HYDROSTATIC TRANSMISSION AND METHOD FOR BRAKING USING THE SAME

Abstract

A hydrostatic transmission with which a braking operation is realized in which at least one traction motor which acts as a pump is supported via a closed circuit on an adjustable axial piston pump which acts as a motor, and which in turn is supported on an internal combustion engine. Since overspeeding of the latter should be avoided, pilot control is performed with a rotational speed that is subcritical for the internal combustion engine, and thereafter closed-loop control is performed with a closed-loop controller which outputs a corrective value in a manner dependent on the rotational speed deviation between the present rotational speed and the maximum and thus ideal rotational speed of the internal combustion engine.

Claims

1. A hydrostatic transmission for a traction drive of a mobile working machine, comprising: a primary unit including a driveshaft coupled to an internal combustion engine of the traction drive; a secondary unit coupled to an output of the traction drive; two working lines of a closed circuit each configured to fluidically connect the primary unit and the secondary unit; an electrical control unit configured to control an adjustable pivot angle and an adjustable swept volume of the primary unit during a braking operation of the hydrostatic transmission; and a closed-loop controller configured to output a corrective value for the pivot angle or the swept volume, wherein the corrective value is added to a pivot angle pilot-controlled by the electrical control unit or swept volume pilot-controlled by the electrical control unit.

2. The hydrostatic transmission according to claim 1, wherein an input variable of the closed-loop controller is a rotational speed difference between an actual rotational speed of the internal combustion engine, or a variable derived from the former, and a maximum rotational speed of the internal combustion engine, or another variable derived from the former.

3. The hydrostatic transmission according to claim 2, wherein the electrical control unit is configured such that the braking operation is initiated by an operating element or based on automatic monitoring of the traveling speed of the mobile working machine or of a variable derived from the former or based on automatic monitoring of the actual rotational speed of the internal combustion engine or of the variable derived from the former.

4. The hydrostatic transmission according to claim 1, further comprising: a first pressure-limiting valve arranged on a first working line of the two working lines; and a second pressure-limiting valve arranged on a second working line of the two working lines, wherein the braking operation is a high-power braking operation in which a first part of the braking power is dissipated via one of the first and the second pressure-limiting valves, while a second part of the braking power is dissipated via the primary unit.

5. The hydrostatic transmission according to claim 4, wherein the first and second pressure-limiting valves each have a flat characteristic curve with regard to their pressure difference as a function of their passed-through volume flow.

6. The hydrostatic transmission according to claim 1, wherein the pivot angle and the swept volume of the primary unit are adjustable in both directions from a zero position.

7. The hydrostatic transmission according to claim 1, wherein: the primary unit is a load-sensing axial piston machine, in a pump operating mode of which forces act in a direction of a decrease of its pivot angle, which forces are dependent on a pressure difference between the two working lines and on a rotational speed of the driveshaft and on the pivot angle, and in the electrical control unit there is stored a characteristic map of the primary unit in which the pressure difference and the rotational speed are assigned a respective pivot angle.

8. The hydrostatic transmission according to claim 7, wherein: the primary unit includes an adjustment device including an electrical setting pressure valve and a setting cylinder, the electrical control unit controls a setting pressure with the electrical setting pressure valve and the setting cylinder of the adjustment device, the setting pressure acts in a direction of an increase of the pivot angle of the primary unit, and the dependency of the setting pressure or of a setting pressure deviation on the pressure difference of the first and second working lines and on the rotational speed of the driveshaft and on the pivot angle or on the swept volume is stored in the characteristic map.

9. The hydrostatic transmission according to claim 8, wherein the pressure differences of the two working lines and the rotational speeds of the driveshaft for the pivot angles or swept volumes in both directions from the zero position are stored in the characteristic map.

10. The hydrostatic transmission according to claim 9, wherein: the setting cylinder is double-acting and has two setting pressure chambers, and the setting pressure in the two setting pressure chambers is controllable by the electrical control unit by a common or a respective electrically adjustable setting pressure valve.

11. The hydrostatic transmission according to claim 1, wherein the pivot angle and the swept volume of the primary unit are adjustable by an adjustment device, which exhibits feedback of the pivot angle.

