Hydrostatic transmission and methods for braking with it

Abstract

Hydrostatic transmission for a drive system, comprising a drive shaft (5) of a primary unit (2) that can be coupled to an internal combustion engine (4) of the drive system and at least one secondary unit (6) that can be coupled to an output (14) of the drive system, wherein the two units (2, 6) are fluidically connected to each other via two working lines (20, 22) of a closed circuit, and wherein the primary unit (2) has an adjustable swivel angle or an adjustable displacement volume (Vg_pump) that can be controlled by an electrical control unit (40) when the hydrostatic transmission is braked, and wherein braking can be initiated automatically via the control unit (40) when an actual travel speed (v_veh_act) of the mobile working machine or a quantity derived therefrom (n_mot_act) reaches a travel speed threshold (v_veh_on) or a derived threshold therefrom (n_mot_on), characterized in thatthat the engagement threshold (v_veh_on; n_mot_on) is variable or adjustable; wherein the engagement threshold (v_veh_on; n_mot_on) is adjustable via a driver request; wherein the engagement threshold (n_mot_on) is equal to the sum of a target speed (n_mot_des) of the secondary unit (6) derived from the driver request and an additional value (n_mot_over_on).

Description

[0001] The invention relates to a hydrostatic transmission with which hydrostatic braking is possible, according to the preamble of claim 1, and a method for braking a vehicle with such a hydrostatic transmission.

[0002] Hydrostatic transmissions for mobile machinery are known from the prior art, in which a hydrostatic pump (primary unit) and at least one hydrostatic motor (secondary unit) are fluidically connected to each other via a closed hydraulic circuit. A primary unit can be located in a closed hydraulic circuit with several secondary units, the secondary units being arranged in parallel to each other. An internal combustion engine, e.g., a diesel engine of the mobile machinery, is coupled to the primary unit, and an output, e.g., an axle or a wheel of the mobile machinery, is coupled to the secondary unit in a rotationally fixed manner. Thus, the mobile machinery has a drive system that incorporates a hydrostatic transmission.

[0003] In publication EP 1 960 699 B1, a hydrostatic transmission is disclosed that can also be used for braking. In this system, the power flows in the opposite direction to traction operation, from the output shaft via the secondary unit, which acts as a pump, and via the primary unit, which acts as a motor, to the internal combustion engine, which is then driven in a passive towing mode. The high-pressure working line of the closed circuit is protected by a pressure relief valve, which also allows a portion of the braking power to be dissipated during braking. However, with this solution, braking is only initiated when the driver explicitly requests it by pressing the brake pedal.

[0004] German patent applications DE 10 2014 211 393 A1 and US 2014 / 0372000 A1 each disclose a hydrostatic transmission that can also be used for braking. This transmission is intended to protect the internal combustion engine from overspeeding. A speed control system is described for this purpose, which detects when the engine's braking power is exceeded and automatically initiates high-performance braking. A portion of the braking power is transferred to the internal combustion engine, while a portion is converted into heat via the pressure relief valve of the affected high-pressure line.

[0005] Furthermore, it is known from the prior art to automatically initiate braking by means of a hydrostatic transmission when the travel speed of the mobile work machine in question becomes too high. Two-point controllers are used for this purpose, so that braking is activated when a maximum speed or engagement threshold is exceeded. A fixed braking torque then acts until the travel speed falls below a disengagement threshold.

[0006] A disadvantage of this type of speed monitoring is that the activation and deactivation thresholds are fixed values. This means that, for example, when the mobile work machine is traveling downhill, the automatic braking may be initiated and terminated repeatedly in an unpleasant and technically suboptimal manner.

[0007] In contrast, the invention is based on the objective of creating a hydrostatic transmission and a method in which these disadvantages are avoided.

[0008] This problem is solved by a hydrostatic transmission having the features of claim 1 and by a method having the features of claim 11.

[0009] Further advantageous embodiments of the invention are described in the dependent patent claims.

