A force-controlled bolt alignment method for robotic arms based on a rotational search strategy

Abstract

This invention discloses a force-controlled bolt alignment method for a robotic arm based on a rotational search strategy, comprising: S1, robotic arm pre-alignment, controlling the robotic arm to move to the pre-alignment position; S2, robotic arm linear motion contacting the bolt, controlling the robotic arm to move along the positive z-axis of the Cartesian tool coordinate system, stopping the robotic arm movement when the force in the negative z-axis direction exceeds a set threshold F1; S3, robotic arm rotational search, controlling the robotic arm to rotate clockwise around the z-axis, stopping the robotic arm rotation when the force in the negative z-axis direction is less than a set threshold F2 or the torque in the counterclockwise z-axis direction exceeds a set threshold T1; S4, robotic arm linear alignment into the bolt, controlling the robotic arm to move in the positive z-axis direction, stopping the robotic arm movement when the force in the negative z-axis direction exceeds a set threshold F3. This invention significantly improves the success rate and operational safety of the alignment process.

Description

Technical Field

[0001] This invention relates to the field of robotics, and in particular to a force-controlled bolt alignment method for a robotic arm based on a rotational search strategy. Background Technology

[0002] In overhead contact line robotic arms tightening bolts, it's typically necessary to align the end effector sleeve of the robotic arm with the bolt first. Traditionally, this is done by setting a waypoint through the robotic arm's position control to directly align it with the bolt. However, in actual operation, the bolt's angle can easily change. If a simple waypoint control is used to align the robotic arm with the bolt, the angle difference between the sleeve and the bolt could cause a rigid collision during alignment, leading to an abrupt stop of the robotic arm. This reduces the safety and success rate of bolt alignment. Summary of the Invention

[0003] To address the issues of low alignment success rate and low safety caused by sleeve and bolt angle errors in traditional bolt alignment methods, this invention proposes a robotic arm force-controlled bolt alignment method based on a rotation search strategy, thus solving the aforementioned problems.

[0004] This application discloses a force-controlled bolt alignment method for a robotic arm based on a rotational search strategy, comprising the following steps: S1. Pre-alignment of the robotic arm: Control the robotic arm to move to the pre-alignment position and make the robotic arm sleeve and the bolt coaxial, without requiring the sleeve to enter the alignment position in a straight line; S2. The robotic arm makes linear motion to contact the bolt, controls the robotic arm to move along the positive z-axis of the Cartesian space tool coordinate system, and acquires real-time force information. When the force acquired in the negative z-axis direction is greater than the set threshold F1, the robotic arm stops moving. S3. Robotic arm rotation search: Control the robotic arm to rotate clockwise around the z-axis of the Cartesian space tool coordinate system and obtain real-time force information. When the force obtained in the negative z-axis direction is less than the set threshold F2 or the torque in the counterclockwise direction around the z-axis is greater than the set threshold T1, stop the robotic arm rotation. S4. The robotic arm is aligned with the bolt in a straight line. The robotic arm is controlled to move along the positive z-axis of the Cartesian space tool coordinate system and real-time force information is obtained. When the force obtained in the negative z-axis direction is greater than the set threshold F3, the movement of the robotic arm is stopped.

[0005] Preferably, the Cartesian space tool coordinate system is oriented in the same direction as the robotic arm flange coordinate system, and the coordinate center point of the Cartesian space tool coordinate system is located at the geometric center point of the bottom surface of the robotic arm sleeve.

[0006] Preferably, step S2 includes the following steps: The coordinate relationship between the current robotic arm sleeve and the robotic arm base is obtained, and can be represented by a homogeneous transformation matrix as follows:

[0007] The 3*3 submatrix in the upper left corner represents the rotation matrix, and the 3*1 submatrix in the upper right corner represents the translation matrix. The pose of the robotic arm relative to the robotic arm base coordinate system after translation along the positive z-axis of the Cartesian tool coordinate system is:

[0008] in,

[0009] In the formula Let be the translation distance of the robotic arm along the positive z-axis, and , This refers to the distance between the robotic arm sleeve and the bolt when the robotic arm reaches the pre-aligned position in S1. Control the robotic arm to move along a straight path to When the robotic arm moves to During the process, force information is acquired in real time through a six-dimensional force sensor. When the force in the negative z-axis direction acquired by the six-dimensional force sensor is greater than the set threshold F1, the movement of the robotic arm is stopped.