12. A method for the closed-loop control of a braking operation with a hydrostatic transmission for a traction drive of a mobile working machine, comprising: pivoting a primary unit back; performing pilot control of the pivot angle or of the swept volume; and performing closed-loop control of the pivot angle or of the swept volume by adding a corrective value to a pilot-controlled pivot angle or to a pilot-controlled swept volume, wherein the hydrostatic transmission includes the primary unit including a driveshaft coupled to an internal combustion engine of the traction drive, a secondary unit coupled to an output of the traction drive, two working lines of a closed circuit each configured to fluidically connect the primary unit and the secondary unit, an electrical control unit configured to control the pivot angle and the swept volume of the primary unit during the braking operation of the hydrostatic transmission, and a closed-loop controller configured to output the corrective value.

13. The method according to claim 12, wherein, during the closed-loop control, a determination of the corrective value is performed in a manner dependent on a rotational speed difference between an actual rotational speed of the internal combustion engine, or of a variable derived from the former, and a maximum rotational speed of the internal combustion engine, or a variable derived from the former.

14. The method according to claim 13, further comprising: initiating the braking operation based on a driver demand or an overshooting of the actual rotational speed of the internal combustion engine or of a variable derived from the former or based on an overshooting of a traveling speed or of a variable derived from the former.

15. The method according to claim 12, wherein the pilot control of the pivot angle or of the swept volume is performed based on a characteristic map of the primary unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In the drawings:

[0037] FIG. 1 shows a circuit diagram of the hydrostatic transmission according to the disclosure as per a first exemplary embodiment,

[0038] FIG. 2 shows a schematic overview of the pilot control with closed-loop control according to the disclosure of the swept volume of the primary unit during the braking operation as per a second exemplary embodiment,

[0039] FIG. 3 shows a diagram of the swept volumes of the primary unit and of the secondary unit of the hydrostatic transmission during the braking operation as per both exemplary embodiments,

[0040] FIG. 4 shows a characteristic map of the primary unit of the hydrostatic transmission as per the first exemplary embodiment,

[0041] FIG. 5 shows a schematic overview of the characteristic-map-based pilot control with closed-loop control according to the disclosure of the swept volume during the braking operation as per the first exemplary embodiment, and

[0042] FIG. 6 shows two diagrams of the characteristic-map-based pilot control with closed-loop control according to the disclosure during the braking operation as per the first exemplary embodiment.

DETAILED DESCRIPTION

[0043] FIG. 1 shows a circuit diagram of the hydrostatic transmission according to the disclosure. Said transmission has a primary unit 1 and a secondary unit 2, which are both designed with an adjustable swept volume. The primary unit 1 is an axial piston machine, to the driveshaft 4 of which there is rotationally conjointly connected a crankshaft of a diesel engine (not shown). The rotational speed of the driveshaft 4 is monitored by means of a rotational speed sensor 6 and an electrical control unit 8. The rotational speed of the crankshaft of the diesel engine is thus also indirectly monitored.

[0044] The primary unit 1 is fluidically connected to the secondary unit 2 via a closed hydraulic circuit, which has two working lines 10. An output (not shown) is rotationally conjointly coupled to a driveshaft 12 of the secondary unit 2. The output is for example a differential transmission of a driven axle of the mobile working machine.

[0045] The hydrostatic transmission according to the disclosure as per FIG. 1 thus forms, together with the diesel engine and the output, a traction drive of a mobile working machine. During traction operation, the driveshaft 4 of the primary unit 1 serves as a driveshaft, and the primary unit 1 operates as a pump, whereas the secondary unit 2 operates as a motor, and the driveshaft 12 of the secondary unit 2 is an output shaft.

[0046] During a braking operation of the hydrostatic transmission according to the disclosure, the output is supported, via the driveshaft 12 and via the secondary unit 2 operating as a pump and via one of the two working lines 10 and via the primary unit 1 operating as a motor and via the driveshaft 4 of the primary unit 1, on the diesel engine, which is then cranked and, by means of its friction and acceleration forces of the pistons, dissipates at least a part of the braking energy of the mobile working machine.

[0047] On each working line 10 there is provided a pressure-limiting valve 14, via which the respective working line 10 can be relieved of pressure to a feed line 16. The latter is filled with feed pressure medium from a tank T by a feed pump 18, which is coupled rotationally conjointly to the driveshaft 4 of the primary unit 1. Furthermore, the feed line 16 can be relieved of pressure to a tank T via a pressure-limiting valve 20.

[0048] The feed line 16 is connected via a respective spring-loaded check valve 22 to the two working lines 10, such that the respectively low-pressure-conducting working line 10 can possibly be supplied with additional pressure medium from the feed line 10. For this purpose, the opening directions of the two check valves 22 are directed from the feed line 16 to the respective working line 10.

