# Research of Calibration Method for Industrial Robot Based on Error Model of Position
## 1 Introduction  
With the wide application of industrial robot, people have higher requirements for its positioning accuracy. However, due to the influence of manufacturing error, assembly error, joint abrasion, and transmission error of robot parts, the positioning accuracy of robot is not satisfactory, which limits the application of robot in high-precision field. Therefore, it is particularly important to study the positioning accuracy improvement method of industrial robot. In fact, the positioning accuracy of industrial robots can be divided into two parts: repeated positioning accuracy and absolute positioning accuracy (APA) [1]. In most cases, we only need to improve the repeated positioning accuracy of the robot. For example, when industrial robot welds a workpiece, the repeated accuracy of the robot normally determines the trajectory accuracy of the workpiece directly. At this time, the APA is meaningless. Therefore, people focus on how to improve the repeat positioning accuracy, which can be improved by many methods such as soft computing and machine learning [2, 3]. Industrial robots normally have the merit of high repeat positioning accuracy. For some small industrial robots, its repeat positioning accuracy even reach 0.01 mm, while the APA of the robots may be beyond 1 mm or worse [4]. So in this field, the research on APA is obviously insufficient. For example, in the application of multi robot cooperation, for kinematics parameter identification and robot off-line programming tasks, it is necessary to calibrate the precise pose of the robot base coordinate frames in the world coordinate system, which requires that the industrial robot must have sufficient APA [5]. The fundamental reason for the low APA of industrial robot is that the pose of TCP (Tool Center Point) are calculated by kinematics mathematical model which contains the structure size of the linkage and motion parameters of joint. Moreover, there are errors between the values taken in calculation and actual parameters of the robot, resulting in a deviation between the calculated results and the actual pose. The following two methods can be used to enhance the robot's APA: (1) using high accuracy machining equipment and advanced assembly method to reduce robot link machining and assembly errors; (2) combining advanced measurement tools and related algorithms to identify and correct robot structural parameters, that is robot calibration method [6]. The first method is expensive and has limited effect on improving the precision of the robot, and it cannot solve the problem of precision decline caused by wear and deformation. The second method is cheaper than first one, and it is more effective in compensating the precision degradation caused by wear or structural deformation [7]. Therefore, robot calibration is the main method to enhance the APA of robot, and it is also the main research direction of many scholars [8]. In the study of robot calibration method, [PERSON] et al. [6] found that the absolute positioning error is mainly caused by the parameter error of robot kinematics model. Therefore, the research of robot calibration technology focuses on the error identification of robot kinematics parameters primarily [9; 10; 11]. In the current research on the reduction of APA caused by kinematic parameter errors, it usually assumes that the robot base coordinate system (RBCS) and the flange coordinate system (FCS) are known or can be directly measured. However, the fact is that most of the robot flange and base coordinate system are \"hidden\"--it cannot be directly measured and need to be obtained through special measurement and calculation methods [12]. In reference [13], the position and pose measurement of robot is realized by using coordinate theodolite, and the APA error of the robot is reduced to 0.28 mm after calibration. This instrument is designed according to the principle of angle detecting, and exhibits good measurement accuracy.