# A Worm-Inspired Robot Flexibly Steering on Horizontal and Vertical Surfaces
## 5 Conclusions and Future Work  
In this paper, a novel earthworm-inspired segmented crawling robot was designed, built, and tested. Its skeleton structure is made of nylon and is divided into four segments. The periodic crawling motion of the structure is actuated by pneumatic power and achieved by sequentially moving each of the segments. Compared with previous research results, this robot has the following novelties:  
(1) A novel steering mechanism based on the underactuated principle is designed. By adjusting the length of tendons on both sides, the steering motion of the robot can be precisely controlled. Compared with previous worm-inspired robots with only liner motion capacity, the above steering mechanism enables our robot to have more flexible motion, similar to worms in nature. Compared with previous snake-like robots or legged robots, which use a large number of actuators to control each of the degrees of freedom independently, our robot uses only two digital servos to control the deflection angles of four segments, which makes it not only much easier to control, but also easier to fit into a miniaturization design. Figure 18: A diagram showing the relationship between the thrust, the deflection angles of each segment, and the cylinder elongation \(x\). Segments 2 and 3 have relatively higher thrust and thus have better load capacity, while Segment 1 and Segment 4 should be specially designed to reduce their weight. (2) With suction cups mounted on the feet, the robot is able to crawl not only on horizontal planes, but also on inclined or vertical planes, which is beyond the capabilities of any of the previous earthworm-inspired robots. The floating platforms that the feet are mounted on help keep the suction cups from abrasion against the wall and make the absorption process more reliable. Moreover, as the feet of the robot adopt modular design, the suction cups can be replaced by other foot structures according to the particular environment in order to achieve better steering efficiency and speed. For example, a foot with spines can be used on porous surfaces, and magnetic feet can be used on metal surfaces. (3) The structure of the robot is designed to be extensible, as its structure can be added to five or even more segments. Robots with different numbers of segments share the similar motion process, as discussed in Section 4.1. Such characteristics make the robot easier to be used as a general purpose modular crawling platform, because segments carrying different devices can be added to its structure in practical application. Basing on the novelties above, we established the theoretical model of the robot during the steering process, and analyzed the theoretical relationship between the length of the tendon and the steering angle, and simulated the trajectory of the robot during the entire movement. Experiments were conducted to verify the theoretical motion characteristics. In addition, by establishing a theoretical model, we calculated the traction force that each segment of the robot can provide, and analyzed the relationship between the traction force and other parameters. We estimated the maximum load that each segment can withstand during motion. Our robot prototype has the ability to move in two dimensions on a plane, but in further exploration, we will try to enable it to move in three dimensions, such as a pitching motion. By increasing the degree of freedom of its skeleton structure and adding a new set of tendons to control, we can enhance the robot's working ability and obstacle avoidance in complex environments. The individual contributions of authors: conceptualization, W.Y. and W.Z. ; methodology, W.Y. ; validation, W.Y. ; formal analysis, W.Y. ; investigation, W.Y. ; resources, W.Z. ; data curation, W.Y. ; writing of original draft preparation, W.Y. ; writing of review and editing, W.Y. and W.Z. ; project administration, W.Z. ; funding acquisition, W.Z. This research was supported by the National Natural Science Foundation of China (No.