Hydrostatic travel drive and method for controlling the hydrostatic travel drive

Abstract

A hydrostatic travel drive includes a hydraulic pump for the purpose of supplying pressure medium to a hydraulic motor of the travel drive that can be coupled to an output, which pump can be coupled to a drive machine. The hydraulic pump has an actuating cylinder with at least one cylinder chamber and a swept volume which can be adjusted via the actuating cylinder, and at least one electrically activatable pressure valve via which the cylinder chamber can be charged with an adjustingly active actuating pressure. The travel drive further includes device via which a pressure of the hydraulic pump can be limited by means of influencing the actuating pressure.

Claims

1. A hydrostatic travel drive comprising: a hydraulic pump coupled to a drive machine and configured to supply pressure medium to a hydraulic motor of the hydrostatic travel drive, the hydraulic motor coupled to an output of the hydrostatic travel drive, the hydraulic pump having an actuating cylinder that includes at least one cylinder chamber and is configured to adjust a swept volume of the hydraulic pump; at least one electrically activatable pressure valve configured to charge the at least one cylinder chamber with an adjustingly active actuating pressure; and an electronic control unit configured to limit an outlet pressure of the hydraulic pump by controlling the actuating pressure, wherein the electronic control unit is configured to limit the outlet pressure using an actuating pressure model, and wherein the actuating pressure model includes a characteristic map of the hydraulic pump, in which the actuating pressure is described as a function of the outlet pressure and at least one of the swept volume of the hydraulic pump and of a variable of the hydraulic pump representative of the swept volume.

2. The hydrostatic travel drive according to claim 1, wherein the actuating pressure model includes a characteristic curve of the hydraulic pump, the characteristic curve representing the actuating pressure as a function of a limit of the outlet pressure and at least one of the swept volume of the hydraulic pump and of a variable of the hydraulic pump representative of the swept volume.

3. The hydrostatic travel drive according to claim 2, wherein the actuating pressure is described in the characteristic curve as a function of a rotational speed of the hydraulic pump.

4. The hydrostatic travel drive according to claim 2, wherein the electronic control unit is configured to determine a maximum permissible actuating pressure from the characteristic curve.

5. The hydrostatic travel drive according to claim 1, wherein the actuating pressure is described in the characteristic map as a function of a rotational speed of the hydraulic pump.

6. The hydrostatic travel drive according to claim 1, further comprising: a request interface configured be signal-connected to the electronic control unit and configured to transmit a speed request to the electronic control unit.

7. The hydrostatic travel drive according to claim 6, wherein the electronic control unit is configured to determine a necessary actuating pressure based upon the speed request and the characteristic map.

8. The hydrostatic travel drive according to claim 7, wherein the electronic control unit is configured to at least one of (i) determine a lower of a maximum permissible outlet pressure and the necessary actuating pressure, and (ii) select the lower of the maximum permissible outlet pressure and the necessary actuating pressure.

9. The hydrostatic travel drive according to claim 1, wherein the electronic control unit includes a valve characteristic curve of the pressure valve, the valve characteristic curve describing an electric activating current as a function of the actuating pressure.

10. The hydrostatic travel drive according to claim 1, wherein the electronic control unit is configured to limit the outlet pressure to a maximum permissible outlet pressure using the actuating pressure model.

11. A method for limiting an outlet pressure of a hydraulic pump of a travel drive that includes (i) the hydraulic pump, which is coupled to a drive machine and configured to supply pressure medium to a hydraulic motor of the hydrostatic travel drive, the hydraulic motor coupled to an output of the hydrostatic travel drive, the hydraulic pump having an actuating cylinder that includes at least one cylinder chamber and is configured to adjust a swept volume of the hydraulic pump, (ii) at least one electrically activatable pressure valve configured to charge the at least one cylinder chamber with an adjustingly active actuating pressure, and (iii) an electronic control unit configured to limit a pressure of the hydraulic pump, and an actuating pressure model, the method comprising: determining, with the electronic control unit, a necessary actuating pressure according to a speed request from a characteristic map of the hydraulic pump in the actuating pressure model, the characteristic map representing the actuating pressure as a function of the outlet pressure and at least one of a swept volume of the hydraulic pump and of a variable of the hydraulic pump representative of the swept volume; and controlling the actuating pressure with the electronic control unit so as to limit the outlet pressure of the hydraulic pump using the determined necessary actuating pressure.

