## 3 Control Strategies for Obstacle-Aided Locomotion
### Obstacle-Aided Locomotion  
Even though obstacle avoidance and obstacle accommodation are useful features for snake robot locomotion in unstructured environments, these control approaches are not sufficient to fully exploit obstacles as means of propulsion. As observed in nature, biological snakes exploit the terrain irregularities and push against them so that a more efficient locomotion gait can be achieved. In particular, the entire snake's body bends itself, and all sections consistently follow the path taken by the head and neck [2]. Snake robots may adopt a similar strategy. A key aspect of practical snake robots is therefore obstacle-aided locomotion [3; 4]. To understand the mechanism underlying the functionalities of biological snakes on the basis of a synthetic approach, a model of a serpentine robot with viscoelastic properties was presented in [38; 39]. It should be noted that the authors adopted the term _scaffold-assisted serpentine locomotion_, which is conceptually similar to the idea of obstacle-aided locomotion. The authors also designed an autonomous decentralised control scheme that employs local sensory feedback based on the muscle length and strain of the snake body, the latter of which is generated by the body's softness. Through modelling and simulations, the authors demonstrated that only two local reflexive mechanisms which exploit sensory information about the stretching of muscles and the pressure on the body wall, are crucial for realising locomotion. This finding may help develop robots that work in undefined environments and shed light on the understanding of the fundamental principles underlying adaptive locomotion in animals. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Obstacle Avoidance** & **Obstacle Accommodation** & **Obstacle-Aided Locomotion** \\ \hline
[32; 33; 34; 35] & [36; 37] & [3; 4; 40; 41; 42; 43; 44; 45; 46; 47; 48] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Snake locomotion in unstructured environments. However, to the best of our knowledge, little research has been done concerning the possibility of applying this locomotion approach to snake robots. For instance, a preliminary study aimed at understanding snake-like locomotion through a novel push-point approach was presented in [40]. In [40], an overview of the lateral undulation as it occurs in nature was first formalised according to the following conditions:  
* it occurs over irregular ground with vertical projections;
* propulsive forces are generated from the lateral interaction between the mobile body and the vertical projections of the irregular ground, called push-points;
* at least three simultaneous push-points are necessary for this type of motion to take place;
* during the motion, the mobile body slides along its contacted push-points. Based on the conditions described in Remark 1, the authors of the same work considered a generic planar mechanism and a related environment that suit to satisfy the fundamental mechanical phenomenon observed in the locomotion of terrestrial snakes. A simple control law was applied and tested via dynamic simulations with the purpose of calculating the contact forces required to propel the snake robot model in a desired direction. Successively, these findings were tested with practical experiments in [41], where closed-loop control of a snake-like locomotion through lateral undulation was presented and applied to a wheel-less snake-like mobile mechanism. To sense the environment and to implement this closed-loop control approach, simple switch sensors located on the side of each module were adopted. A more accurate sensing approach was introduced in [42], where a design process for the electrical, sensing, and mechanical systems needed to build a functional robotic snake capable of tactile and force sensing was presented. Through manipulation of the body shape, the robot was able to move in the horizontal plane by pushing off of obstacles to create propulsive forces.