A workpiece coordinate system determination method for robotic optical element polishing

Abstract

The application discloses a workpiece coordinate system determination method for robot optical element polishing, which comprises the following steps: placing a workpiece on a workbench, controlling a detection probe to move along the edge of the workpiece, collecting the coordinates of the edge points of the workpiece, calculating the position of the center point of the workpiece and the placing posture according to the coordinates of the edge points of the workpiece; controlling the detection probe to move vertically downward at the position of the center point of the workpiece, collecting the position coordinates of the center point of the upper surface of the workpiece, obtaining a preliminary workpiece coordinate system according to the coordinate information of all the collected points; generating a measurement path in an offline programming mode based on the established preliminary workpiece coordinate system, scanning the upper surface of the workpiece along the measurement path by using a displacement sensor, and collecting the coordinates of the scanning process and the displacement sensor data in real time by using an online monitoring device; performing plane linear fitting according to the collected coordinates and the displacement sensor data, compensating the pitch angle error of the preliminary workpiece coordinate system based on the plane linear fitting, and obtaining a final workpiece coordinate system.

Description

Technical Field

[0001] This invention relates to the field of intelligent manufacturing, and more particularly to a method for determining the workpiece coordinate system for robotic polishing. Background Technology

[0002] With the development of industrial technology, industrial robots have been widely used in high-precision machining fields such as optical component polishing. Before machining, the robot needs to set a workpiece coordinate system based on the workpiece's placement position and execute the machining program within this coordinate system. When the workpiece position changes, only the workpiece coordinate system needs to be adjusted without reprogramming, allowing the machining task to proceed. Traditionally, the workpiece coordinate system is set manually by adjusting the robot's and workpiece's poses to achieve "workpiece alignment," thereby determining the workpiece coordinate system. For large workpieces, adjusting their position and orientation is extremely inconvenient and has low accuracy. Therefore, developing automated methods for determining the workpiece coordinate system to improve its efficiency and accuracy is crucial. Summary of the Invention

[0003] This invention provides a method for determining the workpiece coordinate system for polishing robot optical components, which solves the technical problems mentioned above.

[0004] The technical solution of this invention to solve the above-mentioned technical problems is as follows: A method for determining the workpiece coordinate system for robot polishing, based on a detection probe and a displacement sensor set on the robot's end effector, includes the following steps:

[0005] Step 1: Place the workpiece to be processed on the worktable, control the detection probe to move a certain distance along the edge of the workpiece, and sample at least three points towards the edge of the workpiece to obtain data from at least three sampling points;

[0006] Step 2: Based on the obtained set of sampling point data, calculate the center point position and placement posture of the workpiece to be inspected, and control the detection probe to move vertically downward at the center position of the workpiece. The obtained sampling point data is the center point coordinate of the upper surface of the workpiece. Based on all the above sampling point data, calculate the preliminary workpiece coordinate system.

[0007] Step 3: Based on the surface shape of the workpiece's upper surface, a measurement path is generated using offline programming. Based on the preliminary workpiece coordinate system, a displacement sensor is used to scan the upper surface of the workpiece along the measurement path. The online monitoring device collects the coordinates and sensor data during the scanning process in real time.

[0008] Step 4, according to the obtained scanning data, the pitch error of the XY plane of the preliminary coordinate system is calculated and compensated to obtain the final workpiece coordinate system. The position of the final workpiece coordinate system is the center point of the upper surface of the workpiece, the X-axis direction points to the positive direction defined by the workpiece, and the Z-axis points to the outer normal direction corresponding to the center point of the upper surface of the workpiece.

[0009] In a preferred embodiment, the detection probe is a Renishaw probe, the robot is an ABB robot, and the step of collecting the edge points of the workpiece is:

[0010] S101: using the "SreachL" instruction of the robot, the detection probe is controlled to approach the edge of the workpiece at a faster speed, when contact occurs, the detection probe sends a level signal to the IO board card of the robot, after the robot receives the level signal, the contact position coordinates are immediately recorded, the speed is reduced until the movement stops, the "MoveOff" instruction is used to move the robot in reverse until it is separated from the contact, and the contact position coordinates are regarded as the approximate coordinates of the contact point;

[0011] S102: the detection probe stops at a distance away from the contact, and the sampling step described in S101 is re-executed, but the movement speed of the probe is significantly slower, and the new contact position coordinates obtained are the accurate coordinates of the contact point, which are stored in the memory of the robot controller.

