NASU -- Novel Actuating Screw Unit: Origami-inspired Screw-based Propulsion on Mobile Ground Robots

Abstract

Screw-based locomotion is a robust method of locomotion across a wide range of media including water, sand, and gravel. A challenge with screws is their significant number of impactful design parameters that affect locomotion performance. One crucial parameter is the angle of attack (also called the lead angle), which has been shown to significantly impact the performance of screw propellers in terms of traveling velocity, force produced, degree of slip, and sinkage. As a result, the optimal design choice may vary significantly depending on application and mission objectives. In this work, we present the Novel Actuating Screw Unit (NASU). It is the first screw-based propulsion design that enables dynamic reconfiguration of the angle of attack for optimized locomotion across multiple media and use cases. The design is inspired by the kresling unit, a mechanism from origami robotics, and the angle of attack is adjusted with a linear actuator, while the entire unit is spun on its axis to generate propulsion. NASU is integrated into a mobile test bed and experiments are conducted in various media including gravel, grass, and sand. Our experiment results indicate a trade-off between locomotive efficiency and velocity exists in regards to angle of attack, and the proposed design is a promising direction for reconfigurable screws by allowing control to optimize for efficiency or velocity.

Figures (7)

• Figure 1: NASU is a screw-based propulsion unit for robot ground mobility that dynamically adjusts its angle of attack. The use of NASU is to allow control for optimizing efficiency or velocity over a wide array of terrains. It allows for trading off higher traveling velocity and lower efficiency (left) for higher efficiency and lower velocity (right) for resource-constrained robotic deployments.
• Figure 2: From left to right, the figures show NASU at the minimum and maximum angles of attack. The angle of attack is set by linearly actuating the back plate whose position is denoted with parameter $d$. When the linear actuator moves, 2-DOF joints connecting the front and back plates to the struts are passively actuated to set the angle of attack of the blades. Finally, the angle of attack can be computed using $d_0$ and the strut length, denoted as $\ell$.
• Figure 3: From left to right, the figures show an FEA estimating the displacement of the entire NASU and stress on one of the blades and its 2-DOF joints, respectively. The applied force is taken from the maximum amount of force measured in our previous outdoor screw experiments icra2023. Fixture points are set as connections from NASU to the ball screw which drives NASU's ability to change the angle of attacks. NASU was set to its maximum angle, 35°, to simulate the most extreme possible situation and resulted in minimal deflection and stress.
• Figure 4: The figure shows a geometrical breakdown of a top-down view of NASU to find the percentage of blade contact with the environment for an ideal locomotion velocity. The circle on the left figure defines the root radius of NASU, and the struts projected on this top-down view are tangent to this circle. A right triangle is formed from this projection to compute the interior angle, $\phi$, which is used to solve for the percentage of blade contact.
• Figure 5: The mobile test bed was augmented to mount NASU for experimentation. The mobile test bed constrains the motion linearly and measures the resultant propulsion forces and traveling velocity. The pulley system is used to apply a counterweight compensating for the test bed's added mass which leaves the effective mass equal to that of NASU including its driving motor and linear actuator to adjust the angle of attack.