METHOD FOR CONTROLLING A HYDROSTATIC DRIVE

Abstract

The present invention relates to a method for controlling a hydrostatic drive (1) of a mobile work machine, wherein the hydrostatic drive is provided with a first hydraulic machine (2) with an adjustable displacement volume and with a second hydraulic machine (6) with an adjustable displacement volume, wherein the hydraulic machines (2, 6) are fluidically connected, and wherein the first hydraulic machine (2) can be coupled to a drive machine (4) and the second hydraulic machine (6) can be coupled to at least one wheel (12) to be driven or a chain or axle (14) to be driven, a. Detecting that the hydrostatic drive is to be switched from a drive state to a deceleration state or vice versa; b. Determining a target swivel angle of the second hydraulic machine based on estimated losses; c. Adjusting the swivel angle of the second hydraulic machine (6) on the basis of the desired swivel angle determined in step b., so that a rotational speed of the second hydraulic machine (6) can remain constant over the transition between the deceleration status and the drive status or vice versa.

Description

TECHNICAL AREA

[0001] The invention relates to a method for controlling a hydrostatic drive, preferably of a work machine. Furthermore, this invention relates to a control unit, a work machine comprising the control unit, and a computer program. STATE OF THE ART

[0002] Hydrostatic drive systems for mobile work machines are known, in which a hydraulic pump and one or more hydraulic motors are connected in a typically closed hydraulic circuit. The hydraulic pump is driven by a prime mover—e.g., a diesel engine or an electric motor—and the hydraulic motors ultimately drive the mobile work machine—e.g., via a respective wheel.

[0003] The hydraulic pump and hydraulic motors of such travel drives often have adjustable displacement. This allows, for example, the volume flow delivered by the hydraulic pump to be varied in a closed circuit at a constant combustion engine speed, thus adjusting the output speed of the hydraulic motors or wheels—in other words, the travel speed of the mobile machine.

[0004] Hydrostatic drive systems have a wide range of applications, for example, in agriculture, where the machine must perform a working function in addition to its driving function. The forage harvester is one example. The forage harvester is an agricultural device used for collecting, chopping, and loading crops such as grass, alfalfa, or corn, particularly when preparing silage or whole-crop silage.

[0005] In the case of driving operation (i.e., the mobile work machine is driven by the prime mover), the hydraulic pump's volumetric losses result in the actual flow rate being lower than the theoretical flow rate. At the same time, these losses increase the engine's fuel consumption.

[0006] To solve this problem, these losses are very often taken into account when calculating the target swivel angle of the hydraulic pump and hydraulic motor (a type of loss compensation). However, when these losses are taken into account during load reversal (i.e., when the hydrostatic drive is switched from a drive state, in which the mobile machine is driven by the prime mover, to a deceleration state, in which the mobile machine is decelerated by the prime mover, or vice versa), the hydraulic motor will actually accelerate during load reversal. The reason for this is that this compensation adjusts the hydraulic motor in such a way that the internal losses are taken into account. This means that the hydraulic motor consumes more power than without loss compensation.However, as soon as a load reversal occurs, the compensation before the reversal will cause an increase in speed, since the hydraulic motor no longer operates as a hydraulic motor but now as a hydraulic pump. This is particularly disadvantageous.

[0007] In contrast, the object of the invention is to produce a method that enables simple control of the working machine even in the event of a load reversal. SUMMARY

[0008] According to one embodiment of the present invention, a method for controlling a hydrostatic drive (1) of a mobile work machine is provided, wherein the hydrostatic drive is provided with a first hydraulic machine (2) with an adjustable displacement volume and with a second hydraulic machine (6) with an adjustable displacement volume, wherein the hydraulic machines (2, 6) are fluidically connected, and wherein the first hydraulic machine (2) can be coupled to a drive machine (4) and the second hydraulic machine (6) can be coupled to at least one wheel (12) to be driven or a chain or axle (14) to be driven, the method comprising the following steps: a. Detecting that the hydrostatic drive is to be switched from a drive state in which the mobile work machine is driven by the drive machine (4) to a deceleration state in which the mobile work machine is decelerated by the drive machine (4), or vice versa (ie from the deceleration state to the drive state); b. Determining a target swivel angle of the second hydraulic machine or target size (such as target displacement volume or otherwise known as target displacement volume) on the basis of estimated losses in the first and second hydraulic machines (2, 6); c. Adjusting the swivel angle of the second hydraulic machine (6) on the basis of the desired swivel angle determined in step b., so that a rotational speed of the second hydraulic machine (6) can remain constant over the transition between the deceleration status and the drive status or vice versa.