[0012] S103: the detection probe is controlled to move to the vicinity of the next sampling point by the "MoveOff" instruction, the sampling steps described in S101 and S102 are executed, and the new sampling points obtained are recorded in the memory of the robot controller, and the reciprocating execution is performed until the collection of all edge point positions is completed.

[0013] In a preferred embodiment, the measurement and calculation method of the preliminary workpiece coordinate system is: for a circular workpiece, 5 sampling points are set, 4 of which are arbitrary sampling points, the center coordinates of the circle are calculated using the 3-point circle center formula, and the average value is taken as the final workpiece center coordinates, and the 5th sampling point is set in the positive direction of the circular workpiece to determine the placement attitude of the workpiece; for a square workpiece, a total of 8 sampling points are set, two sampling points are set on each side, and the workpiece center point position is determined by the average value of the 8 sampling point coordinates, and the placement attitude is calculated by the two sampling point coordinates of each side and finally determined by taking the average value.

[0014] In a preferred embodiment, the displacement sensor used is a laser ranging sensor, and data collection is performed before the robot polishing, which focuses on quickly and accurately correcting the pitch error of the workpiece before processing.

[0015] In another preferred embodiment, the displacement sensor used is integrated with the active force control flange, and the data acquisition is performed during the robot pre-polishing stage, which focuses on the convenient correction of the workpiece pitch angle during the machining process.

[0016] In a preferred embodiment, the detection path generated by the offline programming is a normal line following detection path generated according to the surface shape of the workpiece, and the displacement sensor stroke direction is always coincident with the normal line of the upper surface of the workpiece.

[0017] In a preferred embodiment, the online monitoring device is a signal collector arranged between the robot, the displacement sensor and the computer, and the collected data includes the coordinates (XYZ) of the TCP of the robot and the pressing stroke H of the displacement sensor. The collected data is ultimately aggregated in the computer and saved as a point cloud data file (XYH).

[0018] In a preferred embodiment, the pitch angle error calculation method of the workpiece coordinate system is:

[0019] S201: Perform linear fitting of the plane based on the least square method on the collected point cloud file (XYH), and obtain the displacement plane equation;

[0020] S202: According to the preliminary workpiece coordinate system attitude data, solve the plane equation of the two-dimensional plane composed of the XY axes and the normal line equation n1 of the plane corresponding to the preliminary workpiece coordinate system;

[0021] S203: Add the displacement plane equation and the XY coordinate system plane equation, and solve the new plane after addition, and the corresponding plane normal equation n2;

[0022] S204: According to the vector calculation formula, calculate the included angle θ and the common normal line n3 of n1 and n2, and based on the general rotation transformation matrix, calculate the transformation matrix T rotating θ angle around n3;

[0023] S205: Multiply the transformation matrix T on the preliminary workpiece coordinate system to obtain the final workpiece coordinate system, and complete the calculation of the pitch angle error of the workpiece coordinate system.

[0024] In a preferred embodiment, the robot movement, sampling point recording and the like are automatically completed under the control of the robot program, and the calculation result is automatically given. The robot program and the calculation method are encapsulated in the robot controller. The offline programming, online monitoring, point cloud calculation, pitch angle error calculation and the like mentioned above are automatically completed by the program in the computer, and the final calculation result is given.

[0025] In a preferred embodiment, the above measurement method can be repeated based on the final workpiece coordinate system to improve the setting accuracy of the workpiece coordinate system.

[0026] Compared with the existing workpiece coordinate system determination method in robot optical element polishing, the technical scheme of the present application has the following beneficial effects:

[0027] 1. The determination process of the workpiece coordinate system is mostly an automatic process under program control, and before the workpiece coordinate system is determined, there is no need to align the position and attitude of the workpiece, which greatly reduces the operation difficulty of the workpiece coordinate system determination and improves the production efficiency.

[0028] 2. The method using the detection probe and the displacement sensor combination realizes the all-around measurement of the position and attitude of the workpiece coordinate system, and especially realizes the measurement of the pitch angle error.

[0029] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the following preferred embodiments of the present application are described in detail below, and the accompanying drawings are described. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the technical scheme in the embodiments, the following briefly introduces the drawings needed to be used in the embodiments.

