Method

Abstract

The invention relates to a method for providing a fluid supply for hydraulic loads (17) using a variable-speed drive (10) for a fluid pump (12) with a variable fluid delivery volume Vg, comprising a control device (58), which changes the fluid delivery volume Vg of the fluid pump (12) on the basis of at least a hydraulic feedback of a pressure, and comprising a sensor system, which consists at least of a pressure sensor (36) for detecting the actual pressure pIst and a sensor (38) for detecting a measurement variable which is proportional to the fluid delivery volume Vg of the fluid pump (12), in particular in the form of a pivot angle aIst. The method is characterized in that at least the actual pressure pIst and the actual pivot angle alst are the respective input variables for an electronic controller (34) which determines at least the rotational speed n of the variable-speed drive (10) as a respective output variable and, as a function of at least one one-dimensional or one multi-dimensional characteristic map, a target pivot angle o-'s oii which serves as a control deviation e for the rotational speed control while being continuously compared with the actual pivot angle a'lst.

Description

[0001] Proceedings

[0002] The invention relates to a method for providing a fluid supply for hydraulic consumers using a variable-speed drive for a fluid pump with a variable fluid delivery volume, with a control device that changes the fluid delivery volume of the fluid pump depending on a hydraulic pressure feedback, and with a sensor system, at least consisting of a pressure sensor for detecting the actual pressure p Ist and a sensor for detecting a measured quantity proportional to the fluid delivery volume of the fluid pump, in particular in the form of a swivel angle a IstDE 2012 009 136 A1 discloses a method for operating a fluid pump in which a pumping unit delivering a certain volume per revolution is driven by a rotating drive, wherein the pumping unit is adjustable and / or the drive is variable in speed, and wherein an actual volume flow rate is regulated to a target volume flow rate and / or an actual delivery pressure is regulated to a target delivery pressure by specifying, in the case of an adjustable pumping unit, a target value for the delivery volume of the pumping unit and, in the case of a variable-speed drive, a target value for the speed of the drive, wherein at least one of the specified target values ​​is applied to a pilot value that depends on the actual delivery pressure. In this way, the operation of a fluid pump, in particular a variable-speed pump, is improved in the event of load fluctuations, and load-dependent leakage losses can also be effectively addressed.

[0003] DE 10 2012 016 780 B4 discloses a method for operating a fluid pump in which a pumping unit delivering a certain volume per revolution is driven by an electric drive rotating at a specific speed. The pumping unit is adjustable, and the drive is variable in speed. The actual flow rate is regulated to a target flow rate, and / or the actual discharge pressure is regulated to a target discharge pressure. For fixed discharge volumes, the rotational speed of the drive is specified as the control variable. In full-load operation of the drive, a first, larger discharge volume is specified, and in partial-load operation of the drive, a second, smaller discharge volume is specified. This improves the energy efficiency of variable-speed pumps, particularly during partial-load phases.

[0004] Based on this prior art, the invention aims to further improve known solutions, in particular to provide a particularly energy-efficient system. A method comprising all the features of claim 1 solves this problem.

[0005] By virtue of the characterizing part of claim 1, at least the actual pressure p lst and the actual swivel angle a Ist the respective input variables for an electronic control unit (ECU), which as its respective output variable is at least the rotational speed n of the variable-speed drive and, depending on at least one one-dimensional or one multi-dimensional characteristic map, a target swivel angle a SoU determined by continuous comparison with the actual swivel angle a istAs a control deviation e for the speed control or control device, a type of demand-based flow control is implemented, which is taken over by a hydraulic-mechanical controller of the fluid pump, in particular in the form of a variable displacement pump. In this way, speed control for the variable-speed drive can be achieved with the aim of being able to regulate the highest possible swivel angle (70-90%) for the fluid or variable displacement pump in an energy-efficient manner.

[0006] In a preferred embodiment of the method according to the invention, it is provided that the target swivel angle a Soü depending on the respective actual pressure p Ist / The target oscillation angle a is specified by means of at least one table of values ​​as a one-dimensional characteristic map, serving as the reference variable for speed control. Preferably, for efficient operation of the fluid supply when using such a table of values, the target oscillation angle a is defined. SoUin the following area: 0.7 a max < a SoU < 0.9 a max Accordingly, the speed setting on the variable-speed drive is used to determine a target swivel angle a. Soli to regulate the fluid pump or the variable displacement pump. The following applies to the main operating range: p < p max the a soii < a max ' st - For p > p max may also apply a Soll < a^. Furthermore, the target swivel angle is a So u as well as the maximum swivel angle a max The scope of care varies depending on the specific circumstances. a The maximum value is determined via the table of values ​​depending on the pressure.