[0010] The claimed hydrostatic transmission is intended for a drive system comprising an internal combustion engine, e.g., a diesel engine, and an output, e.g., a wheel or an axle. The hydrostatic transmission has a drive shaft of a primary unit, which can be coupled to the internal combustion engine of the drive system and operates as a pump during traction, and at least one secondary unit, which can be coupled to an output of the drive system and operates as a motor during traction. One or more secondary units can be assigned to a primary unit; for example, four secondary units on two axles and four wheels in a field sprayer, or one secondary unit in a forklift. The primary unit and the secondary unit(s) are fluidically connected to each other via two working lines of a closed hydraulic circuit.The primary unit has an adjustable swivel angle and thus an adjustable stroke volume, which can be controlled by an electrical control unit during braking of the hydrostatic transmission. Braking can be initiated automatically via the control unit when a current travel speed of the mobile machine, or a value derived therefrom (e.g., the current rotational speed of the secondary unit), reaches a threshold for the travel speed or a threshold derived therefrom (e.g., the rotational speed of the secondary unit). According to the invention, the threshold for the braking is variable or adjustable. This makes it possible, for example, for the automatic braking to be initiated, executed, and terminated in a smooth and technically optimal manner when the mobile machine is traveling downhill.The actual driving speed can also be directly recorded and provided by the control unit instead of monitoring the derived quantity as mentioned above.

[0011] If the engagement threshold can be adjusted via a driver request, particularly via a control element such as a brake pedal, accelerator pedal, drive lever, or cruise control, the driver's foresight can be used to advantage. For example, the driver can reduce the engagement threshold if they know that the mobile work machine is heavily loaded or that the upcoming slope is steep.

[0012] Preferably, the derived entry threshold is equal to the sum of a target speed of the secondary unit derived from the driver's request and an additional value.

[0013] It is preferred if the braking can also be automatically deactivated by a drop in the actual driving speed of the mobile working machine or the quantity derived therefrom below a trigger threshold of the driving speed or a trigger threshold derived therefrom, e.g. a trigger threshold of the rotational speed of the secondary unit, and if the trigger threshold is also variable or adjustable.

[0014] The derived exit threshold is also preferably obtained from the sum of the target speed of the secondary unit derived from the driver's request and an additional value.

[0015] Preferably, the entry threshold and / or exit threshold are also automatically adjustable by the control unit. This could, for example, depend on an automatically detected load situation of the mobile work machine in question or on the actual speed of the combustion engine, which must not be over-revved.

[0016] Then it is advantageous if the control unit is designed in such a way that braking can be initiated and terminated based on automatic monitoring of the actual speed of the combustion engine or a quantity derived from it, e.g., the speed of the primary unit.

[0017] If the secondary unit also has an adjustable swivel angle and thus an adjustable stroke volume, which can be controlled by the control unit during braking, then a braking torque can be adjusted during braking.

[0018] According to a first variant, the braking torque can be controlled or regulated depending on a difference between, on the one hand, the actual driving speed or the quantity derived therefrom, and, on the other hand, a target driving speed or the quantity derived therefrom, in particular the target speed of the secondary unit.

[0019] According to a second variant, the braking torque can be controlled or regulated depending on a difference between, on the one hand, the actual driving speed or a quantity derived from it, and, on the other hand, a maximum driving speed or a quantity derived from it, maximum rotational speed of the secondary unit.

[0020] To enable high-performance braking, it is particularly preferred to have a pressure relief valve on each of the two working lines. A first portion of the braking force can be dissipated via the pressure relief valve, while a second portion can be dissipated via the primary unit and the internal combustion engine. The achievable braking force is particularly high when the first portion is greater than the second.

[0021] During high-performance braking, if the flow rate through the primary unit increases, the flow rate through the affected pressure relief valve decreases. This can cause the pressure in the high-pressure working line to drop. To minimize this pressure reduction or to keep the pressure nearly constant, pressure relief valves with a flat characteristic curve regarding their pressure differential as a function of the flow rate are preferred.

[0022] In a preferred embodiment of the hydrostatic transmission according to the invention, the swivel angle and the stroke volume of the primary unit are adjustable on both sides of a zero position. This allows the affected drive system to be used in both directions of travel of the mobile work machine in towing operation, while maintaining the same direction of rotation of the combustion engine, and to be braked accordingly in both directions according to the invention.

[0023] In a simplified embodiment of the hydrostatic transmission according to the invention, the primary unit is a load-sensitive axial piston machine. During pump operation, forces act in the direction of a reduction in its swivel angle, which depend on a pressure difference between the two working lines, a rotational speed of the drive shaft, and the swivel angle. A characteristic map of the primary unit is then stored in the control unit, in which a respective swivel angle is assigned to the pressure difference and the rotational speed, thus eliminating the need for feedback of the swivel angle.

