ARRANGEMENT FOR CONTROLLING A HYDRAULIC THREE-POINT HITCH

Description

[0001] The invention relates to an arrangement for controlling a hydraulic three-point linkage, comprising a hydraulic cylinder with a working chamber which can be pressurised with hydraulic fluid to change the lifting position of a lower link included in the hydraulic three-point linkage, and first and second seat valves communicating with the working chamber, the first seat valve being designed to establish an inlet connection with a high-pressure source and the second seat valve being designed to establish a return connection with a hydraulic reservoir.

[0002] The use of hydraulic three-point linkages is a common feature, especially on agricultural tractors, for operating a wide variety of attachments and soil cultivation equipment. The three-point linkage, located at the rear or front of the agricultural tractor, typically comprises left and right lower links that are pivoted to a part of the agricultural tractor's chassis in such a way that an attachment attached to the corresponding coupling interfaces of the lower links can be raised and lowered using a hydraulic lifting mechanism. In addition to the two lower links, a central top link is also provided, which guides the attachment during raising and lowering and can be attached to it via another coupling interface.The hydraulic lifting mechanism has at least one hydraulic cylinder, by means of which left and right lever arms, which are connected to one another in a rotationally fixed manner via a rigid shaft, can be pivoted, to the free ends of which the two lower links are in turn articulated via respective lifting struts.

[0003] The hydraulic cylinder is actuated by an electrically controllable valve assembly, which includes a first seat valve supplied with hydraulic fluid from a high-pressure pump for pressurizing a working chamber of the hydraulic cylinder, and a second seat valve for relieving pressure from the working chamber. The second seat valve opens into a hydraulic reservoir. In other words, the first seat valve is used to raise the lower links of the three-point linkage, while the second seat valve is used to lower them.

[0004] Such an arrangement is known, for example, from EP 0 348 854 A1, which serves to control a single-acting hydraulic cylinder. The use of a single-acting hydraulic cylinder results in the contact pressure exerted by an implement on the ground being primarily determined by its weight, so that in certain agricultural applications, such as cultivating or plowing, undesirable variations in the soil cultivation result can occur depending on the soil conditions.

[0005] Further arrangements for controlling a hydraulic three-point linkage or a hydraulic cylinder are known from the documents EP 1 496 009 A1, US 2018 / 209450 A1 and US 2012 / 205563 A1.

[0006] In view of this, it is the object of the present invention to further develop an arrangement for controlling a hydraulic three-point linkage of the type mentioned at the outset with a view to achieving a uniform soil cultivation result.

[0007] This object is achieved by an arrangement for controlling a hydraulic three-point linkage having the features of patent claim 1.

[0008] The arrangement for controlling a hydraulic three-point linkage comprises a hydraulic cylinder with a working chamber that can be pressurized with hydraulic fluid to change the lifting position of a lower link included in the hydraulic three-point linkage, as well as first and second seat valves communicating with the working chamber. The first seat valve is designed to establish an inlet connection with a high-pressure source, and the second seat valve is designed to establish a return connection with a hydraulic reservoir. This is a double-acting hydraulic cylinder with a further working chamber connected to a control valve arrangement, by means of which a predetermined target pressure can be set in the further working chamber.

[0009] By using a double-acting hydraulic cylinder and appropriately specifying the target pressure, it is possible to exert a downward force on the lower link if the second seat valve is in its open position. This allows the contact pressure exerted by the implement to be specifically increased in certain agricultural applications, such as cultivating or plowing, if the weight of the implement alone is not sufficient to achieve the desired soil cultivation result. It is also conceivable to guide the implement along a defined surface contour by appropriately specifying or varying the target pressure and to counteract any resulting tendency of the implement to "float." This is important, for example, when distributing crops in a flat silo or clearing snow with a pusher blade.

[0010] Advantageous further developments of the arrangement according to the invention emerge from the subclaims.

