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Wen | Mo Qingyan
Editor|Mo Qingyan
preface
Robots exhibit extremely high working and execution capabilities in the fields of processing, production and assembly, and are highly adaptable to harsh working environments, despite great advances in robotics.
However, in the face of increasingly complex tasks, the load capacity and working space of a single robot are limited, and some complex large-scale load tasks cannot be completed by a single robot.
In this case, a multi-robot collaborative system is required, and multiple robots have several advantages over a single robot, with better flexibility, collaboration and reliability.
Multi-robot systems have been widely used in medical fields, aerospace, and industrial applications such as welding and assembly, and researchers have proposed a multi-robotic, integral cell design and motion planning optimization method to reduce the pure human work of assembling metal panels.
Widely used
Also using a tactile dual-arm master, from a remote operating system to measure, the interaction between tissues and tools is forced, and direct force feedback is found to minimize tissue damage.
According to the theory, multi-robot systems have been widely used to complete high-precision, large-load and complex tasks, showing high efficiency and ideal results.
The calibration of the dual robot system mainly solves the problem of the coordinate matrix relationship of the robot hand eye, base coordinates and tool flange, among which the calibration of the single robot system also includes the calibration of the robot hand eye and tool flange.
Therefore, the calibration of the coordinate system of the base of the double robot is the basis, premise and necessary condition for the cooperative operation of the two robots, and there are two main calibration methods for the basic coordinate system of the double robot, namely contact and non-contact.
Contact calibration uses special tools for assisted positioning
Non-contact calibration generally obtains the attitude relationship of two robots through external sensors such as cameras and laser trackers, and researchers propose a calibration method for three-dimensional laser sensors and robot attitudes.
Experimental results show that the proposed method has good accuracy in predicting hand-eye transformation covariance, and the researchers propose a linear solution given the transformation from the robot base frame to the robot flange.
By measuring the pose of the robot end effector, the transformation matrix from the robot world to the robot base, from the robot tool to the robot flange coordinate frame can be obtained.
The scientists converted the under-actuated robot hand and soft finger calibration problem into an AX=YB problem and compared three methods based on nonlinear optimization and evolutionary calculations.
In addition, the researchers also used the binocular vision system and coordinate transformation theory to obtain the spatial attitude relationship between the base coordinates and the base coordinates of the grinding robot.
The scientists proposed a two-robot kinematics modeling and base frame calibration method for automated drilling and riveting systems for aircraft panel assemblies.
The relative attitude of the coordination robot was obtained by labeling the image to meet the real-time calibration requirements, and the experimental results showed that the accuracy of the calibration method reached 2 mm and 0.1°, respectively.
In order to reduce the error of the above calibration method, researchers proposed a linear approximation iterative algorithm to solve the matrix equation AXB = YCZ.
The simultaneous calibration of hand-eye, tool flange, robot and robot in the two-robot system is realized, and the two groups of researchers use respectively, iterative and probabilistic methods to solve X, Y and Z in the multi-robot system.
However, the calculation time of the iterative method is long, and the probabilistic method is greatly affected by noise interference, so the researchers combine the closed method based on the Kronecker product with the iterative method of convex function optimization problem to propose a new two-robot calibration method.
This method improves the efficiency and accuracy of iterations, and the researchers propose a DA-based calibration problem, quaternion and singular value factorization algorithms.
Simulation and experiments verify the high calibration accuracy of the method, and the researchers propose a combined solution based on the two-quaternion closed solution and the Levenberg Marquardt iterative solution to solve the unknown parameters in the AXB = YCZ equation, and realize the calibration of the two-robot system in orthopedic surgery, although these non-contact calibration methods can be automated.
However, the complexity and high cost of two-robot systems using this method, and sensor measurement errors lead to calibration errors, which limit their batch application in industrial production.
And to calculate the relative position relationship of the basic coordinate system, the researchers proposed a two-robot base coordinate calibration method that only uses a series of "hand buckle" operations and corresponding joint information, and uses quaternions and Lagrange multiplier methods to iterate the orthogonal normalized rotation matrix.
However, when using these methods, the calculation process is complex, and manual operation amplifies the calibration error, and the researchers propose a method to calibrate the robot's basic coordinate system using unit quaternions.
Improved orthogonality of the rotation matrix, which measured five different tool center points, but the tool center point operation was time-consuming and inaccurate.
