CONTROL OF CONTACT FORCE IN A MACHINE TOOL

Abstract

Method for controlling a contact task, wherein the method comprises: Measuring a contact force (Fc) between a tool (20) and a workpiece (12) along a compliance axis; Measuring at least one dynamic process variable in addition to the contact force (Fc); and Controlling the contact force (Fc) along the compliance axis depending on at least one dynamic process variable.

Description

[0001] The present disclosure relates generally to material removal and surface finishing methods and, more particularly, to force control of automated surface finishing and material removal tools.

[0002] Many manufactured parts require surface finishing such as grinding, sanding, and polishing to provide a smooth surface. These contact tasks are labor-intensive but can be automated with machine tools. Numerous manufacturers offer machine tools that can be mounted on a robotic arm or manipulator to grind, sand, or polish the surface of a manufactured part. Typically, the machine tool is attached to the manipulator with an actuator that moves the machine tool along a single axis, and some form of feedback control is used to maintain a constant contact force between the workpiece and the tool.

[0003] The results produced by the machine tool can vary depending on the applied contact pressure, which can change dynamically even if the contact force remains constant. For example, the area of ​​contact between a grinding tool and a workpiece will decrease if the grinding pad is not parallel to the workpiece surface in the case of a flat surface or not tangent to the workpiece surface in the case of a curved surface. The area of ​​contact can also decrease due to the curvature of the surface, which can vary. As the area of ​​contact is reduced, the contact pressure increases if the contact force is kept constant. In this case, the same force is applied over a smaller area, so the contact pressure across the contact area is increased.

[0004] In addition, the abrasive used in a material removal process wears out over time. As the abrasive wears, friction decreases, and thus more force is required to remove the same amount of material.

[0005] Therefore, a greater contact force may be required as the abrasive wears.

[0006] A skilled operator manually performing a material removal process can compensate for dynamic factors that affect the grinding process. Currently, there is no way to provide similar control for single-axis, force-controlled tools to compensate for dynamic variables in the material removal process.

[0007] A problem to be solved is therefore to provide a way to compensate for dynamic variables in the material removal process caused by changes in contact pressure, contact area or wear of the abrasive, while maintaining a constant contact force.

[0008] Attempts to solve this problem, which are part of the state of the art, are known from the documents DE 10 2018 106 086 A1, DE 10 2017 110 773 A1, DE 10 2016 006 704 B4, DE 10 2015 104 164 B4, DE 10 2014 119 532 A1, DE 10 2011 006 679 A1.

[0009] The problem is solved by the invention according to the independent claims. The dependent claims comprise additional embodiments that enable further optimizations and specific applications.

[0010] The present disclosure provides methods for controlling contact force in a single-axis, force-controlled tool that are responsive to dynamic process variables, such as the orientation of the tool relative to the workpiece, the curvature of the workpiece, the wear of the abrasives, and other dynamic process variables not otherwise controlled by a single-axis, force-controlled actuator. A control system includes sensors to determine a contact force between the tool and the workpiece along a single compliance axis and one or more additional process parameters, such as a tilting moment on the tool due to the orientation of the workpiece and surface curvature, a torque about a rotational axis of the tool, a motor current, etc.The control system uses the measurement of these additional parameters to control the contact force between the tool and the workpiece. Specifically, the invention provides a method for controlling a contact task according to claim 1, an automated tool according to claim 4, and a controller for an automated tool according to claim 9. The dependent claims define preferred and / or advantageous embodiments of the invention. Fig. 1 shows a machine tool mounted on a robot arm for performing a contact task. Fig. 2A and Fig. 2B shows an actuator arranged between the machine tool and a manipulator to control the contact force between the machine tool and the workpiece. Fig. 3 is an exploded perspective view of the actuator. Fig. Figure 4 shows a force / torque sensor arranged between the tool and the actuator. Fig. 5 represents a control circuit for controlling the actuator. Fig. Figure 6 illustrates a scenario where a grinding tool contacts a flat surface of a workpiece at an angle. Fig. Figure 7 illustrates a control circuit in which the contact force is adjusted as a function of a tilting moment due to tool orientation to compensate for a reduced contact area between the tool and the workpiece. Fig. 8A and Fig. 8B illustrates a scenario where a grinding tool contacts a curved surface of a workpiece at an angle. Fig. Figure 9 illustrates a control circuit in which the contact force is adjusted depending on a tilting moment due to tool orientation and surface curvature to compensate for a reduced contact area between the tool and the workpiece. Fig. 10 shows a control circuit in which the contact force is adjusted as a function of a torque about a rotation axis of the tool in order to compensate for wear of an abrasive. Fig. Figure 11 illustrates an exemplary method for controlling the contact force between a tool and a workpiece along a single compliance axis.

