Kulko is an experimental platform for investigating snake robot locomotion in environments with obstacles. The joint modules of Kulko are covered by contact force sensors to allow the robot to sense its environment, and spherical shells that give the robot a smooth outer surface, thereby allowing slithering (gliding) motion in unstructured environments.
Overview of the Snake Robot Design
Snake robot locomotion in an unstructured environment requires that the snake robot can sense its environment, which can be achieved by equipping the robot with contact force sensing capabilities along its body. A force sensing system for a snake robot is challenging since the robot is articulated. In particular, the force sensing capabilities of the robot should be maintained independently of how the joints are flexed, which represents a significant design challenge.
Enabling a snake robot to glide forward in an unstructured environment requires that the body of the robot is sufficiently smooth, i.e. free of obstructive features. In particular, irregularities along the body may potentially induce large friction forces on the robot that obstruct the gliding motion. Obtaining a sufficiently smooth surface combined with contact force sensing along the articulated body is challenging.
The idea behind the design of Kulko, which was conceived with the above challenges in mind, is to encapsulate each joint module by a spherical shell that gives the joint a smooth outer surface independently of how the joint is flexed. Contact force sensing is thereby achieved by mounting force sensors underneath each spherical shell. Kulko consists of a serial connection of 10 identical ball-shaped joint modules. The smooth exterior surface and the force sensing capabilities of the robot are maintained independently of how the joints are flexed.
The Joint Actuation Mechanism
The joint mechanism and internal structure of Kulko is identical to the internal structure of the snake robot Wheeko.
The Head and the Tail Module
The head of the snake robot contains a small wireless camera and two IR distance sensors (Sharp GP2D120). The tail is equipped with a power off button and power connectors for charging the battery located in each joint module.
The Exterior Gliding Surface
The smooth exterior gliding surface of Kulko is obtained by covering each joint module by two hemispherical shells. Each hemispherical shell is 1.5 mm thick, weighs 42 g, and has an outer diameter of 140 mm. The shells were moulded from a plastic material.
The Contact Force Measurement System
A set of force sensing resistors (FSRs) are used to measure the external contact forces applied to each joint module. A FSR is a polymer thick film device that exhibits a decrease in electrical resistance when the force applied to the active surface area of the sensor increases. The FSR chosen for Kulko has a diameter (active sensor area) of 13 mm. A small cotton pad (3 mm thick) is placed over each FSR in order to distribute the applied force across the entire active area of the sensor.
Experiments with Kulko
In order to use Kulko for experiments related to obstacle-aided locomotion, we have developed an experimental setup based on the same camera tracking system that is used to measure the position of the snake robot Wheeko during motion control experiments. The setup also consists of a reconfigurable obstacle course with circular obstacles whose position can easily be changed by means of a grid of mounting holes in the floor.
The following video shows a simulation of a controller for obstacle-aided locomotion based on a snake robot simulator developed by NTNU and SINTEF.
In the next video, the snake robot Kulko is used to experimentally investigate the obstacle-aided locomotion controller.
