Robot system, robot control device, control method, and control program

Abstract

This robot system comprises a robot, a control device that controls the robot, and a tool that is attached to the robot and performs prescribed work on a workpiece. The control device that controls the robot implements, on the basis of an external force acting on the robot, force control for operating the robot in a direction in which the external force acting on the robot is released. The control device switches the extent of the force control so that a load acting on the robot is reduced in a state in which the tool and the workpiece are mutually restrained.

Description

ROBOT SYSTEM, ROBOT CONTROL DEVICE, CONTROL METHOD, AND CONTROL PROGRAM

[0001] The present disclosure relates to a robot system, a robot control device, a control method, and a control program.

[0002] An anchor driving method is known in which an anchor held by an anchor driving tool (hereinafter referred to as the tool) attached to a robot is driven into a pilot hole in a wall (see, for example, Patent Document 1). In this anchor driving method, when inserting the anchor into the pilot hole, the anchor is attached to the tool in a constrained state, and the robot is controlled to insert the anchor so that the moment detected by a force sensor attached to the robot is minimized. After the anchor is inserted into the pilot hole, the anchor and the tool are released from the constrained state, and only an axial impact force is applied to the anchor, thereby reducing the load on the robot's joint axis.

[0003] Japanese Patent Application Laid-Open No. 2022-72055

[0004] However, for example, if the tool is a screw tightening device that tightens a bolt, releasing the bolt from its constrained state by the tool will make it impossible to tighten the bolt. Therefore, it is desirable to reduce the load acting on each axis of the robot even when performing work while maintaining the tool constrained to the work object.

[0005] One aspect of the present disclosure is a robot system comprising a robot, a control device that controls the robot, a tool attached to the robot that performs a predetermined task on a work object, and a sensor that detects an external force acting on the robot, wherein the control device performs force control to move the robot in a direction that will release the external force based on the external force detected by the sensor, and switches the degree of the force control so as to reduce the load acting on the robot when the tool and the work object are mutually constrained.

[0006] 1 is an overall configuration diagram showing a robot system according to an embodiment of the present disclosure; FIG. 2 is a partial vertical cross-sectional view showing a tool tip and a bolt, illustrating an example of force control by the robot system of FIG. 1; FIG. 3 is a flowchart illustrating a method for controlling a robot in the robot system of FIG. 1; FIG. 4 is a diagram schematically illustrating models of a robot and a tool in the robot system of FIG. 1; FIG. 5 is a diagram illustrating a cantilever beam similar to the case where the gain of force control is high in the model of FIG. 4; FIG. 6 is a diagram illustrating a doubly-supported beam similar to the case where the gain of force control is low in the model of FIG. 4; FIG. 7 is a partial vertical cross-sectional view showing a tool tip and a bolt, illustrating another example of force control by the robot system of FIG. 1;

[0007] A robot system 10, a control device 20 for the robot 1, a control method, and a control program according to an embodiment of the present disclosure will be described below with reference to the drawings. As shown in FIG. 1 , the robot system 10 according to this embodiment includes a robot 1, a control device 20 that controls the robot 1, a tool 30 attached to the robot 1, and a sensor 40 that detects an external force Fs.

[0008] 1, the robot 1 is, for example, a vertical six-axis articulated robot. However, the robot 1 is not limited to this and may be a robot of any other configuration.

[0009] The robot 1 includes a base 2 placed on an installation surface B such as a floor, and a rotating body 3 supported rotatably relative to the base 2 about a vertical first axis J1. The robot 1 also includes a first arm 4 supported rotatably relative to the rotating body 3 about a horizontal second axis J2, and a second arm 5 supported rotatably relative to the first arm 4 about a third axis J3 parallel to the second axis J2. The robot 1 also includes a three-axis wrist unit 6 attached to the tip of the second arm 5.

