Hydraulic axial piston unit and method for controlling a hydraulic axial piston unit

Abstract

The invention relates to a hydraulic axial piston unit with a drive mechanism whose displacement volume is adjusted via an adjustable pivot angle of a displacement element. A first control port and a second control port are arranged circumferentially between a first pressure port and a second pressure port. A cylinder bore can be fluidically connected to one of the control ports when the working piston arranged in the cylinder bore is at its inner dead center (IDC) or outer dead center (ODC). The control ports are connected to control valves via control lines. Each control valve is capable of continuously opening and closing the connected control line and, in parallel, continuously connecting the connected control line to the nearest pressure port in the direction of rotation of the cylinder block and to a hydraulic reservoir or a lower system pressure.

Description

[0001] The present invention relates to hydraulic axial piston units and a method for controlling hydraulic axial piston units. In particular, the invention relates to hydraulic axial piston units with a swash plate design and hydraulic axial piston units with a bent-axis design. The invention also relates to a method for controlling both types of hydraulic axial piston units.

[0002] Hydraulic axial piston units with swashplate or bent-axis designs are widely known in the art and are used as fixed or variable displacement units. They can be used in both pump and motor operation. The displacement volume of hydraulic axial piston units can be adjusted / controlled by adjusting / changing the pivot angle of a displacement element, i.e., the swashplate or yoke.

[0003] To convert mechanical power into hydraulic power and vice versa, hydraulic axial piston units have a drive assembly. This drive assembly has a rotatable cylinder block with cylinder bores in which working pistons are reciprocally arranged to deliver hydraulic fluid from a kidney-shaped inlet port to a kidney-shaped outlet port, which are arranged on a valve segment of the hydraulic axial piston unit. When the displacement element is pivoted relative to the drive shaft axis of the hydraulic unit, the working pistons are forced to reciprocate between their inner dead center (IDC) and outer dead center (ODC) as the cylinder block rotates.In a hydrostatic axial piston unit of the swash plate design, a piston is at its inner dead center when the direction of movement of the piston changes from a movement toward the valve segment to a movement toward the displacement element. A piston is at its outer dead center when its direction of movement changes from a movement toward the displacement element to a movement toward the valve segment.

[0004] As is well known, one of the inlet or outlet ports serves as the high-pressure port, and the other as the low-pressure port. Which port serves as the high-pressure port and which port serves as the low-pressure port depends on the operating mode of the hydraulic unit and the hydraulic flow direction.

[0005] Servo units are often used to adjust the swivel angle of a displacement element in a hydraulic axial piston unit. These control and servo arrangements are complex and error-prone due to the high demands placed on manufacturing and operating precision. They are therefore expensive to manufacture and install. In addition, the control and servo arrangements known from the state of the art are bulky and space-consuming due to the variety of parts, which increases the overall size of the hydraulic axial piston units. The known controls of hydraulic axial piston units are developed for specific applications and require application-specific adaptation of the control components, such as specific valve plates and / or valve sections as well as specifically adapted servo and control spools and springs, all of which require tight tolerances.The components of displacement control systems are subject to wear and tear and therefore require constant maintenance or replacement. Furthermore, these specific components are not suitable for replacement during operation, meaning they cannot be adapted to individual load situations after installation and are often not suitable for use in different hydraulic axial piston units with different volume sizes / displacement volumes.

[0006] However, one of the main advantages of using servo units to adjust the pivot angle of a displacement element is that servo units can be easily combined with neutral return mechanisms that force the displacement element into its neutral position when no displacement command is present.

[0007] It is therefore an object of the invention to provide a hydraulic axial piston unit with a control for adjusting and regulating the displacement volume of hydraulic axial piston units, which overcomes the deficiencies of the prior art solutions, wherein it should be easily possible to equip the hydraulic axial piston unit with a neutral return mechanism.

[0008] In particular, the solution according to the invention should have a smaller number of components or at least be of a simpler design, yet be capable of reliably adjusting and controlling the displacement volume of hydraulic axial piston units. Consequently, the hydraulic axial piston unit according to the invention should be more cost-effective and require less installation space than the solutions known from the prior art. The control system according to the invention for a hydraulic axial piston unit should be adaptable to different hydraulic axial piston units, even "on the fly," i.e., without having to dismantle the hydraulic axial piston unit.

[0009] The object is achieved by a hydraulic axial piston unit according to claim 1 and a method for controlling the displacement volume of a hydraulic drive group according to claim 12. Preferred embodiments are specified in the subclaims directly or indirectly dependent thereon.

[0010] A hydraulic axial piston unit according to the invention comprises a drive mechanism whose displacement volume is adjusted via a displacement element with an adjustable pivot angle. The drive mechanism comprises a rotatable cylinder block with cylinder bores in which working pistons are accommodated for reciprocating movement.

[0011] When the cylinder block rotates and the displacement element is pivoted with respect to the cylinder block's axis of rotation and / or the drive shaft axis, the pistons in the corresponding cylinder bores perform a forward and backward movement. During one full revolution of a cylinder bore and the working piston arranged in the cylinder bore, a piston changes its direction of movement twice. At the inner dead center (IDC), the working piston changes its direction of movement from moving toward a fluid exchange opening of the cylinder bore to moving away from the fluid exchange opening of the cylinder bore. Accordingly, the inner dead center within one revolution is the position at which the working piston is closest to the fluid exchange opening of the cylinder bore, i.e., it is inserted furthest into the cylinder bore, and the fluid volume in the cylinder bore is minimal.At outer dead center (ODC), the working piston's movement transitions from moving away from the cylinder bore's fluid exchange port to moving toward the cylinder bore's fluid exchange port. Consequently, at outer dead center, for one full revolution of the cylinder block, the working piston is in the position farthest from the fluid exchange port of the corresponding cylinder bore—i.e., where the working piston is furthest out from the cylinder bore, and the fluid volume in the cylinder bore is the largest allowed by the set swing angle.

