# Research on the Dynamics of the Space Tubular Expandable Structure Driving Deployment Unit
## 4 The Experimental Results and Discussion  
### Inter-Level Transition Test  
To validate the stability of the parallel triangular configuration structure constructed based on the theoretical node displacement description using the absolute nodal coordinate formulation (ANCF), a 1:1 scale experimental prototype was designed and fabricated. The material selection and boundary conditions were set to be consistent with those used in the theoretical and simulation stages. The experimental setup can be referred to in Figure 20.  
Figure 19: The comparison of the displacement trajectories. (**a**) Centroid displacement in the X-direction; (**b**) Centroid displacement in the Y-direction; (**c**) Centroid displacement in the Z-direction; (**d**) Trajectory rotation angle. During the experiment, various parameters of the driving and stowing units were monitored using sensors. Figure 21 illustrates the installation positions of the sensors in the experimental prototype. Displacement sensors, velocity sensors, and angle sensors were utilized to measure the endpoint displacement, axial movement velocity, and rotational angle during the unfolding phase of the driving and stowing units. To simplify the motion structure for the experiment, symmetrical elements were chosen within the variable-radius inner-drive unit, as both the driving and stowing units exhibit a symmetrical structure. The experimental setup involved a stepped surface to simulate the non-continuous interior surface of the cylindrical structure. The driving motor was responsible for providing the driving force to an elastic wheel, while the pre-tensioning motor applied a radial pre-tensioning load, pressing the elastic wheel against the stepped surface. Moreover, the driving arm, pre-tensioning components, and pre-tensioning motor were aligned with the central column and equipped with lubricated ball bearings and guide rails. These modifications facilitated axial unfolding, combining the traction force from the elastic wheel with the radial pre-tensioning load from the pre-tensioning components. The experiment aimed to assess the stability of the parallel triangular configuration structure with thin-walled flexible beam elements after implementing the ANCF method for node displacement description, using the same boundary conditions as in the simulation. Figure 22 illustrates the endpoint displacements in the x and y directions at both the free and constrained ends during the experimental phase. To obtain a more representative result, the data were divided into 40 groups and averaged, resulting in four curves representing the average displacements in the x and y directions for each group. Figure 21: Prototype local diagram. Figure 20: Test prototype diagram. From Figure 22a,b, we can observe a consistent trend in the average displacements in the x and y directions for the four groups of endpoints during the experimental phase. However, noticeable fluctuations occur at the inter-level discontinuities or steps due to the contact between the stowing unit and the inner wall of the variable-radius inner-drive unit using an elastic wheel. Specifically, at 0.25 s, there is a peak displacement in the x-direction, and between 0.25 s to 0.5 s, there is an axial displacement in the y-direction. This phenomenon is attributed to the transitional phase between levels where the elastic component undergoes deformation, and the driving and stowing unit experiences its first non-continuous surface. Upon further comparison with the free-end x and y displacements in Figures 10 and 11, it becomes evident that the displacement trends of any group in Figure 22 more accurately reflect the motion state during the same time period. Therefore, combining the free and constrained ends as a complete motion mode for the driving and stowing unit is a valid approach. Figure 23 illustrates the trajectory angles during the experimental phase.