Method for operating a hydraulic system with two hydraulic circuits

Abstract

A hydraulic system having two hydraulic circuits 6, 7, each having a consumer 14, 15 and a variable pump 2, 3. The two pumps are jointly driven by a common motor 4. The two circuits can be selectively

Description

The present invention relates to a method for operating a hydraulic system having two hydraulic circuits, as well as a computing unit and a computer programme for carrying out the same. Background of the invention In work machines or, for example, construction machines, such as excavators, wheel loaders, or backhoe loaders, double-circuit open-centre systems or single-circuit LUDV systems (LUDV: load-independent flow distribution) can be installed as hydraulic systems. In the case of a single-circuit LUDV system, power regulation or drive torque limitation on the one pump is based on the volumetric flow and the pressure. Depending on how the pump is actuated, the limitation is carried out via the pump pivot angle. Volumetric flow distribution among hydraulic consumers (e.g. hydraulic cylinders and / or hydraulic motors) is carried out via pistons and pressure balances in relation to joystick requirements. In the case of double-circuit open-centre systems, power regulation is usually carried out according to the principle of total power regulation. This allocates a certain proportion of the available power to the two pumps. In this case, the volumetric flows in the two circuits are distributed as a function of the power in the respective circuit. Disclosure of the invention According to the invention, a method for operating a hydraulic system is proposed, having two hydraulic circuits, as well as a computing unit and a computer programme for carrying it out, having the features of the independent claims. Advantageous embodiments are the subject-matter of the dependent claims as well as the following description. The invention avails itself of the following measure: for a hydraulic system having two hydraulic circuits, each of which is associated with a hydraulic pump and a summation device, which is configured so as to optionally, corresponding to an actuation, establish an interconnection between the two circuits, in which the two circuits are hydraulically connected, to determine a respective volumetric flow requirement and pressure requirement for each of the two circuits; based on the determined volumetric flow requirements and pressure requirements, to determine a total power demand or total torque demand that would be needed in the case of interconnection of the two circuits in order to drive the two hydraulic pumps; and, when the total power demand or total torque demand exceeds a maximum power output or a maximum torque of the motor, to prevent the interconnection of the two circuits. With the proposed measure, it can be prevented that, when the hydraulic pump is unable to satisfy the volumetric flow requirement in one (first) of the two circuits and when the interconnection of the circuits would occur in order to obtain an additional (summation) volumetric flow from the other (second) of the circuits due to power or torque limitation of the motor, the conveyed volumetric flow in both is reduced. A motor overload (in which the power or torque limitation of the motor becomes effective) could occur in the event of an interconnection, because generally different pressure levels are present in the two circuits, such that an additional power dissipation occurs in the circuit from which pressure medium is conveyed upon an interconnection into the other circuit. This is true, in particular, in the case of an upwards summation, i.e. when pressure medium is conveyed from the circuit with the lower required pressure value into the circuit with the higher required pressure value, because the pressure level in the circuit with the lower required pressure value must be raised up to the higher required pressure value. The interconnection of the two circuits is to be understood in the sense that when the circuits are interconnected, pressure medium can be exchanged or conveyed between the two circuits, but the two circuits can have different pressure levels (in addition to a pressure drop due to a possible volumetric flow via the summation device, which generally has a finite cross-section or acts as a choke or aperture). Different pressure levels are given, in particular, in the case of downwards summation. A performance or torque limitation exists because the motor has a maximum power (i.e. a maximum power that can be imparted by the motor) and / or a maximum torque (i.e. a maximum torque that can be imparted by the motor). The motor is configured or selected such that the maximum power is less than the sum of all maximum consumer power requirements of the hydraulic loads in the two circuits. Thus, the maximum power and / or the maximum torque of the motor is less than the sum of maximum powers or torques required by the hydraulic pumps (which result from respective maximum consumer requirements). This is expedient for efficiency and cost reasons, because the maximum power is rarely required by consumers at the same time. For example, a less efficient operation of the hydraulic pumps or motor in case of a low partial load is avoided. Likewise, smaller, i.e. more cost-efficient, hydraulic pumps or a less powerful, i.e. an inexpensive, motor can be