Browsing by Author "Chen, Thomas, committee member"

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Open Access
Autonomous robot control: integrated control strategies for a mobile robotic arm
(Colorado State University. Libraries, 2025) Weinmann, Katrina, author; Simske, Steve, advisor; Chen, Thomas, committee member; James, Susan, committee member; Zhao, Jianguo, committee member
Autonomous robots open up a wide range of potential applications for robotic systems beyond the controlled manufacturing environments where they were originally used. These applications can include agriculture, space exploration, search and rescue, fire-fighting, performing tasks in hazardous environments, personal care robots, and much more. Some of these applications have the potential to replace humans in dangerous environments or improve quality of life for elderly or disabled individuals, thus providing great positive societal impacts. However, the technologies needed for robots to operate safely and autonomously in unstructured environments, and especially when interacting with humans, are still being developed. Designing and controlling autonomous robotic systems is a very challenging problem, with some of the major objectives including efficient autonomous navigation in both known and unknown environments; real-time, dynamic obstacle avoidance; real-time and energy-efficient trajectory generation; and safe operation with and/or in close proximity to humans. While all of these topics have been researched in the field of robotics, existing solutions still have limitations which encourage further developments improving on existing autonomous robotic capabilities. Furthermore, each application and configuration of autonomous robotic systems has a unique set of requirements. In this dissertation, the platform of a mobile robotic arm was chosen for its wide range of potential applications achieved from the combined navigation and manipulation abilities of such a robot configuration. Within the scope of autonomous mobile robot arm control, the following topics were identified and chosen for research: (1) indoor target localization; (2) efficient navigation in partially known environments; (3) integrated control (i.e. coordinated base and arm motion) of a mobile robot arm, including both navigation and trajectory generation and tracking. For each topic, a novel or improved methodology was developed, all relevant to a wide range of autonomous robot deployments. The contributions of this dissertation are as follows: (1) Bluetooth-based homing controller for indoor target localization achieving a mean target localization accuracy of 0.12m or less with various levels of simulated sensor noise; (2) modified artificial potential field-based method for efficient navigation in a partially-known environment and for integrated control of a mobile robot arm improving autonomous navigation success rate and efficiency over existing APF-based methods in environments of varying levels of complexity; (3) real-time many objective optimization-based approach trajectory generation method for integrated motion of a mobile robot arm to reach a desired end-effector configuration, demonstrating a 100% success rate in achieving the desired configuration and reaching the configuration in under 30s in 77% of trials; (4) offline trajectory generation for mobile robot arm end-effector trajectory tracking using a sampling-based combinatorial optimization method to generate integrated motion trajectories (coordinated mobile base and robotic arm motion) achieving over 99% success rate in high accuracy (<5mm position tracking error and <0.1 radian orientation tracking error) end-effector trajectory tracking tested on 500 sample trajectories; and (5) integrated controller design for differential drive mobile robot arm trajectory tracking consisting of a fuzzy logic-based differential drive robot (DDR) controller reducing irregular trajectory tracking errors by 2.4X to 6.8X over existing DDR controller designs, and integrated robotic arm-facilitated DDR base tracking error compensation reducing mean maximum end-effector tracking errors by 18%.
Open Access
Exploration of nitric oxide generation from S-nitrosoglutathione for the advancement of anti-fouling glucose biosensor membrane materials
(Colorado State University. Libraries, 2022) Melvin, Alyssa C., author; Reynolds, Melissa M., advisor; Zadrozny, Joseph, committee member; Farmer, Delphine, committee member; Chen, Thomas, committee member
Blood-contacting medical devices such as implantable glucose sensors suffer from biofouling which limits the lifetime of the device and puts the patient at risk of arterial embolism and infection. Researchers have been developing medical device coatings to address the two causes of surface biofouling: thrombus and biofilm formation. One promising strategy is surface-localized production of nitric oxide (NO), a biomolecule with antithrombotic and antibacterial physiological functions, as a multifunctional therapeutic for biofouling prevention. Because NO is a gaseous free radial with a very short physiological half-life, achieving localized NO generation presents a clear challenge. An innovative approach that will be explored herein is incorporating catalysts on the medical device surface that release NO from endogenous NO sources, S-nitrosothiols (RSNOs). A water stable copper-based metal–organic framework (MOF) CuBTTri, Cu(II) benzene-1,3,5-tris(1H-1,2,3-triazoy-5-yl), has been shown to be an effective catalyst for the generation of NO from RSNOs. Two RSNOs, S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP), are commonly used in the development of new NO-generating materials. It is known that RSNOs are susceptible to decomposition by stimuli including heat, light, and trace metal ions which can inadvertently be introduced through basic handling, storage conditions, and experimental setups. Despite their frequent use, there is limited and conflicting literature examining the comparative stability of GSNO and SNAP. In order to accurately characterize and quantify the behavior of NO-generating materials, reliable and robust methods must be developed to prevent spontaneous RSNO decomposition under the desired experimental conditions. In Chapter 2, the comparative stability of GSNO and SNAP was thoroughly examined to inform subsequent experiments in the development of CuBTTri-based anti-fouling materials with RSNOs as the NO source. Though CuBTTri is an effective catalyst for this reaction, solid state material must be immobilized into a flexible, processable scaffold for coating devices. In Chapter 3, the effects of incorporating CuBTTri into a medical grade polyurethane composite material on NO generation is explored. In Chapter 4, the effects of three key parameters of CuBTTri composite materials, MOF particle size, MOF loading, and polymer concentration, on NO generation are evaluated to assess the tunability of these next-generation materials. In Chapter 5, the effects of the CuBTTri/polyurethane composite material on the enzyme function and analytical performance of a glucose biosensor are examined. Though metal ion-promoted NO release from RSNOs is promising strategy for the development of NO-generating materials, the majority of studies focus on copper, and few have surveyed the ability of other common metal ions to produce this effect. Finally, in Chapter 6, NO generation from GSNO by Cu2+ and twenty transition and post-transition metal ions was monitored using NO-selective chemiluminescence-based detection to expand the range of potential metals for the development of NO-based anti-fouling materials.
