Patent Publication Number: US-2022220702-A1

Title: A method of controlling a rotatable load, a hydraulic system and a working machine

Description:
TECHNICAL FIELD 
     The invention relates to a method of controlling a rotatable load by a hydraulic system, a hydraulic system for controlling a rotatable load, and a working machine. 
     The invention is applicable on hydraulic systems of working machines within the fields of industrial construction machines, material handling machines or construction equipment, in particular wheel loaders and excavators. Although the invention will be described with respect to an excavator, the invention is not restricted to this particular machine, but may be used in any working machine comprising a hydraulic system with a high-pressure side and a low-pressure side, such as wheel loaders, articulated or rigid haulers and backhoe loaders. 
     BACKGROUND 
     Hydraulic systems are used in a wide range of applications. For example, working machines typically rely on hydraulic systems to provide power for handling loads. A hydraulic system for a working machine may comprise various hydraulic actuators, for example hydraulic cylinders and rotary hydraulic machines. Hydraulic cylinders may for example be provided in a working device comprising arms and a bucket. Rotary hydraulic machines may for example be used for propulsion of a working machine and/or for a swing function of an excavator. Hydraulic hybrid systems can be used to recuperate energy from the hydraulic actuators and use it later to reduce the loading of an internal combustion engine. 
     Hydraulic systems comprising a dedicated high-pressure side and a dedicated low-pressure side may be referred to as dual pressure hydraulic systems, and are previously known as such. Dual pressure hydraulic systems typically comprise one or more high-pressure accumulators connected to a high-pressure side, and one or more low-pressure accumulators connected to a low-pressure side. Advantages associated with dual pressure hydraulic systems are for example improved energy efficiency and controllability. 
     US 2013098011 A1 discloses a hydraulic system having a first circuit fluidly connecting a first pump to a swing motor in a closed-loop manner, and a second circuit fluidly connecting a second pump to a first travel motor and a first linear tool actuator in a parallel closed-loop manner. The hydraulic system may also have a combining valve configured to selectively fluidly connect the first circuit to the second circuit. 
     SUMMARY 
     An object of the invention is to provide a method of controlling a rotatable load of a hydraulic system, which method is energy efficient, accurate, simple and/or cheap and enables an energy efficient, simple and/or cheap hydraulic system. 
     According to a first aspect of the invention, the object is achieved by a method of controlling a rotatable load according to claim  1 . The method is carried out in a 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. 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. 
     The method enables the use of a relatively cheap two-quadrant hydraulic machine instead of a relatively expensive four-quadrant hydraulic machine. The method can however also be carried out with a four-quadrant hydraulic machine. 
     At high rotational speeds, high power operations, and/or high torque operations of the hydraulic machine, energy losses over the valve arrangement may be relatively high if the rotatable load is controlled to a large extent by controlling the valve arrangement. Throttling over the valve arrangement may therefore be reduced or eliminated in the displacement control mode. Thereby, energy losses over the valve arrangement are reduced or eliminated. In the valve control mode however, i.e. at low rotational speeds, low power operations and/or low torque operations of the hydraulic machine, the energy losses over the valve arrangement are relatively low. 
     In case an over-center four quadrant hydraulic machine is used in a dual pressure hydraulic system without a valve arrangement between the high-pressure side/low-pressure side and the hydraulic machine, it may be difficult to control the rotatable load at start-up by means of only displacement control, for example due to mechanical play between the hydraulic machine and the rotatable load, such as in an intermediate gearbox. The displacement may thereby initially be increased “within the play” without the hydraulic machine generating any substantial torque until the hydraulic machine “hits” a driven part as the backlash is eliminated. 
     Furthermore, in case an over-center four quadrant hydraulic machine is used in a dual pressure hydraulic system without a valve arrangement between the high-pressure side/low-pressure side and the hydraulic machine, the hydraulic machine needs to be positioned in zero displacement at start-up in order to not generate a torque. During start-up of a hydraulic system under the control of the method according to the invention in contrast, the displacement of the hydraulic machine can be arbitrary (for example 100%) since the valve control mode will be active during start-up. By means of the valve control mode, the main pressure drop may be over the valve arrangement at start-up. 
     Conversely, the method according to the invention also enables stopping of the rotatable load with an arbitrary displacement of the hydraulic machine by means of the valve arrangement. For an over-center four quadrant hydraulic machine used in a dual pressure hydraulic system without a valve arrangement between the high-pressure side/low-pressure side and the hydraulic machine in contrast, the displacement of the hydraulic machine needs to be brought to zero when the rotatable load is stopped. 
