlling the pressure limiting device via the control unit in such a way that a limit pressure of the pressure limiting device is set to a value below a specified limit pressure".

[0036] In one variant, this step is already carried out if only one of the electro-hydraulic adjustment units is faulty, or in another variant, it is only carried out if the electro-hydraulic adjustment units of one and the other hydraulic machine, in particular all hydraulic machines of the circuit, are faulty.

[0037] In order to be able to initiate countermeasures as outlined above, which enable the safe operation of the drive, the procedure includes in a further development the step "Predicting and / or detecting a change of the hydraulic machines from engine operation to pump operation and / or a zero crossing of the displacement volumes".

[0038] Depending on the outcome, the following steps or a subset thereof can be carried out as a countermeasure according to the procedure: "Controlling the electro-hydraulic adjustment unit of the hydraulic machine with the changeover such that the dynamics of the adjustment within a time or displacement volume interval extending around the changeover are reduced compared to the dynamics outside this interval." "Controlling the electro-hydraulic adjustment unit of the hydraulic machine that can be coupled to the drive machine such that the working pressure of the working line within a time interval extending around the changeover is increased compared to the working pressure outside this interval." "Limiting the displacement volume within a time interval extending around the changeover to a limit." "Controlling a throttle cross-section of a hydraulic fluid flow path from the hydraulic fluid sink to the hydraulic machine with the changeover."

[0039] Preferably, the method according to at least one aspect of the preceding description is stored in the control unit for execution.

[0040] In the following, an exemplary embodiment of a hydrostatic drive and a method according to the invention are explained in more detail with reference to the drawings. The drawings show: Figure 1 a hydrostatic drive designed as a drive system according to an exemplary embodiment, Figure 2 a drive system in train operation according to Figure 1 hydraulic machine functioning as a hydraulic pump Figure 3 Possible operating quadrants of the drive according to the figures, Figure 4 a proper, error-free operation of the drive according to the figures, Figure 5 the behavior of a conventional drive in the event of an electrical fault in the electro-hydraulic adjustment unit of the hydraulic motor coupled to the output, during forward travel, Figure 6 the behavior of the drive according to the invention Figures 1 to 4 in the same electrical fault situation, during forward travel, Figure 7 the behavior of a conventional drive in the event of an electrical fault in the electro-hydraulic adjustment unit of the hydraulic motor coupled to the output, during reverse travel, and Figure 8 the behavior of the drive according to the invention Figures 1 to 4 and 6 in the same electrical fault situation, during reverse driving.

[0041] According to Figure 1 A hydrostatic drive 1, designed as a drive system, has a first hydraulic machine 4 coupled to a drive machine 2 and a second hydraulic machine 8 fluidically connected to it via a working line 6. The hydraulic machines 4 and 8 each have an electro-hydraulic adjustment unit 24 and 26, respectively, by which their displacement volume can be adjusted. The second hydraulic machine 8 is coupled to an output-side gearbox 14.

[0042] The respective hydraulic machine is specifically designed as an EOC or "Electronified Open Circuit" pump known from the applicant's product range. The actuator 1 is protected against overpressure by a pressure relief valve 16, the setpoint pressure of which can be adjusted by actuating an electromagnet 18. In the case of a high operating pressure as intended and the associated high setpoint pressure, the electromagnet 18 is preferably assigned to a pilot or control valve, via which the setpoint pressure at the pressure relief valve 16 can then be adjusted electrohydraulically by actuating the pilot valve instead of directly electrically. The in Figure 2 The electromagnetic direct actuation of the pressure relief valve 16 shown is, however, suitable in the case of a lower intended pressure range and the associated lower target limit pressure.

[0043] The drive 1 has a hydrostatic working consumer 20, which in the illustrated embodiment is designed as a hydraulic cylinder. For the supply of hydraulic fluid, the working lines 6 are connected to the hydraulic cylinder 20 via a control valve block 22.

[0044] In normal operation, the drive motor 2 rotates in a constant direction, such that according to Figure 1 is symbolized by n = +.