selected. At a given motor speed, the power and torque imparted by the motor are equivalent (power equal to speed times torque). The method is, in particular, a computer-implemented method implemented by an electronic controller of the hydraulic system. According to one design, in particular when the pressure requirement in the second circuit is lower than the pressure requirement in the first circuit, a total volumetric flow requirement is determined as the sum of the volumetric flow requirements of the first and second circuit, and the total power demand is determined from the higher of the pressure requirements and the total volumetric flow, and the total torque demand is determined from the higher of the pressure requirements and a total displacement that is needed in order to achieve the total volumetric flow requirement. The total power demand is given as a product of the total volumetric flow requirement and a pressure value determined based on the higher of the pressure requirements, such that the pressure value is equal to or greater than the higher pressure requirement. A pressure drop can be taken into account via the summation device, for example, which is added to the higher pressure requirement in order to obtain the pressure value. The total torque demand results correspondingly as a product of the total displacement and the pressure value, wherein the total displacement at a given speed can be determined from the total volumetric flow requirement. According to one design, in particular when the pressure requirement in the second circuit is lower than the pressure requirement in the first circuit, the total power demand is determined as the sum of a first and a second individual power demand, wherein the first individual power demand is determined from the volumetric flow requirement of the first circuit and the higher of the pressure requirements, and wherein the second individual power demand is determined from the volumetric flow requirement of the second circuit and the higher of the pressure requirements. Alternatively or additionally, in particular when the pressure requirement in the second circuit is lower than the pressure requirement in the first circuit, the total torque demand is determined as the sum of a first and a second individual torque demand, wherein the first individual torque demand consists of the higher of the pressure requirements and the displacement of the hydraulic pump associated with the first circuit that is needed in order to satisfy the volumetric flow requirement of the first circuit, and wherein the second individual torque demand is determined from the higher of the pressure requirements and the displacement of the hydraulic pump associated with the second circuit that is needed in order to satisfy the volumetric flow requirement of the second circuit. According to one design, when it is determined that, for the first circuit, the hydraulic pump associated therewith is unable to satisfy the volumetric flow requirement without connection to the second circuit, and when the total power demand or total torque demand does not exceed the maximum power or the maximum torque of the motor, the interconnection of the two circuits is established such that a summation volumetric flow from the second circuit into the first circuit occurs via the summation device. If the pressure requirement in the second circuit is higher than the pressure requirement in the first circuit, the summation device or its passage cross-section is set in particular such that an adjustment of the different pressure levels occurs. According to one design, when the pressure requirement of the first circuit lies above the pressure requirement of the second circuit, [there is] an adjustment of the pressure requirement of the second circuit to the pressure requirement of the first circuit. In particular, a pressure drop is taken into account via the summation device. The procedure here can be such that the pressure in the second circuit is initially adjusted, i.e. increased, and the interconnection is subsequently carried out. According to one design, the hydraulic pumps are actuated such that the volumetric flow requirement of the circuit with which they are respectively associated is satisfied or, if a hydraulic pump is unable to satisfy the latter, is satisfied to the extent possible. Thus, at a given speed, the displacement of the respective hydraulic pump is set such that the respective volumetric flow requirement is satisfied. When this is not possible, i.e. when the volumetric flow requirement is greater than the product of the speed and maximum displacement of the hydraulic pump, the displacement can be set to the maximum displacement, such that as large a volumetric flow as possible is obtained. According to one design, when both hydraulic pumps are able to satisfy the volumetric flow requirement of the circuit with which they are associated, having in particular a sufficiently large maximum displacement, the maximum power or the maximum torque of the motor is allotted to the hydraulic pumps, such that the displacements of the hydraulic pumps are or can be adjusted according to the ratio of volumetric flow requirements. By dividing according to the ratio, a simultaneous movement of hydraulic consumers can be achieved. It is assumed again that the speed is given such that the maximum importable volumetric flow of