Joint-repositionable Inner-wireless Planar Snake Robot

TL;DR

The paper tackles the weight and power drawbacks of traditional multi-joint snake robots by introducing a joint-repositionable inner-wireless design in which motor-driven units move along internal rack gears to reconfigure joint coupling. A wireless-charging-enabled soft robot skin powers these joints, enabling untethered operation at approximately 7.6 W input with joints receiving up to 3.6 W, and yields a lightweight 1.3 kg platform. The authors present a variable-length arc-shaped joint model to describe kinematics and two locomotion strategies: a Serpentine, joint-position-free mode and an obstacle-aided, joint-position-based mode, validated through simulations and a 3-joint prototype. Wireless charging shows robust efficiency above 50–60% across bending and minimal EMI with motors, while experiments demonstrate feasible planar locomotion speeds of ~2.25 and ~0.76 length units per time in respective modes. Overall, the work offers a practical path toward energy-efficient, articulated snake robots capable of untethered operation in constrained environments, with clear avenues for 3D locomotion and smarter control emerging in future work.

Abstract

Bio-inspired multi-joint snake robots offer the advantages of terrain adaptability due to their limbless structure and high flexibility. However, a series of dozens of motor units in typical multiple-joint snake robots results in a heavy body structure and hundreds of watts of high power consumption. This paper presents a joint-repositionable, inner-wireless snake robot that enables multi-joint-like locomotion using a low-powered underactuated mechanism. The snake robot, consisting of a series of flexible passive links, can dynamically change its joint coupling configuration by repositioning motor-driven joint units along rack gears inside the robot. Additionally, a soft robot skin wirelessly powers the internal joint units, avoiding the risk of wire tangling and disconnection caused by the movable joint units. The combination of the joint-repositionable mechanism and the wireless-charging-enabled soft skin achieves a high degree of bending, along with a lightweight structure of 1.3 kg and energy-efficient wireless power transmission of 7.6 watts.

Figures (8)

• Figure 1: Concept of joint-repositionable inner-wireless planar snake robot. Our robot enables multi-joint-like locomotion while remaining low-powered and lightweight structure. Inside the robot, joint-repositionable units can move freely, enabling to construct the various joint coupling. The units are powered wirelessly through a soft robot skin.
• Figure 2: Design and operation of a joint-repositionable, inner-wireless planar snake robot. (a) Schematic of the joint-repositionable snake robot, including repositionable joint units and a fixed joint unit. (b) Detailed view of the joint unit components, including a DC motor with an encoder, a flexible rack gear, and a wheel. The joint unit types are categorized as repositionable and fixed, with the basic operations of moving and bending depicted. (c) Two types of locomotion strategies for adapting to different environments: joint-position-free serpentine locomotion for obstacle-free, narrow terrains, and joint-position-based, obstacle-aided locomotion.
• Figure 3: Design overview of a wireless-charging-enabled soft robot skin. (a) Circuit diagram of the soft robot skin.(b) Simulated inductive field of the soft robot skin model, and (c) power transfer efficiency for the soft robot skin geometry. (d) Photograph of the soft robot skin composed of a liquid-metal-based transmitter coil. (e) Photograph of joint units connected to an RX coil. (f) Fabrication process of the soft robot skin.
• Figure 4: Schematic of the variable-length arc-shaped joint model. Our robot, comprising $N$ joint units, is represented as a sequence of arcs with uniform curvature. The rotational movement of the motors ($d$) changes the length ($L$) and angle ($\theta$) of each arc segment.
• Figure 5: Locomotion control strategy using the variable-length arc-shaped joint model. (a) Illustration of joint-position-free serpentine locomotion along a blue-colored serpenoid curve. (b) Illustration of joint-position-based obstacle-aided locomotion, highlighting orange-colored counter forces. When the fixed shape encounters an obstacle, the resulting counterforce propels the robot forward while keeping its blue-colored shape.