Robot controlling method, and robot system

Abstract

To provide a technique for maintaining retention force during non-energization of a piezoelectric actuator.SOLUTION: A method includes the steps of: (a) determining whether or not the retention force during non-energization of a piezoelectric actuator included in a tool part mounted at the tip of a robot arm is sufficient; (b) executing retention force recovery operation of the piezoelectric actuator, when it is determined that the retention force is insufficient; and (c) executing a robot work to a workpiece, when it is determined that the retention force is sufficient.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present disclosure relates to a method for controlling a robot and a robot system.

Background Art

[0002] Patent Document 1 discloses a robot having a piezoelectric actuator at the tip of a robot arm. The piezoelectric actuator is used to compensate for the position error of the robot's end effector.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The piezoelectric actuator has the advantage of being able to maintain a holding force even when not energized. However, when the piezoelectric actuator is not operated, the holding force of the piezoelectric actuator decreases over time. When the holding force of the piezoelectric actuator decreases, there is a problem that the position of the piezoelectric actuator shifts when the inertial force or external force during robot operation is large, and the position accuracy deteriorates.

Means for Solving the Problems

[0005] According to a first aspect of the present disclosure, there is provided a method for controlling a robot. The method includes: (a) determining whether the holding force of a piezoelectric actuator included in a tool unit attached to the tip of a robot arm is sufficient when the piezoelectric actuator is not energized; (b) when it is determined that the holding force is insufficient, performing a holding force recovery operation of the piezoelectric actuator; and (c) when it is determined that the holding force is sufficient, performing a robot operation on a workpiece.

[0006] According to a second aspect of the present disclosure, a robot system is provided. The robot system includes a robot including a robot arm, a tool unit including a piezoelectric actuator and attached to the tip of the robot arm, and a control device. The control device is configured to execute: (a) a process of determining whether the holding force when the piezoelectric actuator is not energized is sufficient; (b) a process of executing a holding force recovery operation of the piezoelectric actuator when it is determined that the holding force is insufficient; and (c) a process of executing a robot operation on a workpiece when it is determined that the holding force is sufficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0008] A. First Embodiment: FIG. 1 is an explanatory diagram showing an example of a robot system in an embodiment. The robot system 10 includes a robot 100, a robot controller 200 that controls the robot 100, an information processing apparatus 300, a first pedestal 510 on which a workpiece WK is placed, and a second pedestal 520 on which a force sensor 710 is placed. The information processing apparatus 300 is, for example, a personal computer.

[0009] In FIG. 1, three axes X, Y, and Z that define an orthogonal coordinate system in a three-dimensional space are depicted. The X-axis and the Y-axis are horizontal axes, and the Z-axis is a vertical axis. In this example, the XYZ coordinate system is a robot coordinate system with a reference point preset in the robot 100 as the origin.

[0010] The robot 100 includes a base 110 and a robot arm 120. A tool unit 600 is attached to the tip 122 of the robot arm 120. The tool unit 600 has a moving device 610 and an ink ejection device 620 as an end effector.

[0011] The moving device 610 has a piezoelectric actuator 611, a movable stage 612 that is driven by the piezoelectric actuator 611 to move, and an attachment 613 fixed to the movable stage 612. The moving device 610 is used, for example, to finely adjust the printing position by the ink ejection device 620.

[0012] As the moving device 610, for example, a single-axis stage capable of moving the movable stage 612 in a single-axis direction can be used. Also, a multi-axis stage capable of moving the movable stage 612 in two or more directions may be used. In the present embodiment, a single-axis stage is used as the moving device 610, and the movable stage 612 is driven by the piezoelectric actuator 611 to be movable along the moving direction Dm. The piezoelectric actuator 611 can maintain a holding force even when not energized. That is, the movable stage 612 is held in position by the holding force of the piezoelectric actuator 611 even when the piezoelectric actuator 611 is not energized. In the present embodiment, the holding force direction Dh of the piezoelectric actuator 611 is a direction parallel to the moving direction Dm of the movable stage 612.