[0045] The hydraulic machines 6 and 8 are both designed with adjustable displacement volume. For this purpose, they feature, according to... Figure 1 Each has an electro-hydraulic adjustment unit 24, 26, from which a swashplate of the hydraulic machines 6, 8, which determines the respective displacement volume, is articulated.

[0046] The hydraulic machines 6, 8 are designed with a pivotable displacement volume, so that a reversal of the volume flow is possible despite the constant connection to the working line 6 and the pressure medium sink T, in particular for reversing the direction of travel and for transitioning from pulling to towing operation and vice versa.

[0047] The changes in volume flow direction and torque direction at the output associated with reversing and transition are in Figure 1 sketched.

[0048] The drive 1 has a control unit 28 for control, which is at least signal-connected to the electro-hydraulic adjustment units 24, 26, the electromagnet 18 of the pressure relief valve 16 and the control valve 22.

[0049] Figure 2Figure 1 shows a more detailed representation of the first hydraulic machine 4 operating as a hydraulic pump in normal train operation. The following considerations also apply to the second hydraulic machine 8, with the difference that its drive shaft is coupled to the gearbox 14 and the drive shaft of the first hydraulic machine 4 is coupled to the drive machine 2.

[0050] According to Figure 2 The electro-hydraulic adjustment unit 24 has an electromagnetically actuated valve 30, controllable by the control unit 28, with a pressure port connected to the working line 6 and a tank port connected to the pressure medium sink T. Furthermore, the valve 30 has a port connected to an actuating pressure chamber of an adjusting cylinder 32 of the electro-hydraulic adjustment unit 24.

[0051] In the de-energized state, a valve body of valve 30 is biased by a spring into an end position in which the working line 6 is connected to an actuating pressure chamber of the adjusting cylinder 32. When the valve 30 is electromagnetically actuated, the actuating pressure chamber is connected to the low-pressure T. Intermediate positions are possible in which the actuating pressure chamber is connected to both the working line 6 and the low-pressure T.

[0052] Alternatively, the valve 30 of the electro-hydraulic adjusting unit 24 can be designed inverted, i.e., in the unenergized state, the valve body of the valve 30 is loaded by the spring into an end position in which the pressure medium sink T is connected to the actuating pressure chamber of the adjusting cylinder 32, whereas when the valve 30 is electromagnetically actuated, the actuating pressure chamber is connected to the working line 6.

[0053] Intermediate positions in which the actuating pressure chamber is connected to both the working line 6 and the low pressure T are of course possible in both variants, for example by means of a negative overlap or undercoverage of control edges.

[0054] The actuating pressure chamber of the actuating cylinder 32 is bounded by an actuating piston, which, when the valve 30 is de-energized, loads the swashplate 34 of the first hydraulic machine 4 in the direction of a (negative) maximum displacement volume -Vgmax. In the present embodiment, the value -Vgmax for the hydraulic machine 4 operating in motor mode is defined such that, for a given direction of rotation and speed of the drive motor 2, the hydraulic machine 4 delivers the maximum negative flow rate Q-max.

[0055] In the present embodiment, the value +Vgmax is defined conversely for the hydraulic machine 4 operating in pump mode such that, for a given direction of rotation and speed of the drive machine 2, the hydraulic machine 4 provides the maximum positive delivery volume flow Q+max.

[0056] A spring-loaded counter-piston of an electro-independent, hydraulic-mechanical adjustment unit 36 ​​engages the swashplate 34 in the opposite direction of adjustment. Its actuating pressure chamber is permanently connected to the working line 6 via hydraulic fluid. In the de-energized state, the adjustment units 24 and 36 act according to the working pressure in the working line 6 such that above a system-specific working pressure, which depends on the piston area ratio, the spring force of the adjustment unit 36, and the rotational speed, the electro-hydraulic adjustment unit 24 adjusts the displacement volume Vg towards the negative maximum (-Vgmax), and below this working pressure, the adjustment unit 36 ​​adjusts the displacement volume Vg towards the positive maximum (+Vgmax).

