## 3 JET Design
### Lower Body Design  
The lower body design goal was to achieve a wide ROM. To accommodate various environments and obstacles tailored to an adult body, a wide lower body workspace is required. The foot workspace can be extended by increasing the leg length or by increasing the joint ROM. However, in the case of JET, the lower body length is limited by the robot actuator. Therefore, to secure a large working space, the ROM of each joint must be increased.  
JET has the same hip joint structure as THORMANG. The hip joint consists of two actuators in roll-pitch order, as shown in Figure 5a, and each axis of rotation is orthogonal at the initial position. To increase the ROM, an S-shape thigh frame was devised, as shown in Figure 5b. This thigh frame reduces interference between the motor and thigh frames, thereby increasing the hip joint ROM, as shown in Figure 5c. Figure 6 compares the JET, THORMANG, THORMANG3, and TALOS ROMs. Because of self-collision, the ROM of one joint is not independent of another joint position. Thus, thigh frame was designed for ROM expansion when both hip joints bend. Hence, the ROM of the two hip joints was expanded by 39.3% and 36.7%, respectively, compared to THORMANG and THORMANG3. The hip joint ROM of TALOS, which is introduced in [18], is also smaller than the ROM of JET. The authors of [21] developed HRP-SP, which has 14.8% larger hip roll ROM than that of JET when the pitch angle is 110\({}^{\circ}\). JAXON3-P, which has a 1.7 m height and 70 kg weight, also has 14.8% larger hip roll ROM than that of JET when the hip pitch is 110\({}^{\circ}\)[24]. To increase the knee ROM, the curves of the back of the thigh and shin frame were designed similarly, and the knee actuator was relocated from the shin to the thigh. These changes allow JET to perform more varied postures than THORMANG.  
THORMANG has a larger upper-to-lower body height ratio than that of a typical young female with similar height, at 1:0.79 compared to 1:1, respectively. The large head, which includes many cameras, is one of the reasons for this difference. Generally, a humanoid robot with a shorter pelvis and lower shoulder height than humans has performance limitations for various tasks involving human life tools such as ladders and stairs.  
Figure 5: Lower limb design: (**a**) lower body standby posture, (**b**) thigh design, (**c**) lower body squatting pose. The lower body frames were designed to avoid self-collisions. In particular, the thigh frames were designed such that the link front surfaces are perpendicular to the ground.  
The JET lower body design is based on the average body proportions of a young female for more natural behavior in industry and service applications. Thus, the lower body is lengthened from 696 to 839 mm compared to THORMANG. The the thigh length to shin length to ankle joint height ratio is 1:1:0.26, which is adjusted to the average ratio for a young female. Figure 3b shows the length of each part. With this leg length increase, the upper-to-lower body height ratio is almost 1:1.  
In addition to the functional advantages, the S-shape thigh frame has aesthetic benefits, having a fuller shape than that of the shin and human-like body proportions. As shown in Figure 5c, the S-shape thigh frame seems to be stretching the knees in the standby pose, because the thigh front surface is perpendicular to the ground.