A METHOD OF CONTROLLING A HYDRAULIC ACTUATOR, A HYDRAULIC ACTUATOR, A HYDRAULIC SYSTEM AND A WORKING MACHINE

Abstract

A method of controlling a hydraulic actuator, wherein the hydraulic actuator includes a linear double-acting output member, and at least three working chambers in fluid connection with the output member, the working chambers having respective effective areas with a non-binary relationship; wherein the method includes selectively fluidly connecting each working chamber to either a high-pressure side or a low-pressure side to provide a plurality of discrete pressurization states of the hydraulic actuator; determining at least one of the pressurization states as a prevented pressurization state; and transitioning between a plurality of allowed pressurization states among the pressurization states while preventing transition to the at least one prevented pressurization state. A hydraulic actuator and a hydraulic system are also provided.

Claims

1. A method of controlling a hydraulic actuator, wherein the hydraulic actuator comprises: a linear double-acting output member, and at least three working chambers in fluid connection with the output member, the working chambers having respective effective areas with a non-binary relationship; wherein the method comprises: selectively fluidly connecting each working chamber to either a high-pressure side or a low-pressure side to provide a plurality of discrete pressurization states of the hydraulic actuator; characterized in that the method further comprises determining at least one of the pressurization states as a prevented pressurization state; and transitioning between a plurality of allowed pressurization states among the pressurization states while preventing transition to the at least one prevented pressurization state.

2. The method according to claim 1, wherein the method is carried out in a hydraulic system comprising: a high-pressure side, a low-pressure side, and a valve arrangement arranged to selectively fluidly connect each working chamber to either the high-pressure side or the low-pressure side to provide the plurality of discrete pressurization states of the hydraulic actuator.

3. The method according to claim 1, wherein at least two of the working chambers have respective effective areas with a substantially binary relationship.

4-5. (canceled)

6. The method according to claim 1, wherein the hydraulic actuator comprises at least four working chambers, and wherein the fourth smallest effective area is 2.75-3.75 times the second smallest effective area.

7. The method according to claim 1, wherein the hydraulic actuator comprises at least four working chambers, and wherein the fourth smallest effective area is 6-7.5 times the smallest effective area.

8. The method according to claim 1, wherein the pressurization states are put in order based on a respective force output of the output member in each pressurization state, and wherein the method further comprises switching less than all working chambers between the high-pressure side and the low-pressure side when transitioning from each allowed pressurization state to an immediately adjacent allowed pressurization state.

9. The method according to claim 1, wherein the pressurization states are put in order based on a respective force output of the output member in each pressurization state, and wherein the method further comprises transitioning between two of the allowed pressurization states by skipping one or more of the at least one prevented pressurization state.

10. The method according to claim 1, further comprising determining one or more of the at least one prevented pressurization state in dependence of a currently adopted pressurization state.

11-14. (canceled)

15. The method according to claim 1, wherein the pressurization states are put in order based on a respective force output of the output member in each pressurization state, and wherein for constant pressures in the high-pressure side and the low-pressure side, a difference between a force output of the output member in one of the allowed pressurization states and a force output of the output member in one of the prevented pressurization states, is smaller than a difference between force outputs of the output member in two immediately adjacent allowed pressurization states.

16. The method according to claim 1, wherein for constant pressures in the high-pressure side and the low-pressure side, a force output of the output member in one of the allowed pressurization states and a force output of the output member in one of the prevented pressurization states are substantially the same.

17. (canceled)

18. A hydraulic actuator comprising: a linear double-acting output member; and at least three working chambers in fluid connection with the output member, the working chambers having respective effective areas with a non-binary relationship; characterized in that at least two of the working chambers have respective effective areas with a substantially binary relationship.

19. The hydraulic actuator according to claim 18, wherein the hydraulic actuator comprises at least four working chambers, and wherein the fourth smallest effective area is 2.75-3.75 times the second smallest effective area.

20. The hydraulic actuator according to claim 18, wherein the hydraulic actuator comprises at least four working chambers, and wherein the fourth smallest effective area is 6-7.5 times the smallest effective area.

21. The hydraulic actuator according to claim 18, wherein the hydraulic actuator comprises at least four working chambers having respective effective areas with a non-binary relationship; and wherein two of the working chambers have respective effective areas with a substantially binary relationship.

22. The hydraulic actuator according to claim 18, wherein the hydraulic actuator comprises at least four working chambers, and wherein at least three of the working chambers have respective effective areas with a substantially binary relationship.

