Research On 6-DOF Parallel Radiotherapy Bed For Radiation Therapy

Tumor is one of the major diseases that threaten human health. With the rapid development in the field of radiation domestic and international, the vast majority of tumors were adopted radiation therapy currently. Some degrees of freedom are restricted when reproducing the patient’s position during CT process of the conventional radiotherapy bed, and the positioning accuracy is generally not high, which hinder the realization of precise radiotherapy. Therefore, taking the advantages of parallel mechanism, combining with the actual needs of the radiotherapy bed, which can make up for the shortage of conventional radiotherapy bed, achieve the purpose of precise radiotherapy.First of all, according to the requirements of the workspace and the dimensions of the structure of the radiotherapy bed in actual use, 6-HTRT parallel robot of parallel drive form is chosen as the radiotherapy bed model. Take research on kinematics of the radiotherapy bed, then the expressions of inverse solution are obtained. Combining with the other inverse solution constraints, the precise analysis of the workspace is taken on the double universal joints parallel radiotherapy bed. The workspace of the radiotherapy bed with zero attitude and ultimate attitude are obtained, and the influence of workspace by changing the rule of geometric parameters has also been obtained, which can provide the basis for the parameter optimization of the radiotherapy bed.Since the radiotherapy bed bears eccentric loading during operation, leading to the suffered force of each link increasing. Then, the suffered force of the link and the input force of the lead screw are analyzed when the position and orientation are given, and the simulation is carried out in ADAMS. The results can guide the design of the specific parts and the selection of the motors and other purchased parts.For the problem of improving the strength and rigidity of the Hooke joint, a method of optimizing the initial mounting angle of the Hooke joint is proposed. To meet the conditions of the workspace, it can effectively reduce the limited angle of Hooke joint. So, the size of the Hooke joint can be reduced. For the requirements of the use of the radiotherapy bed, a multi target coordinated parameters optimization method is proposed with considering the workspace, the forces on it and the Jacobian matrix condition number, as well as constructing the objective function of the universe average of condition number and the fluctuation of condition number. Then, the parameters of the radiotherapy bed are obtained with the smaller condition number with uniform distribution of the Jacobian matrix in the whole workpace.In order to obtain the main factors which affect the position and orientation errors of the radiotherapy bed. Establish the error model of the radiotherapy bed by the closed-loop vector method. Total differential is performed of the error model to get the relationships between the position and orientation error of the radiotherapy bed and the main factors. The sensitive parameters which have greater impact on position and orientation error and the sensitive direction of the position and orientation error of the radiotherapy bed can be obtained by analyzing. The results of the analysis can guide the design precision and assembly precision of the parts. The results also provide a theoretical basis for the main factors which should be considered in the establishment of the calibration model.Finally, according to the theoretical analysis, the system integration of the radiotherapy bed is completed. The experimental prototype of the radiotherapy bed is developed. To improve the positioning accuracy of radiotherapy bed, the kinematic calibration is performed on the radiotherapy bed. The calibration model of the radiotherapy bed is established based on the inverse kinematics. For the problem of the identification of the structural parameters, the method of the combining of the genetic algorithm and minimum and maximum optimizing method is proposed. The combining of the advantages of the two methods for identification can effectively reduce the residuals of the target function. The error transfer of the Jacobi matrix condition number is taken as the basis for selecting the space points. 26 sets of position and orientation are selected. The acquisition method of the position and orientation of the radiotherapy bed is proposed based on the decomposition of Matrix Singular Value, and the laser tracker is taken as the tool. The experimental results show that the positioning accuracy of the radiotherapy bed has been greatly improved after calibration. The radiotherapy bed can realize the precise positioning of the six degrees of freedom when it reproduces the patient’s position under CT examination.