Method of controlling a rotatable load, a hydraulic system and a working machine

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 when 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 when 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 when 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%.

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 when 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 when 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 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. 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 80%.

14. 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 100%.

15. 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 when 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 when 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 when 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.

16. The hydraulic system according to claim 15 , 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%.

17. The hydraulic system according to claim 15 , 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 when 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 when the required displacement is larger than the displacement limitation of the hydraulic machine.

18. The hydraulic system according to claim 17 , 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.

19. The hydraulic system according to claim 17 , 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.

20. The hydraulic system according to claim 17 , 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.

21. The hydraulic system according to claim 15 , 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.

22. The hydraulic system according to claim 15 , 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.

23. The hydraulic system according to claim 15 , 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 80%.

24. The hydraulic system according to claim 15 , wherein the displacement limitation in the valve control mode and/or a displacement of the hydraulic machine in the valve control mode is 100%.