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Characterizing Granular Media for Robotic Exploration
University of California, San Diego
Department of Mechanical and Aerospace Engineering
Spring 2025, MAE 156B: Senior Design Project
Project Overview
Project Overview
Background
Background
The Gravish Lab and Bioinspired Design and Robotics Lab of the UC San Diego Department of Mechanical and Aerospace Engineering, led by Dr. Nicholas Gravish and Dr. Michael Tolley, use research on natural biomechanics to inspire robotic designs. In recent work, their teams have been developing a bio-inspired robot capable of digging and navigation in sand and shallow water, supported by the Office of Naval Research (ONR). The design is intended to facilitate seafloor exploration, monitor environmental contaminants, and support the study of marine organism and underwater terrain. The initial design was modeled after worms and sea turtles, featuring underactuated appendages to thrust its forward motion and terrafoils to ensure a level sand depth. However, the resistive loads acting on the worm during field testing were not accurately represented in the lab environment. This is due to the uniformity of the silica beads used to model the granular medium (GM) in the lab, which in general generate less resistance than beach sand. In sand, drag and lift forces experienced by robots vary greatly with depth and fluid-solid state. Current, standard soil resistance measurement methods are not applicable on the small scale these robots are designed.
Objective
Objective
As such, the sponsors requested the construction of a Resistive Torque Measurement Apparatus (RTMA) to improve sand characterization, and in turn, future performance of this and similar robots in GM. Operating in all regions of the beach, spanning from dry sand to shallow water 76 mm (3 in), the apparatus was tasked with measuring resistive torque with respect to an angular and vertical position. Surveying between penetration depths of 127 mm (5 in) to 254 mm (10 in), the apparatus will measure the torque as an intruder, modeling the wingspan of the bio-inspired worm, is moved through sand. Recording of these measurements necessitates an onboard data logging system, programmed at the push of a button, with GPS and other critical environmental data being stored for data processing.
How Does It Work?
How Does It Work?
Final RTMA design CAD.
High-level overview of electronics package.
The RTMA utilized a reaction torque sensor to measure the resistive torque on a manually rotated intruder submerged in granular media. The RTMA design included four main systems: torque measurement, angular measurement, environmental sensor, and control and data logging. All systems are integrated with a modular tripod frame design. The apparatus can be assembled at a testing site and can be placed at various locations, where the intruder is then manually penetrated into and rotated in granular media. The apparatus utilizes a setup and data collection mode switch to alternate between initial setup, data collection, then sending data for storage and preparing for the next testing location. A high-level overview of the electronics package of the RTMA is illustrated above, including the primary torque and angular measurement sensors, various environmental sensors, user interaction peripherals, and main computing and power distribution components.
Performance Summary
Performance Summary
The completed RTMA was able to produce useful measurements at various depths across various beach sand conditions. Torque, angular position, and various environmental data were measured and stored as CSV data files, allowing for analysis and comparison of the properties of granular media. The RTMA fulfilled all functional requirements of the project, with a full list of capabilities listed below:
Stability and rigidity in various beach sand conditions, including submersion in up to 150 mm (6 in) of water
Convenient and quick collapsibility and portability: <10 minutes to setup and break down, and <5 minutes to perform each test
Precise torque measurement up to 50 Nm, <0.1 Nm sensitivity
Precise angle measurement, <0.1° sensitivity
Environmental sensing capability, measuring GPS coordinates, ambient temperature and humidity, and soil moisture
"Push-of-a-button" testing system, allowing for convenient use by only two operators
Rapid data transfer and analysis, with each test file storing all environmental data along with torque and angle measurements taken at 50 Hz
Purpose built data analyzer, allowing for convenient data comparison and visualization
The results found by the RTMA suggest interesting trends regarding how torque varies with sand conditions, depth, angle, and time. Research using the RTMA will continue under the Gravish Lab to determine resistance trends in various GM conditions, and what they suggest about the future of bio-inspired robotics.
Documentation
Documentation
Who Are We?
Who Are We?
Resistive Torque Apparatus Team [From left:] Emily Brown, Dylan McCulloch, Richard Vallejo Jr, Hrag Ekmekjian
The Resistive Force Measurement Apparatus, sponsored by Dr. Nicholas Gravish and Dr. Michael Tolley, was designed, manufactured, and tested by four UC San Diego Mechanical Engineering students as their Senior Capstone Project. This project would not be possible without the contributions, insight, and support of the sponsors, instructional team, and engineering staff aiding the team throughout the entire process.
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