[0031] Figure 1 is a distribution diagram of sampling points when the detection probe samples the circular and square workpieces;

[0032] Figure 2 is a flowchart of the detection probe determining the preliminary workpiece coordinate system;

[0033] Figure 3 is a flowchart of the displacement sensor determining the pitch angle error of the workpiece coordinate system. DETAILED DESCRIPTION

[0034] The robot used in the present embodiment is an ABB industrial robot, the detection probe is a Renishaw probe, and the displacement sensor is a displacement sensor integrated in the main power control flange. It should be understood that the specific embodiments described in the present specification are only for the purpose of explaining the present application, and are not intended to limit the present application.

[0035] Figure 1 is a path diagram of the detection probe for determining the workpiece coordinate system of the circular workpiece and the square workpiece. The coordinate system O0 represents the relative coordinate system based on which the robot moves in this measurement. The relative coordinate system here is generally the workbench coordinate system, O1 is the workpiece coordinate system to be measured, P n represents the sampling point, n is the sampling point label, and θ represents the included angle between the positive direction of the workpiece and the relative coordinate system, that is, the attitude of the workpiece. For a circular workpiece, 5 sampling points are set, of which 4 sampling points are arbitrary sampling points, corresponding to Figure 1P1-P4, the center coordinates of the circle are calculated using a 3-point circle center formula, and the average value is taken as the final workpiece center coordinates, and the 5th sampling point corresponds to Figure 1 P0 in the circle, which is set in the positive direction of the circular workpiece to determine the placement attitude of the workpiece. Any of the above sampling points is obtained by automatic sampling of the robot, and the 5th sampling point is obtained by manually teaching the robot or installing a stop block at the point and automatically sampling by the robot. For a square workpiece, a total of 8 sampling points are provided, 2 sampling points are provided on each side, and the workpiece center point position is determined by the average value of the 8 sampling point coordinates, and the placement attitude is calculated by the two sampling point coordinates of each side, and finally the average value is determined.

[0036] As shown in Figure 2 , in this embodiment, after the user inputs the basic parameters in the robot, the automatic movement, sampling and calculation are completed by the predetermined robot program, and the calculation result is automatically given. The steps of workpiece edge point collection and coordinate system calculation are:

[0037] S101: The user moves the robot to a certain distance above the workpiece, which can be 20mm, and the position and height allow a certain error, for example, 30mm. The basic information of the workpiece is input in the robot teach pendant, which should include: basic size information of the workpiece and a preset safety distance, which should be greater than the error limit introduced in the manual operation; the robot generates an automatic feed route at the dotted line position in Figure 1 according to the above information, and plans to automatically sample after reaching a certain position of the dotted line;

[0038] S102: After reaching the predetermined sampling position, such as Figure 1 the sampling point P n , the "SreachL" instruction of the robot is automatically executed to control the detection probe to approach the workpiece edge at a relatively fast speed, which can be set to 5mm / s. When contact occurs, the detection probe sends a level signal to the IO board card of the robot, and the robot immediately records the contact position coordinates P' after receiving the level signal, and then slows down and stops moving. The "MoveOff" instruction of the robot is automatically executed to move the robot in reverse until it is separated from the contact, and the coordinate point P' is regarded as the approximate coordinates of the contact point;

[0039] S102: The detection probe stops at a distance from the P' point, which can be 5mm, and the sampling step described in S101 is automatically re-executed, but the probe moves at a significantly slower speed, which can be 1mm / s. The new contact position coordinates obtained are the accurate coordinates P of the contact point, which are stored in the memory of the robot controller. In S101, the first sampling purpose is to quickly find the edge point position, and the second sampling purpose is to accurately measure the coordinate value of the edge point.

[0040] S103: The probe is moved to the vicinity of the next sampling point by the "MoveOff" command, and the sampling steps described in S101 and S102 are performed, and the obtained new sampling point is recorded in the robot controller memory, and the steps are repeatedly performed until the collection of all edge point positions is completed.

[0041] It should be noted that the circular workpiece is provided with 5 sampling points, P1 to P4 are used to determine the position of the workpiece center in the relative coordinate system, and P0 is used to determine the included angle between the positive direction of the workpiece and the positive direction of the relative coordinate system, that is, the placement posture of the workpiece. The square workpiece is provided with 8 sampling points, P1-P8 are used to determine the position and placement posture of the workpiece. In addition, the above method is only used to determine the XY value of the center point of the upper surface of the workpiece;

[0042] S104: After confirming the XY coordinates of the workpiece center, the "MoveOff" command of the robot is executed to move the robot to a certain distance above the workpiece, which can be set to 10mm, and the "SreachL" command of the robot is automatically executed to sample downward, and the obtained Z coordinate of the sampling point is the Z coordinate of the center point of the upper surface of the workpiece;