[0057] The drive 1 has a pressure sensor 38 connected to the working line 6 and a swivel angle sensor 40 coupled to the swashplate 34 for control and / or regulation via the control unit 28.

[0058] Figure 3 shows possible operating quadrants of the drive 1. During driving operation, the hydraulic machines 4, 8 are divided into four quadrants I - IV.

[0059] The first operating quadrant I is an accelerating forward drive at a positive target engine speed nm and a positive target torque Mm (+) of the second hydraulic machine 8 ("engine"). Here, the volume flow Q, or the pump swivel angle ap, is positive (+) and a swivel angle αm of the second hydraulic machine 8 is negative (-).

[0060] The second operating quadrant II is forward braking at a positive target motor speed nm (+) but a negative (-) target torque Mm (braking). The volume flow Q, or the pump swashplate angle aP, is negative (-) and the motor swashplate angle αm is positive (+).

[0061] The third operating quadrant III is an accelerating reverse movement with nm (-), Mm (-), Q, or aP (+) and αm (+).

[0062] The fourth operating quadrant IV is a braking reverse movement with nm (-), Mm (+), Q, or aP (-) and αm (-).

[0063] Figure 4This shows the error-free normal operation of drive 1 using a reversing process, i.e., starting from accelerating forward travel I, the process transitions via decelerating forward travel II to accelerating reverse travel III. The top section shows the time profiles of a speed request according to the position PedIPos of drive 1's accelerator pedal, the engine speed nm, and the position Drvdir of drive 1's direction selector switch. The middle section shows target values ​​for the swivel angle αmsoll and the torque Mmsoll of the second hydraulic machine 8, as well as its actual swivel angle αmist, resulting from a driving strategy. The bottom section shows the target value of the operating pressure psoll, its actual value pist, and the actual swivel angle of the first hydraulic machine 4 αpist, all resulting from the driving strategy.

[0064] The first hydraulic machine 4 is pressure-controlled via the control unit 28 and the associated adjustment unit 24, whereby the working pressure p results from a maximum value of the requirements of the driving and working hydraulics. The second hydraulic machine 8 is torque-controlled and swivel-angle-controlled via the control unit 28 and its associated adjustment unit 26.

[0065] Electrical faults, such as broken cables and short circuits, particularly at the solenoid of valve 30, can occur with a certain probability in both hydraulic machines 4 and 8. With conventional drive and control systems, control over the swivel angles αpsoll and αmsoll is lost, and depending on the driving conditions, an uncontrolled swivel angle αapist and αmist will develop. Fundamentally critical conditions include pulsating pressure oscillations, unintended acceleration or deceleration of the machine, and, in particular, sustained cavitation. Such failures can result in engine damage due to cavitation or a high swivel angle at high speed. In this case, the engine can lift off the swashplate and be damaged. Thus, a seemingly "simple" electrical fault can escalate into significant economic damage, such as gearbox or valve block failure.

[0066] Figure 5This illustrates the problem with conventional drives when there is a fault in the electromagnet of the valve of the electro-hydraulic adjustment unit of the hydraulic motor coupled to the output. In the aforementioned fault condition, this solenoid pivots – due to the operating pressure p in the working line – towards -Vgmax until its displacement flow rate exceeds the delivery flow rate of the hydraulic pump coupled to the drive motor, thus causing the pressure p to collapse. In this de-energized and de-pressured state, both hydraulic motors pivot towards their positive maximum displacement volume +Vgmax until a pressure p builds up again. Therefore, the typical fault pattern is an oscillation that develops according to... Figure 5 , between -Vgmax and +Vgmax, as well as between zero bar and a maximum pressure pmax set at the pressure relief valve. This high-frequency change according to Figure 5This can cause damage due to cavitation and / or overspeed. Furthermore, critical machine conditions, particularly driving conditions, can occur, which may be safety-critical. For example, unintended accelerations can occur if the hydraulic pump is not yet saturated and the hydraulic motor is already approaching its maximum speed (-Vgmax).

