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DTSTART;TZID=America/New_York:20240617T130000
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DTSTAMP:20260403T134754
CREATED:20240603T161004Z
LAST-MODIFIED:20240603T161004Z
UID:10007978-1718629200-1718632800@seasevents.nmsdev7.com
SUMMARY:MEAM Ph.D. Thesis Defense: "Bistable Structures Enable Passive Transitions in Mobile Robots"
DESCRIPTION:Making robots more capable\, agile\, and efficient requires careful design of the robot’s mechanical body to match task requirements. Passive components allow a robot to perform a task without a dedicated actuator\, often improving both power consumption and overall performance. In this thesis\, we investigate robotic applications of bistable mechanisms\, mechanical structures that exhibit two stable equilibria\, to enable passive actuation and locking for systems with discrete task modes. \nThe main underlying idea of this thesis is that “force reversal” provides a practical means of causing bidirectionally passive snap-through of bistable structures\, meaning that in the frame of the bistable mechanism\, force must be applied toward each target equilibrium state. Force reversal can be produced through a variety of means that do not necessarily require direction change in the actuators. In particular\, the design of the mechanism surrounding the bistable structure serves as transmission to redirect actuation forces. We demonstrate this idea in the context of three unique systems: a gripper that uses contact\, a gripper that uses a twisting Kresling origami pattern\, and a morphing aerial vehicle that uses inertial forces. \nMore specifically\, the main theoretical contribution of this thesis is a method for determining the actuation force requirements for dynamically-actuated bistable mechanisms\, where inertial forces are responsible for producing snap-through. In this case\, there is a direct relationship between the inertial forces and the output force of the actuators that produce the associated motions. We find that the minimum actuating force required for snap-through depends on the ratio between the mass on the bistable structure and the robot’s total mass\, and that it also depends on friction but not on viscous damping. The main experimental contribution includes demonstrations of the impact of bistable mechanisms on grasping and flying systems. For perching\, we show that attaching a linkage to a passive bistable structure augments a gripper’s locking strength\, leading to passive grasping with a high strength-to-weight ratio. For aerial reconfiguration\, we demonstrate that the energy cost of passive dynamic transformation can be offset by the efficiency gains of transforming from a quadrotor to a fixed wing mode. Overall\, this thesis shows that passive bistable mechanisms can eliminate the need for task-specific actuators by repurposing existing locomotion actuators.
URL:https://seasevents.nmsdev7.com/event/meam-ph-d-thesis-defense-bistable-structures-enable-passive-transitions-in-mobile-robots/
LOCATION:Towne 319\, 220 S. 33rd Street\, Philadelphia\, 19104\, United States
CATEGORIES:Doctoral,Dissertation or Thesis Defense
ORGANIZER;CN="Mechanical Engineering and Applied Mechanics":MAILTO:meam@seas.upenn.edu
END:VEVENT
BEGIN:VEVENT
DTSTART;TZID=America/New_York:20240618T100000
DTEND;TZID=America/New_York:20240618T113000
DTSTAMP:20260403T134754
CREATED:20240604T154627Z
LAST-MODIFIED:20240604T154627Z
UID:10007979-1718704800-1718710200@seasevents.nmsdev7.com
SUMMARY:MEAM Seminar: "Controlling Contact Transitions for Dynamic Robots"
DESCRIPTION:Legged robots\, robotic manipulators\, and their combined embodiment as humanoid robots have received considerable attention across both academia and industry. However\, with few notable exceptions\, state-of-the-art demonstrations are significantly less dynamic than their biological counterparts. A considerable challenge for performing more dynamic tasks for both legged robots and robotics manipulators lies within controlling contact interactions with their environment. Legged robots are sensitive to impacts with the ground when executing dynamic motions because they undergo large changes in their velocities in a short amount of time with uncertainty in both the impact model and timing. Robotics manipulators often focus on quasistatic models or static contacts to avoid the underactuation that comes with sliding. First\, we will propose a general framework for reducing sensitivity to uncertainty to the impact event\, which we demonstrate on dynamic jumping and running controllers on the 3D bipedal robot\, Cassie. Next\, we explore a dynamic non-prehensile manipulation task that requires the consideration of the full spectrum of hybrid contact modes. We leverage recent methods in contact-implicit MPC to handle the multi-modal planning aspect of the task. We demonstrate\, with careful consideration of integration between the simple model used for MPC and the low-level tracking controller\, how contact-implicit MPC can be adapted to dynamic tasks. Finally\, I propose small modifications to the MPC framework to add a dual-sided margin to the stick-slip boundary.