[0049] The adjustment of the swept volume of the primary unit 1 is realized by means of an adjustment device 24. The latter has a double-acting setting cylinder 26, the two pressure chambers of which act counter to one another on a setting piston which is coupled to a swashplate of the axial piston machine 1.

[0050] In the first exemplary embodiment, each of the two pressure chambers can be charged with pressure medium from the feed line 16 via a separate setting pressure valve 28. The two setting pressure valves 28 are electrically adjusted by the control unit 8. Furthermore, the adjustment device 24 has a spring arrangement (not shown) by means of which the piston of the setting cylinder 26 and the swashplate of the primary unit 1 are preloaded into a central position. From there, the primary unit 1 can be adjusted in both directions.

[0051] Since the primary unit 1 permits four-quadrant operation in both exemplary embodiments of the hydrostatic transmission according to the disclosure, forward travel and forward braking and reverse travel and reverse braking are thus possible.

[0052] In both exemplary embodiments, as already mentioned, the secondary unit 2 is also of adjustable design. For this purpose, use is made of an adjustment device 30 which has a valve with an electrical actuator which is likewise actuated by the control unit 8. Here, the adjustment device 30 has a feedback spring 32, such that closed-loop control of the pivot angle of the secondary unit 2 is possible.

[0053] FIG. 2 shows a schematic overview of the pilot control with closed-loop control according to the disclosure of the swept volume of the primary unit during the braking operation as per the second exemplary embodiment. A rotational speed difference n_eng between, on the one hand, the actual rotational speed n_eng_act of the internal combustion engine or the rotational speed, proportional to the former, of the primary unit n_pump_act and, on the other hand, a maximum rotational speed n_eng_max of the internal combustion engine or the rotational speed, proportional to the former, of the primary unit n_pump_max is an input variable of a PI closed-loop controller 138. The latter outputs a corrective value Vg_pump_add_incr for the swept volume Vg_pump. More specifically, the corrective value Vg_pump_add_incr is added to the pilot-controlled swept volume Vg_pump_brake to give an actuation signal for the adjustment device of the primary unit. Thus, according to the disclosure, the diesel engine is brought from a subcritical range to its maximum rotational speed n_eng_max without overshooting the latter.

[0054] The adjustment device for the pivot angle of the primary unit as per the second exemplary embodiment exhibits feedback of the pivot angle and may be an electroproportional (EP) adjustment device. The adjustment device may correspond to the adjustment device of the secondary unit of the first exemplary embodiment as per FIG. 1.

[0055] FIG. 3 shows, in a diagram, the profile with respect to time of the swept volumes Vg_pump, Vg_mot of the primary unit 1 and of the secondary unit 2 of the two exemplary embodiments. Firstly a transition from traction operation to braking operation, and then a transition from braking operation back to traction operation, are shown. Such a braking operation by means of the hydrostatic transmission according to the disclosure may be initiated by:

a) transmission of a signal from an operating element actuated by a driver, for example a brake pedal, to the control unit 8,
b) a high, limit-value rotational speed of the diesel engine that has been determined by the rotational speed sensor 6 and by the control unit 8,
c) a limit-value traveling speed v_veh of the mobile working machine that has been determined indirectly by a rotational speed sensor 34 of the driveshaft 12 of the secondary unit 2 and transmitted to the control unit 8.

[0056] In both exemplary embodiments, the braking operation is controlled, or controlled in closed-loop fashion, by the control unit 8. For this purpose, firstly, the pivot angle angle_pump and thus the swept volume Vg_pump of the primary unit 1 is reduced in a short time to a low value. Said low value may amount to for example approximately 10% of the maximum pivot angle angle_pump_max or of the maximum swept volume Vg_pump_max of the primary unit 1. Thus, firstly, the diesel engine is reliably protected against an excessively high rotational speed n_eng because said diesel engine, in the short time, not the rotational speed n_eng that would theoretically have to be attained at the small pivot angle angle_pump of the primary unit. Thus, the pressure in the working line 10 that now conducts high pressure increases quickly, and the corresponding high-pressure valve 14 opens a connection to the feed line 16.

[0057] In both exemplary embodiments, the control unit 8 thereupon actuates the adjustment device 30 of the secondary unit 2 such that its pivot angle and thus its swept volume Vg_mot are increased. Here, the pivot angle angle_pump of the primary unit 1 is also increased again somewhat (not illustrated), whereby the diesel engine is accelerated and a part of the braking power is dissipated via the latter.