[0011] With reference to the drawings, the force control principles according to the present disclosure will be described in the context of a material removal device 10. Those skilled in the art will understand that the methods described herein are not limited to material removal methods, but are more generally applicable to any task involving force-controlled contact with a single compliance axis.

[0012] The material removal device 10 generally includes a tool 20 (e.g., a grinder) held by a manipulator 30 for grinding a workpiece 12. The tool 20 includes an electric or pneumatic motor 22 and a sanding pad 24. The tool 20 may comprise a rotary sander or an orbital sander. The manipulator 30 includes a robotic arm 32 having 6 degrees of freedom (DOF). The function of the manipulator 30 is to move the tool 20 into an operating position while grinding the workpiece 12. An actuator 40, mounted between the end effector plate 34 of the robotic arm 32 and the tool 20, controls the contact force between the tool 20 and the workpiece 12 along a single compliance axis. A control system 100 ( Fig. 5) controls the amount of contact force as described below.

[0013] In the Fig. 2A, Fig. 2B and Fig. 3, an exemplary embodiment of the actuator 40 is shown. The actuator 40 includes a housing 42 containing an actuator assembly 80 that provides the driving force for moving the tool 20 along the single compliance axis. The housing 42 includes a base 44 and a top plate 46. The base 44 has a cavity 86 in which the actuator assembly 80 is received. The top plate 46 is fixedly connected to the base 44 by screws (not shown) and has an opening 48.

[0014] The mounting adapter plate (MAP) 50 is slidably mounted on the top plate 46 of the housing 44. A pair of linear guide rails 52 are attached to the upper surface of the top plate 46. The MAP 50 is mounted on carriages 54 that slide along the linear guide rails 52.

[0015] A 6-axis force / torque sensor 60, which is Fig. 4, is arranged between the MAP 50 and a tool adapter plate (TAP) 70. The force / torque sensor 60 includes a central hub 62 connected to an outer ring 64 via three short supports 66. The central hub 62 is securely attached to the TAP 70 with screws (not shown), while the outer ring 64 is securely attached to the MAP 50 with screws (not shown). The sensor 60 has six strain gauges 68; three are attached to the top surface of the respective support 66 and three are attached to one of the vertical surfaces of the respective supports 66. Four of the six strain gauges 68 are in Fig. 4. The strain gauges 68 measure the deformation of the beams 66 when forces are transferred between the TAP 70 and the MAP 50.

[0016] The TAP 70 provides a platform for mounting the tool 20 to the manipulator 30. In the illustrated embodiment, the TAP 70 comprises a generally flat plate securely attached to the hub 62 of the force / torque sensor 60 with screws (not shown). The machine tool 20 is attached to the TAP 70 of the force / torque sensor 60. The MAP 50, the sensor 60, and the TAP 70 together form an interface between the manipulator 30 and the tool 20 with an integrated sensor 60.

[0017] In the Fig. 1, the actuator 40 is mounted on the end effector plate 34 and oriented to move the tool 20 linearly along the compliance axis (e.g., the vertical axis). The actuator assembly 80 is disposed within the housing 42 and moves the tool 20 linearly along the compliance axis (e.g., the vertical axis) to control the amount of force exerted by the tool 20 on the workpiece 12. The actuator assembly 80 includes a pneumatic cylinder 82, a linkage block 84, and a load cell 88. The pneumatic cylinder 82 is secured to the inner wall of the base 44 by the load cell 88. The linkage block 84 is attached to the shaft of the pneumatic cylinder 82. The linkage block 84 extends through the opening 48 in the top plate 46 of the housing 42 and is connected to the MAP 50. By actuating the pneumatic cylinder 82, the MAP 50 moves back and forth along the linear guide rails.In this way, the movement of the tool 20 is limited to the single compliance axis. The opening 48 is sized to provide clearance for the movement of the connecting block 84.

[0018] In the illustrated embodiment, the actuator 40 is supported by the robot arm 32, and the force sensor 60 is disposed between the actuator 40 and the tool 20. Those skilled in the art will understand that other force sensor arrangements are possible. For example, the force sensor 60 may be disposed between the actuator 40 and the end effector plate 34 on the robot arm 32. In another example, the force sensor 60 may be disposed between the actuator 40 and the workpiece 12. Therefore, the description of the force sensor arrangement herein is intended to be illustrative only to facilitate understanding of the disclosure and is not intended to be limiting.

