A METHOD OF CONTROLLING A ROTATABLE LOAD, A HYDRAULIC SYSTEM AND A WORKING MACHINE

Abstract

A method of controlling a rotatable load by a hydraulic system, the method includes determining if the hydraulic system is in a first operating state or a second operating state; controlling the rotatable load in a valve control mode if the hydraulic system is in the first operating state, the valve control mode including controlling the rotatable load by controlling a valve arrangement; controlling the rotatable load in a displacement control mode if the hydraulic system is in the second operating state, the displacement control mode including controlling the rotatable load mainly by controlling the displacement of a hydraulic machine; and applying a non-zero absolute minimum displacement limitation to the hydraulic machine at least in the valve control mode.

Claims

1. A method of controlling a rotatable load by a hydraulic system, the hydraulic system comprising a high-pressure side; a low-pressure side; a variable displacement hydraulic machine for rotationally driving the rotatable load; and a valve arrangement in fluid communication with the high-pressure side, the low-pressure side, a first side of the hydraulic machine, and a second side of the hydraulic machine; wherein the method comprises: determining if the hydraulic system is in a first operating state or a second operating state, the determination being based at least on one of a commanded or actual rotational speed of the hydraulic machine, a commanded or actual torque of the hydraulic machine, and/or a commanded or actual power of the hydraulic machine; controlling the rotatable load in a valve control mode if the hydraulic system is in the first operating state, the valve control mode comprising controlling the rotatable load by controlling the valve arrangement; controlling the rotatable load in a displacement control mode if the hydraulic system is in the second operating state, the displacement control mode comprising controlling the rotatable load mainly by controlling the displacement of the hydraulic machine; and applying a non-zero absolute minimum displacement limitation to the hydraulic machine at least in the valve control mode.

2. The method according to claim 1, wherein the displacement limitation in the valve control mode and/or a displacement of the hydraulic machine in the valve control mode, is at least 60%, such as at least 80%, such as 100%.

3. The method according to claim 1, further comprising: applying the displacement limitation to the hydraulic machine at least when a rotational speed of the hydraulic machine is below a first speed threshold value, when a power of the hydraulic machine is below a first power threshold value, and/or when a torque of the hydraulic machine is below a first torque threshold value; determining a required displacement of the hydraulic machine based on a command for the hydraulic machine and a pressure drop over the hydraulic machine; determining that the hydraulic system is in the first operating state, if the required displacement is smaller than the displacement limitation of the hydraulic machine; and determining that the hydraulic system is in the second operating state, if the required displacement is larger than the displacement limitation of the hydraulic machine.

4. The method according to claim 3, further comprising reducing or eliminating the displacement limitation of the hydraulic machine, or maintaining a constant displacement limitation of the hydraulic machine, when the rotational speed of the hydraulic machine is above a second speed threshold value, higher than or equal to the first speed threshold value, when the power of the hydraulic machine is above a second power threshold value, higher than or equal to the first power threshold value, and/or when the torque of the hydraulic machine is above a second torque threshold value, higher than or equal to the first torque threshold value.

5. The method according to claim 3, further comprising gradually reducing the displacement limitation of the hydraulic machine when the rotational speed of the hydraulic machine increases from the first speed threshold value, when the power of the hydraulic machine increases from the first power threshold value, and/or when the torque of the hydraulic machine increases from the first torque threshold value.

6. The method according to claim 3, further comprising determining the pressure drop over the hydraulic machine based on a pressure difference between the high-pressure side and the low-pressure side.

7. The method according to claim 1, wherein a pressure difference between the first side and the second side is smaller in the valve control mode than in the displacement control mode.

8. The method according to claim 1, further comprising switching a fluid communication between the high-pressure side and the first side to a fluid communication between the high-pressure side and the second side by means of the valve arrangement.

9. The method according to claim 1, wherein the rotatable load is entirely controlled by controlling the displacement of the hydraulic machine in the displacement control mode.

10. The method according to claim 1, further comprising controlling the displacement of the hydraulic machine by means of closed loop control in the displacement control mode.

