Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session S14: Robophysics: Robotics Meets Physics IV: Complex EnvironmentFocus Live
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Sponsoring Units: DBIO DSOFT Chair: Kirsty Wan, University of Exeter |
Thursday, March 18, 2021 11:30AM - 11:42AM Live |
S14.00001: Attractoring-based locomotion Bulcsú Sándor, Michael Nowak, Claudius Gros Locomotion is the result of the complex interplay of the controlling nervous system, the body, and the environment. From a dynamical systems point of view, however, regular locomotion corresponds to limit cycles formed in the phase space of variables describing the robot's or animal's dynamical states. These limit cycles may be generated either centrally by neural circuits called central pattern generators (CPG) or peripherally via closing the local somatosensory loop. |
Thursday, March 18, 2021 11:42AM - 11:54AM Live |
S14.00002: Force-chain evolution in granular packings under a razor-clam inspired penetration Sichuan Huang, Junliang Tao The Atlantic Razor clam facilitates downward burrowing in soils by changing the shapes of the shell and the foot. Here we investigate how the shell opening facilitates the soil burrowing of razor clams using a clam-inspired penetrator and photoelasticity. The penetrator has an expandable ‘shell’ and a protrusible ‘foot’, and penetrates dry granular materials by modeling the sequential shell opening and foot extension of razor clams. The soil-penetrator interactions are intuitively visualized through the evolution of force chains in granular materials. Results imply that the ‘shell’ opening facilitates the ‘foot’ extension by forming a penetration anchor through compressing the surrounding granules; it also relaxes the stresses of granules around the ‘foot’, leading to a reduction of soil resistance applied on the ‘foot’. However, the following ‘foot’ extension causes disappearance of the stress relaxation, resulting in a temporary resistance reduction; it also tends to weaken the penetration anchor. Findings from this study shed lights on developing a ‘living’ clam-inspired burrowing robot for underground explorations. |
Thursday, March 18, 2021 11:54AM - 12:06PM Live |
S14.00003: Better Stair Climbing By Using Symmetry Arun Bishop, Shai Revzen Roboticists often approach problems of control from a brute force modeling perspective. Instead, we construct a solution based on understanding of the problem’s symmetries. We are attempting such a solution to the problem of robust stair-climbing. For a robot executing a slow periodic gait - one that builds up no momentum - the state of the robot’s limbs is captured in one cyclic phase variable. A second “shift” variable, captures the robot’s location relative to the stairs’ periodic pattern. A third “skew” variable, captures the heading angle of the robot relative to the stairs. When climbing an infinite staircase, these three variables complete the robot’s state up to symmetry. |
Thursday, March 18, 2021 12:06PM - 12:18PM Live |
S14.00004: Morphological compliance enables robot locomotion through cluttered terrain Heiko Kabutz, Kaushik Jayaram The remarkable ability of animals, such as mice, cockroaches and spiders, to manoeuvre through challenging cluttered natural terrain has been a primary inspiration for legged robots. Recent research indicates that body reorientation along pathways of minimal energy is a key factor influencing such locomotion. We propose to extend this idea by hypothesizing that soft bodied animals and robots could employ an alternate yet equally effective strategy relying on their distributed body compliance to squeeze through cluttered obstacles. To demonstrate the same, we have developed a palm-scale, origami-based hexapedal robot, Compliant Legged Articulated Robotic Insect (CLARI) with compliant exoskeletal morphology. The robot, fabricated using a multilayer laminate laser micromachining technique, has six independently actuated two degree of freedom legs. Using this strategy, CLARI is able to passively conform to its environment and move through both horizontally and vertically confined spaces and requires only simple leg mechanics. We aim to verify our hypothesis by constructing a series of robots with varying body compliance and experimentally determining the preference for body deformation vs reorientation in a given cluttered environment for identical feedforward control commands. |
Thursday, March 18, 2021 12:18PM - 12:30PM Live |