23. (canceled)

24. A hydraulic system comprising: a hydraulic actuator having a linear double-acting output member, and at least three working chambers in connection with the output member, the working chamber have respective effective areas with a non-binary relationship; a high-pressure side; a low-pressure side; a valve arrangement arranged to selectively fluidly connect each working chamber to either the high-pressure side or the low-pressure side to provide a plurality of discrete pressurization states of the hydraulic actuator; and a control system configured to control the hydraulic actuator by controlling the valve arrangement; characterized in that the control system is configured to: determine at least one of the pressurization states as a prevented pressurization state; and control the hydraulic actuator to transition between a plurality of allowed pressurization states among the pressurization states while preventing transitioning to the at least one prevented pressurization state.

25. The hydraulic system according to claim 24, wherein at least two of the working chambers have respective effective areas with a substantially binary relationship.

26. The hydraulic system according to claim 24, wherein the hydraulic actuator comprises at least four working chambers having respective effective areas with a non-binary relationship; and wherein two of the working chambers have respective effective areas with a substantially binary relationship.

27. The hydraulic system according to claim 24, wherein the hydraulic actuator comprises at least four working chambers, and wherein at least three of the working chambers have respective effective areas with a substantially binary relationship.

28-29. (canceled)

30. A working machine comprising a hydraulic actuator according to claim 18 and/or a hydraulic system comprising: a hydraulic actuator having a linear double-acting output member, and at least three working chambers in connection with the output member, the working chamber have respective effective areas with a non-binary relationship; a high-pressure side; a low-pressure side; a valve arrangement arranged to selectively fluidly connect each working chamber to either the high-pressure side or the low-pressure side to provide a plurality of discrete pressurization states of the hydraulic actuator; and a control system configured to control the hydraulic actuator by controlling the valve arrangement; characterized in that the control system is configured to: determine at least one of the pressurization states as a prevented pressurization state; and control the hydraulic actuator to transition between a plurality of allowed pressurization states among the pressurization states while preventing transitioning to the at least one prevented pressurization state.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0055] In the drawings:

[0056] FIG. 1 is a schematic illustration of a working machine according to the invention comprising a hydraulic system,

[0057] FIG. 2 is a block diagram of the hydraulic system in FIG. 1,

[0058] FIG. 3a is a partial block diagram of the hydraulic system showing a hydraulic actuator and a valve arrangement,

[0059] FIG. 3b is a diagram showing a force output of an output member for a plurality of pressurization states of the hydraulic actuator in FIG. 3a,

[0060] FIG. 4a is a partial block diagram of the hydraulic system showing a further example of hydraulic actuator and a valve arrangement,

[0061] FIG. 4b is a diagram showing a force output of an output member for a plurality of pressurization states of the hydraulic actuator in FIG. 4a, and

[0062] FIG. 5 is a flowchart outlining the general steps of the method according to the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

[0063] In the following, a method of controlling a hydraulic actuator, a hydraulic actuator, a hydraulic system comprising a hydraulic actuator and a working machine comprising a hydraulic actuator and/or a hydraulic system, will be described. The same or similar reference numerals will be used to denote the same or similar structural features.

[0064] FIG. 1 is a schematic illustration of a working machine 18 according to the invention. The working machine 18 comprises a hydraulic system 20 according to the invention. The working machine 18 is exemplified as an excavator. The working machine 18 comprises an upper swing structure 22, a lower travel structure 24 and a working device 26. The working machine 18 further comprises a cab 28 in the upper swing structure 22, and a swing motor 30 between the upper swing structure 22 and the lower travel structure 24. The lower travel structure 24 comprises two travel motors 32 (only one is visible in FIG. 1) for driving a respective crawler track.

[0065] The working device 26 comprises a boom 34, an arm 36 and a bucket 38. The working device 26 further comprises two hydraulic actuators 40 (only one is visible in FIG. 1) exemplified as boom cylinders, a hydraulic actuator 42 exemplified as an arm cylinder and a hydraulic actuator 44 exemplified as a bucket cylinder. The hydraulic actuators 40 operate between the upper swing structure 22 and the boom 34 by means of a linear double-acting output member 46. The hydraulic actuator 42 operates between the boom 34 and the arm 36 by means of a linear double-acting output member 46. The hydraulic actuator 44 operates between the arm 36 and the bucket 38 by means of a linear double-acting output member 46. In this example, each output member 46 is a piston rod.

