Browsing by Subject "Finite element analysis"

Now showing 1 - 12 of 12

Open Access
Algebraic reconstraction for 3D magnetic resonance-electrical impedance tomography (MREIT) using one component of magnetic flux density
(Institute of Physics and Engineering in Medicine, 2004) Ider, Y. Z.; Onart, S.
Magnetic resonance-electrical impedance tomography (MREIT) algorithms fall into two categories: those utilizing internal current density and those utilizing only one component of measured magnetic flux density. The latter group of algorithms have the advantage that the object does not have to be rotated in the magnetic resonance imaging (MRI) system. A new algorithm which uses only one component of measured magnetic flux density is developed. In this method, the imaging problem is formulated as the solution of a non-linear matrix equation which is solved iteratively to reconstruct resistivity. Numerical simulations are performed to test the algorithm both for noise-free and noisy cases. The uniqueness of the solution is monitored by looking at the singular value behavior of the matrix and it is shown that at least two current injection profiles are necessary. The method is also modified to handle region-of-interest reconstructions. In particular it is shown that, if the image of a certain xy-slice is sought for, then it suffices to measure the z-component of magnetic flux density up to a distance above and below that slice. The method is robust and has good convergence behavior for the simulation phantoms used.
Open Access
Design and development of X-band GaN-based low-noise amplifiers
(2022-12) Zafar, Salahuddin
Gallium nitride (GaN) high electron mobility transistor (HEMT) technology emerged as a preferable candidate for high-power applications. GaN HEMTs on silicon carbide (SiC) substrate provide the best combination of speed and power due to high power density, escalated saturated carrier velocity, high efficiency, enhanced electrical breakdown, and superior thermal conductivity. Over the years, GaN technology also started to take its place in low-noise applications due to built-in power handling capability at the receive end of transceivers for compact designs and high linearity. For GaN-based low-noise amplifiers (LNAs), improving the noise figure (NF) and getting it close to other competitive technologies is always challenging. More-over, further improvement in the robustness of GaN-based LNAs in terms of survivability and reverse recovery time (RRT) is needed. For this purpose, NAN-OTAM’s 0.15 µm GaN on SiC HEMT process is used to realize LNAs, one with survivability as high as 42 dBm and the other having NF as low as 1.2 dB. Survivability is investigated in terms of gain compression and forward gate current, while RRT is explored in detail with respect to the RC time constant of transistor and trap phenomenon. In the LNA design, the significance of inductive source degenerated HEMT, and the role of stability networks towards NF improvement are discussed in detail. Furthermore, thermal simulations and infrared (IR) thermographic measurements of the LNA monolithic microwave integrated circuit are correlated to unveil the maximum channel temperature buried inside the two-dimensional electron gas of HEMT.
Open Access
Erratum to "Equivalent circuit-based analysis of CMUT cell dynamics in arrays" [May 13 1016-1024]
(IEEE, 2013-06-03) Oguz, H. K.; Atalar, Abdullah; Köymen, Hayrettin
In the original publication of the paper [1], the funding source was inadvertently omitted from the footnote on page 1016. The footnote should have read "Manuscript received November 20, 2012; accepted February 14, 2013. This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under project grant 110E216. A. Atalar acknowledges the support of the Turkish Academy of Sciences (TUBA)." The authors regret this omission.
Open Access
Experimental results for 2D magnetic resonance electrical impedance tomography (MR-EIT) using magnetic flux density in one direction
(Institute of Physics Publishing, 2003) Birgül, Ö.; Eyüboğlu, B. M.; İder, Y. Z.
Magnetic resonance electrical impedance tomography (MR-EIT) is an emerging imaging technique that reconstructs conductivity images using magnetic flux density measurements acquired employing MRI together with conventional EIT measurements. In this study, experimental MR-EIT images from phantoms with conducting and insulator objects are presented. The technique is implemented using the 0.15 T Middle East Technical University MRI system. The dc current method used in magnetic resonance current density imaging is adopted. A reconstruction algorithm based on the sensitivity matrix relation between conductivity and only one component of magnetic flux distribution is used. Therefore, the requirement for object rotation is eliminated. Once the relative conductivity distribution is found, it is scaled using the peripheral voltage measurements to obtain the absolute conductivity distribution. Images of several insulator and conductor objects in saline filled phantoms are reconstructed. The L2 norm of relative error in conductivity values is found to be 13%, 17% and 14% for three different conductivity distributions.
Open Access
Finite element analysis of the deformation behaviour of pure aluminium in repetitive corrugation-straightening and constrained groove pressing
(Inderscience Enterprises, 2018) Rahimi, F.; Sadeghi, B. M.; Ahmadi, M.