[0058] The further abbreviations or formula terms in the diagram of FIG. 3 have the following meaning:

TABLE-US-00001 phasein_mot_ms Duration of the activation of the secondary unit. phasein_mot_delay_ms Delay of the activation of the secondary unit, if it reacts more quickly than the primary unit. phasein_pump_ms Duration of the activation of the primary unit. phasein_pump_delay_ms Delay of the activation of the primary unit, if it reacts more quickly than the secondary unit. phaseout_mot_ms Duration of the deactivation of the secondary unit. phaseout_mot_delay_ms Delay of the deactivation of the secondary unit, if it reacts more quickly than the primary unit. phaseout_pump_ms Duration of the deactivation of the primary unit. phaseout_pump_delay_ms Delay of the deactivation of the primary unit, if it reacts more quickly than the secondary unit.

[0059] To maximize the rotational speed of the diesel engine and to not exceed a permissible maximum rotational speed in the process, the closed-loop control discussed with reference to the following figures is used in the first exemplary embodiment, and the closed-loop control discussed with reference to FIG. 2 is used in the second exemplary embodiment.

[0060] FIG. 4 shows a characteristic map 36 of the primary unit 1 as per the first exemplary embodiment, which is designed as a load-sensing unit. The characteristic map 36 represents the influence of the pressure difference p between the two working lines 10 and of the actual rotational speed n_pump_act and furthermore of the pivot angle angle_pump of the primary unit 1 on the setting pressure p_st prevailing in the setting cylinder 24 (cf. FIG. 1). More specifically, the relationship is illustrated such that the pressure difference p of the two working lines 10 is plotted along a first axis, wherein for example the positive pressure differences p represent overrun operation and the negative pressure differences p represent the braking operation according to the disclosure. The actual rotational speed of the driveshaft 4 n_pump_act of the primary unit 1 is plotted on a further axis. The different surfaces of the surface set shown apply for example for three positive and three negative pivot angles angle_pump of the swashplate of the primary unit 1. Plotted on the vertical axis is a setting pressure deviation p_st_comp that is required for maintaining the pivot angle angle_pump at the respective operating point. Here, positive pressure values in the abovementioned example represent overrun operation, whereas the negative pressure values apply for the braking operation according to the disclosure. This means that, upon the initiation of the braking operation, in the case of the adjustment device 24 discussed with reference to FIG. 1, a change of the pressure chamber that is charged with setting pressure medium p_st is required.

[0061] In the first exemplary embodiment, the characteristic map 36 is stored in the control unit 8 (cf. FIG. 1), such that, by means of a pressure sensor (not shown) provided on each of the working lines 10, the control unit 8 can control every desired pivot angle angle_pump by virtue of the corresponding setting pressure deviation p_st_comp being set through corresponding actuation of the setting pressure valves 28. Here, by contrast to the hydrostatic transmissions of the prior art, no feedback of the pivot angle angle_pump of the primary unit 1 is necessary.

[0062] As a result of variance in the manufacture of the primary units 1 and as a result of other factors such as wear and viscosity of the pressure medium used, small deviations of the various physical primary units 1 from the characteristic map 36 are possible, such that, in the first exemplary embodiment, the above-described characteristic-map-based control is utilized as pilot control and is additionally refined as per FIG. 5. In said figure, the characteristic map 36 from FIG. 4 is illustrated in the central region, which characteristic map is used to assign a corresponding corrective value p_st_comp to the setting pressure p_st. A safety corrective value is also incorporated into the characteristic map 36 in order to allow for said possible variance of the characteristic of the various primary units 1 and in order, by means of the pilot control, which is simple in terms of apparatus, by means of the characteristic map 36, to always set a swept volume Vg_pump_comp in the case of which the diesel engine reliably does not overspeed and dissipates a high but non-critical braking power even in the event of a failure of the functions described below.

[0063] According to the disclosure, closed-loop control is also superposed on the pilot control, which closed-loop control compares the actual rotational speed n_eng_act of the diesel engine with the maximum admissible rotational speed n_eng_max thereof and generates a further corrective value for the setting pressure p_st. Alternatively, it is also possible for the rotational speed n_pump_act of the primary unit 1 to be compared with a converted maximum admissible rotational speed n_pump_max of the primary unit 1 and for the further corrective value for the setting pressure p_st to be generated.

