Locomotion as Manipulation with ReachBot

Abstract

Caves and lava tubes on the Moon and Mars are sites of geological and astrobiological interest but consist of terrain that is inaccessible with traditional robot locomotion. To support the exploration of these sites, we present ReachBot, a robot that uses extendable booms as appendages to manipulate itself with respect to irregular rock surfaces. The booms terminate in grippers equipped with microspines and provide ReachBot with a large workspace, allowing it to achieve force closure in enclosed spaces such as the walls of a lava tube. To propel ReachBot, we present a contact-before-motion planner for non-gaited legged locomotion that utilizes internal force control, similar to a multi-fingered hand, to keep its long, slender booms in tension. Motion planning also depends on finding and executing secure grips on rock features. We use a Monte Carlo simulation to inform gripper design and predict grasp strength and variability. Additionally, we use a two-step perception system to identify possible grasp locations. To validate our approach and mechanisms under realistic conditions, we deployed a single ReachBot arm and gripper in a lava tube in the Mojave Desert. The field test confirmed that ReachBot will find many targets for secure grasps with the proposed kinematic design.

Figures (12)

• Figure 1: ReachBot exploring an analog martian lava tube. (A) shows a rendering of a full ReachBot configuration overlaid on our field test site in the Lavic Lake volcanic field in the Mojave Desert. (B) shows the single-boom prototype in the same lava tube to demonstrate deployer, perception system, and microspine gripper, which can be seen grasping from another perspective in (C).
• Figure 2: Visualization of ReachBot's kinematic workspace. (A) Each shoulder joint has a pan, $\theta$, and tilt, $\phi$, range of motion in addition to a prismatic boom with extension $b$. The model treats boom deployers as points, with their mass incorporated into the mass of the robot body. A free-body diagram for ReachBot's body (B) includes the actuated pan moment $M_{\text{pan}}$, tilt moment $M_{\text{tilt}}$, and prismatic force $F_{\text{prismatic}}$along the boom, as well as a reaction force $F_{\text{reaction}}$in the plane perpendicular to $F_{\text{prismatic}}$, at each shoulder joint. There is no moment about the boom due to the free wrist. Gravity ($F_{\text{gravity,body}} = m_\text{body}G$) acts at the center of mass. When a gripper is attached, it supports its own weight, but when detached, the shoulder joint must support it. The free-body diagram for an unattached gripper (C) includes interaction forces at the shoulder joint and gravity ($F_{\text{gravity,gripper}} = m_\text{gripper}G$).
• Figure 3: Motion planning simulation based on a lava tube. (A) shows a rendering of ReachBot in a lava tube near Mojave, CA with graspable anchor points identified with green highlighting. (B) shows a sequence of body positions corresponding to vertices in the footstep plan graph. (C) shows a body motion during which all eight grippers remain attached. (D) shows an end effector motion in which a gripper is detached and reattached at a new location. In each continuous phase, the seed trajectory (sequence of static waypoints) is shown as a dotted red line. The dynamically feasible optimized trajectory is shown as a solid blue curve.
• Figure 4: Grasp model and analysis. (A) Geometric parameters for three-finger grasp model.(B) Free body diagram and contact forces. (C) Compliant spine-asperity contact. (D) A 3D limit surface generated from a Monte Carlo simulation with 10,000 samples using parameters derived during the field test; force components are with respect to ($x,y,z$) coordinates in (B), in a frame centered at the wrist. (E) Cross-section polar plot of the magnitude of applied pull force, $F_{pull}$, using parameters from field test, with $(-90^\circ \leq \beta \leq 90^\circ)$, where $\beta$ is the pull angle, and assuming 20 spines engaged (25%). Slight asymmetry in $\alpha$ arises from asymmetry in the rock. Straight sides correspond to failure associated with a finger lifting off. Red triangles mark field test results. Additional details are provided in Supplement \ref{['sup:MC']}. (F) Cross-section polar plot showing the dependence of $F_{pull}$ on grasp force, $F_{int}$.
• Figure 5: Mechanical gripper design. (A) Opening and closing tendons are driven by a single gearmotor and drum (B). Proximal/distal phalanges are connected by ball joints, with permanent magnets to hold them in place when the hand is open (C). The opening and closing tendons respectively are illustrated in (D) and (E). Linearly constrained spines (F) are arranged in compact arrays on carriages that float with respect to the fingertips. Additional details are provided in Supplementary Methods.