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Locomotion

Rolling
The main objective of this project was to explore rolling as a locomotion option for a snake robot. The robot was oriented so that all the axes of rotation were parallel to the ground plane. The robot moves by transitioning between 3 main configuration shapes, the forward, middle, and backward configurations. To move forward, the robot alternates between the middle and forward configuration states unless its initial position was the backward state. In that case, the robot first transitions to the middle state before moving to the forward state. The opposite happens for moving backwards. 



One full step is defined as the robot moving from the middle state to the forward/backward state and back to the middle state. Without any slipping, the robot should move the length of 1 link in 1 step. Each step is broken down into 2 half steps in order to maintain the height of the CG of the robot. With half-step control, the height of the robot remains almost constant for 1 step. With full-step control, there is no defined middle configuration so the CG moves up and down during 1 step. It was found that half-step control was necessary for the robot to traverse certain obstacles.



The 10th joint, which is not motorized, is called the "phantom" joint. The purpose of this joint is to close the robot so it makes a complete loop. The phantom joint consists of a wider bracket and small permanent magnets mounted at each end of the robot. When the robot curls itself in a loop, the magnets snap the joint into place. The magnets also help support the joint when it's on the top of the robot. Without any sort of support, a servo would have to take the full load of a large lever of links when the phantom joint is at the top.

                    




The rolling method of locomotion performed better on flat ground and inclines, but had difficultly going under and over certain obstacles. Please see the overview video for the comparison.



Worm
The worm method of mobility was used to compare the two methods of locomotion in a variety of scenarios. The worm transitions between two main configurations, one with a flat bottom and one with a pointed bottom. These curves propagate through the length of the robot in sequence causing the robot to move forward or backward.






The maximum height of the worm configuration is about 10 cm compared to the 15 cm of the rolling configuration. This allows the worm to pass under obstacles that are too short of the rolling configuration to roll under. Also, the use of a "head" allows the worm to climb over obstacles.





The worm method of locomotion performed better at going under and over obstacles that the rolling method couldn't, but was not as fast and couldn't handle steep inclines. Please see the overview video for the comparison.