Single Actuator Undulation Soft-bodied Robots Using A Precompressed Variable Thickness Flexible Beam

TL;DR

Soft robots often require many actuators to achieve undulation, which increases weight and limits agility. This work designs a tendon-driven variable-thickness flexible beam precompressed into an initial S-shape and driven by a single motor to generate traveling waves along its axis, using a linear thickness profile $d_i = f(l_i) = \frac{d_A - d_B}{L} l_i + d_B$ with $L=140\,\text{mm}$, $H=10\,\text{mm}$, $d_A=6\,\text{mm}$ and $d_B\in\{6,4,2\}\,\text{mm}$, plus precompression offsets $\Delta_L \in \{0,5,10,15\}\,\text{mm}$ and winding amplitudes $\Delta_\tau \in \{15,20,25,30,35,40\}\,\text{mm}$. The authors fabricate the beams via SLA printing, analyze how thickness and precompression affect traveling waves, and demonstrate a snake-like soft robot powered by a single actuator. Key contributions include the design and fabrication of the variable-thickness beam, parametric analysis of precompression and wind/unwind amplitudes on wave propagation, and a practical single-actuator demonstration. This approach reduces actuator count and control complexity for undulatory locomotion and offers potential for locomotion in unstructured or aquatic environments.

Abstract

Soft robots - due to their intrinsic flexibility of the body - can adaptively navigate unstructured environments. One of the most popular locomotion gaits that has been implemented in soft robots is undulation. The undulation motion in soft robots resembles the locomotion gait of stringy creatures such as snakes, eels, and C. Elegans. Typically, the implementation of undulation locomotion on a soft robot requires many actuators to control each segment of the stringy body. The added weight of multiple actuators limits the navigating performance of soft-bodied robots. In this paper, we propose a simple tendon-driven flexible beam with only one actuator (a DC motor) that can generate a mechanical traveling wave along the beam to support the undulation locomotion of soft robots. The beam will be precompressed along its axis by shortening the length of the two tendons to form an S-shape, thus pretensioning the tendons. The motor will wind and unwind the tendons to deform the flexible beam and generate traveling waves along the body of the robot. We experiment with different pre-tension to characterize the relationship between tendon pre-tension forces and the DC-motor winding/unwinding. Our proposal enables a simple implementation of undulation motion to support the locomotion of soft-bodied robots.

Figures (10)

• Figure 1: Top: a 3D printed snake-like soft-bodied robot with a pre-compressed tendon-driven variable thickness flexible beam body. The robot uses only one actuator (a DC motor) to generate traveling mechanical waves along the body to support undulation locomotions. Bottom: snapshots of an undulation motion.
• Figure 2: Parametric design of the variable thickness flexible beam, (a) nonlinear variable thickness, (b) linear variable thickness.
• Figure 3: Design of the variable thickness flexible beam. One end of the beam is tapered. The thicknesses of the two ends of the beam are 6mm-6mm, 6mm-4mm, 6mm-2mm. The beam is initially pre-compressed by connecting two tendons to a motor at the thickest end. (a) Tapered flexible beam with tendons, (b) Pre-compressed flexible beam by shortening both tendons to induce initial buckling state.
• Figure 4: 3D-printed variable thickness flexible beam with different value of morphology parameters
• Figure 5: (a) We measure the tension force of the tendons using two hand-held force gauges. We simulate the winding/unwinding of the motor by moving the two hand-held force gauges in reversed order. We experiment one by one with three types of variable thickness flexible beam $S_{62}$, $S_{64}$, $S_{66}$. (b) Each test will start by precompressing the flexible beam.