[0012] For example, if a working piston of a hydraulic axial piston pump is at outer dead center, the pressure in the corresponding cylinder bore changes from a low inlet pressure to a high outlet pressure, whereas if a piston is at inner dead center, the pressure in the cylinder bore changes from a high outlet pressure (discharge pressure) to a low inlet pressure (suction pressure). In a hydraulic motor, the situation is reversed: At outer dead center, the pressure on a working piston and in the corresponding cylinder bore changes from high inlet pressure to low outlet pressure, whereas at inner dead center, the pressure on the piston and in the corresponding cylinder bore that accommodates the piston changes from low outlet pressure to high inlet pressure.

[0013] The adjustable longitudinal position of the inner dead center and outer dead center of the working pistons, i.e., the position along the rotational axis of the drive assembly, depends on the inclination angle / tilt angle / swivel angle of the displacement element. However, the angular position, i.e., the rotational position, of the inner and outer dead center is fixed by the design of the drive assembly as long as the orientation and position of the swivel axis of the adjusting element are not changed, i.e., independent of the swivel angle of the displacement element.

[0014] The hydraulic axial piston unit according to the invention further comprises a valve segment with a first pressure port and a second pressure port. When a cylinder bore overlaps with the first or second pressure port, hydraulic fluid can be directed to or discharged from the cylinder bores. Furthermore, a first control port and a second control port are arranged circumferentially between the respective circumferential ends of the first pressure port and the second pressure port.

[0015] The first and second control ports are arranged on the valve segment such that a cylinder bore can be fluidically connected to the first control port or the second control port when the associated working piston is at or near its inner dead center (IDC) or at or near its outer dead center (ODC).

[0016] The first control port is connected to a first control valve via a first control line. The second control port is connected to a second control valve via a second control line. Each of the control valves is capable of continuously opening and closing the connected control line and, at the same time, continuously connecting the connected control line to the pressure port next in the direction of rotation of the cylinder block and to a hydraulic reservoir or lower system pressure. For the purposes of this description, the terms hydraulic reservoir and lower system pressure are treated interchangeably. This means that something connected to a hydraulic reservoir can alternatively be connected to a low system pressure, and vice versa.

[0017] The opening size of the connection between a control port and the corresponding pressure port is adjusted by adjusting the position of the control valve in the corresponding control line, e.g., the position of the control valve spool in the corresponding control valve cylinder, to adjust the opening size of the control valve. In this description, the term "adjusting the position of the control valve" is used synonymously to express that the flow rate through the control valve is adjusted. When the position / flow rate is adjusted by a control valve, the connection size of the corresponding control line with a hydraulic reservoir or low system pressure is also adjusted.Preferably, when the size of the connection opening of a control port with the associated pressure port is increased, the size of the connection opening between the control port and the hydraulic reservoir or the lower system pressure is reduced at the same time.

[0018] The size of the connecting opening between a control port and the associated pressure port influences the sum of the static pressure forces acting on the displacement element. The pressures present at the first control port and the second control port each generate a tilting force that acts on the displacement element via the working pistons. Preferably, the first and second control ports are arranged on the valve segment on opposite sides relative to a tilting axis of the displacement element.

[0019] Therefore, a pressure force at the first and / or second control port causes a kit moment / torque with respect to the tilt axis on the displacement element, whereby the moment at the first control port has a different sign than the moment at the second control port. The sum of the resulting tilting moments - including the tilting moments generated by the pressure ports - sets the pivot angle of the displacement element and thus causes - depending on the tilting direction - a corresponding change in the displacement volume of the engine. If the pressure level at the first control port is adjusted relative to the pressure level at the second control port, the resulting tilting moment changes. The resulting tilting moment is also influenced by other parameters and forces, which will be explained later.

[0020] According to the invention, the pivot angle of the displacement element of the hydraulic unit can be adjusted by controlling the size of the connecting opening between the pressure ports and the control ports, or by controlling the ratio of the opening sizes of the ports to each other. The position of the valves can be controlled, for example, by an electronic controller, as explained further below. Static pressure is used to control and adjust the pressure profile encountered by a cylinder bore when it crosses one of the control ports.

[0021] The displacement element's pivot angle and the position of a control valve typically do not change with high frequency. Because static pressure is used to influence the pressure profile along the valve segment to control the displacement element's pivot angle, the frequency with which the valve position needs to be adjusted is relatively low.

[0022] Furthermore, since each of the control valves is capable of connecting a control port to a pressure port and / or a hydraulic reservoir, the displacement element can be pivoted into its initial / neutral position by a neutral return mechanism when both connections between the control ports and the pressure ports are blocked and a connection of the control ports to the hydraulic reservoir is released.

[0023] When the displacement element is pivoted out of its initial position, the position of one of the control valves can be adjusted to establish communication with at least one partially open control line between the associated control port and the associated pressure port, while the other control valve connects the other control port to the hydraulic reservoir. This creates a differential pressure between the two control ports. In closed-circuit applications, there is a minimum system low pressure that can be directed to a control port, and in open-circuit applications, the boost pressure can be used to be directed to one of the control ports via the pressure port.Since the other control port is / can be connected to the tank when the corresponding control valve is in its first position, the pressure difference across the control ports creates a change in the tilting moments on the displacement element. This changed tilting moment is able to overcome the restoring moment of a neutral return mechanism and forces the displacement element to leave its neutral / initial position. Once the displacement element has left its neutral position, i.e. there is a small swivel angle, the rotating hydrostatic axial piston unit creates a higher pressure in one of the pressure ports, which can then be used to variably adjust the swivel angle of the displacement element by variably adjusting the pressure applied to one of the control ports using one of the control valves.The other control valve can maintain the pressure level at the other control port at tank or supply pressure or, by switching the control valve to the second end position, raise the pressure level at this control port to a low system pressure level, for example to enable smoother running of the hydrostatic axial piston unit.

[0024] In a preferred embodiment, the control valves can be three-way, two-position control valves. Preferably, both control valves can have a similar or identical design, thus increasing the number of identical parts and reducing manufacturing costs. A control valve can have a first valve port, a second valve port, and a third valve port. In a first end position of the control valve, the third valve ports are connected exclusively to the respective first valve ports. In a second end position of the control valve, the third valve ports are connected exclusively to the respective second valve ports. The first end position and the second end position can represent the limits for the adjustability of the control valve position.However, intermediate positions of the control valves are also possible, in which all three ports are connected to each other with smaller opening sizes than in the end positions in order to allow an adjustable flow through the control valves.

