# Performance Analysis of Hybrid Kinematic Mechanism for Fusion Reactor Maintenance
## 6 Conclusions  
This paper first introduces an HKM for BB maintenance in a DEMO vacuum vessel, and then establishes the forward and inverse kinematic models of the HKM using a combination of analytical method and numerical iteration. By simulating the inboard and outboard BB remote maintenance process, the trajectory planning and simulation analysis of the HKM are carried out in Cartesian space and joint space, respectively, which validates the feasibility of the structure design. However, the high self-weight of 80 tons, the high working environment temperature, and the radiation environment in the vacuum vessel limited the selections of drive and lubrication systems for the HKM. The thread driving cylinder and dry friction lubrication degrade the spherical joint at one end of the piston into a universal joint, which causes position errors in the HKM end-effector. Through simulation, we calculated the error distribution caused by changing the spherical joint into a universal joint. During the lifting of the left outboard BB, the end-effector produces a maximum absolute error \(\Delta P=1.5\) mm, and during the lifting of the left inboard BB, the end-effector produces a maximum absolute error \(\Delta P=3.1\) mm. Combined with the overall effects of HKM rigid-flexible coupling deformation, assembly, machining, and other factors on the accuracy, the end position error will be further expanded. To compensate for the global errors, we propose a variable parameter error compensation plan based on the LM nonlinear damped least-squares algorithm. Error compensation can be realized through the HKM workspace gridding and offline parameter error identification. The simulation results show that the maximum error, mean error, and standard deviation at the end of the HKM are significantly reduced, which verifies the effectiveness of the compensation algorithm. The HKM has high stiffness, high load, and high precision compared to SKM,  
Figure 14: Lifting left outboard (**a**) and inboard (**b**) BB variable grid density results analysis.  
and a large working space compared to the PKM. It can achieve relatively large positioning and orientation capabilities and is suitable for applications in confined spaces within vacuum vessels.  
In the future, we will further analyze the HKM end position accuracy by combining the dynamics and rigid-flexible coupling characteristics of the HKM. Then, various error factors will be incorporated into the HKM controller design to achieve HKM closed-loop control and online compensation of end position errors.  
The following supporting information can be downloaded at [[https://www.youtube.com/shorts/yzCeUeoBLM](https://www.youtube.com/shorts/yzCeUeoBLM)]([https://www.youtube.com/shorts/yzCeUeoBLM](https://www.youtube.com/shorts/yzCeUeoBLM)), Video S1: HKM lifting BBs.mp4.  
Conceptualization, H.W. and S.B.; methodology, C.L.; software, G.Q.; validation, S.B., H.W., C.L. and G.Q.; formal analysis, C.L. and G.Q.; investigation, H.W. and C.L.; resources, H.W. and A.J.; data curation, C.L.; writing--original draft preparation, C.L. and G.Q.; visualization, H.W., G.Q. and C.L.; supervision, H.W., S.B. and A.J.; project administration, H.W.; funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript.  
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No. 101052200--EUROfusion). Views and opinions expressed, however, are those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.  
Not applicable.  
The data and code used to support the findings of this study are available from the corresponding author upon request.  
The authors declare no conflict of interest.