Working device with a gripper, especially a slotted wall gripper

Abstract

The invention relates to a work device, in particular a cable excavator or a special foundation engineering machine, comprising a grab, in particular a diaphragm wall grab, which is suspended from the work device by a cable, a winch on which the cable is mounted for winding and unwinding the grab, a first measuring device with which a closing value, representing a closing force acting during an excavation operation when the grab buckets close, can be detected, and a control unit connected to the first measuring device by means of which the winch can be actuated. According to the invention, the control unit is configured to control or regulate the winch during an excavation operation based on a setpoint value such that the grab is held with a predetermined residual tensile force, and to change the setpoint value depending on the detected closing value.The invention further relates to a method for carrying out a digging operation using the work device according to the invention and a corresponding computer program product.

Description

[0001] The present invention relates to a working device according to the preamble of claim 1, as well as a method and a computer program product for carrying out a digging operation using such a working device.

[0002] Hydraulic or mechanical grabs are used in a variety of applications, such as bucket dredging, grab handling, and specialist foundation engineering. In the latter, grab solutions are sometimes used as an alternative to trench cutters in the form of hydraulic or mechanical trench grabs. With these two-shell grabs, the opening and closing of the grab buckets is achieved either via one or more hydraulic cylinders (hydraulic closing cylinders) or via a cable system using designated holding and closing cables.

[0003] Diaphragm walls are used in specialist foundation engineering, among other things, to secure excavation pits. To construct such a diaphragm wall, a vertical trench is typically excavated from the ground along guide walls, which is then lined with reinforced concrete. The resulting diaphragm wall supports the further foundation of the structure. A diaphragm wall grab is typically suspended from the carrier machine, usually a cable excavator with a swiveling boom or a specialist foundation engineering machine with a mast, by one or more steel cables (usually a mooring cable for hydraulic diaphragm wall grabs; usually a mooring and a closing cable for mechanical diaphragm wall grabs).

[0004] With such work equipment, it is known to set a static residual tensile force during active free fall, so that the grab does not rest completely on the ground during the digging process, but is held by the winch for the holding cable (holding winch) with the set residual tensile force. However, this solution does not take into account different soil or bulk material properties, so that the grab is sometimes held with a residual tensile force that is too high or too low.

[0005] The present invention is based on the objective of further developing generic work tools in such a way as to achieve an optimized digging process. In particular, the efficiency and user-friendliness of the grab closing process are to be increased.

[0006] According to the invention, this problem is solved by a working device with the features of claim 1, by a method according to claim 12, and by a computer program product according to claim 15. Advantageous embodiments of the invention are described in the dependent claims and the following description.

[0007] Accordingly, a working device is proposed which includes a grab suspended from the working device, in particular from a boom or mast of the working device, by at least one cable. The working device may be, in particular, a cable excavator or a specialized foundation engineering machine. The grab comprises at least two openable and closable grab buckets and is preferably designed as a two-shell grab.

[0008] The grab can be designed as a mechanical grab, in particular a diaphragm wall grab, in which case the working device comprises several cables by which the grab is suspended from the working device, and corresponding winches on which the cables are wound and unwound for spatial adjustment or movement of the grab and for opening and closing the grab buckets. The cables can be a holding cable and a closing cable, as are known from known grab solutions, in particular from known mechanical diaphragm wall grabs. The winches are, in particular, independently operable of one another.

[0009] Alternatively, the grab can be designed as a hydraulic grab, in particular a diaphragm wall grab, in which case the working device comprises a cable (holding cable) and a cable winch (holding cable winch) on which the cable is wound and unwound for spatial adjustment or movement of the grab. The opening and closing of the grab buckets is effected in this case by at least one, preferably two, hydraulic closing cylinders, which are arranged in particular on or in the grab.

[0010] Even with a hydraulic grab, it's possible that the grab is held by more than one support cable, with several support cable winches being operated synchronously to move the grab. In this case, the design regarding the control / regulation of the "cable winch" applies specifically to all existing support cable winches.

