# Research on the Dynamics of the Space Tubular Expandable Structure Driving Deployment Unit
## 4 The Experimental Results and Discussion
### Inter-Level Transition Test
It is observed that the rotational angle exhibits peak changes during the intervals of 0.15-0.25 s, 0.45-0.55 s, and 0.75-0.85 s. This phenomenon is attributed to the transitional phase between levels, involving the rebound effect of the elastic component and the effect of radial pre-tensioning load, which leads to a counterclockwise rotation of the trajectory angle. However, as the driving and stowing unit progresses axially over continuous surfaces and the diameter of the variable-radius inner-drive unit decreases, the trajectory angle undergoes a clockwise rotation, resulting in a further increase in the rotational angle. Figure 23: Trajectory turning angle. Figure 22: The endpoint displacements in the x and y directions. (**a**) The displacement in the X-direction at the endpoints; (**b**) The displacement in the Y-direction at the endpoints. Upon analyzing Figure 24a-c, it is evident that the centroid's displacement in three directions of the driving and expanding unit, utilizing thin-walled flexible beam elements described with the absolute nodal coordinate formulation (ANCF), shows a slight reduction in the experimental phase compared to the theoretical and simulation phases. The maximum displacements at peak stages are reduced by 1.81 mm, 2.71 mm, and 0.47 mm, respectively. This reduction is attributed to the friction between the elastic wheel surface and the inner wall of the spatial tube. However, the results indicate that the displacements in all directions are significantly smaller than the dimensional parameters of the driving and expanding unit and the spatial tube structure, demonstrating a high level of stability while effectively accomplishing the climbing motion on the inner wall of the spatial tube. However, during the experimental phase, the model's beam units were connected using pin connections, which led to some errors attributed to material processing and environmental factors. Nevertheless, these errors were within an acceptable range. Comparing the experimental results with theoretical and simulated data, the maximum error in positional changes during the experimental phase was 0.41%, and the minimum error was 0.32%. These errors occurred during the transition from the first stage to the third stage, likely due to the gradual reduction in the diameter of the spatial sleeve, resulting in more stable mechanical properties. Additionally, Figure 15d indicated that the rotational angle of the parallel triangular configuration of the thin-walled flexible beam unit almost perfectly matched between the simulation and theoretical stages, implying that this error has no significant impact on the trajectory analysis of the driving and retracting unit. Furthermore, considering that the structure operates in a space environment with zero gravity during orbit, such errors can be neglected, especially with the presence of a temperature control system that prevents material properties from affecting the structure. The experimental results confirmed the excellent stability of the driving and expanding unit with a thin-walled flexible beam structure when operating inside the space telescope's collapsible structure. The experimental findings align closely with the theoretical and simulation analysis results, showing a high level of consistency. The adoption of a thin-walled flexible beam structure in the driving and expanding unit not only enhances the structure's ability to undergo rigid-flexible coupling deformation, improves its dynamic characteristics, and enhances motion precision but also facilitates future lightweight design initiatives. For instance, during the experimental phase, the thin-walled flexible beam structure demonstrated remarkable performance in handling varying loads and maintaining stability.