METHOD FOR OPERATING A HYDROSTATIC TRANSMISSION OF A DRIVE TRAIN OF A MOTOR VEHICLE

Abstract

A method for operating a hydrostatic transmission of a drive train of a motor vehicle. An automatic creep function of the motor vehicle is made possible by the hydrostatic transmission. in order to be able to realize the creep function in a reliable manner, a current value of a first parameter that is independent of a pedal position of an accelerator and a current value of a second parameter that is dependent on the pedal position are compared to each another. The creep function is then activated as long as the current value of the first parameter is greater than the current value of the second parameter. In the context of the creep function, the current value of the first parameter is used to determine a current value of a target delivery quantity of a hydrostatic machine of the transmission operated as a pump.

Claims

1-13. (canceled)

14. A method for operating a hydrostatic transmission (12) of a drive train (1) of a motor vehicle in which an automatic creep function of the motor vehicle is made possible by the hydrostatic transmission (12), the method comprising: comparing a current value of a first parameter and a current value of a second parameter to one another, with the first parameter being independent of a pedal position of an accelerator (19) and the second parameter being dependent on a pedal position of an accelerator (19); activating the creep function if the current value of the first parameter is greater than a current value of the second parameter; determining, from the current value of the first parameter, a current value of a target delivery quantity of a hydraulic machine (13) of the transmission (12) operating as a pump in a context of the creep function; taking into account a nominal swivel angle (.sub.1) of the hydrostatic machine (13) that is independent of the pedal position of the accelerator (19) as the first parameter, and taking into account a nominal swivel angle (.sub.2) of the hydrostatic machine (13) that is dependent on the pedal position as the second parameter; and setting a value of a resulting nominal swivel angle (.sub.res) of the hydrostatic machine (13) equal to the larger current value of the first and the second parameters in the context of the creep function.

15. The method according to claim 14, further comprising basing the first parameter on a driving speed of the motor vehicle.

16. The method according to claim 15, wherein a progression of the first parameter is degressive with respect to the driving speed.

17. The method according to claim 15, further comprising taking into account the driving speed with a sign attributed thereto, such that different progressions of the first parameter are specified for positive and for negative values of the driving speed.

18. The method according to claim 14, further comprising altering a progression of the first parameter by a driver of the vehicle.

19. The method according to claim 14, further comprising taken into account an actuation state of a service brake (20) of the motor vehicle, and a progression of the first parameter is altered when the service brake (20) is actuated.

20. The method according to claim 19, further comprising setting, above a defined actuation level of the service brake (20) the current value of the first parameter to zero.

21. A method of operating a hydrostatic transmission (12) of a drive train (1) of a motor vehicle in which an automatic creep function of the motor vehicle is made possible by the hydrostatic transmission (12), the method comprising: comparing a current value of a first parameter and a current value of a second parameter to one another, and the first parameter being independent of a pedal position of an accelerator (19) and the second parameter being dependent on a pedal position of an accelerator (19); activating the creep function as long as the current value of the first parameter is greater than the current value of the second parameter; determining, as a function of the current value of the first parameter, a current value of a target delivery quantity of a hydraulic machine (13) of the transmission (12) operating as a pump in a context of the creep function; taking into account a hypothetical pedal position (Pos.sub.1) of the accelerator (19) as the first parameter, and taking into account an actual pedal position (Pos.sub.2) of the accelerator (19) as a second parameter; and determining, from the larger current value of the first and the second parameters, a current value of a nominal swivel angle (.sub.res) of the hydrostatic machine (13) in the context of the creep function.

22. The method according to claim 21, further comprising basing the first parameter on a driving speed of the motor vehicle.

23. The method according to claim 22, wherein a progression of the first parameter is degressive with respect to the driving speed.

