Hydrostatic traction drive in closed hydraulic circuit and method for controlling the hydrostatic traction drive

Abstract

A hydrostatic traction drive in a closed hydraulic circuit includes two hydraulic machines in the closed hydraulic circuit and a control unit configured to control a braking torque of the traction drive. A method for controlling the hydrostatic traction drive includes controlling the braking torque for the hydrostatic traction drive.

Claims

1. A hydrostatic traction drive, comprising: a first hydraulic machine with a variable displacement volume, the first hydraulic machine configured to be coupled to a drive machine; a second hydraulic machine with a variable displacement volume, the second hydraulic machine configured to be coupled to a wheel to be driven or to a chain or axle to be driven, the first and second hydraulic machines fluidically connected by a first branch line and a second branch line; and a control unit configured to: determine an overrun mode of the traction drive depending on a measured actual value of a revolution rate of the second hydraulic machine and on a calculated theoretical value of the revolution rate of the second hydraulic machine, the theoretical value calculated based on a measured actual value of a revolution rate of the first hydraulic machine; control a braking torque of the traction drive depending on the determined overrun mode; in response to determining that the traction drive is in the overrun mode, reduce the displacement volume of the first hydraulic machine and the displacement volume of the second hydraulic machine, such that a ratio between the displacement volume of the first hydraulic machine and the displacement volume of the second hydraulic machine is maintained, until the traction drive is in a neutral drive state; and in response to the traction drive entering the neutral drive state, increase the displacement volume of the second hydraulic machine.

2. The traction drive according to claim 1, wherein the braking torque is configured to be produced by restricting a partial flow of a return volumetric flow directed from the second hydraulic machine to the first hydraulic machine.

3. The traction drive according to claim 2, further comprising a hydraulic restriction mechanism having a pressure medium input and a pressure medium output, the pressure medium input fluidically connected to a branch line and the pressure medium output fluidically connected to a pressure medium sink of the traction drive.

4. The traction drive according to claim 1, wherein the control unit is configured to determine the overrun mode based on the actual value of the revolution rate of the second hydraulic machine deviating from the theoretical value of the revolution rate by more than a predetermined threshold amount.

5. The traction drive according to claim 4, wherein the control unit is further configured to determine the overrun mode based on an actual value of a revolution rate of the drive machine deviating from a target value for the drive machine by more than a predetermined threshold.

6. The traction drive according to claim 1, wherein the theoretical value is a theoretical loss-free value of revolution speed for the second hydraulic machine.

7. The traction drive according to claim 1, wherein the control unit is further configured to determine the theoretical value depending on a ratio of actual values of displacement volumes of the hydraulic machines.

8. The traction drive according to claim 1, wherein the control unit is configured to determine for at least one of the hydraulic machines an actual value of its displacement volume depending on a target value of its displacement volume.

9. The traction drive according to claim 8, wherein a characteristic field of the actual value of the displacement volume is stored in the control unit as a function of the target value of the displacement volume.

10. The traction drive according to claim 1, further comprising one or more of (i) a first revolution rate detection unit configured to detect the measured actual value of the revolution rate of the first hydraulic machine or a revolution rate that is configured to be derived therefrom and (ii) a second revolution rate detection unit configured to detect the actual value of the revolution rate of the second hydraulic machine.

11. A method for controlling a braking torque of a hydrostatic traction drive including a first hydraulic machine with a variable displacement volume and a second hydraulic machine with a variable displacement volume, the method comprising: determining an overrun mode of the traction drive depending on a measured actual value of a revolution rate of the second hydraulic machine and on a calculated theoretical value of the revolution rate, the theoretical value calculated based on a measured actual value of a revolution rate of the first hydraulic machine; controlling the braking torque depending on the determined overrun mode; in response to determining that the traction drive is in the overrun mode, reducing the displacement volume of the first hydraulic machine and the displacement volume of the second hydraulic machine, such that a ratio between the displacement volume of the first hydraulic machine and the displacement volume of the second hydraulic machine is maintained, until the traction drive is in a neutral drive state; and in response to the traction drive entering the neutral drive state, increasing the displacement volume of the second hydraulic machine.

12. The method according to claim 11, wherein determining the overrun mode of the traction drive includes: determining that a deviation of the actual value of the revolution rate of the second hydraulic machine from the theoretical value of the revolution rate is greater than a predetermined threshold amount.