Repetitive corrugation-straightening (RCS) and constrained groove pressing (CGP) are two promising severe plastic deformation techniques for sheet metallic materials. There are differences in amount of imposed plastic strain, hence, it is important to compare the deformation mode in these two techniques. In the present paper, the effect of RCS and CGP on the deformation behaviour of pure aluminium is studied. The effective parameters are analysed using finite element analysis (FEA) for one pass of deformation. The computer simulations are carried using DEFORM-3D and to verify the FEA results, a comparison with previous literatures is made. It is observed that filling ratio of the die is higher for CGP. Additionally, the deformed work-piece endured more plastic strain in CGP and the effective strain is more homogenously distributed in this technique.
Open Access
Identification of internal process parameters of micro milling considering machined surface topography
(2022-07) Masrani, Abdulrzak
Micro-milling is a fast and versatile machining method that can be used to manufacture three-dimensional parts of a wide range of materials with high accuracy. Modeling of micro-milling processes is complex due to size effects, where the chip thickness becomes comparable to the cutting edge radius. Furthermore, tool runout and deflection effects on the process outputs are amplified and cannot be neglected. As the process is scaled down where micrometer accuracy is required; modeling and identifying the process parameters becomes essential to optimize or monitor the process. This study presents a systematic approach to force modeling and parameter identification of micro-milling processes. Finite element analysis of tool deflection is integrated into mechanistic modeling of micro-milling forces together with considering the trochoidal trajectory of the cutting teeth, tool runout, and chip thickness accumulation due to minimum uncut chip thickness. The internal process parameters are identified using the experimental cutting forces and machined surface topography with a novel methodology. The research results are experimentally validated by slot and side micro-milling tests on commercially pure titanium, using coated carbide micro-end-mills with diameters of 0.2 and 0.4 mm, and accurate predictions of model parameters and cutting forces are obtained. The proposed force models can be used in smart manufacturing and digital twin applications to reduce the time and costs associated with process optimization. The proposed parameter identification techniques can also help to reduce the need for advanced measurement systems.
Open Access
Influence of crown margin design on the stress distribution in maxillary canine restored by all-ceramic crown: a finite element analysis
(Korean Academy of Dental Sciences, 2015) Ozer, Z.; Kurtoglu, C.; Mamedov, A. M.; Özbay, Ekmel
Purpose: To investigate the infl uence of crown margin design on the stress distribution and to localize critical sites in maxillary canine under functional loading by using three dimensional fi nite element analysis. Materials and Methods: The bite force of 100 N, 150 N, and 200 N was applied with an angulation of 45° to the longitudinal axis of tooth. Six models were restored with IPS e.max (Ivoclar Vivadent, Schaan, Liechtenstein) with a different margin design. With lingual ledge and various thicknesses, three different core ceramics were designed in each model. Result: In the core ceramic, the maximum tensile stresses were at the labiocervical region. In the veneering ceramic the maximum tensile stresses were at the area where the force was applied in all models. Conclusion: Shoulder and chamfer margin types are acceptable for all-ceramic rehabilitations. A ledge on the core ceramic at cervical region may affect the strength of all-ceramic crowns.
Open Access
Magnetic resonance electrical impedance tomography (MREIT) based on the solution of the convection equation using FEM with stabilization
(Institute of Physics Publishing, 2012-07-27) Oran, O. F.; Ider, Y. Z.
Most algorithms for magnetic resonance electrical impedance tomography (MREIT) concentrate on reconstructing the internal conductivity distribution of a conductive object from the Laplacian of only one component of the magnetic flux density (∇ 2B z) generated by the internal current distribution. In this study, a new algorithm is proposed to solve this ∇ 2B z-based MREIT problem which is mathematically formulated as the steady-state scalar pure convection equation. Numerical methods developed for the solution of the more general convectiondiffusion equation are utilized. It is known that the solution of the pure convection equation is numerically unstable if sharp variations of the field variable (in this case conductivity) exist or if there are inconsistent boundary conditions. Various stabilization techniques, based on introducing artificial diffusion, are developed to handle such cases and in this study the streamline upwind Petrov-Galerkin (SUPG) stabilization method is incorporated into the Galerkin weighted residual finite element method (FEM) to numerically solve the MREIT problem. The proposed algorithm is tested with simulated and also experimental data from phantoms. Successful conductivity reconstructions are obtained by solving the related convection equation using the Galerkin weighted residual FEM when there are no sharp variations in the actual conductivity distribution. However, when there is noise in the magnetic flux density data or when there are sharp variations in conductivity, it is found that SUPG stabilization is beneficial.