24. A control unit (17), for a hydrostatic transmission (12), comprising a device for regulating a delivery volume of a hydrostatic machine (13) of the transmission (12) operated as a pump, and the device being configured for comparing a current value of a first parameter and a value of a second parameter to one another, and the first parameter being independent of a pedal position of an accelerator (19) and the second parameter being dependent on a pedal position of the accelerator (19), and using the larger of the first and the second current values for determining a current value of a target delivery quantity of the hydrostatic ma.sup.-chine (13), and a nominal swivel angle (.sub.1) of the hydrostatic machine (13) that is independent of the pedal position of the accelerator (19) being taken into account as the first parameter and a nominal swivel angle (.sub.2) of the hydrostatic machine (13) that is dependent on the pedal position of the accelerator (19) being taken into account as the second parameter, and a value of a resulting nominal swivel angle (.sub.res) of the hydrostatic machine (13) being set equal to the larger current value of the first and the second parameters in a context of the creep function.

25. A control unit (17) for a hydrostatic transmission (12), comprising a device for regulating a delivery volume of a hydrostatic machine (13) of the transmission (12) operated as a pump, the device being configured for comparing a current value of a first parameter and a value of a second parameter to each other, the first parameter being independent of a pedal position of an accelerator (19) and the second parameter being dependent on a pedal position of the accelerator (19), and for using the larger of the first and the second current values for determining a current value of a target delivery quantity of the hydrostatic machine (13), and a hypothetical pedal position (Pos.sub.1) of the accelerator (19) being taken into account as the first parameter and an actual position (Pos.sub.2) of the accelerator (19) being taken into account as the second parameter, the larger current value of the first and the second parameters being used in a context of the creep function for determining a current value of a nominal swivel angle (.sub.res) of the hydrostatic machine (13).

26. A computer program product for a control unit (17) according to claim 24, the computer program product being configured to implement a routine for comparing the current values of the first and the second parameters and for determining the current value of the target delivery quantity by suitable control commands stored in a software program for carrying out a method of operating the hydrostatic transmission (12) of the drive train (1) of the motor vehicle in which the automatic creep function of the motor vehicle is made possible by the hydrostatic transmission (12), the method including: comparing the current value of the first parameter and the current value of the second parameter to one another, the first parameter being independent of the pedal position of the accelerator (19) and the second parameter being dependent on the pedal position of the accelerator (19); activating the creep function if the current value of the first parameter is greater than the current value of the second parameter; determining from the current value of the first parameter, the current value of the target delivery quantity of the hydraulic machine (13) of the transmission (12) operating as the pump in the context of the creep function; taking into account the nominal swivel angle (.sub.1) of the hydrostatic machine (13) that is independent of the pedal position of the accelerator (19) as the first parameter, and taking into account the nominal swivel angle (.sub.2) of the hydrostatic machine (13) that is dependent on the pedal position as the second parameter; and setting the value of the resulting nominal swivel angle (.sub.res) of the hydrostatic machine (13) equal to the larger current value of the first and the second parameters in the context of the creep function.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Advantageous embodiments of the invention, which shall be explained in the following, are illustrated in the drawings. Shown in:

[0028] FIG. 1 is a schematic view of a drive train of a motor vehicle;

[0029] FIG. 2 is a flow diagram of a method for operating a hydrostatic transmission of the drive train from FIG. 1, according to a first embodiment of the invention;

[0030] FIG. 3 is an illustrative diagram of a parameter of the method from FIG. 2;

[0031] FIG. 4 is an illustrative diagram of a parameter of the method from FIG. 2;

[0032] FIG. 5 is a flow diagram of a method for operating a hydrostatic transmission of the drive train from FIG. 1, according to a second design option of the invention;

[0033] FIG. 6 is an illustrative diagram of a parameter of the method from FIG. 5; and

[0034] FIG. 7 is an illustrative diagram of a parameter of the method from FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] FIG. 1 shows a schematic view of a drive train 1 of a motor vehicle, which is in particular a commercial vehicle in the form of a mobile work machine such as a wheel loader. The drive train 1 comprises a main engine 2, which in the present case is designed as an internal combustion engine and connected on an output side to a motor vehicle transmission 3. The motor vehicle transmission 3 is composed of a shift group 4, a main group 5, and a range group 6, wherein the shift group 4 is arranged upstream of the main group 5 and the range group 6 is arranged downstream of the main group 5. Specifically, the shift group 4 is coupled to the output side of the main engine 2 and comprises two (not shown here) directional clutches, the selective actuation of which enables either forward or reverse travel of the motor vehicle.

