Hydrostatic Traction Drive with a Pressure Cutoff and Method for Calibrating the Pressure Cutoff

Abstract

A hydrostatic traction drive includes a hydraulic pump for supplying pressure medium to a hydraulic motor of the traction drive. The hydraulic pump has an actuation cylinder with at least one cylinder space and a swept volume which can be adjusted thereby. At least one electrically actuable pressure valve is provided, by means of which an actuation pressure which has an adjusting effect can be applied to the cylinder space. In addition, the traction drive has a device by means of which a pressure of the hydraulic pump can be limited by influencing the actuation pressure.

Claims

1. A hydrostatic traction drive comprising: a hydraulic pump coupled to a drive machine and configured to supply pressure medium to a hydraulic motor, which is coupled to an output of the hydrostatic traction drive, the hydraulic pump having an actuating cylinder that includes at least one cylinder space and is configured to adjust a swept volume of the hydraulic pump; at least one electrically actuable pressure valve configured to apply an actuation pressure which has an adjusting effect to the cylinder space; and a device configured to limit a pressure of the hydraulic pump by influencing the actuation pressure, wherein the device is configured so as to limit the pressure in a controlled fashion, and the controlled limitation of the pressure is calibrated.

2. The hydrostatic traction drive according to claim 1, wherein the device is an electronic control unit.

3. The hydrostatic traction drive according to claim 1, wherein the device includes a characteristic curve of the hydraulic pump, the characteristic curve representing the actuation pressure as a function of a limit of the pressure and at least of the swept volume of the hydraulic pump or of a variable for representing the swept volume of the hydraulic pump.

4. The hydrostatic traction drive according to claim 1, wherein the device includes a characteristic diagram of the hydraulic pump, in which the actuation pressure is described as a function of the pressure and at least of the swept volume of the hydraulic pump or of a variable representing the swept volume of the hydraulic pump.

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

6. The hydrostatic traction drive according to claim 4, wherein the actuation pressure is described in the characteristic diagram as a function of a rotational speed of the hydraulic pump.

7. The hydrostatic traction drive according to claim 3, wherein the device is configured to determine a maximum permissible actuation pressure from the characteristic curve.

8. The hydrostatic traction drive according to claim 4, wherein the device is configured to determine from the characteristic diagram a necessary actuation pressure that is necessary according to a speed request.

9. The hydrostatic traction drive according to claim 1, wherein the device includes a valve characteristic curve of the at least one pressure valve, the valve characteristic curve describing an electrical actuation current as a function of the actuation pressure.

10. A method for calibrating a device of a traction drive that includes (i) a hydraulic pump coupled to a drive machine and configured to supply pressure medium to a hydraulic motor, which is coupled to an output of the hydrostatic traction drive, the hydraulic pump having an actuating cylinder, which includes at least one cylinder space and is configured to adjust a swept volume of the hydraulic pump, (ii) at least one electrically actuable pressure valve configured to apply an actuation pressure which has an adjusting effect to the cylinder space, and (iii) the device, which is configured to limit a pressure of the hydraulic pump by influencing the actuation pressure, wherein the device is configured so as to limit the pressure in a controlled fashion, and the controlled limitation of the pressure is calibrated, the method comprising: actuating the pressure valve with an actuation current according to a ramp function; sensing the pressure as a function of the actuation current; determining a limit actuation current which is assigned to a pressure limit of the pressure if the pressure reaches the pressure limit; and storing the pressure limit and the limit actuation current.

11. The method according to claim 10, further comprising: explicitly specifying the pressure limit.

12. The method according to claim 10, further comprising: implicitly specifying the pressure limit as a pressure offset of an opening pressure of a pressure-limiting valve of the traction drive; determining the opening pressure from a profile of the sensed pressure; determining the pressure limit from the opening pressure and the pressure offset; and actuating the pressure valve with the actuation current according to a falling ramp function, up to the pressure limit.

13. The method according to claim 12, wherein the determining of the opening pressure from the profile of the sensed pressure comprises: determining an opening of the pressure-limiting valve from the profile of the sensed pressure; and maintaining an actuation current which is effective during the opening, during a time period in which the pressure stabilizes at the opening pressure.

14. The method according to claim 12, further comprising: comparing the opening pressure with a set opening pressure which is set at the pressure-limiting valve.