11. The method according to claim 1, wherein the control of the rotatable load comprises controlling one of torque, rotational position, rotational speed, and rotational acceleration of the rotatable load.

12. The method according to claim 1, wherein the hydraulic machine is a two-quadrant hydraulic machine operative as a pump and as a motor.

13. A hydraulic system for controlling a rotatable load, the hydraulic system comprising: a high-pressure side; a low-pressure side; a variable displacement hydraulic machine for rotationally driving the rotatable load; and a valve arrangement in fluid communication with the high-pressure side, the low-pressure side, a first side of the hydraulic machine and a second side of the hydraulic machine; wherein the hydraulic system further comprises a control system configured to: determine if the hydraulic system is in a first operating state or a second operating state, the determination being based at least on one of a commanded or actual rotational speed of the hydraulic machine, a commanded or actual torque of the hydraulic machine, and/or a commanded or actual power of the hydraulic machine; control the rotatable load in a valve control mode if the hydraulic system is in the first operating state, the valve control mode comprising controlling the rotatable load by controlling the valve arrangement; control the rotatable load in a displacement control mode if the hydraulic system is in the second operating state, the displacement control mode comprising controlling the rotatable load mainly by controlling the displacement of the hydraulic machine; and applying a non-zero absolute minimum displacement limitation to the hydraulic machine at least in the first operating state.

14. The hydraulic system according to claim 13, wherein the displacement limitation in the first operating state and/or a displacement of the hydraulic machine in the first operating state, is at least 60%, such as at least 80%, such as 100%.

15. The hydraulic system according to claim 13, wherein the control system is configured to: apply the displacement limitation to the hydraulic machine at least when a rotational speed of the hydraulic machine is below a first speed threshold value, when a power of the hydraulic machine is below a first power threshold value and/or when a torque of the hydraulic machine is below a first torque threshold value; determine a required displacement of the hydraulic machine based on a command for the hydraulic machine and a pressure drop over the hydraulic machine; and determine that the hydraulic system is in the first operating state, if the required displacement is smaller than the displacement limitation of the hydraulic machine; and determine that the hydraulic system is in the second operating state, if the required displacement is larger than the displacement limitation of the hydraulic machine.

16. The hydraulic system according to claim 15, wherein the control system is configured to reduce or eliminate the displacement limitation of the hydraulic machine, or maintain a constant displacement limitation of the hydraulic machine, when the rotational speed of the hydraulic machine is above a second speed threshold value, higher than or equal to the first speed threshold value, when the power of the hydraulic machine is above a second power threshold value, higher than or equal to the first power threshold value, and/or when the torque of the hydraulic machine is above a second torque threshold value, higher than or equal to the first torque threshold value.

17. The hydraulic system according to claim 15, wherein the control system is configured to gradually reduce the displacement limitation of the hydraulic machine when the rotational speed of the hydraulic machine increases from the first speed threshold value, when the power of the hydraulic machine increases from the first power threshold value, and/or when the torque of the hydraulic machine increases from the first torque threshold value.

18. The hydraulic system according to claim 15, wherein the control system is configured to determine the pressure drop over the hydraulic machine based on a pressure difference between the high-pressure side and the low-pressure side.

19. The hydraulic system according to claim 13, wherein the control system is configured to control the valve arrangement to switch a fluid communication between the high-pressure side and the first side to a fluid communication between the high-pressure side and the second side.

20. The hydraulic system according to claim 13, wherein the control system is configured to control the rotatable load entirely by controlling the displacement of the hydraulic machine in the displacement control mode.

21-28. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0060] In the drawings:

[0061] FIG. 1 is a schematic illustration of a working machine according to the invention comprising a hydraulic system according to the invention,

[0062] FIG. 2 is a block diagram of the hydraulic system,

[0063] FIG. 3 is a partial enlarged view of the hydraulic system in FIG. 2,

[0064] FIG. 4 is a partial enlarged view of the hydraulic system in FIG. 2 showing one example of a valve arrangement,

[0065] FIG. 5 is a partial enlarged view of the hydraulic system in FIG. 2 showing a further example of a valve arrangement,

[0066] FIG. 6 is a flowchart outlining the general steps of the method according to the invention,

[0067] FIG. 7 is a diagram showing one example of a displacement limitation as a function of rotational speed of a hydraulic machine, and

[0068] FIG. 8 is a diagram showing a further illustrative example of a displacement limitation as a function of rotational speed of the hydraulic machine.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

[0069] In the following, a method of controlling a rotatable load by a hydraulic system, a hydraulic system for controlling a rotatable load, and a working machine, will be described. The same reference numerals will be used to denote the same or similar structural features.

