# Equivalent Dynamic Analysis of a Cable-Driven Snake Arm Maintainer
## 2 Related Work  
### Underactuated SAM Platform  
As shown in Figure 2, the designed SAM arm is 2300 mm long and contains 10 joints. To suit the narrow environment, the underactuated principle is used for a lightweight and miniaturized design. The SAM designed for the actual needs of the CFETR vacuum chamber includes three main parts: a multijoint snake arm, an end effector, and a cable drive box. The movement of each joint involves the use of three evenly distributed cables along the circumference to achieve a two-degrees-of-freedom rotational movement. The end effector can be equipped with various tools, such as vision, clamping, cutting, and dust-removal tools, whereas the signal transmission is realized through the internal circular hole wiring of the joint unit. Meanwhile, multiple sets of servo motors are installed in the cable drive box, with the screw rod module driven by the servo motors to pull the cables and realize motion control of each joint group of the SAM.  
Figure 2: SAM design model (**a**) and prototype (**b**).  
The principle behind the underactuated design lies in using a single-drive motor group (three motors) to drive multiple joint units to form a joint group to achieve synchronous rotation of the angle. Figure 3a shows the cable distribution of each joint in a single-joint group, where each joint cross-section contains 15 rows of cable through-holes. The first joint group contains three joints driven by cables in three rows of through-holes, denoted by the blue dashed lines in Figure 3a. The distribution of cables along the joint section is shown in Figure 3b. Here, each row of cables is fixed in layers in the through-holes of the first, second, and third joint units in the first joint group from the outside to the inside in the radial direction. The other end of the cable is driven by the screw rod module, as shown in Figure 3c. A transmission gear is installed at the end of each screw rod to realize the movement of the three screw rods in the screw-rod module in a ratio of \(1:2:3\). The screw-rod module drives the multiple joints in a joint group to realize the synchronous movement of the adjacent joints in the space.  
To highlight the novelty of underactuated SAMs, we compared the snake arm parameters produced by OC robotics and SIASUN robotics, as shown in Table 1. The following advantages can be achieved: (1) Under the premise of ensuring high spatial curvature and load capacity, the complexity of the drive system and control system is reduced by multiples. (2) The synchronous movement of adjacent joints considerably simplifies the kinematics model, and the inverse kinematics solution is more convenient and quicker. (3) A precise kinematics model can be established with high position accuracy. (4) The structural design can achieve miniaturization and lightweight, which are highly suitable  
Figure 3: SAM underactuated mechanism and cable distribution. (**a**) Synchronous movement of the underactuated joint group rotation angle. (**b**) Distribution of traction cables for each joint. (**c**) The screw-rod module containing three screw rods moves in a ratio of \(1:2:3\) through the gear set.  
for the application requirements of complex and narrow spaces, such as those involving nuclear power [33].