[0010] Preferably, in step S2, a constant force F4 is used to control the robotic arm to move along the positive z-axis of the Cartesian tool coordinate system, and .

[0011] Preferably, step S3 includes the following steps: The coordinate relationship between the current robotic arm sleeve and the robotic arm base is obtained, and can be represented by a homogeneous transformation matrix as follows:

[0012] The 3*3 submatrix in the upper left corner represents the rotation matrix, and the 3*1 submatrix in the upper right corner represents the translation matrix. Because the bolt is a regular hexagon, and each side corresponds to a central angle of 60°, it can be guaranteed that regardless of the angle difference, the sleeve can be aligned with the bolt after rotating no more than 60°. The pose of the robotic arm relative to the robotic arm base coordinate system after rotating 60° clockwise around the z-axis of the Cartesian tool coordinate system is:

[0013] in,

[0014] Control the robotic arm sleeve to rotate to When the robotic arm sleeve rotates to During the process, force information is acquired in real time through a six-dimensional force sensor. When the force in the negative z-axis direction acquired by the six-dimensional force sensor is less than the set threshold F2, or when the torque in the counterclockwise z-axis direction acquired by the six-dimensional force sensor is greater than the set threshold T1, the rotation of the robotic arm is stopped.

[0015] Preferably, in step S3, a constant force F4 is used to control the robotic arm to move along the positive z-axis of the Cartesian space tool coordinate system.

[0016] Preferably, step S4 includes the following steps: The coordinate relationship between the current robotic arm sleeve and the robotic arm base is obtained, and can be represented by a homogeneous transformation matrix as follows:

[0017] The 3*3 submatrix in the upper left corner represents the rotation matrix, and the 3*1 submatrix in the upper right corner represents the translation matrix. The pose of the robotic arm relative to the robotic arm base coordinate system after translation along the positive z-axis of the Cartesian tool coordinate system is:

[0018] in,

[0019] In the formula Let be the translation distance of the robotic arm along the positive z-axis, and , This refers to the distance between the robotic arm sleeve and the bolt when the robotic arm finishes rotating in S3. Control the robotic arm to move along a straight path to When the robotic arm moves to During the process, force information is acquired in real time through a six-dimensional force sensor. When the negative force in the z-axis direction acquired by the six-dimensional force sensor is greater than the set threshold F3, the movement of the robotic arm is stopped, and the alignment is completed.

[0020] Preferably, in step S4, a constant force F5 is used to control the robotic arm to move along the positive z-axis of the Cartesian tool coordinate system, and .

[0021] The beneficial effects of this invention are: This invention uses information from a six-dimensional force sensor to determine the contact state between the robotic arm and the bolt in real time. During the rotational search process of the sleeve at the end of the robotic arm, the force information fed back by the six-dimensional force sensor can be used to determine whether the sleeve and the bolt are aligned. This allows the system to effectively compensate for possible angular errors between the bolt and the sleeve, thereby significantly improving the success rate of the alignment process and operational safety. Attached Figure Description

