Force-controlled handling device for robot-assisted surface processing

Abstract

This makes it possible to almost reliably maintain a specified machining force in force control. [Solution] a handling device having a linear actuator acting between a first flange connectable to a manipulator and a second flange to which a tool or a machine tool having a tool can be mounted, and exerting a force on the second flange or an end stop in response to a control variable; a force sensor coupled between the second flange and the tool, measuring the force exerted on the tool by the handling device when the tool contacts a surface; and a control unit having a state observer determining an estimate of the force exerted on the tool by the handling device based on the control variable, wherein the control unit further sets the control variable based on the estimate and a target value while contact between the tool and the surface is not detected, and sets the control variable based on the measured force and the target value while contact is detected.

Description

[Technical field]

[0001] The present invention relates to a force-controlled handling device for automated robot-assisted surface machining. Such a handling device can in particular act as an interface between a manipulator (robot) and a machine tool. [Background technology]

[0002] In robot-assisted surface processing, a machine tool (e.g. grinding, drilling, milling, polishing, etc.) is guided by a manipulator, e.g. an industrial robot. The machine tool and the so-called Tool Center Point (TCP) of the manipulator can be coupled in various ways. The manipulator usually has practically free freedom to adjust the position and orientation of the TCP and can move the machine tool, e.g. along a trajectory parallel to the workpiece surface. The industrial robot usually performs position control and can move the TCP precisely along the desired trajectory.

[0003] In order to obtain good results in robot-assisted grinding, polishing or other surface treatment processes, control of the processing force (grinding force) is often necessary, but it is often difficult to achieve sufficient accuracy with conventional industrial robots. Industrial robots have large arms and heavy inertial masses, and their controllers (closed-loop control) cannot react quickly to fluctuations in the processing force. To solve this problem, a handling device can be placed between the TCP of the manipulator and the machine tool, which is smaller (and lighter) than the industrial robot and can couple the TCP of the manipulator and the machine tool. In surface processing, the handling device has, in particular, a linear actuator, which controls only the processing force (contact force between the tool and the workpiece), while the manipulator is position-controlled and moves the machine tool along the desired trajectory together with the linear actuator. Force control allows the handling device to compensate (within a certain range) with the help of the linear actuator for inaccuracies in the position and shape of the workpiece to be processed and inaccuracies in the trajectory of the manipulator. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] US2014005831A1 Summary of the Invention [Problem to be solved by the invention]

[0005] In many surface finishing processes, the quality of the finished product is highly dependent on the forces being kept within the desired specified ranges during the process. For example, excessive grinding (processing) forces applied (even for a short period of time) during a grinding process can severely damage or destroy the workpiece, resulting in expensive repairs.

[0006] The inventors set themselves the task of developing an improved handling device with force control which makes it possible to almost reliably maintain a specified processing force. [Means for solving the problem]

[0007] The above problem is solved by a device according to claim 1 and a method according to claim 7. Various embodiments and further developments are the subject of the dependent claims. An embodiment is a handling device with a linear actuator acting between a first flange connectable to a manipulator and a second flange on which a tool or a machine tool with a tool can be mounted. The linear actuator exerts a force on the second flange or on an end stop depending on a control variable. The device further comprises a force sensor coupled between the second flange and the tool and configured to measure a force exerted by the handling device on the tool upon contact of the tool with the surface. The control unit comprises a state observer for determining an estimate of the force exerted by the handling device on the tool based on the control variable. The control unit further detects contact of the tool with the surface and sets the control variable based on the estimate and a target value while no contact is detected and sets the control variable based on the measured force and a target value while contact is detected.

[0008] Another embodiment is a method of controlling a handling device. The handling device includes a linear actuator acting between a first flange connectable to a manipulator and a second flange to which a tool or a machine tool having a tool can be attached. One embodiment of the method includes controlling the linear actuator with a control variable, the linear actuator exerting a force on the second flange or on an end stop in response to the control variable. Further, this embodiment includes the steps of detecting contact between the tool and the surface, and measuring a force exerted by the handling device on the tool by a force sensor coupled between the second flange and the tool when the tool contacts the surface. Based on the control variable, an estimate of the force exerted by the handling device on the tool is determined. The control variable is set based on the estimate and a target value while no contact is detected, and based on the measured force and the target value while contact is detected. Effect of the Invention

[0009] This makes it possible to almost reliably maintain a specified machining force in force control. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 shows a typical example of a robot-assisted grinding system comprising an industrial robot, a handling device with force control, and a grinding machine.