The researchers used the three-point calibration method to complete the calibration of the coordinate system of the base of the welding process of the double robot, and by adding a calibration strip at the end of the double robot, driving the double robot to position at three points in space, solving the attitude transformation matrix of the double robot, and the researchers measured the coordinates of the three non-collinear calibration points.
To realize the conversion between the basic coordinates of the two robots, they propose an optimization method based on Lie algebraic exponential mapping to ensure the orthogonality of the rotation matrix, and the researchers propose a calibration method based on three non-collinear points to establish a new coordinate system and complete the calibration through the transformation between coordinate systems.
The researchers proposed a three-point calibration method using a laser sensor and a buzzer, and the researchers used an asynchronous calibration method based on planar projection, with two calibration points, to improve calibration efficiency and accuracy, although the contact calibration method is low cost and simple to operate.
However, the calibration tool is not a standard part, its versatility needs to be improved, manual teaching operation will also lead to the formation of robots, specific restraint posture has major errors, errors will accumulate during the calibration process, thereby reducing calibration accuracy.
These analyses show that the non-contact calibration method has high accuracy, but with the increase in cost, while the contact calibration method has low accuracy and cannot meet the needs of processing, production and assembly.
The current study proposes a multi-robot calibration method called "three-point measurement calibration", which obtains the three positions of the terminal adapter relative to the line sensor by adjusting the robot's posture.
Then, the sensor line length is used to establish the relationship between the tooling coordinate system and the coordinate system of the double robot base, and the transformation matrix between the coordinate system of the double robot base is obtained, and the experimental verification is carried out on the platform of the double robot collaborative system.
Multi-robot environment
In the multi-robot collaboration platform, the robots need to work together, therefore, it is necessary to obtain the position relationship between the robots, in the construction of the robot collaboration platform.
Due to the large weight and size of the robot, robot track and other components, there will be deviations from the designed installation size, so it is necessary to calibrate the robot after installation.
Schematic diagram of two-robot collaboration
The multi-robot collaborative platform adopted in this paper is mainly composed of two industrial robots, two robot tracks and a rotating workpiece workbench, and the collaborative platform can realize the functions of robot milling, welding, measurement, assembly and so on by replacing the terminal tools of the robot.
The calibration of the robot tool coordinate system is directly related to the positioning of the tool posture, which will affect the accuracy of the robot machining or assembling parts.
The function of TCF calibration is to teach the robot to the top of the calibration rod through the known information of Euler angle, position and robot joint angle of the tool coordinate system, and obtain the pose transformation matrix from the tool coordinate system to the end of the robot body.
Combined with the existing two-robot calibration method, handshake method and coordinate measuring machine method, the three calibration methods are compared from the aspects of accuracy, cost, human factors and robot working space, limitations, etc.
The handshake method is limited by the robot's workspace, requiring the two robots' workspaces to overlap, and since the operator needs to manually control the connection of the two robot end adapters, the results of this calibration method are affected by human factors and produce errors.
The calibration tool is fixed between the two robots, on the steel plate base, by adjusting the upper knob of the tool, the tool guide rod can be made in two different directions, which can be vertical or horizontal, after adjusting the position of the wire sensor.
The accuracy of the three-point measurement calibration method meets the calibration requirements, and the coordinate data of the robot calibration point is measured from the robot data and wire sensor, which is not affected by the operator's human factors.
This method has no limitation of robot working space and has low cost, so it has good practicality when used in actual factories.
The experiment used 1600 ABB robots against a single robot
conclusion
The calibration of the basic coordinate system of the dual robot is very important to realize the collaborative operation of the two robots in processing, production and assembly, and a dual robot calibration system is established based on the calibration of tooling components.
And the calibration system is easy to operate, by adjusting the vertical or horizontal direction of the tooling guide rod and the position of the line sensor, the conversion matrix between the coordinate system of the base of the double robot can be obtained, and an effective method for the calibration of the base coordinate system of the contact double robot of the "three-point measurement calibration method" is proposed.
By calibrating the three reference measurement points distributed orthogonally on the tooling parts to the mapping relationship between the robot end point and the robot base coordinate system, a rapid solution method for the parameter calibration of the coordinate system of the double robot base is obtained, and the base coordinate transformation matrix of the double robot is further solved
The experimental results show that our calibration method can effectively improve the calibration accuracy and efficiency, and at the same time, by replacing the wire sensor stroke, the base coordinate calibration between robots at any position distance can be realized, and it has good adaptability.
The calibration method can also be applied to the calibration of three or more robot base coordinate systems, providing valuable reference and guidance for robot processing, production and assembly.
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