[0011] According to an exemplary embodiment of the control valve arrangement, it can have a 3 / 2-way pilot valve with an outlet for providing a control pressure corresponding to the predetermined target pressure, wherein the 3 / 2-way pilot valve can be pushed by means of an electric actuator into a first position connecting the outlet to the high-pressure source in order to provide the control pressure corresponding to the predetermined target pressure. By returning the control pressure or the working pressure generated in the further working chamber via a control input in the opposite valve actuation direction, the valve can be pushed into a second position connecting the outlet to the hydraulic reservoir. The electric actuator is controlled by an actuator signal corresponding to the desired target pressure. If the double-acting hydraulic cylinder is to operate in single-acting mode, the electric actuator remains inactive.This connects the additional working chamber of the hydraulic cylinder directly to the hydraulic reservoir. The same applies if the first seat valve is open or both seat valves are closed, preventing the hydraulic cylinder from becoming blocked or an uncontrolled pressure increase or buildup in the additional working chamber.

[0012] In the simplest case, the control pressure provided at the outlet of the 3 / 2-way pilot valve can be used to directly pressurize the other working chamber of the hydraulic cylinder. To achieve sufficiently high travel speeds of the hydraulic cylinder, the cylinder must allow correspondingly high flow rates, which ultimately leads to a valve design with comparatively large dimensions as well as increased power of the electric actuator required to operate it.

[0013] With a view to a cost- and space-optimized implementation of the control valve arrangement, it is therefore possible to provide a 3 / 2-way main valve having an additional outlet communicating with the additional working chamber of the hydraulic cylinder. The control pressure provided by the 3 / 2-way pilot valve can be used to push the 3 / 2-way main valve via a first control input into a first position connecting the additional outlet to the high-pressure source. By returning the working pressure generated in the annular chamber via a second control input in the opposite valve actuation direction, the 3 / 2-way main valve can be pushed into a second position connecting the additional outlet to the hydraulic reservoir. In other words, the 3 / 2-way pilot valve equipped with the electric actuator serves only to pilot the 3 / 2-way main valve, so that no special requirements are placed on the former with regard to the flow rates to be achieved.The 3 / 2-way pilot valve is therefore comparatively compact and cost-effective. The same applies to the 3 / 2-way main valve, as it is operated purely hydraulically via the two control inputs.

[0014] By feeding back the control pressure or the working pressure generated in the other working chamber, a control loop is created in which a target pressure corresponding to the respective actuator signal is automatically set. This eliminates the need for complex active electrical pressure control.

[0015] To implement ground pressure control, the control valve arrangement can further comprise a 4 / 2-way valve, by means of which the outlet of the 3 / 2-way pilot valve can be connected to the working chamber of the hydraulic cylinder instead of to the first control input of the 3 / 2-way main valve, and at the same time the first control input of the 3 / 2-way main valve can be connected to the hydraulic reservoir instead of to the outlet of the 3 / 2-way pilot valve. By increasing the working pressure in the working chamber of the hydraulic cylinder, the ground pressure exerted on the ground by an attachment attached to the three-point linkage can be specifically reduced. This is desirable, for example, for mowers and is currently mostly achieved purely mechanically using appropriate spring arrangements or the like.

[0016] Furthermore, a pressure compensation valve can be installed upstream of each of the two seat valves, depending on its respective flow direction. This ensures that the volume flow of hydraulic fluid passing through the seat valves, and thus the travel speed of the hydraulic cylinder, remains essentially constant and depends exclusively on the current actuation state of the respective seat valve.

[0017] The pressure compensation valve is formed, in particular, by a pressure control valve that compares the hydraulic pressure applied to the seat valve outlet side with the hydraulic pressure applied to the seat valve inlet side and adjusts it to a fixed differential pressure. The pressure compensation valve and seat valve can be integrated into a common valve assembly.

[0018] The inventive arrangement for controlling a hydraulic three-point linkage is described in more detail below with reference to the drawings. Identical reference numerals refer to identical or functionally comparable components. They show: Fig. 1 shows a schematically illustrated embodiment of the arrangement according to the invention for controlling a hydraulic three-point power lift, Fig. 2 shows a first variant of the Fig. 1 illustrated embodiment, and Fig. 3 a second variant of the Fig. 1 illustrated embodiment.

[0019] Fig. 1 shows a schematically illustrated embodiment of the arrangement 10 according to the invention for controlling a hydraulic three-point linkage 12.

[0020] The three-point linkage 12 (not shown in detail) is of conventional design and comprises left and right lower links that are articulated to a chassis section of an agricultural tractor (not shown) in such a way that an implement attached to the corresponding coupling interfaces of the lower links can be raised and lowered in a targeted manner using a hydraulic lifting mechanism 14. In addition to the two lower links, a central top link is also provided, which serves to guide the implement during raising and lowering and can be attached to it via another coupling interface. The three-point linkage 12 is located at the rear or front of the agricultural tractor.