a hydraulic pump substantially results (i.e. neglecting any potential leakage) as a product of the speed and the maximum displacement. The expression “corresponding to the ratio of volumetric flow requirements” is to be understood to mean that the ratio of displacements should be equal to the ratio of volumetric flow requirements or, when a translation ratio of the rotational speeds between the hydraulic pumps is not equal to one, should be equal to the ratio of the volumetric flow requirements, taking this translation ratio into account. When the maximum power or the maximum torque of the motor is not sufficient, the volumetric flows in both circuits are reduced relatively equally (by the same percentage). According to one design, when the hydraulic pump of one of the circuits requires less power or torque than the power or torque allocated thereto in order to satisfy the volumetric flow requirement and the pressure requirement of the circuit with which this hydraulic pump is associated, the power or torque beyond the power or torque allotted thereto is made available to the other hydraulic pump. The simultaneous movement can thus be maintained when the initially allocated power or the initially allocated torque in a circuit is not sufficient, e.g. when different pressure levels are present in the circuits. According to one design, when the hydraulic pump associated with the first circuit is unable to satisfy the volumetric flow requirement of the first circuit, and when the interconnection of the two circuits is prevented, the hydraulic pump associated with the second circuit is actuated according to a reduced volumetric flow requirement. The reduced volumetric flow requirement is determined such that the ratio of the reduced volumetric flow requirement to the volumetric flow actually conveyed by the hydraulic pump associated with the first circuit is equal to the ratio of the original volumetric flow requirements of the two circuits. This is expedient in order to maintain a concurrent movement of consumers in a situation where, due to power and torque limitation, no interconnection occurs, i.e. the interconnection is prevented because the total power demand or total torque demand exceeds a maximum power or maximum torque of the motor. Original volumetric flow requirements refer herein to the initially detected volumetric flow requirements for both circuits, e.g. based on user inputs via an operating device. According to one design, for each circuit, the volumetric flow requirement is determined based on at least one control signal for the at least one consumer of the respective circuit. The at least one control signal is in particular an at least one operating signal detected by an operator device or is based on such a signal. The operator equipment, e.g. a joystick, is configured so as to sense an input of an operator of a work machine in which the hydraulic system is installed. According to one design, for each circuit, the pressure requirement is determined based on a pressure reading, in particular a load pressure reading. The pressure readings can be detected using corresponding pressure sensors. In the event that a pressure control of the hydraulic pump is performed by means of an electronic controller, both a load pressure sensor and a pump pressure sensor can be provided. In the case of a purely hydraulic pressure control via a load pressure line, at least one of a pump pressure sensor or a load pressure sensor can be provided. A computing unit according to the invention, e.g. a control unit of a hydraulic system of a mobile work machine, is configured, in particular in terms of programme technology, so as to carry out a method according to the invention. The implementation of a method according to the invention in the form of a computer programme or computer programme product having programme code for carrying out all of the method steps is also advantageous, because doing so results in especially low costs, in particular if an executing control device is still used for further tasks and is therefore provided in any case. Suitable data carriers for providing the computer programme are in particular magnetic, optical and electrical memories, such as hard drives, flash memories, EEPROMs, DVDs, etc. A download of a programme via computer networks (internet, intranet, etc.) is also possible. Further advantages and embodiments of the invention result from the description and the attached drawings. It is understood that the features specified hereinabove and those yet to be explained hereinafter are usable not only in the combination specified in each case, but also in other combinations, or on their own, without departing from the scope of the present invention. The invention is shown schematically by way of exemplary embodiments in the drawings and will be described in detail hereinafter with reference to the drawings. Description of the drawings Fig. 1 shows a hydraulic system having two hydraulic circuits, in which a method according to the invention can be applied. Fig. 2 shows a flow chart according to one embodiment of the invention. Detailed description of the drawings Fig. 1 shows, in a greatly simplified manner, a hydraulic system having two hydraulic circuits, in which a method according to the invention can be applied. For example, the hydraulic system can be installed in a mobile work machine, such as an excavator