Calibration method for normal measurement of terahertz probe at tail end of mechanical arm

Abstract

The invention discloses a calibration method for normal measurement of a terahertz probe at the tail end of a mechanical arm. The calibration method comprises the steps that the vertical incident angle of the terahertz probe and the initial distance between the terahertz probe and a measured surface are set; laser beams emitted by the four laser displacement sensors are vertically irradiated to a measured surface to generate four calibration points, and three-dimensional coordinates of all the calibration points are obtained; acquiring three-dimensional coordinates of a to-be-measured point of the measured surface, and calculating an actual normal vector of the to-be-measured point of the measured surface according to the calibration point and the three-dimensional coordinates of the to-be-measured point of the measured surface; calculating unit vectors of an ideal normal vector and an actual normal vector of a to-be-measured point of the measured surface, and calculating an angular deviation formed by the two unit vectors; and calibrating the measurement angle of the terahertz probe and the distance between the terahertz probe and the measured surface according to the calculated angle deviation and the obtained three-dimensional coordinate of the point to be measured. According to the invention, the distance between the measurement angle of the terahertz probe and the measured surface can be calibrated, and the accuracy of the measurement of the coating thickness of the terahertz probe is ensured.

Description

Technical Field

[0001] This invention belongs to the field of geometric parameter calibration, specifically relating to a calibration method for measuring the normal of a terahertz probe at the end of a robotic arm. Background Technology

[0002] With the continuous development of modern industrial technology, the requirements for the surface performance of materials are increasing. As an important means to ensure the performance and structural stability of materials, coatings can not only improve the corrosion resistance, wear resistance and thermal stability of the substrate, but also endow it with special optical, electrical and magnetic properties. They have been widely used in key components and equipment in aerospace, weaponry and other fields, which can effectively ensure that equipment and key components can work normally in harsh environments, and play an important supporting role in the development of my country's aerospace, weaponry and other fields.

[0003] The protective and functional effects of a coating are influenced by the uniformity of its thickness. To ensure coating quality, both thickness and uniformity are crucial. Coatings that are too thick or too thin will adversely affect the substrate's performance. For example, in anti-corrosion coatings, a coating that is too thin may fail to effectively isolate corrosive media, while a coating that is too thick may lead to decreased adhesion and increased costs. Furthermore, for certain functional coatings, such as optical interference coatings, the coating thickness directly affects their optical performance. Therefore, during coating preparation, precise control of the coating thickness and accurate, real-time measurement of the coating thickness are essential.

[0004] Currently, commonly used coating inspection technologies can be divided into destructive testing (DPT) and non-destructive testing (NDT) technologies. DPT technologies include scanning electron microscopy (SEM) and metallography. Both SEM and metallography measure coating thickness by observing the cross-section of the test piece under a microscope, requiring the test piece to be well-manufactured during production. Both methods damage the test piece itself and are costly and difficult to implement. Traditional NDT methods include ultrasonic testing, eddy current testing, X-ray testing, and infrared thermography. Ultrasonic testing requires a coupling agent, is sensitive to the sound velocity of the material, and is difficult to measure thin layers. Eddy current testing is limited to non-conductive coatings on conductive substrates and is greatly affected by the electromagnetic properties of the substrate. X-ray testing is costly, its sensitivity is easily affected by multiple factors, it cannot perform real-time monitoring, and it poses radiation safety issues. Infrared thermography's surface emissivity sensitivity greatly affects measurement accuracy. Compared to these NDT technologies, terahertz time-domain spectroscopy is an emerging NDT technology. Terahertz waves, due to their unique spectral characteristics and excellent penetration of most non-metallic coatings, exhibit unique advantages in non-destructive testing. They can perform non-contact, non-destructive, and high-precision measurements of coatings, ensuring the quality of weapon and equipment development and production.

