Hamidreza Marvi

Friction enhancement in concertina locomotion of snakes

Narrow crevices are challenging terrain for most organisms and biomimetic robots. Snakes move through crevices using sequential folding and unfolding of their bodies in the manner of an accordion or concertina. In this combined experimental and theoretical investigation, we elucidate this effective means of moving through channels. We measure the frictional properties of corn snakes, their body kinematics and the transverse forces they apply to channels of varying width and inclination. To climb channels inclined at 60°, we find snakes use a combination of ingenious friction-enhancing techniques, including digging their ventral scales to double their frictional coefficient and pushing channel walls transversely with up to nine times body weight. Theoretical modelling of a one-dimensional n-linked crawler is used to calculate the transverse force factor of safety: we find snakes push up to four times more than required to prevent sliding backwards, presumably trading metabolic energy for an assurance of wall stability.

The effect of up‐armoring of the high‐mobility multi‐purpose wheeled vehicle (HMMWV) on the off‐road vehicle performance

Purpose – A parallel finite‐element/multi‐body‐dynamics investigation is carried out of the effect of up‐armoring on the off‐road performance of a prototypical high‐mobility multipurpose‐wheeled vehicle (HMMWV). The paper seeks to investigate the up‐armoring effect on the vehicle performance under the following off‐road maneuvers: straight‐line flatland braking; straight‐line off‐angle downhill braking; and sharp left turn.Design/methodology/approach – For each of the above‐mentioned maneuvers, the appropriate vehicle‐performance criteria are identified and the parameters used to quantify these criteria are defined and assessed. The ability of a computationally efficient multi‐body dynamics approach when combined with a detailed model for tire/soil interactions to yield results qualitatively and quantitatively consistent with their computational counterparts obtained using computationally quite costly finite element analyses is assessed.Findings – The computational results obtained clearly reveal the comp...

Snakes mimic earthworms: propulsion using rectilinear travelling waves

In rectilinear locomotion, snakes propel themselves using unidirectional travelling waves of muscular contraction, in a style similar to earthworms. In this combined experimental and theoretical study, we film rectilinear locomotion of three species of snakes, including red-tailed boa constrictors, Dumerils boas and Gaboon vipers. The kinematics of a snakes extension–contraction travelling wave are characterized by wave frequency, amplitude and speed. We find wave frequency increases with increasing body size, an opposite trend than that for legged animals. We predict body speed with 73–97% accuracy using a mathematical model of a one-dimensional n-linked crawler that uses friction as the dominant propulsive force. We apply our model to show snakes have optimal wave frequencies: higher values increase Froude number causing the snake to slip; smaller values decrease thrust and so body speed. Other choices of kinematic variables, such as wave amplitude, are suboptimal and appear to be limited by anatomical constraints. Our model also shows that local body lifting increases a snakes speed by 31 per cent, demonstrating that rectilinear locomotion benefits from vertical motion similar to walking.

A finite element analysis of pneumatic‐tire/sand interactions during off‐road vehicle travel

Purpose – The purpose of this paper is to carry out a series of transient, non‐linear dynamics finite element analyses in order to investigate the interactions between a stereotypical pneumatic tire and sand during off‐road vehicle travel.Design/methodology/approach – The interactions were considered under different combined conditions of the longitudinal and lateral slip as encountered during “brake‐and‐turn” and “drive‐and‐turn” vehicle maneuvers. Different components of the pneumatic tire were modeled using elastic, hyper‐ and visco‐elastic material models (with rebar reinforcements), while sand was modeled using the CU‐ARL sand models developed by Grujicic et al. The analyses were used to obtain functional relations between the wheel vertical load, wheel sinkage, tire deflection, (gross) traction, motion resistance and the (net) drawbar pull. These relations were next combined with Pacejka magic formula for a pneumatic tire/non‐deformable road interaction to construct a tire/sand interaction model sui...

Experimental investigation of optimal adhesion of mushroomlike elastomer microfibrillar adhesives

Optimal fiber designs for the maximal pull-off force have been indispensable for increasing the attachment performance of recently introduced gecko-inspired reversible micro/nanofibrillar adhesives. There are several theoretical studies on such optimal designs; however, due to the lack of three-dimensional (3D) fabrication techniques that can fabricate such optimal designs in 3D, there have not been many experimental investigations on this challenge. In this study, we benefitted from recent advances in two-photon lithography techniques to fabricate mushroomlike polyurethane elastomer fibers with different aspect ratios of tip to stalk diameter (β) and tip wedge angles (θ) to investigate the effect of these two parameters on the pull-off force. We found similar trends to those predicted theoretically. We found that β has an impact on the slope of the force-displacement curve while both β and θ play a role in the stress distribution and crack propagation. We found that these effects are coupled and the optimal set of parameters also depends on the fiber material. This is the first experimental verification of such optimal designs proposed for mushroomlike microfibers. This experimental approach could be used to evaluate a wide range of complex microstructured adhesive designs suggested in the literature and optimize them.

