Research On High-precision Positioning Technology Of A Mobile Sit-to-stand Radiotherapy Device

Currently,proton heavy ion radiation therapy has been recognized as one of the most important means of cancer treatment due to its good therapeutic advantages.The fixed beam in particle radiotherapy has low freedom and poor adaptability,and the rotating frame is expensive and has a high operating cost.For these reasons,the development of a mobile sit-to-stand radiotherapy device is imminent.However,the device is characterized by a long drive chain,multiple structural parameters,nonlinearity,and strong coupling,resulting in poor spatial positioning accuracy of the system.To address this problem,this paper takes the mobile sit-to-stand radiotherapy device developed by the group as the research object and starts the research from the aspects of error source analysis,error modeling,posture error analysis,error source sensitivity,and error compensation.The main research contents of the paper are as follows:(1)By analyzing the mechanical structure of the mobile sit-to-stand radiotherapy device,the connection and motion form of the series-parallel mechanism of the device were determined.Based on the study of the spatial posture of the radiotherapy patient-bearing device,the sources of the end-position error of the mobile sit-to-stand radiotherapy device were analyzed.By using the inverse kinematic solution method and the chi-square coordinate transformation theory,the error models of the parallel platform and the series mechanism are established,respectively,and the integrated posture error model of the mobile sit-to-stand radiotherapy device is obtained by integration and superposition.(2)The structural parameters of the mobile sit-to-stand radiotherapy device were set,and the influence law of each error source of the hybrid mechanism on the end-position error of the system was studied based on the comprehensive posture error model with the single-element analysis method using Matlab software.Then,through the analysis of the sensitivity of the error sources,it was clarified that the main sources of the spatial posture error of the system were the drive rod length error and the hinge point position error of the parallel mechanism,and the comprehensive posture error model was simplified on this basis.(3)The advantages and disadvantages of different error compensation control strategies of the parallel mechanism are analyzed with the results of the mobile sit-to-stand radiotherapy device posture error study.Based on the improved posture error model of the parallel platform,a particle swarm optimization algorithm with improved inertia weights and shrinkage factors is proposed to minimize the integrated posture error by finding the optimal parallel platform drive rod length error parameters,so as to achieve the compensation of the end posture error of the system.Through simulation analysis,the theoretical feasibility of the algorithm for improving the end-point accuracy of the parallel platform is verified.(4)The overall structure of the experimental platform is built by a decentralized control method according to the control requirements of error compensation on the six-degree-of-freedom parallel platform.Based on the improved particle swarm algorithm,the error compensation experiments were conducted on the parallel platform,and the positions of the ends of the system before and after the error compensation were measured by laser detection equipment.In this paper,the results of the analysis of spatial position positioning errors of mobile sit-to-stand radiotherapy devices provide a technical solution to improve the adaptability of fixed-beam particle precision radiotherapy as well as a technical reference for studies related to patient precision positioning.