     The valve control mode comprises controlling the rotatable load by controlling a flow through the valve arrangement. Thus, in the valve control mode, the valve arrangement is involved to control the rotatable load. In the valve control mode, the valve arrangement may therefore be said to operate as a flow source. Although the valve control mode comprises control of the rotatable load by controlling the valve arrangement, the valve control mode may also comprise control of the rotatable load by controlling the displacement of the hydraulic machine. For example, in the valve control mode, a pressure drop over the hydraulic machine may constitute up to 50% of the pressure difference between the high-pressure side and the low-pressure side. In the valve control mode, the displacement of the hydraulic machine may either be constant or variable. 
     In the displacement control mode, the pressure at the first side/second side may be at least 60%, such as at least 75%, such as at least 90% of the pressure in the high-pressure side in case a fluid communication is established from the high-pressure side to the first side/second side by means of the valve arrangement. Alternatively, or in addition, a pressure drop over the hydraulic machine may constitute at least 60%, such as at least 70%, such as at least 80% of the pressure difference between the high-pressure side and the low-pressure side in the displacement control mode. In this way, the rotatable load is mainly controlled by controlling the displacement of the hydraulic machine. 
     In the displacement control mode, the valve arrangement may be controlled to provide a maximum pressure drop over the hydraulic machine. Thereby, a torque requested by the hydraulic machine, e.g. based on an error signal in a feedback loop, can be reached by controlling the displacement. The displacement control mode may comprise secondary control of the hydraulic machine. 
     According to one example, the rotatable load is controlled by only controlling the displacement of the hydraulic machine in the displacement control mode, e.g. while maintaining an opening area of each valve of the valve arrangement constant. The valve arrangement may thereby be said to operate as a pressure source in the displacement control mode. For example, each valve of the valve arrangement may be fully open, or open as much as possible, in the displacement control mode in order to establish a smallest possible pressure drop over the valve arrangement. In the valve control mode, the rotatable load may be controlled by controlling both the valve arrangement and the displacement of the hydraulic machine. The valve control mode may thus comprise a blended or mixed control of the valve arrangement and the displacement of the hydraulic machine. 
     In the valve control mode, the pressures at the first side and the second side may be said to be coupled to the rotatable load. In the displacement control mode, the pressures at the first side and the second side may be said to be decoupled from the rotatable load. 
     A non-zero absolute minimum displacement limitation may for example be 5-100%, for example 10%. In case the hydraulic machine has a variable displacement of −100%-0%, the displacement limitation may be set to −10%. The absolute minimum displacement limitation is in this case 10%. The displacement limitation, or a different non-zero absolute minimum displacement limitation, may or may not be applied also in the displacement control mode. 
     The non-zero absolute minimum displacement limitation may be applied by software, e.g. by a control system as described herein. Alternatively, the non-zero absolute minimum displacement limitation may be applied by hardware, e.g. by using an irreversible hydraulic machine with a built-in minimum displacement. 
     As a further example, a non-zero absolute minimum displacement limitation may be 100%. Thereby, the hydraulic machine is forced to a maximum displacement in the first operating state, i.e. at low rotational speeds, low power operations and/or low torque operations. 
     The high pressure in the high-pressure side is higher than the low pressure in the low-pressure side during operation of the hydraulic system. The pressures in the high-pressure side and the low-pressure side are not limited to any specific pressure values. Rather, the terminologies “high pressure” and “low pressure” indicate that these pressure levels are different and that the high pressure is higher than the low pressure. The pressure levels in the high-pressure side and the low-pressure side are selected depending on each configuration. The pressure levels in the high-pressure side and the low-pressure side may vary during operation of the hydraulic system. 
     The high-pressure side may be referred to as a primary side or primary source of hydraulic power arranged to both produce and receive a volume flow at a first pressure level and the low-pressure side may be referred to as a secondary side or secondary source of hydraulic power arranged to both produce and receive a volume flow at a second pressure level, lower than the first pressure level. 
     According to one embodiment, 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%. For example, in case the displacement limitation is set to 80% in the valve control mode, the displacement may be held constant at any value between 80% and 100%, or may vary between 80% and 100%. In this embodiment, the determination if the hydraulic system is in the first operating state or the second operating state may be made based only 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. In this embodiment, a required displacement, as described herein, does not have to be determined in order to decide whether the hydraulic system is in the first operating state or the second operating state. However, a required displacement may nevertheless be determined and used in the displacement control mode. In this embodiment, the displacement of the hydraulic machine may be constant in the valve control mode. 