15. The method according to claim 10, wherein the pressure limit is one of a maximum permissible pressure and a minimum necessary pressure of a driving mode of the traction drive.

16. The method according to claim 10, further comprising: establishing at least one calibration condition before actuating the pressure valve with the actuation current according to the ramp function.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] An exemplary embodiment of a hydrostatic traction drive according to the disclosure and an exemplary embodiment of a method according to the disclosure for calibrating the pressure cutoff thereof are illustrated in the drawings. The disclosure will now be explained in more detail with reference to the figures of these drawings, in which:

[0042] FIG. 1 shows a hydraulic circuit diagram of a hydrostatic traction drive according to an exemplary embodiment,

[0043] FIG. 2 shows a theoretical time diagram of a pressure and of an actuation current according to a calibration method, according to an exemplary embodiment, and

[0044] FIG. 3 shows a measured time diagram of a drive rotational speed, of the pressure and of the actuation current, according to the method according to FIG. 2.

DETAILED DESCRIPTION

[0045] According to FIG. 1, a hydrostatic traction drive 1 has a hydraulic pump 2 which is fluidically connected in the closed hydraulic circuit via the working lines 4 and 6 to a hydraulic motor (not illustrated) in order to supply the latter with pressure medium. In this context, the hydraulic pump 2 is coupled to a drive machine (not illustrated) via a drive shaft 8 in order to transmit a torque. The coupling is not stepped up here so that the rotational speed of the drive machine and of the hydraulic pump 2 are identical. The hydraulic pump 2 is configured as an axial-piston pump of a swash plate design and can be operated in both rotational directions and both in the pump mode and in the motor mode. It has an adjustable expulsion volume V.sub.P and an adjustment device 10 which is configured 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 actuation pressure line 16 to an output 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 port 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 actuation 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 can be actuated electromagnetically, wherein the actuation pressure p.sub.a or p.sub.b which respectively results in the actuation pressure line 16 or 24 is proportional, according to a valve characteristic curve, with an actuation current I.sub.a or I.sub.b of the electromagnet a or b, respectively. By means of the electromagnetic activation of the pressure-reducing valves 18, 26 it is therefore possible to control the actuation pressures p.sub.a, p.sub.b of the cylinder chambers 12, 14 by specifying the actuation currents I.sub.a, I.sub.b. For this purpose, the electromagnets a, b of the pressure-reducing valves 18, 26 have a signal-transmitting connection to an electronic control unit 32 via a respective signal line 28 or 30.

[0046] Furthermore, the hydrostatic traction drive 1 has a rotational-speed-sensing unit 34 via which a rotational speed n.sub.P of the hydraulic pump 2 can be sensed and can be transmitted to the electronic control unit 32 via a signal line 36. Likewise, the traction drive 1 has a rotational-speed-sensing unit (not illustrated) via which the rotational speed n.sub.M of the hydraulic motor can be sensed and can be transferred to the electronic control unit 32 via the signal line 38.

[0047] In order to provide safety-relevant pressure protection of the working lines 4, 6 against overloading, the hydrostatic traction drive 1 has in each case a pressure-limiting valve 40 which is connected to the respective working line 4, 6. The two pressure-limiting valves 40 are connected by their outputs to a feed pressure 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 pressure-limiting valves responding, pressure medium is therefore relaxed into the feed pressure line 44, as a result of which energetic losses are less than if the relaxation took place toward the tank T. The pressure-limiting valves 40 each have a feed function or suction function in the form of a non-return valve.

[0048] The hydrostatic traction drive 1 can be operated both in the traction mode and in the towing mode 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 expulsion volume V.sub.P can be adjusted by means of the adjustment device 10 on both sides of a neutral position with a zero volume V.sub.P=0. As a result, given a constant rotational direction of the drive shaft 8 and of the drive machine (diesel engine) a reversal of the direction of travel is possible.

