## 4 Design of the Origami Crawling Robot
### System Design of the Origami Crawling Robot  
An eight-layer twisted tower is used as the skeleton of the crawling robot. The forward and bending motions of the crawling robot are achieved by operating on the four ropes interspersed in the origami structure. DC motors are installed on the head or tail of the crawling robot. Four sliding feet are used to support the body and increase the friction with the ground. In each contraction and extension cycle, a net forward motion is achieved due to the anisotropic friction between the sliding feet and the ground. The conceptual design of the origami crawling robot is as shown in Figure 13. A 5 V battery is used to supply power for the DC motor array, microcontroller, and camera. The optional camera can be used to capture pictures in narrow environments, such as pipelines or ventilation ducts. In the proposed crawling robot system, the anisotropic friction in different directions is used to generate a motion in the target direction. The side view of the crawling robot \begin{table}
\begin{tabular}{c c} \hline \hline
**Type of DC Motor** & **DS04-NFC** \\ \hline Voltage & 4.8 V–6 V \\ Torque & 5.5 kg/cm \\ Speed & 60\({}^{\circ}\)/0.22 s \\ Weight & 38 g \\ Size & 40.8 mm \(\times\) 20 mm \(\times\) 39.5 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of the DS04-NFC DC motor. Figure 13: Design schematic of the origami crawling robot. and the two types of sliding feet are as shown in Figure 14. The front contact part of the rubber sliding foot is arc-shaped and made of a material with a low friction coefficient. The end contact part is made of a patterned rubber pad to provide greater friction, as shown in Figure 14a. When the ground is made of ferromagnetic materials, a sliding foot with an embedded electromagnet can be used to generate an adhesion force and achieve reliable crawling motions, as shown in Figure 14c. The adhesion force is controlled by the current in the coil of the electromagnet. When the origami robot is crawling on the iron ground with electromagnetic sliding feet, the MCU can make the electromagnets switch between the \"on\" and \"off\" states in order to adjust the friction force of the four sliding feet periodically. The electromagnet with a size of \(\Phi\)20 mm \(\times\) 15 mm can generate a 2.5 kg force, as shown in Figure 14d. Further details of the electromagnet KB-20/15(KBAOELE, Leqin, China) are as shown in Table 2.  
Friction between the sliding feet and the surface plays an important role in the proposed origami crawling robots. Here, the sliding test with an inclined plane was used to estimate the maximum coefficient of static friction. The sliding feet were placed on the wooden or steel platform, and the angle between platform was increased until the object just began to slide on the plane. The angle \(\varphi\) can be measured with the angle meter, and the tangent of this angle \(\mathrm{tg}\varphi\) is the maximum coefficient of static friction [34]. The experimental setup was as shown in Figure 15, and the coefficient of friction between the sliding feet and different surfaces can be found in Table 3. Since there are some difference in design between the rubber front foot and rubber back foot, both types were tested with forward sliding and backward sliding. The experimental results show that the friction coefficient of backward motion is commonly larger than that of forward motion for both rubber feet and electromagnetic feet; this design could help the robot move forward more easily. \begin{table}
\begin{tabular}{c c} \hline \hline
**Type of Electromagnet** & **KB-20/15** \\ \hline Maximum adhesion force & 2.5 kg \\ Size & \(\Phi\)20 mm \(\times\) 15 mm \\ Working voltage & 24 V \\ Working current & 0.1 A \\ Weight & 25 g \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of the KB-20/15 electromagnet.