[0013] An ink ejection device 620 is attached to the movable stage 612 via an attachment 613. The attachment 613 is an attachment auxiliary member for attaching other devices and members to the movable stage 612. However, the attachment 613 may be omitted and the ink ejection device 620 may be directly attached to the movable stage 612. The movable stage 612 and the attachment 613 are holding force acting members on which the holding force of the piezoelectric actuator 611 acts.

[0014] The ink ejection device 620 has an ink ejection head 622. The ink ejection head 622 performs printing by ejecting ink onto a printing area on the surface of the workpiece WK. It is preferable that the ink ejection head 622 can eject a plurality of types of ink to perform color printing. The printing area on the surface of the workpiece WK may not be a flat surface and may include a curved surface portion. As can be understood from this description, the robot system 10 of the present embodiment functions as a DTS (Direct To Shape) printing device that prints on the surface of a three-dimensional object.

[0015] As the end effector, any device or mechanism other than the ink ejection device 620 can be used. For example, as the end effector, a coating device for applying a liquid agent, a welding device for performing welding, a robot hand used for precision assembly, etc. can be used.

[0016] The force sensor 710 is used when checking the holding force of the piezoelectric actuator 611. As the force sensor 710, for example, a load cell can be used. Alternatively, a three-axis force sensor or a six-axis force sensor may be used. The force sensor 710 is supported by an elastic member 712 such as a spring, and thus is devised so that the position displacement of the force sensor 710 can be increased. However, the elastic member 712 may be omitted.

[0017] Near the tip of the robot arm 120, a TCP (Tool Center Point) is set as a control point of the robot 100. In the example of FIG. 1, the TCP is set near the surface of the ink ejection head 622. Note that the control point TCP can be set at an arbitrary position.

[0018] The robot arm 120 is sequentially connected by six joints J1 to J6. Among these joints J1 to J6, three joints J2, J3, and J5 are bending joints, and the other three joints J1, J4, and J6 are twisting joints. In this embodiment, a six-axis robot is illustrated, but it is possible to use a robot having an arbitrary robot arm mechanism having a plurality of joints. Further, although the robot 100 of this embodiment is a vertical articulated robot, a horizontal articulated robot may be used.

[0019] FIG. 2 is a block diagram showing the functions of the information processing apparatus 300. The information processing apparatus 300 includes a processor 310, a memory 320, and an interface circuit 330. An input device 340 and a display device 350 are connected to the interface circuit 330, and a robot controller 200 is also connected thereto. The robot 100, the moving device 610, the ink ejection device 620, and the force sensor 710 are connected to the information processing apparatus 300 via the robot controller 200. However, a part of these may be directly connected to the information processing apparatus 300.

[0020] The processor 310 has a function as a robot work execution unit 312. The robot work execution unit 312 executes a robot work using a robot control program RP. In the present embodiment, the robot work is a work of printing on a workpiece WK using the ink ejection device 620.

[0021] The functions of the robot work execution unit 312 are respectively realized by the processor 310 executing a computer program stored in the memory 320. However, a part or all of the functions of the robot work execution unit 312 may be realized by a hardware circuit.

[0022] The memory 320 stores a robot control program RP created by the teaching process. The robot control program RP is composed of a plurality of instructions for operating the robot 100 and includes coordinate values of a plurality of teaching points.

[0023] As described in the prior art, when the piezoelectric actuator 611 is not operated, the holding force of the piezoelectric actuator 611 decreases over time. Therefore, it is desirable to confirm whether the holding force of the piezoelectric actuator 611 is sufficient at an appropriate timing when no robot work is being performed.

[0024] FIG. 3 is an explanatory diagram showing the state at the time of confirming the holding force in the first embodiment. In this example, by moving the robot arm 120, the attachment 613 is pressed against the force sensor 710, whereby an external force is applied to the attachment 613. The application direction Df of the external force is opposite to the holding force direction Dh of the piezoelectric actuator 611. At this time, the piezoelectric actuator 611 is maintained in a non-energized state. Confirmation of whether the holding force of the piezoelectric actuator 611 is sufficient can be performed using, for example, any of the following confirmation methods M1 and M2.