[0011] The working device comprises a first measuring device with which a closing parameter can be detected, and a control unit connected to the first measuring device, via which the cable winch (or, in the case of a mechanical gripper, at least one of the cable winches) can be operated or controlled. The cable winch can be controlled or regulated via the control unit in order to raise / lower and close / open the gripper in a desired manner.

[0012] When the following refers to the control of the winch (singular), unless otherwise specified, this includes the case where the work equipment has several winches, at least one of which (in particular a holding winch) is controlled. In the case of multiple winches, the control system can preferably control all winches, and in particular, independently of each other.

[0013] The closing force specified above characterizes the closing force acting on the grab buckets during a digging operation. This closing force can be a force acting on a cable, a torque acting on a winch, or another derived quantity such as motor current or hydraulic pressure. The closing force is a measure of the resistance of the grab buckets during closing, i.e., the magnitude of the closing forces during the closing process.

[0014] According to the invention, the control unit is designed to control or regulate the said winch during the digging process using a setpoint value in such a way that the grab is held with a predetermined residual pulling force.

[0015] To optimize the closing process and simultaneously ensure that the closing force does not become too high while the digging process is carried out efficiently, the control unit is designed, according to the invention, to change the setpoint during the digging process, i.e., during the closing process of the gripper blades, depending on the detected closing force. The residual tensile force is therefore not kept constant, but adjusted in certain situations based on the currently occurring closing force, which is reflected in the detected closing force.

[0016] In one possible embodiment, the first measuring device includes a force sensor to detect the closing force. This sensor can, for example, be designed as a load cell that detects a rope force occurring during gripper closing, such as a force in a closing rope in a mechanical gripper solution. The force sensor can, for example, include one or more strain gauges.

[0017] Alternatively or additionally, the first measuring device can include a torque sensor to detect the closing force. This can, for example, detect a torque acting on a winch during gripper closing, such as a closing winch in a mechanical gripper solution. The torque sensor can, for example, include one or more strain gauges.

[0018] Alternatively or additionally to a closing parameter directly dependent on the closing force, a closing parameter indirectly dependent on the closing force can be measured. For this purpose, the first measuring device can include a pressure sensor for measuring hydraulic pressure in a hydraulic system, for example, in the case of a hydraulic gripper solution in a hydraulic system driving at least one hydraulic closing cylinder, or in the case of a mechanical gripper solution in a hydraulic system driving a hydraulic winch motor. In the latter case, this could be the winch motor of a closing winch.

[0019] The measurement of hydraulic pressure is based on the understanding that a higher closing force manifests itself in an increased pressure in the hydraulic system of the drive used to close the gripper blades, so that the measured pressure can be used as a closing parameter.

[0020] If an electric motor is used instead of a hydraulic winch motor in a mechanical gripper solution, the motor current of the electric winch motor can be used as the closing variable instead of the hydraulic pressure.

[0021] Combinations of the aforementioned sensors can also be used to achieve even more precise measurement of the closing force. For example, a direct measurement of the closing force or closing torque could be combined with an indirect measurement derived from pressure or motor current.

[0022] In another possible embodiment, the closing parameter is a force, a torque, a pressure, or a current. If the control unit compares the closing parameter with a corresponding limit value, for example, to activate or deactivate an adaptation or assistance mode, the respective limit value has the same unit.

[0023] In another possible embodiment, the working device includes a second measuring device by means of which an actual value representing the current residual tensile force with which the gripper is held can be recorded. The control unit is connected to the second measuring device and receives the recorded actual value from it. The control unit is configured to compare the actual value with the aforementioned target value and, based on this comparison, to control or regulate at least one of the winches so that the desired residual tensile force is achieved.

[0024] In another possible embodiment, the second measuring device comprises a force sensor and / or a torque sensor. The possible configurations previously described with regard to the first measuring device also apply to the force sensor and / or torque sensor of the second measuring device.

[0025] Alternatively or additionally, the second measuring device can include a pressure sensor for detecting hydraulic pressure in a hydraulic system driving a hydraulic winch motor, or, in the case of an electric winch motor, a sensor for detecting the motor current of the winch motor in order to record the actual value. This applies in particular to the winch motor of a holding cable winch. The possible configurations previously described in relation to the first measuring device also apply to pressure or motor current detection using the second measuring device.