[0025] The first valve ports of the first control valve and the second control valve can be connected to a hydraulic reservoir or to a lower system pressure. The second valve port of the first control valve can be connected to the first pressure port, and the second valve port of the second control valve can be connected to the second pressure port. The third valve port of the first control valve can be connected to the first control port via the first control line, and the third valve port of the second control valve can be connected to the second control port via the second control line.

[0026] Therefore, in a first end position of the first control valve or the second control valve, the respective first or second control port is connected to a hydraulic reservoir or a low system pressure, with the connection to the corresponding pressure port being blocked. In the second end position of the first control valve or the second control valve, the respective first or second control port is preferably only connected to the corresponding pressure port, with the connection to the hydraulic reservoir or the lower system pressure being blocked.

[0027] The first control valve and the second control valve are preferably continuously switchable / adjustable between the first end position and the second end position. If the control valves are in an intermediate position between the first end position and the second end position, the first and second control ports are connected in parallel, i.e., simultaneously, to the hydraulic reservoir / system low pressure and to the corresponding pressure port.

[0028] The pressure level applied to a control port can be adjusted by setting or adjusting the position of the corresponding control valve. Although this control system has fewer components than a comparable servo control system, the pivot angle of the displacement element can be precisely adjusted. Unnecessary components in the control system can be eliminated, while adjusting the control valves allows the displacement element to be tilted from its neutral position, overcoming the restoring torque of a neutral return mechanism.

[0029] If the hydraulic axial piston unit is operated, for example, as a hydraulic pump in a closed circuit, there are situations in which the hydraulic pump must be pivoted out of its neutral position, even if the system pressure difference—i.e., the load or the difference between the pressure at the first pressure port and the pressure at the second pressure port—is minimal. This can be the case, for example, if the displacement element is pivoted out after the hydraulic unit has been started or after the hydraulic unit has been operating in idle mode, or if the axial piston unit transitions from pumping mode to driving mode, or vice versa.

[0030] If the hydraulic axial piston unit receives the command to swing out in such a situation, the control valve connected to the control port at the inner dead center IDC can be commanded to the first end position or to a position close to the first end position or remain there. In this position, the hydraulic fluid is directed from the control port at the IDC via the associated control line to the hydraulic reservoir. The control valve assigned to the opposite control port, i.e. the control port at the outer dead center ODC, can be commanded to the second end position or to a position close to the second end position or remain there, in which the associated control port is connected to the pressure port next in the direction of rotation of the cylinder block. Therefore, the hydraulic fluid is directed from the pressure port (back) to the control port at the outer dead center.The resulting pressure difference between the control ports at the inner dead center and the outer dead center generates tilting moments acting on the displacement element, which can cause a tilting / swivelling movement of the displacement element.

[0031] In a preferred embodiment, the first pressure port is arranged next to and downstream of the first control port in the direction of rotation of the cylinder block, and / or the second pressure port is arranged next to and downstream of the second control port in the direction of rotation of the cylinder block. This means that the pressure ports and the control ports are arranged alternately when considering one revolution of the engine group.

[0032] Preferably, the circumferential distance from the first control port to the first and second pressure ports and the circumferential distance from the second control port to the first and second pressure ports are smaller than the circumferential extent of the cylinder bores. If, during the rotational movement of the cylinder block, a cylinder bore leaves the circumferential region in which the cylinder bore overlaps with the first or second pressure port, the hydraulic fluid remaining in the cylinder bore can be further compressed due to continued movement of the piston. This effect can occur, for example, in a hydraulic pump when the piston is close to its inner dead center but has not yet reached its inner dead center.Further compression of the hydraulic fluid in the cylinder bore leads to a pressure surge or pressure peak in the cylinder bore and consequently at the valve segment, since the hydraulic fluid in the cylinder bore cannot be drained via the first or second pressure port. Pressure peaks, pressure surges, or uneven pressure distribution across the valve segment can also occur in other scenarios and situations, e.g., a type of cavitation near outer dead center. To mitigate these effects, circumferential pressure extension grooves (also called "fishtails") are often provided in the valve segments known from the prior art as an extension of the pressure ports.

[0033] The flow rate through the first control valve and / or the second control valve can be continuously adjusted by means of a hydraulically, pneumatically, or electromechanically generated force. The first control valve and / or the second control valve can be preloaded into one of the end positions, preferably the first end position, by an elastic element, e.g., a spring.

[0034] The hydraulic axial piston unit may include a neutral return mechanism capable of generating a return force or a return moment on the displacement element when the displacement element is pivoted out of its initial or neutral position.

[0035] In a preferred embodiment, the spool of the first control valve is biased into the first end position by a spring-generated biasing force, in which the first control port is connected to the hydraulic reservoir. Similarly, the spool of the second control valve is biased into its first end position by a spring-generated biasing force, in which the second control port is also connected to the hydraulic reservoir. A neutral reset mechanism forces the displacement element into its neutral position. Due to the connection of the first control port and the second control port to the hydraulic reservoir, the tilting moments generated are low, and the displacement element remains in its neutral position.

[0036] Both control valves are equipped with an electrical actuator, e.g., a solenoid valve actuator, capable of applying an actuating force that opposes the preload force generated by the control valve spring. If a lift command is given to the displacement element, a current signal can be supplied to at least one of the electrical actuators, causing a preferably proportional movement of the actuator and moving the corresponding control valve spool toward the second end position. The respective control valve thus establishes at least a partial connection between the pressure port at system or feed pressure and the control port in order to supply a higher pressure to the control port. The resulting pressure difference between the pressure at the first control port and the pressure at the second control port causes a tilting movement of the displacement element.

[0037] Preferably, the size of the connecting opening between the first pressure port and the first control port and / or the size of the connecting opening between the second pressure port and the second control port corresponds to the continuously adjustable position of the slides in the first control valve and / or in the second control valve. In particular, the size of the connecting opening between the control port and the pressure port can be largest in the second end position and smallest in the first end position, so that the opening size decreases when the slide of the first control valve and / or the second control valve is moved from the second end position to the first end position. Conversely, the opening size and thus the flow rate through the control line increases when the first control valve and / or the second control valve is moved from the first end position to the second end position.

[0038] Accordingly, the pivot angle of the displacement element can be adjusted according to the positions of the first control valve and the position of the second control valve.

