Hydrostatic propulsion drive for a vehicle

Abstract

A hydrostatic propulsion drive for a vehicle is disclosed including an electronic control unit and a hydrostatic transmission having a hydraulic pump with an adjustable swept volume and a hydraulic motor with an adjustable swept volume, the hydraulic pump being arranged in a closed hydraulic circuit. The swept volumes of the hydraulic pump and the hydraulic motor are changeable in accordance with a correction factor which is stored in a characteristic diagram and is dependent on the rotational speed and the drive torque of the hydraulic motor, the operating pressure at the hydraulic motor being increased if the correction factor is used. Due to the increased operating pressure, flow-induced pressure drops are reduced by way of volumetric flows, and leakage flows via gaps between moving parts of the hydraulic pump and the hydraulic motor are kept large enough to avoid excessive friction and loss of efficiency.

Claims

1. A hydrostatic propulsion drive for a vehicle, the hydrostatic propulsion drive comprising: a hydrostatic transmission having a hydraulic pump with an adjustable swept volume and a hydraulic motor with an adjustable swept volume, the hydraulic pump being arranged in a hydraulic circuit; and an electronic control unit configured to increase an operating pressure that prevails at the hydraulic motor by changing the swept volume of the hydraulic pump and the swept volume of the hydraulic motor in accordance with at least one correction factor, the at least one correction factor being stored in at least one characteristic diagram and being dependent on both a rotational speed of the hydraulic motor and a drive torque of the hydraulic motor.

2. The hydrostatic propulsion drive according to claim 1, the electronic control unit further configured to: change a transmission ratio of the hydrostatic transmission by performing a follow-up adjustment of the hydraulic pump and the hydraulic motor, the swept volume of the hydraulic pump being first adjusted up to a maximum swept volume and then the swept volume of the hydraulic motor being decreased, starting from a maximum step-down ratio, in which the hydraulic motor is set to a maximum swept volume and the hydraulic pump is set to a minimum swept volume.

3. The hydrostatic propulsion drive according to claim 1, wherein the electronic control unit is configured to store the at least one characteristic diagram.

4. The hydrostatic propulsion drive according to claim 1, the electronic control unit further configured to: superimpose a regulation of the rotational speed of the hydraulic motor on the changing of the swept volume of the hydraulic pump and the swept volume of the hydraulic motor in accordance with the at least one characteristic diagram.

5. The hydrostatic propulsion drive according to claim 1, the electronic control unit further configured to: carry out a switchover between (i) a normal mode, in which the swept volume of the hydraulic pump and the swept volume of the hydraulic motor remain without correction, and (ii) an optimum mode, in which the swept volume of the hydraulic pump and the swept volume of the hydraulic motor are changed in accordance with the at least one correction factor which is stored in the at least one characteristic diagram.

6. The hydrostatic propulsion drive according to claim 1, the electronic control unit further configured to: reduce the swept volume of the hydraulic pump and the swept volume of the hydraulic motor using a gradient limiter in an optimum mode, in which the swept volume of the hydraulic pump and the swept volume of the hydraulic motor are changed in accordance with the at least one correction factor which is stored in the at least one characteristic diagram.

7. The hydrostatic propulsion drive according to claim 1, wherein the at least one characteristic diagram of the at least one correction factor has a rotational speed of the hydraulic motor as a dimension of input.

8. The hydrostatic propulsion drive according to claim 1, the electronic control unit further configured to: adjust the swept volume of the hydraulic pump and the swept volume of the hydraulic motor in each case electro-proportionally.

9. The hydrostatic propulsion drive according to claim 1, the electronic control unit further configured to: adjust the swept volume of the hydraulic motor electro-proportionally.

10. The hydrostatic propulsion drive according to claim 1, wherein the hydraulic circuit is a closed hydraulic circuit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) One exemplary embodiment of a hydrostatic propulsion drive according to the disclosure is shown in the drawings. The disclosure will now be described in greater detail using the figures of said drawings, in which:

(2) FIG. 1 shows a schematic of the exemplary embodiment,

(3) FIG. 2 shows a regulating diagram of the exemplary embodiment, and

(4) FIG. 3 shows a characteristic diagram for the correction factor.