[0070] FIG. 1 is a schematic illustration of a working machine 10 according to the invention. The working machine 10 comprises a hydraulic system 12 according to the invention. In FIG. 1, the working machine 10 is exemplified as an excavator.

[0071] The working machine 10 comprises an upper swing structure 14, a lower travel structure 16 and a working device 18. The working machine 10 further comprises a cab 20 in the upper swing structure 14, and a rotatable load 22 in the form of a swing motor between the upper swing structure 14 and the lower travel structure 16. The lower travel structure 16 comprises two rotatable loads 24 in the form of travel motors (only one is visible in FIG. 1) for driving a respective crawler track.

[0072] The working device 18 comprises a boom 26, an arm 28 and a bucket 30. The working device 18 further comprises two boom cylinders 32 (only one is visible in FIG. 1), an arm cylinder 34 and a bucket cylinder 36. The boom cylinders 32 operate between the upper swing structure 14 and the boom 26. The arm cylinder 34 operates between the boom 26 and the arm 28. The bucket cylinder 36 operates between the arm 28 and the bucket 30.

[0073] FIG. 2 is a block diagram of the hydraulic system 12 in FIG. 1 according to an embodiment of the invention. The hydraulic system 12 comprises a high-pressure side 38 and a low-pressure side 40.

[0074] In the example in FIG. 2, the high-pressure side 38 and the low-pressure side 40 are arranged in a common pressure rail (CPR) architecture. The high-pressure side 38 comprises a high-pressure rail and the low-pressure side 40 comprises a low-pressure rail. The high-pressure side 38 and the low-pressure side 40 may alternatively be referred to as a high-pressure circuit and a low-pressure circuit, respectively. The high-pressure side 38 and the low-pressure side 40 form a dual pressure system comprising two charging circuits at different pressure levels (the high-pressure side 38 and the low-pressure side 40). The hydraulic system 12 thus comprises a dedicated high-pressure side 38 and a dedicated low-pressure side 40. The dual pressure hydraulic system 12 differs from load sensing hydraulic systems where pressure is to a substantially larger extent adjusted depending on load, i.e. a resistive control.

[0075] During operation of the hydraulic system 12, the pressure in the high-pressure side 38 is higher than the pressure in the low-pressure side 40. These pressure levels may vary somewhat during operation of the hydraulic system 12 while the pressure in the high-pressure side 38 is higher than the pressure in the low-pressure side 40. The high pressure in the high-pressure side 38 may for example be 200-350 bars±10%, such as 250 bars±10%, during operation of the hydraulic system 12. The low pressure in the low-pressure side 40 may for example be 15-30 bars±10% during operation of the hydraulic system 12. The high pressure in the high-pressure side 38 may for example be 330 bars when the boom 26 is in a low position and 200 bars when the boom 26 is in a high position.

[0076] The hydraulic system 12 further comprises a high-pressure hydraulic energy storage 42 and a low-pressure hydraulic energy storage 44. The high-pressure hydraulic energy storage 42 is connected to the high-pressure side 38 and the low-pressure hydraulic energy storage 44 is connected to the low-pressure side 40. In FIG. 2, each of the high-pressure hydraulic energy storage 42 and the low-pressure hydraulic energy storage 44 is exemplified as an accumulator. The high-pressure hydraulic energy storage 42 can store/release hydraulic energy from/to the high-pressure side 38. The low-pressure hydraulic energy storage 44 can store/release hydraulic energy from/to the low-pressure side 40. The high-pressure hydraulic energy storage 42 requires a higher energy storage capacity in case the pressure variation in the high-pressure side 38 is low and vice versa. The same applies for the low-pressure hydraulic energy storage 44 with respect to the low-pressure side 40.