[0010] The wrist unit 6 includes a first wrist element 7 supported rotatably relative to the second arm 5 about a fourth axis J4, and a second wrist element 8 supported rotatably about a fifth axis J5 relative to the first wrist element 7. The wrist unit 6 also includes a third wrist element 9 supported rotatably about a sixth axis J6 relative to the second wrist element 8. In other words, the robot 1 includes six rotational joints (hereinafter simply referred to as joints) A1 to A6 that can rotate about six axes J1 to J6, respectively.

[0011] Each of the joints A1 to A6 is equipped with a motor and a reducer (not shown), and is driven with high torque by amplifying the torque generated by the motor using the reducer. Therefore, as will be described later, the torques T1 to T6 generated by each of the joints A1 to A6 correspond to the current values ​​for driving the corresponding motors.

[0012] The tool 30 is attached to the tip of the third wrist element 9, and is a screw tightening tool such as a nut runner that has a tool tip 32 such as a socket into which a bolt head 31a of the bolt 31 is fitted. The tool 30 rotates the tool tip 32 about a screw tightening axis C1, thereby rotating the bolt 31 fitted into the tool tip 32 about its axis C2 and fastening it into a workpiece 33. The tool 30 can switch between low-speed, low-torque rotation and high-speed, high-torque rotation in response to a command signal sent from the control device 20, for example.

[0013] The work object 33 is a device, machine, or member that has one or more screw holes 33a for fastening the bolts 31. The work object 33 is fixed directly or indirectly to an installation surface B such as a floor surface.

[0014] The sensors 40 for detecting the external force Fs are, for example, torque sensors 40a to 40f built into each of the joints A1 to A6 of the robot 1, and detect torques T1 to T6 acting on each of the joints A1 to A6 about the axes J1 to J6. That is, the sensors 40 detect the external force Fs acting on the robot 1 as torques T1 to T6 acting on each of the joints A1 to A6 about the axes J1 to J6.

[0015] The control device 20 according to this embodiment includes at least one processor and at least one memory such as a ROM and a RAM. The processor executes an operation program stored in the memory to operate the robot 1, and also controls the force of the robot 1 according to a control program stored in the memory.

[0016] Here, force control refers to control that moves the robot 1 in a direction that releases the external force Fs when the external force Fs is calculated based on the torques T1 to T6 detected by the sensor 40. The control method for the robot according to this embodiment will be described below using as an example a case where a bolt 31 that is loosely fastened in a screw hole 33a of a work object 33 is retightened, as shown in FIG.

[0017] 3, when the operation program is started, the processor sets the force control gain to a high gain (step S1). Next, the processor determines whether or not the screw tightening operation according to the operation program has been completed (step S2). If the screw tightening operation has not been completed, the processor calculates the external force Fs acting on the tip of the third wrist element 9 based on the torques T1 to T6 detected by the sensor 40 (step S3).

[0018] Specifically, the processor calculates a Jacobian matrix based on the posture information of the robot 1, and calculates an external force Fs acting on the tip of the robot 1 based on the torques T1 to T6 detected by the sensor 40 and the Jacobian matrix. The Jacobian matrix is ​​calculated using Equation 1.

[0019]

[0020] Here, s １ ~s ６ are directional vectors along the axes J1 to J6 of the joints A1 to A6, respectively. １ ~r ６ are position vectors directed from the joints A1 to A6 to the point of action of the external force Fs acting on the tip of the third wrist element 9, respectively.

[0021] Furthermore, a vector V, which is a combination of the angular velocity ω and translational velocity v of the tip of the third wrist element 9 of the robot 1, can be expressed as in Equation 2.

[0022] Here, θ' is a vector that brings together the angular velocities of all of the joints A1 to A6. By utilizing the relationship shown in Equation 2, the external force Fs, which is a combination of the force and moment acting on the third wrist element 9 of the robot 1, can be calculated using Equation 3.

[0023] Here, τ is a vector that collects torques T1 to T6 acting on all of the joints A1 to A6.

[0024] Execution of the operation program operates the tool 30, and the tool tip 32 is rotated around the screw tightening axis C1 at low speed and low torque, while the tool tip 32 is brought close to the bolt head 31 a by the operation of the robot 1. When the tool tip 32 comes into contact with the bolt head 31 a, an external force is generated that pushes the tool tip 32 back from the bolt head 31 a.

