Method for operating an electrohydraulic drive of a hydraulic cylinder, and hydraulic system

Abstract

The invention relates to a method for operating an electrohydraulic drive of a hydraulic cylinder (2), wherein the electrohydraulic drive has a hydraulic machine (14) and an electric machine (16), wherein the hydraulic machine (14) is connected to a pump channel (34) which is connected via a throttle point (36) to a cylinder channel (8) which is connected to a cylinder chamber (6) of the hydraulic cylinder, wherein a tank (32) is connected to the hydraulic machine and is connected via a logic valve (42) to the cylinder channel (8), wherein the logic valve has an actuating device (44) which is adjusted in accordance with a pressure difference between the pump channel (34) and the cylinder channel (8), wherein a position sensor (12) is provided which detects position values (20) of a piston (4) of the hydraulic cylinder, said method comprising the following steps: detecting (110) specified position values (54) for the piston (4); determining (130) a target trajectory (76) from the specified position values, which target trajectory includes target position values (78), wherein a maximum possible speed is taken into account as a speed limitation when determining the target trajectory, which speed is determined depending on a cylinder chamber pressure; and, if a load shedding operation is to take place, carrying out (150) a load shedding control (70) in accordance with the target trajectory in order to determine the target rotational speed (28), and actuating the electric machine (16) at the determined target rotational speed.

Description

[0001] Method for operating an electro-hydraulic drive of a hydraulic cylinder and hydraulic system

[0002] Description

[0003] The present invention relates to a method for operating an electro-hydraulic drive of a hydraulic cylinder, a hydraulic system as well as a computing unit and a computer program for carrying out the method.

[0004] Background of the invention

[0005] Hydraulic control systems have a long tradition in gas and steam turbine construction, for example in power plants. Control and switching valves operated by hydraulic cylinders, so-called "energy cylinders," are used to ensure the control and safety of the turbines. Such an energy cylinder can be controlled via an attached control block. When controlling gas and steam turbines, it can happen that the electrical load drops suddenly. This can lead to dangerously high turbine speeds if the actuated control or switching valve is not closed quickly enough.

[0006] Disclosure of the invention

[0007] According to the invention, a method for operating an electrohydraulic drive of a hydraulic cylinder, a hydraulic system, a computing unit, and a computer program for implementing the method are proposed, having the features of the independent patent claims. Advantageous embodiments are the subject of the dependent claims and the following description.

[0008] The method relates to the operation of an electro-hydraulic drive of a hydraulic cylinder, wherein preset position values ​​for a position of a piston of the hydraulic cylinder are recorded, and a target trajectory with target position values ​​is determined from the preset position values. A maximum possible speed is taken into account as a speed limit, which is determined depending on a cylinder chamber pressure. Furthermore, if load shedding operation is to take place (e.g. in response to a load shedding signal), load shedding control is carried out according to the target trajectory in order to determine the target speed, wherein the electric machine is controlled at the target speed determined by the load shedding control. By taking the pressure-dependent maximum possible speed into account as a speed limit when determining the target trajectory, unfavorable control behavior, which may, for example,which could occur as a result of a rapid change in the default values, can be avoided or at least reduced.

[0009] The electro-hydraulic drive has a hydraulic machine and an electric machine which is coupled to the hydraulic machine and which can be controlled at a desired speed. A pressure connection of the hydraulic machine is connected to a pump channel, which in turn is connected via a throttle to a cylinder channel, which in turn is connected to a cylinder chamber of the hydraulic cylinder. A tank is connected to a tank connection of the hydraulic machine and to the cylinder channel via a logic valve. The logic valve has an actuating device which is adjusted between a closed and an open position according to a pressure difference between the pump channel and the cylinder channel. A position sensor is provided which detects or measures position values ​​of a piston of the hydraulic cylinder. The measured position values ​​(orPosition measurements) are used as actual position values ​​in the control (both in load shedding control and in the normal operation described below).

