# Influence of the Approach Direction on the Repeatability of an Industrial Robot
## 1 Introduction
This is achieved by a measuring system that communicates with the robot controller and the laser tracker, while the robot configurations are chosen with the help of a deep neural network. Laser trackers are often used as a reference measurement to evaluate the properties of other calibration methods [6; 7; 8; 9]. Another use of lasers for robot calibration is in the form of laser interferometry-based sensing and measurement as described in [10]--this method provides real-time dynamic position measurements with high accuracy, high sampling rate, and large working space. A 3D measurement system consisting of a commercially available telescoping ball bar system is proposed in [9]. After calibrating the robot in selected 72 poses of the end-effector with respect to the base, verification in 10,000 random robot configurations made by comparison with a laser tracker confirmed approximately two times better accuracy of the robot than before the calibration. A geometric method of calibration is also described in [11] with a special focus on finding the best calibration poses of the robot to achieve the highest possible improvement in positioning accuracy. The authors in [12] point out a problem that applies to all calibration methods and is related to the influence of backlash of the robot joints. This effect is described in the article together with a proposed method for the elimination of this source of additional error. A commonly used group of methods for robot calibration is based on photogrammetry. One of the approaches is to use a 3D photogrammetry-based measuring device to track the position and orientation of the robot end-point in real-time and use this information as feedback for the robot controller [13]. Photogrammetry is also applied in off-line robot calibration [6]. Research has also been done in the area of use of a commercially-available portable photogrammetry system in industrial robot calibration, the authors in [7] compare this approach with a laser tracking calibration with very good results--the accuracy of photogrammetry is only slightly worse. The research in [14] describes the use of a camera mounted on the robot end-effector and a calibration board placed in the workplace where the camera can see it most of the time. This proposed calibration method also works on-line. A similar solution with a single camera mounted near the robot end-point was introduced also in [15] and the achieved improvement in robot accuracy was between three to six times. As far as high-speed cameras are concerned, authors in [16] utilized the Phantom v2511 camera with up to 25,000 frames per second to ascertain the accuracy of an industrial robot not only in the absolute value but also concerning the movement direction. Such a system can also be used to measure dynamic movement, oscillation, vibration, etc. Application of stereo-vision for robot calibration has also been the subject of research, for example in [17] a method for increasing the accuracy of an industrial milling robot with the help of a stereo camera is described. In addition, another research was done using low-cost systems [18] or laser-camera-triangulation [19]. Unlike photogrammetry, the digital image correlation method enables full-field 3D displacement measurements, allowing the correlation system to be used in both vibration analysis and deformation analysis of components. The main advantage over point measurement is the possibility of including more data in the processing of results. Since the measurement is carried out on a surface, in addition to position information, it is also possible to obtain information about the direction of the surface normal. If the high-speed cameras are used, it is possible to analyze kinematics, dynamics, and vibration of the manipulator in detail, or to measure its modal properties. The study in [20] describes a motion analysis of a two-arm service robot as it passes between two positions. It was a large-scale application designed to reconstruct the movement of the manipulator in space.