[0043] Since the coordinate system position of the workpiece polished by the robot is set at the center point of the upper surface of the workpiece, the X axis points to the positive direction of the workpiece, and the Z axis points to the outer normal direction of the center point of the upper surface of the workpiece, so after the above measurement and calculation, only the direction of the positive direction of the Z axis is not measured. Since the Z axis direction of the relative coordinate system, that is, the normal direction of the workbench and the outer normal direction of the center point of the upper surface of the workpiece, has small error, it is first regarded as the same direction, and the obtained workpiece coordinate system is the preliminary workpiece coordinate system, and the calculation method is: on the basis of the relative coordinate system, first translate X, Y, Z along the x, y, z axes of the relative coordinate system, and then rotate θ angle around the z axis on the basis of the current coordinate system. For ABB robot, the relative coordinate system can be represented as a homogeneous transformation matrix, and after translation and rotation transformation, the coordinate system is converted into position and quaternion input into ABB robot.

[0044] The included angle error between the positive direction of the Z axis and the outer normal of the center point of the upper surface of the workpiece is called the workpiece pitch angle error, and the tool down pressure is required to be constant during robot polishing. When the robot processes according to the constant down pressure path programmed offline, the workpiece pitch angle error will cause the tool down pressure to change linearly in space. Based on the above principle, the down pressure stroke of the displacement sensor is collected by using the online collection device, and the pitch angle error is calculated, so that the error can be corrected. As shown in Figure 3 The displacement sensor integrated in the main force control flange is used to measure the down pressure, and the constant down pressure path planning, data collection, data calculation and other steps are automatically completed by the computer program. The pitch angle error measurement and calculation method of the workpiece coordinate system is:

[0045] S201: The user inputs the workpiece surface shape, and the path planning program generates a measurement path for constant downward pressure based on the workpiece surface shape. For displacement sensors integrated with the active force control flange, a machining path for constant downward pressure can be directly generated, and data is collected during the initial machining process. Here, the relative coordinate system on which the robot moves is generally based is the preliminary workpiece coordinate system determined in the previous step. The collected data includes the XY coordinates of the path points and the downward pressure H of the displacement sensor; all data is saved in XYH point cloud format.

[0046] S202: Perform plane linear fitting based on the least squares method on the acquired point cloud file (XYH) to obtain the displacement plane equation. The purpose of plane linear fitting is to remove the influence of robot vibration and other factors on the measurement and improve the measurement accuracy.

[0047] S203: Based on the attitude data of the relative coordinate system, solve for the plane equation of the XY-axis plane of the relative coordinate system and the normal equation n1 of the plane. Here is a method to convert the quaternion of the ABB robot's coordinate system attitude into the XY plane equation and the corresponding normal: First, convert the quaternion into the corresponding rotation matrix. Then, the three column vectors of the rotation matrix are the unit vectors of the positive directions of the X, Y, and Z axes of the coordinate system. From the unit vectors, the XY plane equation and the normal equation of the coordinate system can be obtained.

[0048] S204: Superimpose the displacement plane equation with the relative coordinate system XY plane equation, and solve for the new plane equation and the corresponding new plane normal equation n2.

[0049] S205: Calculate the angle θ between n1 and n2 and the common normal n3 according to the vector calculation formula. Based on the general rotation transformation matrix, calculate the transformation matrix T of rotating about n3 by an angle θ.

[0050] S206: Multiply the attitude matrix corresponding to the relative coordinate system by the transformation matrix T to obtain the new attitude matrix, which is the workpiece coordinate system attitude matrix T' after pitch angle error correction. For ABB robots, the final workpiece coordinate system T' can be converted into a quaternion and then input into the ABB robot teach pendant.

[0051] From the above description of this embodiment, combined with Figure 2 , Figure 3 As can be seen, most of the work in this method is automatically completed by the robot controller and computer, which greatly improves the efficiency of determining the workpiece coordinate system. In addition, in the preliminary workpiece coordinate system determination and pitch angle error determination steps described in this embodiment, the relative coordinate system on which the robot motion is based does not necessarily have to be based on a specific coordinate system. The accuracy problem caused by different relative coordinate systems can be solved by repeating this method several times.

