517 research outputs found
Determination of dynamic variations in the optical properties of graphene oxide in response to gas exposure based on thin-film interference
No full textWe present an effective yet simple approach to study the dynamic variations in optical properties (such as the refractive index (RI)) of graphene oxide (GO) when exposed to gases in the visible spectral region, using the thin-film interference method. The dynamic variations in the complex refractive index of GO in response to exposure to a gas is an important factor affecting the performance of GO-based gas sensors. In contrast to the conventional ellipsometry, this method alleviates the need of selecting a dispersion model from among a list of model choices, which is limiting if an applicable model is not known a priori. In addition, the method used is computationally simpler, and does not need to employ any functional approximations. Further advantage over ellipsometry is that no bulky optics is required, and as a result it can be easily integrated into the sensing system, thereby allowing the reliable, simple, and dynamic evaluation of the optical performance of any GO-based gas sensor. In addition, the derived values of the dynamically changing RI values of the GO layer obtained from the method we have employed are corroborated by comparing with the values obtained from ellipsometry.This article is published as Tabassum, Shawana, Liang Dong, and Ratnesh Kumar. "Determination of dynamic variations in the optical properties of graphene oxide in response to gas exposure based on thin-film interference." Optics Express 26, no. 5 (2018): 6331-6344. DOI: 10.1364/OE.26.006331. Posted with permission.</p
Development of Wire and Arc Additive Manufacturing Process for Deposition of Ni- Based Super Alloy
No full textWire Arc Additive Manufacturing (WAAM) is an innovative form of 3D printing that uses an electric arc as a heat source to melt metal wire, which is then deposited layer by layer to create complex metal parts. This technique achieves high deposition rates and allows the building of large-scale components with significant cost and material savings compared to traditional manufacturing methods. In the past, a variety of materials have been fabricated through WAAM. However, very little work has been done using WAAM on nickel-based super-alloys. Nickel-based super-alloys are a class of metallic alloys known for their exceptional high-temperature strength, resistance to creep deformation, and resistance to corrosion and oxidation. The excellent combination of these properties makes these materials ideal for use in jet engines, gas turbines, power plants, and other high-temperature applications. Manufacturing these complex design components of nickel-based superalloys using traditional methods is challenging and expensive. To address this issue, the current research focuses on the feasibility and development of the WAAM process as a promising alternative for producing nickel-based components.
The initial focus is on establishing a robust GMAW-based WAAM process. This involves optimizing the gas metal arc welding (GMAW) process parameters, a key element of WAAM. Through numerous trials, the main aim is to identify the ideal settings for factors such as current, voltage, and welding speed. This study uses a bi-directional, low-cost, GMAW-based wire arc additive manufacturing (WAAM) setup to deposit aerospace-grade super-alloy IN718. Orthogonally designed experiments are carried out to optimize process parameters for single and multilayer bead geometries.
Additionally, ANOVA analysis and S/N ratio plots are employed to optimize process parameters for bead characteristics such as width, reinforcement, penetration, dilution, WRFF, WPSF, surface waviness, and effective area. Furthermore, the effect of heat input is discussed. The study results indicate that increased heat input increases bead parameters like bead width, reinforcement, dilution, penetration, and weld form factor while negatively impacting the weld shape factor and wetting angle.
After optimizing process parameters, characterizations of WAAM-deposited materials were done based on metallurgical and mechanical testing. The metallurgical analysis includes optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The mechanical analyses were done based on hardness and tensile testing. These tests provide insights into the microstructure, phases present, elemental composition, grain orientation, and material strength.
The microstructural examination reveals that a high heat input produces a coarse grain structure with an average grain size of 72.29 µm. In contrast, a low heat input produces a finer grain structure with an average grain size of 32.12 µm and smaller Laves phases. The XRD analysis shows the presence of NbC and TiC phases in the low heat input sample, enhancing the deposited material's mechanical properties. The multilayer wall structures fabricated with high heat input demonstrate more uniformity in shape and less waviness than those fabricated with low heat input. Tensile testing results indicate that the low heat input sample exhibits higher strength (775.82 MPa) than the high heat input sample (741.07 MPa). This increased strength is attributed to the smaller grain structure and higher hardness (278.11 HV) observed in the low heat input sample compared to the high heat input sample (257.26 HV). The EBSD analysis further confirms that the low heat input sample has a highly textured surface, more grain boundary lengths, and larger grain boundary orientation, which contributes to its superior mechanical properties.