     Furthermore, this embodiment may comprise a stepwise transition between the valve control mode and the displacement control mode. For example, when the rotational speed of the hydraulic machine increases to over a second speed threshold value (e.g. 300 rpm), it may be determined that the hydraulic system enters the second operating state and the valve control mode is switched stepwise to the displacement control mode, e.g. over a 300 ms time period. In the displacement control mode, the displacement limitation may be maintained, reduced or eliminated. When the rotational speed of the hydraulic machine decreases to below a first speed threshold value (e.g. 200 rpm) lower than the second speed threshold value, it may be determined that the hydraulic system again enters the first operating state and the displacement control mode is switched stepwise to the valve control mode, e.g. over a 300 ms time period. 
     According to one embodiment, the method further comprises 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. 
     The method according to this embodiment may alternatively be defined as applying a non-zero absolute minimum 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; controlling the rotatable load by controlling the valve arrangement, if the required displacement is smaller than the displacement limitation of the hydraulic machine; and controlling the rotatable load mainly by controlling the displacement of the hydraulic machine, if the required displacement is larger than the displacement limitation of the hydraulic machine. 
     Throughout the present disclosure, the control of the rotatable load by controlling the valve arrangement may be referred to as a “valve control mode” and the control of the rotatable load mainly by controlling the displacement of the hydraulic machine may be referred to as a “displacement control mode”. The switching between the valve control mode and the displacement control mode may be made by a continuous or repeated comparison between the required displacement and the displacement limitation. For example, in case the non-zero absolute minimum displacement limitation is 10%, the valve control is applied when the required displacement is below 10% and the displacement control is applied when the required displacement is above 10%. 
     The required displacement may for example be calculated based on the pressure drop over the hydraulic machine and a torque command to the hydraulic machine in a feedback loop, e.g. implemented in the control system as described herein. Instead of a torque command, a rotational speed command to the hydraulic machine or a rotational acceleration command to the hydraulic machine may alternatively be used. The pressure drop over the hydraulic machine may be a difference between the pressure at the first side and the pressure at the second side. The pressure drop over the hydraulic machine may be calculated either directly, e.g. by means of pressure sensors at the first side and the second side. Alternatively, the pressure drop over the hydraulic machine may be calculated indirectly, e.g. based on the high pressure in the high-pressure side, the low pressure in the low-pressure side and pressure drops over the valve arrangement, and/or based on a difference between reference pressures at the first side and the second side. As a further alternative, for the purpose of calculating the required displacement, the pressure drop over the hydraulic machine can be approximated as the pressure difference between the high pressure in the high-pressure side and the low pressure in the low-pressure side, i.e. by ignoring further pressure drops, such as over the valve arrangement. In other words, in order to calculate the required displacement, a desired or possible maximum pressure drop over the hydraulic machine can be used. 
     Each of the first speed threshold value, the first power threshold value, and the first torque threshold value may be either constant or variable. For example, the first speed threshold value, the first power threshold value and/or the first torque threshold value may vary depending on a load condition on the hydraulic machine. In case the hydraulic system is used in a working machine, the first speed threshold value, the first power threshold value and/or the first torque threshold value may for example depend on an angle of a slope travelled by the working machine. In case a constant first speed threshold value is used, this may for example be 20-250 rpm/min, such as 50-200 rpm/min. 
     According to one embodiment, the method further comprises 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. For example, the displacement limitation may be 10% at rotational speeds below the first speed threshold value, decrease at rotational speeds above the first speed threshold value, and be 2% at rotational speeds above the second speed threshold value. The hydraulic machine is thereby allowed to swash down at higher rotational speeds, at higher powers and/or at higher torques. Thereby, pressure losses over the valve arrangement are reduced at high speed/high power/high torque operations. 
     Each of the second speed threshold value, the second power threshold value and the second torque threshold value may be constant. According to one example, the first speed threshold value is 200 rpm and the second speed threshold value is 500 rpm. Alternatively, each of the second speed threshold value, the second power threshold value and the second torque threshold value may be variable. For example, the second speed threshold value, the second power threshold value and/or the second torque threshold value may vary depending on a load condition on the hydraulic machine. However, the second speed threshold value is always equal to or larger than the first speed threshold value, the second power threshold value is always equal to or larger than the first power threshold value and the second torque threshold value is always equal to or larger than the first torque threshold value. 
     According to one embodiment, the method further comprises 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. The displacement limitation may gradually reduce to zero. Alternatively, the displacement may gradually reduce until the second speed threshold value, the second power threshold value and/or the second torque threshold value is reached. 