[0010] The volume flow of pressure fluid delivered by the hydraulic machine corresponds to the target speed, i.e., the delivered volume flow is essentially proportional to the target speed (except for leaks or similar). The electric machine, for example, includes an inverter with an inverter control that regulates the speed according to the target speed.

[0011] The pressure difference between the pump channel and the cylinder channel is caused by a volume flow via the throttle point. The logic valve can therefore be controlled via the speed. The logic valve or its actuating device is set up so that the logic valve is or is moved to the closed position (no flow of pressure medium through the logic valve possible) when the pressure in the pump channel is greater than the pressure in the cylinder channel or in the cylinder chamber. If the pressure in the cylinder channel or in the cylinder chamber is or is moved to the open position by at least one opening pressure threshold (which is in particular greater than zero). The opening pressure threshold is reached, for example, at an opening volume flow (from the cylinder channel to the pump channel) or a corresponding opening speed.

[0012] Unless otherwise stated, the term "connected" or "connection" is to be understood in the sense of "hydraulically connected," i.e., in the sense of hydraulic connections, e.g., in the form of lines, channels, or flow cross-sections. Thus, a flow of pressure medium is possible between (hydraulically) connected elements, whereby hydraulic control or regulating elements, such as valves, can be provided that can influence the flow of pressure medium.

[0013] According to one embodiment, the target speed is determined using a position control system based on the specified position values, and the electric motor is controlled at the target speed determined by the position control system when no load shedding operation is to occur. During this normal operation, i.e., when no load shedding operation is to occur, the target position is typically changed relatively slowly, so that the logic valve remains closed. In particular, the target speed remains below the opening speed during normal operation.

[0014] According to one embodiment, the load shedding control includes a position control according to the target position values, wherein, in particular, a deviation between the actual position values ​​and the corresponding target position values ​​is controlled to zero. Accordingly, a final desired position can be reached according to the specified position values ​​while maintaining the speed limit.

[0015] According to one embodiment, the target trajectory includes target speed values, or target speed values ​​are determined from the target position values. This allows additional information (besides the target position) to be used in the control system, the target speed, thus improving the control behavior.

[0016] According to one embodiment, the load shedding control includes a speed control according to the target speed values, wherein, in particular, a deviation between actual speed values ​​and corresponding target speed values ​​is controlled to zero. In particular, overshoot can be largely avoided by the speed control.

[0017] According to one embodiment, when determining the target trajectory, the target trajectory is determined in such a way that the maximum possible speed is not exceeded. In particular, if necessary, the target trajectory is determined in such a way that the target speed does not exceed the maximum possible speed. This can prevent control according to target specifications with unrealistically high speeds, which could, for example, cause control instabilities.

[0018] According to one embodiment, the maximum possible speed is determined based on a flow cross-section, in particular a maximum flow cross-section, of the logic valve in the open position. In addition to the volume flow from the cylinder channel via the logic valve to the tank, the volume flow delivered by the hydraulic machine between the cylinder channel and the tank is also taken into account.

[0019] According to one embodiment, the target speed values ​​correspond at least partially to a volume flow from the cylinder chamber that is higher than a volume flow via the throttle point, which causes a pressure difference between the pump channel and the cylinder channel, which causes the logic valve to be adjusted to the open position.

[0020] A computing unit according to the invention, e.g. a control unit of a hydraulic drive, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0021] A hydraulic system according to the invention comprises a hydraulic cylinder, an electro-hydraulic drive as described above and a computing unit according to the invention.

[0022] Implementing a method according to the invention in the form of a computer program or computer program product with program code for performing all method steps is also advantageous, as this entails particularly low costs, especially if an executing control unit is also used for additional tasks and is therefore already present. Suitable data storage devices for providing the computer program include, in particular, magnetic, optical, and electrical storage devices, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible.

[0023] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0024] It is understood that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0025] The invention is illustrated schematically in the drawing using exemplary embodiments and is described in detail below with reference to the drawing.