[0007] In a further preferred embodiment of the method according to the invention, it is provided that the target swivel angle a SoUis determined via a multi- or multidimensional characteristic map, using either a 3D characteristic map of a multidimensional function or a model-based method that incorporates state variables resulting from the operation of the

[0008] Fluid pump results, especially in the form of the actual pressure p Ist , the actual rotational speed n Ist / the current consumption l DC , the voltage U DC , and the temperature T

[0009] Accordingly, the target swivel angle a Soü a multidimensional determination is specified, in which at least two quantities are used; in the simplest case a 3D characteristic map, but also a multidimensional function as well as a model I-based method, where one quantity is the pressure p Ist and as at least one further state variable of the speed-variable drive, e.g. the actual speed n lst , the current consumption I DC , the voltage U DCor / or the temperature T is used.

[0010] Furthermore, the target swivel angle a Soü can be specified via a multidimensional determination using at least three quantities; in the simplest case, via a multidimensional function, but also via a model-based method, where one quantity is the pressure p. lst , another of the measured swivel angles a lst and at least one other state variable of the variable-speed drive is used, e.g. the actual speed n lst / the current consumption I DC , the voltage U DC and / or the temperature T.

[0011] Further advantageous embodiments of the method according to the invention are the subject of the dependent claims. The dependent claims also include a device for carrying out the method described above, as well as the use of the method incorporating the device.

[0012] The inventive method is explained in more detail below with reference to exemplary embodiments shown in the drawing. Figure 1 shows, in principle and not to scale, the basic components for controlling a variable displacement pump;

[0013] Figs. 2 to 4, in the form of circuit diagrams, show various variants of methods for controlling the variable displacement pump according to Fig. 1;

[0014] Figs. 5 to 7 are examples of value tables as used in the speed control for the process variants according to Figs. 2 to 4;

[0015] Figs. 8 and 9 show examples of a multidimensional determination of a target swivel angle a. SoU with two or three different input variables;

[0016] Fig. 10 Example of a characteristic curve with an electro-proportional limit of the displacement volume V;

[0017] Fig. 11 Representation of a gradient limit with limitation of an increasing or decreasing gradient / rate of change;

[0018] Figs. 12 to 14 symbolic representation of a controller and two saturation curves for a PI controller and an extended PI controller respectively;

[0019] Figures 15 and 16 illustrate the use of the method for controlling a closed-center with load sensing and an open-center hydraulic circuit, respectively, using two circuit diagrams.

[0020] Fig. 1 shows a variable-speed drive 10 for a fluidic, in particular hydraulic, fluid pump in the form of a variable displacement pump 12, which draws fluid medium from a storage tank 14 and feeds it into a supply circuit 16 with a predefinable fluid volume and pressure, to which at least one hydraulic consumer 17 is regularly connected (see Figs. 15 and 16). The variable-speed drive 10 has, in the usual manner, a frequency converter 18 and an electric motor 20, whose output shaft 22 is connected via a coupling 24, for example in the form of a clutch, to the drive shaft 26 of the variable displacement pump 12 for the purpose of driving the latter.

[0021] Furthermore, a hydraulic-mechanical load-sensing (LS) controller 28 is provided for controlling the variable displacement pump 12, which can also be a rotary angle pump. This controller receives hydraulic load-sensing (LS) signals or hydraulic negative flow control (NFC) signals from the respective hydraulic consumer 17 via a signal line 30 in the usual manner. The hydraulic-mechanical controller 28, or load-sensing (LS) controller, features an electro-proportional limit of the displacement volume V. g via a limiter 32.