13. The method according to claim 12, wherein determining the overrun mode of the traction drive further includes, prior to the determining that a deviation of the actual value of the revolution rate of the second hydraulic machine from the theoretical value of the revolution rate is greater than the predetermined threshold amount: measuring the actual value of the revolution rate of the second hydraulic machine, and calculating the theoretical value of the revolution rate.

14. The method according to claim 13, wherein calculation of the theoretical value of the revolution rate is further based on a ratio of actual values of displacement volumes of the hydraulic machines.

15. The method according to claim 12, wherein determining the overrun mode of the traction drive further includes: determining that a deviation of an actual value of a revolution rate of the drive machine from a target value for the drive machine is more than a predetermined threshold.

16. The method according to claim 11, wherein controlling the braking torque depending on the determined overrun mode includes: adjusting at least one of the displacement volumes of the hydraulic machines.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An exemplary embodiment of a hydrostatic traction drive according to the disclosure and a method according to the disclosure for controlling said traction drive are explained in detail below in two figures. In the figures:

(2) FIG. 1 shows a hydraulic circuit diagram of the traction drive, and

(3) FIG. 2 shows a revolution rate-volumetric-flow time diagram of the traction drive according to FIG. 1, which results from control of the traction drive based on the method.

DETAILED DESCRIPTION

(4) According to FIG. 1 a hydrostatic traction drive 1 has a first hydrostatic hydraulic machine 2, which is primarily operated as a hydraulic pump and is driven by a drive machine 4 that is embodied as a diesel engine. Furthermore, the hydrostatic traction drive 1 has a second hydrostatic hydraulic machine 6 that is coupled by means of a drive shaft 8 to an axle 14 having two wheels 12 and that is primarily operated as a hydraulic motor. Both hydraulic machines 2, 6 are each adjustable in their displacement volume by means of an adjustment unit 16, 18. The first hydraulic machine 2 is fluidically connected in a closed hydraulic circuit to the second hydraulic machine 6 by means of a first branch line 20, which is the feed line in the following discussions, via which pressure medium flows from hydraulic machine 2 to hydraulic machine 6, and via a second branch line 22, which is the second branch line in the following discussions, via which pressure medium flows from hydraulic machine 6 to hydraulic machine 2.

(5) The hydrostatic traction drive 1 has a feed pump 26 connected to a drive shaft 24 of the first hydraulic machine 2, which can deliver pressure medium from a tank T into a feed line 28. The latter divides into three branches, wherein a first branch can be brought into a pressure medium connection with the tank T by means of a pressure relief valve 30. A second or third branch can be brought into a pressure medium connection with branch line 20 or with branch line 22 by means of pressure relief valve 32 or pressure relief valve 34, each comprising an integrated anti-suckback valve 36 or 38.

(6) Both hydraulic machines 2, 6 can be operated in all four quadrants, so that both the flow direction of the pressure medium in the closed hydraulic circuit as well as the direction of rotation of each hydraulic machine 2, 6 is reversible.

(7) The hydrostatic traction drive 1 has a control unit 40, to which a brake operating pedal 44 is connected by means of a signal line 42. The brake operating pedal 44 has a sensor 46 by means of which an actuation force of the brake operating pedal 44 can be detected and transmitted to the control unit 40 by means of the signal line 42. The control unit 40 is connected by means of an electrical signal line 48 to the adjustment device 16 of the hydraulic machine 2 and by means of an electrical signal line 50 to the adjustment device 18 of the hydraulic machine 6. A revolution rate detection unit 54, by means of which the revolution rate of the second hydraulic machine 6 can be detected on the drive shaft 8, is connected by means of an electrical signal line 52 to the control unit 40. A revolution rate detection unit 60, by means of which the revolution rate of the first hydraulic machine 2 can be detected on its drive shaft, is connected to the control unit 40 by means of an electrical signal line 62. The control unit 40 has a memory unit 56, in which a method according to the disclosure is stored according to the preceding description, and a processor unit 58, in which the method can be carried out.