[0067] Figure 6 In contrast, the behavior shown by the drive unit 1 according to the invention in the same fault condition is demonstrated. Figure 6At second 13 of forward travel, the previously described power failure occurs at the electromagnet of valve 30 of the electro-hydraulic adjustment unit 24 of the hydraulic motor 8. Due to the operating pressure p in the working line 6 and the power failure, the hydraulic motor 8 initially pivots towards -Vgmax, as in the conventional case, so that its displacement flow rate is briefly greater than the delivery flow rate of the hydraulic pump 4. The operating pressure p drops, and the displacement volume of the hydraulic motor 8 is adjusted back towards +Vgmax by the spring of its current-independent hydraulic adjustment unit 36.

[0068] According to the invention, the control unit 28 continuously monitors the electro-hydraulic adjustment units 24 of the hydraulic machines 4, 8 for their correct function, in particular for their current flow. Specifically, it monitors the correct electrical function of the electromagnets of the valves 30.

[0069] In the event of a fault as shown in the figure, the control unit 28 determines the fault at the electromagnet of the valve 30 of the hydraulic motor 8. The control unit contains a method according to the invention for its execution, which provides a substitute control according to the invention for the other hydraulic machine 4 in the event of a fault in the electro-hydraulic adjustment unit 24 of the hydraulic motor 8, and vice versa.

[0070] In case of error according to Figure 6 The valve 30 of the functioning electro-hydraulic control unit 24 of the hydraulic machine 4 is then controlled so that the output torque of the hydraulic machine 8 follows the direction of zero despite its failed (faulty) electro-hydraulic control unit. In principle, a substitute control system replaces the intended control system when a fault occurs.

[0071] In the aforementioned fault condition, the control unit 28 modifies the requested target operating pressure or target driving pressure and actuates the valve 30 of the electro-hydraulic adjustment unit 24 of the hydraulic pump 4 with a control current corresponding to the modified target. The modified target operating pressure is dimensioned such that the swashplate 34 of the hydraulic motor 8 pivots back to zero due to the changing force equilibrium acting on it – consisting of the actuating force of the hydraulic adjustment unit, its spring, and the engine forces acting on it resulting from the operating pressure. This type of substitute control prevents cavitation in the system and keeps the pivot angle of the hydraulic motor 8 small, as an excessively large pivot angle at a given rotational speed could damage the engine.

[0072] The following Figure 7 , 8They show the same fault - failure of the electro-hydraulic adjustment unit 24 of the hydraulic motor 8 - but in reverse. Figure 7 demonstrates the behavior of a conventional drive system, Figure 8 In contrast, this describes the behavior of the drive according to the invention.

[0073] In this case, according to Figure 8 The control unit 28, configured according to the invention, is further enhanced by an additional step: reducing the limit pressure set at the pressure relief valve 16 to a value far below the intended limit pressure, for example, to 20 bar. The effect is that with lower working pressure in the working line 6, the adjustment dynamics of the respective displacement volumes Vgm and Vgp, or of the swashplates 34, decrease and are limited. This prevents the hydraulic machines 4, 8 from reaching their limits too quickly. Vgmax and +Vgmax pivot before drive 1 is regulated. A further advantage is, of course, that overpressure in the hydraulic circuit is avoided and that this limitation of the dynamics restricts a sudden change in torque at output 14, thereby preventing critical driving situations.

[0074] In summary, the control unit according to the invention functions as follows: In the event of a fault in the output-side hydraulic machine 8, the control unit 28 has a substitute control device with a substitute input value for the drive-side hydraulic machine 4, wherein the substitute input value corresponds to the actual displacement volume Vgmist of the output-side hydraulic machine 8. The substitute input value is, for example, the actual swivel angle αmist of the swashplate detected by the swivel angle sensor 40 or a detected travel distance of the adjustment unit of the output-side hydraulic machine 8. Via the substitute control device, the working pressure p of the working line 6 can then be controlled by actuating the electro-hydraulic adjustment unit 24 of the drive-side hydraulic machine 4 such that the substitute input value (swivel angle αmist) follows a value corresponding to a zero displacement volume Vg0, i.e., for example, a swivel angle αmist of 0°.