Open Access
Nonlinear modeling of an immersed transmitting capacitive micromachined ultrasonic transducer for harmonic balance analysis
(IEEE, 2010) Oguz, H. K.; Olcum, S.; Senlik, M. N.; Taş, V.; Atalar, Abdullah; Köymen, Hayrettin
Finite element method (FEM) is used for transient dynamic analysis of capacitive micromachined ultrasonic transducers (CMUT) and is particularly useful when the membranes are driven in the nonlinear regime. One major disadvantage of FEM is the excessive time required for simulation. Harmonic balance (HB) analysis, on the other hand, provides an accurate estimate of the steady-state response of nonlinear circuits very quickly. It is common to use Mason's equivalent circuit to model the mechanical section of CMUT. However, it is not appropriate to terminate Mason's mechanical LC section by a rigid piston's radiation impedance, especially for an immersed CMUT. We studied the membrane behavior using a transient FEM analysis and found out that for a wide range of harmonics around the series resonance, the membrane displacement can be modeled as a clamped radiator. We considered the root mean square of the velocity distribution on the membrane surface as the circuit variable rather than the average velocity. With this definition, the kinetic energy of the membrane mass is the same as that in the model. We derived the force and current equations for a clamped radiator and implemented them using a commercial HB simulator. We observed much better agreement between FEM and the proposed equivalent model, compared with the conventional model.
Open Access
Optimization of the gain-bandwidth product of capacitive micromachined ultrasonic transducers
(IEEE, 2005-12) Olcum, S.; Senlik, M. N.; Atalar, Abdullah
Capacitive micromachined ultrasonic transducers (cMUT) have large bandwidths, but they typically have low conversion efficiencies. This paper defines a performance measure in the form of a gain-bandwidth product and investigates the conditions in which this performance measure is maximized. A Mason model corrected with finite-element simulations is used for the purpose of optimizing parameters. There are different performance measures for transducers operating in transmit, receive, or pulse-echo modes. Basic parameters of the transducer are optimized for those operating modes. Optimized values for a cMUT with silicon nitride membrane and immersed in water are given. The effect of including an electrical matching network is considered. In particular, the effect of a shunt inductor in the gain-bandwidth product is investigated. Design tools are introduced, which are used to determine optimal dimensions of cMUTs with the specified frequency or gain response.
Open Access
Reducing anchor loss in micromechanical extensional Mode resonators
(IEEE, 2010-02) Tas, V.; Olcum, S.; Aksoy, M. D.; Atalar, Abdullah
In this work, we propose a novel method to increase the quality factor of extensional mode micromechanical resonators. The proposed resonator topology is suitable for integration in a silicon-based process to fabricate micromechanical filters and oscillators. It is a half-wavelength-long strip excited longitudinally by electrostatic forces, and it is isolated from the substrate by alternating with bars of a quarter wavelength long. This structure causes a large impedance mismatch between the resonator and the substrate and hence reduces the anchor loss considerably. The performance of the resonator is determined by finite element simulations. We introduce an equivalent electrical circuit to predict the performance of the resonator. The electrical model gives results consistent with the finite element simulations. The proposed resonator is expected to have a very small anchor loss resulting in a very high Q.
Open Access
Unveiling Tmax inside GaN HEMT based X-band low-noise amplifier by correlating thermal simulations and IR thermographic measurements
(IEEE, 2022-12-20) Zafar, Salahuddin; Durna, Yılmaz; Koçer, Hasan; Akoğlu, Büşra Çankaya; Aras, Yunus Erdem; Odabaşı, Oğuz; Bütün, Bayram; Özbay, Ekmel
This paper presents a method to reveal the channel temperature profile of high electron mobility transistors (HEMTs) in a multi-stage monolithic microwave integrated circuit (MMIC). The device used for this study is a two-stage X-band low-noise amplifier fabricated using 0.15 m GaN-on-SiC technology with 4x50 m and 4x75 m HEMTs at the first and the second stage, respectively. The surface temperature measured through infrared (IR) thermography has a diffraction-limited resolution. Moreover, it is impossible to measure sub-surface Tmax residing inside the two-dimensional electron gas of HEMT using IR thermographic measurements. Finite element analysis (FEA) thermal simulations are performed in this study to acquire the surface and sub-surface temperature profiles of the whole MMIC. IR measurements and FEA simulations are integrated through a correlation-based method verifying the accuracy of the FEA-based temperature profiles. This method leads to accurately finding the hotspots in the MMIC, thus revealing the Tmax of both stages. The correlation method using two filters approach to match the measurements and simulated temperature profiles of all the stages finds its application in MMICs’ high-temperature operating lifetime reliability tests.