[0077] The hydraulic system 12 further comprises a main pump 46. In FIG. 2, the main pump 46 is connected to the high-pressure side 38 and the low-pressure side 40. The main pump 46 is arranged to pressurize the high-pressure side 38. The main pump 46 is here exemplified as a variable displacement hydraulic machine operative as both pump and motor.

[0078] The hydraulic system 12 further comprises an auxiliary pump 48. In the example in FIG. 2, the auxiliary pump 48 is arranged to supply pressurized fluid from a tank 50 to the high-pressure side 38. The auxiliary pump 48 of this example is a fixed displacement pump. The main pump 46 and the auxiliary pump 48 are connected to a common drive shaft driven by an internal combustion engine 52 of the working machine 10.

[0079] The hydraulic system 12 of this example further comprises a pressure relief valve 54 connected between the low-pressure side 40 and the tank 50. The hydraulic system 12 further comprises a fan motor 56, and a fan 58 arranged to be driven by the fan motor 56.

[0080] The hydraulic system 12 further comprises three variable displacement hydraulic machines 60, 62. The hydraulic machine 60 is arranged to rotationally drive the rotatable load 22 and each of the two hydraulic machines 62 is arranged to rotationally drive a respective rotatable load 24. In this example, each hydraulic machine 60, 62 is a two-quadrant hydraulic machine operative as a pump and a motor, i.e. an irreversible variable displacement hydraulic machine. Each hydraulic machine 60, 62 has a displacement variable between 0% and 100%. The hydraulic system 12 needs to be able to control the speed of each hydraulic machine 60, 62 while the pressure in the high-pressure side 38 is kept within a high pressure interval.

[0081] The hydraulic system 12 further comprises three gearboxes 64. Each gearbox 64 is arranged between a hydraulic machine 60, 62 and a rotatable load 22, 24, and is driven by a drive shaft 66 of a respective hydraulic machine 60, 62.

[0082] The hydraulic system 12 further comprises three valve arrangements 68. Each valve arrangement 68 is associated with one of the rotatable loads 22, 24. Each valve arrangement 68 is in fluid communication with the high-pressure side 38, the low-pressure side 40, a first port or first side 70 of an associated hydraulic machine 60, 62 and a second port or second side 72 of an associated hydraulic machine 60, 62.

[0083] Furthermore, each valve arrangement 68 is configured to selectively establish a fluid communication between the high-pressure side 38 and the first side 70, selectively establish a fluid communication between the high-pressure side 38 and the second side 72, selectively establish a fluid communication between the low-pressure side 40 and the first side 70, and selectively establish a fluid communication between the low-pressure side 40 and the second side 72. Thus, each valve arrangement 68 is configured to switch a fluid communication between the high-pressure side 38 and the first side 70 to a fluid communication between the high-pressure side 38 and the second side 72.

[0084] The hydraulic system 12 further comprises a control system 74. The control system 74 comprises a data processing device and a memory having a computer program stored thereon, the computer program comprising program code which, when executed by the data processing device causes the data processing device to perform various steps, or command execution of various steps, as described herein.

[0085] FIG. 3 is a partial enlarged view of the hydraulic system 12 in FIG. 2. In connection with FIG. 3, a method of controlling the rotatable load 22 by the hydraulic system 12 will be described. Each of the rotatable loads 24 can also be controlled in the same way.

[0086] As illustrated in FIG. 3, the control system 74 is arranged to receive a reference signal or command 76 for the hydraulic machine 60, a signal indicative of a rotational speed of the hydraulic machine 60, and signals indicative of the pressures in the high-pressure side 38, the low-pressure side 40, the first side 70 and the second side 72. The control system 74 is further configured to control the valve arrangement 68, and to control the displacement of the hydraulic machine 60 by means of a displacement controller 78.

[0087] The method of this example comprises applying a non-zero absolute minimum displacement limitation to the hydraulic machine 60 when the rotational speed of the hydraulic machine 60 is below a first speed threshold value. Since the hydraulic machine 60 has a displacement variable between 0% and 100%, a non-zero absolute minimum displacement limitation of for example 10% is applied. When the displacement limitation is applied to the hydraulic machine 60, the displacement can thus vary between 10% and 100%.