(8) FIG. 2 shows, summarized in a diagram, a time profile of an actual value n.sub.M,i, and of a theoretical value n.sub.M,c of the revolution rate n.sub.M of the second hydraulic machine 6, a time profile of an actual value n.sub.A,i, and of a target value n.sub.A,s of the revolution rate n.sub.A of the drive machine 4 specified by the operator by means of the brake operating pedal 44 and a time profile of an actual value V.sub.M,i of the displacement volume V.sub.M of the second hydraulic machine 6 and of an actual value V.sub.P,i of the displacement volume V.sub.P of the first hydraulic machine 2. The value n.sub.M,c is determined for this by means of the control unit 40 according to FIG. 1 by means of the relationship
n.sub.M,c=n.sub.P,iV.sub.P,i/V.sub.M,i
assuming no losses. Here n.sub.P,i, is the actual value of the revolution rate n.sub.P of the first hydraulic machine 2 detected by means of the revolution rate detection unit 60 according to FIG. 1 and corresponds to the actual value n.sub.A,i of the revolution rate n.sub.A of the drive machine 4 because of the coupling of the drive shafts of the drive machine 4 and the first hydraulic machine 2. A proportional relationship can also exist here because of a gearbox being interposed. In FIG. 2, in addition the deviation (n.sub.M,in.sub.M,c) determined by the control unit 40 is plotted at a point in time t.sub.2. Starting from a point in time t.sub.0, all plotted values n.sub.M,i, n.sub.M,c, n.sub.A,i, n.sub.A,s, V.sub.M,i, V.sub.P,i, are constant. The traction drive 1 is operating in a stationary manner, as is the case for example during horizontal straight ahead travel without acceleration or braking.

(9) At a point in time t.sub.1 the vehicle driven by the traction drive 1 according to FIG. 1 is now travelling on a slope and changes to the overrun mode. Regardless of the slope, the theoretical value n.sub.M,c of the revolution rate n.sub.M of the second hydraulic machine 6 and the target value n.sub.A,s of the drive machine 4 specified at the gas pedal (not shown) continue to remain constant during this. By contrast however, the actual value n.sub.M,i of the revolution rate n.sub.M of the second hydraulic machine 6 and the actual value n.sub.A,i of the drive machine 4 increase because of the acceleration that is acting owing to gravity. A limit value S1 for the deviation (n.sub.M,in.sub.M,c) for a first switch-on criterion and a limit value S2 for a deviation (n.sub.A,in.sub.A,s) for a second switch-on criterion are stored in the control unit 40. If both deviations (n.sub.M,in.sub.M,c), (n.sub.A,in.sub.A,s) exceed the associated limit values S1, S2, as is the case for both by chance in the example shown at the point in time t.sub.2, the control unit 40 intervenes and switches a hydraulic braking torque M.sub.B on. At the point in time t.sub.2, the control unit 40 determines that on the one hand a hydraulic slip of the second hydraulic machine 6 is too large and that moreover the drive machine 4 is turning too rapidly. It is also conceivable that under certain circumstances or for certain machines fitted with the traction drive according to the disclosure, only exceeding the one limit value or only exceeding the other limit value or exceeding one of the two limit values results in an intervention by the control unit 40 and a hydraulic braking torque being switched on. That both conditions must be fulfilled appears particularly favorable however, and in most application cases is a guarantee that braking is only initiated when desired. I.e., if only limit value S1 is exceeded, then the triggering of braking may not be desired even if there is a relatively large hydraulic slip but the diesel engine does not significantly exceed its target revolution rate. If only limit value S2 is exceeded, then the triggering of braking may not be desired if for example there is a significant positive revolution rate difference (n.sub.A,in.sub.A,s) as a result of the influence of the working hydraulics or during rapid reduction of the target diesel revolution rate, but the vehicle is not in the braking state.

(10) The control unit 40 now performs the task of providing the hydrostatic braking torque M.sub.B by automatically controlling the adjustment devices 16, 18, i.e. independently of the operator. For this purpose, it first controls the adjustment devices 16, 18 in a brief period of time between t.sub.2 and t.sub.3 such that the displacement volumes V.sub.P,i, and V.sub.M,i of both hydraulic machines 2, 6, are reduced simultaneously while maintaining a hydraulic transmission ratio. The objective is initially in said period of time to produce a neutral drive state, in which neither driving nor braking occurs, and to prevent torque peaks from acting on the drive machine 4. Said transition is parameterized in the control unit 40.

(11) From point in time t.sub.3 the control unit increases the displacement volume (feed volume) V.sub.M,i of the second hydraulic machine 6 further. Because said second hydraulic machine 6 is operating in pumping mode during the overrun mode, the return volumetric flow that it delivers by means of the second branch line 22 to the first hydraulic machine 2 increases accordingly. At the same time the displacement volume (volumetric displacement) V.sub.P,i of the first hydraulic machine 2 is already reduced so much however, that the same cannot receive the full return volumetric flow. As a result the pressure in the second branch line 22 rises until the pressure relief valve 34 opens and pressure medium is released into the tank T or into the first branch line 20. The braking torque M.sub.B results from the partial flow discharging via the pressure relief valve 34 and the pressure difference and the braking energy is released in order to the drive machine 4.