[0026] Character description

[0027] Figure 1 shows a hydraulic system and an associated control scheme according to embodiments of the invention.

[0028] Figures 2A, 2B show exemplary time courses of a target position, an actual position and a speed which are obtained when applying a method according to the invention in the load shedding case.

[0029] Figure 3 shows a flowchart according to embodiments of the invention.

[0030] Detailed description of the drawing

[0031] Figure 1 shows a hydraulic system or control scheme according to embodiments of the invention. The figure depicts a hydraulic cylinder 2 or energy cylinder, which is driven by an electro-hydraulic drive, which is controlled or regulated according to the invention, e.g., by a computing unit 50.

[0032] The hydraulic cylinder 2 or its piston 4 is preloaded in one direction, e.g., into an extended state, by means of a spring element. Pressurizing a cylinder chamber 6 of the hydraulic cylinder 2 with pressurized medium (i.e., hydraulic fluid, e.g., hydraulic oil) via a cylinder channel 8 (or cylinder chamber channel) causes the piston 4 to move in the opposite direction, e.g., into a retracted state. A piston rod 10 of the hydraulic cylinder 2 is coupled, e.g., to an actuating device of a regulating or control valve of a gas or steam turbine (not shown) in order to open and close the latter (the closed state of the regulating or control valve being when the hydraulic cylinder is extended).

[0033] A position of the piston 4 or the piston rod 10 is measured with a position sensor 12 and position measurement values ​​20 (actual position x ist) are transmitted to the computing unit 50, which records them. If the piston speed is used in the control, the current speed 22 (actual speed v ist ) can be measured by the position sensor 12 or calculated from the position measurement values ​​and transmitted to the computing unit 50. Alternatively or additionally, if the piston speed is used in the control, the computing unit 50 can calculate the actual speed 22 from the position measurement values ​​20.

[0034] The electro-hydraulic drive comprises a hydraulic machine 14 or hydraulic pump and an electric machine 16, which is coupled to the hydraulic machine 14 in order to drive it. This coupling is rotationally fixed, so that a speed of the hydraulic machine 14 is equal to a speed of the electric machine 16 or, if a coupling gear with a gear ratio other than one is used, so that the speed of the hydraulic machine 14 is equal to the gear ratio times the speed of the electric machine 16. Windings of the electric machine 16 are supplied with electrical currents, for example, via an inverter. The current speed 24 (actual speed n ist ) of the electric machine 16 can be measured with a speed sensor 18 and adjusted by a speed controller 26 to a target speed 28 (target speed n soU). The speed controller 26 can be implemented, for example, in an inverter controller of the electric machine 26, so that the computing unit 50 transmits the target speed 28 to the speed controller 26 in the inverter controller (approximately as shown). Alternatively, the speed controller 26 can be implemented in the computing unit 50.

[0035] A first connection or tank connection of the hydraulic machine 14 is connected to a tank channel 30, which in turn is connected to a tank 32. A second connection or pressure connection of the hydraulic machine 14 is connected to a pump channel 24 (or pressure channel). The electric machine 16 and the hydraulic machine 14 are rotatable in both directions, so that pressure medium can be pumped through the hydraulic machine 14 in both directions. For example, a speed greater than zero corresponds to the pumping direction from the tank channel 30 to the pump channel 34, and a speed less than zero corresponds to the pumping direction from the pump channel 34 to the tank channel 30. The pump channel 34 is connected to the cylinder chamber channel 8 via a throttle point 36, so that pressure medium can be pumped between the tank 32 and the cylinder chamber 6 of the hydraulic cylinder 2 via the throttle point 36 by means of the hydraulic machine 14.A pressure sensor 38 can be provided on the cylinder channel 8 or on the cylinder chamber 6, which measures the cylinder chamber pressure 40 (or cylinder chamber pressure p. z ), ie the pressure of the pressure medium present in the cylinder chamber 6, and transmits it to the computing unit 50.