[0011] [Diagram 2] FIG. 2 shows an exemplary embodiment of a handling device (not shown is the control unit contained therein).

[0012] [Diagram 3] FIG. 2 is a diagram showing an example of a control device of a handling system in which force control is realized.

[0013] [Figure 4] 1 is a flow chart illustrating an example of a method for controlling a handling device for robot-assisted surface treatment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Various embodiments are described in more detail below using illustrated examples, which are not necessarily to scale and the invention is not limited to the illustrated embodiments, but rather the emphasis is on explaining the principles underlying the invention.

[0015] Before describing various embodiments of the present invention in detail, a general example of a robot-assisted grinding machine will first be described, as the concepts described herein are transferable to other types of surface finishing (e.g., polishing, milling, drilling) and are not limited to grinding.

[0016] According to FIG. 1, the robot-assisted grinding device comprises a manipulator 80, for example an industrial robot, and a grinding machine 50 with a rotating grinding tool 51. The grinding machine 50 is coupled to the so-called Tool Center Point (TCP) of the manipulator 1 via a linear actuator 100, which in this embodiment is generally called a handling device. Strictly speaking, the TCP is not a point but a vector, which can be described for example by three spatial coordinates (position) and three angles (directions). In robotics, generalized coordinates in the configuration space (usually the six joint angles of the robot) are sometimes used to describe the position of the TCP. The position and orientation of the TCP are sometimes called "pose". The position (including the orientation) of the TCP defines the movement of the grinding tool as a function of time, and this movement is called a trajectory. The TCP is often defined as the center of the end effector flange of the robot, but this is not necessarily the case. The TCP can be any point (theoretically even outside the robot) at which the robot can adjust its position and orientation. The TCP can also define the origin of the tool coordinate system.

[0017] In the case of an industrial robot having six degrees of freedom, the manipulator 80 has joints G 11 , G 12 , G 13 The first segment 82 is usually (but not necessarily) fixedly (rigidly) connected to the foundation 81. The joint G 11 connects the segments 82 and 83. 11 The joint G may be biaxial, allowing the segment 83 to rotate about a horizontal axis of rotation (elevation) and a vertical axis of rotation (azimuth). 12 Joint G connects segment 83 and segment 84 and allows pivotal movement of segment 84 relative to the position of segment 83. 13 Joint G connects segment 84 and segment 85. 13 can be biaxial, so (joint G11 The TCP has a fixed relative position with respect to the segment 85, which typically includes a revolute joint (not shown) that allows a revolute movement of an end effect flange 86 arranged on the segment 85 about a longitudinal axis A (shown in dashed lines in FIG. 1 and corresponding to the axis of rotation of the grinding tool in the illustrated example). Each axis of the joint is assigned an actuator (e.g. an electric motor) by which a rotational movement about the respective joint axis can be caused. The actuators of the joints are controlled by the robot control 70 according to a robot program. Various industrial robots / manipulators and associated controls are known and will not be described further here.

[0018] The manipulator 80 is usually position-controlled, i.e. the robot controller determines the pose (position and orientation) of the TCP and can move it along a predefined trajectory. In FIG. 1, the longitudinal axis of the segment 85 on which the TCP is located is denoted as A. When the linear actuator of the handling device 100 is at its end stop, the pose of the TCP also defines the pose of the grinding machine 50 (and the grinding disk 51). As mentioned at the beginning, the handling device 100 sets the contact force (processing force) between the tool (e.g., the grinding disk 51) and the workpiece 60 to a desired value during the grinding process. Direct force control by the manipulator 80 is usually too inaccurate for grinding applications. This is because the high mass inertia of the segments 83-85 of the manipulator 80 makes rapid correction of force peaks (e.g., when placing the grinding tool on the workpiece 60) practically impossible with conventional manipulators. For this reason, the robot control unit 70 is configured to control the posture (position and direction) of the TCP of the manipulator 80, and the handling device 100 is configured to control the force entirely.