[0022] Furthermore, an electronic control unit (ECU) 34 is provided, which can also be integrated as a processing unit in the inverter 18. A pressure sensor 36 is connected in the supply circuit 16, which transmits the respective actual pressure in the supply circuit 16 on the fluid outlet side of the variable displacement pump 12 to the input side of the electronic control unit 34. The variable displacement pump 12 is also equipped with a swivel angle sensor 38, which measures the actual swivel angle a. Ist as a further input variable to the electronic control unit 34. This in turn is connected to the inverter 18 via a data exchange line 40 and allows bidirectional data exchange of data such as the target speed n Soll , the actual rotational speed n / st , the control current I DC , the control voltage U DCas well as the temperature T. An electro-proportional flow rate control together with a load-sensing controller, as designated 28, 32 in Fig. 1, is state of the art and is shown by way of example in the HYDAC International product catalog “Axially piston pump PPV100M” with publication number DE2.918.2 / 09.22. Overall, Fig. 1 shows the so-called “hardware setup” for the variable-speed pump 12.

[0023] Figure 2 shows, in the form of a circuit diagram, a variant for the speed control of the variable displacement pump 12. The circuit diagram symbolically shows the pressure sensor 36 and the swivel angle sensor 38, which receives the actual pressure p as a pressure signal. lst as well as the actual swivel angle signal a lstThe electronic control unit (ECU) 34 receives input data. Three filters 42 are stored in the ECU 34, which filter the input data accordingly and can be designed as high-pass or low-pass filters. The simplest form of filtering is the determination of the moving average over a certain number of data points according to the formula planned.

[0024] Furthermore, the controller 34 contains two value tables 44 and 46, each as a one-dimensional characteristic map (1 and 2), as exemplified in Figures 5 and 6, respectively. These tables use the pressures p' and p" as input variables and represent the swivel angle profile a as a function of the corresponding input pressures p' and p". The output variables for value table (1) 44, as shown in Figure 5, are a maximum swivel angle a. maxand for the table of values ​​(2) 46 according to Fig. 6 a target swivel angle a SoU If one overlays the two value tables 1 and 2 as shown in Fig. 7, it is noticeable that the maximum swivel angle a max always above the target swivel angle a So u according to the table of values ​​(2). The determined a max The values ​​from the table of values ​​(1) 44 form a control current I according to Fig. 2. a max for the limiter 32, which provides electro-proportional limitation of the displacement volume V g for the fluid or variable displacement pump 12. Following the table of values ​​(2) 46, as shown in Fig. 6, a gradient limit 48 preferably follows, as exemplified in Fig. 11, which defines the course of the target swivel angle a. SoU over time t with a right-angled ramp-like rising and falling gradient 50 or 52, which is represented by dashed lines 54 as a 'Soli is shown. In this respect, Fig. 11 illustrates the limitation for both the rising and falling gradients for the swivel angle α.

[0025] According to the illustration in Fig. 2, the two swivel angle values ​​a' SoU and a' lst At a node 56, the input values ​​are compared, and the difference value is passed as a control deviation e to the input side of a controller 58 as part of the control device, usually in the form of a PI controller. The controller 58 outputs a speed value. SoU The electric motor 20 is supplied with power, which is output to the inverter 18, which has integrated speed control. A PID controller can also be used instead of the PI controller 58.

[0026] The controller 58, symbolically represented in Fig. 12, for example in the form of a conventional PI controller which can also be used multiple times, can, in addition to the control deviation e on the input side, also the maximum and minimum speed values ​​n max or n min as output variables, which on the output side of the controller 58 result in a regulated target speed n Soa for the electric motor 20 with the inverter 18 interposed. Accordingly, it further follows from the illustration in Fig. 13 that the active operating or control range 60 of the controller 58 takes place between two saturation ranges 62, with speed values ​​above n max or rotational speed values ​​below n min Figure 14, however, refers to the use of a so-called extended PI controller 58, in which the maximum rotational speed n max depending on the pump pressure p lstat the output of the variable displacement pump 12. Here too, the controller 58 operates within its control range within the saturation limits. Speed ​​values ​​above and below n max or lie outside the corresponding control range 60, which is based on a maximum speed value n max increases, with the rotational speeds varying depending on the pressure p.

[0027] In the above-presented method according to variant 1 of the speed control, the value tables (1) 44 and (2) 46 according to Figs. 5 and 6 are in any case coordinated with each other and stored in the control such that the following applies to the main working range of the variable displacement pump 12:

[0028] In addition, it may be provided that for p > p max Additionally, the following should apply: SoU < a max .

[0029] Furthermore, the tables of values ​​(1) and (2) are preferably coordinated such that for

[0030] P Pmax gi't- 0.7 U max ^ Q-setpoint <0.9 d max .