[0025] The processor determines whether any of the components of the calculated external force Fs exceeds a predetermined threshold value (step S4). The predetermined threshold value is stored in the memory for each component.

[0026] If none of the components of the external force Fs exceeds the threshold, the processor operates the robot 1 in a direction to release the external force Fs based on the calculated external force Fs (copy control: step S5). For example, as shown in FIG. 2, assume that an external force Fz in a direction along the screw tightening axis C1 and external forces Fx and Fy in directions perpendicular to the external force Fz are generated. In this case, the tool 30 is moved in the same direction as the external forces Fx and Fy in the directions perpendicular to the screw tightening axis C1 (the direction of arrow D). Force control is not performed for the external force Fz.

[0027] This allows the screw tightening axis C1 of the tool tip 32 to be aligned with the axis C2 of the bolt 31, and the tool tip 32 to be fitted into the bolt head 31 a. When the tool tip 32 is fitted into the bolt head 31 a, the processor outputs a command signal in the operating program to operate the tool 30 at high speed and with high torque.

[0028] As the tool tip 32 continues to rotate due to the operation of the tool 30, the bolt 31 is tightened into the screw hole 33a, and the bolt head 31a abuts against the workpiece 33. At this point, the tool 30 and the workpiece 33 are mutually constrained about the screw tightening axis C1, and the moment about the screw tightening axis C1 increases rapidly.

[0029] If it is determined in step S4 that any component of the external force Fs exceeds a predetermined threshold, the processor reduces the gain of the force control (step S6). That is, the processor performs the force control and switches the degree of force control to a lower level in a state in which the tool 30 and the workpiece 33 are mutually constrained.

[0030] The processor then repeats the process from step S2, and if it is determined in step S2 that the screw tightening is complete, the processor ends the screw tightening operation. The completion of the screw tightening can be determined based on the duration of fastening using the high-speed, high-torque screw tightening tool, etc.

[0031] The operation of the robot system 10 according to this embodiment configured as described above will be described below. With the robot system 10 according to this embodiment, the control device 20 performs force control of the robot 1 with a relatively high gain until the tool tip 32 fits into the bolt head 31 a.

[0032] That is, the control device 20 performs highly responsive force control to move the robot 1 in a direction that releases the external force Fs until the tool tip 32 fits into the bolt head 31 a. This allows the robot 1 to quickly align the screw tightening axis C1 with the axis C2 of the bolt 31.

[0033] When the screw tightening axis C1 coincides with the axis C2 of the bolt 31, the tool tip 32, which is rotating at a low speed, fits into the bolt head 31a at any phase around the screw tightening axis C1. The fitting of the tool tip 32 into the bolt head 31a can be detected from the coordinate values ​​of each of the joints A1 to A6 of the robot 1.

[0034] On the other hand, the control device 20 reduces the gain of the force control and the responsiveness of the force control after detecting an increase in the tightening torque by the tool tip 32. As a result, even if the moment around the screw tightening axis C1 increases suddenly, the robot 1 does not actively move in a direction to reduce the moment.

[0035] As a result, during the operation of tightening the bolt 31 with the tool 30, the movement of the robot 1 in a direction that relieves the moment acting around the screw tightening axis C1 is restricted, and the force acting on the tool 30 in a direction perpendicular to the screw tightening axis C1 increases. However, the inventors have found that switching the gains in this way significantly reduces the moments acting around the axes J1 to J6 of the robot 1 compared to when the gains are not switched.

[0036] 4 is a schematic diagram of a model during a screw tightening operation using a tool 30 attached to the tip of the third wrist element 9 of the robot 1. The robot 1 is constrained to an installation surface B at the base 2 at its lower end. When the tool tip 32 is fitted into the bolt head 31 a, the fitting of the tool tip 32 with the bolt head 31 a also constrains the tool tip 32 at the tip to the installation surface B.