[0036] The cylinder channel 8 is further connected to the tank channel 30 via a logic valve 42, which is designed, for example, as a seat valve. The logic valve 40 has a closed position in which no volume flow of pressure medium through the logic valve 42 is possible, and an open position in which a volume flow of pressure medium through the logic valve 42 is possible. The logic valve 42 has a hydraulic actuating device 44, which is connected to the cylinder channel 8 and the pump channel 34 via control channels or control lines shown as dashed lines. The logic valve 42, in particular its actuating device 44, is designed such that pressurization via the control channel, which is connected to the pump channel 34, in which the pressure medium has a pump pressure p Phas, is connected, causes an adjustment (of the logic valve) or force in the direction of the closed position and that a pressure is applied via the control channel, which is connected to the cylinder channel 8, in which the pressure medium the cylinder chamber pressure p z is connected, causes an adjustment or force towards the open position. Accordingly, the logic valve 42 is located depending on the pressure difference between the pump pressure p P and cylinder chamber pressure p z in the closed or open position. A pressure difference between pump pressure p P and cylinder chamber pressure p z results from the pressure drop caused by a volume flow of pressure medium via the throttle point 36, whereby the volume flow via the throttle point 36 can be controlled by a respective speed.

[0037] If the pump pressure p P greater than the cylinder chamber pressure p z, the logic valve 42 is in the closed position. The logic valve 42 is particularly preloaded into the closed position, e.g., by a spring element. Accordingly, the logic valve 42 is also in the closed position when the cylinder chamber pressure p z greater than the pump pressure p P and the difference (p z - p P ) between cylinder chamber pressure p z and pump pressure p P is less than an opening pressure threshold corresponding to the bias of the logic valve 42 in the closed position. If the logic valve is not biased in the closed position, the opening pressure threshold is zero. If the cylinder chamber pressure p z greater than the pump pressure p P and the difference (p z - p P ) between cylinder chamber pressure p z and pump pressure p Pis greater than the opening pressure threshold, the logic valve 42 is in the open position or assumes this position. The flow cross-section in the open position can be variable, e.g., with increasing difference between the cylinder chamber pressure p z and the pump pressure p P gain weight.

[0038] During normal operation, position control occurs, with the target speed 28 for the electric machine 16 or its speed controller 26 being determined by a position controller 52 (or position control) based on a specified position, which generally varies over time, or a temporal series of specified position values ​​54, which is or are specified, for example, by a higher-level controller of the mechanical system in which the hydraulic cylinder 2 is used (e.g., for actuating a regulating or control valve of a gas or steam turbine). As shown, for example, the difference between the respective specified position value 54 and the actual position 20 (or actual position value) is determined and regulated to zero by the speed controller 26. The symbolic switch 56 is in the lower position, i.e., it connects the output of the computing unit 50, at which the target speed 28 is output, to the speed controller 26.During normal operation, the logic valve 42 is typically in the closed position, ie the volume flows occurring during normal operation (which correspond to the rate of change of the specified position) do not result in a pressure difference across the throttle point that is above the opening pressure threshold.

