## 3 Methods and Results
### Repeatability and Hysteresis Test  
During the tracking test, shown in Figure 16a, the OptiTrack optical motion tracking system was utilized to track the position of the robot's tip. Four reflective markers were affixed to the bench to determine the base coordinate system. The robot scaffold was fixed on the bench using double-sided tape. Six syringe pumps were used to drive the actuators. Commands from the laptop were transmitted via a USB to two Arduino Mega2560 boards, with each manipulating three syringe pumps. The open-loop controlled robot tracked a circle with a diameter of 43 mm, repeated ten times (Supplementary Materials). The tracking results are depicted in Figure 16b, while Figure 16c showcases the tracking errors within one cycle. The maximum, mean, and standard deviation for tracking errors amounted to 11.05 mm, 6.00 mm, and 6.65 mm, respectively. The tracking error and the observed  
Figure 14: The measured robot workspace. (**a,b**) Along the X axis. (**c,d**) Along the Z axis. (**e,f**) Along the Y axis.  
Figure 15: The scaffold stiffness test. (**a**) The stiffness test’s setup. (**b**) The variable stiffness test results.  
trajectory shape may be attributed to several factors: (1) The deformation of the robot: the inherent flexibility of the inflated beams on the scaffold may deform, causing the actuator to deviate from its intended pose. (2) Unexpected actuator behavior: despite being assumed to have two rigid linkages connected by a 1-DOF revolute joint, the actuator may bend off-plane. The positions of the revolute joints may deviate from their intended positions. (3) The control expression of each actuator is based on a test setup that does not fully reflect the constraints imposed on the actuator by the robot scaffold, which may also introduce slight errors. (4) Manufacturing errors: as seen in Section 2.6, the edge welded manually along the green lines differs from other edges, which introduce slight asymmetries in the scaffold. Nevertheless, the test shows the robot has very high repeatability. The repeatability error was tested by comparing each point to the average value of ten corresponding points in ten cycles. Table 1 shows the maximum, mean, and root mean square errors (RMSEs) for repeatability, indicating that both the mean error and RMSE are smaller than 0.5 mm.  
In theory, the robot should not exhibit hysteresis since the cable is directly connected from the actuator to the effector without any intermediary contact. The hysteresis test procedure involved controlling the tip to execute a circular motion for ten repetitions and then returning to its initial position. The distance between the final and initial positions was used to quantify hysteresis. Only the two rotational DOFs were active during this process, with the rotation center located at the effector axis and the midpoint of the two groups of attachment points. Figure 16d shows this process, and Figure 16e provides an enlarged view around the initial point. Table 2 presents the results of the hysteresis test, where circles of different diameters were tracked. The hysteresis values were all below 2.5 mm. When converted to angle hysteresis, calculated by dividing the position hysteresis by the distance between the robot tip and the rotation center, all four hysteresis angles were below 2\({}^{\circ}\).  
\begin{table}
\begin{tabular}{c c c c} \hline  & **Maximum** & **Mean** & **RMSE** \\ \hline Error X/mm & 1.1311 & 0.2661 & 0.3219 \\ Error Y/mm & 0.8929 & 0.1918 & 0.2279 \\ Error Z/mm & 0.4326 & 0.0818 & 0.0993 \\ Absolute error/mm & 1.3551 & 0.3645 & 0.4186 \\ \hline \end{tabular}
\end{table}
Table 1: The repeatability test results.  
Figure 16: Robot tracking. (**a**) The test setup. (**b**) The results of tracking a 43 mm diameter circle. (**c**) The tracking errors. (**d**) The results of tracking a 36 mm diameter circle. (**e**) An enlarged view around the initial point.