12. The method according to claim 11 further comprising: at least one of (i) determining, with the electronic control unit, a traction mode or braking mode of the travel drive, and (ii) determining, with the electronic control unit, a direction of travel of the travel drive; and selecting, with the electronic control unit, at least one of a characteristic curve and a characteristic map of the hydraulic pump and/or of the pressure valve in dependence on the determined mode and/or on the determined direction of travel.

13. The method according to claim 11, further comprising: determining, with the electronic control unit, a maximum permissible actuating pressure from a characteristic curve of the hydraulic pump, the characteristic curve representing the actuating pressure as a function of a limit of the outlet pressure and at least one of a swept volume of the hydraulic pump and of a variable of the hydraulic pump representative of the swept volume.

14. The method according to claim 13 further comprising: determining, with the electronic control unit, a lower of the necessary actuating pressure and the maximum permissible actuating pressure; based on the determined lower of the necessary actuating pressure and the maximum permissible actuating pressure, determining, with the electronic control unit, an electric activating current of the pressure valve from a valve characteristic curve of the pressure valve, the valve characteristic curve representing the electric activating current as a function of the actuating pressure; and with the electronic control unit, activating the pressure valve with the determined electric activating current.

15. The method according to claim 11, wherein controlling the actuating pressure with the electronic control unit so as to limit the outlet pressure of the hydraulic pump using the actuating pressure model comprises: controlling the actuating pressure with the electronic control unit so as to limit the outlet pressure of the hydraulic pump to below a maximum permissible outlet pressure limit using the actuating pressure model.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Two exemplary embodiments of a hydrostatic travel drive according to the disclosure and one exemplary embodiment of a method according to the disclosure for controlling it are illustrated in the drawings. The disclosure will now be explained in more detail with reference to the figures of these drawings, in which

(2) FIG. 1a shows a hydraulic circuit diagram of a hydrostatic travel drive according to a first exemplary embodiment,

(3) FIG. 1b shows a hydraulic circuit diagram of a hydrostatic travel drive according to a second exemplary embodiment,

(4) FIG. 2 shows a simple block diagram of a method for controlling the hydrostatic travel drive with input and output variables according to one exemplary embodiment,

(5) FIGS. 3a and 3b show characteristics of the hydrostatic travel drive according to FIGS. 1a and 1b, and

(6) FIG. 4 shows a detailed block diagram of the method according to FIG. 2.

DETAILED DESCRIPTION

(7) According to FIG. 1a, a hydrostatic travel drive 1 has a hydraulic pump 2 which is fluidically connected in a closed hydraulic circuit via the working lines 4 and 6 to a hydraulic motor (not shown) to supply said drive with pressure medium. Here, the hydraulic pump 2 is coupled to a drive machine (not shown) via a drive shaft 8 for transmitting a torque. Here, the coupling is not stepped up, with the result that the rotational speed of the drive machine and of the hydraulic pump 2 are identical. The hydraulic pump 2 is designed as an axial piston pump of swashplate construction and can be operated in both directions of rotation and both in the pump mode and in the motor mode. It has an adjustable displacement volume V.sub.P and an adjusting device 10 designed as a double-acting hydraulic cylinder. The hydraulic cylinder 10 has a first cylinder chamber 12 and a second cylinder chamber 14 which counteracts the first. The first cylinder chamber 12 is connected via a first actuating pressure line 16 to the outlet of a first pressure-reducing valve 18. The latter is connected to a control pressure line 20 which can be supplied with control pressure medium via a control pressure connection p.sub.S and via a feed pump 22 which is seated on the same drive shaft 8 as the hydraulic pump 2. In the same way, the second cylinder chamber 14 is connected via a second actuating pressure line 24 to a second pressure-reducing valve 26 which is connected to the control pressure line 20. The pressure-reducing valves 18, 26 are electromagnetically actuatable, wherein the actuating pressure p.sub.a or p.sub.b resulting in each case in the actuating pressure line 16 or 24 is, according to a valve characteristic curve, proportional to an activating current I.sub.a or I.sub.b of the electromagnet a orb. The actuating pressures p.sub.a, p.sub.b of the cylinder chambers 12, 14 can thus be controlled via the electromagnetic actuation of the pressure-reducing valves 18, 26 via the specification of the activating currents I.sub.a, I.sub.b. For this purpose, the electromagnets a, b of the pressure-reducing valves 18, 26 are signal-connected to an electronic control unit 32 via a respective signal line 28 or 30.

