Challenge of snake robot

Snake robot can be used for reconnaissance, examination of pipe structure in power plant or factory. But there are some challenges to use snake robot in real situations.

1. Lack of design standard
Currently, some researchers design and make snake robot. But there is no standardized methodology. There is no guide line to determine module number, module length, wiring, actuation method and so on. So it is difficult to make snake robot and there is no criterion to check snake robot's performance or reliability. Actually, these standard can be constructed by analyzable mathematical model for control theory of snake robot. But currently researchers can't construct such mathematical model of snake robot.

2. Lack of environmental sensing
Measuring contact force or external force acting on body is challenging. Because robot's modules are articulated. One solution of this problem is using textile sensor on surface of robot. Kulko use this method to sense external force to pass obstacles. But Kulko can't sense force around linkage between two modules.

3. SLAM
SLAM is simultaneous localisation and mapping. It enable a robot to create a map of environment and determine its own position in the map simultaneously. Snake robot's movement highly depend on interaction between environment and robot body. So it is essential that know about environment. It can help path planing of robot and give an answer to how robot can achieve the mission. Vision from camera and sensors on body can collect information to do SLAM.

4. Untethered operation
Robots that I presented at previous text are all tethered robot. Wire supply power and deliver information to robot or human operator. So on board power source and wireless communication ability is necessary for untethered operation. It is challenge because snake robot has limited payload capability. Wireless communication is also difficult. Because most of applications of snake robot is in inaccessible by human like nuclear power plant or disaster area. Reliable wireless communication in that environment is difficult. So robot must have ability that operate autonomously when communication with operator is lost until communication is reconnected.

5. Robust and strong actuation mechanisms
Snake robot is very sensitive to weight. Because most operation of snake robot need to lift their parts like sticking its head out of the break to find survivors in disaster area. It requires high ratio between the strength of the actuators and the weight of the robot. Also it must not over heat during operation and robust to the impact.

If these challenges will be solved, snake robot can be used for many applications.

Reference : Limbless Locomotion: Learning to Crawl with a Snake Robot
                 Snake Robots : Modelling, Mechatronics, and Control

Snake robot - S-Bot (KAIST)

Second type of snake robot is S-Bot. It was made by KAIST student in laboratory of prof. Soo-Hyun Kim.

Yoo Sang Jun, Modular type robot for field moving and tree climbing, 2009

This robot consist of two kinds of modules. One is for climbing and the other is for moving in soft terrain.

Yoo Sang Jun, Modular type robot for field moving and tree climbing, 2009

Spiky wheels scratch the pole and get reaction force to climb up along helical trajectory. Screw type module generate thrust in soft terrain like sand or mud.

Snake robots - Modular snake robot(CMU)

As I mention before, snake use coiling motion to get enough friction force. This climbing strategy is used for climbing robots. I found two kinds of robot.

http://informationng.com/wp-content/uploads/2014/01/carnegie-mellon-robotic-snake.jpg

First kind of snake robot is made by CMU(Carnegie Mellon University). It's name is 'Modular snake robot'. It consist of many module describe in lower figure.

Design and Architecture of the Unified Modular Snake Robot

 It can climb up the human leg using their special rolling motion.

The important part is that robot’s movement mainly based on coiling motion of snake. It can also make stable coiling position like this. Major application of this robot is reconnaissance.

Source of holding force

 As we saw in previous texts, snake hold pole during climbing. The source of holding force is skin structure and helical movement.

Yoo Sang Jun, Modular type robot for field moving and tree climbing, 2009

Snake's skin is covered by unidirectional scales like above picture. This scales generate low friction when snake moves forward and generate large friction when snake slip backward.

http://www.eplantscience.com/index/general_zoology/characteristics_of_reptiles_that_distinguish_them_from_amphibians.php

Scales make bumpy surface like upper picture to generate asymmetric friction coefficient. Asymmetric friction coefficient help climbing of snake.

First source of holding force, skin structure, can improves climbing ability of robot. But I can't find research team that study about snake skin for robots. Current research only focus on second source of holding force, helical motion.

Yoo Sang Jun, Modular type robot for field moving and tree climbing, 2009

When snake coil the vertical pole, we can model that situation like this picture. Every joint generate torque using their muscles and torque make stable contact between pole and snake. It also increase normal forces (N1, N2, N3, N4). It means snake get larger friction force from the pole. This phenomenon make snake climb up the vertical pole.

Motion analysis of snake

http://www.qldzoo.com/snakes-move/

First of all, there are two kind of snake movement. Of course some snake move by sidewinding motion which is slide and jump to side or Caterpillar movement which is movement like caterpillar according to their size. But most snake use Serpentine movement and Concertina movement when they move forward. Serpentine movement is like upper diagram. Red part push the ground or obstacles to get reaction forces. Reaction forces form vector sum and resultant force make snake move forward.

When we see serpentine movement as 3D graphics, only force generation part contact to the ground and rest of part is lifted.

Second moving method is concertina movement.

http://www.qldzoo.com/snakes-move/

This movement is pushing and pulling. Snake push their frontal body as support with tail part. After that frontal body contact to the ground and pull the tail part to move forward. When snake climb up the pole, it use two motion simultaneously.

First snake push up frontal body by Serpentine movement and coil the pole using their frontal body. Then it pull their tail part as holding the pole like concertina movement, Snake coil the pole using their tail part and push up frontal body again. Climbing motion consist of holding and moving. Important part is holding motion. It generates holding force based on skin and coiling force.

Introduction - snake robot

Snake shape robot is that doesn't use adhesive force to climb up the vertical structures.
Currently, many research groups research about snake robot to use for climbing as well as swimming, moving around harsh terrain and moving in narrow space. 

Snake Robots : Modelling, Mechatronics, and Control

Snake robot can be used for fire fighting in road tunnel, search and rescue in disaster area, inspection and maintenance inside of plant.
In this report we search snake robot as subcategory of climbing robot. So I focus on climbing snake robot.