URL:https://seasevents.nmsdev7.com/event/meam-seminar-controlling-contact-transitions-for-dynamic-robots/
LOCATION:Room 337\, Towne Building\, 220 South 33rd Street\, Philadelphia\, PA\, 19104\, United States
CATEGORIES:Seminar,Doctoral
ORGANIZER;CN="Mechanical Engineering and Applied Mechanics":MAILTO:meam@seas.upenn.edu
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BEGIN:VEVENT
DTSTART;TZID=America/New_York:20240618T120000
DTEND;TZID=America/New_York:20240618T120000
DTSTAMP:20260403T134754
CREATED:20240611T122503Z
LAST-MODIFIED:20240611T122503Z
UID:10007985-1718712000-1718712000@seasevents.nmsdev7.com
SUMMARY:ESE PhD Thesis Defense: "Engineering copper-vacancy color centers in zinc sulfide for quantum defect discovery"
DESCRIPTION:Photoluminescent point defects\, or color centers\, in wide-bandgap semiconductors are important platforms for quantum information science because they can be operated as solid-state quantum spin-light interfaces. Implementing so-called defect qubits in an expanded variety of materials systems is beneficial for applications\, since the host-defect material properties determine operating parameters such as emission wavelength\, spin coherence time\, and pathways for device integration. A key challenge is obtaining materials that contain defects of interest\, and at sufficiently low concentrations to allow observation of quantum emission. This thesis concerns the creation of copper-vacancy complexes for quantum defect studies in zinc sulfide\, a material in which there is no known defect qubit. Zinc sulfide\, as the host material\, possesses a wide bandgap and a low concentration of nuclear spins\, enabling the operation of an electronically isolated spin-light interface with low magnetic background noise. The copper-vacancy center\, as the point defect of interest\, has been shown to exhibit favorable characteristics including radiative transitions between isolated states inside the zinc sulfide bandgap\, a paramagnetic ground state\, and a C3V-symmetric impurity-vacancy structure which results in favorable orbital and spin characteristics for several known defect qubits. We use both chemical synthesis and focused ion beam (FIB) implantation to obtain copper-vacancy color centers in zinc sulfide. FIB implantation of copper followed by annealing creates localized arrays of copper-vacancy color centers in single-crystal zinc sulfide. Studies of copper-vacancy center activation in bulk zinc sulfide reveal new evidence regarding the origins of the associated emission\, and provide bright ensembles of centers sharing a single crystal lattice for field-dependent measurements. However\, the background emission in commercially available zinc sulfide poses a barrier to observing quantum emission from copper-vacancy color centers. This barrier is overcome by the successful activation of copper-vacancy centers in colloidal nanocrystals of zinc sulfide\, which we can sufficiently dilute using solution processing methods to the extent that we are able to measure photon antibunching from copper-vacancy centers. We discuss the templated assembly and isolation of colloidal nanocrystals of zinc sulfide containing copper-vacancy color centers\, which can withstand liftoff and ligand-exchange procedures without quenching of the copper-vacancy luminescence. We further discuss techniques uniquely developed for the spin-optical characterization of these copper-vacancy centers as potential defect qubits. These include time-gating photoluminescence scans to improve the visibility of copper-vacancy centers based on the long-lived emission components we measure in ensemble studies\, and 2D\, room-temperature optically-detected magnetic resonance spectroscopy capabilities compatible with time-gating. Prior to the work presented here to gain access to red-emitting copper-vacancy color centers for their attractive properties as a defect qubit candidates\, there has not been an intensive effort to create or understand red-emitting copper-vacancy color centers (R-Cu centers) in zinc sulfide since the mid-20th century. As a result\, they have never been created using ion beam implantation\, and there is only one report of copper-doped zinc sulfide nanocrystals which emit a red peak assigned to these color centers. In providing routes for obtaining arrays of localized emission from copper-vacancy color centers in both bulk and colloidal nanocrystal zinc sulfide\, this thesis provides new understanding of the red emission from the copper-vacancy color centers and proposes a solution to inconsistencies in reports of their emission mechanism and peak energy. We find that the R-Cu emission arises from thermally activated carrier transfer between two radiative manifolds\, producing an anomalous plateau in the thermal quenching profile and blueshifted luminescence upon increasing temperature. Understanding of these characteristics and their relationship to the charge and spin states of the R-Cu center can inform the development of protocols for operating the center as a quantum spin-light interface. We further demonstrate the powerful advantages of quantum defect exploration using colloidal nanocrystals in place of bulk single-crystals or powders.