[0024] Preferably, the control unit can control a control pressure via an electric control pressure valve and a control cylinder of an adjusting device of the primary unit, wherein the control pressure acts in the direction of an increase in its swivel angle, and wherein the dependence of the control pressure or a control pressure deviation on the pressure difference of the two working lines and the speed of the drive shaft and the swivel angle or the stroke volume is stored in the characteristic map.

[0025] In another embodiment of the hydrostatic transmission according to the invention, the swivel angle and the stroke volume of the primary unit are adjustable via an adjusting device that provides feedback of the swivel angle. The adjusting device can be an electroproportional (EP) adjusting device.

[0026] The method according to the invention serves to control braking with the aforementioned hydrostatic transmission and comprises the following steps: first, the engagement threshold of the travel speed or the engagement threshold derived therefrom is adjusted. Then, the actual travel speed of the mobile working machine or the value derived therefrom reaches the engagement threshold, whereupon the braking of the hydrostatic transmission is automatically initiated via the control unit. This makes it possible, for example, for the automatic braking of the mobile working machine during a downhill run to be initiated, carried out, and terminated in a smooth and technically optimal manner.

[0027] It is particularly preferred if the braking is carried out with a variable or adjustable braking torque.

[0028] A preferred further development of the method includes the following additional steps: the exit threshold of the travel speed, or the exit threshold derived from it, is also adjusted. After initiating braking and braking with variable or adjustable braking torque, the actual travel speed of the mobile working machine, or the value derived from it, reaches the exit threshold, whereupon the braking of the hydrostatic transmission is automatically terminated via the control unit.

[0029] An embodiment of the transmission according to the invention is shown in the drawings. The invention will now be explained in more detail with reference to the figures in these drawings.

[0030] They show Fig. 1 a circuit diagram of a drive system with a hydrostatic transmission according to the invention in the exemplary embodiment and Fig. 2 a diagram of the rotational speeds of the primary unit of the gearbox Fig. 1.

[0031] According to Fig. The hydrostatic drive system comprises a hydrostatic transmission 1 with a hydrostatic primary unit 2, which primarily operates as a hydraulic pump and is driven by an internal combustion engine 4, designed as a diesel engine, via a drive shaft 5. Furthermore, the hydrostatic transmission 1 has a hydrostatic secondary unit 6, which is coupled via a drive shaft 8 to an axle 14 having two wheels 12 and primarily operates as a hydraulic motor. More precisely, the drive shaft 8 is coupled to a differential gear 10 of the axle 14.

[0032] Both hydraulic machines 2, 6 have adjustable stroke volumes Vg_pump and Vg_mot via respective adjusting devices 16 and 18. The first hydraulic machine 2 is fluidically connected to the secondary unit 6 in a closed hydraulic circuit via a first working line 20, which is the supply line in the following considerations and through which hydraulic fluid flows from the primary unit 2 to the secondary unit 6, and via a second working line 22, which is the return line in the following considerations and through which hydraulic fluid flows from the secondary unit 6 to the primary unit 2.

[0033] The hydrostatic transmission 1 has a feed pump 26 connected to the drive shaft 5 of the primary unit 2, which can pump hydraulic fluid from a tank T into a feed line 28. The latter branches into three branches, the first of which can be connected to the tank T via a pressure relief valve 30. A second and a third branch can be connected to branch line 20 and branch line 22, respectively, via pressure relief valves 32 and 34, each of which has an integrated suction check valve 36 and 38.

[0034] Both units 2, 6 can be operated in all four quadrants, so that both the flow direction of the pressure medium in the closed hydraulic circuit and the direction of rotation of each of the units 2, 6 are reversible.

[0035] The hydrostatic transmission 1 has a control unit 40 to which a brake pedal 44 is connected via a signal line 42. The brake pedal 44 has a sensor 46 which detects the force applied to the brake pedal 44 and transmits this information to the control unit 40 via the signal line 42. The control unit 40 is connected via an electrical signal line 48 to the adjusting device 16 of the primary unit 2 and via an electrical signal line 50 to the adjusting device 18 of the secondary unit 6.

[0036] An electrical signal line 52 connects a speed sensing unit 54, which detects the actual speed n_mot_act of the secondary unit 6 at the drive shaft 8, to the control unit 40. An electrical signal line 62 connects a speed sensing unit 60, which detects the actual speed n_pump_act of the primary unit 2 at its drive shaft 5, to the control unit 40. Due to the integral design of the drive shaft 5 with the crankshaft of the internal combustion engine 4, the speed sensing unit 60 also detects the actual speed n_eng_act of the internal combustion engine 4.

