Research On Kinematic Calibration Of Industrial Robot Based On Displacement Sensor

With the expansion of industrial robot application fields and the promotion of off-line programming technology,the requirement for absolute positioning accuracy of robots is becoming higher and higher.Calibration technology of industrial robots can improve the absolute positioning accuracy of robots at low cost without replacing components which is of great importance for improving the working efficiency and quality of robots.In this paper,addressing the problems existing in current calibration systems,such as expensive price and easy to be limited by the site,a new method of robot calibration with displacement sensors was proposed.Based on this,relevant theoretical research was carried out and a set of robot kinematic calibration system was developed.Firstly,according to the new method,solutions of kinematics model and flexible workspace of 6-DOF joint industrial robots were studied.Addressing the problem that classical DH parameters had a sudden change when the adjacent links were parallel,the modified DH model and simplified model were established,and the reduction ratio was introduced to make models closer to actual robots.Furthermore,the weighting matrix was introduced to normalize Jacobian matrix addressing the problem that the flexibility index was inconsistent under different dimensions.The simulation result showed that the improved index eliminated the effect of dimensions.Therefore,the flexible workpace of robots was screened to provide a basis for the placement of the calibration plate.Secondly,robot error models were established combining kinematics model and differential transformation method.Due to the fact that the traditional position error model was not suitable for this situation where the conversion between the robot base and measuring device was not fixed,this paper took the conversions between the measuring device,the calibration plate and the robot as unknown parameters and derived the position error model in the measuring coordinate system,which realized the calibration of the geometric parameter errors and the conversions at the same time.Considering that the distance error model didn't involve the conversion between the calibration plate and the robot,the distance square error model was deduced to solve the problem that there were too many parameters to be solved in the position error model.The error models provided a theoretical basis for the development of the calibration system.Thirdly,simulation experiments were carried out to verify the validity of error models.Addressing the problem that error models were too complex to solve,a sequential quadratic programming algorithm with good convergence and high computational efficiency was used for parameter identification.Addressing the problem that the accuracy and efficiency of the optimization algorithm were susceptible to initial values,an initial value selection strategy was proposed to obtain the conversions between the measuring device,the calibration plate and the robot preliminarily.The simulation results showed that under the same conditions,the error model based on MDH model was better than that based on simplified model and the identification effect of the position error model was better than that of the distance square error model.Moreover,the identification effect of angular errors and reduction ratio error was better than that of length errors.The absolute positioning accuracy of robots can be improved by more than 70% when the angular errors and reduction ratio error account for a large proportion of the total errors.Finally,the hardware and software of the robot calibration system were designed according to the calibration scheme.On the one hand,the calibration plate and the measuring device were designed.Simulations were carried out to analyze the influences of sensor error and installation angle on the accuracy of the measuring system to obtain the performance requirements of sensors and optimize the installation angle.The error of the measuring system designed in this paper was about 0.1mm after the calibration of CMM.On the other hand,a universal workflow for calibration of 6-DOF joint industrial robots was designed.And the sensor data communication system and robot calibration interface were developed.The calibration experiment showed that the absolute positioning accuracy of the robot was improved by 55.2% after error compensation,which preliminarily verified the validity of the calibration system.