[0066] FIG. 2 is a block diagram of the hydraulic system 20 in FIG. 1 according to an embodiment of the invention. The hydraulic system 20 comprises a high-pressure side 48 and a low-pressure side 50. In the example in FIG. 2, the high-pressure side 48 and the low-pressure side 50 are arranged in a common pressure rail (CPR) architecture. The high-pressure side 48 comprises a high-pressure rail and the low-pressure side 50 comprises a low-pressure rail. The high-pressure side 48 and the low-pressure side 50 may alternatively be referred to as a high-pressure circuit and a low-pressure circuit, respectively. The high-pressure side 48 and the low-pressure side 50 form a dual pressure system comprising two charging circuits at different pressure levels (the high-pressure side 48 and the low-pressure side 50). The hydraulic system 20 thus comprises a dedicated high-pressure side 48 and a dedicated low-pressure side 50. The dual pressure hydraulic system 20 differs from load sensing hydraulic systems where pressure is to a substantially larger extent adjusted depending on load, i.e. a resistive control.

[0067] During operation of the hydraulic system 20, the pressure in the high-pressure side 48 is higher than the pressure in the low-pressure side 50. These pressure levels may vary somewhat during operation of the hydraulic system 20 while the pressure in the high-pressure side 48 is higher than the pressure in the low-pressure side 50. The high pressure in the high-pressure side 48 may for example be 200-350 bars±10%, such as 250 bars±10%, during operation of the hydraulic system 20. The low pressure in the low-pressure side 50 may for example be 15-30 bars±10% during operation of the hydraulic system 20. The high pressure in the high-pressure side 48 may for example be 330 bars when the hydraulic actuators 40 are in a low position and 200 bars when the hydraulic actuators 40 are in a high position.

[0068] The hydraulic system 20 further comprises a high-pressure hydraulic energy storage 52 and a low-pressure hydraulic energy storage 54. The high-pressure hydraulic energy storage 52 is connected to the high-pressure side 48 and the low-pressure hydraulic energy storage 54 is connected to the low-pressure side 50. In FIG. 2, each of the high-pressure hydraulic energy storage 52 and the low-pressure hydraulic energy storage 54 is exemplified as an accumulator. The high-pressure hydraulic energy storage 52 can store/release hydraulic energy from/to the high-pressure side 48. The low-pressure hydraulic energy storage 54 can store/release hydraulic energy from/to the low-pressure side 50. The high-pressure hydraulic energy storage 52 requires a higher energy storage capacity in case the pressure variation in the high-pressure side 48 is low and vice versa. The same applies for the low-pressure hydraulic energy storage 54 with respect to the low-pressure side 50.

[0069] The hydraulic system 20 further comprises a main pump 56. In FIG. 2, the main pump 56 is connected to the high-pressure side 48 and the low-pressure side 50. The main pump 56 is arranged to pressurize the high-pressure side 48. The main pump 56 is here exemplified as a variable displacement hydraulic machine operative as both pump and motor.

[0070] The hydraulic system 20 further comprises an auxiliary pump 58. In the example in FIG. 2, the auxiliary pump 58 is arranged to supply pressurized fluid from a tank 60 to the high-pressure side 48. The auxiliary pump 58 of this example is a fixed displacement pump. The main pump 56 and the auxiliary pump 58 are connected to a common drive shaft driven by an internal combustion engine 62 of the working machine 18.

[0071] The hydraulic system 20 of this example further comprises a pressure relief valve 64 connected between the low-pressure side 50 and the tank 60. The hydraulic system 20 further comprises a fan motor 66, and a fan 68 arranged to be driven by the fan motor 66.

[0072] The hydraulic system 20 further comprises three variable displacement hydraulic machines 70, 72. The hydraulic machine 70 is arranged to rotationally drive the swing motor 30 and each of the two hydraulic machines 72 is arranged to rotationally drive a respective travel motor 32.

[0073] The hydraulic system 20 further comprises three gearboxes 74. One gearbox 74 is arranged between a hydraulic machine 70 and the swing motor 30, and one gearbox 74 is arranged between each hydraulic machine 72 and a respective travel motor 32. Each gearbox 74 is driven by a drive shaft 76 of a respective hydraulic machine 70, 72.