[0025] <Confirmation method M1 of holding force> (1a) While applying an external force along the direction Df opposite to the holding force direction Dh to the attachment 613, which is the holding force acting member, the external force is measured by the force sensor 710 to obtain a force measurement value. At this time, it is preferable to gradually increase the external force and unload when the force measurement value reaches a predetermined reference value. (1b) Using the force measurement value, it is determined whether the holding force of the piezoelectric actuator 611 when non-energized is sufficient. This determination can be performed, for example, as follows. (1b-1) When the force measurement value reaches the reference value without decreasing, it is determined that the holding force of the piezoelectric actuator 611 is sufficient. If the measured force value decreases before reaching the reference value, calculate the amount of decrease. (1b-3) If the amount of decrease in the measured force value is less than a preset decrease threshold value and the measured force value then increases and reaches the reference value, it is determined that the holding force of the piezoelectric actuator 611 is sufficient. (1b-4) If the amount of decrease in the measured force value is greater than or equal to the decrease threshold value, it is determined that the holding force of the piezoelectric actuator 611 is insufficient.

[0026] <Method M2 for Confirming Holding Force> (2a) Apply an external force to the attachment 613, which is a holding force acting member, along the direction Df opposite to the holding force direction Dh. At this time, it is preferable to gradually increase the external force and unload when the measured force value measured by the force sensor 710 reaches a predetermined reference value. (2b) While applying the external force, use the position sensor 614 included in the moving device 610 to measure the position of the movable stage 612, which is a holding force acting member. Since the attachment 613 is fixed to the movable stage 612, the position of the movable stage 612 can be considered as the position of the attachment 613. As the position sensor 614, for example, an encoder can be used. (2c) Use the position measurement value by the position sensor 614 to determine whether the holding force of the piezoelectric actuator 611 when not energized is sufficient. This determination can be executed, for example, as follows. (2c-1) Measure the amount of change in the position of the movable stage 612 before and after the application of the external force. (2c-2) If the amount of change in the position of the movable stage 612 is less than a preset change threshold value, it is determined that the holding force of the piezoelectric actuator 611 is sufficient. (2c-3) If the amount of change in the position of the movable stage 612 is greater than or equal to the change threshold value, it is determined that the holding force of the piezoelectric actuator 611 is insufficient.

[0027] Note that many force sensors 710 such as load cells sense force with only a slight displacement, and there is a possibility that a large force may be generated just by slightly moving the robot arm 120. For this reason, the elastic member 712 is used to push the force sensor 710 toward the attachment 613 side, so that the displacement of the force sensor 710 can be increased, and a structure is devised so that an excessively large force does not occur. Further, a sliding component such as a linear guide may be used to configure the force sensor 710 to be slidable. Further, instead of applying an external force to the attachment 613 by moving the robot arm 120, an external force may be applied to the attachment 613 by moving the force sensor 710 using an actuator that moves the force sensor 710. In this case, the elastic member 712 can be omitted.

[0028] In the example of FIG. 3, the holding force is confirmed for one holding force direction Dh, but a plurality of force sensors 710 having different directions may be arranged to confirm the holding force for each of the plurality of holding force directions. Further, by using only one force sensor 720 and changing the joint angle of the robot arm 120, the holding force for each of the plurality of holding force directions may be confirmed. For example, in the configuration of FIG. 3, by rotating the wrist joint J6 of the robot arm 120 by 180 degrees, it is possible to confirm the holding force in one holding force direction Dh and the holding force in the opposite direction, respectively.

[0029] FIG. 4 is a flowchart showing the processing procedure of the robot work. In step S10, a new workpiece WK is prepared as a work target. The preparation of the new workpiece WK may be performed, for example, by another robot or by an operator.