[0005] With the increasing production demands in the aerospace and weaponry sectors, traditional manual measurement methods can no longer meet current needs. There is a need to use robotic arms combined with terahertz probes to achieve automated measurement. To ensure the accuracy of terahertz probe coating thickness measurement, the terahertz probe needs to maintain a certain distance from the surface of the workpiece, and the terahertz waves emitted during measurement must be incident on the workpiece surface parallel to the normal direction. Current technology cannot meet these requirements, thus compromising the accuracy of terahertz probe coating thickness measurement. Summary of the Invention

[0006] The purpose of this invention is to provide a calibration method for normal measurement of a terahertz probe at the end of a robotic arm. This method can monitor in real time whether the terahertz probe is in the normal direction of the workpiece surface and maintains a specific distance from the measured surface during measurement. It can also perform online calibration of the distance between the measurement angle of the terahertz probe at the end of the robotic arm and the measured surface, ensuring the accuracy of the terahertz probe coating thickness measurement.

[0007] One aspect of the present invention provides a calibration method for normal measurement of a terahertz probe at the end effector of a robotic arm, comprising: Step S1: Fix four laser displacement sensors on the housing of the terahertz probe, and the installation positions of the four laser displacement sensors are located on the same plane. Set the vertical incident angle of the terahertz probe and the initial distance from the surface to be measured. Step S2: Four laser displacement sensors emit laser beams that are perpendicularly irradiated onto the surface being measured to generate four calibration points, and the three-dimensional coordinates of each calibration point are obtained. Step S3: Obtain the three-dimensional coordinates of the test point on the measured surface, and calculate the actual normal vector at the test point on the measured surface based on the calibration point and the three-dimensional coordinates of the test point on the measured surface. Step S4: Calculate the unit vectors of the ideal normal vector and the actual normal vector of the measured point on the surface, and calculate the angular deviation formed by the two unit vectors. Step S5: Based on the calculated angle deviation and the obtained three-dimensional coordinates of the point to be measured, calibrate the measurement angle of the terahertz probe and the distance between it and the surface to be measured.

[0008] According to the calibration method for normal measurement of terahertz probe at the end of a robotic arm as described above, it is possible to monitor in real time whether the terahertz probe is in the normal direction of the workpiece surface and maintains a specific distance from the surface being measured. It is also possible to perform online calibration of the distance between the measurement angle of the terahertz probe at the end of the robotic arm and the surface being measured, thereby ensuring the accuracy of the coating thickness measurement of the terahertz probe. Attached Figure Description

[0009] To more clearly illustrate the technical solutions of the present invention, the accompanying drawings used in the description of the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort: Figure 1 This is a schematic diagram illustrating an application scenario of a calibration method for normal measurement of a terahertz probe at the end of a robotic arm, according to an embodiment of the present invention. Figure 2 This is a schematic diagram illustrating the measurement principle of a calibration method for measuring the direction of a terahertz probe at the end of a robotic arm, according to an embodiment of the present invention. Detailed Implementation

[0010] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0011] One embodiment of the present invention provides a calibration method for normal measurement of a terahertz probe at the end effector of a robotic arm. In this embodiment, as... Figure 1 As shown, the terahertz probe 5 is securely mounted on the probe mounting flange 4 via the first probe mounting fixture 2 and the second probe mounting fixture 3. The first probe mounting fixture 2 and the second probe mounting fixture 3 are L-shaped adapter plates, fixed to the probe mounting flange 4 with screws. The probe mounting flange 4 is then fixed to the end of the robotic arm 1 with screws. Four laser displacement sensors 6 are fixed to the housing of the terahertz probe 5 with screws, and the four laser displacement sensors are mounted on the same plane.

[0012] like Figure 2 As shown, during measurement, the laser beam from the laser displacement sensor 6 vertically illuminates the surface being measured, generating various calibration points and measuring the coordinates of these points in the z-direction. The x and y coordinates of the calibration points are determined by the installation position of the laser displacement sensor relative to the terahertz probe. The x and y coordinates of the points to be measured on the surface can be directly read from the robotic arm system. The z-coordinate is the difference between the initial height of the laser beam emitting end face of the laser displacement sensor from the points to be measured on the surface and the distance between the terahertz probe's detection wave emitting end face and the points to be measured on the surface.