[0021] The hydraulic lifting mechanism 14 has left and right hydraulic cylinders 16, by means of which left and right lever arms, which are connected to one another in a rotationally fixed manner via a rigid shaft, can be pivoted, to the free ends of which the two lower links are in turn articulated via respective lifting struts.

[0022] For reasons of clarity, Fig. 1 only a single one of the two hydraulic cylinders 16 is shown, each of which has a first working chamber 18 (so-called lifting chamber) which can be pressurized with hydraulic fluid to change the lifting position of the lower links.

[0023] More precisely, the hydraulic cylinders 16 are actuated by an electrically controllable valve arrangement 20, which has a first seat valve 24, supplied with hydraulic fluid from a high-pressure source 22 of the agricultural tractor, for pressurizing the first working chambers 18, which are connected in parallel hydraulically, and a second seat valve 26, provided for relieving pressure from the first working chambers 18. The second seat valve 26 opens into a hydraulic reservoir 28. Thus, the first seat valve 24, which communicates with the first working chambers 18, serves to raise the lower links of the three-point linkage 12 by establishing an inlet connection 30 with the high-pressure source 22, while the second seat valve 26, which communicates with the first working chambers 18, serves to lower the lower links of the three-point linkage 12 by establishing a return connection 32 with the hydraulic reservoir 28.

[0024] The high-pressure source 22 in this case is a high-pressure pump 36, which is load-controlled via an associated sensor line 34 and is supplied with hydraulic fluid drawn from the hydraulic reservoir 28. The necessary pressure relief of the sensor line 34 is conventionally provided via the high-pressure source 22 and is not shown in detail here.

[0025] To avoid overload-related pressure peaks in the first working chambers 18 of the hydraulic cylinders 16, these communicate with a pressure relief valve 38, by means of which the two seat valves 24, 26 can be bypassed in the direction of the hydraulic reservoir 28. If a pressure threshold value, typically in the range of 230 to 250 bar, is exceeded, the pressure relief valve 38 diverts the hydraulic fluid in the piston chambers 18 towards the hydraulic reservoir 28 until the pressure falls below the threshold value. Furthermore, a one-way valve 40 is provided, which is located in the inlet connection 30 between the first seat valve 24 and the first working chambers 18 of the hydraulic cylinders 16. The one-way valve 40 prevents the lower links from suddenly dropping when the first seat valve 24 opens due to a pressure deficit on the high-pressure source 22 side.

[0026] To control the two seat valves 24, 26, an electric motor-driven eccentric disc 42 is provided, by means of which one or the other of the two seat valves 24, 26 can be pushed into an open position against a restoring spring force.

[0027] As further Fig. 1 As can be seen, a pressure compensation valve 48, 50 is connected upstream of each of the two seat valves 24, 26 with respect to its respective flow direction 44, 46. In this way, it is ensured that the volume flow of the hydraulic fluid passing through the seat valves 24, 26 and thus the travel speed of the hydraulic cylinders 16 remains essentially constant and depends exclusively on the current actuation state of the respective seat valve 24, 26.

[0028] For example, each of the pressure compensation valves 48, 50 is formed by a pressure control valve 52, 54, which compares a hydraulic pressure applied to the respective seat valve 24, 26 on the outlet side with a hydraulic pressure applied to the inlet side and adjusts it to a fixed differential pressure. The pressure compensation valve 48, 50 and the seat valve 24, 26 can be integrated into a common valve assembly.

[0029] In the present case, these are double-acting hydraulic cylinders 16 with respective second working chambers 56 (so-called pressure chambers), which are hydraulically connected in parallel to a control valve arrangement 58, by means of which a predetermined target pressure can be set in the second working chambers 56.

[0030] By appropriately specifying the target pressure, it is possible to exert a downward force on the two lower links by means of the hydraulic cylinders 16 if the second seat valve 26 is in its open position.