[0026] In another possible embodiment, the setpoint is a force setpoint, i.e., in particular, a setpoint for the residual tractive force. For this purpose, the actual value of the residual tractive force can be obtained directly, for example, via a force measurement. The control unit then regulates the residual tractive force to the setpoint. Alternatively, the actual value could be derived or calculated from a measurement of torque and / or hydraulic pressure or motor current. Alternatively, the setpoint could be a torque setpoint, a pressure setpoint, or a current setpoint.

[0027] In a preferred embodiment, the setpoint is a pressure setpoint, wherein the actual value is determined from a pressure measurement in the hydraulic system of a hydraulic winch motor.

[0028] In another possible embodiment, the control unit is configured to increase the residual pulling force by controlling or regulating the winch, in particular a holding winch, when the detected closing force increases. If the closing force, and thus the closing force, increases during the digging process or the grapple closing, an increase in the residual pulling force leads to a reduction in the closing force. This reduces the load on the grapple blades or the load on the drive responsible for the closing process.

[0029] Preferably, the control unit is further configured to reduce the residual pulling force again if the detected closing force decreases. If the closing force, and thus the closing force, decreases again after an increase in the residual pulling force or due to changing soil conditions, the residual pulling force can also be reduced to ensure efficient digging.

[0030] In another possible embodiment, the control unit is configured to adjust the residual pulling force by controlling or regulating the winch based on a functional relationship depending on the detected closing value. In particular, the setpoint is changed based on this functional relationship, so that the residual pulling force adjusts accordingly. Preferably, this is a linear relationship (i.e., the residual pulling force is changed proportionally to the closing value, i.e., increased or, if necessary, decreased).

[0031] Preferably, the control unit is configured to keep the residual tensile force below an upper force limit. This prevents excessive stress on the winch responsible for holding the gripper (in particular, a holding winch). The upper force limit is, in particular, lower than the maximum tensile force of the winch.

[0032] The upper force limit can correspond to a fraction (e.g., 40%, 50%, or 60%) of the maximum pulling force of the winch. The upper force limit can be fixed in the control unit or adjustable, in particular via an input unit connected to the control unit.

[0033] Preferably, a lower force limit also exists, wherein a functional, in particular linear, adjustment of the setpoint occurs between the lower and upper force limits depending on the closing force. The lower force limit can correspond to a fraction (e.g., 20%, 25%, or 30%) of the maximum pulling force of the winch. The lower force limit can be permanently stored in the control unit or adjustable, in particular via an input unit connected to the control unit.

[0034] In another possible embodiment, the control unit is configured to perform a closing force-dependent adjustment of the residual pulling force only in an adjustment or assistance mode. This assistance mode can preferably be activated and deactivated via an input unit connected to the control unit. The input unit can be located in the operator's cab of the work equipment and, for example, be designed as a touchscreen display. This allows the operator of the work equipment to activate or deactivate the closing force-dependent residual pulling force adjustment as needed.

[0035] It may also be provided that the closing force-dependent residual pull adjustment (i.e., the assistance mode) is automatically activated or deactivated when certain conditions are met (for example, reaching certain limits through detected dimensions of the digging process or the work equipment), for example, when a particularly hard surface is detected.

[0036] In another possible embodiment, the control unit is configured to change the residual pulling force only when the detected closing force is above a lower closing force limit and / or below an upper closing force limit. Preferably, an adjustment interval is defined by a lower closing force limit and an upper closing force limit, wherein preferably below this interval either no setting of a specific residual pulling force is made, or a constant residual pulling force is set (e.g., 10% of the maximum pulling force of a winch), and / or wherein preferably above this interval a constant residual pulling force is set, which may, for example, correspond to an upper force limit (e.g., 40%, 50%, or 60% of the maximum pulling force of a winch).

[0037] The setpoint is preferably adjusted between the lower and upper closing size limits according to a functional relationship, in particular according to a linear function, so that the residual tensile force also adjusts itself according to a functional relationship depending on the closing size.

