Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C50: Robophysics II |
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Sponsoring Units: DBIO Chair: Daniel Goldman, Georgia Inst of Tech Room: LACC 511B |
Monday, March 5, 2018 2:30PM - 2:42PM |
C50.00001: Robots that grow like plants Barbara Mazzolai, Emanuela Del Dottore, Ali Sadeghi, Alessio Mondini, Francesca Tramacere Morphogenesis, remodeling, and growth are three processes at the base of generation and evolution of all living beings. These natural abilities to adapt body shapes and material properties to external conditions have captured the interest of scientists and engineers since long. With the relatively new approach of soft robotics, morphological adaptation in robots is reached by exploiting properties of soft materials and actuators taking inspiration from natural systems. In this context, growth is a very interesting feature of living beings that can inspire a generation of robots with new and unpredictable abilities of movement. Noteworthy, plants represent an alternative model of movement in robotics, which is not animal-like and muscle-based. Plants’ movement is mostly a consequence of an indeterminate growth, which occurs for their entire life. For the first time in robotics, we proposed a growing robot inspired by movements and behaviors of plant roots, able to create its own structure exploiting a 3D printer-like system integrated into its tip and depositing a thermoplastic material. Taking inspiration from plants, we can generate new, unexplored abilities in robotics, which can better adapt to external, unstructured environments, move purposively, effectively and efficiently. |
Monday, March 5, 2018 2:42PM - 2:54PM |
C50.00002: A Biological and Robophysical Investigation of Root Circumnutation through Heterogeneous Substrates Erin McCaskey, Christian Hubicki, Kevin Lehner, Philip Benfey, Daniel Goldman Circumnutation, the circular motion exhibited by the tip of a growing root, occurs in a diversity of plants, but its function is not understood. To investigate hypotheses about substrate penetration benefits of such motion, we constructed a simple robotic model of root growth. A robotic arm, instrumented with a load cell, was outfitted with a rotating, compliant end effector (a hot glue gun stick spun by a stepper motor). Existing work observed force reduction effects from circumnutation in homogeneous granular material [Dottore et al. 2016]. We tested the hypothesis that circumnutation aids in a root’s ability to penetrate heterogeneous substrates, e.g. hard obstacles, by plunging the robotic root into a lattice of rough cylinders. Systematic variation of initial root positions revealed that non-rotating roots were significantly more likely to become pinned to obstacles and unable to progress further (75% blocked) compared to a rotating root (10% blocked). Further, the rotating root required less mechanical work (~40%) to penetrate the lattice, suggesting root circumnutation benefits the plant via greater penetration capability with reduced energetic expense. |
Monday, March 5, 2018 2:54PM - 3:06PM |
C50.00003: Experimentally Determining the Mobility Matrix of a Helical Flagellum as a Function of Boundary Distance Bruce Rodenborn, Philip Lockett, Grant Giesbrecht, Hong Ni Previous laboratory studies that used macroscopic robots to model swimming with a helix at low Reynolds number assumed that the linear system of equations representing the motion of a solid body could be determined by exploring each degree of freedom independently (Rodenborn et al., PNAS 2013). However, our recent work finds that this methodology is more complicated when the robot is near a boundary. Axial rotation of the helix creates an attractive force towards the boundary because it pumps fluid parallel to the boundary. This force creates an asymmetry in our data between CW and CCW rotation of the helix and is predicted only if the helix is rotating and translating (Lauga et al. Biophys. J. 2006). However, we can still determine the coefficient of the mobility matrix by averaging the CW and CCW values, and the asymmetry allows us to also determine the attractive force matrix element by taking the difference between the values. We also find that the elements of the mobility matrix determined in this manner display an approximately exponential decrease as a function of the boundary distance with an interaction length about four times the helical radius. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C50.00004: Are rigid robots discrete mechanical systems subject to unilateral constraints? Andrew Pace, Sam Burden Robots built from rigid struts and joints that interact with hard terrain are commonly modeled as discrete mechanical systems subject to unilateral constraints. When part of the robot impacts the terrain in such models, the velocity must reset to satisfy the unilateral constraints. Many impact laws have been proposed, but few have been tested empirically. To test a variety of impact laws, we constructed an automated jumping robot testbed with a 1-leg robot constrained to move vertically. The robot’s leg is a pantograph mechanism constructed from rigid struts and rigid pin joints. If the robot were a discrete mechanical system subject to unilateral constraints, limb and body velocities would be discontinuous at touchdown. In contrast, velocities appear continuous when computed using a variety of schemes for signal differentiation from empirical motor angle or motion capture data measured at 1kHz. Nevertheless, by directly measuring touchdown to within 1msec using an electrical switch, we are able to assess the extent to which a variety of impact laws predict how velocities change from moments immediately preceding to moments immediately following touchdown, and compare these results with simulated data from the corresponding models. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C50.00005: Power Limits of Repeatable Movement in Small, Fast Organisms: Guiding Principles for Engineering Design Mark Ilton, Saad Bhamla, Xiaotian Ma, Suzanne Cox, Leah L. Fitchett, Yongjin Kim, Je-sung Koh, Deepak Krishnamurthy, Chi-Yun Kuo, Fatma Zeynep Temel, Alfred Crosby, Manu Prakash, Gregory P. Sutton, Robert J. Wood, Emanuel Azizi, Sarah Bergbreiter, S. N. Patek Many biological systems incorporate spring and latch elements to enhance power output. These power-amplified biological systems can exceed current engineering performance: they produce high accelerations that can be continuously fueled through metabolic processes, and are used repeatedly with minimal performance degradation. In this work, we establish a framework for analyzing power amplified systems. We model how power enhancement emerges through the dynamic coupling of motors, springs, and latches, each of which displays its own force-velocity behavior. This approach reveals a rich and tunable performance landscape for spring-actuated movement that is applies to biological and synthetic systems. By including non-ideal springs and latches, we identify critical transitions in mass that depend on the materials properties and geometry of the spring and latch components. Analyzing the components as a single, integrated system, reveals the necessity for tuning and inherent tunability of the system. The integration of mathematical, physical, engineering, and biological approaches illuminates the interdependence of power enhancing components and their effects in biological and engineered systems. |
Monday, March 5, 2018 3:30PM - 3:42PM |
C50.00006: Spider Web Inspired Vibration Localization Andrew Otto, Damian Elias, Ross Hatton Orb weaving spiders rely on web vibrations to locate prey trapped in their webs. We created physical and computational models of spider webs to better understand how the spider localizes vibrations. From these models, we identified vibrational cues for indicating the location of a stimulus by measuring the vibrations arriving at points corresponding to the feet of the spider. We found that on a network of tensioned strings, a vibration stimulus’ heading can be inferred from the energy spectral density and the correlation of vibration signals at each foot and that for a web-inspired tension distribution, the vibration range was encoded by the spectral centroid. We implemented these cues as several different real-time localization strategies on our artificial web and tested their performance across a range of web configurations. Our results show that combining these cues enables successful localization of vibration stimuli, and that certain web configurations are more directional than others. These findings demonstrate that location information can be transmitted via vibrations in a network of tensioned strings (i.e. a web), and provide direction for further analysis of the localization process carried out by orb-weaving spiders. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C50.00007: Step Selection in Data-Driven Geometric Gait Optimization Brian Bittner, Ross Hatton, Shai Revzen Geometric gait optimization has opened a frontier for the improvement of robotic gaits and analysis of animal behavior. Borrowing tools from the data-driven modeling of oscillators, we have developed a streamlined method for producing a data-driven geometric model of the dynamics in the neighborhood of a gait. This model informs a gradient ascent optimizer, for which the user can specify a variety of locomotive goal functions. We have found that step size selection has a strong effect on convergence and the quality of selected gaits. In our work, a new gait is constrained geometrically such that it lies within the neighborhood of trajectories collected in the experimental data of the previous gait. This prevents the system from venturing into unknown volumes of the gait design space. We select a step size by performing a line search along the gradient and within the sampled data. For a nine-link Purcell swimmer with injected noise, this step size selector allows optimization of gaits under a variety of system noise regimes. We anticipate that this algorithm's ability to handle noise and step safely will enable its application to hardware in the loop optimization on a broad class of mechanisms. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C50.00008: Crawling strategies and uncertainty handling Juncal Arbelaiz, Anette Hosoi Strategies for locomotion are assorted in nature. Many crawlers use asymmetrical interactions with the substrate to transmit forces through gripping or friction. We present a simple lumped-parameter model, including the effects of inertia, muscle elasticity and interaction with the substrate in order to isolate key features of crawling strategies. Different muscle activation patterns are considered, as well as the role of Central Pattern Generators (CPG) and proprioception in locomotion. Finally, we study the impact of environmental noise and uncertainty on crawling efficacy and complexity of the control strategy. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C50.00009: Discrete Ground Contact Events as a Gait Synthesis Mechanism in Legged Robots George Council, Shai Revzen Periodic legged locomotion tasks for robotic walkers require persistent interaction with the ground. |
Monday, March 5, 2018 4:18PM - 4:30PM |
C50.00010: The Dynamics of Wheeled Locomotion in Granular Media Andras Karsai, Daniel Goldman We have developed an automated experimental setup for studying the dynamics of wheeled locomotion in dry granular substrates. The apparatus enables systematic tests of quasistatic rheological models of intruder-substrate interaction, such as Resistive Force Theory and Terramechanics, in dynamic regimes where inertial effects can cause deviation from theory and changes in the rheological behavior. For a rigid wheel with circumferential protrusions (grousers), we observe that increased wheel rotational acceleration α leads to increased sinkage into the media during transit, suggesting transient effects cause the granular media to fail under rapid shear. Low α and intermediate terrain pressure can minimize wheel slippage and maintain constant wheel sinking depth d over time. For a given final constant rotational speed ω, increasing the ramp-rate to ω results in increased d and slip (the normalized ratio between translational and rotational speed) relative to a slower ramp, suggesting reduced reaction force when the grains under the wheel are more quickly accelerated from rest. Wheel slip and d also sharply increase above a threshold in ω. These transient effects show that time-independent models are insufficient for capturing high inertia wheeled locomotion. |
