A mechanical arm sampling control method in a weak gravity environment

Abstract

The application provides a mechanical arm end segment impedance motion control method. In a weak gravity environment, whether the end touches the ground is judged according to the torque value of each joint of the mechanical arm; if not, the mechanical arm moves according to the end linear planning mode, stops moving after moving to the expected end position; if yes, the mechanical arm moves according to the end impedance mode until a stop instruction is received and the motion is stopped. The method can achieve an effect equivalent to that of the traditional end impedance control method with a control frequency of 100 Hz at a control frequency of 20 Hz, improves the calculation efficiency, saves the calculation resources, superimposes a dynamic feedforward compensation component in the end impedance mode, reduces the error in the end motion process, shortens the end motion stabilization time, adopts a friction torque compensation mode of anti-drive reverse compensation and non-anti-drive positive compensation on the basis of the joint expected torque calculated in the end impedance mode, and improves the touch force control precision in the weak gravity touch sampling process of the mechanical arm end.

Description

Technical Field

[0001] This invention relates to a sampling control method for a robotic arm in a weak gravity environment, belonging to the field of robotics technology. Background Technology

[0002] Robotic arms are commonly used in extraterrestrial object exploration. Numerous interplanetary exploration missions, both domestically and internationally, utilize robotic arms for sampling. The operating environments of these probes are mostly the Moon or Mars. While the gravity of the Moon or Mars is weaker than Earth's (approximately 1 / 6 and 3 / 8 of Earth's gravity, respectively), the large mass of the probes (generally at least several hundred kilograms) makes the reaction force of the robotic arm during sampling negligible compared to the probe's gravity. Therefore, the robotic arm only needs to focus on providing the necessary trajectory and force for sampling. Even if the robotic arm's trajectory interacts with the surface of the planet, generating a force, it is insignificant compared to the probe's gravity, and precise control of the reaction force is unnecessary.

[0003] In existing technologies, OSIRIS targets asteroids and operates in a weak gravitational environment. However, OSIRIS employs a "touch-and-go" sampling method, relying on springs in the sampling device to depress the device and bounce the probe upon contact with the asteroid surface. This eliminates the need to consider the active movement of the robotic arm towards the surface and its reaction force on the probe (for the robotic arm, the contact process is passive). Therefore, it is necessary to optimize the control technologies for the sampling tool's trajectory and contact force, taking into account the weak gravitational environment and unknown conditions inherent in extraterrestrial asteroid exploration missions. Summary of the Invention

[0004] The technical problem solved by this invention is to overcome the shortcomings of the prior art and propose a sampling control method for a robotic arm in a weak gravitational environment, which solves the problem of force-position hybrid control of the end effector of a robotic arm on the surface of a weak gravitational celestial body.

[0005] The technical solution of this invention is: a sampling control method for a robotic arm in a weak gravity environment, comprising: In a weak gravity environment, the torque value of each joint of the robotic arm is used to determine whether the end effector touches the ground; If it does not touch the ground, the robotic arm moves according to the end-effector linear planning pattern and stops moving after reaching the desired end-effector pose. If it has touched the ground, the robotic arm will move in end-effector impedance mode until it receives a stop command and stops moving.

[0006] The step of determining whether the end effector is in contact with the ground based on the torque value of each joint of the robotic arm includes: If the torque value of any joint of the robotic arm is greater than the set threshold, it is judged to be in contact with the ground; otherwise, it is in a non-contact state.

[0007] If the object does not touch the ground, it will move according to the end-line planning mode, including: Calculate the desired speed of each joint of the robotic arm; Each joint moves at the desired speed, driving the distal end to move toward the desired distal end pose; Once the end effector is detected to have moved to the desired end effector pose, the robotic arm stops moving and maintains its current position.

[0008] The desired velocities of each joint of the robotic arm are calculated by multiplying the desired end-effector velocity by the inverse of the Jacobian matrix.

[0009] After contact with the ground, the movement follows an end-impedance mode, including: After contact with the ground, the desired torque of the joint is calculated based on the desired end force. ; Obtain the desired torque of the joint Then, friction torque compensation is performed. The robotic arm joints begin to move according to the new expected joint torque.

[0010] The desired torque of the joint

[0011] In the formula: For the desired torque of the joint, The calculated components of the desired end force. This is the feedforward compensation component for the robotic arm's dynamics.

[0012] The friction torque compensation process includes: when the joint is in reverse drive, the desired torque is subtracted from the friction torque to obtain the new desired joint torque; when the joint is in forward drive or stationary, the desired torque is added to the friction torque to obtain the new desired joint torque.

[0013] The advantages of this invention compared to the prior art are: This invention employs a segmented control method combining end-point linear programming and end-point impedance modes. It achieves the same effect as the traditional end-point impedance control method with a control frequency of 100Hz at a control frequency of 20Hz, thereby improving computational efficiency and saving computational resources.

[0014] This invention superimposes a dynamic feedforward compensation component on the calculated component of the desired end force, thereby reducing the error in the end motion process and shortening the end motion stabilization time.

[0015] Based on the desired joint torque, this invention adopts a friction torque compensation method of reverse drive and reverse compensation and non-reverse drive and positive compensation, which improves the ground contact force control accuracy during the weak gravity ground contact sampling process at the end of the robotic arm. Attached Figure Description

[0016] Figure 1This is a coordinate system diagram of the robotic arm.

[0017] Figure 2 This is the posture change curve of the robotic arm's end effector.