or wheel loader. The hydraulic system comprises two adjustable hydraulic pumps, e.g. a hydraulic pump 2 and a hydraulic pump 3, which are driven together by a motor 4 (drive motor). The motor 4 is, for example, an internal combustion motor (in particular a diesel motor) or an electric motor. The motor 4 is mechanically coupled to the two hydraulic pumps 2, 3 so as to drive them. This coupling comprises at least one shaft and / or at least one transmission. Therefore, for each of the two hydraulic pumps, their speed is equal to the speed of the motor (i.e. the motor speed) or equal to the motor speed times a translation ratio (if a transmission is provided). It is also possible that the speeds of the hydraulic pumps can be different (depending on the assembly of a transmission). Each of the hydraulic pumps has a suction port connected to a tank and a pressure port connected to a respective hydraulic pressure line. For example, the hydraulic pump 2 is connected to a pressure line 6 and the hydraulic pump 3 is connected to a pressure line 7. The hydraulic pumps 2, 3 are configured such that, when driven by the motor 4, they convey pressure medium (i.e. hydraulic fluid, for example a hydraulic oil) from the suction port to the pressure port, i.e. they convey pressure medium from the tank into the respective pressure line 6, 7. At the respective pressure port or in the respective pressure line, the pressure medium has a pressure referred to as the pump pressure, e.g. a pump pressure measured by a pump pressure sensor 22 at the pressure port of the hydraulic pump 2 or in the pressure line 6 and a pump pressure measured by a pump pressure sensor 23 at the pressure port of the hydraulic pump 3 or in the pressure line 7. The hydraulic pumps 2, 3 are adjustable or configured as variable displacement pumps, i.e. their displacement is adjustable, approximately corresponding to a pivot angle. In the context of this application, the term “hydraulic pump” is used for ease of use, rather than saying “adjustable hydraulic pump” or “variable displacement pump”. The term displacement (or swallowing capacity or conveyance volumes) refers to the volume of pressure medium conveyed by the hydraulic pump per revolution. The two hydraulic pumps 2, 3 can, for example, be configured as axial piston machines in a swash plate or swash axis design. The setting of the displacement of the hydraulic pumps 2, 3 is performed, e.g. electromagnetically, hydraulically or hydraulically with electromagnetic pilot control. In the case of a hydraulic setting, a load pressure or load pressure signal (LS) is tapped on hydraulic consumers, passed to the hydraulic pump via a load pressure line (LS line) and the displacement is thus set. For example, a setting piston is provided that causes the displacement of the hydraulic pump to be set, wherein the load pressure is used in order to actuate a pressure scale across which the pressure medium is conducted to the setting piston, such that a pressure difference between the load pressure and the pump pressure remains constant. In the case of a hydraulic setting with electromagnetic pilot control, the flow of pressure medium to the setting piston is controlled via an electromagnetically controlled pressure control valve. In the case of an electromagnetic setting or a hydraulic setting with electromagnetic pilot control, corresponding control signals, e.g. in the form of electrical control currents with which electromagnets are impinged, can be generated by a control unit or an electronic controller 10 (computing unit) of the hydraulic system, or their generation can be initiated. For this purpose, the electronic controller 10 can detect pump pressure readings and / or load pressure readings measured with pump pressure sensors and load pressure sensors, i.e. receive them from the pressure sensors. Electrical lines for control signals and sensor signals are not shown for the sake of visual clarity. The pressure lines 6, 7 are connected to respective valve assemblies 12, 13, e.g. the pressure line 6 is connected to a valve assembly 12 and the pressure line 7 is connected to a valve assembly 13. The valve assemblies 12, 13 each comprise one or more valves (not shown in detail), via which the volumetric flows of pressure medium can be guided from the pressure line 6, 7 to a respective at least one hydraulic consumer 14, 15 (for the sake of simplicity, referred to as a consumer) and then from the at least one hydraulic consumer 14, 15 back to the tank. The control of the volumetric flow is done, for example, with electromagnetic control valves controlled by the electronic controller 10. For example, at least one hydraulic consumer 14 is connected to the valve assembly 12 and at least one hydraulic consumer 15 is connected to the valve assembly 13. The hydraulic consumers each independently comprise at least one hydraulic cylinder and / or at least one hydraulic motor. For example, hydraulic cylinders that cause the movement of a boom of a work machine. With the assembly described above, two hydraulic circuits are formed: a (hydraulic) circuit including the hydraulic pump 2 and the at least one hydraulic consumer 14, as well as the pressure line 6 and the valve assembly 12, and a (hydraulic) circuit including the hydraulic pump 3 and the at least one hydraulic consumer 15, as well as the pressure line 7 and the valve assembly 13. The