| Compared with traditional open surgery,minimally invasive surgery(MIS)has the advantages of small incision,light pain,less bleeding and faster recovery,which shorten the patients' discharge time.Minimally invasive surgery robot can assist the doctors to complete the operation quickly and improve the doctor's working environment,which is a hot spot in the field of medical robotics at present.According to the operating space and flexibility requirements of MIS,the end effector of a MIS robot needs the characteristic of a remote center of motion with four degrees-of-freedom(DOFs)including three rotational DOFs(rotate around the fixed point)and one translation DOF(translate along an axis which passes the point),which is called RCM mechanism.Compared with the existing multi-link and parallel RCM mechanisms,the planetary gear trains(PGTs)RCM mechanism has the advantages of reliable transmission,simple and compact structure.It solves the problems of low flexibility and complexity of the existing RCM mechanism,and meets the development requirements of compactness and portability.Based on the existing topology map of PGTs and combined with the motion characteristics of RCM mechanism,the structure of RCM mechanism is synthesized in this paper.According to the synthesis results,a new RCM mechanism with variable remote center is proposed,which can adjust the position of remote center by changing the PGTs ratio.The main research contents of this paper are as follows:1)The relationship between the virtual center(VC)mechanism and the RCM mechanism is analyzed,and the synthesis problem of RCM mechanism with gear trains is converted into the synthesis of VC mechanism.The similarity of the VC mechanism and the closed-loop linkage mechanism is used to analyze the single carrier and double carrier VC mechanisms under different linkage lengths to obtain the linkage length condition that meets the requirements.2)According to the motion characteristics of the end output link of the VC mechanism,the labeled graphs screening criteria is established,and the existing PTG topology map is screened to obtain the structure with the potential possibility of constructing the RCM mechanism.There are 30 structures that meet the requirements within a single degree of freedom of six components.3)Combining the length conditions with the structure obtained by the screening,the variable remote center of motion mechanism with noncircular planetary gear trains is selected as the research object.The variable remote center scheme and the internal structure design are determined,and the relationship between the end point attitudes and the input parameters is obtained by kinematic analysis.The PGTs ratio of variable remote center of motion mechanism with noncircular planetary gear trains is solved.4)The criterion of pitch curve convexity of noncircular gear is established,and the kinematic characteristics(angular displacement,non-circular gear,working space)of the mechanism under different linkage lengths are analyzed by taking the working space requirements and the non-circular gear convexity as the constraint conditions.The linkage length ratio of the mechanism is 1.3650 to 3.8637(the ratio of the end output link length to the length of the planet carrier),and a conical working space with a half cone angle of 300 to 94.21° is obtained.5)Three appropriate linkage length ratios are selected as the mechanism parameter of the RCM mechanism with variable remote center,and the 3D modeling and simulation analysis are carried out.Finally,the prototype assembly and test are completed,and the experimental remote center position and working space are compared to verify the correctness of the theory. |
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