[0088] In this example, the displacement limitation is applied by means of software in the control system 74. To this end, it is determined whether a rotational speed of the hydraulic machine 60 is below the first speed threshold value. The first speed threshold value may be either constant, for example 200 rpm, or variable. A variable first speed threshold value may depend on operating conditions of the working machine 10. However, a hardware displacement limitation on the hydraulic machine 60 may alternatively be employed.

[0089] The displacement limitation may or may not also be applied at rotational speeds above the first speed threshold value. Naturally, a hardware displacement limitation on the hydraulic machine 60 will be present at all rotational speeds. In case a software limitation is employed, the displacement limitation may be reduced, eliminated or maintained constant above a second speed threshold value, higher than or equal to the first speed threshold value. Between the first speed threshold value and the second speed threshold value, the displacement limitation may be gradually reduced. Also the second speed threshold value may be either constant, for example 500 rpm, or variable. Also a variable second speed threshold value may depend on operating conditions of the working machine 10. Various types of control of the displacement limitation is possible as long as a displacement limitation, by means of hardware or software, is applied to the hydraulic machine 60 at low power operations, low torque operations and/or low speed operations, e.g. at rotational speeds below the first speed threshold value.

[0090] The method of this example further comprises determining a required displacement of the hydraulic machine 60 based on the command 76 for the hydraulic machine 60 and a pressure drop over the hydraulic machine 60. The command 76 may for example be a torque command in a feedback loop of the control system 74. Alternatively, the command 76 may be a rotational speed command or a rotational acceleration command to the hydraulic machine 60. In the example in FIG. 3, pressure sensors are provided for sensing the pressures at the first side 70 and the second side 72 of the hydraulic machine 60. Thereby, the pressure drop over the hydraulic machine 60 can be directly calculated based on these pressures. However, the pressure drop over the hydraulic machine 60 can be calculated or determined in alternative ways. For example, the pressure drop over the hydraulic machine 60 can be estimated based on target pressures at the first side 70 and the second side 72. These target pressures may in turn be calculated based on the high pressure in the high-pressure side 38, the low pressure in the low-pressure side 40, and valve openings of one or more valves in the valve arrangement 68. As a further example, the required displacement can be calculated by approximating the pressure drop over the hydraulic machine 60 as the difference between the high pressure in the high-pressure side 38 and the low pressure in the low-pressure side 40. Thus, a desired or possible maximum pressure drop over the hydraulic machine 60 may be used when calculating the required displacement.

[0091] The method of this example further comprises comparing the required displacement with the displacement limitation of the hydraulic machine 60. If the required displacement is smaller than the displacement limitation, e.g. lower than 10%, it is determined that the hydraulic system 12 is in a first operating state. In the first operating state, the rotatable load 22 is controlled by controlling the valve arrangement 68, i.e. by means of a valve control mode as described herein. If the required displacement is larger than the displacement limitation, e.g. higher than 10%, it is determined that the hydraulic system 12 is in a second operating state. In the second operating state, the rotatable load 22 is controlled by mainly controlling the displacement of the hydraulic machine 60, i.e. by means of a displacement control mode as described herein.

[0092] The control system 74 is configured to continuously or repeatedly determine a required displacement of the hydraulic machine 60 and to continuously or repeatedly compare the required displacement with the current minimum displacement limitation. The control of the hydraulic machine 60 will thus switch between the valve control mode and the displacement control mode during various operation cycles of the working machine 10.

[0093] In the valve control mode, a control of the flow through the valve arrangement 68 is involved to control the rotatable load 22. However, a control of the displacement of the hydraulic machine 60 may also be involved to control the rotatable load 22 in the valve control mode. In the valve control mode, the pressure drop over the hydraulic machine 60 may be less than 50% of the pressure difference between the high-pressure side 38 and the low-pressure side 40. Thus, a major part of the pressure drop will occur over the valve arrangement 68 in the valve control mode.