DETAILED DESCRIPTION

(5) The hydrostatic propulsion drive 10 which is shown comprises a hydraulic pump 11 with an adjustable swept volume and an adjusting apparatus 12, and a hydraulic motor 13 with a likewise adjustable swept volume and an adjusting apparatus 14. Here, swept volume is understood to mean the pressure medium quantity which is conveyed by the hydraulic pump during a single revolution of a drive shaft or which is sucked in by the hydraulic motor during a single revolution of its driveshaft. The hydraulic pump 11 and the hydraulic motor 13 are connected fluidically to one another in a closed hydraulic circuit via two operating lines 15 and 16. It is generally known that, in a constellation of this type, the hydraulic motor can also operate as a pump and the hydraulic pump can also operate as a motor in order to brake the vehicle. The hydraulic pump can be adjusted in two opposed directions from a neutral position, in which the swept volume is zero, with the result that the driving direction of the vehicle can be changed solely by way of adjustment of the hydraulic pump across zero. The hydraulic pump and the hydraulic motor are usually axial piston machines. The hydraulic pump 11 can be driven by a diesel engine 18 via a shaft 17. An output shaft 19 which is connected mechanically to two wheels of the vehicle in a way which is not shown in greater detail is driven by the hydraulic motor 13.

(6) The pressure in the operating line 15 is detected by way of a pressure sensor 20. The pressure in the operating line 16 is detected by way of a pressure sensor 21. The rotational speed of the output shaft 19 is detected by way of a rotational speed sensor 22. The sensors 20 to 22 convert the detected variables into electric signals.

(7) Furthermore, an electronic control unit 30 belongs to the hydrostatic propulsion drive 10, to which electronic control unit 30 the variables which are detected by the sensors 20 to 22 are fed as electric signals. The electronic control unit 30 is configured for controlling the hydrostatic transmission, the essential components of which are the hydraulic pump 11 and the hydraulic motor 13. For the purpose of said control, the control unit 30 is connected via an electric line 31 to the adjusting apparatus 12 of the hydraulic pump 11 and via an electric line 32 to the adjusting apparatus 14 of the hydraulic motor 13. The adjusting apparatus 12 of the hydraulic pump 11 preferably operates electro-proportionally and then has a regulating valve with a regulating piston which can be loaded with a force in the one direction by a first proportional electromagnet and in the opposite direction by a second proportional magnet, and to which regulating piston the position of an actuating piston of the adjusting apparatus is fed back as a spring force. The regulating piston always assumes a middle regulating position when the spring force which is produced by way of the position of the actuating piston is exactly as great as the magnetic force. In an adjusting apparatus of this type, there is an electro-proportional (EP) adjustment. However, the adjusting apparatus can also have, for example, two pressure regulating valves and an actuating piston which is adjoined by two actuating chambers which are loaded with different actuating pressures via the two pressure regulating valves. In a case of this type, the hydraulic pump is load-sensing, because its swept volume is dependent on the pump pressure and on the rotational speed at a given actuating pressure.

(8) The adjusting apparatus of the hydraulic motor 13 is preferably likewise what is known as an EP adjustment, in which a pivot angle of the axial piston machine is set proportionally to the magnitude of an electric current, with which an electromagnet of a regulating valve is loaded, the pivot angle being fed back to the regulating valve, converted with the aid of a compression spring, as a force which acts counter to the force of the electromagnet. The hydraulic motor 13 can be adjusted only between a minimum and a maximum swept volume, but not across a swept volume of zero.

(9) The electronic control unit 30 comprises a pump controller 33 and a motor controller 34, at the inputs of which an electric signal prevails which corresponds to a setpoint speed v.sub.soll of the vehicle and therefore to a setpoint rotational speed of the hydraulic motor 13. The pump controller 33 and the motor controller 34 output setpoint signals .sub.Pmp,soll and .sub.Mot,soll for the pivot angles of the hydraulic pump and of the hydraulic motor.

(10) Furthermore, the control unit 30 comprises a speed regulator 35, to which a regulating deviation between the setpoint speed v.sub.soll and the actual speed v.sub.ist of the vehicle or between the setpoint rotational speed and the actual rotational speed of the hydraulic motor 13 is fed, and which speed regulator 35 influences both the signal of the pump controller and the signal of the motor controller by way of an actuating variable. Speed regulation is therefore superimposed on the pump controller 33 and the motor controller 34.