[0038] The lower and / or upper closing size limit can be permanently stored in the control unit or adjustable via an input unit connected to the control unit.

[0039] In another possible embodiment, the grab is a hydraulic grab and the grab buckets can be opened and closed via at least one, preferably two, hydraulic cylinders (closing cylinders). The grab is held by a mooring cable on a boom or mast of the work equipment, and the residual pulling force is adjusted by a corresponding control system for a winch that supports the mooring cable.

[0040] Alternatively, the grab can be a mechanical grab, wherein the grab is suspended from a boom or mast of the work equipment via a holding rope and a closing rope, wherein the holding rope is mounted on a holding rope winch for adjusting or moving the grab, particularly in a substantially vertical manner, and the closing rope is mounted on a closing rope winch for actuating, i.e. opening and closing, the grab buckets.

[0041] In the mechanical grab solution, both cables can be used to adjust the grab and / or to open or close the grab buckets. Specifically, to adjust (raise or lower) the grab, both cables are wound or unwound together. Furthermore, the grab buckets can be opened either by unwinding the closing cable, by winding the retaining cable, or a combination of both. Similarly, the grab buckets can be closed either by winding the closing cable, by unwinding the retaining cable, or a combination of both. Preferably, the closing cable is used to actuate the grab buckets, and this cable is guided, in particular, by a cable pull mechanism of the diaphragm wall grab to enable a high closing force.

[0042] In the mechanical gripper solution, the control unit is designed to control or regulate at least the holding winch, preferably both winches (i.e., the holding winch and the closing winch), based on the setpoint value, such that the gripper is held with the specified residual tensile force.

[0043] In another possible embodiment, the work device includes an output unit, in particular a display located in the operator's cab, on which the current residual tractive force and / or the closing force and / or the setpoint can be displayed numerically and / or graphically. Alternatively or additionally, the aforementioned functional relationship can be displayed graphically. Alternatively or additionally, the aforementioned lower and / or upper force limit and / or the lower and / or upper closing force limit can be displayed numerically and / or graphically. This allows the operator of the work device to quickly and intuitively grasp the current situation during the digging process. The output unit can simultaneously function as an input unit or be connected to an input unit via which one or more of the aforementioned parameters can be adjusted.

[0044] The invention further relates to methods for carrying out a digging operation using the work device according to the invention. The same properties, features, and advantages arise as already described with regard to the work device according to the invention. Therefore, a repetitive description is largely omitted. In particular, the work device used for the digging operation within the framework of the method according to the invention can be configured in any combination according to each of the previously described embodiments.

[0045] The method involves detecting a closing force, which represents the closing force acting during the digging process when the grab buckets close. During the digging process, a winch (in particular a holding winch) is controlled or regulated based on a setpoint such that the grab is held with a predetermined residual tensile force. During the digging process, the setpoint is changed depending on the detected closing force. In particular, the setpoint, and thus the resulting residual tensile force, is increased if the closing force increases during the grab closing process. Preferably, if the closing force decreases, the setpoint, and thus the resulting residual tensile force, is also decreased.

[0046] In one possible embodiment of the method, it is provided that above a lower closing size limit and / or below an upper closing size limit, the setpoint is automatically changed based on a functional and, in particular, linear relationship depending on the detected closing size.

[0047] In another possible embodiment, it is provided that the setpoint and thus also the residual tensile force is automatically increased when the detected closing force increases and preferably automatically decreased when the closing force decreases, wherein the increase or decrease of the setpoint or the residual tensile force is carried out automatically only in an activated assistance mode.

[0048] The aforementioned steps are carried out in particular by the control unit of the work equipment described above.

[0049] The invention further relates to a computer program product which includes commands which, when the program is executed by the control unit of the device according to the invention, cause the steps of the method according to the invention to be carried out (which can be carried out by the control unit, i.e. automatically).

