Mechanics of locomotion of a double screw crawling robot

This paper analyzes the locomotion of a double screw-like robot composed of a right hand screw and a left hand screw attached via a rotating motor. Since the screws rotate in opposite directions, both screws produce propulsion in the same direction to advance. The advantage of this mechanism is that it can be used to crawl over deformable surfaces such as inside biological vessels and in granular media. We first develop a general analytical model of this double screw-like motion and describe its different modes of locomotion (raising-raising, raising-lowering, lowering-raising, lowering-lowering). We then use the model to determine the speed and forces acting on the robot as a function of the rotation speed, the screw geometry such as the diameter of the screw and its pitch, and the friction coefficient of the surface. The model is then used to evaluate the energy consumption of the locomotion and motor torques. The findings show that the surface forces acting on the two screws vary significantly if the coefficients of friction on each are different. The numerous new singular cases in which the screw forces can become infinite are discussed.