# Performance Analysis of Hybrid Kinematic Mechanism for Fusion Reactor Maintenance
## 1 Introduction  
Under the European Fusion Development Agreement (EFDA), the EU is conducting a series of concept studies and development activities based on the Demonstration Fusion Power Plant (DEMO) [1]. Remote maintenance of the breeding blanket (BB) is a technical challenge that must be addressed before DEMO can operate commercially [2, 3]. As shown in Figure 1, the Tokamak is currently designed to be maintained through 16-18 vertical ports from the top of the vacuum vessel, with each vertical port responsible for maintaining the corresponding five BBs [4, 5]. The main functions of the BB are the following: (1) to achieve tritium multiplication and maintain the tritium needed for the fusion reactor; (2) to convert the energy produced by the fusion reactor into usable heat, electricity, etc. ; (3) radiation shielding, to reduce the diffusion of reflective material and protect the magnet material. The gap between adjacent BBs in a vacuum vessel is only 20 mm, and with the largest BBs reaching a height of 10 m and a mass of about 80 tons, it is challenging to install and remove them precisely. The expected gamma radiation dosage during vacuum vesselmaintenance is about 2000 Gy/h, which means that all operations must be remote, and the radiation level limits the selection of the technical solution. The current design lifetime of BB in the DEMO vacuum vessel is 5 years. To determine the optimal maintenance design, DEMO conducted a large and extensive architectural research study, which concluded that Multi-Module Blanket Segments (MMS) and Vertical Maintenance Schemes (VMS) are the preferred layouts for maintenance [6,7]. For the design of the BB transporter, extensive brainstorming was conducted at an early stage, mainly on the comparison of the series kinematic mechanism (SKM), the parallel kinematic mechanism (PKM), and the hybrid kinematic mechanism (HKM) [8,9]. Through the preliminary evaluation of different structures and the feasibility analysis of the structures' functions, an alternative BB maintenance scheme is proposed based on the HKM. As shown in Figure 1b, the configuration of the HKM is developed based on the classic Tricept parallel mechanism, which is a 3 UPS-PU parallel mechanism with 3-DOFs. The end of the HKM is a series mechanism rotating around three axes, mounted on a Tricept parallel mechanism kinetic platform, which gives the HKM 6-DOFs the ability to meet the end-effector position and orientation control requirements. The maintenance task of the inboard and outboard BBs can be finished separately by the trajectory control of the HKM. The special advantage of the HKM is that it has relatively high stiffness, high load capacity, and high position accuracy [10], and at the same time has a relatively large working space and simple design of the series robot [11,12]. The HKM can achieve relatively large positioning and orientation capabilities; therefore, it is suitable for applications in confined spaces within vacuum vessels. There are hundreds of existing PKMs, however, not many PKMs are used in the actual industrial production process. The main reason is that PKMs generally require a large number of passive joints in their design, and these passive joints will greatly increase the deformation and geometric errors of the whole machine under disturbance forces and overload conditions [13,14]. Therefore, many researchers have been exploring various hybrid mechanism design options. The development of the HKM based on the Tricept platform can achieve a large working space and positioning requirements, which have attracted a lot of attention [15]. This interest led to the development of the HKM and a large number of related papers. [PERSON] et al. [16,17] added 2-3-degree of freedom (DOF) wrists to the Tricept kinetic platform to form a 5-6-DOF hybrid robot and proposed an elastic dynamic modeling and dimensional synthesis method. Based on helical theory and structural dynamics, the kinetic and elastic potential energies of the Tricept mechanism and the series wrist were calculated [18].