[0094] In the displacement control mode, the rotatable load 22 is mainly controlled by controlling the displacement of the hydraulic machine 60. For example, in case the pressure drop over the hydraulic machine 60 constitutes at least 80% of the pressure difference between the high-pressure side 38 and the low-pressure side 40, the rotatable load 22 may be said to be mainly controlled by controlling the displacement of the hydraulic machine 60. According to one example, the rotatable load 22 is entirely controlled by controlling the displacement of the hydraulic machine 60 in the displacement control mode, e.g. an opening area of each valve of the valve arrangement 68 is kept stationary, preferably as large as possible to reduce pressure drop over the valve arrangement 68. However, a control of the flow through the valve arrangement 68 may optionally also be involved to control the rotatable load 22 in the displacement control mode. In any case, a pressure difference between the pressure at the first side 70 and the pressure at the second side 72 is higher in the displacement control mode than in the valve control mode, at least at most times, e.g. except for short times after switching a fluid communication between the high-pressure side 38 and the first side 70 to a fluid communication between the high-pressure side 38 and the second side 72.

[0095] FIG. 4 is a partial enlarged view of the hydraulic system 12 in FIG. 2 showing one example of a valve arrangement 68. The valve arrangement 68 of this example comprises four electro-hydraulic proportional valves 80a-80d. Each valve 80a-80d comprises a plurality of different positions establishing a plurality of different opening areas. The first valve 80a is arranged between the high-pressure side 38 and the first side 70. The second valve 80b is arranged between the high-pressure side 38 and the second side 72. The third valve 80c is arranged between the low-pressure side 40 and the first side 70. The fourth valve 80d is arranged between the low-pressure side 40 and the second side 72. The valve arrangement 68 of the example in FIG. 4 further comprises a PWM (Pulse Width Modulation) controller 82 for controlling the valves 80a-80d based on commands from the control system 74.

[0096] In the valve control mode, the second valve 80b and the third valve 80c may be closed, and the opening area of the first valve 80a and the opening area of the fourth valve 80d may be controlled in order to control rotation of the rotatable load 22 in a first direction. In this case, a pressure difference between the high-pressure side 38 and the low-pressure side 40, minus the pressure drop over the first valve 80a, minus the pressure drop over the fourth valve 80d, may constitute the pressure drop over the hydraulic machine 60.

[0097] Furthermore, in the valve control mode, the first valve 80a and the fourth valve 80d may be closed, and the opening area of the second valve 80b and the opening area of the third valve 80c may be controlled in order to control rotation of the rotatable load 22 in a second direction, opposite to the first direction. In this case, a pressure difference between the high-pressure side 38 and the low-pressure side 40, minus the pressure drop over the second valve 80b, minus the pressure drop over the third valve 80c, may constitute the pressure drop over the hydraulic machine 60.

[0098] In the displacement control mode, the second valve 80b and the third valve 80c may be closed, and the opening area of the first valve 80a and the opening area of the fourth valve 80d may be set and maintained as large as possible while rotation of the rotatable load 22 in the first direction is controlled by controlling the displacement of the hydraulic machine 60. Thereby, a maximum pressure drop over the hydraulic machine 60 is provided. Furthermore, in the displacement control mode, the first valve 80a and the fourth valve 80d may be closed, and the opening area of the second valve 80b and the opening area of the third valve 80c may be set and maintained as large as possible while rotation of the rotatable load 22 in the second direction is controlled by controlling the displacement of the hydraulic machine 60.

[0099] FIG. 5 is a partial enlarged view of the hydraulic system 12 in FIG. 2 showing a further example of the valve arrangement 68. The valve arrangement 68 comprises a valve 84. The valve 84 of this example is a 4/3 spool valve. Similar to the valve arrangement 68 in FIG. 4, also the valve arrangement 68 in FIG. 5 is configured to establish a fluid communication between the high-pressure side 38 and the first side 70 to rotationally drive the rotatable load 22 in the first direction, and to establish a fluid communication between the high-pressure side 38 and the second side 72 to rotationally drive the rotatable load 22 in the second direction.

