Hydrostatic Axial Piston Pump for a Hydrostatic Traction Drive

Abstract

A hydrostatic axial piston pump for a hydrostatic traction drive has an adjusting unit for adjusting the stroke volume. The adjusting unit has an actuating cylinder with two counteracting actuating pressure chambers. An actuating pressure can be set in each actuating pressure chamber via a separate pressure reducing valve, the said actuating pressure depending on and preferably being proportional to a current intensity on a respective solenoid of the respective pressure reducing valve. The two current intensities can be calculated via an electronic control unit on a model basis as a function of a speed and of an operating pressure of the axial piston pump and as a function of an operating element for a driver's request, e.g. an accelerator pedal.

Claims

1. A hydrostatic axial piston pump for a hydrostatic traction drive, the axial piston pump comprising: an adjusting unit configured to adjust a stroke volume, the adjusting unit including an actuating cylinder defining a first actuating pressure chamber in which a first actuating pressure is set via a first pressure reducing valve, the first actuating pressure depending on a first current intensity applied to a first solenoid of the first pressure reducing valve; and an electronic control unit configured to determine the first current intensity based on an actual rotational speed, an operating pressure of the axial piston pump, and an operating element that receives a driver's request.

2. The hydrostatic axial piston pump according to claim 1, wherein the actual rotational speed and the first actuating pressure acts in a first direction of an increase in the stroke volume, while the operating pressure acts in a second direction of a reduction in the stroke volume.

3. The hydrostatic axial piston pump according to claim 1, wherein: the hydrostatic piston pump is configured to be operated both as a motor and as a pump, the actuating cylinder defines a second actuating pressure chamber, which acts counter to the first actuating pressure chamber and in which a second actuating pressure is set via a second pressure reducing valve, the said actuating pressure depending on a second current intensity applied to a second solenoid of the second pressure reducing valve, and the electronic control unit is configured to determine the second current intensity based on the actual rotational speed, the operating pressure, and the operating element.

4. The hydrostatic axial piston pump according to claim 1, wherein the first pressure reducing valve is proportional such that the first actuating pressure is proportional to the first current intensity.

5. The hydrostatic axial piston pump according to claim 3, wherein the first and second pressure reducing valves are proportional such that the respective first and second actuating pressures are proportional to the respective first and second current intensities.

6. The hydrostatic axial piston pump according to claim 1, wherein the electronic control unit is configured to determine the first current intensity based on a characteristic map stored in the electronic control unit.

7. The hydrostatic axial piston pump according to claim 1, wherein the electronic control unit is further configured to determine a target pivoting angle of the axial piston pump on a model basis as a function of the operating element.

8. The hydrostatic axial piston pump according to claim 7, wherein the electronic control unit is further configured to determine the target pivoting angle using a volume flow balance.

9. The hydrostatic axial piston pump according to claim 1, wherein the electronic control unit is further configured to determine an output-guided actuating pressure, which is minimally linked with the first actuating pressure.

10. The hydrostatic axial piston pump according to claim 1, wherein feedback is provided, with which a speed controller is formed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] An exemplary embodiment of an axial piston pump according to the disclosure is illustrated in the figures, in which:

[0019] FIG. 1 shows a circuit diagram of the exemplary embodiment of the axial piston pump according to the disclosure,

[0020] FIG. 2 shows a first logic diagram of an electronic control unit of the axial piston pump from FIG. 1,

[0021] FIG. 3 shows a further logic diagram of the electronic control unit of the axial piston pump from the preceding figures,

[0022] FIG. 4 shows a characteristic map of the electronic control unit of the axial piston pump from the preceding figures,

[0023] FIG. 5 shows a characteristic curve of the electronic control unit of the axial piston pump from the preceding figures, and

[0024] FIG. 6 shows a characteristic map of the electronic control unit of the axial piston pump from the preceding figures.

DETAILED DESCRIPTION

[0025] FIG. 1 shows a circuit diagram of the exemplary embodiment of the axial piston pump according to the disclosure. Only the components of the axial piston pump that are relevant to the disclosure are described. The axial piston pump has a casing 1, on which there are formed two working connections A, B, to each of which a working line (not shown) of a closed circuit is connected. A traction drive for a mobile working machine (not shown) is formed therewith.

[0026] The axial piston pump is implemented with a swash plate 2, of which the pivoting angle .sub.pmp can be set via an adjusting unit 4. Here, use is made of a double-acting actuating cylinder 6 which has a first actuating pressure chamber 8.sub.1 and a second actuating pressure chamber 8.sub.2 acting counter thereto.