[0019] A control circuit 100 controls the pneumatic cylinder 82 to maintain a constant force along the single compliance axis between the tool 20 and the workpiece 12 during operation. A generic embodiment of the control circuit 100 is shown in Fig. 5. The control circuit 100 includes a processing circuit 102 having an inner loop method 104 and an outer loop method 106. A first sensor or set of sensors 110 provides feedback to the processing circuit 102 to determine the measured contact force Fc between the tool 20 and the workpiece 12 along the single compliance axis. The inner loop method 104 controls the actuator 40 to maintain the measured contact force Fc equal to a setpoint S. A second sensor or set of sensors 112 provides additional feedback that is used by the outer loop method 106 to determine the setpoint S used by the inner loop method 104.As described below, the outer loop process 106 adjusts the setpoint S used by the inner loop process 104 depending on one or more dynamic process variables.

[0020] In the Fig. 2A, Fig. 2B and Fig. 3, the contact force Fc along the single compliance axis is measured by the load cell 88 and fed back to the inner control loop process 104. In some embodiments, the load cell 88 may be replaced with a pressure sensor for measuring the contact force or clamp force along the compliance axis. Other variables or parameters (e.g., overturning moment) are measured by the force / torque sensor 60 and fed back to the outer control loop process 106. In some embodiments, the contact force Fc may be determined from the measurements provided by the force / torque sensor 60. In some embodiments, the motor current and / or motor torque may also be fed back to the outer control loop process 106. In other embodiments, the force / torque sensor 60 may measure the torque about the rotational axis of the tool 20.The outer loop process 106 uses the measurements related to these dynamic process variables to set the setpoint S for the inner loop process 104.

[0021] Fig. 6 shows an exemplary application of the control method for compensating the orientation of the tool 20 relative to the workpiece 12. As in Fig. 6, the abrasive pad 24 of the tool 20 contacts the surface of the workpiece 12 at a slight angle. The contact force Fc causes some deflection of the abrasive pad 24, so that the area of ​​the contact region between the abrasive pad 24 and the workpiece 12 varies depending on the contact angle, i.e., the area of ​​the contact region decreases as the contact angle increases. Because the actuator 40 is designed to control the contact force Fc along a single compliance axis and not the orientation of the tool 20 relative to the workpiece 12, the pressure across the contact region increases inversely proportional to the area of ​​the contact region. Contact pressure is believed to be an important factor in the quality of the finished surface. In the worst case, excessive contact pressure could gouge the surface of the finished product.

[0022] According to one embodiment, the data from the 6-axis force / torque sensor 60 is used to determine a tilting moment Mx of the tool 20 and dynamically adjust the contact force Fc. The measurements from the force / torque sensor 60 are in addition to the measurement of the contact force Fc along the single compliance axis. These additional measurements can be used in conjunction with the measurement of the contact force Fc to improve the result of the method. For example, the additional measurements provided by the force / torque sensor 60 can be used to determine the tilting moment Mx on the tool 20, which is related to the tool orientation. The contact force Fc exerted on the workpiece 12 can be varied depending on a tilting moment Mx.In one embodiment, the contact force Fc is controlled to maintain a predetermined setpoint S until the tipping moment Mx reaches or exceeds a threshold. When the tipping moment Mx reaches the threshold, the setpoint S used by the inner-loop control process 104 is adjusted to prevent the tipping moment Mx from exceeding the threshold.

[0023] Fig. Figure 7 illustrates an implementation of the controller 100 for compensating for the tilting of the tool 20 relative to the workpiece 12. In this embodiment, the load cell 88 measures the contact force Fc between the tool 20 and the workpiece 12 and feeds the contact force Fc back to the inner loop process 104. The force / torque sensor 60 measures the tilting moment Mx and feeds the tilting moment Mx back to the outer loop process 106. The inner loop process 104 maintains Fc equal to a predetermined setpoint S until the tilting moment Mx reaches a threshold. Once the threshold is reached, the outer loop process 106 modifies the setpoint S to prevent the tilting moment Mx from exceeding the threshold.

