## 3 JET Design
### Design for Load Alleviation  
Greater leg length increases the moment of inertia as well as the gravity load of the same joint space configuration. As JET employs the same actuators as THORMANG, it was essential to alleviate the load in accordance with the actuator capacity. The frame weight was reduced by adding polygon holes. Through finite element analysis (FEA), a lightweight link was created that could maintain the strength to withstand an applied load without excessive deformation. Figure 8 shows the overall weight proportions of each JET component. The actuators occupy more than 50% of the total robot weight. As the ratio of the actuators to the total weight is significant, the actuator arrangement played an important role in load reduction. In various studies, four bar linkages or ball screws have been used to position the actuators close to the first limb joint [27, 28]. Although transmission helps reduce inertia, these approaches increase the overall weight and mechanical element number. Such changes would complicate achievement of the design goals in this case. Therefore, the JET actuators were placed as close as possible to the parent joint. However, a simple structure is maintained. As shown in Figure 9, various actuator arrangements are possible. The JET actuator positions are colored blue, and the worst cases, with maximum inertia, are colored orange. The inertia values at the hip and shoulder calculated for each case are listed in Table 3. Note that all values were calculated from the initial pose, with full extension of the robot legs and arms. Figure 8: JET weight proportions. Figure 7: Exploded view of computer module. The modularized computer board can be sideways-disassembled by removing four bolts. For example, the total inertia of the entire leg at the hip roll joint is 2.53 kgm\({}^{2}\) for the blue case, with a knee-actuator inertia of 0.22 kgm\({}^{2}\). For the orange case, however, the knee actuator contributes 0.34 kgm\({}^{2}\) to the total inertia with respect to the hip roll joint. Therefore, the inertia due to the knee actuator increases by 50%, and the total inertia increases by 5%. The foot has a major effect on the total inertia. When the robot bends its knees, the proportion of inertia due to the knee actuator increases. Note that the total inertia decreases when the knees bend, but the inertia due to the knee actuator is independent of the joint configuration. Therefore, the proposed design has a greater impact in the walking configuration. Similar to the knee actuator, the forearm and arm actuator positions affect the inertia at the shoulder joint. Without weight reduction, inertia reduction is achieved through actuator rearrangement. Although each JET link was lengthened, the entire link weight is 3 kg less than that of THORMANG. We designed a frame with polygonal-shaped holes, not only for weight reduction but also to enhance the actuator connector accessibility. The actuator modules were utilized as the frame itself to reduce the weight. Thus, in the final design, most actuator modules are exposed. Hence, most actuators can be easily disassembled and replaced. For safety, every frame was assessed via FEA to confirm a yield strength with a gravity load. Most frames were designed to have flat plate shapes for cheap fabrication. These flat frames were manufactured by laser cutting, which is faster and cheaper than CNC milling. Sanding is essential for laser cut frames to remove the roughness of the laser cut surface. However, both rough and sanded surfaces are too inaccurate for assembly. Therefore, we designed flat frames that do not require high edge tolerance for assembly. This design