Abstract-In this paper we present Salamandra robotica II, an amphibious salamander robot, that is able to walk and swim. The robot has four legs and an actuated spine that allow it to perform anguilliform swimming in water and walking on ground. The paper first presents the new robot hardware design, which is an improved version of Salamandra robotica I. We then address several questions related to body-limb coordination in robots and animals that have a sprawling posture like salamander and lizards as opposed to the erect posture of mammals (e.g., in cats and dogs). In particular, we investigate how the speed of locomotion and curvature of turning motions depend on various gait parameters such as the body-limb coordination, the type of body undulation (offset, amplitude and phase lag of body oscillations), and the frequency. Comparisons with animal data are presented, and our results show striking similarities with the gaits observed with real salamanders in particular concerning the timing of body's and limbs' movements and the relative speed of locomotion.

Robots are increasingly used as scientific tools to investigate animal locomotion. However, designing a robot that properly emulates the kinematic and dynamic properties of an animal is difficult because of the complexity of musculoskeletal systems and the limitations of current robotics technology. Here, we propose a design process that combines high-speed cineradiography, optimization, dynamic scaling, three-dimensional printing, high-end servomotors and a tailored dry-suit to construct Pleurobot: a salamander-like robot that closely mimics its biological counterpart, Pleurodeles waltl. Our previous robots helped us test and confirm hypotheses on the interaction between the locomotor neuronal networks of the limbs and the spine to generate basic swimming and walking gaits. With Pleurobot, we demonstrate a design process that will enable studies of richer motor skills in salamanders. In particular, we are interested in how these richer motor skills can be obtained by extending our spinal cord models with the addition of more descending pathways and more detailed limb central pattern generator networks. Pleurobot is a dynamically scaled amphibious salamander robot with a large number of actuated degrees of freedom (DOFs: 27 in total). Because of our design process, the robot can capture most of the animal's DOFs and range of motion, especially at the limbs. We demonstrate the robot's abilities by imposing raw kinematic data, extracted from X-ray videos, to the robot's joints for basic locomotor behaviours in water and on land. The robot closely matches the behaviour of the animal in terms of relative forward speeds and lateral displacements. Ground reaction forces during walking also resemble those of the animal. Based on our results, we anticipate that future studies on richer motor skills in salamanders will highly benefit from Pleurobot's design.

Salamanders have captured the interest of biologists and roboticists for decades because of their ability to locomote in different environments and their resemblance to early representatives of tetrapods. In this article, we review biological and robotic studies on the kinematics (i.e., angular profiles of joints) of salamander locomotion aiming at three main goals: (i) to give a clear view of the kinematics, currently available, for each body part of the salamander while moving in different environments (i.e., terrestrial stepping, aquatic stepping, and swimming), (ii) to examine what is the status of our current knowledge and what remains unclear, and (iii) to discuss how much robotics and modeling have already contributed and will potentially contribute in the future to such studies.

Salamanders propel themselves by proper coordination of limb movements and body undulations. This type of locomotion is interesting for robotics to design robots capable of locomotion on water and land. In this work we identify the control and structural parameters that contribute to forward terrestrial locomotion. We introduce a kinematic model of Salamandra robotica II, a new salamander robot, to explore how the stride length varies with different limb sizes and different types of body oscillations. We also perform systematic tests using a dynamic model built in a physics-based simulator to analyze the locomotion performance in terms of forward speed and power consumption. The results show that it is beneficial to use body undulations with variable curvature along the body, and that the tail can serve as a fifth limb to provide thrust on ground. Experiments using the real robot validate the simulation results and the contribution of the proposed control strategies.

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