[0039] In addition to normal operation, load shedding operation is provided as a further operating mode. In load shedding operation, pressure medium should be diverted from the cylinder chamber 6 as quickly as possible, i.e. the hydraulic cylinder 2 should be extended as quickly as possible, for example by changing the specified position from an initial position to an end position. Whether load shedding operation should take place can be determined in the present case by a load shedding detection 58. The load shedding detection 58 can be implemented, for example, as a module (in particular as a computer program module) in the processing unit 50. Alternatively, the load shedding detection can also be implemented in another processing unit, in particular in the higher-level control system. The load shedding detection 58 determines or recognizes whether load shedding operation should take place or be triggered. For this purpose, the load shedding detection 58 can, for example, evaluate the temporal series of specified position values ​​54.For example, load shedding operation can be triggered when the temporal rate of change of the set position (i.e. the temporal series of set position values) is above a predetermined threshold (and the sign of the rate of change corresponds to the direction of movement of the piston when pressure medium is discharged from the cylinder chamber). Additionally or alternatively, the load shedding detection, particularly when implemented in the higher-level control system, can evaluate signals from the mechanical system in which the hydraulic cylinder 2 is used, on the basis of which, for example, the set position is determined, in order to recognize that load shedding operation is to take place. If a control or regulating valve of a gas or steam turbine is adjusted by the hydraulic cylinder, the load shedding case in which load shedding operation is to take place can consist of the electrical load at a point controlled by the gas or steam turbine.Steam turbine driven generator is eliminated, so that the gas or steam supply should be stopped (corresponding to an extension of the hydraulic cylinder 2) in order to prevent damage to the turbine, for example due to excessive speeds.

[0040] In load shedding mode, the target speed 28 for the speed controller 26 is determined by a load shedding controller 70 (or a load shedding control). The symbolic switch 56 is in the upper position, i.e., it connects the output of the computing unit 50, at which the target speed 28 is output, to the load shedding controller 70. To determine the target speed 28, the load shedding controller 70 uses the actual position 20 and / or the actual speed 22, as well as a target trajectory 72 for a target position and / or a target speed or for the pair of target position and target speed. The load shedding controller 70 can be designed as a pure P controller (P: proportional), PD follower controller (P: proportional, D: differential), or sliding regime follower controller. Follower controller means that the control signal, ie here the setpoint speed 28, is calculated depending on a position and speed error.

[0041] The target trajectory 72 is determined by a pre-filter 74 from the default position values ​​54. The pre-filter 74 calculates the target trajectory 72, e.g. (x so u, v so u), where the (in terms of magnitude) maximum possible speed (of the piston or piston rod) is determined as a function of the measured cylinder chamber pressure p z(for example by the pressure sensor 38) is taken into account as a speed limitation in the pre-filter. The pressure in the tank 32 is known, e.g., approximately 1 bar, so that the pressure difference between the cylinder chamber and the tank, assuming the logic valve 42 is open, is known and the possible volume flow (and thus the possible piston speed) can be determined. Corresponding data on the maximum possible speed used by the pre-filter 74 can, for example, be stored in the form of a function and / or a characteristic map or can be obtained by means of a model. In order to determine the target trajectory 72, the time series of the specified position values ​​54 is filtered such that the speed corresponding to the target trajectory or the target speed does not exceed the maximum possible speed (in terms of absolute value).

[0042] The target trajectory is determined in particular such that the associated volume flow via the throttle point 36 leads to a pressure difference between the cylinder channel 8 and the pump channel 34 that exceeds the opening pressure threshold of the logic valve 42, causing the latter to switch to the open position. Accordingly, a high speed can be achieved because pressure medium can flow through the logic valve. It is assumed that, at typical cylinder chamber pressure values, the volume flow possible via the logic valve in its open position is much greater, or several times greater, than the volume flow delivered by the hydraulic machine at its rated speed.By using the target trajectory, which takes the speed limitation into account, instabilities, in particular overshoot, can be avoided, which can occur, for example, if the target position values ​​were changed immediately, or in a very short time, from the starting position to the end position in the case of load shedding.

[0043] Figures 2A and 2B show exemplary time profiles of a target position 78, an actual position 20, and a rotational speed 24 obtained when applying a method according to the invention in the case of load shedding. In Figure 2A, the position 82 is plotted against time 80 on the vertical axis. In Figure 2B, the rotational speed 84 is plotted against time 80 on the vertical axis. The profiles are shown beginning with a load shedding signal 81.