[0074] The hydraulic system 20 further comprises a plurality of valve arrangements 78, 80. Each valve arrangement 78 is associated with one of the hydraulic actuators 40, 42, 36. One valve arrangement 80 is associated with the swing motor 30 and one valve arrangement 80 is associated with each travel motor 32. Each valve arrangement 78, 80 is in fluid communication with the high-pressure side 48 and the low-pressure side 50.

[0075] The hydraulic system 20 further comprises a control system 82. The control system 82 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.

[0076] FIG. 3a is a partial block diagram of the hydraulic system 20 showing one of the hydraulic actuators 40 and the associated valve arrangement 78. One, several or each of the hydraulic actuators 40, 42 may be of the same design as in FIG. 3a. The hydraulic actuator 40 comprises four variable volume working chambers 84-A, 84-B, 84-C, 84-D, i.e. a first working chamber or “A-chamber” 84-A, a second working chamber or “B-chamber” 84-B, a third working chamber or “C-chamber” 84-C and a fourth working chamber or “D-chamber” 84-D. Each working chamber 84-A, 84-B, 84-C, 84-D may also be referred to with reference numeral “84”.

[0077] The working chambers 84 in FIGS. 3a and 3b have respective effective areas with a non-binary relationship of 6.5:4:2:1. Thus, the first working chamber 84-A has an effective area that is 6.5 times the effective area of the fourth working chamber 84-D, the second working chamber 84-B has an effective area that is four times the effective area of the fourth working chamber 84-D, and the third working chamber 84-C has an effective area that is two times the effective area of the fourth working chamber 84-D. Although the four working chambers 84 have effective areas with a non-binary relationship, the second working chamber 84-B, the third working chamber 84-C and the fourth working chamber 84-D have a binary relationship of 4:2:1.

[0078] The valve arrangement 78 is configured to selectively fluidly connect each working chamber 84 to either the high-pressure side 48 or the low-pressure side 50. Thereby, the hydraulic actuator 40 can adopt 16 discrete pressurization states. The valve arrangement 78 of this example comprises eight proportional valves 86-A, 86-B, 86-C, 86-D, 88-A, 88-B, 88-C, 88-D. Each valve 86-A, 86-B, 86-C, 86-D may also be referred to with reference numeral “86” and each valve 88-A, 88-B, 88-C, 88-D may also be referred to with reference numeral “88”.

[0079] The valve 86-A is provided between the high-pressure side 48 and the first working chamber 84-A, the valve 88-A is provided between the low-pressure side 50 and the first working chamber 84-A, the valve 86-B is provided between the high-pressure side 48 and the second working chamber 84-B, the valve 88-B is provided between the low-pressure side 50 and the second working chamber 84-B, the valve 86-C is provided between the high-pressure side 48 and the third working chamber 84-C, the valve 88-C is provided between the low-pressure side 50 and the third working chamber 84-A, the valve 86-D is provided between the high-pressure side 48 and the fourth working chamber 84-D, and the valve 88-D is provided between the low-pressure side 50 and the fourth working chamber 84-D. Although the proportional valves 86, 88 provide 16 discrete pressurization states in the hydraulic actuator 40, hydraulic fluid may be throttled into or out from each working chamber 84 by means of an associated valve 86, 88 to alter the discrete force output. The throttling however generates losses.

[0080] Transition losses occurs when switching a connection to any of the working chambers 84 between the high-pressure side 48 and the low-pressure side 50. The highest transition losses occur when transitioning the first working chamber 84-A between the high-pressure side 48 and the low-pressure side 50 since the first working chamber 84-A has the largest effective area.

[0081] FIG. 3b is a diagram showing a force output 90 (in kN) of the output member 46 for a plurality of pressurization states 1-16 of the hydraulic actuator 40 in FIG. 3a. The pressurization states 1-16 are also referred to with reference numeral “92”. The two hydraulic actuators 40 of the working machine 18 may have an offset in force to increase the force resolution from 16 discrete force levels to 32 discrete force levels.

[0082] When the first working chamber 84-A is fluidly connected to the high-pressure side 48, the pressure within the first working chamber 84-A generates a force on the output member 46 in an extending direction (to the right in FIG. 3a). When the second working chamber 84-B is fluidly connected to the high-pressure side 48, the pressure within the second working chamber 84-B generates a force on the output member 46 in a retracting direction (to the left in FIG. 3a). When the third working chamber 84-C is fluidly connected to the high-pressure side 48, the pressure within the third working chamber 84-C generates a force on the output member 46 in the extending direction. When the fourth working chamber 84-D is fluidly connected to the high-pressure side 48, the pressure within the fourth working chamber 84-D generates a force on the output member 46 in the retracting direction.