(11) A characteristic diagram 36 is stored in the control unit 30, which characteristic diagram 36 comprises correction factors K.sub.korr for the pivot angles of the hydraulic pump 12 and of the hydraulic motor 13, which correction factors K.sub.korr are dependent on the rotational speed n.sub.mot of the hydraulic motor 13 and on the load which is standardized to a nominal magnitude and under which the hydraulic motor 13 operates. Here, the load which is standardized to a nominal magnitude results from the product of the pressure difference dp, which is determined with the aid of the two pressure sensors 20 and 21 and prevails across the hydraulic motor 13, and the pivot angle .sub.Mot,soll of the hydraulic motor. The respectively determined correction factor of less than/equal to 1 prevails at a first input of a switch 37 which also has a second input, at which a 1 prevails. An output of the switch 37 is connected to the input of a gradient limiter 38. Both the output signal of the motor controller 34 and the output signal of the pump controller 33 are multiplied by the value which prevails at an output of the gradient limiter 38.

(12) FIG. 3 shows the characteristic diagram 36 with a plurality of isolines for the correction factor.

(13) It is now to be assumed that the hydraulic motor 13 is to rotate at a defined rotational speed. The rotational speed of the hydraulic motor 13 results from the delivery quantity of the hydraulic pump 11 and the displacement of the hydraulic motor 13. In the normal mode, the switch 37 is switched in such a way that a 1 is output instead of a correction factor K.sub.korr. In this way, the controller is not influenced. Normal mode means that the pivot angles are not to be reduced for the benefit of an improved degree of efficiency. This can be the case, for example, during road driving, decelerating and maneuvering. The condition for this can be different depending on the vehicle type. The electronic control unit 30 can then act, with consideration of the rotational speed of the hydraulic pump 12, in such a way that, at low rotational speeds, the hydraulic motor 13 is set to its maximum displacement and the desired rotational speed of the output shaft 19 is obtained by way of corresponding adjustment of the hydraulic pump. For rotational speeds higher than a rotational speed, at which the hydraulic pump is pivoted out completely, the hydraulic motor 13 is adjusted to displacements which are smaller than its maximum displacement.

(14) Under certain other conditions, it is better to operate in what is known as an optimum mode. The switch 37 is then moved into the switching position which is not shown in FIG. 2. In a combine harvester, for example, a switchover is carried out for fieldwork from the normal mode to the mode which is optimized in terms of the degree of efficiency. The values which are output by the pump controller 33 for the pivot angle of the hydraulic pump and by the motor controller 34 for the pivot angle of the hydraulic motor are then multiplied with a correction factor K.sub.korr which is read out from the characteristic diagram 36, slowly via ramps of the gradient limiter 38, in order to reduce the pivot angles of a hydraulic pump and a hydraulic motor by the same factor until the optimum ratio of leakage, pressure drop across the lines and ducts, and friction is set. The percentage proportion which is necessary for this is stored in the characteristic diagram in a manner which is dependent on the absolute value Abs(n.sub.mot) of the hydraulic motor rotational speed and the hydraulic motor torque (Abs(dp)*alpha_rel) which is standardized to a nominal magnitude, and has been determined in advance in a computer-aided manner from characteristic diagrams of the degree of efficiency of the hydrostats. Abs(dp) is the absolute pressure difference between the two values which are detected by the pressure sensors 20 and 21. alpha_rel is the pivot angle of the hydraulic motor specified in percent of the maximum pivot angle. Here, the correction factor becomes active via ramps of the gradient limiter 38, with the result that the pivot angles are reduced slowly. If, however, the output load rises suddenly, the pivot angles are moved back again to the follow-on adjustment via a steep ramp.

(15) The speed drops as a result of the rise of the leakage. This is compensated for by way of the superimposed speed regulator which acts on the speed by way of an increase of the pump volumetric flow. In the normal mode, the regulator acts on the hydraulic pump so as to increase its swept volume and on the hydraulic motor so as to reduce its displacement. In the optimum mode, the regulator acts only on the pump, because the pump angle which has already been reduced is increased again in this case.

LIST OF REFERENCE NUMERALS

(16) 10 Hydrostatic propulsion drive 11 Hydraulic pump 12 Adjusting apparatus of 11 13 Hydraulic motor 14 Adjusting apparatus of 13 15 Working line 16 Working line 17 Shaft 18 Diesel engine 19 Output shaft 20 Pressure sensor 21 Pressure sensor 22 Rotational speed sensor 30 Control unit 31 Electric line 32 Electric line 33 Pump controller 34 Motor controller 35 Speed regulator 36 Characteristic diagram 37 Switch 38 Gradient limiter v.sub.Soll Setpoint speed .sub.Pmp,soll Setpoint pivot angle, hydraulic pump .sub.Mot,soll Setpoint pivot angle of the hydraulic motor v.sub.ist Actual speed n.sub.mot Rotational speed of the hydraulic motor M.sub.Mot, Drive torque of the hydraulic motor K.sub.korr Correction factor dp Pressure difference