[0030] In step S11, the robot work execution unit 312 checks the holding force of the piezoelectric actuator 611 when it is not energized. In step S12, the robot work execution unit 312 determines whether or not the holding force of the piezoelectric actuator 611 is sufficient. The processes of step S11 and step S12 can be executed according to the above-described confirmation method M1 or M2.

[0031] When the holding force of the piezoelectric actuator 611 is sufficient, the process proceeds to step S13 described later. On the other hand, when the holding force of the piezoelectric actuator 611 is insufficient, the process proceeds to step S17, and the robot work execution unit 312 executes a holding force recovery operation of the piezoelectric actuator 611. The holding force recovery operation is an operation of driving the piezoelectric actuator 611 to repeat the reciprocating motion of the movable stage 612. The movement stroke of the piezoelectric actuator 611 in the holding force recovery operation is preferably larger than the movement stroke of the piezoelectric actuator 611 in the robot work.

[0032] Steps S13 to S15 are robot work. In step S13, the robot work execution unit 312 moves the tool unit 600 to the printing start position of the next printing area. Here, it is assumed that one or more printing areas are set on the surface of the work WK. Also, when a plurality of printing areas are set, it is assumed that they are set at positions separated from each other. It is preferable to maintain the piezoelectric actuator 611 in a non-energized state during the movement of the tool unit 600 in step S13.

[0033] In step S14, the robot work execution unit 312 performs printing on the printing area using the ink ejection device 620. At this time, fine adjustment of the printing position may be performed using the piezoelectric actuator 611. Specifically, for example, when performing printing in one printing range, it is conceivable to execute a "line feed operation" of moving the ink ejection device 620 in the sub-scanning direction during printing. This line feed operation may be executed by the operation of the robot arm 120, or may be executed using the moving device 610. In any of these cases, if fine adjustment of the printing position is performed using the piezoelectric actuator 611, the line feed operation can be performed with high accuracy.

[0034] However, during the period when ink is being ejected from the ink ejection head 622, it is preferable to keep the piezoelectric actuator 611 de-energized. That is, generally, during the execution of the work by the end effector, it is preferable to operate the end effector with the piezoelectric actuator 611 kept de-energized. By doing so, during the execution of the work by the end effector, the position of the end effector can be held using the holding force of the piezoelectric actuator 611.

[0035] In step S15, the robot work execution unit 312 determines whether there is a next printing area where printing has not been completed. If there is a next printing area, it returns to step S13 and re-executes the robot work of steps S13 to S15. On the other hand, if there is no next printing area, it proceeds to step S16, and the robot work execution unit 312 moves the tool unit 600 to the home position by operating the robot arm 120. The home position is the initial position and initial posture in the state before the start of the work and is preset according to the type of work. During the movement in step S16, it is preferable to keep the piezoelectric actuator 611 de-energized. After step S16, it returns to step S10, a new workpiece WK is prepared, and the processes after step S11 are executed again.

[0036] The confirmation of the holding force of the piezoelectric actuator 611 in steps S11 and S12 described above and the holding force recovery operation in step S17 are executed at a timing when no robot work is being performed. In this way, if the holding force confirmation and recovery operations are performed at a timing when no robot work is being performed, even if a holding force confirmation process or a holding force recovery process is added, the production capacity can be maintained without being reduced.

[0037] In step S14 indicated by the double frame, the piezoelectric actuator 611 is energized and operates. Also, as described above, in step S17 as well, the piezoelectric actuator 611 may be energized and operate. In other steps, it is preferable that the piezoelectric actuator 611 is maintained in a non-energized state. If the holding force of the piezoelectric actuator 611 when non-energized has decreased, there is a possibility that the position and orientation of the ink ejection device 620 may change along with the operation of the robot arm 120 in step S13 or step S16. Therefore, if the holding force of the piezoelectric actuator 611 is confirmed and the recovery operation is performed as described above, it is possible to prevent the problem that the position and orientation of the ink ejection device 620 change during the operation of the robot arm 120.