[0013] The calibration method for terahertz probe orientation measurement at the end of a robotic arm according to embodiments of the present invention includes steps S1 to S7.

[0014] Step S1: Fix four laser displacement sensors on the housing of the terahertz probe, and the installation positions of the four laser displacement sensors are located on the same plane. Set the vertical incident angle of the terahertz probe and the distance from the surface to be measured, and move the robotic arm to the initial position.

[0015] Step S2: Obtain the three-dimensional coordinates of each calibration point. The calibration points are generated by the laser beam from the laser displacement sensor perpendicularly illuminating the surface being measured.

[0016] The three-dimensional coordinates of the calibration point are x, y, and z coordinates. The x and y coordinates of the calibration point are determined by the installation position of the laser displacement sensor relative to the terahertz probe, and the z coordinate is measured by the laser displacement sensor. The z coordinate of the calibration point = HH i .

[0017] The three-dimensional coordinates of the calibration point are:

[0018]

[0019]

[0020]

[0021] In the formula, A i (i=1, 2, 3, 4) are four calibration points on the surface being measured; H The initial height value of the laser beam emitting end face of the laser displacement sensor from the measured point on the surface is given. The end faces of the four sensors are on the same plane and at the same height from the measured point on the surface. H 0 represents the distance between the terahertz probe's transmitting end face and the measured point on the surface being measured; this is the initial setting value. H i (i=1, 2, 3, 4) represents the distance of the laser displacement sensor from the calibration point.

[0022] Step S3: Obtain the three-dimensional coordinates of the test point on the measured surface, and calculate the normal vector at the test point on the measured surface based on the calibration point and the three-dimensional coordinates of the test point on the measured surface.

[0023] The x and y coordinates of the measured point on the surface are directly read from the robotic arm system. The z-coordinate is the difference between the initial height of the laser beam emitting end face of the laser displacement sensor from the measured point and the distance between the terahertz probe's detection wave emitting end face and the measured point. The z-coordinate of the measured point on the surface is... HH 0.

[0024] The three-dimensional coordinates of the test point on the surface under test are:

[0025] In the formula, B The test point is the surface to be tested.

[0026] The equation of the quadratic surface at point B on the measured surface is: In the formula, a , b , c , d , e is a coefficient.

[0027] Substituting the three-dimensional coordinates of the calibration point and the measured point on the surface into the quadratic surface equation yields the following matrix:

[0028] Solving the matrix yields the coefficients. a 0, b 0, c 0, d 0, e 0 。

[0029] The equation of the quadratic surface at point B on the measured surface is:

[0030] At the test point on the surface being measured B The normal vector is .

[0031] Step S4: Calculate the unit vectors of the ideal normal vector and the actual normal vector of the measured point on the measured surface, and calculate the angular deviation between the ideal normal vector and the actual normal vector of the measured point on the measured surface.

[0032] The terahertz probe should be perpendicular to the ideal normal vector of the measured point on the surface. The actual normal vector at point B on the surface to be measured is: .

[0033] The unit vector of the ideal normal vector The unit vector of the actual normal vector

[0034] The angular deviation formed by the two vectors

[0035] Step S5: Based on the acquired angle and distance data, perform online calibration of the distance between the terahertz probe at the end of the robotic arm and the measured surface.

[0036] The working principle of the calibration method for normal measurement of a terahertz probe at the end of a robotic arm according to this invention is as follows: The vertical incident angle of the terahertz probe and its distance from the surface being measured are set, and the robotic arm is moved to its initial position. During measurement, the laser beam from the laser displacement sensor vertically illuminates the surface being measured, generating various calibration points, and the coordinates of these calibration points in the z-direction are measured. The x and y coordinates of the calibration points are determined by the installation position of the laser displacement sensor relative to the terahertz probe. The x and y coordinates of the point to be measured on the surface are directly read from the robotic arm system. The z-coordinate is the difference between the initial height of the laser beam emitting end face of the laser displacement sensor from the point to be measured on the surface and the distance between the terahertz probe's detection wave emitting end face and the point to be measured on the surface. The normal vector at the point to be measured on the surface is calculated based on the three-dimensional coordinates of the calibration points and the point to be measured on the surface. The distance between the terahertz probe measuring the angle at the end of the robotic arm and the surface being measured is calibrated online based on the acquired angle and distance data.