[0019] As already mentioned, during the grinding process, the contact force F between the grinding tool (grinding plate 51 of the grinding machine 50) and the workpiece 60K is the contact force F between the grinding plate 51 and the workpiece 60 (in the direction of the longitudinal axis A). K The contact force F can be set by the handling device 100 and the force control (which can be realized, for example, in the control unit 70) so that F corresponds to a predetermined value. K is the actuator force F with which the handling device 100 presses the surface of the workpiece. A 2. In the absence of contact between the workpiece 60 and the tool 51, the actuator (see FIG. 2) included in the handling device 100 moves to an end stop (not shown since it is integrated in the actuator 2) due to the lack of contact force on the workpiece 60 and presses it with a defined force. In this case, the force control is active all the time. Therefore, in this situation (no contact), the actuator displacement is maximum and the handling device 100 is at its end. The force with which the (linear) actuator (included in the handling device 100) presses against the end stop can be regulated to be very small or even zero (theoretically) in order to allow the smoothest possible contact with the workpiece surface.

[0020] The position control of the manipulator 80 (which can also be realized by the control unit 70) can be performed completely independently of the force control of the handling device 100. The handling device 100 is not used for positioning the grinding machine 50 but rather for controlling the desired contact force F during the grinding process. K , and to detect contact between the tool 51 and the workpiece 60. This contact is easily recognized, for example, by a linear actuator included in the handling device moving away from its end position (the actuator deflection a is equal to the end maximum deflection a MAX is less than.

[0021] Figure 2 shows a schematic diagram of an example of a handling device 100. Components known to those skilled in the art and not necessary for the following discussion (valves, linear guides, etc.) have been omitted from Figure 2 in order not to overwhelm the figure. The actuator 153 contained in the handling device 100 can be a pneumatic actuator, for example a double-acting pneumatic cylinder. However, other pneumatic actuators can also be used, such as bellows cylinders or air muscles. Also, an electric direct drive (gearless) can be used.

[0022] The direction of movement of the actuator / handling device 100 and the axis of rotation of the grinding machine 50 do not necessarily have to coincide with the longitudinal axis A of the segment 85 of the manipulator 80. In the case of pneumatic actuators, the force control can be realized in a known manner by means of control valves, regulating devices (for example implemented in the control unit 70), compressed air accumulators or compressors. To take into account gravity (i.e. the weight of the grinding machine 50), the tilt with respect to the vertical is relevant, so that the actuator 2 includes a tilt sensor (not shown) or the information of the tilt with respect to the vertical can be determined based on the joint angle of the manipulator 80. The determined tilt is taken into account by a force regulating device (see also the description of FIG. 3). The handling device 100 not only allows a reliable mechanical decoupling between the manipulator 80 and the workpiece 60, but also compensates for inaccuracies in the positioning of the TCP.

[0023] The handling device comprises a deflection sensor, which can be configured, for example, as an inductive sensor or a potentiometer, next to the linear actuator 153 (pneumatic cylinder). Essentially, the deflection sensor is configured to measure the deflection of the linear actuator 153. The maximum deflection (a = a MAX), where the linear actuator presses against an end stop. The linear actuator connects two flanges 101 and 102. A change in the distance between the two flanges 101, 102 corresponds to a change in the deflection of the linear actuator 153. The upper flange 102 (FIG. 2) can be connected (for example by a screw) to an end effector flange of the robot (see FIG. 1, end effector flange 86). The machine tool 50 can be attached (directly or indirectly) to the lower flange 101, and in the illustrated example a force sensor 150 is arranged between the handling device and the machine tool 50. This force sensor 150 can be configured, for example, as a load cell and allows a direct measurement of the forces acting between the handling device and the machine tool 50.