S14.00005: Testing whether potential energy landscape can predict stochastic obstacle traversal Bokun Zheng, Qihan Xuan, Chen Li Animals excel at traversing large obstacles in complex terrain by transitioning between locomotor modes, an ability that robots still lack. Our recent study established a quasi-static potential energy landscape approach to locomotor transitions (Othayoth, Thoms, Li, 2020, PNAS). Here we tested whether potential energy landscapes can statistically predict stochastic traversal, specifically, that traversal probability should increase with decreasing potential energy barrier. We developed a dynamic simulation of a simplistic 2-D model system, a self-propelled circular body traversing two horizontal elastic beam obstacles with different stiffness. We assumed each body to be rigid and interacted via collision and continuous contact with no friction. We found that the body had a finite probability to be trapped in an attractive landscape basin in front of two barriers resulting from the two obstacles. Increasing the random force and self-propulsive force increased the probability to escape from this basin and move along trajectories that overcame the lower barrier. These results showed that the potential energy landscapes can predict the probability distribution of stochastic traversal, thus providing a simple, generalizable model for designing robots to traverse complex terrain. |
Thursday, March 18, 2021 12:30PM - 12:42PM Live |
S14.00006: A potential energy landscape based dynamic model of locomotion in complex 3-D terrain Qihan Xuan, Chen Li It is challenging to model locomotion in complex terrain. Recently, our lab successfully used a quasi-static potential energy landscape approach (Othayoth, Thoms, Li, 2020, PNAS) to model locomotor transitions in complex 3-D terrain. To add dynamics, here we developed an energy landscape based dynamic model and tested it using an ellipsoid body traversing beam obstacle. The model simplifies traversal as a self-propelled particle moving on a potential energy landscape in 3-D position and 3-D orientation space, whose dynamics follows a Langevin equation. The model contains conservative forces (weight and elastic forces) described by the potential energy landscape, plus propulsive forces, viscous forces, and random forces, which model self-propulsion, damping, and stochasticity, respectively. Because it is challenging to calculate torques of conservative forces from energy landscape gradient using Euler angles, we used virtual rotation to calculate them. Although our dynamic model neglected collisional dynamics and used simple non-conservative forces, it well simulated locomotion in complex terrain and matched experimental observations. Our model is useful not only for understanding dynamic locomotor transitions but also for robot control during locomotion in complex 3-D terrain. |
Thursday, March 18, 2021 12:42PM - 12:54PM Live |
S14.00007: A robophysical model to study physical sensing of obstacles in legged traversal of complex 3-D terrain Yaqing Wang, Ratan Sadanand Othayoth Mullankandy, Chen Li Terrestrial animals transition between locomotor modes to traverse complex 3-D terrain, an ability that robots still lack. For example, the discoid cockroach traverses grass-like beam obstacles by transitioning from the pitch mode (pitches body up while pushing across the beams) to the roll mode (rolls body to maneuver through the gap). Our recent study discovered that body oscillation facilitates this transition by using kinetic energy fluctuation to probabilistically cross a barrier on a potential energy landscape (Othayoth, Thoms, Li, 2020, PNAS). Besides body oscillation, the animal also flexed its head and abdomen and adjusted its hind leg movement during transition. We hypothesized that the animal sensed physical interaction by flexing its head against the obstacles, then adjusted the abdomen and leg motions accordingly to facilitate pitch-to-roll transition. To study this, we developed a robophysical model with actuated flexible head and abdomen and underactuated body pitch and roll control which mimicked the effect of leg motions. To measure terrain contact forces, we added a force sensor in its head. We are doing experiments to discover how the sensed resistance relate to the potential energy landscape and guides abdomen and leg adjustments to facilitate transition. |
Thursday, March 18, 2021 12:54PM - 1:06PM Live |
S14.00008: Landing Energy Management Sean Gart, Chad Kessens, Jason Pusey, John Nicholson, Daniel E. Koditschek To survive a fall, an animal or robot must dissipate and/or convert excess impact kinetic energy (KE) while limiting peak forces. Humans do this using a drop landing technique in which leg muscles and knee flexion act as a spring and damper to absorb energy. For high drops, paratroopers and parkour athletes perform landing rolls, which convert KE from vertical to rotational, and extend impact time to reduce peak body loads and dissipate energy. In this study, we investigate the role of leg springs and dampers, body orientation, and geometry on landing energy management. We modeled drop landing using a spring-mass-damper system and corroborated prior literature establishing that peak leg force is minimized when leg damping ratio is near 0.4 and leg spring stiffness is such that the leg spring just reaches maximum compression without exceeding it. To model a landing roll, we replaced the mass with a rigid body in a revolute-prismatic-revolute configuration. We found that with appropriate leg stiffness and body orientation, a passive system can convert 50% of pre-impact KE to rotational KE and maintain a lower peak leg force than drop landing. The result is improved understanding of fall survivability and more rapid and efficient transition to horizontal movement through rolling. |
