Addition of a peristaltic wave improves multi-legged locomotion performance on complex terrains

TL;DR

The paper addresses obstacle-climbing limits in multi-segment, two-wave locomotion by introducing a five-segment cable-driven robot that couples a vertical serpenoid wave with a linear peristaltic wave. Through controlled phasing and amplitude of the peristaltic motion, the authors demonstrate on flat ground a moderate gain in speed ($0.25 \pm 0.02$ BL/cyc with proper phasing) and a substantial improvement in rugose terrain performance, including obstacle climbing and reduced jamming. The key contributions are the design of a hybrid actuation scheme, a gait template that combines vertical and peristaltic waves, and quantitative evidence that peristalsis markedly enhances all-terrain locomotion. This work suggests a viable path toward all-terrain multi-legged robots with potential applications in search-and-rescue, environmental monitoring, and planetary exploration.

Abstract

Characterized by their elongate bodies and relatively simple legs, multi-legged robots have the potential to locomote through complex terrains for applications such as search-and-rescue and terrain inspection. Prior work has developed effective and reliable locomotion strategies for multi-legged robots by propagating the two waves of lateral body undulation and leg stepping, which we will refer to as the two-wave template. However, these robots have limited capability to climb over obstacles with sizes comparable to their heights. We hypothesize that such limitations stem from the two-wave template that we used to prescribe the multi-legged locomotion. Seeking effective alternative waves for obstacle-climbing, we designed a five-segment robot with static (non-actuated) legs, where each cable-driven joint has a rotational degree-of-freedom (DoF) in the sagittal plane (vertical wave) and a linear DoF (peristaltic wave). We tested robot locomotion performance on a flat terrain and a rugose terrain. While the benefit of peristalsis on flat-ground locomotion is marginal, the inclusion of a peristaltic wave substantially improves the locomotion performance in rugose terrains: it not only enables obstacle-climbing capabilities with obstacles having a similar height as the robot, but it also significantly improves the traversing capabilities of the robot in such terrains. Our results demonstrate an alternative actuation mechanism for multi-legged robots, paving the way towards all-terrain multi-legged robots.

Figures (7)

• Figure 1: Overview of multi-segment robot that locomotes using a combination of peristaltic and vertical waves. (A) A side-view picture of the robot in resting position on the flat terrain. (B) Computer Aided Design (CAD) zoomed-in representation of a robot two-DoF joint. (C) Side-view diagram showing the actuation mechanism for each DoF.
• Figure 2: Geometry of an individual joint.$\mathcal{L}_{upper}$ and $\mathcal{L}_{lower}$ refer to the length of the upper and lower cables. $D$ and $L_c$ are dimensional constants of the robot. $l$ and $\alpha$ are the peristaltic length and the joint angle controlled by the gait equation, respectively.
• Figure 3: Illustration of basic robot capabilities on flat ground. (A) Snapshots of a joint in (A.1) extension and (A.2) compression states. Compression and extension are labelled with different arrows. (B) Comparison between (B.1) robot using two-DoF joints, capable of propagating a peristaltic wave along the robot body together with the vertical wave, and (B.2) robot using pitch-only joints, capable of propagating a pure vertical wave along the robot body. (C) Comparison between (C.1) the two-DoF joint with both pitch and compression capabilities and (C.2) the pitch-only joint with one rotational DoF in the sagittal plane.
• Figure 4: Vertical-peristaltic wave synchronization. (A) Speed (body length traveled per cycle) is plotted as a function of $\varphi$. Error bars represent standard deviations. (B) (top) The displacement as a function of time over three cycles with (i) the inappropriate phase $\varphi = \pi$ and (ii) the appropriate phase $\varphi = 3\pi/2$. (bottom) Snapshots of robot performing (i) inappropriately and (ii) appropriately synchronized gait. Detrimental synchronization happens when joint contracts while being lifted and straight. Beneficial synchronization happens when joint extends while being lifted and straight.
• Figure 5: Peristaltic wave to improve forward speed on flat ground. (A.1) Locomotion speed of robot (blue) with peristaltic wave and (green) without peristaltic wave, both as a function of $A_{vert}$. Locomotion speed was measured as the net displacement normalized by the body length of the robot over a gait cycle. (A.2) Peristalsis ratio, defined as the speed improvement by peristalsis normalized by the peristalsis wave amplitude, as a function of $A_{vert}$. The peristalsis ratio is significantly greater than 1. (B) Series of time-lapse snapshots over one cycle for (i) $A_{vert}=70^\circ$ and $dl=1cm$, and for (ii) $A_{vert}=70^\circ$ and $\Delta l=0cm$.