# A Worm-Inspired Robot Flexibly Steering on Horizontal and Vertical Surfaces
## 3 Motion Process Analysis and Experiment on Horizontal Platform  
### Worm-Inspired Motion Process  
Similar to the worm movement in nature, the motion of this robot is in cycles. During each movement cycle, each segment of the worm-like robot moves forward once one by one to achieve the forward movement of the overall position. In particular, for this robot prototype, its motion cycle is divided into three phases. Assume that the state in the initial phase is as follows: cylinders in Segment 1 and Segment 2 are in the shortened state, the cylinder in the Segment 3 is in the extended state, and each foot is adsorbed on the working surface. Then, the following movements occur during the next motion cycle:  
Phase 1: The feet of Segment 1 and Segment 4 are off the work surface and lifted; the remaining feet are still attached to the work surface. Then, the cylinders of Segment 1 extend and the cylinder of Segment 3 shortens; thus, Segment 1 and Segment 4 move forward until the limit stroke of the cylinder. After that, the motor drives the feet of Segment 1 and Segment 4 to be lowered, and a negative pressure is generated in the suction cups, which is then adsorbed on the working surface.  
Phase 2: The feet of Segment 2 are off the work surface and lifted; the remaining feet are still attached to the work surface. Then, the cylinders of Segment 1 shorten, and the cylinder of Segment 2 extends; thus, Segment 2 moves forward until the limit stroke of the cylinder. After that, the motor drives the feet of Segment 2 to be lowered, and a negative pressure is generated in the suction cups, which is then adsorbed on the working surface.  
Phase 3: The feet of Segment 3 are off the work surface and lifted; the remaining feet are still attached to the work surface. Then, the cylinders of Segment 3 extend, and the cylinder of Segment 2 shortens; thus, Segment 3 moves forward until the limit stroke of the cylinder. After that, the motor drives the feet of Segment 3 to be lowered, and a negative pressure is generated in the suction cups, which is then adsorbed on the working surface.  
The steering motion of the worm-like robot takes place in Phase 1. When the feet of Segment 1 and Segment 4 are separated from the work surface and lifted, Segment 1 and Segment 4 are deflected to one side, centering on the hinge point by controlling the length of the tendons on the left and right sides. In subsequent movements, the robot will achieve a steering creep movement in one direction. The specific parameter control will be analyzed in Section 3.2.  
Figure 4: A diagram of the air circuit that consists of two pumps, nine valves, three sets of cylinders, and four pairs of suction cups: yellow represents the positive pressure generated by the first air pump, blue represents the negative pressure generated by the second air pump, gray represents the atmospheric pressure, and the direction of the arrow is the transmission direction of the air pressure signal. This figure does not cover the up and down motion of the feet controlled by motors.  
The specific work processes of linear crawling and steering crawling are shown in Figures 5 and 6, respectively. Figure 7 shows the input signal variation of the valve system shown in Figure 4 during a single motion cycle. It can be seen that in Phase 1, there are two segments in motion at the same time, which is a little different from phases 2 and 3, where there is only one segment in motion, respectively.  
Figure 5: Four motion phases in a single cycle of the straight crawling process.  
Figure 6: Steering crawling process in a single cycle.  
Figure 7: The solenoid valves’ control signal during the motion cycle of the robot. As shown in Figure 4, valves 1–3 control the pressure in suction cups, and valves 4–9 control the pressure in the cylinders of segments 1–3. If the solenoid valve receives high potential, the cylinder will be connected to the air pump, and if it’s low potential, the cylinder will be connected to the atmosphere.