[0037] The calibration method for normal measurement of a terahertz probe at the end of a robotic arm, according to embodiments of the present invention, has the following beneficial effects: 1. This invention can be used to monitor in real time whether the terahertz probe is in the normal direction of the workpiece surface and maintains a specific distance from the measured surface during measurement. It can also perform online calibration of the distance between the terahertz probe measurement angle at the end of the robotic arm and the measured surface, ensuring the accuracy of the terahertz probe coating thickness measurement.

[0038] 2. This invention is applicable to real-time monitoring of the distance between the workpiece surface normal and the measured surface of the terahertz probe at the end of different types of robotic arms. The calibration process is simple, quick, and easy to implement, which can improve the efficiency and accuracy of automatic coating thickness measurement.

[0039] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A calibration method for measuring the normal direction of a terahertz probe at the end effector of a robotic arm, characterized in that, include: Step S1: Fix four laser displacement sensors on the housing of the terahertz probe, and the installation positions of the four laser displacement sensors are located on the same plane. Set the vertical incident angle of the terahertz probe and the distance from the surface to be measured. Step S2: Four laser displacement sensors emit laser beams that are perpendicularly irradiated onto the surface being measured to generate four calibration points, and the three-dimensional coordinates of each calibration point are obtained. Step S3: Obtain the three-dimensional coordinates of the test point on the measured surface, and calculate the actual normal vector at the test point on the measured surface based on the calibration point and the three-dimensional coordinates of the test point on the measured surface. Step S4: Calculate the unit vectors of the ideal normal vector and the actual normal vector of the measured point on the surface, and calculate the angular deviation formed by the two unit vectors. Step S5: Based on the calculated angle deviation and the obtained three-dimensional coordinates of the point to be measured, calibrate the measurement angle of the terahertz probe and the distance between it and the surface to be measured.

2. The method as described in claim 1, characterized in that, In step S1, the terahertz probe is fastened onto the probe mounting flange using a first probe mounting fixture and a second probe mounting fixture. The first and second probe mounting fixtures are L-shaped adapter plates. The probe mounting flange is then fixed to the end of the robotic arm with screws.

3. The method as described in claim 1 or 2, characterized in that, In step S2, the three-dimensional coordinates of the four calibration points are as follows: in, A i These are four calibration points on the surface being measured, i = 1, 2, 3, 4; x i , y i These are the x-coordinates and y-coordinates of the four calibration points, respectively. H This represents the initial height value between the laser beam emitting end face of the laser displacement sensor and the point to be measured on the surface being measured. H i The distances of the four laser displacement sensors from the calibration point.

4. The method as described in claim 3, characterized in that, The x and y coordinates of the four calibration points are determined by the installation position of the laser displacement sensor relative to the terahertz probe, and the z coordinate is measured by the laser displacement sensor.

5. The method as described in claim 4, characterized in that, In step S3, the three-dimensional coordinates of the test point on the measured surface are: in, B The test point is the surface to be tested. x 0、 y 0 represents the x-coordinate and y-coordinate of the point to be measured on the surface being measured. HH 0 represents the z-coordinate of the point to be measured on the surface. H 0 represents the distance between the terahertz probe's transmitting end face and the measured point on the surface.

6. The method as described in claim 5, characterized in that, In step S4, the angular deviation formed by the two unit vectors is calculated as follows: ， Wherein, the unit vector of the ideal normal vector is , The unit vector of the actual normal vector is , Let be the equation of the quadratic surface at the point to be measured on the surface being tested. a 0、 b 0、 c 0、 d 0、 e 0 represents the coefficient.

7. The method as described in claim 6, characterized in that, The coefficients are obtained by solving the following matrix. a 0、 b 0、 c 0、 d 0、 e 0: 。

Images