(12) Once there is a drive state in the further profile, in which the value is below the limit value S1, the previously described intervention of the control unit 40 ends and the braking is deactivated. This is the case for example at point in time t.sub.4 according to FIG. 2. In other versions the deviation (n.sub.A,in.sub.A,s) could also be used to deactivate the braking. A switch-off criterion could be if the deviation falls below a limit value S3. Here S3 is somewhat smaller than the switch-on criterion S2 associated with the deviation (n.sub.A,in.sub.A,s). The switch-off criterion could also be that either the deviation (n.sub.M,in.sub.M,c) or the deviation (n.sub.A,in.sub.A,s) or one of the two deviations falls below a limit value. However, it appears to be particularly favorable to use the deviation (n.sub.M,in.sub.M,c) as the switch-off criterion.

(13) Based on the definition for the volumetric efficiency level .sub.P of the first hydraulic machine 2 and .sub.M of the second hydraulic machine 6
.sub.P=n.sub.P,c/n.sub.P,i, and .sub.M=n.sub.M,i/n.sub.M,c
the actual value n.sub.M,i of the revolution rate n.sub.M of the second hydraulic machine 6 (motor) in the driving case, i.e. in the traction mode of the traction drive 1, is below the theoretical value n.sub.M,c of the revolution rate n.sub.M with
n.sub.M,i=n.sub.M,c.sub.M.sub.P
and is above the theoretical value n.sub.M,c of the revolution rate n.sub.M in the braking case, i.e. in the overrun mode, with
n.sub.M,i=n.sub.M,c1/.sub.M1/.sub.P.

(14) Because volumetric losses of the respective hydraulic machine 2, 6 among other things increase approximately in proportion to a pressure difference across the respective hydraulic machine 2, 6, the deviation (n.sub.M,in.sub.M,c) between the theoretical value n.sub.M,c and the actual value n.sub.M,i of the revolution rate n.sub.M of the second hydraulic machine 6 is at the same time an indicator of the magnitude of the currently driving or decelerating torque M on the second hydraulic machine 6.

(15) A function
M.sub.B=k(n.sub.M,in.sub.M,c)
is stored in the control unit 40, by means of which depending on the deviation (n.sub.M,in.sub.M,c) the braking torque M.sub.B is determined that can be automatically produced by means of the control unit 40 by controlling the adjustment devices 16, 18 as described. Here k is a constant. In the exemplary embodiment shown, the control by the control unit 40 is carried out solely based on the deviation (n.sub.M,in.sub.M,c) and/or on the deviation (n.sub.A,in.sub.A,s) and regardless of whether the operator has demanded braking by means of the brake operating pedal 44. However, if the operator intervenes in braking, then because of the mechanical braking effect the switch-off criterion S1 or S3 would of course be reached at an earlier point in time and the intervention according to the disclosure by the control unit would end before point in time t.sub.4.

(16) Furthermore, the fact that the revolution rate n.sub.A,i of drive machine 4 lies below its target value n.sub.A,c in the driving case and lies above its target value n.sub.A,c in the braking case can be used.

(17) A hydrostatic traction drive is disclosed with two hydraulic machines disposed in a closed hydraulic circuit, of which one is provided for driving a wheel or a chain or an axle. A difference between an actual revolution rate of said hydraulic machine and a theoretical revolution rate of said hydraulic machine that is determined under the assumption of no losses can be determined by means of a control unit of the traction drive. A driving state of the traction drive, in particular an overrun mode or a traction mode or an idle mode, can be identified by means of the control unit depending on the deviation and a braking torque of the traction drive can be controlled depending on the identified driving state.

(18) Furthermore, a method for controlling the braking torque of the traction drive is disclosed.

REFERENCE CHARACTER LIST

(19) 1 hydrostatic traction drive 2 first hydraulic machine 4 drive machine 6 second hydraulic machine 8 drive shaft 10 differential 12 wheel 14 axle 16, 18 adjustment device 20 first branch line 22 second branch line 24 drive shaft 26 feed pump 28 feed line 30, 32, 34 pressure relief valve 36, 38 anti-suckback valve 40 control unit 42 signal line 44 brake operating pedal 46 sensor 48, 50, 52 signal line 54 revolution rate detection unit 56 memory unit 58 processor unit 60 revolution rate detection unit 62 signal line