Calibration method for a vision guiding-based laser-tracking measurement system

Abstract

Laser-tracking measurement systems (laser trackers) based on a vision-guiding device are widely used in industrial fields, and their calibration is important. As conventional methods typically have many disadvantages, such as difficult machining of the target and overdependence on the retroreflector, a novel calibration method is presented in this paper. The retroreflector, which is necessary in the normal calibration method, is unnecessary in our approach. As the laser beam is linear, points on the beam can be obtained with the help of a normal planar target. In this way, we can determine the function of a laser beam under the camera coordinate system, while its corresponding function under the laser-tracker coordinate system can be obtained from the encoder of the laser tracker. Clearly, when several groups of functions are confirmed, the rotation matrix can be solved from the direction vectors of the laser beams in different coordinate systems. As the intersection of the laser beams is the origin of the laser-tracker coordinate system, the translation matrix can also be determined. Our proposed method not only achieves the calibration of a single laser-tracking measurement system but also provides a reference for the calibration of a multistation system. Simulations to evaluate the effects of some critical factors were conducted. These simulations show the robustness and accuracy of our method. In real experiments, the root mean square error of the calibration result reached 1.46 mm within a range of 10 m, even though the vision-guiding device focuses on a point approximately 5 m away from the origin of its coordinate system, with a field of view of approximately 200 mm × 200 mm.

Automatic Guidance Method for Laser Tracker Based on Rotary-Laser Scanning Angle Measurement

“…Then, converting coordinates of retroreflector in TCS to LCS, where and X are coordinates of retroreflector in TCS and LCS, respectively. Converting Cartesian coordinates into angle value in the spherical coordinate system, the mathematical transformation is as follows [ 7 ], where ( , , ) are the Cartesian coordinates of the target point in the LCS, r represents the distance between the target and origin of the LCS, and and are the horizontal and elevation angles that the laser tracker needs to turn to reach the retroreflector, respectively.…”

“…The pointing direction of the measuring laser can be calculated by coordinate transformation, and the tracking head is guided to the reflector precisely and automatically. If the laser beam was temporarily blocked, tracking and measuring can be restored automatically in a Field of View (FOV) of ±5° [ 6 , 7 ]. Compared with the monocular camera, the binocular stereo camera can measure the coordinates of specific targets directly, and the requirement for target recognition is reduced while FOV is enlarged.…”

Combined 3D scanning measurement method based on guidance of full-space laser positioning system

“…13 There are also globally guided point-cloud acquisition techniques, 14 laser tracker-guided 3D measurement methods, 15 and techniques based on the triangulation method. 16 Using a single point-cloud acquisition method for smaller amplitude increases the number of splices, consequently amplifying the cumulative error making it challenging to achieve high-precision measurements for large equipment. 17 Although the multisystem combination measurement method compensates for this issue, it is constrained by the single-point tracking operation of the laser tracker.…”

“…Common methods include single-system point-cloud acquisition, 10 3D laser scanning, 11 photogrammetric point-cloud acquisition, 12 and structured light scanners 13 . There are also globally guided point-cloud acquisition techniques, 14 laser tracker-guided 3D measurement methods, 15 and techniques based on the triangulation method 16 . Using a single point-cloud acquisition method for smaller amplitude increases the number of splices, consequently amplifying the cumulative error making it challenging to achieve high-precision measurements for large equipment 17 .…”

Fast and reliable control of steering mirrors with application to free-space communication

“…Referring to the existing literature, a great deal of interest has been devoted to the control of fast steering mirrors that have been used as key tools for laser beam alignment and correction, see e.g., [1], [2], [3], [4], [5]. Laser-tracking measurement systems based on a vision-guiding device has been proposed in [6]. Laser beam jitter control using recursive-least-squares adaptive filters is discussed in [7].…”