[0031] Overall, the solution described above provides a method for supplying fluid to hydraulic consumers 17 using a variable-speed drive 10 for a fluid or variable displacement pump 12 with a variable fluid delivery volume, with a control device in the form of at least one controller 58 that controls the fluid delivery volume V g the variable displacement pump 12 is changed depending on a hydraulic pressure feedback and is equipped with a sensor system, at least consisting of a pressure sensor 36 for detecting the actual pressure p Ist and a sensor 38 for detecting a measured quantity that is proportional to the fluid delivery volume V g the fluid pump 12 is, in particular in the form of a swivel angle a' Ist According to the invention, it is provided that at least the actual pressure p lst and the actual swivel angle a Istthe respective input variables for the electronic control unit (ECU) 34, which as respective output variables is at least the rotational speed n of the variable-speed drive 10 and, depending on at least one one-dimensional characteristic map in the form of the table of values ​​(2) 46, a target swivel angle a Soü determined by continuous comparison with the actual swivel angle a Ist as a control deviation e for the speed control of the variable speed drive 10.

[0032] In this context, reference should also be made to Figure 10, which shows an example of a characteristic curve of the displacement volume or delivery volume V. a shows the course of the control current. of the limiter 32 is applied.

[0033] The further embodiments and process variants, particularly those shown in Figures 3 and 4, are described only insofar as they differ significantly from the preceding embodiment shown in Figure 2. For the same components as specified above, the same reference numerals are used, and the statements made so far also apply to the modified embodiments. Thus, in variant 2 according to Figure 3, the first value table 44 is still used as a one-dimensional characteristic map; however, instead of the second characteristic map 46, a multi-dimensional characteristic map 64 is used, as shown by way of example in Figure 8, with two inputs 66 for input variables, as shown by way of example in Figure 8, where the output variable is again a target swivel angle a. SoThe multidimensional characteristic map 64 thus specifies a processing function F, depending on the input variables shown as examples in Figure 8. This can be a 3D characteristic map, a multidimensional function, or even a model-based method. Accordingly, according to circuit diagram variant 2 in Figure 3, the target swivel angle a is... SoU specified via a multi-dimensional characteristic map 64, in which at least two quantities are used as input variables at the inputs 66, wherein preferably one quantity is the pressure p" as it results on the output side after the filter 42, which on the input side measures the actual pressures p lst The pressure sensor 36 receives the input signal. The subsequent input variable at the lower input 66 is then another state variable of the variable-speed drive 10, for example in the form of the actual rotational speed n. Ist , the current consumption l' DC , the voltage U' DCor the temperature T', which are each forwarded via the bidirectional data exchange line 40 from the converter 18 to a fourth filter 42, which forwards its filtered output data as state variables to the lower input 66 of the multidimensional characteristic map 64 according to Figure 8. The statements made so far regarding the filters 42 also apply to the further fourth filter 42 according to Figure 3, which is connected to the data exchange line 40 upstream of the multidimensional characteristic map 64.

[0034] The embodiment according to Figure 4 is modified compared to Figure 3 in that, instead of the multidimensional characteristic map 64, a multi-dimensional characteristic map 68 is used, as shown in Figure 9, with a further additional input 66, via which the actual swivel angles a' Istare passed on to the multidimensional characteristic map 68. These are filtered in a branch 70 after the actual swivel angles a detected by the swivel angle sensor 38 have been filtered. lst forwarded. Otherwise, these modified swivel angle values ​​a'j will be st After filtering using the lower filter 42 as already described, the data is forwarded to node 56 to determine the control deviation e. Compared to variant 2 according to Figure 3, the multidimensional determination using the associated characteristic map 68 around the actual swivel angle a' is therefore different. Ist extended by the associated filter 42.