Thursday, March 18, 2021 1:06PM - 1:18PM Live |
S14.00009: Simple untethered flagellated robot in fluids and granular media Yayun Du, Andrew Miller, Mohammad Khalid Jawed Rotating flexible flagella have been ubiquitously adopted by cells, microorganisms, and soft robots for propulsion. We report a simple untethered flagellated robot that can swim in both granular media and fluids at low Reynolds number. The robot consists of a motor embedded in a cylindrical head and multiple flagella made of elastomeric rods. We experimentally analyze the effect of various physical parameters, such as angular speed of the motor, head size, tail length, and number of tails, on the performance of the robot. In parallel with experiments, we develop a numerical simulation tool for this fluid-structure interaction problem based on the discrete elastic rods algorithm (for the structures part) and resistive force theory (for the fluidic forces). The simulations are quantitatively compared with experimental data. The computational efficiency of the simulator can be leveraged for data-driven learning of the robot’s dynamics. It can also serve as a design tool to optimize the structural design of the robot. |
Thursday, March 18, 2021 1:18PM - 1:30PM Live |
S14.00010: Parapodia inspired soft appendages enable robot propulsion in granular media Shivam Chopra, Saurabh Jadhav, Michael T Tolley, Nick Gravish Along with the high penetration forces, the characteristic nature of granular media varying between a fluid and a solid poses a challenging environment for locomotion and navigation of robots. Many animals like bristle worms, sea turtles, and mole crickets use asymmetric movements of their soft appendages (parapodia in bristle worms) to efficiently burrow in granular media. Soft robots can undergo large deformations relying on intrinsic compliance but their interaction with granular media is not well understood. Here, we investigate the role of inherent bending stiffness of soft appendages for thrust generation in robots. We design the appendage with passive anisotropic stiffness such that it experiences asymmetric drag from the granular media leading to forward propulsion. We present experimental results where the appendage is oscillated in a controlled granular environment and show that propulsion is highly dependent on the bending stiffness, increasing with the increase in stiffness until it reaches a maximum value and then starts decreasing. We demonstrate the use of these appendages on a functional robot with two appendages for steering and propulsion. Our results advance the understanding of appendage enabled robot burrowing as well as the forces involved in the interaction |
Thursday, March 18, 2021 1:30PM - 2:06PM On Demand |
S14.00011: Bioinspired Embodied Control of Locomotion in Complex Environments Invited Speaker: Robert Full Robophysics is an indispensable approach to discovering general principles of robots operating in complex environments. Fortunately, physics principles governing non-equilibrium processes where motion emerges by internal actuation in synergy with robotic physical models meeting real world environments can generate quantifiable, testable hypotheses of complex biological systems not otherwise possible. In biology, robophysics has led to simple system-level models (templates) that collapse staggering complexity, while exploring how behavior emerges from more representative models (anchors). A next step is to examine the parallel and serial composition of templates and their dynamic reconfiguration. Robophysics provides the opportunity to determine how: biologically embodied mechanical computation works with neural computation; stability occurs in complex environments using soft-matter physics; performance of multiple tasks results without change in structure; multifunctional hierarchical and heterarchical control networks emerge; properties of fail-safe and fault tolerant performance evolve; and how learning can occur using a reduced set of control modules. In turn, robophysics approach to biological locomotion can inspire the design of novel robots and applied mathematics. |
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