[0024] The Fig. 8A and Fig. 8B show another example of the force control method used to compensate for a curvature of the surface of the workpiece 12. Fig. Figure 8A shows the tool 20 contacting a curved surface of a workpiece 12. Fig. Figure 8B shows that the area of ​​the contact area can vary depending on the curvature of the workpiece 12 and the orientation of the tool 20 relative to the workpiece 12. In general, increasing curvature correlates with a smaller contact area. An unfavorable orientation of the tool 20 relative to the workpiece 12 causes a shift of the contact area relative to the grinding axis. In this case, it is also beneficial to reduce the contact force Fc if the area of ​​the contact area is reduced. The area of ​​the contact area can be estimated by measurements from the force / torque sensor 60. The ratio of the contact force Fc and the torque T about the grinding axis is a function of the area of ​​the contact area and the position of the contact area, measured from the compliance axis to the center of gravity of each contact area. The relative position of the contact area can be approximated by the two tilting moments Mx and My.By eliminating the contribution of the position of the contact area from the force / torque ratio (Fc / T), the area of ​​the contact area can be approximately determined. The calculated area of ​​the contact area can be used by the outer control loop process 106 to control the setpoint S for the inner control loop process 104 to maintain a constant pressure in the contact area.

[0025] Fig. 9 shows an implementation of the controller 100 for compensating for the curvature of the workpiece 12 and the orientation of the tool 20 relative to the workpiece 12. In this embodiment, the force / torque sensor 60 measures the contact force Fc between the tool 20 and the workpiece 12 and feeds the contact force Fc back to the inner loop control process 104. Alternatively, the contact force could be measured by the load cell 88. The force / torque sensor 60 measures the tilting moments Mx and My and feeds the tilting moments Mx and My back to the outer loop control process 106. The torque T about the rotational axis of the tool 20 is also fed back from the motor 22 to the outer loop control process 106. Alternatively, the torque T can be measured by the force sensor 60. The inner loop control process 104 maintains Fc equal to a predetermined setpoint S.The outer control loop process 106 adjusts the setpoint S based on the tilting moments Mx and My and the motor torque T to compensate for the curvature of the workpiece 12 and the tilting of the tool 20 relative to the workpiece.

[0026] As another example, the force control method can be used to compensate for abrasive wear. Generally, the abrasive used in grinding wears over time, so more force is required to achieve the same amount of material removal as the abrasive wears. These variations in the amount of material removed can affect the quality of the finished product. A skilled craftsperson performing this task manually can visually and tactilely assess the workpiece 12 and adjust the force or duration of material removal to compensate for abrasive wear. As with tool orientation and workpiece curvature, conventional single-axis force control methods do not compensate for abrasive wear.

[0027] According to one embodiment of the present disclosure, the torque T about the sander axis or the oscillating side forces in an orbital sander are measured and provided to the outer loop control process 106. As the abrasive wear increases, the friction between the sanding pad 24 and the workpiece 12 decreases, thereby decreasing the motor torque T. The amount of material removed is a function of the motor torque T and the contact force Fc. The ratio of the motor torque T to the contact force Fc can be used as a measure of abrasive wear. The amount of contact force Fc can be adjusted depending on the ratio of the sanding torque to the contact force to compensate for wear.

[0028] Fig. 10 shows an implementation of the controller 100 for compensating for abrasive wear. In this embodiment, the force / torque sensor 60 measures the contact force Fc between the tool 20 and the workpiece 12 and feeds the contact force Fc back to the inner loop control process 104. Alternatively, the contact force Fc may be measured by the load cell 88. The motor torque T is also fed back from the motor 22 to the outer loop control process 106. The inner loop control process 104 maintains the Fc equal to a predetermined setpoint S. The outer loop control process 106 adjusts the setpoint S based on the motor torque T to compensate for abrasive wear. In one variation, measurement data from the force sensor 60 may be used to determine a torque about the rotational axis of the tool 20 to use instead of or in addition to the motor torque T.

[0029] Fig.11 shows an exemplary method 200 for controlling a contact task, such as grinding, performed by an automated tool. The method 200 includes contacting a surface of a workpiece 12 with a tool 20 (210), measuring a contact force between the workpiece 12 and the tool 20 along a single compliance axis (220), measuring at least one dynamic process variable in addition to the contact force (230), and controlling the contact force along the single compliance axis based on the at least one dynamic process variable (240).

[0030] In some embodiments of method 200, measuring a contact force between the workpiece and the tool includes measuring the contact force with a load cell on the tool.

[0031] In some embodiments of the method 200, measuring a contact force between the workpiece and the tool includes measuring the contact force with a force sensor disposed between the workpiece 12 or the tool 20 and an actuator 40 that controls the relative position of the tool 20 and the workpiece 12 along the single compliance axis.

[0032] In some embodiments of the method 200, the dynamic process variable includes a linear force or torque between the workpiece or tool and an actuator that controls the relative position of the tool and workpiece along the single compliance axis.

[0033] In some embodiments of the method 200, measuring the dynamic process variable includes measuring the linear force or torque with a force sensor disposed between the workpiece 12 or the tool 20 and the actuator 40.