[0024] The bellows 121 protects the components in the handling device from dust etc. while at the same time allowing the movement in the actuation direction of the pneumatic cylinder 153. The bellows 121 acts like a spring, whose characteristic curve can be taken into account in the force control. The (spring) force component due to the bellows 121 can be determined, for example, based on the deflection a measured by the deflection sensor 151. In the simplest case, the (spring) force component due to the bellows 121 is proportional to the deflection (has a linear spring characteristic). In some embodiments, the actual spring characteristic of the bellows 121 is determined by a calibration measurement.

[0025] In addition to direct force measurements with the load cell 150, indirect force measurements are made by measuring the pressure p in the pneumatic cylinder 153 with a pressure sensor 152 that can be pneumatically coupled to the compressed air line of the pneumatic cylinder 153. This force is given by the pressure p multiplied by the effective piston area A in the cylinder (F A = P x A). If electromechanical actuators are used instead of pneumatic actuators, the force can also be determined from the current draw of the electromechanical actuator. In this case, a current measurement is made instead of a pressure measurement. The actuator force can be calculated from the measured current value.

[0026] Known systems generally do not use redundant force measurements using direct force sensors such as load cells, since the cylinder pressure (or current in the case of electromechanical actuators) is available as a measurement in the controlled pneumatic system. In this context, the example described here uses a direct force measurement (force F) by the force sensor 150. M ) simply does not provide a redundant measurement for the indirect force measurement (force p × A). If the indirect force measurement (using pressure or current measurements) is replaced by a direct force measurement using load cells, the respective force sensor must be arranged to measure the actuator force exerted by the actuator (pneumatic cylinder) on the flange 101 of the handling device 100. If the movement of the flange 101 (relative to the flange 102) is prevented, for example, by an end stop, the force sensor will measure the actuator force acting against the end stop, even though in this case there is no contact with the workpiece. However, this is not the case in the example of FIG. 2. Since the force sensor 150 is not inside the handling device 100 (between the pneumatic cylinder 153 and the flange 101) but outside the flange 101, the force sensor will only measure the force F acting between the machine tool 50 and the handling device. M When there is no contact with the workpiece, in the illustrated example, the force sensor 150 measures if the pneumatic cylinder 153 is pressed against the end stop and what force F A This would measure only the weight of the machine tool 50, regardless of whether it is pressed against an end stop by the force sensor 14 or by the force sensor 16. That is, in the example described here (where there is no contact), a direct force measurement (force F M ) and indirect force measurement (force p × A) are not redundant and in principle measure different forces.

[0027] Indirect force measurement (F for pneumatic cylinders) A = P × A) A and the directly measured force F M The correlation between the contact force F and the workpiece can be formulated only when there is contact between the workpiece and the machine tool.K (process forces) act on the handling device, F K =F M +F G applies, where F G is the weight force of the machine tool acting on the workpiece surface, and F M is the directly measured force with which the handling device presses the machine tool. If the grinding machine is driven upside down, the applied force F G It should be noted at this point that F can also be negative. In the case of contact, the directly measured force is still F M =F A +ΔF=p×A+ΔF, where the offset ΔF includes all interference forces (friction, hysteresis effects, etc.). For contact / process forces, F K =F A +F G +ΔF=F M +F G whereby the offset ΔF, which depends on the state of the handling device, can be determined during operation (eg based on a mathematical model and / or calibration measurements).

[0028] Fig. 3 is a block diagram showing an example of a control device that can be used to drive the handling device 100. The control unit of Fig. 3 consists of a state observer 160, also known as a state estimator, which is supplied with control variables, which in this example represent the setpoint or measured actual values ​​of the cylinder pressure. The state observer 160 is also configured to receive sensor data (e.g. the measured deflection a of the handling device 100, the acceleration of the handling device, the inclination of the handling device relative to the vertical, etc.) and system parameters (e.g. the weight of the machine tool mounted on the handling device, etc.) and to estimate from the information supplied (sensor data and control variables) the state of the handling device, in particular the force FA+ΔF (estimated actual process force) acting on either the end stop (in the absence of contact) or the workpiece (in the presence of contact), effectively supplied by the actuator (pneumatic cylinder). For state estimation, the state observer may include a mathematical model that models the physical behavior of the handling device (e.g. the spring properties of the bellows, friction, etc.).