[0038] As described above, in the first embodiment, it is determined whether the holding force of the piezoelectric actuator 611 when non-energized is sufficient. If it is determined that the holding force is insufficient, a holding force recovery operation of the piezoelectric actuator 611 is executed. Also, when it is determined that the holding force is sufficient, a robot operation on the workpiece WK is executed. As a result, the robot operation can be executed while maintaining the holding force of the piezoelectric actuator 611 when non-energized.

[0039] B. Second Embodiment: FIG. 5 is an explanatory diagram showing the state at the time of holding force confirmation in the second embodiment. In the second embodiment, a force sensor 720 is provided at the tip of the robot arm 120. As this force sensor 720, for example, a 6-axis force sensor may be used.

[0040] A block 714 supported by an elastic member 712 is placed on the second pedestal 520. The block 714 is used to apply an external force to the attachment 613 of the tool part 600. When applying an external force to the attachment 613, the block 714 may be moved, or the tool part 600 may be moved using the robot arm 120.

[0041] The device configuration of the second embodiment is the same as that of the first embodiment except for the force sensor and its installation position. Also in the second embodiment, the confirmation methods M1 and M2 described in the first embodiment can be applied. Further, the processing procedure described with reference to FIG. 4 is also applicable to the second embodiment. The second embodiment also has substantially the same effects as the first embodiment.

[0042] C. Third Embodiment: FIG. 6 is an explanatory diagram showing the state at the time of confirming the holding force in the third embodiment. In the third embodiment, the force sensor is omitted, and instead, an acceleration sensor 730 is installed in the attachment 613. However, the acceleration sensor 730 may be installed at other positions of the tool unit 600. Although it is generally possible to specify acceleration as a robot operation, the acceleration applied to the attachment 613 may not necessarily match the acceleration specified as the robot operation. Therefore, it is preferable to accurately measure the acceleration of the attachment 613 using the acceleration sensor 730.

[0043] As a method for confirming the holding force of the piezoelectric actuator 611 in the third embodiment, for example, the following method can be used. <Confirmation Method M3 of Holding Force> (3a) While operating the robot arm 120 so as to apply acceleration along the direction Da opposite to the holding force direction Dh of the piezoelectric actuator 611 to the movable stage 612 and the attachment 613 which are the holding force acting members, an acceleration measurement value is acquired using the acceleration sensor 730. In this case, the robot arm 120 is accelerated so that the acceleration reaches a preset acceleration reference value. (3b) Using the acceleration measurement value, it is determined whether or not the holding force of the piezoelectric actuator 611 when non-energized is sufficient. This determination can be executed, for example, as follows. (3b-1) The amount of change in the position of the movable stage 612 before and after the addition of acceleration is measured using the position sensor 614 included in the moving device 610. Since the attachment 613 is fixed to the movable stage 612, the position of the movable stage 612 can be considered as the position of the attachment 613. As the position sensor 614, for example, an encoder can be used. (3b-2) When the amount of change in the position of the movable stage 612 is less than a preset change threshold value, it is determined that the holding force of the piezoelectric actuator 611 is sufficient. (3b-3) When the amount of change in the position of the movable stage 612 is greater than or equal to the change threshold value, it is determined that the holding force of the piezoelectric actuator 611 is insufficient.

[0044] Note that instead of acceleration, the robot arm 120 may be accelerated so that the inertial force calculated by multiplying the acceleration and the load weight becomes a preset force reference value. In the above-described confirmation method M3, since the holding force of the piezoelectric actuator 611 can be confirmed without the force sensor 710 and its pressing structure described in FIG. 3, the structure of the entire device becomes simple and it is easy to save space.

[0045] Note that it is also possible to confirm the holding force in both the left and right directions using both acceleration and deceleration during one operation of the robot arm 120. Further, the confirmation of the holding force along a plurality of directions may be executed using a plurality of operations of the robot arm 120 instead of one operation of the robot arm 120. For example, with the wrist joint J6 of the robot arm 120 rotated 180 degrees to reverse the direction of the movable stage 612, the robot arm 120 can be moved in the same direction to confirm the holding force along a plurality of directions.

