Tendon‐Driven Compliant Wheel‐Less Snake Robot for Undulatory Locomotion Using Conformable Ground Contacts

This paper contributes to the understanding of snake robot locomotion by employing nonlinear system analysis tools for investigating fundamental properties of snake robot dynamics. The paper has five contributions: 1) a partially feedback linearized model of a planar snake robot influenced by viscous ground friction is developed. 2) A stabilizability analysis is presented proving that any asymptotically stabilizing control law for a planar snake robot to an equilibrium point must be time-varying. 3) A controllability analysis is presented proving that planar snake robots are not controllable when the viscous ground friction is isotropic, but that a snake robot becomes strongly accessible when the viscous ground friction is anisotropic. The analysis also shows that the snake robot does not satisfy sufficient conditions for small-time local controllability (STLC). 4) An analysis of snake locomotion is presented that easily explains how anisotropic viscous ground friction enables snake robots to locomote forward on a planar surface. The explanation is based on a simple mapping from link velocities normal to the direction of motion into propulsive forces in the direction of motion. 5) A controller for straight line path following control of snake robots is proposed and a Poincaré map is investigated to prove that the resulting state variables of the snake robot, except for the position in the forward direction, trace out an exponentially stable periodic orbit.

Snakes can move through almost any terrain. Similarly, snake robots hold the promise as a versatile platform to traverse complex environments such as earthquake rubble. Unlike snake locomotion on flat surfaces which is inherently stable, when snakes traverse complex terrain by deforming their body out of plane, it becomes challenging to maintain stability. Here, we review our recent progress in understanding how snakes and snake robots traverse large, smooth obstacles such as boulders and felled trees that lack "anchor points" for gripping or bracing. First, we discovered that the generalist variable kingsnake combines lateral oscillation and cantilevering. Regardless of step height and surface friction, the overall gait is preserved. Next, to quantify static stability of the snake, we developed a method to interpolate continuous body in three dimensions (3D) (both position and orientation) between discrete tracked markers. By analyzing the base of support using the interpolated continuous body 3-D kinematics, we discovered that the snake maintained perfect stability during traversal, even on the most challenging low friction, high step. Finally, we applied this gait to a snake robot and systematically tested its performance traversing large steps with variable heights to further understand stability principles. The robot rapidly and stably traversed steps nearly as high as a third of its body length. As step height increased, the robot rolled more frequently to the extent of flipping over, reducing traversal probability. The absence of such failure in the snake with a compliant body inspired us to add body compliance to the robot. With better surface contact, the compliant body robot suffered less roll instability and traversed high steps at higher probability, without sacrificing traversal speed. Our robot traversed large step-like obstacles more rapidly than most previous snake robots, approaching that of the animal. The combination of lateral oscillation and body compliance to form a large, reliable base of support may be useful for snakes and snake robots to traverse diverse 3-D environments with large, smooth obstacles.

Snake robots are highly articulated mechanisms that can perform a variety of motions that conventional robots cannot. Despite many demonstrated successes of snake robots, these mechanisms have not been able to achieve the agility displayed by their biological counterparts. We suggest that studying how biological snakes coordinate whole-body motion to achieve agile behaviors can help improve the performance of snake robots. The foundation of this work is based on the hypothesis that, for snake locomotion that is approximately kinematic, replaying parameterized shape trajectory data collected from biological snakes can generate equivalent motions in snake robots. To test this hypothesis, we collected shape trajectory data from sidewinder rattlesnakes executing a variety of different behaviors. We then analyze the shape trajectory data in a concise and meaningful way by using a new algorithm, called conditioned basis array factorization, which projects high-dimensional data arrays onto a low-dimensional representation. The low-dimensional representation of the recorded snake motion is able to reproduce the essential features of the recorded biological snake motion on a snake robot, leading to improved agility and maneuverability, confirming our hypothesis. This parameterized representation allows us to search the low-dimensional parameter space to generate behaviors that further improve the performance of snake robots.

In this paper we present a new gesture based control for snake robot using a custom developed accelerometer based data glove. This Snake robot which was biologically inspired was developed for the purpose of search and rescue and serve as a research platform for its locomotion analysis. The locomotion of snakes is by a differential [8] curve traveling in its body from the head to the tail and the traction it generates from the surface because of such a traveling wave. This has several controllable parameters like amplitude, frequency, phase difference, angular velocity etc. By changing these parameters the Snake robot can change its gait from sidewinding (as used by the desert snakes), crawling (like caterpillars) or even lift its hood up to look behind an obstacle.