This study also provides insights into cooling environments and thermal management techniques for WAAM of IN718 components. The material was deposited under four different heat-input conditions, using either air or water cooling. The layers were deposited in a normal atmospheric environment with air cooling, while water cooling involved depositing the material inside a water tank with varying water levels. To validate the air- and water-cooling thermal management techniques, IN718 single-pass, and multilayer linear walls were deposited under four different heat-input conditions. Temperature profiles were recorded during single-layer depositions, and geometric and microstructural features were examined. The SEM analysis revealed that the microstructure in the building direction was non-homogeneous compared to that in the deposition direction.
Additionally, water cooling significantly influenced bead characteristics, such as wall width and height. The grain size and anisotropy of the mechanical properties decreased with water cooling. Therefore, water cooling is economical and efficient in mitigating excessive heat accumulation in WAAM-deposited IN718 components.
Further, this study also examines the effects of shielding gases at various heat inputs on the bead geometry, microstructure evolution, and mechanical properties of Inconel 718 deposited by WAAM. Bead-on-plate experiments were conducted with the CMT technique of GMAW, utilizing Taguchi’s L9 orthogonal array. The factor effects, their contributions, and optimal levels were analyzed using ANOVA and S/N ratio approaches. To determine heat input, the dynamic characteristics of the welding power source were recorded and processed.
The study found increased heat input led to more pronounced bead geometry characteristics and a wider heat-affected zone (HAZ). Increasing the CO2 concentration in the Ar + CO2 mixture altered the weld texture from silver metallic (with pure Ar) to grey with 2.5% CO2 and light yellow with 20% CO2. Higher bead width, reinforcement, penetration, and dilution were observed with an Ar 20% CO2 mix. Beads formed with 2.5% CO2 were continuous and smoother at all heat inputs. Welds prepared in a 100% Ar shielding environment exhibited higher hardness than those with 2.5% CO2 and 20% CO2 mixtures. Higher concentrations of Nb-rich precipitates, identified through SEM-EDS analysis, contributed to increased strength.
Finally, the project culminated in a comparative analysis of two WAAM variants: CMT-WAAM and GMAW-WAAM. CMT-WAAM provides a more controllable arc with lower spatter, potentially reducing porosity and improving bead geometry. A comparison of surface waviness and mechanical properties shows that waviness is lower, whereas strength is higher in CMT WAAM components. Understanding these distinctions empowers us to decide the most suitable WAAM approach for specific applications
Foaming of Friction Stir Processed Al Precursor by Microwave Heating
No full textA substance with pores (voids) is known as a porous medium or material. The skeleton element of the substance is frequently referred to as the "matrix" or "frame." Many naturally occurring materials, including rocks and soil (such as aquifers and petroleum reservoirs), zeolites, biological tissues (such as bones, wood, and cork), and man-made materials, including metal, cement, ceramics, and polymers, can be categorized as porous media. These cellular materials are typically utilized in cases where weight needs to be reduced. Metals have a high energy absorption capacity during deformation caused by dislocation motion. As a result, metallic foams might offer an engineering material with the beneficial mechanical characteristics of polymeric foams, making them useful in applications where higher yield stresses and energy absorption are required. For many years, material scientists and technologists have worked to develop porous metals and metal foams. Metal foam is mainly composed of liquid and solid phases that coexist in a specific structure. A gas source is utilized to introduce pores into the structure. This gas source could be a blowing agent powder added to a liquid metal at a specific temperature, direct gas bubbles introduced to a liquid metal, or a blowing agent powder imbedded in the metal that expands when heated.