[0049] The electronic control unit 32 is connected via a signal line 46 to an operator interface in the form of an accelerator pedal (not illustrated). In this context, a speed request is transferred to the electronic control unit 32 from a driver via the accelerator pedal. Said speed request can relate both to reverse travel and to forward travel. If the accelerator pedal is activated, this therefore corresponds to the traction mode or pump mode of the hydraulic pump 2, and if the accelerator pedal is, on the other hand, released this corresponds to the braking mode or motor mode of the hydraulic pump 2. The activation of a travel brake (not illustrated) also corresponds to the braking mode or motor mode of the hydraulic pump 2. The control unit is configured in such a way that it can determine the corresponding mode by reference to the specified action. In order to select a travel direction, the hydrostatic traction drive 1 has in addition a travel direction switch (not illustrated) which can be actuated and which has a signal-transmitting connection to the electronic control unit 32 via a signal line 48. Depending on its position, the actuation of the hydraulic pump 2 takes place in a reversed or non-reversed adjustment range, that is to say on one or the other side of the neutral position of the swept volume of the hydraulic pump 2. For further consideration of this, reference is made to the following travel states:

[0050] Forward travel, traction mode: application of the first actuation pressure p.sub.a to the first cylinder chamber 12 via the first actuation pressure line 16 and the first pressure-reducing valve 18 by actuating the first pressure-reducing valve 18 with the actuation current I.sub.a via the control unit 32 via the first signal line 28.

[0051] Forward travel, braking mode: application of the second actuation pressure p.sub.b to the second cylinder chamber 14 via the second actuation pressure line 24 and the second pressure-reducing valve 26 by actuating the second pressure-reducing valve 26 with the actuation current I.sub.b via the control unit 32 via the signal line 30.

[0052] Reverse travel, traction mode: application of pressure to the second cylinder chamber 14 via the chain 24, 26, 30, 32.

[0053] Reverse travel, braking mode: application of pressure to the first cylinder chamber 12 via the chain 16, 18, 28, 32.

[0054] In the illustrated exemplary embodiment of the hydrostatic traction drive 1, the hydraulic pump 2 is configured in such a way that the pressure p which is present in that line of the working lines 4, 6 which conducts high pressure and which counteracts the actuation pressure p.sub.a or p.sub.b which is then effective and is effective in the direction of its own reduction. For this purpose, the hydraulic pump 2 has a structurally implemented control loop. In the present case, the hydraulic pump 2 which is configured as an axial-piston pump of a swash plate design is implemented in such a way that a control disc of the hydraulic pump 2 is arranged twisted with respect to a rotational axis of its cylinder drum. Junctions of the same cylinder which are connected to the pressure nodule control disc having the pressure (high pressure) are as a result arranged in an asymmetrically distributed fashion with respect to a pivoting axis of the swash plate. The end sections, supported on the swash plate, of the working pistons which are guided in the cylinders are also then arranged in an asymmetrically distributed fashion. A torque which swings back in the pump mode and swings out in the motor mode results from the supporting forces, therefore acting asymmetrically, of the working pistons on the swash plate. As a consequence, a relationship in the form of a pump characteristic curve or a characteristic diagram of pump characteristic curves of the hydraulic pump 2 is produced in which the respective actuation pressure p.sub.a, p.sub.b can be described as a function of the pressure p and of the swept volume V.sub.P of the hydraulic pump 2 as well as the rotational speed n.sub.P thereof. These characteristic curves or characteristic diagrams are measured and are stored in the electronic control unit 32 for processing, in particular for executing, the method which will be described later.

[0055] There follows the description of a normal driving mode of the hydrostatic traction drive 1. The starting point of the description will be taken to be a non-activated accelerator pedal and a drive machine which rotates in the idling mode at the idling speed. Therefore, initially activation of accelerator pedal occurs by the operator, as a result of which the rotational speed of the drive machine (diesel) is increased from the idling mode to the rated rotational speed. Accordingly, an actuation signal or actuation current I.sub.a for the hydraulic pump 2, to be more precise for the first pressure-reducing valve 18 thereof is issued by means of the electronic control unit 32 as a function of the rotational speed of the diesel engine. When the rated rotational speed of the drive machine is reached, a maximum velocity of the traction drive 1 is obtained. Accordingly, the first actuation pressure p.sub.a is increased in accordance with a characteristic diagram, stored in the electronic control unit 32, of the hydraulic pump 2. Since there is still no load acting, the hydraulic pump 2 swings completely out to its maximum swept volume V.sub.Pmax and supplies its maximum volume flow Q.sub.max in the case of a rated rotational speed.