[0009] This invention is particularly advantageous because it enables the mobile work machine to operate with predictable handling, even during load reversals. In particular, the method enables smooth transitions between driving and braking without disruptive torque drops. This is perceived by the driver, for example, as improved responsiveness of the hydrostatic braking system. SHORT DESCRIPTION OF THE CHARACTERS

[0010] The present invention is described with reference to the accompanying figures, wherein like reference numerals refer to like parts and / or similar parts and / or corresponding parts of the system. Regarding the figures: Fig. 1 shows a hydraulic circuit diagram of a hydrostatic drive according to the state of the art, Fig. 2 shows a method for controlling a hydrostatic drive according to the prior art; Fig. 3 shows a method for controlling a hydrostatic drive according to an embodiment of the present invention. DETAILED DESCRIPTION

[0011] The present invention will now be described with reference to specific embodiments as shown in the accompanying figures. Nevertheless, the present invention is not limited to the specific embodiments described in the following detailed description and shown in the figures; rather, the described embodiments merely illustrate some aspects of the present invention, the scope of which is defined by the claims.

[0012] Further modifications and variations of the present invention will be apparent to those skilled in the art. The present description thus encompasses all modifications and / or variations of the present invention, the scope of which is defined by the claims.

[0013] According to Fig. 1, a hydrostatic drive 1 has a first hydrostatic hydraulic machine 2, which operates primarily as a hydraulic pump and is driven by a drive engine 4, which is designed as a diesel engine. Furthermore, the hydrostatic drive 1 has a second hydrostatic hydraulic machine 6, which is coupled via a drive shaft 8 to an axle 14 having two wheels 12 and is operated primarily as a hydraulic motor. The displacement volumes of both hydraulic machines 2, 6 are each adjustable via an adjustment unit 16, 18.The first hydraulic machine 2 is fluidically connected to the second hydraulic machine 6 in a closed hydraulic circuit via a first branch line 20, which in the further considerations is the feed line, via which pressure medium flows from the hydraulic machine 2 to the hydraulic machine 6, and via a second branch line 22, which in the further considerations is the second branch line, via which pressure medium flows from the hydraulic machine 6 to the hydraulic machine 2.

[0014] The hydrostatic drive 1 has a feed pump 26 connected to a drive shaft 24 of the first hydraulic machine 2, which feed pump can pump pressure medium from a tank T into a feed line 28. The latter branches into three branches, a first branch being connectable to the tank T via a pressure relief valve 30. A second or third branch can be connected to the branch line 20 or the branch line 22 via a pressure relief valve 32 or a pressure relief valve 34, each of which has an integrated suction check valve 36 or 38.

[0015] Both hydraulic machines 2, 6 are operable 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 hydraulic machines 2, 6 are reversible. However, it is not necessary for the invention that the two hydraulic machines be located in a closed hydraulic circuit.

[0016] The hydrostatic drive 1 has a control unit 40, to which a brake actuation pedal 44 is connected via a signal line 42. The latter has a sensor 46, via which the actuation force of the brake actuation pedal 44 can be detected and transmitted to the control unit 40 via the signal line 42. This is connected to the actuation device 16 of the hydraulic machine 2 via an electrical signal line 48 and to the actuation device 18 of the hydraulic machine 6 via an electrical signal line 50. A speed detection unit 54, via which the speed of the second hydraulic machine 6 on the drive shaft 8 can be detected, is connected to the control unit 40 via an electrical signal line 52. A speed detection unit 60, via which the speed of the first hydraulic machine 2 on its drive shaft can be detected, is connected to the control unit 40 via an electrical signal line 62.The control unit 40 has a memory unit 56 in which a method according to the invention is stored, and a processor unit 58 in which the method can be executed.

[0017] With reference to Fig. 2, a method according to the prior art is now described so that the technical problem already described in the introduction to the present patent application becomes clearer.

[0018] In Fig. 2, a speed of the first hydraulic machine 2 is shown by curve 101 and the speed of the second hydraulic machine 6 is shown by curve 102. Line 103 describes a desired speed of the second hydraulic machine 6. Time t1 describes the moment at which the system switches from a drive state, in which the mobile work machine is driven by the drive machine 4, to a deceleration state, in which the mobile work machine is decelerated by the drive machine 4.

[0019] As already mentioned, in the driving case, the first hydraulic machine 2 produces less than the theoretical flow rate due to volumetric losses. At the same time, the second hydraulic machine 6 consumes correspondingly more than the theoretical flow rate for the same reason.