[0050] Further features, details and advantages of the invention will become apparent from the exemplary embodiments explained below with reference to the figures. The figures show: Fig. 1a: a side view of a first embodiment of the working device according to the invention with a mechanical gripper; Fig. 1b: a schematic side view of a hydraulic gripper of a second embodiment of the working device according to the invention; Fig. 2: an example of a functional relationship between set residual tensile force and closing size; and Fig. 3: an example of a display on a display unit of the working device.

[0051] In the Figure 1a A first embodiment of the work device 10 according to the invention is shown in a side view, with the ground and a trench 1 created in the ground shown in a schematic sectional view. The trench can be filled with a support fluid. The work device 10 of the embodiment shown here is a cable excavator 10 with an undercarriage 13 comprising a crawler track, a superstructure 14 rotatably mounted thereon, and a boom 15 pivotably attached to the superstructure 14. The superstructure 14 can have a driver's cab 16.

[0052] A mechanical diaphragm wall grab 20 is suspended from the boom 15 by two steel cables, which are led over the boom 15, in particular over rollers on a boom head, to the superstructure 14: a holding cable 21, which is mounted on a holding cable winch 11 attached to the superstructure 14 so as to be wound up and unwound, and a closing cable 22, which is mounted on a closing cable winch 12 also arranged on the superstructure 14 so as to be wound up and unwound.

[0053] The diaphragm wall grab 20 comprises a grab frame at the lower end of which two grab blades 24 are pivotally mounted. These blades can be opened and closed by actuating the closing cable 22 (or the closing cable winch 12). The grab blades 24 can be articulated to a grab carriage via rods. The closing cable 22 is preferably driven by a pulley system or cable pulley system within the grab 20 to achieve a higher closing force and is attached at one end to the grab carriage of the diaphragm wall grab 20. The retaining cable 21 is preferably attached directly (i.e., without a cable pulley system) to the grab carriage.

[0054] The diaphragm wall grab 20 hangs freely on the boom 15, so that all movements (lifting, lowering, opening / closing the grab buckets 24 and especially also rotating the grab 20 about a vertical axis) are preferably carried out only by appropriate actuation of the holding and closing winches 11, 12.

[0055] The dynamic residual pull adjustment according to the invention can be applied not only to such a mechanical diaphragm wall gripper 20, but also to different gripper types. The following explanations are independent of the exact design of the working device 10. Figure 1a .

[0056] The working device 10 has a control unit via which the winches 11, 12 can be controlled. Furthermore, the working device has a first measuring device with at least one sensor for detecting a closing variable, which represents the closing force acting when the gripper closes. It is conceivable to detect the closing force directly, for example by measuring the cable tension in the closing cable 22, or to detect an indirect variable dependent on the closing force, such as a torque. Preferably, the winches 11, 12 have hydraulic winch motors supplied by a corresponding hydraulic system, and the closing variable is a hydraulic pressure in the hydraulic system, which depends on the closing force. In particular, the hydraulic pressure present at the winch motor of the closing winch 12 can be detected.

[0057] The control unit is specifically designed to regulate the cable winches 11, 12 (especially the holding cable winch 11) to achieve a desired residual tensile force with which the gripper 20 is held. This regulation is based on a setpoint, which can be a residual tensile force setpoint. This residual tensile force is then adjusted at specific closing forces by changing the setpoint depending on the detected closing force.

[0058] The working device 10 has, in particular, a second measuring device with at least one sensor for recording an actual value representing the current residual tensile force, which is compared with the target value. Here, the residual tensile force can again be measured directly, e.g., by measuring the tensile force in the holding cable 21. Preferably, a hydraulic pressure in a hydraulic system, in particular a hydraulic pressure present at the winch motor of the holding cable winch 21, is recorded.

[0059] In the Figure 1b A second embodiment of a gripper 20' of the working device according to the invention is shown schematically, wherein, in contrast to the embodiment of the Figure 1a This is a hydraulic diaphragm wall grab.

[0060] An example of how the residual tensile force can be adjusted depending on the closing size is in Figure 2 shown. Here, the closing parameter, for example the hydraulic pressure measured via a hydraulic pressure sensor as the closing parameter representing the current closing force, is plotted on the x-axis, and the setpoint for the control of the residual tensile force is plotted on the y-axis.

