# A Worm-Inspired Robot Flexibly Steering on Horizontal and Vertical Surfaces
## 1 Introduction
Another method is based on microstructures such as micro hooks to provide friction force against working surface. For example, the Kirigami skin structure of the robot described in [16] imitates the scales of snakes and prevents the robot from unidirectional sliding on the working surface. The above-mentioned method is very suitable for rough and soft working surfaces; however, as the friction force provided by microstructures is hugely affected by the pressure and roughness of the surface, its performance on vertical and large angle inclined planes is limited. For most earthworm-inspired robots with only liner motion capacity, the structure and control system design are simple. The robot has a segmented structure and liner telescopic actuators between two adjacent segments. The robot will move forward when those actuators work in a predesigned sequence. For example, the earthworm-inspired friction-controlled soft robot described in [16] uses two independent elastic airbags at the front and rear ends, in which the friction coefficient with the working surface changes with the air pressure. The third airbag is located in the middle, which will perform telescopic movement. During motion, the pressure in each airbag is independently controlled and works in a predesigned sequence. Considering the elasticity of the airbag, the whole structure can be regarded as a double mass-spring-damper model, and thus its position, velocity, and posture can be solved at any moment during motion. In order to have the capacity of more flexible motion, such as two-dimensional steering motion, the structure and control system design of the robot have to be much more complex. For pipeline inspection robots, the ability to pass through elbow pipes is necessary. Therefore, earthworm-inspired pipe robots are usually designed with elastic structures that can help them move along curved pipes [9; 10]. For example, the pipe inspection robot introduced in [9] is designed to pass through complex elbow pipe systems. On the one hand, each segment of the robot is made of elastic materials such as rubber and springs to gain flexibility. On the other hand, two different motion patterns are developed in order to adapt to different pipe situations such as continuous elbow pipes and straight pipes. However, such design does not have the ability to conduct steering motion independently, which may become a serious problem when these robots reach the intersection points in complex pipeline systems. Based on the motion principle of bionic earthworms, we designed and fabricated a novel worm-like robot that can overcome the above limitations. By adopting the principle of underactuation [22], this robot is able to creep on a plane that is inclined at any angle from horizontal to vertical with a controllable and autonomous steering capability. The organization of this paper is as follows. Section 2 gives an introduction of the structure of the robot, Section 3 analyzes the motion process of the robot on the horizontal ground based on theoretical analysis and experiments, and Section 4 analyzes the working performance of the robot on vertical walls, combining the theory and experiments.