     According to one embodiment, the method further comprises determining the pressure drop over the hydraulic machine based on a pressure difference between the high-pressure side and the low-pressure side. The pressure drop over the hydraulic machine may for example be determined as the pressure difference between the high-pressure side and the low-pressure side minus the pressure drops over the valve arrangement. The pressure drops over the valve arrangement may be measured, calculated or estimated. 
     According to one embodiment, a pressure difference between the first side and the second side is smaller in the valve control mode than in the displacement control mode, at least during steady state operation of the hydraulic system. This is a consequence of controlling the valve arrangement to a larger extent, e.g. with a higher pressure drop over the valve arrangement, in the valve control mode than in the displacement control mode. In other words, a pressure difference between the first side and the second side is smaller when controlling the rotatable load by controlling the valve arrangement than when controlling the rotatable load mainly by controlling the displacement of the hydraulic machine. Thus, the total pressure drop over the valve arrangement, e.g. from the high-pressure side and to the low-pressure side, is larger in the valve control mode than in the displacement control mode. However, for short time periods of non-steady state operation of the hydraulic system, for example during short periods of deceleration, the pressure difference between the first side and the second side may be larger in the valve control mode than in the displacement control mode. 
     According to one embodiment, the method further comprises 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. Thereby, the pressurized side of the hydraulic machine can be switched by means of the valve arrangement. For example, when driving the hydraulic machine as a motor and supplying fluid from the high-pressure side to the first side and from the second side to the low-pressure side by means of the valve arrangement, a braking action of the hydraulic machine can be initiated by switching the high-pressure side to the second side and switching the first side to the low-pressure side. The hydraulic machine will then start to operate as a pump that pumps hydraulic fluid to the high-pressure side to recuperate energy as the rotatable load decelerates. This switching can be made both in the valve control mode and in the displacement control mode. 
     According to one embodiment, the rotatable load is entirely controlled by controlling the displacement of the hydraulic machine in the displacement control mode. For example, the rotatable load may be entirely controlled by controlling the displacement of the hydraulic machine, if the required displacement is larger than the displacement limitation of the hydraulic machine. That is, each valve of the valve arrangement may be kept still, preferably in a maximum open position, in the displacement control mode to reduce pressure losses over the valve arrangement. 
     According to one embodiment, the method further comprises controlling the displacement of the hydraulic machine by means of closed loop control in the displacement control mode. For example, a closed loop control of the hydraulic machine may be used when the required displacement is larger than the displacement limitation of the hydraulic machine. 
     According to one embodiment, the control of the rotatable load comprises controlling one of torque, rotational position, rotational speed, and rotational acceleration of the rotatable load. For example, in order to control the torque of the rotatable load, the pressure drop over the hydraulic machine and/or the displacement may be controlled. 
     According to one embodiment, the hydraulic machine is a two-quadrant hydraulic machine operative as a pump and as a motor. Such hydraulic machine may alternatively be referred to as a positive displacement hydraulic machine or an irreversible two-quadrant hydraulic machine, for example having a displacement of 0% to 100%. An over-center hydraulic machine in contrast is reversible, for example having a displacement of −100% to 100%. The displacement of an irreversible hydraulic machine cannot pass the zero position while the displacement of a reversible hydraulic machine can pass the zero position. 
     According to a second aspect, the object is achieved by 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. 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 valve control mode. 
     Since the hydraulic system comprises a high-pressure side and a low-pressure side, the hydraulic system forms a dual pressure system. The hydraulic system may be a common pressure rail hydraulic system. 
     According to one embodiment, 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%. 
     According to one embodiment, 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. 
     The control system according to this embodiment may alternatively be defined as configured to apply configured to apply a non-zero absolute minimum 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; control the rotatable load by controlling the valve arrangement, if the required displacement is smaller than the displacement limitation of the hydraulic machine; and control the rotatable load mainly by controlling the displacement of the hydraulic machine, if the required displacement is larger than the displacement limitation of the hydraulic machine. 
     According to one embodiment, 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. 
     According to one embodiment, 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. 
     According to one embodiment, 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. 
     According to one embodiment, 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. 
     According to one embodiment, the control system is configured to control the rotatable load entirely by controlling the displacement of the hydraulic machine in the displacement control mode. For example, the control system may be configured to control the rotatable load entirely by controlling the displacement of the hydraulic machine, if the required displacement is larger than the displacement limitation of the hydraulic machine. 
     According to one embodiment, the control system is configured to control the displacement of the hydraulic machine by means of closed loop control in the displacement control mode. For example, the control system may be configured to control the displacement of the hydraulic machine by means of closed loop control when the required displacement is larger than the displacement limitation of the hydraulic machine. 