[0022] Figure 1 This is a flowchart of a robotic arm force-controlled bolt alignment method based on a rotation search strategy according to an embodiment of the present invention. Figure 2 This is a schematic diagram showing the dimensions of the sleeve and bolt according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the Cartesian space tool coordinate system according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the linear motion contact bolt of the robotic arm according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the robotic arm sleeve rotation search according to an embodiment of the present invention; Figure 6 This is a schematic diagram illustrating the successful rotational search of the robotic arm sleeve according to an embodiment of the present invention. Figure 7 This is a schematic diagram illustrating the linear entry of the robotic arm sleeve into the bolt contact point and stopping according to an embodiment of the present invention. Figure 8 This is a schematic diagram illustrating the relationship between three attitude changes during the alignment process according to an embodiment of the present invention; Figure 9 This is a schematic diagram of the forces acting on the sleeve during the rotational search process without force-position hybrid control, according to an embodiment of the present invention. Figure 10 This is a schematic diagram of the force applied when the aligned torque stops rotating, according to an embodiment of the present invention, without the use of force-position hybrid control. Figure 11 This is a schematic diagram of the force on the sleeve during the rotational search process using force-position hybrid control, according to an embodiment of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments.

[0024] This application example uses UR's e-series robot. The end of the UR robot's robotic arm is equipped with a sleeve, and the tool flange connection of the robotic arm is equipped with a six-dimensional force sensor. The six-dimensional sensor can measure the force and torque load in the X, Y, and Z directions and feed it back to the UR robot.

[0025] The dimensions of the simple sleeve and bolt structures involved are as follows: Figure 2 As shown, the sleeve edge distance is 19.5mm, the bolt edge distance is 18mm, and the edge distance tolerance of the sleeve and bolt is 1.5mm. If the tolerance is too large, the sleeve may not be able to provide the tightening torque and may slip. If the tolerance is too small, it will reduce the alignment tolerance and reduce the alignment success rate. In this application, the tolerance range between the sleeve and bolt is controlled to be greater than 1mm and less than 3mm according to different bolt sizes.

[0026] This application discloses a force-controlled bolt alignment method for robotic arms based on a rotational search strategy, such as... Figure 1 As shown, it includes the following steps: S1. Pre-align the robotic arm, control the robotic arm to move to the pre-aligned position, and make the robotic arm sleeve and the bolt coaxial.

[0027] Set the Cartesian space tool coordinate system to be aligned with the robotic arm flange coordinate system. The center point of the Cartesian space tool coordinate system should be located at the geometric center of the bottom surface of the robotic arm sleeve. Figure 3 As shown. Ensure the Z-axis of the sleeve is coaxial with the center axis of the bolt to be aligned, and keep the sleeve 2 to 5 cm away from the bolt surface. It is not necessary for the sleeve to enter the alignment position in a straight line. Figure 4 As shown.

[0028] S2. The robotic arm moves linearly to contact the bolt, controlling the robotic arm to move along the positive z-axis of the Cartesian tool coordinate system and acquiring real-time force information. When the force acquired in the negative z-axis direction is greater than the set threshold F1, the robotic arm stops moving.

[0029] The coordinate relationship between the current robotic arm sleeve and the robotic arm base is obtained, and can be represented by a homogeneous transformation matrix as follows:

[0030] The 3x3 submatrix in the upper left corner represents the rotation matrix, and the 3x1 submatrix in the upper right corner represents the translation matrix. , , These represent the offsets along the x-axis, y-axis, and z-axis, respectively.

[0031] The pose of the robotic arm relative to the robotic arm base coordinate system after translation along the positive z-axis of the Cartesian tool coordinate system is:

[0032] in,

[0033] In the formula Let be the translation distance of the robotic arm along the positive z-axis, and , In this embodiment, the distance between the robotic arm sleeve and the bolt is the distance when the robotic arm reaches the pre-alignment position in S1. b is 2 to 5 centimeters.

[0034] like Figure 4 As shown, the MoveL command controls the robotic arm to move along a straight path to... When the robotic arm moves to During the process, force information is acquired in real time through a six-dimensional force sensor. When the force in the negative z-axis direction acquired by the six-dimensional force sensor is greater than the set threshold F1 (15N in this implementation), the MoveL command is stopped for 0.2s to ensure that the contact force between the robotic arm and the bolt is stable at the threshold F3 in the next stage.