[0037] Alternatively, a speed signal can be used, which is provided by the combustion engine as a CAN bus signal. This also requires a sensor. However, this sensor is read by an engine control unit and provided via CAN bus.

[0038] The control unit 40 has a storage unit 56 in which the braking method according to the invention is stored, and a processor unit 58 in which the braking method can be executed.

[0039] When braking by means of the hydrostatic transmission 1 according to the invention, the axle 14 is supported via the drive shaft 8 and via the secondary unit 6 which acts as a pump and via one of the two working lines 22 and via the primary unit 2 which acts as a motor and via the drive shaft 5 of the primary unit 2 on the internal combustion engine 4, which is then dragged along and reduces at least part of the braking energy of the mobile working machine via its friction and acceleration forces of the pistons.

[0040] Furthermore, a cruise control unit 64, an accelerator pedal 66, and a drive lever 68 are electrically connected to the control unit 40 via their respective signal lines. The accelerator pedal and drive lever can also be provided as alternatives.

[0041] During operation of the hydrostatic transmission 1 according to the invention, the control unit 40 calculates a target rotational speed m_mot_des of the drive shaft 8 of the secondary unit 6 from the setting of the cruise control 64 or the position of the accelerator pedal 66 or the drive lever 68, all of which represent a driving request, since this is proportional to the target travel speed v_veh_des of the mobile working machine in question. The actual travel speed v_veh_act is then derived from the actual rotational speed n_mot_act of the secondary unit 6, and if there is an excessive deviation, the braking according to the invention is initiated via the control unit 40.

[0042] Fig.Figure 2 shows a sequence of events, first an entry point (the automatic initiation of braking) and then an exit point (the automatic termination of braking). The speed n_mot of secondary unit 6 is represented over time. More precisely, the actual speed n_mot_act of secondary unit 6 is shown schematically. It can be seen that as the actual speed n_mot_act of secondary unit 6 increases, the entry point occurs when it reaches an entry threshold n_mot_on. This threshold is the sum of the target speed n_mot_des and an additional value n_mot_over_on. In other words, braking is activated by the intervention trigger. n_mot_act−n_mot_des>n_mot_over_on.

[0043] If the driving speed has been reduced by this braking and / or a topology change and / or the use of an additional (not shown) mechanical brake to a speed whose derived value n_mot_act is only slightly above or below the value n_mot_des derived from the required target driving speed v_veh_des by an additional value n_mot_over_off, then the braking is deactivated again. More precisely, the exit trigger is: n_mot_act−n_mot_des <n_mot_over_off.

[0044] In the case of component protection against overspeed, an upper speed threshold still exists, which can also be referred to as n_mot_des, and braking is activated when this threshold is exceeded. Deactivation occurs when a speed is reached that is slightly below or above the maximum design speed of the mobile machine.

[0045] A hydrostatic transmission for a drive system is disclosed, wherein a variable displacement pump and one or more motors are connected in a closed hydraulic circuit. Braking can be initiated and released via the transmission by means of an electronic control unit when the driving speed reaches a threshold value. According to the invention, this threshold value is adjustable automatically by the control unit or depending on a driver request. Preferably, the braking torque is also adjustable. Reference symbol list 1 hydrostatic transmission 2 Primary unit 4 Internal combustion engine 5 Drive shaft 6 Secondary unit 8 Drive shaft 10 Differential gears 12 wheel 14 Output / Axle 16 Adjustment device 18 Adjustment device 20 Work management 22 Work management 24 Drive shaft 26 Feed pump 28 Feed line 30 Pressure relief valve 32 Pressure relief valve 34 Pressure relief valve 36 Suction check valve 38 Suction check valve 40 Control unit 42 Signal line 44 Brake pedal 46 Sensor 48 Signal line 50 Signal line 52 Signal line 54 Speed ​​detection unit 56 storage units 58 processor units 60 speed detection unit 62 Signal line 64 Cruise control 66 Accelerator pedal 68 Driving levers n_mot_act Actual rotational speed of the secondary unit n_mot_des Target speed of the secondary unit n_mot_max maximum speed of the secondary unit derived from the maximum driving speed n_mot_min minimum speed of the secondary unit n_mot_on Entry threshold of the speed of the secondary unit n_mot_over_on Addition value of the speed of the secondary unit at jump start n_mot_off Exit threshold of the speed of the secondary unit n_mot_over_off Addition value of the rotational speed of the secondary unit at exit v_veh_act Actual driving speed v_veh_des target speed v_veh_max maximum driving speed v_veh_on Entry threshold of driving speed v_veh_off Exit threshold of driving speed T Tank