[0018] Figure 3 This is the contact force curve between the end effector of the robotic arm and the satellite display. Detailed Implementation

[0019] This invention discloses a sampling control method for a robotic arm in a weak gravity environment, the specific implementation of which is as follows: 1. Determine whether the end effector is in contact with the ground based on the torque value of each joint of the robotic arm, specifically: When the torque value of any joint of the robotic arm ( (Joint number), greater than the set joint ground contact detection threshold. If the signal is positive, the state is considered to be in contact with the ground. Otherwise, the state is considered not in contact with the ground.

[0020] Once the system is determined to be in contact with the ground, it will remain in contact with the ground regardless of changes in the joint torque value, and will not return to the non-contact state.

[0021] 2. If the current state is not in contact with the ground, then the movement follows the end-line planning mode, specifically: Based on the desired end effector velocity, calculate the desired velocities of each joint of the robotic arm. The calculation formula is as follows:

[0022] In the formula: For the desired speed of the robotic arm joint, It is the inverse of the Jacobian matrix. The desired terminal velocity.

[0023] The joint moves at the calculated desired speed. The movement drives the end effector to move toward the desired end effector position.

[0024] Once the end effector is detected to have moved to the desired end effector pose, the robotic arm stops moving and maintains its current position.

[0025] 3. If the current state is in contact with ground, it will move according to the end impedance mode, specifically: 1) After contact with the ground, calculate the expected torque of the joint based on the expected end force, using the following formula:

[0026] In the formula: For the desired torque of the joint, The calculated components of the desired end force. This is the feedforward compensation component for the robotic arm's dynamics.

[0027] The calculation formula is as follows:

[0028] In the formula: This is the transpose of the Jacobian matrix. Let be the desired stiffness matrix. Let be the desired damping matrix. For the desired terminal velocity, This is the actual end velocity. For the desired end pose, This represents the actual end-effector pose. The desired end force.

[0029] The calculation is performed recursively using the Newton-Euler equations.

[0030] 2) Obtain the desired torque of the joint Then, friction torque compensation is performed, following these steps: In reverse drive mode, if :

[0031] in For joint serial numbers, For the first The expected torque of each joint, For the first The frictional torque of each joint.

[0032] if :

[0033] In other states (forward motion or stationary): if :

[0034] if :

[0035] 3) The robotic arm joints are arranged according to their respective serial numbers. Output torque.

[0036] A preliminary simulation model of the robotic arm was built using the MATLAB simmechanics environment, and the process of the robotic arm's end effector moving to contact the star table was simulated.

[0037] The robotic arm used in this example is a 4-DOF robotic arm, and its coordinate system is established as follows: Figure 1As shown in the figure, the unit of length is millimeters.

[0038] The main parameter values ​​are shown in Table 1: Table 1. Values ​​of Main Parameters

[0039] Figure 2 The results show that before contact with the satellite surface, without dynamic feedforward, the maximum attitude angle change is approximately 4° (compared to the initial attitude of -10°). After adding dynamic feedforward, the maximum attitude angle change is approximately 2°, and the attitude error at the end point is reduced by 50%. After contact with the satellite surface, without dynamic feedforward, the attitude angle at the end point stabilizes at 0° after two oscillation cycles, finally settling at approximately 22 seconds. With dynamic feedforward, the attitude angle at the end point begins to stabilize towards 0° immediately after contact with the satellite surface and stabilizes at approximately 14 seconds, reducing the stabilization time by approximately 30%. Therefore, dynamic feedforward has a significant effect on improving both the attitude error and stabilization time of the end-point motion.

[0040] During the contact process, the contact force between the end and the star surface is as follows: Figure 3 As shown, except at the moment of contact, the contact force after stabilization is less than 5N.

[0041] Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention based on the disclosed technical content without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A sampling control method for a robotic arm in a weak gravity environment, characterized in that, include: In a weak gravity environment, the torque value of each joint of the robotic arm is used to determine whether the end effector touches the ground; If it does not touch the ground, the robotic arm moves according to the end-effector linear planning pattern and stops moving after reaching the desired end-effector pose. If it has touched the ground, the robotic arm will move in end-effector impedance mode until it receives a stop command and stops moving. After contact with the ground, the movement follows an end-impedance mode, including: After contact with the ground, the desired torque of the joint is calculated based on the desired end force. ; Obtain the desired torque of the joint Then, friction torque compensation is performed. The robotic arm joints begin to move according to the new expected joint torque; The desired torque of the joint In the formula: For the desired torque of the joint, The calculated components of the desired end force. This is the feedforward compensation component for the robotic arm's dynamics.

2. The sampling control method for a robotic arm in a weak gravity environment according to claim 1, characterized in that, The step of determining whether the end effector is in contact with the ground based on the torque value of each joint of the robotic arm includes: If the torque value of any joint of the robotic arm is greater than the set threshold, it is judged to be in contact with the ground; otherwise, it is in a non-contact state.

3. The sampling control method for a robotic arm in a weak gravity environment according to claim 1, characterized in that, If the object does not touch the ground, it will move according to the end-line planning mode, including: Calculate the desired speed of each joint of the robotic arm; Each joint moves at the desired speed, driving the distal end to move toward the desired distal end pose; Once the end effector is detected to have moved to the desired end effector pose, the robotic arm stops moving and maintains its current position.

4. The sampling control method for a robotic arm in a weak gravity environment according to claim 3, characterized in that, The desired velocities of each joint of the robotic arm are calculated by multiplying the desired end-effector velocity by the inverse of the Jacobian matrix.

5. The sampling control method for a robotic arm in a weak gravity environment according to claim 1, characterized in that, The friction torque compensation process includes: when the joint is in reverse drive, the desired torque is subtracted from the friction torque to obtain the new desired joint torque; when the joint is in forward drive or stationary, the desired torque is added to the friction torque to obtain the new desired joint torque.

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