[0031] This allows for targeted increases in the contact pressure exerted by the implement in certain agricultural applications, such as cultivating or plowing, when the implement's weight alone is insufficient to achieve the desired soil cultivation result. It is also conceivable to guide the implement along a defined surface contour by appropriately specifying or varying the target pressure, thereby counteracting any tendency of the implement to "float." This is important, for example, when distributing crops in a flat silo or clearing snow with a pusher blade.

[0032] According to the Fig. 1 In the embodiment of the control valve arrangement 58 shown, it has a 3 / 2-way pilot valve 60 with an outlet 62 for providing a control pressure corresponding to the predetermined target pressure. To provide the control pressure corresponding to the predetermined target pressure, the 3 / 2-way pilot valve 60 can be pushed by means of an electric actuator 64 into a first position 66a connecting the outlet 62 to the high-pressure source 22 and, by returning the control pressure or the working pressure generated in the second working chambers 56 via a control inlet 68 in the opposite valve actuation direction, into a second position 66b connecting the outlet 62 to the hydraulic reservoir 28. The electric actuator 64 is controlled by an actuator signal corresponding to the desired target pressure. If the double-acting hydraulic cylinders 16 are to operate in a single-acting mode, the electric actuator 64 remains inactive.As a result, the second working chambers 56 of the hydraulic cylinders 16 are directly connected to the hydraulic reservoir 28. The same applies if the first seat valve 24 is open or both seat valves 24, 26 are closed, in order to prevent the hydraulic cylinders 16 from becoming blocked or an uncontrolled pressure increase or buildup in the second working chambers 56.

[0033] Starting from Fig. 1 The control pressure provided at the outlet 62 of the 3 / 2-way pilot valve 60 is used to directly pressurise the second working chambers 56 of the hydraulic cylinders 16. In order to achieve sufficiently high travel speeds of the hydraulic cylinders 16, this must allow correspondingly high flow rates, which ultimately leads to a valve design with comparatively large dimensions as well as to an increased power of the electrical actuator 64 required for its actuation.

[0034] A cost and space-optimized implementation of the control valve arrangement 58 is possible within the scope of the two variants according to Fig. 2 or 3 can be achieved. In each case, an additional 3 / 2-way main valve 70 is provided, which has a further outlet 72 communicating with the second working chambers 56 of the hydraulic cylinders 16, wherein by means of the control pressure provided by the 3 / 2-way pilot valve 60, the 3 / 2-way main valve 70 can be pushed via a first control inlet 74 into a first position 76a connecting the further outlet 72 to the high-pressure source 22 and by returning the working pressure generated in the second working chambers 56 via a second control inlet 78 in the opposite valve actuation direction into a second position 76b connecting the further outlet 72 to the hydraulic reservoir 28. In other words, the 3 / 2-way pilot valve 60 equipped with the electric actuator 64 serves only to pilot the 3 / 2-way main valve 70, so that no special requirements are placed on the former with regard to the flow rates to be achieved.The 3 / 2-way pilot valve 60 is therefore comparatively compact and cost-effective. The same applies to the 3 / 2-way main valve 70, as it is actuated purely hydraulically via the two control inputs 74 and 78.

[0035] As can be seen, the variants differ only in the type of pressure feedback to the control input 68 of the 3 / 2-way pilot valve 60. Accordingly, Fig. 2 the control pressure applied to the outlet 62 of the 3 / 2-way pilot valve 60 is fed back directly to its control input 68, whereas according to Fig. 3 a return of the working pressure actually generated in the second working chambers 56 takes place.

[0036] By feeding back the control pressure or the working pressure generated in the second working chambers 56, a control loop is formed in which a target pressure corresponding to the respective actuator signal is automatically set. This eliminates the need for complex active electrical pressure control.

[0037] Furthermore, in conjunction with the two aforementioned variants, a contact pressure control is implemented on the agricultural tractor 10. For this purpose, the control valve arrangement 58 comprises an electrically switchable 4 / 2-way valve 80, by means of which the outlet 62 of the 3 / 2-way pilot valve 60 can be connected to the first working chambers 18 of the hydraulic cylinders 16 instead of to the first control inlet 74 of the 3 / 2-way main valve 70, and at the same time the first control inlet 74 of the 3 / 2-way main valve 70 can be connected to the hydraulic reservoir 28 instead of to the outlet 62 of the 3 / 2-way pilot valve 60. The check valve 82 ensures that a connection of the second working chambers 56 of the hydraulic cylinders 16 in the direction of the hydraulic reservoir 28 is interrupted if the contact pressure control is not active, which is the case here when the 4 / 2-way valve 80 is in its unactuated basic position.The shut-off valve 82 assumes its open position as soon as a connection to the outlet 62 of the 3 / 2-way pilot valve 60 is established via the 4 / 2-way valve 80 and a certain minimum pressure is reached there, which is typically 5 to 10 bar.