[0044] Figure 2A shows the curves of the actual position 20, as measured by position sensor 12, and the target position 78 according to the target trajectory. The curve of the target position 78 and, with some delay, the curve of the actual position 20 begin at an initial position 90 and are changed to an end position 92. It can be seen that overshoot practically occurs in the actual position 20. The temporal curve of the specified position values ​​(not shown) is approximately such that they are changed immediately (or at a higher speed than the target trajectory, which, for example, is the maximum possible speed for a given cylinder chamber pressure) from the initial position 90 to the end position 92.

[0045] Figure 2B shows the corresponding curve of the rotational speed 24 of the electric motor and the hydraulic motor, respectively. In particular, a reversal of the direction of rotation can be seen at the time when the target position 78 reaches the end position 92, which is caused in particular by taking the speed error into account in the load shedding control.

[0046] Figure 3 shows a flow chart according to embodiments of the invention. The method illustrated can, as described above, be carried out by a computing unit (e.g., computing unit 50 of Figure 1) or a control unit of the hydraulic drive or hydraulic system (approximately as shown in Figure 1). As already described above, the electro-hydraulic drive has a hydraulic machine and an electric machine which is coupled to the hydraulic machine and which can be controlled at a target speed. A pressure connection of the hydraulic machine is connected to a pump channel, which in turn is connected to a cylinder channel via a throttle point. The cylinder channel is connected to a cylinder chamber of the hydraulic cylinder, and a tank is connected to a tank connection of the hydraulic machine and to the cylinder channel via a logic valve.The logic valve has an actuating device that is adjusted between a closed and an open position according to a pressure difference between the pump channel and the cylinder channel. A position sensor is provided that measures the position values ​​of a piston of the hydraulic cylinder to use these as actual position values ​​in the control system.

[0047] In step 110, default position values ​​or a time series of default position values ​​are recorded, which are specified, for example, by a higher-level controller as described above.

[0048] In optional step 120, the cylinder chamber pressure is recorded, e.g., as a pressure measurement from a pressure sensor. Alternatively, the cylinder chamber pressure could be known in the higher-level controller and transmitted by it. In step 130, a target trajectory that includes target position values ​​or a target position trajectory and optionally target speed values ​​or a target speed trajectory is determined from the specified position values. The time series of the specified position values ​​is filtered, with the maximum possible speed, which depends on the cylinder chamber pressure, being taken into account as a speed limit. For example, the time series of the specified position values ​​can be adjusted so that the maximum possible speed is not exceeded (in terms of absolute value) in order to obtain the target position trajectory or (time series of) the target position values.The target speed values ​​are accordingly (in terms of amount) below the maximum possible speed.

[0049] In step 140, it is determined whether load shedding operation should occur, using, for example, load shedding detection as described above. If this is not the case, normal operation takes place in step 160, in which, for example, position control is carried out according to the specified position or the specified position values, i.e., a deviation between detected actual position values ​​(e.g., measured by a position sensor) and the specified position values ​​is controlled to zero. The position control determines a target speed at which the electric machine is controlled.

[0050] If load shedding operation is to take place, in step 160 a load shedding control is carried out according to the target trajectory, i.e. according to the target position trajectory (temporal series of target position values) and, if applicable, the target speed trajectory (temporal series of target speed values). In this case, a position control of the actual position takes place according to the target position trajectory and, if applicable, a speed control of the actual speed (which is calculated, for example, from recorded actual position values) takes place according to the target speed trajectory. A speed error between the actual speed values ​​and the target speed values ​​is therefore regulated to zero. The load shedding control determines the target speed at which the electrical machine is controlled. As mentioned, the load shedding control orThe load shedding controller can be designed, in particular, as a P controller, PD follower controller, or sliding regime follower controller. Load shedding control can prevent unfavorable control behavior (e.g., severe overshoot) caused by an excessively high rate of change of the specified position values.