[0035] For both method variant 2 and method variant 3 according to Figures 3 and 4 respectively, the table of values ​​(1) 44 according to Figure 5 and the multi-dimensional determination of the target swivel angle a are SoUaccording to the multidimensional characteristic map 64 or the multidimensional characteristic map 68, such that in every case the following applies: a soii < a max for p < p max and in addition for p > p max applies

[0036] The multidimensional or multidimensional characteristic maps 64, 68 mentioned here are known from the control and regulation of so-called non-linear processes, such as those that occur in engine management in motor vehicles, which are used to take non-linear effects into account in the process. Further details are shown, for example, in the INCA V7.4 I Tutorial R03 DE I 09.2023 manual and in a technical article by Henning Tolle in METHODEN (at 3 / 2004), entitled "On some locally generalizing memories and their further development, Part 1: CMAC / AMS and MIAS," which describes such multidimensional or multidimensional characteristic maps in the form of topographies. While in a multidimensional function, according to the representation in Figure 3, the target swivel angle a SoUIf the function of the state variables required for the calculation is, for example, pressure, rotational speed, current, voltage, temperature, or the actual swivel angle, then a purely mathematical description is preferable, whereas in multidimensional or model-based methods, according to the principle diagram in Figure 4, a predefinable model is used to calculate the target swivel angle a. SoU is used, whereby the model takes physical effects in the process into account.

[0037] The following describes the use of a previously described

[0038] The method, along with the device shown, is explained in more detail using two application examples according to Figures 15 and 16. Figure 15 shows two hydraulic motors 72 as consumers 17, each of which can be stopped via a corresponding valve control 74, corresponding to the intermediate switching position shown. Depending on the valve position to the right or left of this, the associated hydraulic motor 72 can be controlled in both directions of rotation. At a changeover valve 76, which is connected in bypass to the associated hydraulic motor 72, the highest pressure is transmitted as load pressure via the load-sensing or signal line 30 in the form of a load-sensing signal to the hydraulic-mechanical LS controller 28 with the electro-hydraulic limiter 32.The variable speed drive 10 used in Figure 15 according to Figure 1 is therefore used for a so-called closed-center solution with load-sensing tap via the associated signal line 30.

[0039] The embodiment shown in Figure 16 is a further supply solution for hydraulic consumers 17; 72, this time designed as a so-called open-center solution and in a so-called NFC (Negative Flow Control) version, in which, when the valve control 74 is de-energized, a central channel 78 to the storage tank 14 is opened, and the signal line 30 is connected to transmit measurement signals to the aforementioned central channel 78, specifically before the fluid enters an orifice or throttle 80 with a spring-loaded check valve 82 running parallel to it. In this way, the fluid pressure in the supply circuit 16 downstream of the pressure sensor 36, when the valve control 74 is in its middle position, is transmitted as a measurement signal (Negative Flow Control) via the signal line 30 to the hydraulic-mechanical NFC controller 28 together with limiter 32, as already shown in Figure 1.

[0040] Within the scope of the present invention description, the swivel angle a is equivalent to the displacement volume or fluid delivery volume V. g a fluid or variable displacement pump 12. Accordingly, the swivel angle a represents the ratio of the displacement volume V g to the geometrically maximum achievable displacement volume V g max This means that the swivel angle 'a' is fundamentally defined independently of the pump design and does not necessarily correspond to a measured angle. For some pump types, such as axial piston pumps with an inclined design, a linear relationship for the swivel angle can be assumed to a good approximation based on the measured angle of the swashplate.

[0041] As an example, a pump 12 with a fluid delivery volume of 18 cm³ 3 The following should be mentioned:

[0042] A swivel angle of 100% or 1 corresponds to a fluid flow volume V to a good approximation. g of 18 cm 3

[0043] A swivel angle of 50% or 0.5 corresponds to a fluid flow volume V to a good approximation. g of 9 cm 3

[0044] A swivel angle of 10% or 0.1 corresponds to a fluid flow volume V to a good approximation. g of 1.8 cm3.

[0045] Another significant feature of the presented control concept is the use of the hydraulic-mechanical controller 28 (a control device that changes the fluid delivery volume of the fluid pump depending on a hydraulic pressure feedback), thus eliminating the need for information about the pressure or volume flow rate as a digital signal for the control. Furthermore, there is no need to explicitly address the control of the swivel angle. This has no equivalent in the prior art.