[0029] The state observer 160 is also configured to detect contact between the machine tool and the tool and send a signal. Since the actuator (pneumatic cylinder) is pressed against the end stop when there is no contact, for example, the actuator is not allowed to leave the end stop (maximum deflection a at the end stop). MAX The contact can be detected by detecting the contact (the deflection is smaller than the deflection of the sensor).

[0030] Another component of the control device shown in Fig. 3 is the process control and monitoring unit 161, where the regulation of the process forces takes place. For this purpose, the process control and monitoring unit 161 receives information on the estimated actual process forces and the contacts from the state observer 160 as well as system parameters (e.g. the weight of the machine tool 50), the target process force F S and the actual process force F, which is measured directly by the force sensor 150 and, as mentioned above, is a meaningful measurement variable only when contact is present. M Receive the target process power FS and the actual force F, which is directly measured and / or estimated. M or F A +ΔF, based on weight force F G Taking into account the theoretical control deviation, the control variable with which the actuator is controlled (in the case of a pneumatic actuator, this is the cylinder pressure p as mentioned above) is calculated by a control algorithm. Suitable control algorithms are already known and will not be described further here. If the theoretical control deviation is zero, the control variable (e.g. pressure in the case of a pneumatic actuator, actuator current in the case of an electromechanical actuator) is calculated as the process force F at the time of contact. K is set to be applied to: K =F M +F G =F S This means that the process forces (contact forces) correspond to a (possibly varying) target force.

[0031] The process control and monitoring unit 161 is further configured to select the "source" of the actual process force (force sensor 150, state observer 160) depending on whether contact is detected or not. If there is no contact, the state observer 160 is selected, if contact is detected the force sensor 150 is selected. Ideally, if contact is detected, both sources should provide the same force value, but the estimated value F A +ΔF includes certain influence parameters due to calibration that may change over time, whereas the direct measurement F M may vary since x always measures the actual force (assuming force sensor 150 is working properly).

[0032] Plausibility check: The process control and monitoring unit 161 furthermore checks the directly measured force values ​​F M and the force value F provided by the state observer A Based on +ΔF, a plausibility check can be configured to be performed during the surface treatment process (i.e., at the time of contact). For this purpose, the process control and monitoring unit 161 determines two values ​​F A +ΔF and FM and, for example, report an error if they are different. In some situations, two values ​​F A +ΔF and F M Based on the deviations between the force values ​​F and F = ... M If F no longer follows the value estimated by the state observer, one likely cause is that the linear guides in the handling device are jammed or friction has increased significantly. M With smaller deviations, the estimate F A If +ΔF is being followed, there may be a slight increase in friction in the pneumatic cylinder (actuator 153) or linear guide (not shown), and maintenance needs to be performed.

[0033] Some aspects and features of the embodiments described herein are now summarized below. The following is not a definitive list, but merely an exemplary summary. The present embodiments relate to a system and method for controlling a handling device comprising a first flange and a second flange and comprising a linear actuator acting between the first and second flanges. The first flange is attached to a manipulator during operation (e.g. attached to a flange of an end effector, see FIG. 1), and a tool (or a machine tool with a tool) is attached to the second flange during operation. While supported by the first flange, the linear actuator can exert a force on the second flange depending on a control variable (see FIG. 2, flanges 101 and 102, linear actuator 152). In the case of a pneumatic actuator (pneumatic cylinder), the control variable is air pressure, and in the case of an electromechanical actuator, the control variable is current through the actuator.