[0038] Different configurations are provided for the connection of the shut-off valve 82 with the 4 / 2-way valve 80. One in Fig. 2 The first embodiment shown provides for the outlet 62 of the 3 / 2-way pilot valve 60 to be connected directly to the check valve 82. The 4 / 2-way valve 80 serves exclusively to actuate the check valve 82. According to a Fig. 3In the second embodiment shown, the outlet 62 of the 3 / 2-way pilot valve 60 is also connected to the shut-off valve 82 via the 4 / 2-way valve 80. The first embodiment results in lower pressure losses than the second and also allows a more compact design of the 4 / 2-way valve 80. The above description is to be understood merely as an example, in that both embodiments can be used equally well in conjunction with the first and second variants of the control valve arrangement 58.

Claims

1. Arrangement for controlling a hydraulic three-point hitch, comprising a hydraulic cylinder (16) with a working chamber (18), to which pressurized hydraulic fluid can be admitted to change the lifting position of a lower arm comprised by the hydraulic three-point hitch (12), and also first and second seat valves (24, 26) communicating with the working chamber (18), the first seat valve (24) being designed for producing a flow connection (30) to a high-pressure source (22) and the second seat valve (26) being designed for producing a return connection (32) to a hydraulic reservoir (28), characterized in that it involves a double-acting hydraulic cylinder (16) with a further working chamber (56), which is connected to a control valve arrangement (58) by means of which a specified target pressure can be set in the further working chamber (56).

2. Arrangement according to Claim 1, characterized in that the control valve arrangement (58) has a 3 / 2-way pilot valve (60) with an outlet (62) for providing an adjusting pressure corresponding to the specified target pressure, wherein, for providing the adjusting pressure corresponding to the specified target pressure, the 3 / 2-way pilot valve (60) can be pushed by means of an electrical actuator (64) into a first position (66a), connecting the outlet (62) to the high-pressure source (22), and, by feeding back the adjusting pressure or the working pressure induced in the further working chamber (56) by way of a control input (68), can be pushed in the opposite valve actuating direction into a second position (66b), connecting the outlet (62) to the hydraulic reservoir (28).

3. Arrangement according to Claim 2, characterized in that the adjusting pressure provided at the outlet (62) of the 3 / 2-way pilot valve (60) serves for directly pressurizing the further working chamber (56) of the hydraulic cylinder (16).

4. Arrangement according to Claim 2, characterized in that a 3 / 2-way main valve (70) is provided, which has a further outlet (72) communicating with the further working chamber (56) of the hydraulic cylinder (16), wherein the 3 / 2-way main valve (70) can be pushed by means of the adjusting pressure provided by the 3 / 2-way pilot valve (60) by way of a first control input (74) into a first position (76a), connecting the further outlet (72) to the high-pressure source (22), and, by feeding back the working pressure induced in the further working chamber (56) by way of a second control input (78), can be pushed in the opposite valve actuating direction into a second position (76b), connecting the further outlet (72) to the hydraulic reservoir (28).

5. Arrangement according to Claim 4, characterized in that the control valve arrangement (58) comprises a 4 / 2-way valve (80), by means of which the outlet (62) of the 3 / 2-way pilot valve (60) can be connected to the working chamber (18) of the hydraulic cylinder (16) instead of to the first control input (74) of the 3 / 2-way main valve (70) and at the same time the first control input (74) of the 3 / 2-way main valve (70) can be connected to the hydraulic reservoir (28) instead of to the outlet (62) of the 3 / 2-way pilot valve (60).

6. Arrangement according to at least one of the preceding claims, characterized in that a pressure compensation valve (48, 50) is connected upstream of each of the two seat valves (24, 26) with respect to its respective through-flow direction (44, 46).

7. Agricultural tractor with a three-point hitch (12) and also with an arrangement (10) for controlling the hydraulic three-point hitch (12) according to at least one of the preceding claims.