Monday, March 5, 2018 4:30PM - 4:42PM |
C50.00011: Systematic study of limb-body coordination during sandfish burial Veronica Paez, Sarah Sharpe, Daniel Goldman The ~10 cm long sandfish lizard can bury into dry granular media within a half-second. Unlike its subsurface sand-swimming behavior, in which propulsion is generated by a head-to-tail traveling-wave of body bending, sand-diving uses both undulatory body motion (typically 1-2 cycles) and a stereotyped pattern of limb use (and disuse). In previous work [Maladen et al, ICRA 2011], a limbless sandfish robot could bury 75% of its body within 5 undulation cycles. To gain insight into the importance of limb use, we performed biological and robophysical experiments. In animal studies, when all limbs were bound (taped to the body), the animal could not effectively bury; hind limb binding increased burial (~4 cycles) but not effectiveness, whereas forelimb binding decreased burial probability (40% success rate using ~6 cycles). To systematically study the morphology, timing and coordination of the sandfish’s limbs and body undulations, we constructed a new sandfish robot with five position-controlled servo motors which generate a travelling body wave, and mounted to the body four rotational servo motors with 3D printed limbs. We expect that a properly coordinated limb use pattern in our robot will improve burial performance. |
Monday, March 5, 2018 4:42PM - 4:54PM |
C50.00012: Geometric Motion Planning for Inertial Systems Hossein Faraji, Ross Hatton The locomotion of many systems is a cyclic pattern which is achieved by internal joint motions to move through the environment. The main goal of this research is to develop an effective tool for motion planning for robots with multiple links in which they balance the benefit of pushing against their environment with the cost of doing so, where the cost of motion is inertial. Standard numerical optimizers can find efficient gaits for given systems, but do not capture the energy landscape that shapes these gaits. Therefore, we wish to understand the inertial geometry of the system in terms of the curves and accelerations that produce the most efficient gaits. This is achieved using two tools: constructing metric fields and constructing geodesics. The metric field in mechanical systems is obtained from the system's inertia which illustrates where it is easier to move in parameterized space. The geodesic illustrates the natural dynamic path of the system which is defined as the straightest path on the manifold. Using these tools, this research develops a variational principle for generating optimal gaits for inertia-based locomoting systems that provides visual insights on the optimal gait. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C50.00013: Effects of Hydrodynamic Coupling Within a Self-Propelling Array of Pitching Hydrofoils Scott Kelly, Rakshit Bhansali, Rodrigo Abrajan-Guerrero The dynamics of a body propelling itself through a fluid can be altered appreciably by the presence of additional self-propelling bodies nearby. Hydrodynamic coupling within an array of self-propelling bodies sharing a particular strategy for propulsion can cause the propulsive efficacy or energy efficiency of this strategy to deviate for the array overall from the efficacy or efficiency that would be observed by a body executing this strategy in isolation. This talk will address the self-propulsion of a planar array of hydrofoils pivoting periodically about their leading edges, mimicking a school of fishlike swimmers in a simplified way. Changes in the relative spacing of foils and phasing of oscillations within the array will be shown to influence the overall efficacy and efficiency of particular individual pivoting motions. Previous studies of similar systems have focused primarily on fixed arrays of hydrofoils in steady background flows rather than self-propelling arrays. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C50.00014: Abstract Withdrawn
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Monday, March 5, 2018 5:18PM - 5:30PM |
C50.00015: The Geometry of Single Smarticle Locomotion William Savoie, Shengkai Li, Daniel Goldman We investigate an active granular medium composed of “smarticles” (smart, active particles), simple, low cost, programmable three-link, two motor robots. These robots can perform self-deformations, enabling individuals to locomote and to repel/attract each other via (dis)entanglement of arms, thereby allowing on-demand formation of gas, fluid, and solid-like states. As a result, a collection of smarticles can exhibit behaviors not typically associated with granular materials such as supporting both compressive and tensile loads. Smarticles, when placed upright on a flat surface can only change their shape, they cannot translate on their own, however, they can move collectively when confined with several other robots. When placed on their side, smarticles can crawl along flat ground while performing different gaits--sequencing their limbs in a periodic fashion. We analyzed and optimized single smarticle locomotion using the geometric mechanics framework (Shapere & Wilczek, PRL, 1987; Hatton & Choset, EPL, 2015). This theory captures and rationalizes both the direction of movement and effectiveness of different gaits along a flat surface. |
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