hydraulic circuits are used in particular as LUDV circuits or LUDV systems (LUDV: load-independent flow distribution), i.e. the valve assemblies are each designed such that the volumetric flow to individual consumers is independent of the load present on the respective consumer, e.g. the extension speed of a hydraulic cylinder is then independent of the load moved by the hydraulic cylinder. This can be achieved, e.g. as is known to the person skilled in the art, by means of pressure scales (so-called individual pressure scales) provided for each of the consumers, wherein the respective pressure scale controls the pressure drop via a control valve that guides the flow of pressure medium to the respective consumer, such that the pressure drop remains constant, in particular takes on a preset value, such as by means of a spring. The volumetric flow to the consumer is then only dependent on the setting, i.e. the flow cross-section, of the control valve, which is in particular performed electromagnetically by the electronic controller 10. The control signals for the valve assemblies 12, 13 are determined by the electronic controller 10 based on operating signals for the respective at least one consumer 14, 15. The operating signals are determined by an operating device or user interface (not shown, e.g. a joystick) based on input from an operator. The control signals can also be generated at least in part according to automatic functions. For example, in such a way that a particular motion curve is traversed by machine elements moved by the at least one hydraulic consumer, e.g. in response to a triggering (pressing a button or the like) by the operator. The electronic controller 10 is configured so as to determine, for each of the two hydraulic circuits, a volumetric flow requirement or a required volumetric flow value and a pressure requirement or a required pressure value. The volumetric flow requirement or the required volumetric flow value for a circuit can be determined from the operating signals for the at least one hydraulic consumer 14, 15 in this circuit and / or the corresponding control signals for the valve assembly 12, 13 in this circuit. For example, as a volumetric flow of the at least one hydraulic consumer of the circuit in order to achieve the desired movement (as indicated by the operating signal), wherein, in the case of a plurality of consumers in a circuit, the individual volumetric flows are added together. To determine the pressure requirement or the required pressure value of a circuit, a prevailing load pressure can be measured by means of a pressure sensor in the respective circuit (for example, by load pressure sensors 20, 21) (e.g. on a load pressure line provided in the respective valve assembly) and transmitted as a load pressure measurement value to the electronic controller 10. For example, as the pressure requirement or required pressure value of a circuit, the load pressure measurement value itself can be used, a required pump pressure value necessary to achieve the load pressure reading, wherein a pressure drop caused by the volumetric flow requirement can be taken into account via the valve assembly, or a corrected required pump pressure value equal to the required pump pressure value plus a tolerance (e.g. 5% of the required pump pressure value or a nominal pressure of the hydraulic pump), in particular in the form of a control reserve. In particular, the electronic controller 10 (when the hydraulic pumps are set in an electro-magnetic manner or in a hydraulic electro-magnetic manner with pilot control) carries out pressure control of the hydraulic pumps according to the pressure requirement, wherein the pump pressure is set to a desired pressure according to the pressure requirement. The respective pump pressure can be measured with pressure sensors (for example, with the pump pressure sensors 22, 23 in the particular circuit). The target pressure can be, depending on the configuration, the required pressure value or the required pressure value plus a control distance (a positive pressure difference value). The motor 4 has a maximum available power (e.g. in kW), also referred to as a maximum power, or a maximum available torque (e.g. in Nm), also referred to as a maximum torque. Power and torque are generally speed-dependent. It is assumed here that the speed of the motor and thus the speeds of the hydraulic pumps are of a given value. It is further assumed that the rotational speeds of the hydraulic pumps are equal to the motor speed (a possible translation ratio unequal to one could also be considered, for example, in the displacement of the respective hydraulic pump). Because the power and the torque are proportional to each other (with the speed as the proportionality factor), then, at the given speed, the power and the torque can be considered equivalent. The maximum available power or the maximum available torque will in particular be released to the hydraulic pumps or the circuits according to the ratio of the two volumetric flow requirements. In so doing, it can be determined whether the actual required torque (which is substantially proportional to the product of displacement and pump pressure) in a circuit is less than the torque released for that circuit. If this is the case, then, if the actual required torque of the other circuit is higher than the torque released for it, the difference (i.e. the actual required torque released) can be provided to the other circuit or hydraulic pump. If a volumetric flow requirement of zero is required (i.e. if no hydraulic consumer is to be moved) or a very low torque is required in a circuit, it can additionally be provided that a predetermined minimum torque is provided to this circuit or its hydraulic pump. As a result, for example, an idle pressure can be maintained. The procedure described above relates to operating situations in which the hydraulic circuits are supplied with pressure medium independently from one another, despite being driven together by the motor. It can also occur that one of the hydraulic pumps, even when set to its maximum possible displacement, is unable to satisfy the volumetric flow requirement in the circuit in which this hydraulic pump is included or associated (e.g. when the volumetric flow requirement is greater than the product of maximum possible displacement and speed). In this case, the circuits can be hydraulically connected such that pressure medium can be exchanged between them, in particular between the pressure lines. The hydraulic system of Fig. 1 further comprises a summation device 30, with which the two circuits can be hydraulically connected optionally, i.e. by means of corresponding control, e.g. by the electronic controller 10, such that pressure medium can be exchanged between the two circuits. The summation device 30 in particular comprises a control valve or proportional valve, e.g. a spool valve, which is set in an electromagnetic manner or in a hydraulic electro-magnetic manner with pilot control. The control of the summation device 30 is done in particular by the electronic controller 10, which generates the respective control signals or causes them to be generated. The summation device 30 is connected to the pressure lines of the two circuits and the pressure ports of the two hydraulic pumps, respectively. In the case of an interconnection (i.e. in an interconnected state), the summation device is actuated to open a flow opening between the two circuits or the two pressure lines, such that pressure medium can flow from one circuit to the other (depending on the pressure ratios present). A cross-section of the flow opening in the control valve of the summation device is in particular controllable or adjustable. In the unconnected state, the flow opening of the summation device is closed, such that no pressure medium can be exchanged. The unconnected state is, for example, the normal state of the summation device, wherein for example the control valve is biased to a corresponding normal position. In addition to optionally interconnecting the pressure lines, it can additionally be provided that the summation device is configured so as to also join or interconnect load pressure lines of the two circuits when the two circuits are interconnected. For each circuit, it can be determined whether the hydraulic pump of the respective circuit is able to satisfy the volumetric flow requirement of that circuit. This is done in particular by comparing the volumetric flow requirement or the required volumetric flow value to a maximum volumetric flow of the hydraulic pump, which is given as a product of maximum possible displacement and speed of the hydraulic pump. When a hydraulic pump is unable to satisfy the volumetric flow requirement, and when the other hydraulic pump has not reached its maximum volumetric flow and / or the torque provided to it is not yet utilised, it can be provided that the two circuits are optionally interconnected (as part of the method according to the invention) by means of the summation device 30 and the conveyed volumetric flow of the other hydraulic pump is increased, such that a summation volumetric flow is supplied via the summation device to the circuit whose hydraulic pump is unable to satisfy the volumetric flow requirement, which is given in particular as a difference between the volumetric flow requirement and the maximum volumetric flow of the hydraulic pump. Because different pressure levels or pump levels may exist in the two circuits, due to the different pressure requirements, additional power dissipation generally occurs during interconnection. This is the case in particular when an upward summation is carried out, i.e. when pressure medium is to be conveyed from the circuit with the smaller pressure requirement into the circuit with the higher pressure requirement, because in this case the pressure medium in the circuit with the smaller pressure requirement must be increased. This pressure increase includes the pressure medium that serves to supply the consumers in this circuit itself and for which the lower pressure level would actually be sufficient. The power dissipation generally occurs as heat on valves that reduce the pressure again to the necessary level (in the case of a downward summation, e.g. also at the summation device). When the cross-section of the flow opening in the control valve of the summation device is controllable or adjustable, different pressure levels can be maintained in the event of a downward summation. The summation device can also comprise a pressure scale. This pressure scale (or LUDV pressure scale) ensures that the pressure difference is kept constant via the summation device or its control valve in the event of a downward summation (and thus not too much