[0037] It can be considered that when the force control gain is large, the degree of constraint between the tool tip 32 and the installation surface B is small, and when the force control gain is small, the degree of constraint between the tool tip 32 and the installation surface B is large. This is similar to the relationship between a cantilever beam and a doubly supported beam shown in Figures 5A and 5B.

[0038] 5A, one end 50a of the beam 50 is fixed, and the other end 50b is not fixed. Therefore, when a moment M acts on the beam 50, the other end 50b is largely displaced, causing the beam 50 to deform significantly. As a result, a large stress acts on the one end 50a of the beam 50.

[0039] 5B, both ends 50a, 50b of the beam 50 are fixed, so when a moment M acts on the beam 50, a force F acts on both ends 50a, 50b of the beam 50, reducing the deformation of the beam 50. As a result, the stress acting on one end 50a of the beam 50 is reduced compared to the case of a cantilever beam. In other words, the force F acting on both ends 50a, 50b of the beam 50 supports the beam 50, thereby reducing the stress acting on the beam 50.

[0040] By lowering the force control gain while the tool tip 32 and the bolt head 31a are engaged, it is possible to constrain both ends of the robot 1, that is, the base 2 and the tool tip 32, to the installation surface B. This allows the forces acting on both ends of the robot 1 to support the robot 1 in the same way as a doubly supported beam, and suppresses deformation occurring at each of the joints A1 to A6 of the robot 1. As a result, it is possible to reduce the load acting on each of the joints A1 to A6 compared to when the force control gain is high.

[0041] As described above, according to this embodiment, by switching the degree of force control, it is possible to quickly engage the tool tip 32 with the bolt head 31a and reduce the load on each of the joints A1 to A6 during the screw tightening operation. According to this embodiment, the moment about the fifth axis J5 and the sixth axis J6 is likely to increase due to a sudden increase in torque about the screw tightening axis C1 caused by tightening the bolt 31. By reducing the moment about the fifth axis J5 and the sixth axis J6, it is possible to prevent the moment about each of the axes J1 to J6 from exceeding the allowable moment, thereby maintaining the robot 1 in a healthy state.

[0042] In this embodiment, the external force Fs acting on the tip of the robot 1 is estimated based on the torques T1 to T6 acting on each of the joints A1 to A6 detected by a sensor 40 consisting of torque sensors 40a to 40f built into each of the joints A1 to A6. Alternatively, a six-axis force sensor may be placed at any position on the robot 1, and the external force Fs may be estimated based on the six-axis forces and moments detected by the force sensor. The six-axis force sensor is capable of detecting six force components consisting of forces in three mutually orthogonal axis directions and moments about the three axes, and may be placed, for example, only between the third wrist element 9 and the tool 30.

[0043] Alternatively, a secondary encoder may be provided for each of the joints A1 to A6. The external force Fs can be estimated based on the difference between the position of each of the joints A1 to A6 output by the secondary encoder and the position of each of the joints A1 to A6 calculated based on the encoder provided in the motor. The secondary encoder is provided separately from the encoder provided in the motor, and is a detector that directly detects the amount of displacement of each of the joints A1 to A6.

[0044] Furthermore, it is not necessary to provide a sensor 40 for detecting the external force Fs, and the external force Fs may be estimated based on the current values ​​of the motors for driving the joints A1 to A6, etc. Estimation of the external force Fs based on the forces and moments of the six axes, the positional deviations of the joints A1 to A6, the current values ​​of the motors, etc. may also be performed using a Jacobian matrix, similar to the case where it is based on the torques T1 to T6.

[0045] In this embodiment, all components of the external force Fs are monitored, and the force control gain is switched when any of the components exceeds a predetermined threshold. Alternatively, only specific external force components that increase particularly due to the screw tightening operation may be monitored. An example of an external force component that increases particularly due to the screw tightening operation is an external force component in a direction perpendicular to the screw tightening axis C1. The force control gain may also be switched based on the moment about the screw tightening axis C1 detected by the sensor 40.