Claims

Claims 1. A method for operating an electro-hydraulic drive of a hydraulic cylinder (2), wherein the electro-hydraulic drive comprises a hydraulic machine (14) and an electric machine (16) which is coupled to the hydraulic machine and which can be controlled at a desired speed, wherein a pressure connection of the hydraulic machine (14) is connected to a pump channel (34), wherein the pump channel (34) is connected to a cylinder channel (8) via a throttle point (36), wherein the cylinder channel (8) is connected to a cylinder chamber (6) of the hydraulic cylinder, wherein a tank (32) is connected to a tank connection of the hydraulic machine (14) and to the cylinder channel (8) via a logic valve (42), wherein the logic valve (42) has an actuating device (44) which is adjusted between a closed and an open position in accordance with a pressure difference between the pump channel (34) and the cylinder channel (8),wherein a position sensor (12) is provided which detects position values ​​(20) of a piston (4) of the hydraulic cylinder, the method comprising: Detecting (110) default position values ​​(54) for the piston (4); Determining (130) a target trajectory (76) with target position values ​​(78) from the specified position values, wherein, when determining the target trajectory, a maximum possible speed is taken into account as a speed limitation, which is determined as a function of a cylinder chamber pressure; and, if load shedding operation is to take place, carrying out (150) a load shedding control (70) according to the target trajectory in order to determine the target speed (28), and controlling the electric machine (16) at the target speed determined by the load shedding control.

2. Method according to claim 1, wherein, if no load shedding operation is to take place, the target speed (28) is determined by a position control (52) in accordance with the preset position values ​​(54) and the electric machine (16) is controlled at the target speed determined by the position control (160).

3. The method according to claim 1 or 2, wherein the load shedding control (70) includes a position control according to the target position values ​​(78); in particular, a deviation between actual position values ​​and corresponding target position values ​​is controlled to zero.

4. Method according to one of the preceding claims, wherein the target trajectory (70) includes target speed values ​​or target speed values ​​are determined from the target position values.

5. The method according to claim 4, wherein the target speed values ​​correspond at least partially to a volume flow from the cylinder chamber (6) which is higher than a volume flow via the throttle point (36), which causes a pressure difference between the pump channel (34) and the cylinder channel (8) which causes the logic valve (42) to be adjusted to the open position.

6. The method according to claim 4 or 5, wherein the load shedding control (70) includes a speed control according to the target speed values; in particular, a deviation between actual speed values ​​and corresponding target speed values ​​is controlled to zero.

7. Method according to one of claims 4 to 6, comprising determining (130) the target trajectory (76) such that the target speed does not exceed the maximum possible speed.

8. Method according to one of the preceding claims, comprising determining (130) the desired trajectory (76) such that the maximum possible speed is not exceeded.

9. Method according to one of the preceding claims, wherein the maximum possible speed is determined based on a flow cross-section, in particular a maximum flow cross-section, of the logic valve (42) in the open position.

10. A computing unit (50) comprising a processor configured to carry out the method according to any one of the preceding claims.

11. Hydraulic system comprising a hydraulic cylinder (2), wherein a position sensor (12) is provided which measures position values ​​of a piston (4) of the hydraulic cylinder (2);an electro-hydraulic drive, wherein the electro-hydraulic drive includes a hydraulic machine (14) and an electric machine (16) which is coupled to the hydraulic machine and which can be controlled at a desired speed (28), wherein a pressure connection of the hydraulic machine is connected to a pump channel (34), which is connected via a throttle point (36) to a cylinder channel (8) which is connected to a cylinder chamber (6) of the hydraulic cylinder, wherein a tank (32) is connected to a tank connection of the hydraulic machine (14) and via a logic valve (42) to the cylinder channel (8), wherein the logic valve has an actuating device which is adjusted between a closed and an open position in accordance with a pressure difference between the pump channel (34) and the cylinder channel (8); and a computing unit (50) according to claim 10; 12. Hydraulic system according to claim 11, wherein the hydraulic cylinder (2) or its piston (4) is preloaded by a spring element.

13. A computer program comprising instructions which, when executed by a computer, cause the computer to carry out the method according to claims 1 to 9.

14. A computer-readable data carrier on which the computer program according to claim 13 is stored.

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