(8) Furthermore, the hydrostatic travel drive 1 has a rotational speed detection unit 34 via which a rotational speed n.sub.P of the hydraulic pump 2 can be detected and via which a signal line 36 can be transmitted to the electronic control unit 32. The travel drive 1 likewise has a rotational speed detection unit (not shown) via which the rotational speed n.sub.M of the hydraulic motor can be detected and via which signal line 38 can be transmitted to the electronic control unit 32.

(9) For safety-relevant pressure safeguarding of the working lines 4, 6 against overload, the hydrostatic travel drive 1 has in each case a pressure-limiting valve 40 which is connected to the respective working line 4, 6. Both pressure-limiting valves 40 are connected by their outlets to a feed pressure line 44 which is connected to the feed pump 22. The feed pressure line 44 is fluidically connected via a throttle 42 to the control pressure line 20. In the case of the activation of the pressure-limiting valves, pressure medium is thus expanded into the feed pressure line 44, with the result that energy losses are lower than if the expansion occurred toward the tank T. The pressure-limiting valves 40 each have a feeding or replenishing function in the form of a nonreturn valve.

(10) The hydrostatic travel drive 1 can be operated both in the traction mode and in the overrun or braking mode. In the traction mode, the hydraulic pump 2 operates in the pump mode, and in the braking mode it operates in the motor mode. In addition, the hydraulic pump 2 is reversible, that is to say its displacement volume V.sub.P is adjustable via the adjusting device 10 on both sides of a neutral position with zero volume V.sub.P=0. As a result, a reversal of direction of travel is possible with the direction of rotation of the drive shaft 8 and of the drive machine (diesel engine) remaining the same.

(11) The electronic control unit 32 is connected via a signal line 46 to an operator interface in the form of an accelerator pedal (not shown). Here, a speed request is transmitted to the electronic control unit 32 by a driver via the accelerator pedal. This can concern both the reverse travel and the forward travel. If the accelerator pedal is actuated, this corresponds to the traction or pump mode of the hydraulic pump 2, and if, by contrast, the accelerator pedal is moved back, this corresponds to the braking or motor mode of the hydraulic pump 2. The actuation of a service brake (not shown) also corresponds to the braking or motor mode of the hydraulic pump 2. The control unit is designed in such a way that it can determine the corresponding mode on the basis of said actuation. To select a direction of travel, the hydrostatic travel drive 1 additionally has an actuable direction of travel switch (not shown) which is signal-connected via a signal line 48 to the electronic control unit 32. In dependence on its position, the activation of the hydraulic pump 2 occurs in its reversed or nonreversed adjusting region, that is to say on either side of the neutral position of the swept volume of the hydraulic pump 2. The following driving states may be defined for further consideration:

(12) Forward travel, traction mode: pressurizing the first cylinder chamber 12 via the first actuating pressure line 16 and the first pressure-reducing valve 18 with the first actuating pressure p.sub.a by activating the first pressure-reducing valve 18 with the activating current I.sub.a via the control unit 32 via the first signal line 28.

(13) Forward travel, braking mode: pressurizing the second cylinder chamber 14 via the second actuating pressure line 24 and the second pressure-reducing valve 26 with the second actuating pressure p.sub.b by activating the second pressure-reducing vale 26 with the activating current I.sub.b via the control unit 32 via the signal line 30.

(14) Reverse travel, traction mode: pressurizing the second cylinder chamber 14 via the chain 24, 26, 30, 32.

(15) Reverse travel, braking mode: pressurizing the first cylinder chamber 12 via the chain 16, 18, 28, 32.