[0061] The control unit can define a lower closing force limit x1 and an upper closing force limit x2. Below the lower closing force limit x1, a constant setpoint value (in particular, a residual pulling force corresponding to a specific fraction, such as 10% of the maximum winch pulling force, although other values ​​are conceivable) can be set. Between the lower and upper closing force limits x1 and x2, the setpoint value, and thus the residual pulling force, is adjusted according to a linear relationship. The higher the closing force, the higher the setpoint value to which the residual pulling force is adjusted. Above the upper closing force limit x2, the setpoint value remains constant.

[0062] The values ​​to which the target value is set at the lower and upper closing force limits x1 and x2 are designated as y1 and y2. Alternatively, y1 and y2 could represent the residual tensile forces set at x1 and x2. These could each correspond to a force limit for the residual tensile force stored in the control unit, which represents a specific fraction (e.g., 25% and 50%, although other values ​​are of course conceivable) of a maximum winch tensile force.

[0063] In the Figure 3Figure 16 shows an example of how the relevant parameters are displayed on a display unit in a driver's cab for a specific embodiment of dynamic residual pulling force adjustment. Of course, other values ​​or display options are conceivable. The lower and upper closing force limits x1, x2 are set to 100 bar and 250 bar, respectively (the closing force here is the measured hydraulic pressure as the "grip closing pressure"). The upper and lower force limits, between which the residual pulling force is linearly adjusted, are set to 25% and 50% of the maximum winch pulling force. Below the lower closing force limit x1, the residual pulling force is set to 10% of the maximum winch pulling force.

[0064] If, during the closing process of the grapple buckets 24, the grapple closing pressure reaches the range of the set pressures x1, x2 (these can be fixed in the control unit or, if necessary, adjustable by the operator via an input unit), the retraction force is increased proportionally to the set percentage values ​​of the maximum winch pulling force y1, y2 (these can also be fixed or adjustable via an input unit). This reduces the force on the grapple buckets 24. If the grapple closing pressure drops again, for example, due to the bucket geometry or the soil conditions, the retraction force is reduced proportionally to ensure optimal digging performance. This guarantees a continuous and optimal / efficient closing process, which increases performance and ease of use.

[0065] The display unit can simultaneously serve as an input unit (e.g., a touch display). One or more controls for input can be displayed. For example, in the exemplary embodiment of the Figure 3 A control element 32 for activating / deactivating the dynamic residual tractive force adjustment is shown. This allows the operator to manually switch the assistance system on or off as needed.

[0066] The constant retraction force below the lower closing size limit x1 can be adjusted via a potentiometer.

[0067] The values ​​of x1 and x2 and / or y1 and y2 can be set directly via the input / output unit. Reference symbol list:

[0068] 1 Ground slot 10 Working device 11 Holding winch 12 Closing winch 13 Undercarriage 14 Superstructure 15 Boom 16 Operator's cab 20 Grab 20'Grab (hydraulic grab) 21 Holding cable 22 Closing cable 24 Grab bucket 26 Hydraulic closing cylinder 30 Functional relationship between setpoint and closing size 32 Control element

Claims

1. Working equipment (10), in particular a cable excavator or special foundation engineering equipment, comprising a grab (20), in particular a diaphragm wall grab, with at least two openable and closable grab buckets (24), which is suspended on the working equipment (10) by a cable (21), a cable winch (11) on which the cable (21) is mounted so as to be wound up and unwound for moving the grab (20), a first measuring device with which a closing value, representing a closing force acting during an excavation process when closing the grab buckets (24), can be detected, and a control unit connected to the first measuring device by means of which the cable winch (11) can be operated. characterized by that the control unit is designed to: - control or regulate the winch (11) on the basis of a setpoint value in such a way that the grab (20) is held with a predetermined residual pulling force, and - change the setpoint value depending on the detected closing force.