[0034] In one embodiment, a force sensor measures the force F exerted on the tool by the handling device when the tool contacts the surface. MThe contact force between the tool and the surface is measured using a force F M and the gravitational force F acting on the surface due to the weight of the machine tool and the tool (which depends on the angular position). G corresponds to the superposition of

[0035] In the absence of contact, the machine tool hangs from the handling device and the force sensor measures the force F G Only the force sensor measures the force F provided by the linear actuator (force control) pressing against the end stop. In such a situation, the aforementioned force sensor cannot be used for force control. Therefore, for example, the force F provided by the linear actuator can be measured based on the control variables (e.g., target pressure or actual pressure) using a state observer that can be implemented in the control unit. A +ΔF is determined. The control unit may also be configured to detect contact between the tool and the surface. Further, the control unit may determine an estimate of F if no contact is detected. A Set the control variable (e.g., pressure p) of the linear actuator based on +ΔF and the target value, and once contact is detected (as soon as and while contact is detected), the measured force F M and a target value, the control variable is set based on the target value. That is, the force information used for the force control depends on whether or not a contact is detected.

[0036] An example of the concept described here is summarized below with the aid of the flow diagram in FIG. 4. FIG. 4 relates to a method for controlling a handling device with a linear actuator (see FIG. 2, pneumatic cylinder 154), which acts between a first flange (see FIG. 1, flange 102) connectable to a manipulator and a second flange (see FIG. 1, flange 101) on which a tool or a machine tool with a tool is mounted. The method comprises the steps of controlling the linear actuator with a control variable (e.g. air pressure p) such that the linear actuator exerts a force (dependent on the control variable) on the second flange (in case of contact between the tool and the surface) or on an end stop (in case of no contact) (see FIG. 4, step S1). The method further comprises detecting contact between the tool and the surface (see FIG. 4, step S2) and determining a force F exerted by the handling device on the tool between the second flange and the tool in case of contact between the tool and the surface. M using a mechanically coupled force sensor (see FIG. 4, step S3).

[0037] The method further comprises: determining a force F exerted on the tool by the handling device based on the control variable; M The estimated value of F A The method includes determining +ΔF (with or without contact with the surface) (see FIG. 4, step S4). The control variables are set based on the estimated and target values ​​when and as long as contact is not detected (see FIG. 4, step S6), and based on the measured force and target values ​​when and as long as contact is detected (see FIG. 4, step S5). Note that the steps shown in FIG. 4 may be partially performed in parallel. Also, the arrows in the flow chart do not necessarily imply a chronological order.

[0038] In particular, step S4 is performed regardless of whether contact is detected or not. In the absence of contact, a force estimate is needed to set the force with which the linear actuator presses against the end stop. For a gentle contact, this force should be as small as possible (ideally zero or a few Newtons). When the tool touches the surface, the actuator moves away from the end stop and the force control is determined by the directly measured force F M Nevertheless, the estimate F A +ΔF is also determined during the surface preparation process (at contact) for process validation and defect detection. Before contact is made, the actuator presses the end stop with the smallest (minimum) force possible. In theory, this minimum force can be adjusted to zero newtons. In practice, values ​​below 10 newtons or even below 1 newton are used to contact the surface very gently. As soon as contact is made, the target force can be increased at a defined rate until the desired machining (grinding) force is reached.

[0039] In addition to the control variable (pressure in case of pneumatic actuators), other sensor data related to the state of the actuators and / or the handling device can be included in the determination of the estimate, such as the actuator displacement, which can be measured using a potentiometer or an inductive displacement sensor coupled to the actuator (see FIG. 3, state observer 160). G =m×g×cos(θ) is, for example, the force control (see FIG. 3, process control unit 161) from the target force to the weight force F G (where m is the mass of the machine tool including the tool, g is the acceleration of gravity and θ is the angular deviation from the vertical (tilt). Alternatively, the force of gravity can be taken into account in direct and indirect force measurements. The tilt angle θ can be measured or calculated from the (generalized) coordinates of the TCP of the manipulator. Since the robot controller "knows" the angular position of the TCP, it also "knows" the angular positions of the handling device and the tool.