[0020] For this reason, compensation is often required. This compensation comes from a volume flow balance: Vg,P⋅nP⋅ηP⋅=Vg,M⋅nMηM where V g,P and V g,M the displacement of the first and second hydraulic machines (maximum displacement multiplied by the swivel angle of the first / second hydraulic machine), with n p and n M the speed of the first and second hydraulic machine and with η P and η Mwhere the efficiency of the first and second hydraulic machines is taken into account. From this equation, the compensated swivel angle of the second hydraulic machine can be determined in order to achieve the desired speed of the second hydraulic machine.

[0021] The efficiency of the first and second hydraulic machines is already known (from measured data or models) and is generally dependent on the pressure at the respective hydraulic machine and the speed of the respective hydraulic machine. It is clear to those skilled in the art that, as an alternative to Equation 1, the actual losses can be estimated. This means that, as an alternative to the efficiencies, the hydraulic losses are estimated (as absolute values). These are, of course, also known from measured data or models.

[0022] During load reversal, as already mentioned, the second hydraulic machine 6 assumes a new role as a pump to decelerate the mobile work machine. Now operating as a pump, the second hydraulic machine 6 produces less flow than desired. This causes the second hydraulic machine to accelerate, as more flow than desired flows through the second hydraulic machine 6. Thus, the transition from the drive state to the deceleration state, with the displacement Vg of the adjustment units remaining unchanged, leads to an increase in the speed of the second hydraulic machine 6.

[0023] Analogous to what has already been described, this means that when the braking process is initiated, when the second hydraulic machine 6 is continuously adjusted to a larger volume from normal drive, a delayed load reversal will occur. This will be perceived by the vehicle operator as a delayed onset of the braking effect. This is obviously a disadvantage.

[0024] With reference to Fig. 3, a method according to an embodiment of the present invention will now be described, which comprises the method described with reference to Fig. 2 can solve the problems described.

[0025] In a first step, it is detected that the hydrostatic drive needs to be switched from a drive state to a deceleration state or vice versa. The detection that the hydrostatic drive needs to be switched from the drive state to the deceleration state or vice versa is preferably achieved by means of a pressure in the first and second branch lines 20, 22.

[0026] In a second step, a target swivel angle of the second hydraulic machine 6 is determined on the basis of estimated losses in the first and second hydraulic machines 2, 6.

[0027] As an example, one possible way to account for losses is described. It will be clear to those skilled in the art that this is not the only way to account for losses.

[0028] Based on equation 1, the displacement of the second hydraulic machine is calculated using the following formulas, depending on whether the second hydraulic machine operates as a motor (equation 2) or as a pump (equation 3): Vg,M=Vg,P⋅nP⋅ηP⋅ηMnM Vg,M=⋅Vg,P⋅nPnM⋅ηP⋅ηM

[0029] This means that when it is detected that a switch is being made between a drive state and a deceleration state, or vice versa, the system switches from equation 2 to equation 3, or vice versa. This means that a new target swivel angle (or target displacement) of the second hydraulic machine 6 is calculated. The target swivel angle of the second hydraulic machine 6 represents a jump compared to the current actual swivel angle of the second hydraulic machine 6 (since switching from one equation to another).

[0030] In a third step, the swivel angle of the second hydraulic machine 6 is adjusted on the basis of the determined target swivel angle, so that a rotational speed of the second hydraulic machine 6 can remain constant over the transition between the deceleration status and the drive status or vice versa.

[0031] Since, as already mentioned, the desired swivel angle of the second hydraulic machine 6 represents a jump in comparison to a current actual swivel angle of the second hydraulic machine 6, the swivel angle of the second hydraulic machine 6 is preferably varied continuously starting from the current actual swivel angle up to the determined desired swivel angle.

[0032] As in Fig. 3, after it is detected that the hydrostatic drive must be switched from the drive state to the deceleration state (time t1), the determined target swivel angle of the second hydraulic machine 6 will be greater than its actual value, so that the speed of the second hydraulic machine 6 can remain constant over the transition between the deceleration state and the drive state.

[0033] The line 104 from Fig.4 describes the speed of the second hydraulic machine that would be used during the transition without the described method. In contrast, line 102 describes the actual speed of the second hydraulic machine. By adjusting the swivel angle (or generally a displacement Vg) of the second hydraulic machine, the goal of keeping the actual speed 102 constant is achieved. Without adjusting the swivel angle of the second hydraulic machine 107 (as in the prior art), this would have caused an acceleration of the second hydraulic machine. The swivel angle of the first hydraulic machine, on the other hand, was kept constant.