2. Working device (10) according to claim 1, wherein the first measuring device comprises a force sensor and / or a torque sensor and / or wherein the first measuring device comprises a pressure sensor for detecting a hydraulic pressure in a hydraulic system driving a hydraulic winch motor or a hydraulic cylinder (26) or a sensor for detecting a motor current of an electric winch motor in order to detect the closing variable.

3. Working device (10) according to claim 1 or 2, wherein the closing parameter is a force, a torque, a pressure or an amperage.

4. Working device (10) according to one of the preceding claims, comprising a second measuring device by means of which an actual value representing the current residual tensile force can be detected, wherein the control unit is configured to compare the actual value with the target value and to control or regulate the winch (11) on the basis of this comparison.

5. Working device (10) according to the preceding claim, wherein the second measuring device comprises a force sensor and / or a torque sensor and / or wherein the second measuring device comprises a pressure sensor for detecting a hydraulic pressure in a hydraulic system driving a hydraulic winch motor or a sensor for detecting a motor current of an electric winch motor in order to detect the actual value.

6. Working device (10) according to one of the preceding claims, wherein the setpoint is a force setpoint, a torque setpoint, a pressure setpoint or a current setpoint.

7. Working device (10) according to one of the preceding claims, wherein the control unit is configured to increase the residual pulling force by controlling or regulating the winch (11) when the detected closing quantity increases, wherein the control unit is preferably further configured to decrease the residual pulling force again when the detected closing quantity decreases.

8. Working device (10) according to one of the preceding claims, wherein the control unit is configured to adjust the residual pulling force by controlling or regulating the winch (11) on the basis of a functional and in particular linear relationship (30) depending on the detected closing quantity, wherein the control unit is preferably configured to keep the residual pulling force below an upper force limit (y2), which is in particular smaller than a maximum pulling force of the winch (11).

9. Working device (10) according to one of the preceding claims, wherein the control unit is configured to perform an adjustment of the residual tensile force dependent on the closing size only in an assistance mode, wherein the assistance mode is preferably activatable and deactivatable via an input unit connected to the control unit.

10. Working device (10) according to one of the preceding claims, wherein the control unit is configured to change the residual tensile force only if the detected closing size is above a lower closing size limit (x1) and / or below an upper closing size limit (x2), wherein the lower and / or the upper closing size limit (x1, x2) are preferably fixed in the control unit or adjustable via an input unit connected to the control unit.

11. Working device (10) according to one of the preceding claims, wherein the grab (20) is a hydraulic grab and the grab buckets (24) can be opened and closed via at least one hydraulic cylinder (26), or wherein the grab (20) is a mechanical grab which is suspended from a boom (15) or mast of the working device (10) via a holding cable (21) and a closing cable (22), wherein the holding cable (21) for adjusting the grab (20) in a substantially vertical manner is mounted on a holding cable winch (11) and the closing cable (22) for actuating the grab buckets (24) is mounted on a closing cable winch (12) so as to be wound and unwound, wherein the control unit is configured to control or regulate at least the holding cable winch (11), preferably both cable winches (11, 12) based on the setpoint such that the grab (20) is The specified residual tensile force is maintained.

12. Method for carrying out a digging operation using a work device (10) according to one of the preceding claims, wherein a closing parameter representing a closing force acting during the digging operation when closing the grab buckets (24) is detected, wherein during the digging operation the winch (11) is controlled or regulated on the basis of a setpoint such that the grab (20) is held with a predetermined residual tensile force, wherein the setpoint is changed during the digging operation depending on the detected closing parameter, in particular increased.

13. Method according to the preceding claim, wherein above a lower closing size limit (x1) and / or below an upper closing size limit (x2) the setpoint is automatically changed based on a functional and in particular linear relationship (30) depending on the detected closing size.

14. Method according to one of the two preceding claims, wherein the residual tensile force is automatically increased when the detected closing force is increased and preferably automatically decreased when the closing force is decreased, wherein the increase / decrease of the residual tensile force is in particular only carried out automatically in an activated assistance mode.

15. Computer program product comprising instructions which, when the program is executed by the control unit of a working device (10) according to one of claims 1 to 11, cause the steps of the method according to one of claims 12 to 14 to be executed.

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