[0035] Optionally, as the robotic arm moves along a straight path to... During the process, a constant force F4 (15N in this embodiment) is used to control the robotic arm to move along the positive z-axis of the Cartesian tool coordinate system, and When the negative z-axis force obtained by the six-dimensional force sensor is greater than the set threshold F1 (15N in this implementation), the robot is stopped for 1 second by the StopL command to ensure that the contact force between the robot arm and the bolt is stable at the threshold F1 in the next stage.

[0036] S3. Robotic arm rotation search: Control the robotic arm to rotate clockwise around the z-axis of the Cartesian space tool coordinate system and obtain real-time force information. When the force obtained in the negative z-axis direction is less than the set threshold F2 or the torque in the counterclockwise direction around the z-axis is greater than the set threshold T1, stop the robotic arm rotation.

[0037] Since S2 is interrupted when the contact force between the robotic arm and the bolt exceeds the set threshold F1, the robotic arm in the current pose has a contact force with the sleeve that exceeds the set threshold F1. The coordinate relationship between the current robotic arm sleeve and the robotic arm base is obtained, and represented by a homogeneous transformation matrix as follows:

[0038] The 3x3 submatrix in the upper left corner represents the rotation matrix, and the 3x1 submatrix in the upper right corner represents the translation matrix. , , These represent the offsets along the x-axis, y-axis, and z-axis, respectively.

[0039] Because the bolt is a regular hexagon, and each side corresponds to a central angle of 60°, it can be guaranteed that regardless of the angle difference, the sleeve can be aligned with the bolt after rotating no more than 60°. The pose of the robotic arm relative to the robotic arm base coordinate system after rotating 60° clockwise around the z-axis of the Cartesian tool coordinate system is:

[0040] in,

[0041] like Figure 5 As shown, the MoveL command controls the robotic arm's sleeve to rotate to... When the robotic arm sleeve rotates to During the process, force information is acquired in real time through a six-dimensional force sensor. When the force in the negative z-axis direction acquired by the six-dimensional force sensor is less than the set threshold F2 (5N in this embodiment), it can be determined that there is no contact between the bolt and the robotic arm sleeve, that is, the robotic arm sleeve and the bolt are aligned in the current pose. The robotic arm rotation is stopped by the StopL command. Alternatively, when the torque in the counterclockwise z-axis direction acquired by the force sensor is greater than the set threshold T1 (1.5Nm in this embodiment), it indicates that the sleeve has been aligned and entered the bolt. If rotation continues, it may damage the bolt and the robotic arm, so the robotic arm rotation is stopped by the StopL command. The positional relationship between the sleeve and the bolt in this state is as follows: Figure 6 As shown, the force curve during this process is as follows: Figure 10 As shown in the figure, Fz represents the force along the z-axis, with positive values ​​along the positive z-axis and negative values ​​along the negative z-axis. Frz represents the torque, with positive values ​​along the clockwise direction and negative values ​​along the counterclockwise direction.

[0042] Optionally, a force-position hybrid control method can be used when the robotic arm sleeve rotates to... During the process, a constant force F4 controls the robotic arm to move along the positive z-axis of the Cartesian tool coordinate system. Currently, the positive z-axis is controlled by a constant force of magnitude F4, while the other directions are controlled by position. This means that during the movement, the homogeneous attitude transformation matrix of the sleeve... Only change the value of the top-left 3x3 submatrix. , , The value will not be affected by right multiplication. Change, and The value is determined by a constant force F4 in the positive z-axis direction, ensuring that the contact force between the robotic arm sleeve and the bolt in the z-axis direction is F4. Based on real-time force feedback from the six-dimensional force sensor, if the current force along the positive z-axis is less than the set threshold F2, it can be determined that there is no contact between the bolt and the robotic arm sleeve, meaning the robotic arm sleeve and bolt are aligned in the current pose, and the robotic arm rotation is stopped using the StopL command. If the current counterclockwise torque along the z-axis is greater than the set threshold T1 (1.5 Nm), it indicates that the robotic arm sleeve has aligned and entered the bolt; continued rotation may damage the bolt and the robotic arm, so the robotic arm rotation is also stopped using the StopL command. The advantage of using a force-position hybrid control method for rotational search and alignment is that the sleeve maintains a contact force of approximately F4 with the bolt during the search process, avoiding misjudgments of alignment position due to insufficient contact between the robotic arm sleeve and the bolt, thus improving the alignment success rate. Simultaneously, the constant force control at the moment of contact between the robotic arm sleeve and the bolt can buffer the impact force between the bolt and the robotic arm sleeve, improving alignment safety.