pressure medium flows to the other pump or to the other circuit). In the case of the upward summation, the pressure scale ensures that the pressure medium does not flow in the wrong direction and that the pump pressure in which the conveyance is to take place is reported as the load pressure in the other pump circuit. Due to the power dissipation that occurs in addition to the power required in each circuit by the at least one consumer (substantially by the product of pressure requirement and volumetric flow requirement), an interconnection can occur in the event that a power limitation or torque limitation, i.e. the maximum power or the maximum torque, of the motor is reached or exceeded. The result is that the power provided by the motor, which only provides power and torque within these limitations, is less than the actual power required, such that the movements of all consumers are subsequently slowed down against the desired movement. According to the invention, a method is proposed by which this problematic situation is avoided. The method will be described below with reference to Fig. 2. The method can be performed, for example, by the electronic controller 10 of the hydraulic system. Fig. 2 shows a flow chart of a method for operating a hydraulic system having two hydraulic circuits according to one embodiment of the invention. The hydraulic system is structured as described above in connection with Fig. 1. That is to say, each of the circuits comprises at least one hydraulic consumer, wherein two adjustable hydraulic pumps are provided and each of the two circuits is associated with one of the two hydraulic pumps, which provide the pressure medium in the respective circuit, wherein the two hydraulic pumps are driven together by one motor, and wherein a summation device is provided, which is configured so as to optionally, corresponding to an actuation, establish an interconnection between the two circuits, in which the two circuits are hydraulically connected. In step 100, for each of the two circuits, a respective volumetric flow requirement and pressure requirement are determined. The volumetric flow requirement is determined from, for example, control signals for the consumer or the respective valve assembly. The pressure requirement can be determined from pressure readings, in particular a load pressure. In step 110, it is determined whether for one of the circuits, referred to as the first circuit, the hydraulic pump associated therewith is unable to satisfy the volumetric flow requirement without interconnecting with the other of the circuits, referred to as the second circuit. In particular, it is determined whether the hydraulic pump associated with the first circuit can satisfy or not satisfy the volumetric flow requirement of the first circuit, even at maximum displacement. In step 120, based on the determined volumetric flow requirements and pressure requirements, a total power demand or total torque demand is determined that would be needed in the case of interconnection of the two circuits in order to drive the two hydraulic pumps. For this purpose, e.g. if an upward summation were to take place, that is to say, when the pressure requirement in the second circuit is lower than the pressure requirement in the first circuit, initially the volumetric flow requirements are added and multiplied by the higher of the two pressure requirements, i.e. in the event of an upward summation by the pressure requirement in the first circuit (wherein possibly prior to multiplication, a pressure difference which takes into account in particular a pressure drop via the summation device in the case of interconnection, can be added to the higher pressure requirement), in order to determine the total power demand. The total torque demand can be determined analogously, wherein initially a total displacement or effective displacement needed in order to achieve the total volumetric flow requirement, i.e. the sum of the two volumetric flow requirements, is determined by means of the speed (substantially, i.e. ignoring leakage, as the total displacement is equal to total volumetric flow requirement by speed). In step 130, it is checked whether the total power demand or total torque demand exceeds a maximum power output or torque of the motor. The maximum power or torque can be known or transmitted by a motor controller. In step 140, when it is determined in step 130 that the total power demand or total torque demand exceeds the maximum power output or the maximum torque of the motor, the interconnection of the two circuits is prevented. Optionally, when the hydraulic pump associated with the first circuit is unable to satisfy the volumetric flow requirement of the first circuit, the volumetric flow requirement of the second circuit can additionally be reduced to a lesser volumetric flow requirement, such that the ratio of reduced volumetric flow requirement of the second circuit to the actual conveyed volumetric flow in the first circuit is equal to the ratio of the original volumetric flow requirements of the two circuits. In the optional step 150, when the hydraulic pump associated with the first circuit is unable to satisfy the volumetric flow requirement of the first circuit and when the total power demand or total torque demand does not exceed the maximum power or the maximum torque of the motor, the two circuits are interconnected.