(16) In the two depicted exemplary embodiments of a hydrostatic travel drive 1; 101, the hydraulic pump 2 is designed in such a way that the pressure p which prevails in the high-pressure-carrying one of the working lines 4, 6 counteracts the then active actuating pressure p.sub.a or p.sub.b and is active in the direction of its own reduction. For this purpose, the hydraulic pump 2 has a structurally realized control loop. In the present case of the hydraulic pump 2 designed as an axial piston pump of swashplate construction, this is realized in such a way that a control disk of the hydraulic pump 2 is arranged in a twisted manner with respect to an axis of rotation of its cylinder drum. Mouths of those cylinders which are connected to the pressure kidney control disk having the pressure (high pressure) are thus arranged so as to be asymmetrically distributed with respect to a pivot axis of the swashplate. Also asymmetrically distributed are then the end portions, supported on the swashplate, of the working pistons guided in the cylinders. From the thus asymmetrically acting supporting forces of the working pistons there results, on the swashplate, a pivoting-back torque in the pump mode and a pivoting-out torque in the motor mode. There consequently occurs a relationship in the form of a pump characteristic curve or of a characteristic map of pump characteristic curves of the hydraulic pump 2 in which the respective actuating pressure p.sub.a, p.sub.b can be described in dependence on the pressure p and on the swept volume V.sub.P of the hydraulic pump 2, and on its rotational speed n.sub.P. These characteristic curves or characteristics maps are measured and stored in the electronic control unit 32 for processing, in particular for carrying out the subsequently described method.

(17) There follows the description of a driving mode of the hydrostatic travel drive 1; 101 with reference to FIGS. 3a, 3b. FIG. 3a illustrates a characteristic of the hydraulic pump 2 with open-loop-control, that is to say with directly controlled displacement or swept volume V.sub.P. The pressure p, to be more precise the pressure difference Δp between the working lines 4, 6, is plotted in dependence on the displacement volume V.sub.P. The first actuating pressure p.sub.a of the first cylinder chamber 12 is plotted as a parameter. This pressure increases starting from the origin 0/0. As limiting power, the nominal power of the diesel drive machine P.sub.nomeng is illustrated as a dashed curve. The starting point of the description is taken to be an unactuated gas or accelerator pedal and a drive machine rotating in idle at idle rotational speed. According to the arrow 1, there first occurs an actuation of the gas pedal by the operator, with the result that the rotational speed of the drive machine (diesel) is increased from idle to nominal rotational speed. Accordingly, an activating signal or activating current I.sub.a for the hydraulic pump 2, to be more precise for its first pressure-reducing valve 18, occurs via the electronic control unit 32 in dependence on the rotational speed of the diesel engine. On reaching the nominal rotational speed of the drive machine, a maximum driving speed of the travel drive 1 is obtained. Accordingly, the first actuating pressure p.sub.a is increased according to a characteristic map of the hydraulic pump 2 that is stored in the electronic control unit 32 according to FIG. 1a. Since there is still no load acting, the hydraulic pump 2 pivots out fully to its maximum swept volume V.sub.Pmax and delivers its maximum volumetric flow Q.sub.max at nominal rotational speed.

(18) As a result of occurring driving resistances, a pressure or load pressure p, for example 250 bar, is established when driving on level ground. This operation is symbolized in FIG. 3a by the arrow designated by the number 2. In the diagram according to FIG. 3a, a point Q is then reached which lies on the curve P.sub.nomeng. At this point, the first actuating pressure p.sub.a at the nominal rotational speed is dimensioned such that the hydraulic power p.sub.Qmax of the hydraulic pump 2 corresponds to the nominal power P.sub.nomeng.

(19) If then the load on the travel drive 1; 101 increases, for example when traveling uphill or, in the case of a wheeled loader, when loaded with gravel, the pressure p increases. By virtue of the aforementioned design of the hydraulic pump 2 in which, in the traction mode of the hydraulic pump 2 in forward travel, the working pressure p counteracts the first actuating pressure p.sub.a in the direction of a reduction of the swept volume V.sub.P, the pressure p pivots back the pivot cradle of the hydraulic pump 2, with the result that the travel slows. The first actuating pressure p.sub.a is not changed in the meantime and corresponds to the straight line (arrow 3) which intersects the point Q and which represents the reduction of the swept volume V.sub.P and simultaneous increase in the pressure p or the pressure difference Δp.