[0036] On the output side, the motor vehicle transmission 3 is connected to an axle drive 7 of a drive axle 8 of the motor vehicle, wherein drive movement of the main engine 2 transmitted by the motor vehicle transmission is distributed to two drive wheels 9 and 10 of the drive axle 8.

[0037] As indicated in FIG. 1 in the present case the main group 5 of the motor vehicle transmission 3 is designed as a power-split transmission and is composed of a mechanical transmission 11 and a hydrostatic transmission 12. The mechanical transmission 11 (not illustrated in any further detail) is preferably embodied as a stepped transmission and can be a spur gear transmission or a planetary transmission.

[0038] The hydrostatic transmission 12 comprises two hydrostatic machines 13 and 14, which are interconnected in a hydraulic circuit 15 and which can each be operated either as a hydraulic pump or as a hydraulic motor. The two hydrostatic machines 13 and 14 are each swash plate machines, wherein a given delivery volume or displacement volume of the individual hydrostatic machine is defined by means of a currently set swivel angle of the hydrostatic machines 13 or 14, respectively. In the present case, the swivel angle can be varied in the hydrostatic machine 13 as well as in the hydrostatic machine 14 in order to after the delivery or displacement volumes, respectively.

[0039] A plurality of control units are assigned to the drive train 1, of which, in the present case, one is a control unit 16 of the main engine 2 and the other is a control unit 17 of the motor vehicle transmission 3. The control units 16 and 17 are integrated in a data bus system 18 of the motor vehicle, via which they obtain different information including, among other things, a current pedal position of an accelerator 19, the actuation state of a service brake 20 and a speed of an output 21 of the motor vehicle transmission 3.

[0040] In the present case, a creep function of the motor vehicle can be realized by means of the hydrostatic transmission 12, this being achievable according to the flow diagram in FIG. 2 or according to the flow diagram from FIG. 5 as an alternative.

[0041] In this case, the flow diagram for operating the hydrostatic transmission 12 according to a first embodiment of the invention is presented in FIG. 2: first, in step S1, the current output speed n.sub.Ab, of the motor vehicle transmission 3 and a current pedal angle .sub.P, of the accelerator 19 are read in, and a current pedal position Pos.sub.P expressed as a percentage is determined from the latter. In a subsequent step S2, two nominal swivel angles .sub.1 and .sub.2 of the hydrostatic machine 13 of the hydrostatic transmission 12 are determined, of which the nominal swivel angle .sub.1 is a parameter that is dependent on a driving speed of the motor vehicle and thus also on the output speed n.sub.Ab and the nominal swivel angle .sub.2 is a parameter that is dependent on the pedal position Pos.sub.P. A determination of current values of the nominal swivel angles .sub.1 and .sub.2 is carried out specifically on the basis of the diagrams in FIG. 3 and FIG. 4.

[0042] In FIG. 3 the progression of the nominal swivel angle .sub.1 is plotted against the output speed n.sub.Ab and in the diagram in FIG. 4 the nominal swivel angle .sub.2 is plotted against the pedal position Pos.sub.P of the accelerator 19. In the diagram presented in FIG. 3, the actuation of the service brake 20 also has an effect on the nominal swivel angle .sub.1, wherein increasing actuation brings about a parallel shift of the progression of the nominal swivel angle .sub.1 towards smaller values, as indicated by the arrow and the dashed line in FIG. 3.