[0043] S4. The robotic arm is aligned with the bolt in a straight line. The robotic arm is controlled to move along the positive z-axis of the Cartesian space tool coordinate system and real-time force information is obtained. When the force obtained in the negative z-axis direction is greater than the set threshold F3, the movement of the robotic arm is stopped.

[0044] The coordinate relationship between the current robotic arm sleeve and the robotic arm base is obtained, and can be represented by a homogeneous transformation matrix as follows:

[0045] The 3x3 submatrix in the upper left corner represents the rotation matrix, and the 3x1 submatrix in the upper right corner represents the translation matrix. , , These represent the offsets along the x-axis, y-axis, and z-axis, respectively.

[0046] The pose of the robotic arm relative to the robotic arm base coordinate system after translation along the positive z-axis of the Cartesian tool coordinate system is:

[0047] in,

[0048] In the formula Let be the translation distance of the robotic arm along the positive z-axis, and In this embodiment , This is the distance between the robotic arm sleeve and the bolt when the robotic arm finishes rotating in S3.

[0049] like Figure 7 As shown, the MoveL command controls the robotic arm to move along a straight path to... When the robotic arm moves to During the process, force information is acquired in real time through a six-dimensional force sensor. When the negative force in the z-axis direction acquired by the six-dimensional force sensor is greater than the set threshold F3 (15N in this embodiment), the movement of the robotic arm is stopped by the StopL command, and the alignment is completed.

[0050] Optionally, a constant force F5 is used to control the robotic arm to move along the positive z-axis of the Cartesian tool coordinate system, and When the negative z-axis force detected by the six-dimensional force sensor exceeds the set threshold F3, the robotic arm stops moving via the StopL command, and alignment is completed.

[0051] A schematic diagram of the three attitude changes during the alignment process is shown below. Figure 8 As shown. By comparing the force curves of the sleeve throughout the entire process. Figure 9 and Figure 11 It can be observed that adding constant force control can significantly reduce the impact force during contact, and the force is more stable during the contact process.

[0052] In summary, the force-controlled bolt alignment method for robotic arms based on a rotational search strategy disclosed in this application addresses the problems of low alignment success rate and low safety caused by sleeve and bolt angle errors in traditional alignment methods by incorporating force-controlled rotational search during coaxial bolt alignment. This application optimizes the bolt alignment process for robotic arms, improves the success rate and safety of bolt alignment, and is suitable for high-precision positioning scenarios such as contact wire bolt tightening.

[0053] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for force-controlled bolt alignment using a robotic arm based on a rotational search strategy, characterized in that, Includes the following steps: S1. Pre-alignment of the robotic arm: control the robotic arm to move to the pre-alignment position and make the robotic arm sleeve coaxial with the bolt; S2. The robotic arm makes linear motion to contact the bolt, controls the robotic arm to move along the positive z-axis of the Cartesian space tool coordinate system, and acquires real-time force information. When the force acquired in the negative z-axis direction is greater than the set threshold F1, the robotic arm stops moving. S3. Robotic arm rotation search: Control the robotic arm to rotate clockwise around the z-axis of the Cartesian space tool coordinate system and obtain real-time force information. When the force obtained in the negative z-axis direction is less than the set threshold F2 or the torque in the counterclockwise direction around the z-axis is greater than the set threshold T1, stop the robotic arm rotation. S4. The robotic arm is aligned with the bolt in a straight line. The robotic arm is controlled to move along the positive z-axis of the Cartesian space tool coordinate system and real-time force information is obtained. When the force obtained in the negative z-axis direction is greater than the set threshold F3, the movement of the robotic arm is stopped.