[0064] By means of the pilot control as per the characteristic map 36 (cf. FIGS. 4 and 5), the primary unit 1 is thus already pivoted into the vicinity of the ideal value. According to the prior art, the primary unit 1 however still remains at an altogether somewhat excessively large swept volume Vg_pump (displacement volume), which would lead to a somewhat excessively high rotational speed n_eng_act of the diesel engine. In the case of the excessively small pivot angle angle_pump used according to the disclosure, the braking capability of the diesel engine would be only partially utilized. Thus, although a reliable braking function is duly already basically possible with the described load-sensing primary unit 1, that part of the braking power that flows via the primary unit is however further improved.

[0065] The final deviation from the ideal value is ultimately compensated by means of the superposed closed-loop control with the PID closed-loop controller 38 of the first exemplary embodiment, shown in FIG. 4, or with the PI closed-loop controller 138 of the second exemplary embodiment, shown in FIG. 2, which utilizes the last actual rotational speed n_eng_act of the diesel engine of its maximum admissible rotational speed n_eng_max as input variable and outputs an increased swept volume Vg_pump_add_incr. Thus, a swept volume Vg_pump_brake_corr that is optimum for the braking operation under the conditions of the physical primary units 1 is determined.

[0066] The effect is illustrated by way of example in FIG. 6. Uniform utilization of the diesel engine during the braking operation is realized without the latter reaching the inadmissible rotational speed range.

[0067] A hydrostatic transmission is disclosed with which a braking operation can be realized in which at least one traction motor which acts as a pump is supported via a closed circuit on an adjustable axial piston pump which acts as a motor, and which in turn can be supported on an internal combustion engine. Since overspeeding of the latter should be avoided, pilot control is performed with a rotational speed that is subcritical for the internal combustion engine, and thereafter closed-loop control is performed with a closed-loop controller which outputs a corrective value in a manner dependent on the rotational speed deviation between the present rotational speed and the maximum and thus ideal rotational speed of the internal combustion engine. Thus, effective braking with a subcritical rotational speed of the internal combustion engine is ensured at all times even in the event of a failure of the closed-loop controller.

LIST OF REFERENCE DESIGNATIONS

[0068] 1 Primary unit [0069] 2 Secondary unit [0070] 4 Driveshaft [0071] 6 Rotational speed sensor [0072] 8 Control unit [0073] 10 Working line [0074] 12 Driveshaft [0075] 14 Pressure-limiting valve [0076] 16 Feed line [0077] 18 Feed pump [0078] 20 Pressure-limiting valve [0079] 22 Check valve [0080] 24 Adjustment device [0081] 26 Setting cylinder [0082] 28 Setting pressure valve [0083] 30 Adjustment device [0084] 32 Feedback spring [0085] 34 Rotational speed sensor [0086] 36 Characteristic map [0087] 38 Closed-loop controller [0088] 138 Closed-loop controller [0089] angle_pump Pivot angle of the primary unit [0090] angle_pump_max Maximum pivot angle of the primary unit [0091] n_eng_act Actual rotational speed of the internal combustion engine [0092] n_eng_max Maximum admissible rotational speed of the internal combustion engine [0093] n_mot_act Actual rotational speed of the secondary unit [0094] n_pump_act Actual rotational speed of the primary unit [0095] n_pump_max Rotational speed of the primary unit derived from the maximum rotational speed of the internal combustion engine [0096] p_st Setting pressure of the primary unit [0097] p_st_comp Setting pressure deviation of the primary unit [0098] phasein_mot_ms Duration of the activation of the secondary unit [0099] phasein_mot_delay_ms Delay of the activation of the secondary unit [0100] phasein_pump_ms Duration of the activation of the primary unit [0101] phasein_pump_delay_ms Delay of the activation of the primary unit [0102] phaseout_mot_ms Duration of the deactivation of the secondary unit [0103] phaseout_mot_delay_ms Delay of the deactivation of the secondary unit [0104] phaseout_pump_ms Duration of the deactivation of the primary unit [0105] phaseout_pump_delay_ms Delay of the deactivation of the primary unit [0106] Vg_mot Swept volume of the secondary unit [0107] Vg_pump Swept volume of the primary unit [0108] Vg_pump_add_incr Increased swept volume of the primary unit for braking [0109] Vg_pump_brake Pilot-controlled swept volume of the primary unit for braking [0110] Vg_pump_brake_corr Setpoint swept volume of the primary unit for braking [0111] Vg_pump_comp Swept volume deviation of the primary unit [0112] Vg_pump_max Maximum swept volume of the primary unit [0113] v_veh Traveling speed [0114] n_eng Rotational speed difference between the setpoint rotational speed and the actual rotational speed of the internal combustion engine [0115] p Pressure difference between the two working lines [0116] T Tank