[0046] The device configuration of the third embodiment is the same as that of the first embodiment except that a force sensor and its pressing structure are not required. Also, the processing procedure described in FIG. 4 is applicable to the third embodiment. The third embodiment also has substantially the same effects as the first and second embodiments.

[0047] · Other forms: The present disclosure is not limited to the above-described embodiments, and can be realized in various forms without departing from the gist thereof. For example, the present disclosure can also be realized by the following aspects (aspect). The technical features in the above embodiments corresponding to the technical features in each of the following aspects can be appropriately replaced or combined in order to solve some or all of the problems of the present disclosure, or to achieve some or all of the effects of the present disclosure. Further, if the technical feature is not described as essential in this specification, it can be appropriately deleted.

[0048] (1) According to the first aspect of the present disclosure, a method for controlling a robot is provided. This method includes: (a) a step of determining whether the holding force when the piezoelectric actuator included in the tool part attached to the tip of the robot arm is sufficient; (b) a step of executing a holding force recovery operation of the piezoelectric actuator when it is determined that the holding force is insufficient; and (c) a step of executing a robot operation on the workpiece when it is determined that the holding force is sufficient. According to this method, the robot operation can be executed while maintaining the holding force of the piezoelectric actuator when it is not energized.

[0049] (2) In the above method, the tool part may include a holding force acting member on which the holding force of the piezoelectric actuator acts. The step (a) may include: (a1) a step of obtaining a force measurement value, which is a measured value of the external force by a force sensor, while applying an external force to the holding force acting member along a direction opposite to the holding force direction in which the holding force acts; and (a2) a step of determining whether the holding force is sufficient using the force measurement value. According to this method, it is possible to determine whether the holding force is sufficient using a force sensor.

[0050] (3) In the above method, the tool part may include a holding force acting member on which the holding force of the piezoelectric actuator acts and a position sensor for measuring the position of the holding force acting member. The step (a) may include: (a1) a step of obtaining a position measurement value, which is a measurement value of the position sensor, while applying an external force to the holding force acting member along a direction opposite to the holding force direction in which the holding force acts; and (a2) a step of determining whether the holding force is sufficient using the position measurement value. According to this method, it is possible to determine whether the holding force is sufficient by using a position sensor that measures the position of the holding force acting member.

[0051] (4) In the above method, the tool part may include a holding force acting member on which the holding force of the piezoelectric actuator acts. The step (a) may include: (a1) a step of obtaining an acceleration measurement value, which is a measurement value of the acceleration by an acceleration sensor, while operating the robot arm so as to apply an acceleration to the holding force acting member along a direction opposite to the holding force direction in which the holding force acts; and (a2) a step of determining whether the holding force is sufficient using the acceleration measurement value. According to this method, it is possible to determine whether the holding force is sufficient by using an acceleration sensor.

[0052] (5) In the above method, the holding force recovery operation may be executed such that the movement stroke of the holding force acting member on which the holding force of the piezoelectric actuator acts is larger than the movement stroke of the holding force acting member in the robot work. According to this method, the holding force of the piezoelectric actuator can be recovered.

[0053] (6) In the above method, the step (c) may include a step of executing work by the end effector of the robot while maintaining the piezoelectric actuator in a non-energized state. According to this method, during the execution of the operation by the end effector, the holding force of the piezoelectric actuator can be utilized to hold the position of the end effector.

[0054] (7) In the above method, the tool part may include an ink ejection head, and the robot operation may be a process of performing printing on a printing area including a curved surface part of the surface of the workpiece using the ink ejection head. According to this method, direct-to-shape printing can be accurately executed.