[0075] In the event of a fault in the drive-side hydraulic machine 4, pressure control is no longer possible. The control unit 28 therefore has a substitute control device with a substitute setpoint for the displacement volume Vgm of the output-side hydraulic machine 8 as a substitute input value. Since the electro-hydraulic adjustment unit 26 of the output-side hydraulic machine 8 is non-faulty, i.e., intact and controllable, it can be directly controlled with the substitute setpoint, which corresponds to Vgmset = 0, regardless of the requested torque. Thus, in the aforementioned fault condition, the requested torque at the output 14 is "ignored" and instead regulated to zero.

[0076] In the event of a failure of the electro-hydraulic adjustment units 24, 26 of both the drive-side and the output-side hydraulic machine(s) 4, 8, control via the electro-hydraulic adjustment units 24, 26 is no longer possible. To prevent critical driving situations, critical pressure peaks, and / or cavitation, the control unit 28 is designed according to the invention to control the pressure relief valve 16 in such a way that the pressure fluctuations in the drive system are stabilized by the pressure relief valve 16. In particular, pressure peaks are avoided and the output torque is limited by the pressure relief valve 16.

[0077] It is known that the engine requires back pressure on the low-pressure side during operation to achieve high speeds. One way to create this back pressure is with a check valve. However, during pump operation (reversing, torque reversal), the oil must be drawn in with as little restriction as possible. Therefore, this element should be designed to be unlockable to prevent restriction during pump operation when drawing in oil. The inventor has developed a component which, based on a control signal (electrical or hydraulic), generates a pressure drop on the low-pressure side during engine operation and ensures unrestricted intake of the output-side unit during pump operation.

[0078] A hydrostatic drive is disclosed, comprising at least two hydraulic machines arranged in an open hydraulic circuit and designed with a displacement volume adjustable to zero. These hydraulic machines can be electrically and / or electronically controlled in at least one of their adjustment directions via a control unit of the drive. According to the invention, the control unit is designed such that a failure of the electrical and / or electronic components controlling the adjustment can be detected. In the event of a failure, the corresponding functioning hydraulic machine can be controlled such that the torque on the malfunctioning hydraulic machine approaches zero. This prevents damage to the drive that could result from the failure.

[0079] Disclosed in particular is a secondary-controlled drive system consisting of two or more pivoting axial piston machines in a swashplate design, each with an electrically / electronically controlled valve of an electro-hydraulic adjustment unit acting on the swashplate and an electrically independent adjustment unit acting in opposition to it, wherein the swashplates pivot back from their maximum displacement volume in the de-energized state and under operating pressure, pivot out to their maximum displacement volume in the de-energized state and without pressure, and an equilibrium displacement volume is established at a lower standby pressure, wherein a requested pressure can be regulated by means of a secondary control unit via the axial piston machine coupled to a drive machine and at the axial piston machine coupled to an output, based on a requested torque at the output,A swivel angle can be adjusted. According to the invention, an electrical fault in the axial piston machines can be detected via the control unit, and depending on this, the intended control can be converted into a substitute control of the non-faulty, i.e., still intact, axial piston machine(s), in such a way that critical conditions with regard to the driving state and component damage are prevented, in particular in such a way that torque-free switching remains possible.

[0080] Furthermore, a corresponding method for controlling the drive has been revealed.

Claims

1. Hydrostatic drive comprising hydraulic machines (4, 8), one (4) of which can be coupled to a drive machine (2) and another (8) of which can be coupled to an output (14) and which are firstly fluidically connected via a working line (6) and secondly are fluidically connected to a pressure medium sink (T) and are each configured with a displacement volume (Vgp, Vgm) which can be adjusted to zero and either side of zero, wherein the hydraulic machines, for adjustment, each have at least one electrohydraulic adjustment unit (24, 26) which can be connected to the working line (6) and to low pressure (T), characterized in that a control unit (28) of the drive (1) is designed such that it can be used to determine a faulty electrohydraulic adjustment unit from amongst the electrohydraulic adjustment units (24, 26) and to control a non-faulty hydraulic machine (4, 8) from amongst the hydraulic machines (4, 8) in such a way that a torque (M) of the hydraulic machine (4, 8) with a faulty electrohydraulic adjustment unit (24, 26) tends towards zero.