Claims

1. A method for operating a hydraulic system having two hydraulic circuits, each of which comprises at least one hydraulic consumer (14, 15), wherein two adjustable hydraulic pumps (2, 3) are provided and each of the two circuits is associated with one of the two hydraulic pumps, which provide the pressure medium in the respective circuit, wherein the two hydraulic pumps are driven together by one motor (4), and wherein a summation device (30) is provided, which is configured so as to selectively, corresponding to an actuation, establish an interconnection between the two circuits, in which the two circuits are hydraulically connected; wherein, for each of the two circuits, a respective volumetric flow requirement and pressure requirement are determined (100);wherein it is determined (110) whether, for a first of the circuits, the hydraulic pump (2, 3) associated therewith is unable to satisfy the volumetric flow requirement without interconnecting with the other (the second) of the circuits;wherein, based on the determined volumetric flow requirements and pressure requirements, a total power demand or total torque demand is determined (120) that would be needed in the case of interconnection of the two circuits in order to drive the two hydraulic pumps (2, 3); and,wherein, when the total power demand or total torque demand exceeds a maximum power output or a maximum torque of the motor (4), the interconnection of the two circuits is prevented (140).

2. The method according to claim 1, wherein, in particular when the pressure requirement in the second circuit is lower than the pressure requirement in the first circuit, a total volumetric flow requirement is determined as the sum of the volumetric flow requirements of the first and second circuit, and the total power demand is determined from the higher of the pressure requirements and the total volumetric flow, and the total torque demand is determined from the higher of the pressure requirements and a total displacement that is needed in order to achieve the total volumetric flow requirement.

3. The method according to claim 1, wherein, in particular when the pressure requirement in the second circuit is lower than the pressure requirement in the firstcircuit, the total power demand is determined as the sum of a first and a second individual power demand, wherein the first individual power demand is determined from the volumetric flow requirement of the first circuit and the higher of the pressure requirements, and wherein the second individual power demand is determined from the volumetric flow requirement of the second circuit and the higher of the pressure requirements; and / or wherein the total torque demand is determined as the sum of a first and a second individual torque demand, wherein the first individual torque demand consists of the higher of the pressure requirements and the displacement of the hydraulic pump associated with the first circuit that is needed in order to satisfy the volumetric flow requirement of the first circuit, and wherein the second individual torque demand is determined from the higher of the pressure requirements and the displacement of the hydraulic pump associated with the second circuit that is needed in order to satisfy the volumetric flow requirement of the second circuit.

4. The method according to any one of the preceding claims, wherein, when it is determined that, for the first circuit, the hydraulic pump (2, 3) associated therewith is unable to satisfy the volumetric flow requirement without connection to the second circuit, and when the total power demand or total torque demand does not exceed the maximum power or the maximum torque of the motor (4), the interconnection of the two circuits is established such that a summation volumetric flow from the second circuit into the first circuit occurs via the summation device (30).

5. The method according to claim 4, wherein, when the pressure requirement of the first circuit lies above the pressure requirement of the second circuit, an adjustment of the pressure requirement of the second circuit to the pressure requirement of the first circuit is made; wherein, in particular, a pressure drop via the summation device is taken into account.

6. The method according to any one of the preceding claims, [wherein] the hydraulic pumps (2, 3) are actuated such that the volumetric flow requirement of the circuit with which they are respectively associated is satisfied or, if a hydraulic pump is unable to satisfy the latter, is satisfied to the extent possible.

7. The method according to any one of the preceding ciaims, wherein, when both hydraulic pumps (2, 3) are able to satisfy the volumetric flow requirement of the circuit with which they are associated, having in particular a sufficiently large maximum displacement, the maximum power or the maximum torque of the motor (4) is allotted to the hydraulic pumps, such that the displacements of the hydraulic pumps are or can be adjusted according to the ratio of volumetric flow requirements.

8. The method according to claim 7, wherein, when the hydraulic pump (2, 3) of one of the circuits requires less power or torque than the power or torque allocated thereto in order to satisfy the volumetric flow requirement and the pressure requirement of the circuit with which this hydraulic pump is associated, the power or torque beyond the power or torque allotted thereto is made available to the other hydraulic pump.

9. The method according to any one of the preceding claims, wherein, when the hydraulic pump (2, 3) associated with the first circuit is unable to satisfy the volumetric flow requirement of the first circuit, and when the interconnection of the two circuits is prevented, the hydraulic pump (3, 2) associated with the second circuit is actuated according to a reduced volumetric flow requirement; wherein the reduced volumetric flow requirement is determined such that the ratio of the reduced volumetric flow requirement to the volumetric flow actually conveyed by the hydraulic pump associated with the first circuit is equal to the ratio of the original volumetric flow requirements of the two circuits.

10. The method according to any one of the preceding claims, wherein, for each circuit, the volumetric flow requirement is determined based on at least one control signal for the at least one consumer of the respective circuit; wherein the at least one control signal is in particular at least one operating signal detected by an operating device or is based on such a signal.

11. The method according to any one of the preceding claims, wherein, for each circuit, the pressure requirement is determined based on a pressure reading, in particular a load pressure reading.

12. A computing unit (10) comprising a processor configured so as to carry out the method according to any one of the preceding claims.

13. A computer programme comprising instructions which, when executed by a computer, prompt the latter to carry out the method according to claims 1 to 11.

14. A computer-readable data carrier on which the computer programme according to claim 13 is stored.

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