Open Access
Gallium nitride high electron mobility transistors in chip scale packaging: evaluation of performance in radio frequency power amplifiers and thermomechanical reliability characterization
(Colorado State University. Libraries, 2017) Shover, Michael Andrew, author; Collins, George, advisor; Chandrasekar, Venkatachalam, committee member; Chen, Thomas, committee member; Ackerson, Chris, committee member
Wide bandgap semiconductors such as Gallium Nitride (GaN) have many advantages over their Si counterparts, such as a higher energy bandgap, critical electric field, and saturated electron drift velocity. These parameters translate into devices which operate at higher frequency, voltage, and efficiency than comparable Si devices, and have been utilized in varying degrees for power amplification purposes at >1 MHz for years. Previously, these devices required costly substrates such as sapphire (Al2O3), limiting applications to little more than aerospace and military. Furthermore, the typical breakdown voltage ratings of these parts have historically been below ~200 V, with many targeted as replacements for 50 V Si LDMOS as used in cellular infrastructure and industrial, scientific, and medical (ISM) applications between 1 MHz and 1 GHz. Fortunately within the past five years, devices have become commercially available with attractive key specifications: GaN on Si subtrates, with breakdown voltages of over 600 V, realized in cost effective chip scale packages, and with inherently low parasitic capacitances and inductances. In this work, two types of inexpensive commercially available AlGaN/GaN high electron mobility transistors (HEMTs) in chip scale packages are evaluated in a set of three interconnected experiments. The first explores the feasibility of creating a radio frequency power amplifier for use in the ISM bands of 2 MHz and 13.56 MHz, at power levels of up to 1 kW, using a Class E topology. Experiments confirm that a DC to RF efficiency of 94% is easily achievable using these devices. The second group of experiments considers both the steady state and transient thermal characterization of the HEMTs when installed in a typical industrial application. It is shown that both types of devices have acceptable steady state thermal resistance performance; approximately 5.27 °C/W and 0.93 °C/W are achievable for the source pad (bottom) cooled and top thermal pad cooled device types, respectively. Transient thermal behavior was found to exceed industry recommended maximum dT/dt by over 80x for the bottom cooled devices; a factor of 20x was noted with the top cooled devices. Extrapolations using the lumped capacitance method for transient conduction support even higher initial channel dT/dt rates. Although this rate of change decays to recommended levels within one second, it was hypothesized that the accumulated mechanical strain on the HEMTs would cause early life failures if left uncontrolled. The third set of experiments uses the thermal data to design a set of experiments with the goal of quantifying the cycles to failure under power cycling. It is confirmed that to achieve a high number of thermal cycles to failure as required in high reliability industrial systems, the devices under test require significant thermal parameter derating to levels on the order of 50%.
Open Access
Material validation and part authentication process using hardness indentations with robotic arm implementation
(Colorado State University. Libraries, 2021) Weinmann, Katrina J., author; Simske, Steve, advisor; Chen, Thomas, committee member; Ma, Kaka, committee member; Zhao, Jianguo, committee member
In today's global economy, there are many levels of validation and authentication which must occur during manufacturing and distribution processes to ensure sufficient cyber-physical security of parts. This includes material inspection and validation during manufacturing, a method of track-and-trace for the entire supply chain, and individual forensic authentication of parts to prevent counterfeiting at any point in the manufacturing or distribution process. Traditionally, each level of validation or authentication is achieved through a separate step in the manufacturing or distribution process. In this work, a process is presented that uses hardness testing and the resulting indentations to simultaneously provide three critical functions for part validation and authentication: (i) material property validation and material property mapping achieved by administering multiple hardness tests over a given area on the part, (ii) part serialization that can be used for track-and-trace through administering hardness tests in a specific 'barcode' pattern, and (iii) the opportunity for forensic-level authentication through use of high-resolution images of the indents. Additionally, a fourth manufacturing advantage is gained in the provision of improved bonding potential for adhesive joints provided by the increase in surface area and surface roughness resulting from the addition of indents to the adherend surface. A methodology for implementing this process using a robotic arm with an end-effector-mounted portable hardness tester is presented. Implementation using a robotic arm allows a high degree of customizability of the process without changes in setup, making this process ideal for additive manufactured parts, which are often custom or low-batch and require a higher level of material validation. As a whole, this work presents a highly-customizable, single-step process that provides multi-level quality control, validation, authentication, and cyber-physical security of parts throughout the manufacturing and distribution processes