Introduction to discrete modelling : Modelling slithering locomotion

The study of animal locomotion is vast since animals have various shapes and modes of locomotion. It finds multiple applications especially in robotics. In this report, the focus will be put on snake locomotion. However snakes still have various techniques for moving their bodies such as unilateral contraction/extension of their belly, accordion-like, side-winding or lateral undulation of the body. This report will focus on the last gait mentioned. Studies have already shown how snakes move around push points and applications to wheeled-snake robots have been made. However in some cases snakes may have to move their body over flat surfaces without any rocks or branches to push on. From that observation, the need for a new model considering frictional anisotropy is raised. A mathematical model for slithering locomotion based on frictional anisotropy with all the assumptions implied will firstly be addressed. Then some elements of the work will be kept and focus particularly on the friction force will be put in order to get a better understanding of its need and origin through two models. To conclude, the anisotropy will be seen in the context of biotribology.

Online optimization of swimming and crawling in an amphibious snake robot

Alessandro Crespi, Auke Ijspeert

An important problem in the control of locomotion of robots with multiple degrees of freedom (e.g., biomimetic robots) is to adapt the locomotor patterns to the properties of the environment. This article addresses this problem for the locomotion of an amphibious snake robot, and aims at identifying fast swimming and crawling gaits for a variety of environments. Our approach uses a locomotion controller based on the biological concept of central pattern generators (CPGs) together with a gradient-free optimization method, Powell’s method. A key aspect of our approach is that the gaits are optimized online, i.e., while moving, rather than as an off-line optimization process. We present various experiments with the real robot and in simulation: swimming, crawling on horizontal ground, and crawling on slopes. For each of these different situations, the optimized gaits are compared with the results of systematic explorations of the parameter space. The main outcomes of the experiments are: 1) optimal gaits are significantly different from one medium to the other; 2) the optimums are usually peaked, i.e., speed rapidly becomes suboptimal when the parameters are moved away from the optimal values; 3) our approach finds optimal gaits in much fewer iterations than the systematic search; and 4) the CPG has no problem dealing with the abrupt parameter changes during the optimization process. The relevance for robotic locomotion control is discussed.

2008

Compliant snake robot locomotion on horizontal pipes

Auke Ijspeert, Kamilo Andres Melo Becerra, Mehmet Hasan Mutlu, Massimo Vespignani

In this paper we introduce a body-compliant Modular Snake Robot executing rolling gaits on different cylindrical geometries. In the state of the art it is considered that an active shape adaptation to the terrain while a gait is executed produces better performances than a simple pre-programmed stiff motion without feedback. Several attempts to reproduce such behaviors in snake robots range from compliant shape controllers (acting in joint space) to torque control strategies of elastic actuated joints. In our proposal, we incorporate compliant elements in a modular snake robot structure to passively adapt the robot’s shape to the environment. The gait control remains simple by acting directly in the robot’s joint space with known gait generation schemes. To validate our results we performed experiments with compliant modular snake robots rolling on pipes with different geometry characteristics such as different diameters, smooth surfaces, surfaces with presence of obstacles (terrain bumps), and considerable changes in diameter in a single robot run. We evaluated the performance across different robot’s body-compliance values, measuring the speed of locomotion as well as the power consumption. Our results show that providing a good selection of body compliant elements is a way to maintain high locomotion performance (at least while rolling on pipes) without including additional complex control artifacts to the simple open-loop cyclic gait controller.

2015

Investigating Sensorimotor Control in Locomotion using Robots and Mathematical Models

Robin Thandiackal

Locomotion is a very diverse phenomenon that results from the interactions of a body and its environment and enables a body to move from one position to another. Underlying control principles rely among others on the generation of intrinsic body movements, adaptation and synchronization of those movements with the environment, and the generation of respective reaction forces that induce locomotion. We use mathematical and physical models, namely robots, to investigate how movement patterns emerge in a specific environment, and to what extent central and peripheral mechanisms contribute to movement generation. We explore insect walking, undulatory swimming and bimodal terrestrial and aquatic locomotion. We present relevant findings that explain the prevalence of tripod gaits for fast climbing based on the outcome of an optimization procedure. We also developed new control paradigms based on local sensory pressure feedback for anguilliform swimming, which include oscillator-free and decoupled control schemes, and a new design methodology to create physical models for locomotion investigation based on a salamander-like robot. The presented work includes additional relevant contributions to robotics, specifically a new fast dynamically stable walking gait for hexapedal robots and a decentralized scheme for highly modular control of lamprey-like undulatory swimming robots.

EPFL2017