Claims

Patent claims 1. Method for providing a fluid supply for hydraulic consumers (17) using a variable speed drive (10) for a fluid pump (12) with a variable fluid delivery volume, with a control device (58) that controls the fluid delivery volume the fluid pump (12) is changed depending on at least one hydraulic pressure feedback, and with a sensor system, consisting at least of a pressure sensor (36) for detecting the actual pressure p lst and a sensor (38) for detecting a measured quantity proportional to the fluid flow volume V g the fluid pump (12) is, in particular in the form of a swivel angle a Istl characterized by the fact that at least the actual pressure p lst and the actual swivel angle a Istthe respective input variables for an electronic control unit (34) are, which as respective output variables are at least the rotational speed n of the variable-speed drive (10) and, depending on at least one one-dimensional (44, 46) or one multi- or multidimensional characteristic map (64, 68), a target swivel angle a' So ii determined, which is continuously compared with the actual swivel angle a' Ist serves as the control deviation e for speed control.

2. Method according to claim 1, characterized in that the target swivel angle a Soli depending on the current pressure p lst , is specified by means of at least one table of values ​​(44, 46) as a one-dimensional characteristic map and as a reference variable for speed control.

3. Method according to claim 1 or 2, characterized in that for efficient operation of the fluid supply when using a Table of values ​​(44, 46), the target swivel angle aSo u in the following range 0.7 < a Soll < 0.9 a max lies.

4. Method according to one of the preceding claims, characterized in that the target swivel angle a SoU is determined via a multi- or multidimensional characteristic map (64, 68) using a 3D characteristic map of a multidimensional function, or a model I based method including state variables such as those resulting from the operation of the fluid pump, in particular in the form of the actual pressure p Ist , the actual rotational speed n / st , the current consumption I DC / the voltage U DC , and the temperature T 5. Method according to one of the preceding claims, characterized in that the maximum swivel angle a max depending on the current pressure p Ist is variable via at least one table of values ​​(44, 46) as the one-dimensional characteristic map.

6. Method according to one of the preceding claims, characterized in that the controller (58), which on the output side specifies the speed setting n Soll for the variable speed drive (10) at least a PI controller, preferably a PID controller, is supplied.

7. Method according to one of the preceding claims, characterized in that the controller (58) has a limited operating or control range (60) which is between a minimum speed «Tnin and a maximum speed n^r and outside the control range (60) the controller (58) is in a saturation (62) where the output value is no value below the minimum speed and above the maximum speed n TOax assumes.

8. Method according to any of the preceding claims, characterized in that the control range (60) is determined by a variable maximum speed n^y- depending on the respective actual pressure p. lstvia at least one table of values ​​as the one-dimensional characteristic map variable rst.

9. Method according to one of the preceding claims, characterized in that the respective table of values ​​(44, 46) is generated using the input variable p Ist after prior digital signal processing by at least one filter (42) the output quantities a max or a SoU This results in a table of values ​​1 (44) generally producing larger values ​​than a table of values ​​2 (46) on its own.

10. Method according to one of the preceding claims, characterized in that for the digital signal processing of the respective actual pressure p Ist at least one filter (42) is used which calculates a moving average over a predefinable number of data points, in particular according to the formula 1 1. Method according to one of the preceding claims, characterized in that the respective output value of the value table 2 (46), which corresponds to the target swivel angle a S(M corresponds to a digital limit of the gradient (54) before the control deviation (e) for the rotational speed control is formed.

12. Device for carrying out the method according to one of the preceding claims, characterized in that the control device (28) of the fluid pump (12), which controls the fluid delivery volume V g depending on a hydraulic pressure feedback, via an electro-hydraulic limit (32) of the maximum fluid delivery volume V g max has and is controlled by the control unit (34) such that the achievable fluid delivery volume V g in the operating range (60) the maximum swivel angle a max corresponds.

13. Device according to claim 12, characterized in that at least one variable speed drive (10) with an inverter (18) and an electric motor (20) connected thereto is provided, which drives a fluid pump (12) with a variable fluid delivery volume drives and with an electronic control unit (34) that controls the inverter (18) and the electro-hydraulic limiting (32) of the fluid delivery volume V g the fluid pump (12) and with a sensor system, at least consisting of a pressure sensor (36) for detecting the actual pressure p Ist and a sensor (38) for detecting a measured quantity proportional to the fluid delivery volume V g the fluid pump (12) is, in particular in the form of an actual swivel angle a lst , which serves as at least one of the respective input variables for the electronic control unit (34).

14. Use of a method according to one of claims 1 to 11 and a device according to claim 12 or 13, characterized in that a control device (28) which changes the fluid delivery volume of a fluid pump (12) depending on a hydraulic feedback of a pressure is used to be employed in closed-center with load sensing or open-center supply circuits (16).

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