[0056] As a result of driving resistances which occur, a pressure or load pressure p, for example of 250 bar, occurs when driving on the flat. An operating point which lies on a curve of maximum power P.sub.nomeng of the drive machine is then reached. At this operating point, the first actuation pressure p.sub.a at the rated rotational speed is dimensioned in such a way that the hydraulic power PQ.sub.max of the hydraulic pump 2 corresponds to the rated power P.sub.nomeng.

[0057] If the load on the traction drive 1 then increases, for example during uphill travel or when a wheel loader is taking on grit, the pressure p increases. Owing to the abovementioned configuration of the hydraulic pump 2, in which during forward travel in the traction mode of the hydraulic pump 2 the working pressure p counteracts the first actuation pressure p.sub.a in the direction of a reduction in the swept volume V.sub.P, the pressure p swings back the adjustable cradle of the hydraulic pump 2, as a result of which the travel slows down. The first actuation pressure p.sub.a is not changed during this time, as a result of which there is a subsequent further reduction in the swept volume V.sub.P when the pressure p is increased further or when there is a pressure difference p.

[0058] When a maximum permissible pressure p.sub.max or cutoff pressure or a maximum permissible pressure difference p.sub.max is reached, the electronic control unit 32 ensures that this limit p.sub.max, p.sub.max is not exceeded. Accordingly, despite a further increasing load, there is no further increase in the pressure p since the first actuation pressure p.sub.a is decreased by means of the control unit 32 via the pressure-reducing valve 18 according to FIG. 1 in such a way that the pressure p.sub.max is not exceeded. Therefore, if, 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 engages according to the pressure cutoff which controls according to the disclosure and reduces the first actuation pressure p.sub.a. As a result, even when the load increases further the maximum permissible pressure p.sub.max can be prevented from being exceeded.

[0059] At least the following are input variables of a method according to the disclosure: a swinging angle .sub.P of the hydraulic pump 2 which is proportional to the swept volume V.sub.P, the rotational speed n.sub.P of which hydraulic pump 2 is equal to or proportional to the rotational speed n.sub.eng of the drive machine in the exemplary embodiments, and the limit p.sub.max, which is to be defined or is predetermined, of the maximum permissible working pressure, that is to say what is referred to as the cutoff pressure.

[0060] FIG. 2 shows a timing diagram of a pressure profile p(t) and actuation current profile I.sub.a(t) of an exemplary embodiment of a method according to the disclosure. There are at least two exemplary embodiments of a method to be noted. Both have in common the fact that firstly calibration conditions are established. This is done, on the one hand, by a pump rotational speed n.sub.P being driven or set to a value which is relevant in the driving mode. This corresponds, in particular, for example to the rated rotational speed n.sub.eng of the drive machine. In addition, the hydraulic pump 2 is driven under the so-called blocking condition. This means that the hydraulic motor which is supplied with a pressure medium by said hydraulic pump 2 is set either to a displacement volume V.sub.N which is equal to zero or, in the case of a hydraulic motor which is configured as a constant machine, is secured by a brake. In both cases, the shaft power of the hydraulic motor is zero and the hydraulic pump 2 generates, through its pumping power, only a leak in the hydraulic circuit.

[0061] In the next step, continuous raising of the actuation current I.sub.a occurs over a ramp which is configured in a continuous or incremental fashion. In this way, the limit p.sub.min or p.sub.max for which the assigned actuation current I.sub.amax is to be determined in the sense of a calibration is approached. At this point, the two exemplary embodiments are then divided into different branches.

[0062] According to a first exemplary embodiment of the method, the raising of the actuation current I.sub.a is continued until the previously explicitly defined limit or the previously explicitly defined cutoff pressure p.sub.min or p.sub.max is reached and sensed. The actuation current I.sub.a which is then predefined at this time is then stored as a reference value for the pump control in the device 32. Tolerances of the hydraulic pump 2 and of the pressure-reducing valve 18 are then compensated with this reference value. These steps are also carried out for the traction mode during reverse travel in a way analogous to the specified steps which represent the traction mode during forward travel.