Actively controlled fibrillar friction surfaces

In this letter, we propose a technique by which we can actively adjust frictional properties of elastic fibrillar structures in different directions. Using a mesh attached to a two degree-of-freedom linear stage, we controlled the active length and the tilt angle of fibers, independently. Thus, we were able to achieve desired levels of friction forces in different directions and significantly improve passive friction anisotropies observed in the same fiber arrays. The proposed technique would allow us to readily control the friction anisotropy and the friction magnitude of fibrillar structures in any planar direction.

SCALYBOT: A SNAKE-INSPIRED ROBOT WITH ACTIVE CONTROL OF FRICTION

Snakes are one of the world’s most versatile locomotors, at ease slithering through rubble or ratcheting up vertical tree trunks. Their adaptations for movement across complex dry terrain thus serve naturally as inspirations for search-and-rescue robotics. In this combined experimental and theoretical study, we perform experiments on inclined surfaces to show a snake’s scales are critical anatomical features that enable climbing. We find corn snakes actively change their scale angle of attack by contracting their ventral muscles and lifting their bodies. We use this novel paradigm to design Scalybot, a two-link limbless robot with individually controlled sets of belly scales. The robot ascends styrofoam plates inclined up to 45°, demonstrating a climbing ability comparable to that of a corn snake in the same conditions. The robot uses individual servos to provide a spatial and temporal dependence of its belly friction, effectively anchoring the stationary part of its body while reducing frictional drag of its sliding section. The ability to actively modulate friction increases both the robot’s efficiency over horizontal surfaces and the limiting angles of inclination it can ascend.Copyright

Fiberbot: A miniature crawling robot using a directional fibrillar pad

Vibration-driven locomotion has been widely used for crawling robot studies. Such robots usually have a vibration motor as the actuator and a fibrillar structure for providing directional friction on the substrate. However, there has not been any studies about the effect of fiber structure on robot crawling performance. In this paper, we develop Fiberbot, a custom made mini vibration robot, for studying the effect of fiber angle on robot velocity, steering, and climbing performance. It is known that the friction force with and against fibers depends on the fiber angle. Thus, we first present a new fabrication method for making millimeter scale fibers at a wide range of angles. We then show that using 30° angle fibers that have the highest friction anisotropy (ratio of backward to forward friction force) among the other fibers we fabricated in this study, Fiberbot speed on glass increases to 13.8±0.4 cm/s (compared to ν = 0.6±0.1 cm/s using vertical fibers). We also demonstrate that the locomotion direction of Fiberbot depends on the tilting direction of fibers and we can steer the robot by rotating the fiber pad. Fiberbot could also climb on glass at inclinations of up to 10° when equipped with fibers of high friction anisotropy. We show that adding a rigid tail to the robot it can climb on glass at 25° inclines. Moreover, the robot is able to crawl on rough surfaces such as wood (ν = 10.0±0.2 cm/s using 30° fiber pad). Fiberbot, a low-cost vibration robot equipped with a custom-designed fiber pad with steering and climbing capabilities could be used for studies on collective behavior on a wide range of topographies as well as search and exploratory missions.

Animal and Robotic Locomotion on Wet Granular Media

Most of the terrestrial environments are covered with some type of flowing ground; however, inadequate understanding of moving bodies interacting with complex granular substrates has hindered the development of terrestrial/all-terrain robots. Although there has been recent performance of experimental and computational studies of dry granular media, wet granular media remain largely unexplored. In particular, this encompasses animal locomotion analysis, robotic system performance, and the physics of granular media at different saturation levels. Given that the presence of liquid in granular media alters its properties significantly, it is advantageous to evaluate the locomotion of animals inhabiting semi-aquatic and tropical environments to learn more about effective locomotion strategies on such terrains. Lizards are versatile and highly agile animals. Therefore, this study evaluated the brown basilisk, which is a lizard species from such habitats that are known for their performance on wet granular media. An extensive locomotion study was performed on this species. The animal experiments showed that on higher saturation levels, velocity of the animal was increased due to an increase in the stride length. A basilisk-inspired robot was then developed to further study the locomotion on wet granular media and it was observed that the robot can also achieve higher velocities at increased saturation levels. This work can pave the way for developing robotic systems which can explore complex environments for scientific discovery, planetary exploration, or search-and-rescue missions.