[0034] In some embodiments of the method 200, the force sensor includes a 6-axis force / torque sensor 60 between the tool and the actuator.

[0035] In some embodiments of the method 200, the parameter includes a motor current or a motor torque for the tool.

[0036] In some embodiments of the method 200, controlling the contact force along the single compliance axis includes controlling a relative position of the workpiece 12 and the tool 20.

[0037] In some embodiments of the method 200, controlling a relative position of the workpiece 12 and the tool 20 along the single compliance axis includes linearly translating the tool 20 along the single compliance axis.

[0038] In some embodiments of the method 200, controlling a relative position of the workpiece 12 along the single compliance axis and the tool 20 based on the parameter includes linearly translating the workpiece 12 along the single compliance axis.

[0039] The techniques and methods described here make it possible to control the contact force in a single-axis, force-controlled tool as a function of dynamic process variables, such as the orientation of the tool relative to the workpiece, the curvature of the workpiece, the wear of the abrasive, and other dynamic process variables not otherwise controlled by a single-axis, force-controlled actuator. These control methods improve the quality of the finished surface of machined parts and reduce the number of defects in the machined parts. Reducing material defects, in turn, increases the quality of the finished part and leads to lower costs because fewer parts are discarded.

Claims

[1] A method for controlling a contact task, the method comprising: Measuring a contact force (Fc) between a tool (20) and a workpiece (12) along a compliance axis; Measuring at least one dynamic process variable in addition to the contact force (Fc); and Controlling the contact force (Fc) along the compliance axis depending on the at least one dynamic process variable. [2] The method of claim 1, wherein measuring the contact force (Fc) between the workpiece (12) and the tool (20) comprises measuring the contact force (Fc) with one of a load cell (88) on the tool (20), a force sensor (60) arranged between the workpiece (12) or the tool (20) and an actuator (40) controlling the relative position of the tool (20) and the workpiece (12) along the single compliance axis, and a force sensor (60) arranged between the actuator (40) controlling the relative position of the tool (20) and the workpiece (12) along the single compliance axis, and an end effector plate (34) of a robot arm (32). [3] The method of claim 1 or claim 2, wherein the dynamic process variable comprises one of a linear force or torque between the tool (20) and an actuator (40) that controls the relative position of the tool (20) and the workpiece (12) along the single compliance axis, a motor current for the tool (20), and a motor torque (T) of the tool (20). [4] Automated tool (10) comprising: a tool (20) for machining a workpiece (12); an actuator (40) for moving the tool (20) relative to the workpiece (12) along a single compliance axis; a first sensor (60, 88) for determining a contact force (Fc) between the tool (20) and the workpiece (12) along the compliance axis; a second sensor (60) for determining a dynamic process variable in addition to the contact force (Fc) along the compliance axis; a processing circuit (102) for controlling the actuator (40), wherein the processing circuit (102) is configured to control the contact force (Fc) along the compliance axis depending on the dynamic process variable. [5] The automated tool (10) of claim 4, wherein the first sensor comprises one of a load cell (88) on the tool (20), a force sensor (60) disposed between the workpiece (12) or the tool (20) and the actuator (40) along the single compliance axis, and a force sensor disposed between the actuator (40) and an end effector plate (34) of a robot arm (32). [6] The automated tool (10) of claim 4 or claim 5, wherein the dynamic process variable comprises one of a linear force or torque between the tool (20) and the actuator (40) that controls the relative position of the tool (20) and the workpiece (12) along the single compliance axis, a motor current for the tool (20), and a motor torque (T) of the tool (20). [7] Automated tool (10) according to any one of claims 4-6, wherein the second sensor comprises a force sensor (60) arranged between the tool (20) and the actuator (40). [8] Automated tool (10) according to any one of claims 4-7, wherein the processing circuit (102) is configured to: to determine a setpoint (S) depending on the dynamic process variable; and to control the actuator (40) depending on the measured contact force (Fc) and the setpoint (S) in order to keep the contact force (Fc) at the setpoint (S). [9] A controller (100) for an automated tool (10) configured to perform a contact task, the controller (100) comprising a processing circuit (102) configured to: to determine a contact force (Fc) between a tool (20) of the automated tool (10) and a workpiece (12) depending on first sensor data; to determine a dynamic process variable depending on second sensor data; to determine a control setpoint (S) depending on the dynamic process variable; to control the contact force (Fc) according to the control setpoint (S). [10] Controller (100) according to claim 9, wherein the processing circuit (102) is designed to determine the setpoint value (S) as a function of a tilting moment (Mx, My) of the tool (20), a torque about a rotational axis of the tool (20) and a motor current or a motor torque.

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