[0027] A first control pressure p.sub.st1 acts in the first actuating pressure chamber 8.sub.1 in the direction of an increase in the pivoting angle .sub.pmp and therefore in the direction of an increase in the stroke volume Vol.sub.pmp of the axial piston pump. This is counteracted by a second actuating pressure p.sub.st2 in the second actuating pressure chamber 8.sub.2 in the direction of a reduction in the pivoting angle .sub.pmp and therefore in the direction of a reduction in the stroke volume Vol.sub.pmp of the axial piston pump. It is therefore possible to define an actuating pressure difference p.sub.st=p.sub.st1p.sub.st2, wherein this actuating pressure difference p.sub.st by definition always acts in the direction of an increase in the pivoting angle .sub.pmp and in the stroke volume Vol.sub.pmp.

[0028] Via a drive shaft 10 of the axial piston pump, its drive mechanism 12 and, going beyond this, also a feed pump 14 is driven. The drive shaft 10 is driven by a diesel engine (not shown) and rotates at a rotational speed n.sub.pmp. This rotational speed n.sub.pmp acts together with the actuating pressure difference p.sub.st in the direction of an increase in the pivoting angle .sub.pmp. More precisely, an increase in the rotational speed n.sub.pmp acts in this way.

[0029] If the axial piston pump shown supplies multiple traction motors of the mobile working machine via its working connections A, B, let it be assumed that B is the high-pressure connection during forward travel, so that the channel connected to the working connection B is characterized by high pressure HD, while the other channel, connected to the working connection A, is characterized by low pressure ND. The high pressure HD, which is also designated as operating pressure, acts in the direction of a reduction in the pivoting angle .sub.pmp. These relationships are designated as a characteristic of the axial piston pump and are stored in an electronic control unit 16 as formulas and/or as characteristic maps or characteristic curves. In addition, the rotational speed n.sub.pmp and the actuating pressure difference p.sub.st and the high pressure HD are measured. Therefore, it is possible to move to operating points of the axial piston machine according to the disclosure without any feedback in the sense of a control loop being necessary for this purpose.

[0030] The two actuating pressures p.sub.st1, p.sub.st2 are controlled via two pressure reducing valves 18.sub.1, 18.sub.2. These each have an electric solenoid a, b, which are connected to the electronic control unit 16 via a respective electric line 20.sub.1, 20.sub.2. The two pressure reducing valves 18.sub.1, 18.sub.2 are designed in such a way that the respective actuating pressure p.sub.st1, p.sub.st2 is proportional to the respective current intensity I.sub.1, I.sub.2.

[0031] The inlets of the two pressure reducing valves 18.sub.1, 18.sub.2 are supplied by the feed pump 14 via a feed pressure line 22.

[0032] FIG. 2 shows a preparatory first logic diagram of the electronic control unit 16 from FIG. 1. A ramp over time is calculated from an absolute speed limit and from the position of the operating element 24 configured as an accelerator pedal. From this, by incorporating an actual speed v.sub.act of the mobile working machine, a speed limit is calculated.

[0033] According to FIG. 3, the speed limit, together with a rotational speed n.sub.eng of the internal combustion engine (not shown) is converted via a formula 26 into a target pivoting angle .sub.pmp of the axial piston pump.

[0034] The hydraulic motor or motors (not shown) of the traction drive concerned is/are either constant-displacement motors, or the pivoting angle .sub.mot is determined and set in accordance with the formula 28.

[0035] PmpSpdLim is calculated by an addition of three values, which are determined with respective characteristic curves 30, 32, 34. In the first characteristic curve 30, the target pivoting angle .sub.pmp previously calculated with the formula 26 is taken into account. In the second characteristic curve 32, the high pressure HD present in the working line is taken into account. In the third characteristic curve 34, the actual rotational speed n.sub.pmp of the drive shaft 10 (cf. FIG. 1) of the axial piston pump is taken into account.

[0036] The logic diagram of FIGS. 2 and 3 therefore shows the calculation of the target pivoting angle .sub.pmp on the basis of the volume flow balance Q.sub.pmpQ.sub.leak=Q.sub.mot. From the target pivoting angle .sub.pmp, an actuating pressure is determined via the spring characteristic curve 30 and the characteristic curve 32 mapping the high pressure. In addition, via the rotational speed-dependent characteristic curve 34, the rotational speed-depended restoring forces are compensated. Instead of these characteristic curves 30, 32, 34, formulas (functional equations) can also be used.

[0037] The resultant actuating pressure acts in a limiting manner on a pump activation PmpCtrl, which limits the output as an overload safeguard.

[0038] The target value for the speed can be calculated simply from the operating element 24 and the maximum speed. In addition, the maximum dynamics are limited.