[0040] According to one embodiment, it is also possible to automatically check the validity of the surface treatment process and ensure its validity at the end of the process. For example, the force F measured (directly by the force sensor) during the surface treatment process can be M and the estimated value F (determined by the state observer). A +ΔF can be compared and the deviation between the measured and estimated values ​​for a particular surface treatment process can be recorded. At the end of the process or during the process, the recorded data can be analyzed to check the validity of the process and indicate errors. For example, a specific error can be determined based on the deviation between the directly measured force and the estimated value (and possibly other sensor data such as the deviation of the actuator). For example, if, upon contact with the surface and with a high estimated value, the measured force does not become similarly high, it is very likely that the linear guide (e.g. arranged parallel to the actuator) or the actuator itself does not move or at least the friction in the linear actuator or in the linear guide is abnormally high. In this case, the next time the surface is touched, a gentle contact is no longer guaranteed. Additionally or alternatively, the deviation between the target force value and the measured force can also be analyzed. [Explanation of symbols]

[0041] 50…Machine tools 51...Tools 80…Manipulator 100...Handling device 101...Second flange 102...First flange 150...Force sensor 153...Linear actuator 160…State observer 161...Process control and monitoring unit

Claims

1. a handling device having a linear actuator (153) acting between a first flange (102) connectable to a manipulator and a second flange (101) on which a tool or a machine tool having a tool can be attached, the linear actuator (153) exerting a force on the second flange (101) or on an end stop depending on a control variable (p); A force (F) is coupled between the second flange (101) and the tool, the force (F) exerted by the handling device on the tool upon contact of the tool with a surface. Ｍ a force sensor configured to measure a force of A control unit having a state observer (160), the state observer (160) detecting the force (F Ｍ ) estimate (F Ａ +ΔF) based on said control variable (p); having The control unit further comprises: detecting contact between the tool and the surface; While the contact is not detected, the estimate (F Ａ +ΔF) and a target value based on which the control variable (p) is set; While the contact is detected, the measured force (F Ｍ ) and set the control variable (p) based on the target value. system.

2. The system of claim 1 , wherein the target value is variable and is increased from a minimum value after the contact is detected.

3. The system of claim 1 or claim 2, wherein the linear actuator (153) pushes against the end stop while the contact is not detected.

4. The control unit is configured to control the measured force (F Ｍ ) and the estimated value (F Ａ 4. The system of claim 1, further configured to compare the measured value of the measured value with the measured value of the measured value (+ΔF) and indicate or record an error based on their deviation.

5. The control unit is configured to detect the contact and to control the measured force (F Ｍ ) and the estimated value (F Ａ 4. The system of claim 1, further configured to perform a check of the validity of the surface treatment process based on the measured value of the surface treatment process (ΔF) and to determine a possible cause of error if the check fails.

6. The system according to any one of claims 1 to 5, wherein the state observer (160) is configured to determine the estimated value (FA+ΔF) based on the control variable (p) and further sensor data relating to the state of the linear actuator (153), in particular the deflection of the actuator.

7. A method for controlling a handling device having a linear actuator (153) acting between a first flange (102) connectable to a manipulator and a second flange (101) on which a tool or a machine tool having a tool can be mounted, the method comprising the steps of: controlling the linear actuator (153) with a control variable (p), such that the linear actuator (153) exerts a force on the second flange (101) or on an end stop in response to the control variable (p); detecting contact between the tool and a surface; The force exerted by the handling device on the tool (F) is measured by a force sensor coupled between the second flange (101) and the tool when the tool comes into contact with the surface. Ｍ ) measuring the Based on the control variable (p), the force (F) exerted by the handling device on the tool is calculated. Ｍ ) estimate (F Ａ +ΔF); While the contact is not detected, the estimate (F Ａ +ΔF) and setting the control variable (p) based on a target value; While the contact is detected, the measured force (F Ｍ ) and setting the control variable (p) based on the target value; The method according to claim 1,

8. The estimated value (F Ａ 8. The method of claim 7, wherein further sensor data relating to the state of the linear actuator (153), in particular the deflection of the linear actuator, are taken into account when determining the linear actuator (153).

9. 9. The method of claim 7 or claim 8, wherein the contact between the tool and a surface is detected by the linear actuator moving away from the end stop.

10. During the surface treatment process, the tool contacts the surface and the force (F Ｍ ) based on which the validity of the surface treatment process is checked and the measured force (F Ｍ 10. The method according to claim 7, further comprising determining, in case of a deviation between the estimated value and the estimated value, a possible cause of error.

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