2. The robotic arm force-controlled bolt alignment method based on a rotational search strategy according to claim 1, characterized in that, The Cartesian space tool coordinate system is aligned with the robotic arm flange coordinate system, and the coordinate center point of the Cartesian space tool coordinate system is located at the geometric center point of the bottom surface of the robotic arm sleeve.

3. The robotic arm force-controlled bolt alignment method based on a rotational search strategy according to claim 2, characterized in that, S2 includes the following steps: The coordinate relationship between the current robotic arm sleeve and the robotic arm base is obtained, and can be represented by a homogeneous transformation matrix as follows: The 3*3 submatrix in the upper left corner represents the rotation matrix, and the 3*1 submatrix in the upper right corner represents the translation matrix. The pose of the robotic arm relative to the robotic arm base coordinate system after translation along the positive z-axis of the Cartesian tool coordinate system is: in, In the formula Let be the translation distance of the robotic arm along the positive z-axis, and , This refers to the distance between the robotic arm sleeve and the bolt when the robotic arm reaches the pre-aligned position in S1. Control the robotic arm to move along a straight path to When the robotic arm moves to During the process, force information is acquired in real time through force sensors. When the force sensor acquires a force in the negative z-axis direction that is greater than the set threshold F1, the movement of the robotic arm is stopped.

4. The robotic arm force-controlled bolt alignment method based on a rotational search strategy according to claim 3, characterized in that, In S2, a constant force F4 is used to control the robotic arm to move along the positive z-axis of the Cartesian tool coordinate system, and .

5. The robotic arm force-controlled bolt alignment method based on a rotational search strategy according to claim 4, characterized in that, S3 includes the following steps: The coordinate relationship between the current robotic arm sleeve and the robotic arm base is obtained, and can be represented by a homogeneous transformation matrix as follows: The 3*3 submatrix in the upper left corner represents the rotation matrix, and the 3*1 submatrix in the upper right corner represents the translation matrix. The pose of the robotic arm relative to the robotic arm base coordinate system after rotating 60° clockwise around the z-axis of the Cartesian tool coordinate system is: in, Control the robotic arm sleeve to rotate to When the robotic arm sleeve rotates to During the process, force information is acquired in real time through force sensors. When the force sensor acquires a force in the negative z-axis direction that is less than the set threshold F2, or when the force sensor acquires a torque in the counterclockwise z-axis direction that is greater than the set threshold T1, the robotic arm rotation is stopped.

6. The robotic arm force-controlled bolt alignment method based on a rotational search strategy according to claim 5, characterized in that, In S3, a constant force F4 is used to control the robotic arm to move along the positive z-axis of the Cartesian space tool coordinate system.

7. The robotic arm force-controlled bolt alignment method based on a rotational search strategy according to claim 6, characterized in that, S4 includes the following steps: The coordinate relationship between the current robotic arm sleeve and the robotic arm base is obtained, and can be represented by a homogeneous transformation matrix as follows: The 3*3 submatrix in the upper left corner represents the rotation matrix, and the 3*1 submatrix in the upper right corner represents the translation matrix. The pose of the robotic arm relative to the robotic arm base coordinate system after translation along the positive z-axis of the Cartesian tool coordinate system is: in, In the formula Let be the translation distance of the robotic arm along the positive z-axis, and , This refers to the distance between the robotic arm sleeve and the bolt when the robotic arm finishes rotating in S3. Control the robotic arm to move along a straight path to When the robotic arm moves to During the process, force information is acquired in real time through force sensors. When the force sensor acquires a force in the negative z-axis direction that is greater than the set threshold F3, the movement of the robotic arm is stopped, and the alignment is completed.

8. The robotic arm force-controlled bolt alignment method based on a rotational search strategy according to claim 7, characterized in that, In S4, a constant force F5 is used to control the robotic arm to move along the positive z-axis of the Cartesian space tool coordinate system, and .

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