[0100] FIG. 6 is a flowchart outlining the general steps of the method according to the invention. The method comprises determining 51 if the hydraulic system 12 is in a first operating state or a second operating state, the determination being based at least on one of a commanded or actual rotational speed 88 of the hydraulic machine 60, 62, a commanded or actual torque of the hydraulic machine 60, 62, and/or a commanded or actual power of the hydraulic machine 60, 62; controlling S2 the rotatable load 22, 24 in a valve control mode if the hydraulic system 12 is in the first operating state, the valve control mode comprising controlling the rotatable load 22, 24 by controlling the valve arrangement 68; controlling S3 the rotatable load 22, 24 in a displacement control mode if the hydraulic system 12 is in the second operating state, the displacement control mode comprising controlling the rotatable load 22, 24 mainly by controlling the displacement 90 of the hydraulic machine 60, 62; and applying S4 a non-zero absolute minimum displacement limitation 86 to the hydraulic machine 60, 62 at least in the first operating state.

[0101] FIG. 7 is a diagram showing one illustrative example of the displacement limitation 86 as a function of the rotational speed 88 of the hydraulic machine 60. The rotational speed 88 may for example be a measured rotational speed 88. In the diagram, the abscissa shows the rotational speed 88 and the ordinate shows the displacement 90. In the example in FIG. 7, the first speed threshold value 92 is set to 200 rpm and the second speed threshold value 94 is set to 500 rpm. At rotational speeds 88 below the first speed threshold value 92, the displacement limitation 86 is set to 10%. At rotational speeds 88 above the second speed threshold value 94, the displacement limitation 86 is set to 2%.

[0102] The displacement limitation 86 linearly decreases from 10% to 2% as the rotational speed 88 increases from the first speed threshold value 92 to the second speed threshold value 94.

[0103] FIG. 7 further shows three examples of determined required displacements 96 for the hydraulic machine 60. In operation point A at a rotational speed 88 of 100 rpm, the required displacement 96 is 5%, i.e. smaller than the displacement limitation 86 of 10% at a rotational speed 88 of 100 rpm. Thereby, in operation point A, it is determined that the hydraulic system 12 is in the first operating state and the valve control mode of the hydraulic machine 60 is consequently selected.

[0104] In operation point B at a rotational speed 88 of 100 rpm, the required displacement 96 is 15%, i.e. larger than the displacement limitation 86 of 10% at a rotational speed 88 of 100 rpm. Thereby, in operation point B, it is determined that the hydraulic system 12 is in the second operating state and the displacement control mode of the hydraulic machine 60 is consequently selected.

[0105] In operation point C at a rotational speed 88 of 600 rpm, the required displacement 96 is 5%, i.e. larger than the displacement limitation 86 of 2% at a rotational speed 88 of 600 rpm. Thereby, in operation point C, it is determined that the hydraulic system 12 is in the second operating state and the displacement control mode of the hydraulic machine 60 is selected.

[0106] FIG. 8 is a diagram showing a further illustrative example of the displacement limitation 86 as a function of rotational speed 88 of the hydraulic machine 60 according to an alternative embodiment. Mainly differences will be described. In FIG. 8, a speed threshold value 94 is used as a basis for determining whether the hydraulic system 12 is in the first operating state or the second operating state. The speed threshold value 94 may be constituted by a second speed threshold value according to the present disclosure. In this case, the first speed threshold value and the second speed threshold value may for example adopt the same value. When the rotational speed 88 is lower than the speed threshold value 94, it is determined that the hydraulic system 12 is in the first operating state and the valve control mode is consequently adopted by the hydraulic system 12. When the rotational speed 88 is higher than the speed threshold value 94, it is determined that the hydraulic system 12 is in the second operating state and the displacement control mode is consequently adopted by the hydraulic system 12. Thus, in this embodiment, it is not necessary to determine a required displacement of the hydraulic machine 60 in order to decide whether the hydraulic system operates in the first operating state or the second operating state.

[0107] Furthermore, in the example in FIG. 8, a displacement limitation 86 of 100% is set in the valve control mode, i.e. at rotational speeds below the speed threshold value 94. In the displacement control mode, i.e. above the speed threshold value 94, the displacement limitation 86 is removed.

[0108] It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.