Claims

[1] Hydrostatic transmission for a drive system, comprising a drive shaft (5) of a primary unit (2) that can be coupled to an internal combustion engine (4) of the drive system and at least one secondary unit (6) that can be coupled to an output (14) of the drive system, wherein the two units (2, 6) are fluidically connected to each other via two working lines (20, 22) of a closed circuit, and wherein the primary unit (2) has an adjustable swivel angle or an adjustable displacement volume (Vg_pump) that can be controlled by an electrical control unit (40) when braking the hydrostatic transmission, and wherein braking can be initiated automatically via the control unit (40) when an actual travel speed (v_veh_act) of the mobile working machine or a quantity derived therefrom (n_mot_act) reaches a travel speed threshold (v_veh_on) or a derived threshold therefrom (n_mot_on), characterized by, that the engagement threshold (v_veh_on; n_mot_on) is variable or adjustable; wherein the engagement threshold (v_veh_on; n_mot_on) is adjustable via a driver request; wherein the engagement threshold (n_mot_on) is equal to the sum of a target speed (n_mot_des) of the secondary unit (6) derived from the driver request and an additional value (n_mot_over_on). [2] Hydrostatic transmission according to claim 1, wherein the braking can be automatically deactivated by lowering the actual driving speed (v_veh_act) of the mobile working machine or the quantity derived therefrom (n_mot_act) below a break-in threshold of the driving speed (v_veh_off) or a break-in threshold derived therefrom (n_mot_off), characterized by , that the exit threshold (v_veh_off; n_mot_off) is variable or adjustable. [3] Hydrostatic transmission according to one of the preceding claims, wherein the control unit (40) is designed such that braking can be automatically initiated and terminated based on automatic monitoring of an actual rotational speed (n_eng_act) of the internal combustion engine (4) or a quantity derived therefrom (n_pump_act). [4] Hydrostatic transmission according to one of the preceding claims, wherein the secondary unit (6) has an adjustable swivel angle or an adjustable stroke volume (Vg_mot) which is controllable by the control unit (40) during braking. [5] Hydrostatic transmission according to one of the preceding claims, wherein a braking torque is variable or adjustable during braking. [6] Hydrostatic transmission according to claim 5, wherein the braking torque is controllable or adjustable depending on a difference (Δv_veh; Δn_mot) between the actual driving speed (v_veh_act) or the quantity derived therefrom (n_mot_act) and a target driving speed (v_veh_des) or a quantity derived therefrom (n_mot_des). [7] Hydrostatic transmission according to claim 5, wherein the braking torque is controllable or adjustable depending on a difference (Δv_veh; Δn_mot) between the actual driving speed (v_veh_act) or the quantity derived therefrom (n_mot_act) and a maximum driving speed (v_veh_max) or a quantity derived therefrom (n_mot_max). [8] Hydrostatic transmission according to one of the preceding claims, wherein a pressure relief valve (32, 34) is arranged on each of the two working lines (20, 22), and wherein the braking is a high-performance braking system in which a first part of the braking power can be reduced via one of the pressure relief valves (32, 34), while a second part of the braking power can be reduced via the primary unit (2). [9] Hydrostatic transmission according to claim 8, wherein the pressure limiting valves (32, 34) each have a flat characteristic curve with respect to their pressure difference as a function of their volume flow rate. [10] Hydrostatic transmission according to one of the preceding claims, wherein the swivel angle and the stroke volume (Vg_pump) of the primary unit (2) are adjustable on both sides of a zero position. [11] Method for controlling a braking system with a hydrostatic transmission according to one of the preceding claims comprising the steps: - Determining the entry threshold of the driving speed (v_veh_on) or the derived entry threshold (n_mot_on), - Reaching the actual driving speed (v_veh_act) or the derived value (n_mot_act) of the entry threshold (v_veh_on; n_mot_on), and - automatic initiation of braking of the hydrostatic transmission (1) via the control unit (40). [12] Method according to claim 11 comprising the steps: - Determining the exit threshold of the driving speed (v_veh_off) or the derived exit threshold (n_mot_off), - Reaching the actual driving speed (v_veh_act) or the derived value (n_mot_act) of the exit threshold (v_veh_off; n_mot_off), and - automatic release of the braking of the hydrostatic transmission (1) via the control unit (40).

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