[0034] After the transition, the known deceleration process will take place. This means that first, the swivel angle of the second hydraulic machine is further increased until a certain travel speed of the mobile work machine is reached (usually 1 / 3 of the initial speed). Then, the swivel angle of the first hydraulic machine is reduced until a standstill is reached. This process is not shown in the figures, as it is already prior art and its explanation is not relevant to the implementation of the method of the present invention.

[0035] The described method is stored in the memory unit 56 and is executed by the processor unit 58.

[0036] It is noted that even though only the transition between the drive state and the deceleration state has been explicitly described in the figures, this invention can also be used in the transition between the deceleration state and the drive state.

[0037] The described method can be used in various types of work machines. Essentially, the method can be used in any work machine with a hydrostatic drive and a controllable prime mover (such as a diesel engine). Examples of applications include a forage harvester, a combine harvester, a snow blower, or a road tiller.

[0038] While the present invention has been described with reference to the embodiments described above, it will be apparent to those skilled in the art that it is possible to make various modifications, variations and improvements to the present invention in light of the above teachings and within the scope of the appended claims without departing from the scope of the invention.

[0039] Furthermore, the areas in which those skilled in the art would be familiar have not been described here in order not to unnecessarily obscure the invention described.

[0040] Accordingly, the invention is not to be limited by the specific illustrative embodiments, but only by the scope of the appended claims.

Claims

[1] Method for controlling a hydrostatic drive (1) of a mobile work machine, wherein the hydrostatic drive is provided with a first hydraulic machine (2) with an adjustable displacement volume and with a second hydraulic machine (6) with an adjustable displacement volume, wherein the hydraulic machines (2, 6) are fluidically connected, and wherein the first hydraulic machine (2) can be coupled to a drive machine (4) and the second hydraulic machine (6) can be coupled to at least one wheel (12) to be driven or a chain or axle (14) to be driven, the method comprising the following steps: a. detecting that the hydrostatic drive is to be switched from a drive status in which the mobile work machine is driven by the drive machine (4) to a deceleration status in which the mobile work machine is decelerated by the drive machine (4), or vice versa; b. Determining a target swivel angle of the second hydraulic machine or a target variable which is dependent thereon on the basis of estimated losses in the first and in the second hydraulic machine (2, 6); c. Adjusting the swivel angle of the second hydraulic machine (6) on the basis of the desired swivel angle or desired size determined in step b., so that a rotational speed of the second hydraulic machine (6) can remain constant over the transition between the deceleration status and the drive status or vice versa. [2] Method according to claim 1, wherein in step b. the desired swivel angle of the second hydraulic machine (6) or desired size represents a jump in comparison to a current actual swivel angle of the second hydraulic machine (6) or actual size, wherein in step c. the swivel angle of the second hydraulic machine (6) is varied continuously from the current actual swivel angle up to the determined desired swivel angle. [3] Method according to claim 1 or 2, wherein in step b. the estimation of the losses is carried out by means of pre-stored data. [4] Method according to one of claims 1 to 3, wherein the losses of the first and the second hydraulic machine (2, 6) are hydraulic losses which are dependent on a current pressure and a current rotational speed of the first and the second hydraulic machine (2, 6). [5] Method according to one of claims 1 to 4, wherein if it is detected in step a. that the hydrostatic drive must be switched from the drive state to the deceleration state, the desired swivel angle of the second hydraulic machine (6) determined in step b. is greater than its actual value, so that the speed of the second hydraulic machine (6) can remain constant over the transition between the deceleration state and the drive state. [6] Method according to one of claims 1 to 5, wherein the first and the second hydraulic machine (2, 6) are fluidically connected via a first branch line (20) and a second branch line (22), wherein in step a. the recognition that the hydrostatic drive must be switched from the drive state to the deceleration state or vice versa takes place by means of a pressure in the first and in the second branch line (20, 22). [7] Control unit which is arranged to carry out a method according to one of claims 1 to 6. [8] Work machine comprising a hydrostatic travel drive (1), wherein the hydrostatic travel drive is provided with a first hydraulic machine (2) with an adjustable displacement volume and with a second hydraulic machine (6) with an adjustable displacement volume, wherein the hydraulic machines (2, 6) are fluidically connected, and wherein the first hydraulic machine (2) can be coupled to a drive machine (4) and the second hydraulic machine (6) can be coupled to at least one wheel (12) to be driven or a chain or axle (14) to be driven, wherein the work machine comprises a control unit according to claim 7. [9] Computer program adapted to execute and / or control the method according to any one of claims 1 to 6. [10] A machine-readable storage medium having stored thereon a computer program according to claim 9.

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