(20) On reaching a point L in the diagram according to FIG. 3a, a maximum permissible pressure p.sub.max or cut-off pressure, or a maximum permissible pressure difference Δp.sub.max, is achieved. The task of the electronic control unit 32 is then to ensure that this limit p.sub.max, Δp.sub.max is not exceeded. Accordingly, with further increasing load, there occurs no further increase in the pressure p by virtue of the fact that the first actuating pressure p.sub.a is reduced via the control unit 32 via the pressure-reducing valve 18 according to FIG. 1a in such a way that the pressure p.sub.max is not exceeded. Accordingly, a path is followed according to FIG. 3a along a blocking or limit curve which extends horizontally to the left from the point L with constant pressure p.sub.max or constant pressure difference Δp.sub.max. If thus, for example, a maximum permissible pressure p.sub.max of, for example, 450 bar is set in the control unit 32, the control unit 32 intervenes according to the pressure cut-off according to the disclosure and takes back the first actuating pressure p.sub.a. It is thus possible, even with further increasing load, to prevent the maximum permissible pressure p.sub.max from being exceeded.

(21) FIG. 2 shows a block diagram of one exemplary embodiment of a method 52 according to the disclosure. The following can be mentioned as input variables in the method 52: a pivot angle α.sub.P, proportional to the swept volume V.sub.P, of the hydraulic pump 2, its rotational speed n.sub.P and the limit p.sub.max, which is to be defined or is predetermined, of the maximum permissible working pressure. An electronic pressure cut-off function on the data base of an inverted pump characteristic occurs via the method 52. On the one hand, the actuating pressures p.sub.a, p.sub.b for the normal driving mode and p.sub.amax, p.sub.bmax as maximum permissible actuating pressures in dependence on the current driving mode (α.sub.P, n.sub.P) and on the limit p.sub.max are obtained as output variable via the method 52. This applies both to the traction or pump mode P and to the braking or motor mode M of the hydraulic pump 2.

(22) The pivot angle α.sub.P or the swept volume V.sub.P can be duly provided to the method 52, for example by detection. Alternatively, however, the method 52 stored in the control unit 32 for execution can have a step of balancing. For this purpose, the pivot angle α.sub.P is determined via the control unit 32 from the detected pump rotational speed n.sub.P, hydraulic motor rotational speed n.sub.M, and from the swallowing volume V.sub.M of the hydraulic motor that is known from the electroproportional direct activation. Since no leakage is included in this simple balance, this way of determining the pivot angle α.sub.P represents an estimation.

(23) FIG. 3b shows a characteristic of the vehicle or of the travel drive 1; 101, in which the tractive effort of the travel drive is plotted over the vehicle speed, wherein the nominal rotational speed of the drive machine is represented as a parameter.

(24) FIG. 1b shows a second exemplary embodiment of a hydrostatic travel drive 101 which is substantially identical to that according to FIG. 1a. Only the differences from the aforementioned exemplary embodiment according to FIG. 1a will therefore be discussed. There is now illustrated, in addition, a drive machine 54 which is coupled to the hydraulic pump 2 via the drive shaft 8 according to FIG. 1a and which has a drive machine control device 56. The latter is connected to the electronic control unit 32 via a CAN bus 58 and via a bus 60. Also connected to the bus 60 is a service interface 62 via which an operator or maintenance personnel has/have access to all the components connected to the bus 60. These components are, in addition, a direction of travel switch 64, a brake pedal 66, an accelerator or gas pedal 68, a driving mode selection switch 70, a speed limit switch 72, a speed limit selection switch 74 and a manual throttle switch 76.

(25) An axle or an output 80 is coupled to the hydraulic motor 78 illustrated. The hydraulic motor 78 is arranged via the working lines 4, 6 with the hydraulic pump 2 in the closed hydraulic circuit. It is designed as an axial piston machine of oblique axis construction with adjustable displacement volume V.sub.M. Here, its displacement volume V.sub.M is directly controlled in an electroproportional manner and therefore behaves proportionally to an activating current I.sub.M which is output by the electronic control unit 32.