[0063] The second exemplary embodiment of the method does not adopt the approach of detecting the pressure cutoff point (explicitly predefined limit) but rather uses the function of the pressure-limiting valve 40. After the two steps of creating calibration conditions and continuously raising the pump actuation, which are identical to the first exemplary embodiment, the actuation current I.sub.a is raised according to FIG. 2 until an opening of the pressure-limiting valve 40 according to FIG. 1 can be measured at the point 1. For this purpose, by means of the control unit 32 the pressure p is sensed according to FIG. 2, top diagram, said pressure continuously increasing owing to the step-wise raising of the actuation current I.sub.a according to the bottom part of FIG. 2. As long as a gradient p/t is positive, the pressure-limiting valve 40 has not yet opened. If the control unit 32 detects a negative gradient p/t, an opening point 1 of the pressure-limiting valve 40 is sensed. Starting from the opening point 1, the pressure-limiting valve 40 exhibits a behaviour in the form of a characteristic drop in the pressure p. Beyond the opening pressure 1, the pressure p then stabilizes according to the top part of FIG. 2 in a plateau which is sensed as an opening pressure N.sub.B of the pressure-limiting valve 40 and is stored in the control unit 32.

[0064] The actuation current I.sub.a subsequently drops in a step-wise fashion according to the bottom part of FIG. 2 until, starting from the opening pressure p.sub.DB, the pressure p has dropped to the limit p.sub.max which is calculated from the pressure interval p.sub.DB. The actuation current I.sub.amax which is then applied by the control unit 32 in FIG. 2 is stored together with the limit p.sub.max in the control unit 32, as a result of which tolerances of the hydraulic pump 2 and of the pressure-reducing valve are compensated.

[0065] FIG. 3 shows time profiles of the actuation current I.sub.a, of the pressure p, of the filtered pressure p.sub.f, of the drive rotational speed n.sub.eng which are measured under real conditions as set point values and actual values, as well as of the drive torque M.sub.eng. The second exemplary embodiment of the method is illustrated with the detection of an opening pressure 1, and of a pressure-limiting point 2 at which the pressure p(p.sub.f) behaves in a steady-state fashion after the opening. According to the top part of FIG. 3, a setpoint rotational speed n.sub.engsoll of the drive machine is predefined and set at 2000 revolutions. The actual value is n.sub.eng. According to the central part of FIG. 3, the actuation current I.sub.a is increased in a step-wise fashion. Accordingly, the pressure p and the filtered pressure p.sub.f follow according to the bottom part of FIG. 3. At the opening point 1, the pressure p and the filtered pressure p.sub.f drop rapidly. This is sensed by the control unit 32 and detected as an opening pressure 1. A characteristic of the pressure-limiting valve 40 is stored in the control unit 32, and the pressure profile of the pressure-limiting valve 40 is known from said characteristic as a function of the time t after the opening point 1. The control unit 32 therefore reacts starting from the opening point 1 by keeping the actuation current I.sub.a constant so that the pressure p or p.sub.f can stabilize. This occurs when the gradient p (in particular p.sub.f)/t is equal to zero, which is the case in the bottom part of FIG. 3 at the so-called pressure-limiting point 2. The pressure N.sub.B and the associated current I.sub.aDB are stored in the control unit 21. The limit p.sub.amax can therefore be calculated directly by means of the offset or the pressure interval p.sub.DB which has been previously stored in the control unit 32. Subsequent to the plateau of the actuation current I.sub.aDB, the pressure-reducing valve 18 is actuated with an actuation current I.sub.a with a step-wise dropping ramp function by means of the control unit 32. A renewed rise in the pressure when the pressure-limiting valve 40 closes is apparent. After this there is a renewed drop in the pressure p, p.sub.f until the pressure p, p.sub.f reaches the previously calculated limit p.sub.amax. The value pair composed of p.sub.amax and the assigned T.sub.amax is stored in the control unit 32. The tolerances of the hydraulic pump 2 and of the pressure-reducing valve 18 are therefore compensated.

[0066] A hydrostatic traction drive with a hydraulic pump with an adjustable swept volume is disclosed, wherein the adjustment takes place by means of an actuation pressure which is made available in proportion to actuation current, by means of which adjustment a hydraulic motor can be supplied with a pressure medium. According to the disclosure, a pressure limitation or pressure cutoff is provided on the basis of a controlled limitation of the actuation pressure as well as automated calibration of the pressure cutoff, by means of an electronic control unit of the traction drive.

[0067] Furthermore, a method for calibrating the specified pressure cutoff by means of the device is disclosed, with steps of driving along a ramp of the actuation current, sensing the pressure at the limit and sensing the assigned actuation current as well as storing this value pair in the device.