In the current technique, Titanium Hydride (TiH2: blowing agent), Aluminum powder (Al powder) and Calcium Carbonate (CaCO3: stabilizing agent) is embedded in the Aluminum AA7075 and AA6063 matrix using Friction Stir Processing (FSP) as the starting procedure. At this point, the metal is referred to as a precursor. This precursor creates a porous structure after being heated in a furnace/microwave at a specific time and temperature for the decomposition of the TiH2. As a result of this decomposition, hydrogen gas is released, which remains in the matrix and forms pores. Metal foam is created when these holes have a spherical shape or a design that causes the density to be significantly reduced.
The results of various experiments such as powder filling techniques, tool pin profiles, and a new precursor development technique called as Friction Stir Deposition (FSD) are reported in this research work in order to ensure uniform TiH2 distribution throughout the aluminum matrix for producing closed cell aluminum metal foam and its characterization. This indirect foaming method prevents the early melting of the material. This procedure opens the way for the creation of effective localized foam parts.
Based on the characteristic of the developed foam, it has been observed that the distribution of foamable mixture improved in the buried hole technique by creating an entrapped space to restrict the mixture to flow away and produce a porous Aluminum with a porosity of approximately 81% at 4-passes with 40% TiH2 composition. As the number of FSP passes increases, pore size decreases. The percentage decrease in pore size from one pass to four passes during microwave heating is observed as 17.78 % as compared to 14.76 % in furnace heating. The time required for the development of foam in microwave heating is lower than that of furnace heating and the time needed for the development of foam in microwave heating is only 16.67% of the furnace heating. It is also observed from the results that the porosity in sandwich and groove techniques is 69% and 48%, respectively. The buried hole technique has higher porosity and improvement in porosity is of 17.92% and 71.25% as compared to sandwich and groove techniques, respectively.
It is also observed that during different tool pin profiles, the distribution of the foaming mixture improves with continuous material deformation by straight threaded cylindrical (STC) tool pin compared to the pulsating effect produced by square (SQ) and triangular (TR) pin. The foam developed at straight threaded cylindrical (STC) pin shows uniform deformation behavior during compression test than the foam developed by other tool profiles.
The foam developed by friction stir deposition (FSD) technique shows uniform distribution of foaming powder in 4 and 5 numbers of holes in the consumable rod as compare to other. The hardness of the deposited precursor is lower than the base metal. The deposited material shows equiaxed fine grains that occurred due to dynamic recrystallization during deposition also have longer plateau stress due to optimum pore size which absorbs energy for longer duration
Special Issue on Recent Advances in Petri Nets, Automata, and Discrete-Event Hybrid Systems
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‘InfraRivChange’: A Web Application to Monitor River Migration at Sites of Critical Bridge Infrastructure in the Philippines
No full textNo abstract available
Directed control of discrete event systems
No full textFor the control of discrete event systems, the notion of directed control refines that of supervisory control. A directed controller is one that selects at most one controllable event to be enabled at any state (without disabling any uncontrollable event), which is in fact how a discrete event control is implemented. In contrast, a supervisory controller computes a maximal allowable set of controllable events at each state, leaving undecided exactly which one is to be enabled.
We model discrete event systems using the automaton formalism. Under directed control, our first goal is to achieve logical correctness of the controlled system behavior as specified by safety and nonblocking. Subsequently we address the best performance issue by providing an optimization based framework. The optimization task is to direct a system in such a way that regardless of the history of evolution, it accomplishes a pending task in a minimal cost.
