Dissertation
HIDEX: a state-of-the-art hexapod for advancing hydrodynamic experimental testing
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Summer 2025
DOI: 10.25820/etd.008080
Abstract
The modeling and control of uncrewed underwater vehicles (UUVs) with atypical hullforms remains a challenging endeavor, with direct relevance to naval autonomy in the nearshore and deep-water regions alike. One of the principle hurdles is the rapid and accurate identification of hydrodynamic components (i.e., added mass and added damping) that impact the maneuvering attributes of these vehicles (Konstantinidis, 2013). Traditional experimental methods, such as the Planar Motion Mechanism (PMM), are limited by the number of simultaneously articulated DOFs and are limited to single-frequency testing, making such systems impractical for resolving frequency-dependent added mass or damping matrices. Thus, to address this and enhance hydrodynamics research at the University of Iowa, a six-degree-of-freedom hexapod (also known as Gough-Stewart platform) is designed, fabricated, controlled and commissioned with impressive capabilities.
The 6-DOF platform, termed a hexapod, overcomes the limitations of PMMs by being able to test models in a broad-branded frequency spectra in multiple degrees of freedom in a single experiment. This thesis discusses various aspects of the hexapods, including its kinematics, mechanical design, limitations, and control strategies. To optimize its performance and enable precise control, a Digital Model and a Digital Shadow of the platform were developed using MATLAB App. The Digital Model is used to simulate and evaluate the platform’s range of motion and dynamic positioning accuracy, demonstrating its ability to support large payloads, operate at high speeds, and achieve sub-millimeter translational and sub-0.1 degree dynamic positioning errors, verified using a precision motion capture system. Building on the Digital Model, a camera-based tracking system is presented, incorporating a hybrid fiducial marker and a hand-eye calibration to precisely estimate the hexapod’s pose. The Digital Shadow enables real-time visualization and validation of the u-joint kinematics for a hexapod with universal joints at both ends of its actuators. Thus, this state-of-the-art hexapod, developed at the University of Iowa, addresses the need for a more efficient platform for identifying hydrodynamic components, while also contributing to ongoing advancements in hexapod-base research.
Details
- Title: Subtitle
- HIDEX: a state-of-the-art hexapod for advancing hydrodynamic experimental testing
- Creators
- Juliana Danesi Ruiz
- Contributors
- Rachel Vitali (Advisor)Casey M. Harwood (Advisor)Tyler Bell (Committee Member)Hiroyuki Sugiyama (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Mechanical Engineering
- Date degree season
- Summer 2025
- DOI
- 10.25820/etd.008080
- Publisher
- University of Iowa
- Number of pages
- xvi, 136 pages
- Copyright
- Copyright 2025 Juliana Danesi Ruiz
- Grant note
The work of this thesis was in part supported by the U.S. Office of Naval Research (ONR) through grant N00014-22-1-2097, managed by Troy Hendricks. And, it is also supported by the U.S. Department of Education through grant EDP116S210005.
(iii)- Comment
- This thesis has been optimized for improved web viewing. If you require the original version, contact the University Archives at the University of Iowa: https://www.lib.uiowa.edu/sc/contact/
- Language
- English
- Date submitted
- 07/25/2025
- Description illustrations
- Illustrations, tables, graphs, charts
- Description bibliographic
- Includes bibliographical references (pages 103-113).
- Public Abstract (ETD)
The added mass is a key concept in fluid mechanics that affect maneuverability and performance of underwater vehicles. Added mass occurs when an object accelerates or decelerates in water, as the water can not occupy the same space as the object, the water surrounding it accelerates at varying magnitudes and directions. Understanding this component is critical for designing precise control systems for underwater vehicles. However, current traditional experimental method used to measure added mass are time-consuming and limited in motion.
To address this, a benchmark experiment was first conducted to validate a streamlined approach for capturing added mass data using a single trial with multiple random vibrations. This method allowed data collection across a range of frequencies with one experiment, significantly reducing testing time. The results closely matched theoretical predictions, validating the method.
Building on this benchmark, a custom-designed hexapod (also known as a Stewart Platform) was designed and build to take advantage of multi-frequency test and multi-directional testing. Hexapods are parallel manipulators known for their superior stiffness, accuracy, and ability to move in all six degrees of freedom. This project leverages state-of-the-art hexapod research to create a platform capable of high-precision positioning and control. While hexapods have historically been used for tire testing and flight simulators (Stewart, 1965), this work represents the first known application of a Stewart Platform within a towing tank facility for experimental fluid mechanics research in North America.
- Academic Unit
- Mechanical Engineering
- Record Identifier
- 9984948427902771
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