[0039] As mentioned above, the positions (of the spool) of the first control valve and / or the position (of the spool) of the second control valve can be proportional to electrical signals sent from an electronic control unit to an actuator of the first control valve and / or the second control valve. The electronic control unit can be capable of, for example, converting operator commands into an electrical signal that can be transmitted to an electrical actuator of the first and / or second control valve. This ensures precise yet rapid adjustment of the position of the first and second control valves.

[0040] Preferably, the electronic control unit may have a signal connection to at least one sensor selected from a group of sensors comprising a swivel angle sensor, a shaft position sensor, a pressure sensor, a flow sensor, a speed sensor, a temperature sensor, a direction sensor, a torque sensor, an acceleration sensor, or any other sensor capable of monitoring at least one operating parameter of the hydraulic unit. This hardware device enables the control of the position of the displacement element.

[0041] In a preferred embodiment, the hydraulic axial piston unit can be operated as a hydraulic pump in a closed hydraulic circuit.

[0042] The invention also relates to a method for variably controlling the displacement volume of a drive mechanism of a hydraulic axial piston unit. The hydraulic axial piston unit can, for example, be designed according to one embodiment as specified above.

[0043] The procedure includes the following steps: a) Draining or supplying hydraulic fluid from or to the passing cylinder bores via the first control port, the first control valve and the first control line, b) supplying or discharging hydraulic fluid to or from the passing cylinder bores via the second control port, the second control valve and the second control line, c) adjusting the position of at least one of the control valves towards the second end position in order to adjust the pivot angle of the displacement element and to control the displacement volume of the hydraulic drive group.

[0044] In some cases, particularly when the control ports are arranged symmetrically on the valve segment with respect to the tilt axis, the amount of hydraulic fluid supplied to one of the control ports is substantially complementary to the amount of hydraulic fluid discharged from the other control port to which no hydraulic fluid is supplied.

[0045] The pressure in the cylinder bores can be increased if hydraulic fluid is fed under high pressure into the passing cylinder bores at the outer dead center (in hydraulic pumps) or at the IDC control port (in hydraulic motors). Due to the higher pressure, the force on the working piston, which closes the cylinder bores, increases. This increased force is transferred from the piston to the displacement element and supported there. According to the principle of "actio = reactio", the supporting force influences the force and moment equilibrium acting on the displacement. If a high pressure is fed into the through cylinder bore, e.g. of a hydraulic pump, at the outer dead center of the control, an increased tilting force is generated on the displacement element.If the tilting moments (tilting moments) on the displacement element are greater than the restoring forces / moments that push the displacement element back to its initial or neutral position, the pivot angle of the displacement element increases.

[0046] In the exemplary case of a hydraulic pump, adjusting a control valve toward the second end position, in which the pressure port with the higher system pressure is connected via the control line to the control port near the ODC, leads to a higher pressure at this control port and in the passing cylinder bore. This leads to an increase in the pivot angle of the displacement element. In the other case, if the control port is the IDC, the higher pressure in the passing cylinder bore would cause a reduction in the pivot angle of the displacement element.

[0047] Preferably, the hydraulic axial piston unit is operated as a hydraulic pump, and to increase the displacement volume of the hydraulic drive group, step c) can be one of the following: c.1) Adjusting the position of the control valve connected to the control port, which is located at or near the outer dead center (ODC), towards the second end position; and / or c.2) Adjusting the position of the control valve connected to the other control port located at or near the inner dead center (IDC) towards the first end position; c.3) Adjustment of the position of both control valves towards the second end position.

[0048] Steps c.1) and / or c.2) are preferably performed when the pressure difference between the first pressure port and the second pressure port, i.e., the load on the hydraulic unit, is relatively low or minimal. In this context, the position of the control valve connected to the control port at or near the IDC can be actively adjusted toward the first end position, e.g., by an electronic actuator, or the position can be passively adjusted toward the first end position, e.g., due to a preload force of an elastic element.

[0049] Carrying out step c.3) may be preferred if the displacement element of the hydraulic unit has already been pivoted out of its neutral position and if the load is high enough, i.e., above a required minimum pressure difference. In this case, both control valves can be actuated from the first end position and / or in or near the second end position, in which the control ports are connected to the respective pressure ports. To further increase the displacement volume of the drive unit, one or both control valves can be positioned further towards the respective second end position in order to enlarge the size of the connecting opening between the control ports and the associated pressure ports.

[0050] The method according to the invention may further comprise the step of processing a command from a control device, e.g., a joystick or other user input device, using an electronic control unit (ECU). The electronic control unit may comprise a microcontroller for adjusting the position of the first control valve and the second control valve in order to adjust / control the pressure in the control lines, the control port, and finally in the cylinder bores to control the displacement volume of the hydraulic axial piston unit. In principle, each of the control valves or all of the control valves can be controlled by the electronic control unit, preferably individually.

[0051] In a preferred embodiment, the method may further comprise the step of detecting at least one operating parameter of the hydraulic axial piston unit by means of a sensor selected from a group of sensors comprising a swivel angle sensor, a shaft position sensor, a pressure sensor, a flow sensor, a speed sensor, a temperature sensor, a direction sensor, a torque sensor, an acceleration sensor or any other sensor capable of monitoring at least one operating parameter of the hydraulic unit.

[0052] The method may further or alternatively comprise the step of transmitting data to an electronic control unit that reflects a detected position of the displacement element based on the detected operating parameter. The transmitted data may directly indicate the position of the displacement element, e.g., in the case where the position of the displacement element is measured directly by a sensor. Alternatively, the position of the displacement element may be derived from the transmitted data. In this case, the position of the displacement element may be calculated based on the transmitted data.

[0053] Preferably, the method may comprise the following steps - Determining a commanded position of the displacement element, which is commanded by a control device or by an operator by means of an electronic control unit; and / or - Calculating a position delta between the commanded (theoretical) position of the displacement element and a detected position of the displacement element represented by the transmitted data; and / or - Deriving a correction signal from the calculated position delta; and / or - Sending the correction signal for adjusting the position of the first control valve and / or the second control valve based on the calculated position delta to at least one of the actuators of the first control valve and / or the second control valve to adjust the pressure in the cylinder bores to adjust the actual displacement of the engine group to correspond to the commanded (desired) displacement.