Control, state estimation and motion planning of highly articulated snake robots have been challenging tasks for researchers. As a result, formulating gaits for the modular structure, for motion on flat trajectories as well as overcoming obstacles is mathematically complicated. This paper presents a novel design of a Compliant Omni-directional snake robot (COSMOS) consisting of mechanically and software linked spherical robot modules. This design eliminates the problems of planar snake robots to handle versatile motions with complex gait analysis, by leveraging Omni-directional motion capabilities of spherical robots. The robot is also capable of climbing smooth obstacles by introducing compliant joints in the links interconnecting adjacent modules. This paper also presents the basic robot gaits and their motion analysis which clearly demonstrate the robot design advantages such as fast turn speed and simpler motion planning strategies. Experimental results that verify the effectiveness of this robot architecture and gaits that have been designed to traverse flat terrain are included. Also, the spring stiffness of the passive joints which provide the vertical compliance in the links joining the modules is calculated and simulation results for obstacle climbing are included.

Snake robot locomotion in a cluttered environment where the snake robot utilises a sensory-perceptual system to perceive the surrounding operational environment for means of propulsion is defined as perception-driven obstacle-aided locomotion (POAL). From a control point of view, achieving POAL with traditional rigidly-actuated robots is challenging because of the complex interaction between the snake robot and the immediate environment. To simplify the control complexity, compliant motion and fine torque control on each joint is essential. Accordingly, intrinsically elastic joints have become progressively prominent over the last years for a variety robotic applications. Commonly, elastic joints are considered to outperform rigid actuation in terms of peak dynamics, robustness, and energy efficiency. Even though a few examples of elastic snake robots exist, they are generally expensive to manufacture and tailored to custom-made hardware/software components that are not openly available off-the-shelf. In this work, Serpens, a newly-designed low-cost, open-source and highly-compliant multi-purpose modular snake robot with series elastic actuator (SEA) is presented. Serpens features precision torque control and stereoscopic vision. Only low-cost commercial-off-the-shelf (COTS) components are adopted. The robot modules can be 3D-printed by using Fused Deposition Modelling (FDM) manufacturing technology, thus making the rapid-prototyping process very economical and fast. A screw-less assembly mechanism allows for connecting the modules and reconfigure the robot in a very reliable and robust manner. The concept of modularity is also applied to the system architecture on both the software and hardware sides. Each module is independent, being controlled by a self-reliant controller board. The software architecture is based on the Robot Operating System (ROS). This paper describes the design of Serpens and presents preliminary simulation and experimental results, which illustrate its performance.

Snake robots, sometimes called hyper-redundant mechanisms, can use their many degrees of freedom to achieve a variety of locomotive capabilities. These capabilities are ideally suited for disaster response because the snake robot can thread through tightly packed volumes, accessing locations that people and conventional machinery otherwise cannot. Snake robots also have the advantage of possessing a variety of locomotion capabilities that conventional robots do not. Just like their biological counterparts, snake robots achieve these locomotion capabilities using cyclic motions called gaits. These cyclic motions directly control the snake robot's internal degrees of freedom which, in turn, causes a net motion, say forward, lateral and rotational, for the snake robot. The gaits described in this paper fall into two categories: parameterized and scripted. The parameterized gaits, as their name suggests, can be described by a relative simple parameterized function, whereas the scripted cannot. This paper describes the functions we prescribed for gait generation and our experiences in making these robots operate in real experiments.

In this paper, kinematic and dynamic modelling of a wheel-less snake robot that has more potential for adapting to the environment is implemented in Matlab/Simulink and effect of friction between its body with the ground, which plays a highly important role in some snake locomotion such as lateral undulation on motion is analyzed. Moreover, the relationship between number of links of the snake robot and its forward velocity is investigated in this study. The simulation results validate that snake robot can move faster in environments where the anisotropy in friction is large enough. The results also show that the forward velocity of the snake robot is directly proportional to its number of links.

The snake robot, which mimics the mechanism of real snakes, is expected to be used in various environments. This article addresses the challenge of snake robots autonomously adapting to and moving inside and outside pipes having complex shapes using a helical rolling motion. We consider that an irregular helical form can be defined as a combination of three components, namely the axis, cross-sectional shape, and pitch angle, and that it is easier to consider deformations that have compliance only in a specific direction. Our proposed method deforms the cross-sectional shape of the snake robot locally so that it adopts the shape of the pipe accurately. Experiments were conducted using our developed snake robot to verify the effectiveness of the proposed method, and it was demonstrated that the proposed method can be used for a snake robot moving inside and outside of straight pipes and outdoor tree, including those having noncircular and varying cross sections.