     According to one embodiment, the control of the rotatable load comprises controlling one of torque, rotational position, rotational speed, and rotational acceleration of the rotatable load. 
     According to one embodiment, the valve arrangement comprises at least one valve having at least two variable positions establishing at least two different opening areas. By varying the opening area, the at least one valve can control the flow therethrough. The pressure drop is higher through a smaller opening area than through a larger opening area. 
     According to one embodiment, the hydraulic machine is a two-quadrant hydraulic machine operative as a pump and as a motor. Since the first side can be connected to the high-pressure side and the second side can be connected to the low-pressure side, and vice versa, by means of the valve arrangement, the hydraulic system provides the same functions as a four-quadrant hydraulic machine, while only requiring a cheaper two-quadrant hydraulic machine. 
     According to one embodiment, the pressure on the high-pressure side varies above 50 bars, and the pressure on the low-pressure side varies below 50 bars, during operation of the hydraulic system. For example, the pressure on the high-pressure side may vary between 51-500 bars, such as between 200-350 bars, and the pressure on the low-pressure side may vary between 1-50 bars, such as between 15-30 bars, during operation of the hydraulic system. Thus, the high pressure may be kept within a high pressure interval and the low pressure may be kept within a low pressure interval, with no overlap between the high pressure interval and the low pressure interval. 
     According to one embodiment, the hydraulic system further comprises a main pump arranged to pressurize the high-pressure side. The main pump may comprise one port connected to the high-pressure side and one port connected to the low-pressure side. 
     According to one embodiment, the hydraulic system further comprises at least one high-pressure hydraulic energy storage connected to the high-pressure side and/or at least one low-pressure hydraulic energy storage connected to the low-pressure side. The high-pressure hydraulic energy storage and/or the low-pressure hydraulic energy storage may comprise a hydraulic accumulator. 
     The invention also relates to a working machine comprising a hydraulic system according to the invention. 
     Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. 
       In the drawings: 
         FIG. 1  is a schematic illustration of a working machine according to the invention comprising a hydraulic system according to the invention, 
         FIG. 2  is a block diagram of the hydraulic system, 
         FIG. 3  is a partial enlarged view of the hydraulic system in  FIG. 2 , 
         FIG. 4  is a partial enlarged view of the hydraulic system in  FIG. 2  showing one example of a valve arrangement, 
         FIG. 5  is a partial enlarged view of the hydraulic system in  FIG. 2  showing a further example of a valve arrangement, 
         FIG. 6  is a flowchart outlining the general steps of the method according to the invention, 
         FIG. 7  is a diagram showing one example of a displacement limitation as a function of rotational speed of a hydraulic machine, and 
         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 
     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. 
       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. 
     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. 
     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 . 
       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 . 
     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. 
     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. 
     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 . 
     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. 
     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 . 
     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 . 
     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. 
     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 . 
     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 . 
     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 . 
     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. 
       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. 
     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 . 
     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%. 
     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. 
     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. 
     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. 
     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. 
     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 . 
     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. 
     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 . 
       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  80   a - 80   d . Each valve  80   a - 80   d  comprises a plurality of different positions establishing a plurality of different opening areas. The first valve  80   a  is arranged between the high-pressure side  38  and the first side  70 . The second valve  80   b  is arranged between the high-pressure side  38  and the second side  72 . The third valve  80   c  is arranged between the low-pressure side  40  and the first side  70 . The fourth valve  80   d  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  80   a - 80   d  based on commands from the control system  74 . 
     In the valve control mode, the second valve  80   b  and the third valve  80   c  may be closed, and the opening area of the first valve  80   a  and the opening area of the fourth valve  80   d  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  80   a , minus the pressure drop over the fourth valve  80   d , may constitute the pressure drop over the hydraulic machine  60 . 
     Furthermore, in the valve control mode, the first valve  80   a  and the fourth valve  80   d  may be closed, and the opening area of the second valve  80   b  and the opening area of the third valve  80   c  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  80   b , minus the pressure drop over the third valve  80   c , may constitute the pressure drop over the hydraulic machine  60 . 
     In the displacement control mode, the second valve  80   b  and the third valve  80   c  may be closed, and the opening area of the first valve  80   a  and the opening area of the fourth valve  80   d  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  80   a  and the fourth valve  80   d  may be closed, and the opening area of the second valve  80   b  and the opening area of the third valve  80   c  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 . 
       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. 
       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 S 2  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 S 3  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 S 4  a non-zero absolute minimum displacement limitation  86  to the hydraulic machine  60 ,  62  at least in the first operating state. 
       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%. 
     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 . 
       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. 
     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. 
     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. 
       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. 
     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. 
     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.