TerraSkipper: A Centimeter-Scale Robot for Multi-Terrain Skipping and Crawling

Abstract

Mudskippers are unique amphibious fish capable of locomotion in diverse environments, including terrestrial surfaces, aquatic habitats, and highly viscous substrates such as mud. This versatile locomotion is largely enabled by their powerful tail, which stores and rapidly releases energy to produce impulsive jumps. Inspired by this biological mechanism, we present the design and development of a multi-terrain centimeter-scale skipping and crawling robot. The robot is predominantly 3D printed and features onboard sensing, computation, and power. It is equipped with two side fins for crawling, each integrated with a hall effect sensor for gait control, while a rotary springtail driven by a 10mm planetary gear motor enables continuous impulsive skipping across a range of substrates to achieve multi-terrain locomotion. We modeled and experimentally characterized the tail, identifying an optimal length of 25mm that maximizes the mean propulsive force (4N, peaks up to 6N) for forward motion. In addition, we evaluated skipping on substrates where fin based crawling alone fails, and varied the moisture content of uniform sand and bentonite clay powder to compare skipping with crawling. Skipping consistently produced higher mean velocities than crawling, particularly on viscous and granular media. Finally, outdoor tests on grass, loose sand, and hard ground confirmed that combining skipping on entangling and granular terrain with crawling on firm ground extends the operational range of the robot in real-world environments.

Figures (9)

• Figure 2: TerraSkipper with representative challenging terrains shown in the surrounding circular pictures, on which the robot can locomote. A U.S. quarter and a ruler indicate scale.
• Figure 3: Exploded view of the CAD model of TerraSkipper, highlighting key components: tail motor with propeller, spring steel element, hall effect sensor placement with embedded fin magnets, fin motors, chassis, battery, custom PCB, and PCB holder.
• Figure 4: The four transition phases of the spring steel during the flicking motion of the jumper mechanism. Free Rotation: the spring rotates with the motor shaft without contacting the curved wall; Load: the spring tip engages the hollow cylinder and begins to bend, storing elastic energy; Latch: the spring is fully conformed and constrained inside the 270° arc, storing maximum energy; Unlatch: the spring tip exits the arc boundary, releasing stored energy as a rapid snapping motion that propels the robot forward.
• Figure 5: A) Experimental setup for tail characterization. The robot was mounted to the end effector of a UR5e robot arm via a 3D-printed mount. A force/torque sensor fixed on the table with an additional flat mount is used to record the tail’s impact force and frequency. B) Force measured by the force/torque sensor for different tail lengths. For each length, points show all peaks from 10s recording; the square marker indicates the mean with a 95% bootstrap confidence interval. The annotation $n$ reports the number of peaks (tail strikes) detected within the 10s window.
• Figure 6: Top-down trajectories of TerraSkipper tracked from overhead video on a smooth surface. For each condition, Encoder (Sync gait), Encoder (Async gait), and No encoder, three trials were performed and plotted to illustrate deviation from a straight path on a smooth acrylic surface.