[0046] Furthermore, instead of switching the gain of the force control, the force control may be switched between enabled and disabled. Switching the force control to disabled also switches the degree of the force control. Furthermore, instead of switching the gain of the force control, the force control conditions may be switched. For example, switching between pressing control and no-pressing control, or switching the pressing force, etc. may be possible.

[0047] Furthermore, although the degree of force control is switched by monitoring the external force Fs, the degree of force control may alternatively be switched by monitoring the engagement between the tool tip 32 and the bolt head 31a. The presence or absence of engagement may be detected by monitoring the coordinate values ​​of the robot 1 or the amount of movement (movement distance) of the robot 1. Furthermore, if the time required from the start to the end of engagement by force control can be estimated to some extent, the operation time may be monitored.

[0048] In addition, in this embodiment, the case where the tool tip 32 consisting of a socket is fitted to the bolt head 31a of a hexagonal bolt or the like has been exemplified, but instead, a hexagonal wrench may be fitted to the bolt head of a hexagonal socket head bolt. Furthermore, the present invention is not limited to fastening the bolt 31, but may also be applied to fastening a nut to a stud bolt or the like.

[0049] Also, the example shows a case where a socket is fitted onto a bolt 31 loosely fastened in a threaded hole 33a of a workpiece 33 and then the bolt 31 is fastened. Alternatively, as shown in Fig. 6, the present invention may be applied to a case where a screw tightening tool supports a bolt 31 having a fitting outer surface 31b, and the bolt 31 is tightened into a threaded hole 33a having a fitting inner surface 33b provided in the workpiece 33. In this case, the fitting outer surface 31b is fitted into the fitting inner surface 33b by force control, and after the fitting, the gain of the force control is reduced and the screw tightening operation is performed.

[0050] Furthermore, although the present embodiment has been described with reference to the robot system 10 that performs screw tightening work using a screw tightening tool, the present invention is not limited to this. The robot system 10, the control device 20, and the control method according to the present embodiment may be applied to any other work in which the posture of the tool 30 is first adjusted by force control and then the degree of force control is changed.

[0051] Other operations include, for example, drilling holes with a drill, tapping with a tapping tool, polishing with a polishing machine, stamping with a stamping machine, or operations using a tracer arm, moon lifter, etc.

[0052] In this embodiment, the gain of the force control is reduced when the external force component in the direction perpendicular to the screw tightening axis C1 exceeds a threshold. In this case, the stiffness matrix may be calculated according to the posture of the robot 1, and the amount of load reduction per amount of gain change may be estimated based on the stiffness matrix to search for the optimal gain.

[0053] Furthermore, although the gain of the force control is reduced after fitting of the tool tip 32 to the bolt head 31a compared to before fitting, the direction of switching the gain of the force control may be changed depending on the posture of the robot 1 when performing the screw tightening work. The load acting on each of the joints A1 to A6 due to an external force changes depending on the posture of the robot 1.

[0054] For example, when the robot 1 is in a position where external forces have a greater effect on the moments about the axes J5 and J6, the load on the joints A5 and A6 can be reduced by reducing the gain of the force control, as described above. Conversely, when the robot 1 is in a position where external forces have a greater effect on the moments about the axes J1 to J4 than on the axes J5 and J6, the gain of the force control can be increased. As a result, instead of increasing the moment about the axes J5 and J6, the moments about the axes J1 to J4 can be reduced, and the load factors of all the joints A1 to A6 can be kept below 100%.

[0055] Furthermore, when the force control of the robot 1 is switched to disabled after the tool tip 32 has been fitted into the bolt head 31a by the force control, the robot 1 may be operated to further increase the force about the screw tightening axis C1. In this case, for example, the robot 1 may be operated by positive feedback or positive offset. Furthermore, a component other than the force about the screw tightening axis C1 may be increased.

[0056] Since the optimal gain for force control varies depending on the mounting orientation of the tool 30 attached to the tip of the third wrist element 9 and the orientation of the robot 1, the gain that minimizes the load may be learned by performing the test multiple times. The system may also be provided with an input means that allows the user to select whether or not to switch the degree of force control. The input means may be any input device, such as a switch button displayed on the monitor of the control device 20 or teaching console, a keyboard, or a mouse.

