Effective self-righting strategies for elongate multi-legged robots

Abstract

Centipede-like robots offer an effective and robust solution to navigation over complex terrain with minimal sensing. However, when climbing over obstacles, such multi-legged robots often elevate their center-of-mass into unstable configurations, where even moderate terrain uncertainty can cause tipping over. Robust mechanisms for such elongate multi-legged robots to self-right remain unstudied. Here, we developed a comparative biological and robophysical approach to investigate self-righting strategies. We first released \textit{S. polymorpha} upside down from a 10 cm height and recorded their self-righting behaviors using top and side view high-speed cameras. Using kinematic analysis, we hypothesize that these behaviors can be prescribed by two traveling waves superimposed in the body lateral and vertical planes, respectively. We tested our hypothesis on an elongate robot with static (non-actuated) limbs, and we successfully reconstructed these self-righting behaviors. We further evaluated how wave parameters affect self-righting effectiveness. We identified two key wave parameters: the spatial frequency, which characterizes the sequence of body-rolling, and the wave amplitude, which characterizes body curvature. By empirically obtaining a behavior diagram of spatial frequency and amplitude, we identify effective and versatile self-righting strategies for general elongate multi-legged robots, which greatly enhances these robots' mobility and robustness in practical applications such as agricultural terrain inspection and search-and-rescue.

Figures (7)

• Figure 1: Centipede, centipede robot, and experimental setup. (a) An adult S. polymorpha centipede used in the experiments. (b) The biological experimental setup: self-righting in a glass tank with an acrylic floor is recorded by top and side view high speed cameras. (c) An inverted full-size multi-legged robot necessitating self-righting.
• Figure 2: Sequential and One-shot Self-righting Snapshots (a) Top and side view snapshots of the centipede falling and self-righting using the sequential strategy. The four snapshots indicate the releasing, upside down, initiating self-righting, and successful self-righting respectively. (b) Top and side view of the centipede falling onto its back and self-righting using the one-shot strategy.
• Figure 3: Self-righting Gaits (a) The model is composed of a chain of alternating lateral and vertical servomotors forming lateral and vertical joints, two of which are labelled here. (b) The joint angle prescription of (left) lateral and (right) vertical joints as a function of gait fraction. Color indicates the joint index. The amplitude of the joint angle is denoted by $A$. The behavior lag between adjacent lateral joints (and identical to the phase lag between adjacent vertical joints) is denoted by $2\pi \xi$/N. (c) Definition of rolling $\gamma$ in (left) limbless and (right) legged systems.
• Figure 4: The Robophysical Model and Experimental Setup (a) The robophysical model in the limbless configuration, showing the red markers on the ends for position tracking and yellow markers on the top for orientation tracking. (b) The model in the limbed configuration. (c) The static limbs are attached to the bottom of the model with 3D-printed brackets. (d) The robophysical experimental setup, consisting of a floor of hard-flat particleboard recorded from above with a webcam.
• Figure 5: Limbed Self-righting (a) A behavior diagram characterized by (horizontal axis) the body amplitude and (vertical axis) the spatial frequency. We illustrate the top-view postures of the robophysical model over the behavior diagram. (b)(i) Successful one-shot self-righting at a body amplitude of $\pi/4$. (b)(ii) Unsuccessful one-shot self-righting at a body amplitude of $\pi/12$, indicating the need of sufficient amplitude for success. (b)(iii) Successful sequential self-righting at a body amplitude of $\pi/4$.