Terrain adaptability gives snake robots an edge over wheeled mobile robots. Snake robot's applications expansion to disaster management is due to its limbless and modular design. Replicating the navigation and locomotion of a biological snake have been in the spotlight for many researchers. In this study, a robust localization algorithm of a snake robot is developed. A sensor fusion-based algorithm using probabilistic approaches: UKF (Unscented Kalman Filter) and EKF (Extended Kalman Filter) has been proposed. Odometry is fused with one and two IMUs (Inertial Measurement Units) with both probabilistic approaches. Evaluation of results in a simulation environment showed that the fusion of two IMUs and odometry using EKF outer performs in terms of accuracy when odometry is fused with one IMU using EKF. Furthermore, the fusion of two IMUs and odometry with UKF is computationally expensive resulting in a large convergence time which is not the best suitable approach that can be utilized for the snake robot.

Arboreal environments pose many functional challenges for animal locomotion including fitting within narrow spaces, balancing on cylindrical surfaces, moving on inclines, and moving around branches that obstruct a straight path. Many species of snakes are arboreal and their elongate, flexible bodies appear well-suited to meet many of these demands, but the effects of arboreal habitat structure on the locomotion of snakes are not well understood. We examined the effects of 108 combinations of surface shape (cylinder vs. rectangular tunnel), surface width, incline, and a row of pegs on the locomotion of corn snakes (Elaphe guttata). Pegs allowed the snakes to move on the widest and steepest surfaces that were impassable without pegs. Tunnels allowed the snakes to move on steeper inclines than cylinders with similar widths. The mode of locomotion changed with habitat structure. On surfaces without pegs, most snakes used two variants of concertina locomotion but always moved downhill using a controlled slide. Snakes used lateral undulation on most surfaces with pegs. The detrimental effects of increased uphill incline were greater than those of increased surface width on maximal velocity. Snakes moved faster in tunnels than on cylinders regardless of whether pegs were present. Depending on the surface width, the addition of pegs to horizontal cylinders and tunnels resulted in 8-24-fold and 1.3-3.1-fold increases in speed, respectively. Thus, pegs considerably enhanced the locomotor performance of snakes, although similar structures such as secondary branches seem likely to impede the locomotion of limbed arboreal animals.

This paper presents the design and manufacture process of a wheel-less, modular snake robot with series elastic actuators to reliably measure motor torque signal and investigate the effectiveness of active stiffness control for achieving adaptive snake-like locomotion. A polyurethane based elastic element to be attached between the motor and the links at each joint was designed and manufactured using water jet cutter, which makes the final design easier to develop and more cost-effective, compared to existing snake robots with torque measurement capabilities. The reliability of such torque measurement mechanism was examined using simulated dynamical model of pedal wave motion, which proves the efficacy of the design. A distributed control system was also designed, which with the help of an admittance controller, enables active control of the joint stiffness to achieve adaptive snake robot pedal wave locomotion to climb over obstacles, which unlike existing methods does not require prior information about the location of the obstacle. The effectiveness of the proposed controller in comparison to open-loop control strategy was verified by the number of experiments. The results show the capability of the robot to successfully climb over obstacles with the height of more than 55% of the diameter of the snake robot modules.

International audience

The term perception-driven obstacle-aided locomotion (POAL) was proposed to describe locomotion in which a snake robot leverages a sensory-perceptual system to exploit the surrounding operational environment and to identify walls, obstacles, or other structures as a means of propulsion. To attain POAL from a control standpoint, the accurate identification of push-points and reliable determination of feasible contact reaction forces are required. This is difficult to achieve with rigidly actuated robots because of the lack of compliance. As a possible solution to this challenge, our research group recently presented Serpens, a low-cost, open-source, and highly compliant multi-purpose modular snake robot with a series elastic actuator (SEA). In this paper, we propose a new prototyping iteration for our snake robot to achieve a more dependable design. The following three contributions are outlined in this work as a whole: the remodelling of the elastic joint with the addition of a damper element; a refreshed design for the screw-less assembly mechanism that can now withstand higher transverse forces; the re-design of the joint module with an improved reorganisation of the internal hardware components to facilitate heat dissipation and to accommodate a larger battery with easier access. The Robot Operating System (ROS) serves as the foundation for the software architecture. The possibility of applying machine learning approaches is considered. The results of preliminary simulations are provided.

This paper presents an analysis of snake locomotion that explains how non-uniform viscous ground friction conditions enable snake robots to locomote forward on a planar surface. The explanation is based on a simple mapping from link velocities normal to the direction of motion into propulsive forces in the direction of motion. From this analysis, a controller for a snake robot is proposed. A Poincare map is employed to prove that all state variables of the snake robot, except for the position in the forward direction, trace out an exponentially stable periodic orbit.