URL:https://seasevents.nmsdev7.com/event/ese-phd-thesis-defense-engineering-copper-vacancy-color-centers-in-zinc-sulfide-for-quantum-defect-discovery/
LOCATION:Raisler Lounge (Room 225)\, Towne Building\, 220 South 33rd Street\, Philadelphia\, PA\, 19104\, United States
CATEGORIES:Dissertation or Thesis Defense
ORGANIZER;CN="Electrical and Systems Engineering":MAILTO:eseevents@seas.upenn.edu
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BEGIN:VEVENT
DTSTART;TZID=America/New_York:20240619T100000
DTEND;TZID=America/New_York:20240619T110000
DTSTAMP:20260403T134754
CREATED:20240605T173156Z
LAST-MODIFIED:20240605T173156Z
UID:10007982-1718791200-1718794800@seasevents.nmsdev7.com
SUMMARY:MEAM Ph.D. Thesis Defense: "Computational Fluid-Structure Interaction Modeling of the Cardiovascular System"
DESCRIPTION:Patient-specific computational modeling and simulation has become a routine part of cardiovascular clinical research. These techniques leverage medical imaging to construct subject-specific models that can be used to study disease processes\, design and evaluate medical devices\, perform predictive surgery\, and aid in clinical decision-making. Modern cardiovascular simulations often require millions of elements and tens of thousands of time steps. Incorporation of additional physics only contributes to these costs and increases model complexity. Due to the presence of complex pulsatile hemodynamics potentially coupled with deformable vessel walls or heart valves\, development of accurate\, robust\, and efficient cardiovascular simulation tools remains a challenging task. In this thesis\, I present several improvements to existing finite element solver technologies for computational modeling of the cardiovascular system\, all of which were implemented in a new computational FSI framework I developed in the Modular Finite Elements Methods (MFEM) C++ library. First\, I describe a block preconditioning technique for implicit time discretization of the Navier-Stokes equations monolithically coupled to reduced dimension models of the cardiovascular system (e.g. Windkessel model). Mass conservation properties of various solution algorithms are investigated in a patient-specific aorta model. Next\, I show how these improved techniques can be leveraged to simulate FSI problems\, such as blood flow through deformable vessels\, using the arbitrary Lagrangian-Eulerian method combined with a quasi-Newton solution procedure. Lastly\, I present an immersed approach for computational modeling of fluid-structure interaction. A fully implicit monolithic coupling method is described\, as well as several discretization improvements targeted for immersed thin structures. I demonstrate the potential of the method to simulate heart valves dynamics over the cardiac cycle using an idealized problem and two extensions: heterogeneous valves as a simplified model for calcification\, as well as an anisotropic Fung type constitutive model for the leaflets.
URL:https://seasevents.nmsdev7.com/event/meam-ph-d-thesis-defense-computational-fluid-structure-interaction-modeling-of-the-cardiovascular-system/
LOCATION:Towne 319\, 220 S. 33rd Street\, Philadelphia\, 19104\, United States
CATEGORIES:Doctoral,Dissertation or Thesis Defense
ORGANIZER;CN="Mechanical Engineering and Applied Mechanics":MAILTO:meam@seas.upenn.edu
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