(26) The preceding and the following considerations pertaining to the method 52 according to the disclosure apply to both hydrostatic travel drives 1; 101 according to FIGS. 1a and 1b. There follows the somewhat detailed description of the method 52 according to the disclosure with reference to FIG. 4.

(27) As already mentioned, the input variables of the method are the determined or estimated swept volume V.sub.P or the corresponding pivot angle α.sub.P of the hydraulic pump 2, its rotational speed n.sub.P, and a predetermined limit of the pressure or working pressure p.sub.max, in this case of 450 bar.

(28) According to FIGS. 1a and 1b, an actuating pressure model in the form of a pump characteristic or pump characteristic curve 82 for the operation of the hydraulic pump 2 in the traction mode (pump mode) and 84 for the braking or motor mode of the hydraulic pump 2 is stored in the electronic control unit 32 for each pressure limit p.sub.max. What is described here is in each case the respective actuating pressure p.sub.a or p.sub.b in dependence on the pump rotational speed n.sub.P and the pump pivot angle α.sub.P (in percent of the maximum pivot angle). The actuation pressures p.sub.a and p.sub.b are also represented in percent of the maximum available control or actuating pressure p.sub.smax in the control pressure line 20. Here, the scale of the second actuating pressure p.sub.b provided in the motor mode for the second actuating chamber 14 extends from zero to −50%. The plus and minus sign represents the different direction of action of the pressure on account of the counteracting actuating chambers 12, 14.

(29) According to the method 52, at first the respectively maximum permissible actuating pressure p.sub.amax for the traction mode and p.sub.bmax for the braking mode is determined in dependence on the current values n.sub.P, α.sub.P and p.sub.max of the operating state of the travel drive. This occurs permanently anew at fixed limit p.sub.max, since in particular α.sub.P changes in operation on account of its load dependency. For the traction mode there then occurs the matching of a requested first actuating pressure p.sub.a68 which originates from the actuation of the accelerator pedal 68 according to FIG. 1b (also valid for FIG. 1a). The last-mentioned actuating pressure and the determined maximum permissible first actuating pressure p.sub.amax are compared by the control unit 32 and the lower one is selected in step 86. The same analogously occurs in step 88 with the maximum permissible second actuating pressure p.sub.bmax. The in each case lower of the actuating pressures selected from the steps 86 and 88 is then again multiplied by the maximum available control pressure p.sub.smax, whereby an actual actuating pressure p.sub.a or p.sub.b results from the actuating pressure indicated until then in percent. Said actual actuating pressure enters into a respective valve characteristic curve 90 or 92 for the pressure-reducing valve 18 or 26, from which the associated activating current I.sub.a or I.sub.b for activating the pressure-reducing valve 18 or 26 is then determined. In this way there are determined, according to the disclosure, the actuating pressures p.sub.a, p.sub.b required in the normal driving mode and the maximum permissible actuating pressures p.sub.amax, p.sub.bmax from the rotational speed n.sub.P, the pivot angle α.sub.P, the pump characteristic curves 82, 84, the maximum permissible pressure p, the requested actuating pressure p.sub.a68 and the valve characteristic curves 90, 92, wherein the respectively lower actuating leads to the activating current I.sub.a, I.sub.b. Should thus the actuating pressure p.sub.a68 output according to the driver's demand lie above the limit p.sub.amax or p.sub.bmax, the actuating pressure p.sub.a or p.sub.b is limited or cut off.

(30) What is disclosed is a hydraulic machine which can be operated as a pump with a swept volume which can be hydraulically adjusted by means of actuating pressure under electrical control. Here—both for a pump mode and motor mode of the pump—an actuating pressure of the pump for adjusting the swept volume can be limited under electronic control with knowledge of pump physics in dependence on a current pump rotational speed and on a current pump volume under all operating conditions and a maximum limit of a pressure built up by the pump can always be maintained.

(31) Also disclosed is a travel drive with the pump and a method for controlling said travel drive.