[0054] In other words, the electronic control unit can function as a controller that controls the position / swivel angle / swivel movement of the displacer. The position delta, calculated, for example, by comparing the commanded position with the detected (actual) position of the displacer, can be considered an error signal. The correction signal can be derived from the position delta or calculated based on the known design of the controller. To name a few non-limiting examples, a P, PI, or PID controller can be used. However, other linear or nonlinear control algorithms / controllers can also be used.

[0055] For example, if the swivel angle of the displacement element changes due to the set position of the first and second control valves and the displacement element approaches the desired / commanded position, the electrical signals sent to the actuators of the control valves can be adjusted by a controller. The controller continuously monitors the delta between the desired and actual position of the displacement element and can adjust the electrical signal to reduce the delta and, ideally, bring it to zero.

[0056] The valve opening area is preferably proportional to the electrical signal sent to the actuator of the control valve. An increase in the electrical signal, e.g., the current, leads to a reduction / closure of the size of the connecting opening between the control port and the hydraulic reservoir and to an opening / enlargement of the connecting opening between the control port and the pressure port, i.e., an increase in the flow rate through the control line.

[0057] The method according to the invention can further comprise the step of continuously monitoring the operating parameters of the hydraulic axial piston unit in order to smooth pressure transitions between the first and second pressure connections and vice versa, and / or to control the pressure level in the cylinder bores and thus the pressure curve in the cylinder bores on their path around the axis of rotation of the hydraulic axial piston unit, i.e., the pressure curve as a function of the angle of rotation of the cylinder block. The method according to the invention further enables the adjustment of the pivot angle of the displacement element by controlling the pressure level present at the control connections by adjusting the position of the control valves between the first and second end positions.For this purpose, measured operating parameters of the hydraulic unit can be processed by the electronic control system in order to give the actuators of the control valve a corresponding signal for (further) opening and closing the control valve connections connected to the control lines.

[0058] Some or all steps of the method described above may be computer-implemented and / or executed by a computer. Descriptions made in connection with features of the hydraulic unit are also intended to apply to the explanation of the method. Conversely, what has been disclosed in connection with the method can also be implemented as a feature of the hydraulic unit according to the invention.

[0059] With reference to the attached figures, preferred embodiments of a hydraulic axial piston unit according to the invention are explained in more detail to improve understanding of the basic concept of the invention. These embodiments do not limit the scope of the inventive concept, but merely represent possible alternative designs to which modifications may be made within the knowledge of a person skilled in the art without departing from the scope of the invention. Therefore, all such modifications and changes are covered by the claimed invention. The figures show the following: Fig. 1 an exemplary structure of a hydraulic axial piston unit; Fig. 2 schematically shows a hydraulic axial piston unit in a first switching state; Fig. 3 schematically shows a hydraulic axial piston unit in a second switching state; and Fig. 4 shows schematically a hydraulic axial piston unit in a third switching state.

[0060] For better readability, the same reference numbers are used throughout the figures for the same components of different embodiments.

[0061] Fig. Figure 1 shows an exemplary embodiment of a hydraulic axial piston unit with a swash plate design. As already mentioned, the invention can also be applied to a hydrostatic axial piston unit of the bent-axis type, in which the working piston bases are supported on a flange of the drive shaft and the displacement element is a yoke that tilts the cylinder block with respect to the axis of the drive shaft.

[0062] The Fig. The swash plate axial piston unit shown in Figure 1 can be used, for example, as a hydraulic pump and has a swash plate as the displacement element 4, which can be tilted relative to a tilting axis 9 to adjust the displacement volume of a drive unit 2 of the hydraulic unit. The drive unit 2 has a cylinder block 3 which can be rotated about a rotation axis 13 and has cylinder bores 5 in which working pistons 6 are accommodated so that they can move back and forth between an outer dead center (ODC) and an inner dead center (IDC). The working pistons 6 are supported on the displacement element 4 via sliding shoes. On the other side of the cylinder block 3, a valve segment 20 is arranged which has a kidney-shaped first pressure connection 21 and a kidney-shaped second pressure connection 22, which serve as interfaces for connecting the hydraulic unit to an open or closed hydraulic circuit.The valve segment 20 further includes a first control port 23 located near the angular position on the valve segment in which the working pistons 6 are at the outer dead center / IDC of their reciprocating motion during one revolution of the cylinder block. A second control port 24 is located near the opposite IDC / ODC angular position of the working pistons 6.

[0063] The Fig. 2, Fig. 3 and Fig. 4 schematically show a hydraulic axial piston unit in various switching states. A hydraulic axial piston pump is shown here as an example. The hydraulic axial piston unit has a valve section 20 with a first pressure connection 21 and a second pressure connection 22. The hydraulic axial piston unit can, for example, be integrated into a closed hydraulic circuit, so that a hydraulic working line is connected to each of the first pressure connection 21 and the second pressure connection 22.

[0064] In embodiments in which a hydraulic pump is arranged in a closed hydraulic circuit, the pump can theoretically rotate in one or both directions of rotation. However, as a rule and in most applications, the pump drive, e.g. a drive motor, always drives the hydrostatic axial piston unit in the same direction of rotation. Furthermore, the displacement element 4 of the pump can be pivoted in both directions, i.e. in a positive and negative angular position relative to a neutral position, in order to enable the supply of hydraulic fluid under pressure to both connections of the hydraulic pump without having to change the direction of rotation of the pump drive shaft. Depending on the pivot direction of the displacement element, i.e. whether the displacement element is pivoted in a positive or negative direction, the angular positions of the inner dead center IDC and the outer dead center ODC are interchangeable.

[0065] Preferably, the control connections of a hydrostatic axial piston unit driven in one direction of rotation are arranged on the valve segment a few angular degrees after the angular position which coincides with the outer dead center or the IDC.