[0083] As shown in FIG. 3b, the pressurization states 1-16 are put in order based on the corresponding force output 90. Pressurization state 1 provides the lowest force output 90 (a negative force output 90) and pressurization state 16 provides the highest force output 90 etc.

[0084] A negative force output 90 may for example be required for “return to dig”, i.e. in order to rapidly accelerate the boom 34 downwards with an empty bucket 38. If the force that the hydraulic actuator 40 can produce while maintaining the first working chamber 84-A connected to the high-pressure side 48 is not low enough, the first working chamber 84-A may need to be switched to the low-pressure side 50.

[0085] With the non-binary area relationship of 6.5:4:2:1 of the working chambers 84, the step size (i.e. the difference in force output 90) between pressurization states 7-8, 8-9, and 9-10 is half of the step size between pressurization states 1-2, 2-3, 3-4, 4-5, 5-6, 6-7, 10-11, 11-12, 12-13, 13-14, 14-15 and 15-16. Furthermore, in comparison with a binary coded hydraulic actuator, the hydraulic actuator 40 in FIGS. 3a and 3b can produce lower force outputs 90 while maintaining the first working chamber 84-A connected to the high-pressure side 48. Thereby, energy recovery by means of the high-pressure hydraulic energy storage 52 can be improved.

[0086] In the method of controlling the hydraulic actuator 40, at least one of the pressurization states 92 is determined as a prevented pressurization state to which the hydraulic actuator 40 is prevented from transitioning. Each of the remaining pressurization states 92 is an allowed pressurization state to which the hydraulic actuator 40 is allowed to transition. In order to provide a target force output 90 in the output member 46, the hydraulic actuator 40 may be controlled to transition to the allowed pressurization state associated with a force output 90 that most closely matches the target force output 90, while preventing transition to any of the at least one prevented pressurization state.

[0087] The target force output 90 may for example be calculated base on target position, target speed and/or target acceleration of the output member 46, e.g. by means of the control system 82. The control system 82 may control the valve arrangement 78 to switch pressurization of at least one of the working chambers 84 in order to effect a transition of the hydraulic actuator 40 between two pressurization states 92. The control system 82 may also contain various logic functions for determining which pressurization state 92 that is/are currently prevented, e.g. given certain operating conditions of the hydraulic actuator 40 and/or of the working machine 18, a certain operating pattern and/or a posture of the working machine 18.

[0088] To accelerate the boom 34 (see FIG. 1) downwards, the force output 90 of the output member 46 is reduced. If the force output 90 that the hydraulic actuator 40 can produce while maintaining the first working chamber 84-A in fluid connection with the high-pressure side 48 is not low enough for the requested acceleration, the first working chamber 84-A will transition from the high-pressure side 48 to the low-pressure side 50. A further transition of the first working chamber 84-A may occur when a target velocity is reached and the acceleration becomes close to zero.

[0089] According to one example, pressurization state 9 is prevented when the hydraulic actuator 40 adopts any of pressurization states 8 or 10-16, i.e. when the first working chamber 84-A is connected to the high-pressure side 48. If the control system 82 determines that the force output 90 of pressurization state 9 would be most suitable, the control system 82 may instead command a transition to pressurization state 8 since pressurization state 9 is prevented. Although the force output 90 of pressurization state 8 does not give an as good force match as pressurization state 9, the energy costly depressurization of the first working chamber 84-A will be avoided. When transitioning from any of pressurization states 8 or 10-16 to an adjacent pressurization state among the allowed pressurization states 1-8 or 10-16, less than all working chambers 84 are switched between the high-pressure side 48 and the low-pressure side 50. For example, when transitioning from pressurization state 10 to pressurization state 8, pressurization state 9 is skipped and only the fourth working chamber 84-D is switched.

[0090] In this example, a difference between the force outputs 90 of the allowed pressurization state 10 and the prevented pressurization state 9 is smaller than a difference between the force outputs 90 of, for example, the immediately adjacent pressurization states 10 and 11. Furthermore, a difference between the force outputs 90 of the allowed pressurization state 10 and the immediately adjacent prevented pressurization state 9 is approximately 50% of the difference between the force outputs 90 of the immediately adjacent allowed pressurization states 10 and 11.