[0039] FIG. 4 shows a characteristic map 36, in which the relationship between the pivoting angle .sub.pmp (x axis) and the first actuating pressure p.sub.st1 and the actuating pressure difference p.sub.st (y axis) and the operating pressure HD (family of curves) mapped. The relationship of these three values that is shown is based on the real physical properties of the axial piston pump. Thus, for example, with a given operating pressure HD (e.g. 300 bar) at a target pivoting angle .sub.pmp (e.g. 50%), determined via the operating element, a necessary actuating pressure p.sub.st1 or an actuating pressure difference p.sub.st (about 17 bar in this example) can be determined by the electronic control unit 16, in order to supply the solenoids a, b of the two pressure reducing valves 18.sub.1, 18.sub.2 with the respective current intensity I.sub.1, I.sub.2 proportionally thereto.

[0040] Also illustrated in FIG. 4 are the boundaries of a pressure cut-off 38 and the boundaries of an output limit 40.

[0041] From rotational speed n.sub.pmp and calculated pivoting angle .sub.pmp, in accordance with a pump characteristic curve the maximum permissible actuating pressure p.sub.St1 or the maximum permissible actuating pressure difference p.sub.st is determined which would necessitate the maximum permissible high operating pressure HD on the working connection A, B. If the actuating pressure output in accordance with the driver's request lies above this actuating pressure, it is limited.

[0042] If, therefore, the maximum permissible operating pressure HD (e.g. 450 bar) of the traction drive is reached, the pressure cut-off intervenes and cuts back the actuating pressure p.sub.st1 or the actuating pressure difference p.sub.st. As a result, even with a load rising further, exceeding the maximum operating pressure HD can be avoided.

[0043] FIG. 5 shows a single characteristic curve on the principle of the characteristic map from FIG. 4. At a predetermined operating pressure HD, the previously described relationship between the pivoting angle .sub.pmp and the actuating pressure p.sub.st1 or the actuating pressure difference p.sub.st is plotted.

[0044] The diagram shows spring forces: limitation of the angular dependence.

[0045] FIG. 6 shows a single characteristic curve, which illustrates the relationship between the high pressure HD of the axial piston pump (x axis) and the actuating pressure difference p.sub.st on the double-acting actuating cylinder 6. Here, account is taken of the fact that the axial piston pump can also act as a motor during overrun operation of the traction drive concerned. The pressure side in the axial piston pump is therefore changed, which is expressed by the negative operating pressure values HD. Accordingly, the second actuating pressure p.sub.st2 must outweigh the first actuating pressure p.sub.st1, which is illustrated by the negative values of the actuating pressure difference p.sub.st.

[0046] 1. By actuating the operating element 24 equipped as an accelerator pedal, the driver increases the rotational speed n.sub.eng of the diesel engine from idling to nominal rotational speed. In accordance with the eDA control, depending on the rotational speed n.sub.eng, an activation signal is generated for the pressure reducing valves 18.sub.1, 18.sub.2 or their ET controllers. At nominal rotational speed, the maximum speed of travel is to be obtained, therefore the actuating pressure difference p.sub.st is raised to a specific value. Since there is still no load, the axial piston pump pivots out completely and supplies maximum volume flow Q.sub.max.

[0047] 2. As a result of the driving resistances, during driving on the level a load pressure is established, for example 250 bar (Q point, the actuating pressure is set at nominal rotational speed such that the hydraulic output P*Q now corresponds to the nominal output of the diesel engine).

[0048] 3. Under higher load (e.g. the mobile working machine is traveling uphill, picking up gravel), the operating pressure HD rises and the axial piston pump pivots back as a result of the restoring forces. The actuating pressure difference p.sub.st in the ET controller is, however, not reduced by pressure reducing valves 18.sub.1, 18.sub.2.

[0049] Disclosed is a hydrostatic axial piston pump for a hydrostatic traction drive having an adjusting unit for adjusting the stroke volume. The adjusting unit has an actuating cylinder with two counteracting actuating pressure chambers. An actuating pressure can be set in each actuating pressure chamber via a separate pressure reducing valve, the said activating pressure depending on and preferably being proportional to a current intensity on a respective solenoid of the respective pressure reducing valve. The two current intensities can be calculated via an electronic control unit on a model basis as a function of a rotational speed and of a operating pressure of the axial piston pump and as a function of an operating element for a driver's request, e.g. an accelerator pedal. In principle, the disclosure can also be applied to axial piston pump having only a single-acting actuating cylinder if no overrun operation is required, by the axial piston pump changing to a motor mode.