[0057] Furthermore, in order to confirm the effect of switching the degree of force control, a load factor (%), which is the ratio of the moment about the axis J1 to J6 of each joint A1 to A6 to the respective allowable moment, may be displayed or stored. The load factor may be displayed or stored for each joint, or may be displayed or stored for only specific joints A1 to A6. Alternatively, the load factor may be displayed or stored as the sum or average of the load factors of all joints A1 to A6, or as an evaluation value calculated using any other function.

[0058] Instead of the moments of the joints A1 to A6, the load rates of other arbitrary components may be displayed or stored. It is also preferable to display or store the name of the operation program and the line number of the operation program in which the load rate or the like is maximized in association with each other.

[0059] In this embodiment, the control device 20 that controls the robot 1 calculates the external force Fs based on the torques T1 to T6 detected by the torque sensors 40 a to 40 f. Alternatively, the calculation of the external force Fs based on the torques T1 to T6 may be performed by a processing device different from the control device 20.

[0060] The robot system 10, the control device 20 for the robot 1, and the control method according to this embodiment have been described above, but a control program for causing a computer to execute the above-described control method may also be configured.

[0061] The following supplementary notes are further disclosed regarding the above-described embodiments and modifications: (Supplementary Note 1) A robot system comprising a robot, a control device for controlling the robot, and a tool attached to the robot for performing a predetermined task on a work object, wherein the control device performs force control based on an external force acting on the robot to operate the robot in a direction to release the external force, and switches the degree of the force control so as to reduce the load acting on the robot when the tool and the work object are mutually constrained.

[0062] (Supplementary Note 2) The robot system according to Supplementary Note 1, further comprising a sensor that detects an external force acting on the robot.

[0063] (Supplementary Note 3) The robot system according to Supplementary Note 1, wherein the control device calculates an external force acting on the robot.

[0064] (Supplementary Note 4) The robot system according to any one of Supplementary Note 1 to Supplementary Note 3, wherein the load acting on the robot is at least one of the loads acting on each joint of the robot, and is calculated based on the external force and the posture of the robot when the external force is detected.

[0065] (Supplementary Note 5) The robot system according to Supplementary Note 4, wherein the load acting on each of the joints is a force or moment in multiple directions.

[0066] (Supplementary Note 6) The robot system according to any one of Supplementary Note 1 to Supplementary Note 5, wherein the control device switches the degree of the force control based on at least one component of the external force.

[0067] (Supplementary Note 7) The robot system according to any one of Supplementary Note 1 to Supplementary Note 5, wherein the control device switches the degree of the force control based on an operation time or a movement distance of the robot.

[0068] (Supplementary Note 8) The robot system according to any one of Supplementary Note 1 to Supplementary Note 7, wherein the switching of the degree of the force control is switching of a gain of the force control.

[0069] (Supplementary Note 9) The robot system according to any one of Supplementary Note 1 to Supplementary Note 7, wherein the switching of the degree of force control is switching of a target value of the force control.

[0070] (Supplementary Note 10) The robot system according to any one of Supplementary Note 1 to Supplementary Note 7, wherein the switching of the degree of force control is switching between enabling and disabling the force control.

[0071] (Supplementary Note 11) The robot system according to any one of Supplementary Note 1 to Supplementary Note 10, wherein the control device determines a direction in which the force control is switched based on a posture of the robot.

[0072] (Supplementary Note 12) The robot system according to any one of Supplementary Note 1 to Supplementary Note 11, wherein the tool is a screw tightening tool, and the control device switches the degree of the force control based on at least a moment around a screw tightening axis.

[0073] (Supplementary Note 13) The robot system according to any one of Supplementary Note 1 to Supplementary Note 11, wherein the tool is a screw tightening tool, and the control device switches the degree of the force control based on at least a force in a direction perpendicular to a screw tightening axis.