[0066] As in the Fig. 2 to 4, a first control port 23 and a second control port 24 are arranged on the valve segment 20 between the first pressure port 21 and the second pressure port 22. The first control port 23 and the second control port 24 are arranged in the region of the angular position of the inner dead center IDC and the outer dead center ODC. The first control port 23 and the second control port 24 can be arranged directly at the position of the inner dead center IDC and the outer dead center ODC, respectively, or with a relatively small offset from the angular positions of the inner dead center IDC and the outer dead center ODC, respectively. However, in the case of a hydrostatic axial piston unit driven in one direction of rotation, a small offset of the control ports is preferred. This ensures that the pressure signal introduced into the cylinder bore of a passing working piston 6 only occurs during one direction of the reciprocating movement of the working pistons.

[0067] The first control port 23 is connected to a first control line 27, which connects the first control port 23 to a third valve port 30.3 of a first control valve 30. The first control valve 30 further has a first valve port 30.1, which is connected to a hydraulic reservoir 100 or to a lower system pressure. A second valve port 30.2 of the first control valve 30 is connected to the first pressure port 21.

[0068] The second control port 24 is connected to a second control line 28, which connects the second control port 24 to a third valve port 40.3 of a second control valve 40. The second control valve 40 further has a first valve port 40.1, which is connected to a hydraulic reservoir 100 or to a system low pressure. A second valve port 40.2 of the second control valve 40 is connected to the second pressure port 22.

[0069] Therefore, each of the control ports 23, 24 can be connected to the pressure port 21, 22 that follows the respective control port 23, 24 in the direction of rotation 80 of the hydraulic pump. The connection can be established via the control lines 27, 28, the third valve ports 30.3, 40.3, and the second valve ports 30.2, 40.2 of the control valves 30, 40.

[0070] The two control valves 30, 40 are constructed similarly: The control valves 30, 40 have a first end position in which the third valve ports 30.3, 40.3 are connected to the respective first valve ports 30.1, 40.1. When the control valves 30, 40 are in the first end position, a fluid connection is established between the respective control port 23, 24 and a hydraulic reservoir 100. The control valves 30, 40 also have a second position in which the third valve ports 30.3, 40.3 are connected to the respective second valve ports 30.2, 40.2. When the control valves 30, 40 are in the second end position, a fluid connection is established between the control ports 23, 24 and the associated pressure ports 21, 22 via the control lines 27 and 28.

[0071] Preferably, the control valves 30, 40 are continuously adjustable to intermediate positions between the first and second end positions. When the position of the control valves 30, 40 is set between the first end position and the second end position, the control ports 23, 24 are therefore partially connected to the hydraulic reservoir 100 and, in parallel, partially connected to the corresponding first and second pressure ports 21, 22, since the control valve ports do not allow a fully open connection of the control lines 27, 28 to either the pressure ports 21, 22, the tank, or the low system pressure. The relationship between the size of the connection opening to the hydraulic reservoir 100 and the size of the connection opening to the pressure ports 21, 22 is determined by the position of the control valve spools 34, 44 of the control valves 30, 40.

[0072] If a control valve 30, 40 is moved closer to the first end position, the size of the connection opening to the hydraulic reservoir 100 is increased and the size of the connection opening to the pressure port 21, 22 is reduced. Conversely, if a control valve 30, 40 is moved closer to the second end position, the size of the connection opening to the hydraulic reservoir 100 is reduced and the size of the connection opening to the pressure port 21, 22 is increased.

[0073] The control valves 30, 40 are forced into the first end position by a preload force caused by an elastic element, e.g., an elastic control valve spring 35, 45. Therefore, in a basic position of the hydraulic unit, the control valves 30, 40 are in the first end position, as shown in Fig. 2 with the control valve 40. In this basic state, the first control port 23 and the second control port 24 are connected to a hydraulic reservoir 100.

[0074] In order to move the control valves 30, 40 between the first end position and the second end position, each control valve 30, 40 is equipped with a solenoid valve 36, 46, which is capable of, for example, continuously moving the control valve spools 34, 44 of the control valves 30, 40 towards the second end position. A person skilled in the art will easily recognize that the Fig. 2 to 4 as proportional 2-position 3-way valves, can be of any other design that allows control of the flow rate through the control lines 27 and 28, whereby hydraulic fluid can be supplied from one of the pressure ports 21 or 22 to the control ports 23 and 24 or hydraulic fluid can be drained from one or both of the control ports 23 and 24. An electronic control unit (ECU) supplies an electrical signal, e.g., a current signal, to the electromagnetic actuator 36, 46, which causes a proportional movement of the respective control valve spool 34, 44. In the figures, the signal connection between the ECU and the electromagnetic actuators 36, 46 is shown by a dashed line.

[0075] In detail, Fig. 2 shows an example of a first switching state of a hydraulic unit according to the invention. This switching state can occur, for example, when the hydraulic unit is idling or not running.

[0076] The control valves 30, 40 are pushed into the first end position by the control valve springs 35 and 45, so that both control ports 23, 24 are connected to a hydraulic reservoir 100. The electrical signal sent from the ECU to the electromagnetic actuators does not cause any change in the position of the valves 30, 40. For example, a current signal received by the electromagnetic actuators 36, 46 may be zero or below a certain limit, so that the current does not cause any movement of the control valve spools 34 and 44.

[0077] In this switching state, the tilting moment, which results from no or a slight pressure difference between the control ports 23, 24, is smaller than the restoring moment of the neutral return mechanism. The displacement element 4 therefore remains in its neutral position. If the displacement element 4 is pivoted out of its neutral position, a return mechanism of the hydraulic unit can exert a restoring force / moment that forces the displacement element 4 to return to its neutral position. A return mechanism can be implemented, for example, by a small offset of the rotation axis 13 of the plane defined by the swash plate pivot axis and the swash plate center axis, or by a simple arrangement of return springs that press the displacement element 4 - a swash plate or a yoke - into the neutral position.

[0078] Fig. 3 shows a second switching state of a hydraulic unit according to the invention. The second switching state can occur, for example, after the hydraulic unit has been put into operation or when the hydraulic unit has switched from pumping mode to motor mode, or vice versa. In the second switching state, the hydraulic unit is commanded to the working stroke, i.e., the displacement volume of the hydraulic drive group 2 is to be increased. For this purpose, the displacement element 4 is tilted further out of its neutral position. The tilting / pivoting movement is generated by tilting moments that are greater than the restoring force generated by the neutral return mechanism.