[0052] Those skilled in the art can easily understand that the above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for determining the workpiece coordinate system for polishing robot optical components, characterized in that, Based on the detection probe and displacement sensor installed on the robot's end effector, the following steps are included: Step 1: Place the workpiece to be processed on the worktable, control the detection probe to move a certain distance along the edge of the workpiece, and sample at at least three points towards the edge of the workpiece to obtain data from at least three sampling points; Step 2: Based on the obtained set of sampling point data, calculate the center point position and placement posture of the workpiece to be inspected, and control the detection probe to move vertically downward at the center position of the workpiece. The obtained sampling point data is the coordinate of the center point of the upper surface of the workpiece. Based on all the above sampling point data, calculate the preliminary workpiece coordinate system. Step 3: Based on the surface shape of the workpiece's upper surface, a measurement path is generated using offline programming. Based on the preliminary workpiece coordinate system, a displacement sensor is used to scan the upper surface of the workpiece along the measurement path. The online monitoring device collects the coordinates and sensor data during the scanning process in real time. Step 4: Based on the obtained scanning data, calculate and compensate for the pitch angle error of the XY plane of the preliminary workpiece coordinate system to obtain the final workpiece coordinate system. The preliminary method for calculating the pitch angle error of the workpiece coordinate system is as follows: S201: Perform plane linear fitting based on the least squares method on the collected displacement point cloud file to obtain the displacement plane equation; S202: Based on the preliminary workpiece coordinate system attitude data, solve the XY coordinate system plane equation and XY plane normal equation n1 corresponding to the preliminary workpiece coordinate system attitude; S203: Add the displacement plane equation to the XY coordinate system plane equation, solve for the new plane after addition, and the corresponding plane normal equation n2; S204: Calculate the angle θ between n1 and n2 and the common normal n3 according to the vector calculation formula. Based on the general rotation transformation matrix, calculate the transformation matrix T of rotating about n3 by an angle θ. S205: Multiply the initial workpiece coordinate system by the transformation matrix T to obtain the final workpiece coordinate system.

2. The method for determining the workpiece coordinate system for polishing robot optical components according to claim 1, characterized in that, The detection probe used is a contact probe, and the steps for acquiring workpiece edge points are as follows: S101: The detection probe approaches the edge of the workpiece at a relatively fast speed. When contact occurs, the detection probe sends an electrical signal to the robot's IO board. The robot immediately records the contact position coordinates, decelerates and stops, and reverses to disengage from the contact. These coordinates are considered to be the approximate coordinates of the contact point. S102: After the detection probe has disengaged from the contact point, it moves back towards the edge of the workpiece at a small speed to perform a second measurement. The steps are the same as in S101, and the new coordinates obtained are the precise coordinates of the contact point.

3. The method for determining the workpiece coordinate system for polishing robot optical components according to claim 1, characterized in that, The offline programming involves generating a detection path that follows the normals based on the workpiece surface shape.

4. The method for determining the workpiece coordinate system for polishing robot optical components according to claim 1, characterized in that, The displacement sensor is either a contact displacement sensor integrated into the end effector of the polishing robot that can extend and retract with the polishing head, or a non-contact laser rangefinder sensor.

5. The method for determining the workpiece coordinate system for polishing robot optical components according to claim 1, characterized in that, The online monitoring device is a signal acquisition unit set between the robot, displacement sensor and computer. The acquired data is finally aggregated in the computer and saved as point cloud data.

6. The method for determining the workpiece coordinate system for polishing robot optical components according to claim 1 or 2, characterized in that, The preliminary workpiece coordinate system measurement and calculation method is as follows: the three sampling points are generally not used; for circular workpieces, five sampling points are set, of which four sampling points are arbitrary sampling points. The center coordinates are calculated using the three-point center determination formula, and the average value is taken as the final workpiece center coordinates. The fifth sampling point is set in the positive direction of the circular workpiece to determine the workpiece's placement posture; for square workpieces, a total of eight sampling points are set, with two sampling points set on each side. The workpiece center point position is determined by the average value of the eight sampling point coordinates, and the placement posture is calculated by the coordinates of the two sampling points on each side, and the final average value is taken.

7. The method for determining the workpiece coordinate system for polishing robot optical components according to claim 1, characterized in that, Based on the final workpiece coordinate system, repeat the above measurement method to improve the setting accuracy of the workpiece coordinate system.

8. The method for determining the workpiece coordinate system for polishing robot optical components according to claim 1, characterized in that, The robot movement and sampling point recording mentioned are all completed automatically under the control of the robot program, and the calculation results are automatically given. The robot program and calculation method are encapsulated in the robot controller. The offline programming, online monitoring, point cloud computing, and pitch angle error calculation mentioned are all completed automatically by program control in the computer, and the final calculation results are given.

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