2. Drive according to Claim 1, wherein the control unit (28) can be used to control the non-faulty hydraulic machine (4) from amongst the hydraulic machines (4, 8) in such a way that the displacement volume (Vgm) of the hydraulic machine (8) with a faulty electrohydraulic adjustment unit (26) tends towards zero (Vg0).

3. Drive according to Claim 1 or 2, wherein the working line (6) is fluidically connected to a pressure limiting device (16) which can be controlled and adjusted by the control unit (28).

4. Drive according to Claim 3, wherein the control unit (28) is designed such that it can be used, in particular in the event of failure of both hydraulic machines (4, 8), to adjust a limit pressure of the pressure limiting device (16) to below a specified limit pressure.

5. Drive according to any of the preceding claims, wherein a throttle device which can be controlled by the control unit with an adjustable throttle cross section is provided at least in a pressure medium flow path from the other hydraulic machine to the pressure medium sink.

6. Drive according to Claim 5, wherein the control unit is designed such that it can be used to reduce the throttle cross section in motor operation of the other hydraulic machine and in contrast to increase the throttle cross section in pump operation.

7. Drive according to any of the preceding claims, wherein the control unit (28) is designed such that it can be used to detect and / or predict a respective changeover of the hydraulic machines (4, 8) from motor to pump operation and / or a zero crossing of the displacement volume (Vgp, Vgm).

8. Drive according to Claim 7, wherein the control unit (28) is designed such that it can be used to reduce and / or limit a dynamic of the changeover and / or the zero crossing.

9. Drive according to any of the preceding claims, wherein the control unit (28) is designed such that it can be used to reduce and / or limit dynamics of the adjustment of at least that hydraulic machine (4, 8) which is in pump operation.

10. Drive according to Claim 8 or 9, wherein the control unit (28) is designed such that it can be used to reduce and / or limit the dynamics as a function of a speed (n) and / or the displacement volume (Vgp, Vgm).

11. Method for controlling a drive, in particular a traction drive (1), of the hydraulic machines (4, 8), one (4) of which can be coupled to a drive machine (2) and another (8) of which can be coupled to an output (14) and which are firstly fluidically connected via a working line (6) and secondly are fluidically connected to a pressure medium sink (T) and are each configured with a displacement volume (Vgp, Vgm) which can be adjusted to zero and either side of zero, wherein the hydraulic machines, for adjustment, each have an electrohydraulic adjustment unit (24, 26) which can be connected to the working line (6) and to low pressure (T), comprising a step of - controlling the electrohydraulic adjustment units (24, 26) of the hydraulic machines (4, 8) via the control unit (28), characterized by steps of - determining a faulty electrohydraulic adjustment unit (24, 26) from amongst the electrohydraulic adjustment units (24, 26) via the control unit (28), and - controlling a non-faulty electrohydraulic adjustment unit (26, 24) from amongst the electrohydraulic adjustment units (24, 26) via the control unit (28) in such a way that a torque (M) of the hydraulic machine (4, 8) with the faulty electrohydraulic adjustment unit (24, 26) drops, in particular to or at least towards zero.

12. Method according to Claim 11, wherein the working line (6) is fluidically connected to a pressure limiting device (16) which can be controlled and adjusted by the control unit (28), comprising a step of - controlling the pressure limiting device (16) via the control unit (28) in such a way that a limit pressure of the pressure limiting device (16) is adjusted to a value below a specified limit pressure.

13. Method according to Claim 12, wherein the step according to Claim 12 is already carried out when only one of the electrohydraulic adjustment units (24, 26) is faulty, or is carried out only when the electrohydraulic adjustment units (24, 26) of one (4) and the other (8) hydraulic machine are faulty.