In a state-based setting, we formulate and study the existence and synthesis problems with the above objectives. We first show that the existence and the synthesis of a safe and nonblocking directed controller are both solvable in polynomial complexity. Then we present a novel approach with polynomial complexity for the synthesis (and the existence) of an optimal director, thus providing a complete solution to the problems in study.</p
Decentralized/distributed failure diagnosis and supervisory control of discrete event systems
No full textDiscrete event systems (DESs) are event-driven systems, which change their discrete states upon asynchronous occurrence of certain events. This dissertation addresses decentralized/distributed failure diagnosis and supervisory control of DESs.;In a decentralized diagnosis architecture, a local diagnoser performs failure diagnosis completely based on its own observations without communicating with others. A notion of codiagnosability is introduced to capture the property that a system should satisfy such that its failure behaviors are diagnosable by one of the local diagnosers within a bounded delay of their occurrences. Algorithms with polynomial complexity in the size of system/specification models are presented for verifying codiagnosability, computing diagnosis delay bound, synthesizing local diagnosers, and online diagnosis using them. Further diagnosis properties are investigated through the introduction of strong-(co)diagnosability and safe-codiagnosability.;In a distributed diagnosis architecture, local diagnosers exchange their individual observation with each other to perform failure diagnosis collaboratively. Finite automata models are constructed to capture communication delays, and the system/specification/sensing models are augmented with respect to the communication delay models. Via those augmented models, a distributed diagnosis problem is converted to a decentralized diagnosis problem. This allows distributed diagnosis analysis to be performed in same as decentralized diagnosis analysis. Also, in the unbounded delay case decidability of the problem is established.;For supervisory control of DESs, prioritized synchronous composition (PSC) based decentralized control and nondeterministic decentralized control are introduced. A PSC based decision fusion rule is more general than the conventional conjunctive/disjunctive decision fusion rule since it has control-authority besides control-capability. Algorithms are presented for existence and synthesis of PSC based supervisors. Computational complexity of the former is polynomial in the size of both system and specification models, while complexity of the latter is polynomial in the size of systems model, and exponential in the size of specification model. By using nondeterministic supervisors, a weaker condition than the condition of controllability together with co-observability is obtained for decentralized control. Algorithms of polynomial complexity are presented for both existence and synthesis of nondeterministic supervisors in target control and range control problems.</p
Supervisory control of discrete event systems for bisimulation or simulation equivalence
No full textThe supervisory control of discrete event systems provides a framework for control of event-driven systems. Applications of supervisory control theory include protocol design for communication processes, control logic synthesis in manufacturing systems, and collision avoidance in human-computer interaction systems.;When designing a system at a certain level of abstraction, lower level details of the system and its specification are normally omitted to obtain higher level models that may be (non-deterministic) event-driven systems. Nondeterministic systems exhibit both branching and sequential behaviors and are captured using bisimulation equivalence (the traditional language equivalence only captures sequential behaviors). Simulation equivalence is more expressive than language equivalence but captures only the universal fragment of branching behaviors.;This dissertation presents supervisory control of discrete event systems for enforcing bisimulation equivalence or simulation equivalence with respect to given specifications. We show that in the general setting of nondeterministic systems and specifications, the complexity for bisimilarity enforcing control is doubly exponential and for similarity enforcing control remains polynomial solvable. So the choice of behavioral equivalence used depends on the application at hand and there is a trade-off between the expressivity and the complexity. We further show that the bisimilarity enforcing control problem becomes polynomially solvable when the system model is deterministic and there is complete observability of events. When the complete observability requirement is relaxed, the control existence problem remains polynomially solvable and the control synthesis problem becomes singly exponential. These complexities are similar to the ones for control under partial observation in completely deterministic setting Tsitsiklis (1989).;We introduce various notions of state-controllability (SC), state-recognizability (SR), state-achievability (SA), state-controllable-similar (SCS), state-controllability-bisimilar (SCB), and state-achievability-bisimilar (SAB) for deterministic system model. SC is a property of a controlled system under complete observation. Under partial observation, an additional property of a controlled system due to the partial observation is SR. The combined property of SC and SR is called SA. We show that properties of SC, SR and SA are not preserved under bisimulation equivalence and therefore cannot be served as a necessary condition for the existence of a bisimilarity enforcing supervisor. We introduce the notions of SCB and SAB, which are preserved under bisimulation, as part of the necessary and sufficient condition for the existence of a supervisor under complete and partial observation, respectively. We show that SC is not preserved under simulation equivalence and introduce SCS as a necessary and sufficient condition