[0074] (Supplementary Note 14) A control device for a robot equipped with a tool that performs a predetermined task on a work object, the control device comprising at least one processor, the processor performing force control to operate the robot in a direction that relieves an external force acting on the robot based on the external force acting on the robot, and switching the degree of the force control so as to reduce the load acting on the robot when the tool and the work object are mutually constrained.

[0075] (Supplementary Note 15) A control method for a robot equipped with a tool that performs a predetermined task on a work object, the control method performing force control to move the robot in a direction that releases an external force acting on the robot based on the external force, and switching the degree of the force control so that the load acting on the robot is reduced when the tool and the work object are mutually constrained.

[0076] (Appendix 16) A control program for a robot equipped with a tool that performs a predetermined task on a work object, the control program causing a computer to perform force control that causes the robot to move in a direction that relieves an external force acting on the robot, and to switch the degree of the force control so that the load acting on the robot is reduced when the tool and the work object are mutually constrained.

[0077] REFERENCE SIGNS LIST 1 Robot 10 Robot system 20 Control device 30 Tool 33 Work object 40 Sensor 40a, 40b, 40c, 40d, 40e, 40f Torque sensor A1, A2, A3, A4, A5, A6 Rotational joint (joint) C1 Screw tightening axis Fs External force

Claims

1. A robot system comprising: a robot; a control device for controlling the robot; and a tool attached to the robot and performing a specified task on a work object, wherein the control device performs force control to operate the robot in a direction to relieve an external force acting on the robot, based on the external force acting on the robot, and switches the degree of the force control so as to reduce the load acting on the robot when the tool and the work object are mutually constrained.

2. The robot system according to claim 1, further comprising a sensor for detecting an external force acting on the robot.

3. The robot system according to claim 1, wherein the control device calculates an external force acting on the robot.

4. A robot system according to any one of claims 1 to 3, wherein the load acting on the robot is at least one of the loads acting on each joint of the robot, and is calculated based on the external force and the posture of the robot when the external force is detected.

5. A robot system according to claim 4, wherein the load acting on each of said joints is a force or moment in multiple directions.

6. A robot system according to any one of claims 1 to 5, wherein the control device switches the degree of the force control based on at least one component of the external force.

7. A robot system according to any one of claims 1 to 5, wherein the control device switches the degree of the force control based on the operation time or travel distance of the robot.

8. A robot system according to any one of claims 1 to 7, wherein the switching of the degree of force control is a switching of a gain of the force control.

9. A robot system according to any one of claims 1 to 7, wherein the switching of the degree of force control is a switching of a target value of the force control.

10. A robot system according to any one of claims 1 to 7, wherein the switching of the degree of force control is switching between enabling and disabling the force control.

11. A robot system according to any one of claims 1 to 10, wherein the control device determines a direction in which the force control is switched based on the posture of the robot.

12. A robot system according to any one of claims 1 to 11, wherein the tool is a screw tightening tool, and the control device switches the degree of the force control based on at least a moment about a screw tightening axis.

13. A robot system according to any one of claims 1 to 11, wherein the tool is a screw tightening tool, and the control device switches the degree of the force control based on at least a force in a direction perpendicular to a screw tightening axis.

14. A control device for a robot equipped with a tool that performs a specified task on a work object, the control device having at least one processor, the processor performing force control to move the robot in a direction to relieve an external force acting on the robot, based on the external force acting on the robot, and switching the degree of the force control so as to reduce the load acting on the robot when the tool and the work object are mutually constrained.

15. A control method for a robot equipped with a tool that performs a specified task on a work object, comprising the steps of: implementing force control to move the robot in a direction that relieves an external force acting on the robot, based on the external force acting on the robot; and switching the degree of the force control so as to reduce the load acting on the robot when the tool and the work object are mutually constrained.

16. A control program for a robot equipped with a tool that performs a specified task on a work object, the control program causing a computer to carry out force control to move the robot in a direction that relieves an external force acting on the robot, and to switch the degree of the force control so as to reduce the load acting on the robot when the tool and the work object are mutually constrained.

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