[0091] Furthermore, pressurization state 8 is prevented when the hydraulic actuator 40 adopts any of pressurization states 1-7 or 9, i.e. when the first working chamber 84-A is connected to the low-pressure side 50. When transitioning from any of pressurization states 1-7 or 9 to an adjacent pressurization state among the allowed pressurization states 1-7 or 9-16, less than all working chambers 84 are switched between the high-pressure side 48 and the low-pressure side 50. For example, when transitioning from pressurization state 7 to pressurization state 9, pressurization state 8 is skipped and only the fourth working chamber 84-D is switched.

[0092] In this example, a difference between the force outputs 90 of the prevented pressurization state 8 and the allowed pressurization state 7 is smaller than a difference between the force outputs 90 of, for example, the immediately adjacent pressurization states 1 and 2. Furthermore, a difference between the force outputs 90 of the prevented pressurization state 8 and the immediately adjacent pressurization state 7 is 50% of the difference between the force outputs 90 of the immediately adjacent allowed pressurization states 6 and 7.

[0093] In the above two examples, the plurality of allowed pressurization states and the at least one prevented pressurization state are determined in dependence of whether the first working chamber 84-A is connected to the high-pressure side 48 or to the low-pressure side 50. Moreover, the plurality of allowed pressurization states and the at least one prevented pressurization state are different when the first working chamber 84-A is connected to the high-pressure side 48 and when the first working chamber 84-A is connected to the low-pressure side 50.

[0094] FIG. 4a is a partial block diagram of the hydraulic system 20 showing a further example of hydraulic actuator 40 and a valve arrangement 78. One, several or each of the hydraulic actuators 40, 42 may be of the same design as in FIG. 4a. FIG. 4b is a diagram showing a force output 90 of an output member 46 for a plurality of pressurization states 92 of the hydraulic actuator 40 in FIG. 4a. With collective reference to FIGS. 4a and 4b, mainly differences with respect to FIGS. 3a and 3b will be described.

[0095] The working chambers 84 in FIGS. 4a and 4b have respective effective areas with a non-binary relationship of 7:4:2:1. Although the four working chambers 84 have effective areas with a non-binary relationship, the second working chamber 84-B, the third working chamber 84-C and the fourth working chamber 84-D have a binary relationship of 4:2:1.

[0096] As can be seen in FIG. 4b, the force output 90 of the output member 46 is the same in pressurization states 8 and 9. According to one example, pressurization state 8 is prevented when the hydraulic actuator 40 adopts any of pressurization states 9-16. Thus, if a force output 90 corresponding to pressurization states 8 and 9 is requested when the first working chamber 84-A is connected to the high-pressure side 48, pressurization state 9 will be selected since pressurization state 8 is prevented. In this way, transition of the first working chamber 84-A from the high-pressure side 48 to the low-pressure side 50 is avoided. If a force output 90 close to the force output 90 of pressurization state 7 or lower is requested, the hydraulic actuator 40 transitions from any of pressurization states 9-16 to any of pressurization states 1-7 while skipping pressurization state 8.

[0097] According to the same example, pressurization state 9 is prevented when the hydraulic actuator 40 adopts any of pressurization states 1-8. Thus, if a force output 90 corresponding to pressurization states 8 and 9 is requested when the first working chamber 84-A is connected to the low-pressure side 50, pressurization state 8 will be selected since pressurization state 9 is prevented. In this way, transition of the first working chamber 84-A from the low-pressure side 50 to the high-pressure side 48 is avoided. If a force output 90 close to the force output 90 of pressurization state 10 or higher is requested, the hydraulic actuator 40 transitions from any of pressurization states 1-8 to any of pressurization states 10-16 while skipping pressurization state 9. Thus, pressurization state 8 is skipped during force decrease and pressurization state 9 is skipped during force increase. In this way, a hysteresis is introduced in the control of the hydraulic actuator 40, which reduces the number of switches of working chambers 84 between the high-pressure side 48 and the low-pressure side 50 and improves energy efficiency.

[0098] FIG. 5 is a flowchart outlining the general steps of the method according to the invention. The method comprises selectively fluidly connecting S1 each working chamber 84 to either a high-pressure side 48 or a low-pressure side 50 to provide a plurality of discrete pressurization states 92 of the hydraulic actuator 40; determining S2 at least one of the pressurization states 92 as a prevented pressurization state; and transitioning S3 between a plurality of allowed pressurization states among the pressurization states 92 while preventing transition to the at least one prevented pressurization state. Step S2 may be carried out before and/or after step S1.

[0099] 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.