[0079] To generate such tipping moments, one of the control valves 30, 40 is moved towards its second end position. In a hydraulic pump, the control valve 30, 40, which is connected to the control port 23, 24 at the outer dead center ODC, can be switched to the second end position. By adjusting the position of the control valve 30, 40, the control port 23, 24 at the outer dead center ODC is at least partially connected to the corresponding pressure port 21, 22, so that hydraulic fluid can be directed from the pressure port 21, 22 to the control port 23, 24 at the outer dead center ODC. At the same time, the other control valve 40, 30, which is connected to the control port 24, 23 at the inner dead center IDC, remains in its first end position, e.g. because the actuator 46, 36 of the control valve 40, 30 remains de-energized.Thus, the control port 24, 23 at the inner dead center IDC remains connected to the hydraulic reservoir 100 and hydraulic fluid can be fed to the hydraulic reservoir 100 via the control port 24, 23 at the inner dead center IDC.

[0080] The differential pressure at control ports 23, 24 at the outer dead center (ODC) and the inner dead center (IDC) generates tilting moments on the displacement element 4. These tilting moments can be so strong that they overcome the restoring moment of the neutral return mechanism. In this state, the tilting moments force the displacement element 4 to leave its neutral position or increase its pivot angle.

[0081] For illustrative purposes only, Fig. 3 assumes that the control port 24 is the outer dead center control port and the control valve 30 is switched to the second end position, whereby the pressure port 22 is connected to the control port 24. The IDC control port, here control port 23, remains connected to the tank 100. This results in a pressure difference between the two control ports 23, 24, which leads to a tilting moment to increase the pivot angle of the displacement element 4 because the higher pressure is introduced into the passing cylinder bores whose working pistons are near or shortly after the outer dead center, and the hydraulic pressure from the control port 23, here the IDC control port, is diverted to the tank 100 or to the lower system pressure.

[0082] Fig.4 shows a third switching state of the hydraulic unit. In this state, the displacement element 4 of the hydraulic unit has already been pivoted out of its neutral position. In this case, the load, i.e. the pressure difference at the pressure connections 21, 22, is above a required minimum value. Both the first control valve 30 and the second control valve 40 can be at least partially displaced into the second end position due to a current signal sent by the ECU to the electromagnetic actuators 36, 46 of the first control valve 30 and the second control valve 40. Preferably, the setting of the position of the first control valve 30 and the second control valve 40 is proportional to the current signal sent by the ECU to the actuators 36, 46.

[0083] In order to further increase the displacement volume of the engine 2, one of the valves 30, 40 or both valves 30, 40 should be moved further towards the second end position in order to increase the size of the connecting opening between the pressure ports 21, 22 and the control ports 23, 24 and to increase the flow rate through the corresponding control line 27 and / or 28.

[0084] In any case, i.e. regardless of the respective operating state, when the pivot angle of the displacement element 4 is to be changed, e.g. due to the adjustment of the position of the first and second control valves 30, 40, or when the displacement element 4 approaches its desired / commanded position, the electrical signals sent from the ECU to the electromagnetic actuators must also be adjusted. This can be done, for example, by a control algorithm, i.e. control software, executed on the ECU. The control algorithm can be adapted to continuously monitor the delta between the desired position of the displacement element 4 and the actual position of the displacement element 4. The actual position of the displacement element can be measured directly or calculated or derived from measurements of other operating parameters of the hydraulic unit.The control algorithm adjusts the electrical signal sent to the electromagnetic actuators to reduce the delta between the desired position and the actual position.

[0085] From the above description and the attached figures as well as the claims, it is clear that the hydraulic axial piston unit according to the invention offers many possibilities and advantages over the prior art. A person skilled in the art will further recognize that further modifications and changes known in the prior art can be made to a hydraulic axial piston unit according to the invention without departing from the actual spirit of the present invention. Therefore, all such modifications and changes fall within the scope of the claims and are covered by them. Furthermore, it is to be understood that the examples and embodiments described above are for illustrative purposes only and that various modifications, changes or combinations of embodiments that may be suggested to a person skilled in the art are within the spirit and scope of this application. List of Reference Numerals 2 engines 3 cylinder block 4 Displacement element 5 cylinder bores 6 working pistons 7 Work management 8 Work management 9 Swivel axis 13 Rotating axis 20 valve segment 21 First pressure connection 22 Second pressure connection 23 First control connection 24 Second control connection 27 First control line 28 Second control line 30 First control valve 30.1 First valve connection 30.2 Second valve connection 30.3 Third valve connection 34 control valve spool 35 Control valve spring 36 Actuator 40 Second control valve 40.1 First valve connection 40.2 Second valve connection 40.3 Third valve connection 44 control valve spool 45 Control valve spring 46 Actuator 80 Direction of rotation 100 Hydraulic Reservoir ECU Electronic Control Unit IDC Inner Dead Center ODC Outer Dead Center