[0043] After determining the current values for the nominal swivel angles .sub.1 and .sub.2, these values are compared to one another in step S3, wherein a transition to a step S4 takes place for the case in which the current value of the nominal swivel angle .sub.1 is greater than the current value of the nominal swivel angle .sub.2, otherwise a switch to a step S5 takes place, In step S4, a value of a resulting swivel angle .sub.res to be specified for the hydrostatic machine 13 is set equal to the current value of the nominal swivel angle .sub.1 and then in step S6, this nominal swivel angle is set in the hydrostatic machine 13 before going back to step S1. In contrast, in step S5 the value of the nominal swivel angle .sub.res is set equal to the current value of the nominal swivel angle .sub.2, and then this is also set before going back to step S1.

[0044] The flow diagram of a method for operating the hydrostatic transmission 12 presented in FIG. 5 corresponds to a second design option of the invention. In this case, the current output speed n.sub.Ab and a current pedal angle .sub.P of the accelerator 19 are read in, whereupon current values of a hypothetical accelerator position Pos.sub.1 and of an actual accelerator position Pos.sub.2 are determined therefrom in a step S8. The hypothetical accelerator position Pos.sub.1 is thus a parameter that is dependent on a driving speed of the motor vehicle and therefore also on the output speed N.sub.Ab, whereas the actual accelerator position Pos.sub.2 is dependent on the current pedal angle .sub.P.

[0045] The determination of the current values of the hypothetical accelerator position Pos.sub.1 and of the actual accelerator position Pos.sub.2 takes place according to the diagrams presented in FIG. 6 and in FIG. 7, wherein FIG. 6 shows the progression of the hypothetical accelerator position Pos.sub.1 plotted against the output speed n.sub.Ab, whereas the progression of the actual accelerator position Pos.sub.2 is plotted against the pedal angle .sub.P in FIG. 7. As can also be discerned in FIG. 6, the actuation of the service brake 20 has in addition an effect on the progression of the pedal position Pos.sub.1, as in this case a parallel shift towards lower values occurs, as indicated by the arrow and the dashed line in FIG. 6.

[0046] In step S9, the two pedal positions Pos.sub.1 and Pos.sub.2 are compared to one another, wherein a transition to step S10 occurs if the hypothetical pedal position Pos.sub.1 is greater than the actual pedal position Pos.sub.2. A switch to step S11 occurs if the reverse is true.

[0047] In step S10, a current value of a resulting nominal swivel angle .sub.res of the hydrostatic machine 13 is determined from the hypothetical accelerator position Pos.sub.1, whereas in the case of step S11, the current value of the nominal swivel angle .sub.res is determined from the actual pedal position Pos.sub.2. Lastly, in a step S12 the respective value of the nominal swivel angle .sub.res is specified as the swivel angle to be set in the hydrostatic machine 13 before going back to step S7.

[0048] With the method according to the invention, a creep function can be reproduced in a motor vehicle by means of a hydrostatic transmission.

LIST OF REFERENCE SIGNS

[0049] 1 Drive train
2 Main engine
3 Motor vehicle transmission
4 Shift group
5 Main group
6 Range group
7 Axle drive
8 Drive axle
9 Drive wheel
10 Drive wheel
11 Mechanical transmission
12 Hydrostatic transmission
13 Hydrostatic machine
14 Hydrostatic machine
15 Hydraulic circuit
16 Control unit
17 Control unit
18 Data bus system

19 Accelerator

[0050] 20 Service brake

21 Output

[0051] n.sub.Ab Current output speed
.sub.P Current pedal angle
Pos.sub.P Current pedal position
.sub.1 Nominal swivel angle
.sub.2 Nominal swivel angle
.sub.res Resulting nominal swivel angle
Pos.sub.1 Hypothetical accelerator position
Pos.sub.2 Actual accelerator position
S1 to S12 Individual steps