[0055] (8) According to a second aspect of the present disclosure, a robot system is provided. This robot system includes a robot including a robot arm, a tool part including a piezoelectric actuator and attached to the tip of the robot arm, and a control device. The control device is configured to execute: (a) a process of determining whether the holding force when the piezoelectric actuator is non-energized is sufficient; (b) a process of executing a holding force recovery operation of the piezoelectric actuator when it is determined that the holding force is insufficient; and (c) a process of executing a robot operation on a workpiece when it is determined that the holding force is sufficient.

[0056] The present disclosure can also be realized in various forms other than the above. For example, it can be realized in the form of a robot system including a robot and a robot control device, a computer program for realizing the functions of the robot control device, a non-transitory storage medium recording the computer program, and the like.

Description of Reference Numerals

[0057] 10... Robot system, 100... Robot, 110... Base, 120... Robot arm, 122... Tip, 200... Robot controller, 300... Information processing device, 310... Processor, 312... Robot work execution unit, 320... Memory, 330... Interface circuit, 340... Input device, 350... Display device, 510... First pedestal, 520... Second pedestal, 600... Tool unit, 610... Moving device, 611... Piezoelectric actuator, 612... Movable stage, 613... Attachment, 614... Position sensor, 620... Ink ejection device, 622... Ink ejection head, 710... Force sensor, 712... Elastic member, 714... Block, 720... Force sensor, 730... Acceleration sensor

Claims

1. A method for controlling a robot, comprising: (a) determining whether the holding force when the piezoelectric actuator included in the tool part attached to the tip of the robot arm is sufficient; (b) when it is determined that the holding force is insufficient, performing a holding force recovery operation of the piezoelectric actuator; (c) when it is determined that the holding force is sufficient, performing a robot operation on the workpiece. A method including the above.

2. The method according to claim 1, wherein the tool part includes a holding force acting member on which the holding force of the piezoelectric actuator acts, and the step (a) includes: (a1) obtaining a force measurement value, which is a measurement value of the external force by a force sensor, while applying an external force to the holding force acting member along a direction opposite to the holding force direction in which the holding force acts; (a2) determining whether the holding force is sufficient using the force measurement value. A method including the above.

3. The method according to claim 1, wherein the tool part includes a holding force acting member on which the holding force of the piezoelectric actuator acts and a position sensor for measuring the position of the holding force acting member, and the step (a) includes: (a1) obtaining a position measurement value, which is a measurement value of the position sensor, while applying an external force to the holding force acting member along a direction opposite to the holding force direction in which the holding force acts; (a2) determining whether the holding force is sufficient using the position measurement value. A method including the above.

4. The method according to claim 1, wherein the tool part includes a holding force acting member on which the holding force of the piezoelectric actuator acts, and the step (a) includes: (a1) obtaining an acceleration measurement value, which is a measurement value of the acceleration by an acceleration sensor, while operating the robot arm so as to apply an acceleration to the holding force acting member along a direction opposite to the holding force direction in which the holding force acts; (a2) determining whether the holding force is sufficient using the acceleration measurement value. A method including the above.

5. The method according to claim 1, wherein the holding force recovery operation is performed such that the movement stroke of the holding force acting member on which the holding force of the piezoelectric actuator acts is larger than the movement stroke of the holding force acting member in the robot operation.

6. The method according to claim 1, wherein The method, wherein step (c) includes a step of performing an operation by the end effector of the robot while maintaining the piezoelectric actuator in a non-energized state.

7. The method according to claim 1, wherein the tool part includes an ink ejection head, and the robot operation is a process of performing printing on a printing area including a curved surface part of the surface of the workpiece using the ink ejection head.

8. A robot system, comprising: a robot including a robot arm; a tool part including a piezoelectric actuator and attached to the tip of the robot arm; a control device; and the control device is configured to perform a process of determining whether a holding force when non-energized is sufficient for the piezoelectric actuator, perform a process of executing a holding force recovery operation of the piezoelectric actuator when it is determined that the holding force is insufficient, and perform a process of executing a robot operation on a workpiece when it is determined that the holding force is sufficient. A robot system.

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