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1. Stepping pattern changes in the caterpillar Manduca sexta : the effects of orientation and substrate

   https://doi.org/10.1242/jeb.220319  

   Metallo, Cinzia; Mukherjee, Ritwika; Trimmer, Barry A. (July 2020, The Journal of Experimental Biology)

   null (Ed.)

   ABSTRACT Most animals can successfully travel across cluttered, uneven environments and cope with enormous changes in surface friction, deformability and stability. However, the mechanisms used to achieve such remarkable adaptability and robustness are not fully understood. Even more limited is the understanding of how soft, deformable animals such as tobacco hornworm Manduca sexta (caterpillars) can control their movements as they navigate surfaces that have varying stiffness and are oriented at different angles. To fill this gap, we analyzed the stepping patterns of caterpillars crawling on two different types of substrate (stiff and soft) and in three different orientations (horizontal and upward/downward vertical). Our results show that caterpillars adopt different stepping patterns (i.e. different sequences of transition between the swing and stance phases of prolegs in different body segments) based on substrate stiffness and orientation. These changes in stepping pattern occur more frequently in the upward vertical orientation. The results of this study suggest that caterpillars can detect differences in the material properties of the substrate on which they crawl and adjust their behavior to match those properties. 

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2. The bumpy road ahead: the role of substrate roughness on animal walking and a proposed comparative metric

   https://doi.org/10.1242/jeb.245261  

   Clifton, Glenna; Stark, Alyssa Y.; Li, Chen; Gravish, Nicholas (April 2023, Journal of Experimental Biology)

   Outside laboratory conditions and human-made structures, animals rarely encounter flat surfaces. Instead, natural substrates are uneven surfaces with height variation that ranges from the microscopic scale to the macroscopic scale. For walking animals (which we define as encompassing any form of legged movement across the ground, such as walking, running, galloping, etc.), such substrate ‘roughness’ influences locomotion in a multitude of ways across scales, from roughness that influences how each toe or foot contacts the ground, to larger obstacles that animals must move over or navigate around. Historically, the unpredictability and variability of natural environments has limited the ability to collect data on animal walking biomechanics. However, recent technical advances, such as more sensitive and portable cameras, biologgers, laboratory tools to fabricate rough terrain, as well as the ability to efficiently store and analyze large variable datasets, have expanded the opportunity to study how animals move under naturalistic conditions. As more researchers endeavor to assess walking over rough terrain, we lack a consistent approach to quantifying roughness and contextualizing these findings. This Review summarizes existing literature that examines non-human animals walking on rough terrain and presents a metric for characterizing the relative substrate roughness compared with animal size. This framework can be applied across terrain and body scales, facilitating direct comparisons of walking over rough surfaces in animals ranging in size from ants to elephants. 

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3. Real‐Time Remodeling of Granular Terrain for Robot Locomotion

   https://doi.org/10.1002/aisy.202200119  

   Karsai, Andras; Kerimoglu, Deniz; Soto, Daniel; Ha, Sehoon; Zhang, Tingnan; Goldman, Daniel I. (December 2022, Advanced Intelligent Systems)

   Terrain irregularities in natural environments present mobility challenges for autonomous robots and vehicles. Loosely consolidated sandy slopes flow unpredictably when perturbed, often leading to locomotion failure. Systematic experiments with various robot morphologies on flowable terrains feature open‐loop quasistatic gait strategies that remodel the terrain to aid locomotor kinematics. On a sloped terrain of granular media near the critical angle, a laboratory‐scale rover robot induces a flow via a localized fluidization gait to remodel local terrain and succeed in locomotion. A Bayesian optimization machine learning approach that modulates this gait strategy then finds a pattern of selectively fluidizing and solidifying terrain to climb slopes rapidly. In a biped walker robot, a cleated foot design dynamically manipulates the stress fields of flowable slopes. The deeply submerged cleats remodel the shear response of the material by creating jammed regions behind them which then improve forward progression by reducing slip when compared to a flat foot. The “robophysics” approach of systematic experiments exploring terrain reconfiguration combined with future machine learning models of flowable terrain evolution can augment gait discovery for future robots. 

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4. Directionally Compliant Legs Enabling Crevasse Traversal in Small Ground‐Based Robots

   https://doi.org/10.1002/aisy.202200258  

   Lathrop, Emily; Tolley, Michael_T; Gravish, Nick (January 2023, Advanced Intelligent Systems)

   Small ground‐based robots show promise for locomotion on complex surfaces. A critical application area for such robots is movement over complex terrain and within constricted space such as narrow gaps in rubble. To contend with this terrain complexity, robots typically require high degree‐of‐freedom (DOF) limbs. However, for small robot platforms, this approach of high DOF legs is impractical due to actuator limitations. This presents an opportunity to design robots whose morphology enables the outsourcing of computational tasks to the robot body through the use of compliant elements (morphological computation). Herein, a novel robot appendage is developed that can passively compress in a programmed direction in response to environmental constrictions. A robot equipped with these appendages can enter narrow spaces down to 72% of the robot's sprawled body width as well as low ceilings down to 68% its freestanding height. The robot is able to step onto and over small terrain features ( hip height) and navigate various natural terrain types with ease. The results show that these compressible appendages enable versatile robot locomotion for robot exploration in previously unmapped environments. 

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5. Self-propulsion via slipping: Frictional swimming in multilegged locomotors

   https://doi.org/10.1073/pnas.2213698120  

   Chong, Baxi; He, Juntao; Li, Shengkai; Erickson, Eva; Diaz, Kelimar; Wang, Tianyu; Soto, Daniel; Goldman, Daniel I. (March 2023, Proceedings of the National Academy of Sciences)

   Locomotion is typically studied either in continuous media where bodies and legs experience forces generated by the flowing medium or on solid substrates dominated by friction. In the former, centralized whole-body coordination is believed to facilitate appropriate slipping through the medium for propulsion. In the latter, slip is often assumed minimal and thus avoided via decentralized control schemes. We find in laboratory experiments that terrestrial locomotion of a meter-scale multisegmented/legged robophysical model resembles undulatory fluid swimming. Experiments varying waves of leg stepping and body bending reveal how these parameters result in effective terrestrial locomotion despite seemingly ineffective isotropic frictional contacts. Dissipation dominates over inertial effects in this macroscopic-scaled regime, resulting in essentially geometric locomotion on land akin to microscopic-scale swimming in fluids. Theoretical analysis demonstrates that the high-dimensional multisegmented/legged dynamics can be simplified to a centralized low-dimensional model, which reveals an effective resistive force theory with an acquired viscous drag anisotropy. We extend our low-dimensional, geometric analysis to illustrate how body undulation can aid performance in non–flat obstacle-rich terrains and also use the scheme to quantitatively model how body undulation affects performance of biological centipede locomotion (the desert centipede Scolopendra polymorpha ) moving at relatively high speeds (∼0.5 body lengths/sec). Our results could facilitate control of multilegged robots in complex terradynamic scenarios. 

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