for the existence of a similarity enforcing supervisor under complete observation.;The aforementioned results use strict synchronous composition (SSC) of the system and supervisor as a mechanism of control. In SSC, it is required that individual systems synchronously execute all events. Prioritized synchronous composition (PSC) relaxed such synchronization requirements and this has been shown to enrich the control capability when the plant is non-deterministic. (The presence of nondeterminism in a plant model may cause the current state to be known with ambiguity, and allowing the flexibility of not synchronizing an event at all the candidate states that plant may have reached provides for additional benefits.) This dissertation introduces a notion of prioritized synchronous composition under mask (PSCM) to account for partial observation. We study the supervisory control when PSCM is adopted as a mechanism of interaction for both language and bisimulation equivalences. We show that the control & observation-compatibility requirements are removed of a supervisor. For control to achieve a language equivalence, the existence condition is given by achievability that is weaker than controllability and observability combined. (The weaker condition is required since we allow supervisors to be nondeterministic.) This suggests that the notion of PSCM is an appropriate generalization of PSC to account for partial observation.</p
Overview of Sepsis and Sepsis Biomarker Detection
No full textSepsis being a fatal physiological state due to an imbalance in the immune system caused by infection, and one of the most common cause for millions of deaths in the non-coronary intensive care unit worldwide requires special attention in its diagnostic methods and cure. Therefore an understanding of literature related to sepsis is of utmost importance. With the advent of inter-disciplinary research, the study and diagnosis of sepsis problem are not limited to the medical field, rather it requires interventions and active participation of other fields of science and technology. However, often subject matter from interdisciplinary research is expounded in an abstruse manner and hence it becomes elusive for a researcher from different research domain to understand it, leading to loss of quality and efficiency in research. In this survey report, the material is presented in a form that facilitates easy comprehension for the non-medical researchers and has been focused on introducing sepsis, it's causes, extent, comparison of diagnosis techniques: conventional labeled and label-free detections; with special emphasis on sepsis biomarkers to help researchers from multi-disciplinary domain to develop and fabricate devices and ideas to compliment the existing sepsis diagnosis system present in the medical field. A future direction of sepsis diagnosis along with the implementation of novel techniques for sepsis biomarker quantification is also reported.</p
A broadband bistable piezoelectric cantilever-based vibration energy harvester with nonlinear high power extraction
No full textThis work presents a nonlinear vibration energy harvester, which combines a nonlinear bistable broadband piezoelectric cantilever used to transduce ambient vibration energy, with synchronized capture for efficient harvesting over broadband sources. An accurate model of the bistable transducer, that augments the Butterworth van Dyke piezoelectric model to capture the external magnetic force added as a bias to the external vibrations, is presented. Its validity has been demonstrated through physical implementation and experimental validation against simulation of the mathematical model. For efficient extraction of the transduced energy, nonlinear extraction circuits, namely synchronous charge extraction (SCE) and parallel synchronized switch harvesting on inductor (SSHI), are employed. The switching in these circuits is implemented using a fully self-propelled, low-power electronic breaker circuit, capable of detecting extrema in the waveform to perform switching. Both simulated and experimental power outputs from the bistable harvester have been presented, with the SCE and parallel-SSHI providing average outputs with more than one-hundred (100) fold increase over the harvested power reported in literature for the same input, and further, even more significant gains are observed for broadband excitations.
For the above mentioned harvester, bistability is introduced through the use of two repelling magnets, one mounted on the cantilever tip and the other at a fixed location opposite it. Excitations that can overcome the repulsive magnetic force cause the cantilever to snap between its two equilibrium states, increasing amplitude and velocity of vibration, resulting in higher power outputs. This improved performance is observed whenever the cantilever operates in the bistable mode. Lower-amplitude excitations are unable to overcome the repulsive force, causing the cantilever to vibrate around one of its equilibrium states, and with smaller amplitudes in the presence of the opposing repulsion. To circumvent this issue, the second part of the work presents a completely mechanical way of increasing the range of excitation amplitudes over which the system remains bistable, by spring-loading the previously fixed-positioned magnet, and restricting its motion in the horizontal direction, towards and away from the cantilever. Then, whenever the cantilever moves towards the spring-loaded magnet, the latter is pushed away due to the repulsive force, increasing the distance between the magnets, thereby reducing the repulsive force required to be overcome for bistable operation. The opposite occurs when the cantilever moves away. Thus, the role of spring-loading is to introduce a type of negative feedback, through the self-adjustment of the distance between the magnets, favoring bistable operation over a larger range of excitations, and this is accomplished without an added energy cost. A 90% gain in power output levels over the fixed magnet system was observed.</p
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