Claims

[1] Hydraulic axial piston unit with a drive mechanism (2), the displacement volume of which is adjusted by an adjustable pivot angle of a displacement element (4) relative to a drive shaft axis, wherein the drive mechanism (2) has a cylinder block (3) rotatable about a rotational axis (13) with cylinder bores (5) in which working pistons (6) are reciprocally movably mounted, and has a valve segment (20) with a first pressure connection (21) and a second pressure connection (22), wherein a first control connection (23) and a second control connection (24) are arranged in the circumferential direction between the respective circumferential ends of the first pressure connection (21) and the second pressure connection (22), wherein a cylinder bore (5) is fluidically connectable to one of the control connections (23, 24) when the working piston (6) mounted in the cylinder bore (5) is at or near its inner dead center (IDC) or at or near its outer dead center (ODC),wherein the first control port (23) is connected to a first control valve (30) via a first control line (27) and the second control port (24) is connected to a second control valve (40) via a second control line (28), each control valve (30, 40) being capable of continuously opening and closing the connected control line (27, 28) and, at the same time, of continuously adjustably connecting the connected control line (27, 28) to the pressure port (21, 22) following in the direction of rotation of the cylinder block (3) and to a hydraulic reservoir (100) or a low system pressure. [2] Hydraulic axial piston unit according to claim 1, wherein the control valves (30, 40) are three-way two-position control valves having a first valve port (30.1, 40.1), a second valve port (30.2, 40.2) and a third valve port (30.3, 40.3), wherein in a first position of the control valves the third valve ports (30.3, 40.3) are connected to the respective first valve ports (30.1, 40.1) and in a second position the third valve ports (30.3, 40.3) are connected to the respective second valve ports (30.2, 40.2), wherein the first valve ports (30.1, 40.1) of the first control valve (30) and the second control valve (40) are connected to a hydraulic reservoir (100) or a low system pressure, the second Valve connection (30.2) of the first control valve (30) is connected to the first pressure connection (21) and the second valve connection (40.2) of the second control valve (40) is connected to the second pressure connection (22), and wherein the third valve connection (30.3) of the first control valve (30) is connected to the first control connection (23) via the first control line (27) and the third valve connection (40.3) of the second control valve (40) is connected to the second control connection (24) via the second control line (28). [3] Hydraulic axial piston unit according to claim 1 or 2, wherein the first control connection (23) is arranged adjacent to and in front of the first pressure connection (21) in the direction of rotation of the cylinder block (3) and / or the second control connection (24) is arranged adjacent to and in front of the second pressure connection (22) in the direction of rotation of the cylinder block (3). [4] Hydraulic axial piston unit according to one of claims 1 to 3, wherein the distance in the circumferential direction from the first control connection (23) to the first and second pressure connection (21, 22) and the distance in the circumferential direction from the second control connection (24) to the first and second pressure connection (21, 22) is smaller than the extension of the cylinder bores (5) in the circumferential direction. [5] Hydraulic axial piston unit according to one of the preceding claims, wherein the first control valve (30) and / or the second control valve (40) is continuously adjustable by means of a hydraulically, pneumatically or electromechanically generated force. [6] Hydraulic axial piston unit according to one of the preceding claims, wherein the first control valve (30) and / or the second control valve (40) is prestressed into the first end position by an elastic element. [7] Hydraulic axial piston unit according to one of the preceding claims, which has a neutral return mechanism which is capable of generating a return force on the displacement element (4) when the displacement element (4) is pivoted out of its neutral position. [8] Hydraulic axial piston unit according to one of the preceding claims, wherein the opening cross-section through the first control valve (30) for connecting the first pressure connection (21) to the first control connection (23) and / or the opening cross-section through the second control valve (40) for connecting the second pressure connection (22) to the second control connection (24) corresponds to the continuously adjustable position of the first control valve (30) and / or the second control valve (40).of the second control valve (40) for connecting the second pressure connection (22) to the second control connection (24) corresponds to the continuously adjustable position of the first control valve (30) and / or the second control valve (40), wherein in particular the opening cross-section is largest in the second end position and smallest in the first end position, so that the opening cross-section decreases when the first control valve (30) and / or the second control valve (40) is moved from the second end position to the first end position, wherein the pivot angle of the displacement element (4) is adjusted according to the positions of the first and second control valves (30, 40). [9] Hydraulic axial piston unit according to claim 8, wherein the position of the first control valve (30) and / or the position of the second control valve (40) is proportional to an electrical signal sent by an electronic control unit (ECU) to an actuator of the first control valve (30) or the second control valve (40). [10] Hydraulic axial piston unit according to claim 9, wherein the electronic control unit (ECU) is in signal communication with at least one sensor selected from a group of sensors comprising a swivel angle sensor, a shaft position sensor, a pressure sensor, a flow sensor, a speed sensor, a temperature sensor, a direction sensor, a torque sensor, an acceleration sensor or any other sensor capable of monitoring at least one operating parameter of the hydraulic unit. [11] Hydraulic axial piston unit according to one of the preceding claims, which is operated as a hydraulic pump in a closed hydraulic circuit. [12] Method for variable control of the displacement volume of a drive group (2) of a hydraulic axial piston unit according to one of the preceding claims, the method comprising the following steps: a) Draining or supplying hydraulic fluid from or to the passing cylinder bores (5) via the first control port (23), via the first control valve (30) and the first control line (27), b) supplying or draining hydraulic fluid to or from the passing cylinder bores (5) via the second control port (24), via the second control valve (40) and the second control line (28), c) adjusting the position of at least one of the control valves (30, 40) towards the second end position in order to adjust the pivot angle of the displacement element (4) and to control the displacement volume of the hydraulic drive group (2). [13] Method according to claim 12, wherein the hydraulic axial piston unit is operated as a hydraulic pump, and to increase the displacement volume of the hydraulic drive unit (2), step c) comprises: c.1) adjusting the position of the control valve (30, 40) connected to the control port (23, 24) located at or near the outer dead center (ODC) towards the second end position; c.2) adjusting the position of the control valve (40, 30) connected to the control port (24, 23) located at or near the inner dead center (IDC) towards the first end position; and / or c.3) Adjust the position of both control valves (30, 40) towards the second end position. [14] A method according to any one of claims 12 or 13, further comprising the following steps: - detecting at least one operating parameter of the hydraulic axial piston unit by means of a sensor selected from a group of sensors comprising a swivel angle sensor, a shaft position sensor, a pressure sensor, a flow sensor, a speed sensor, a temperature sensor, a direction sensor, a torque sensor, an acceleration sensor or any other sensor capable of monitoring at least one operating parameter of the hydraulic unit; - Transmitting data representing a detected position of the displacement element based on the detected operating parameter to an electronic control unit (ECU). [15] The method of claim 14, further comprising the steps of: - Determining a commanded position of the displacement element, which is commanded by a controller or by an operator by means of the electronic control unit (ECU); - Calculating a position delta between the commanded displacement element position and the detected displacement element position represented by the transmitted data; - Deriving a correction signal from the calculated position delta; and - Sending the correction signal for adjusting the position of the first control valve (30) and / or the second control valve (40) on the basis of the calculated position delta to an actuator of the first control valve (30) and / or the second control valve (40) in order to adjust the pressure in the cylinder bores (5) around the current displacement volume of the engine (2) so that it corresponds to the commanded displacement volume. [16] Method according to one of claims 12 to 15, further comprising the step: - continuous monitoring of the operating parameters of the hydraulic axial piston unit for smoothing pressure transitions between the first and second pressure connections (21, 22) and vice versa, and / or for adjusting the pressure in the cylinder bores (5), and / or for adjusting the pivot angle of the displacement element (4).

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