Faculty of Engineering

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Open Access
3-D dynamic walking trajectory generation for a bipedal exoskeleton with underactuated legs: A proof of concept
(IEEE, 2019-06) Soliman, Ahmed Fahmy; Şendur, Polat; Uğurlu, Regaip Barkan; Mechanical Engineering; ŞENDUR, Polat; UĞURLU, Regaip Barkan; Soliman, Ahmed Fahmy
This paper presents a framework to address three dimensional (3-D) dynamic walking for a bipedal exoskeleton with underactuated legs. To achieve this goal, the framework is constructed via a trajectory generator and an optimized inverse kinematics algorithm that can cope with underactuation. In order to feasibly attain task velocities with underactuated legs, the inverse kinematics algorithm makes use of a task prioritization method via the exploitation of null space. In doing so, the tasks with lower priority, e.g., swing foot orientation, are attained as much as possible without disrupting the higher priority tasks, such as CoM trajectory. Meanwhile, the trajectory generator utilizes the ZMP concept analytically and ensures the acceleration continuity throughout the whole walking period, regardless of the contact and phase changes. The proposed method is verified via a lumped human-bipedal exoskeleton model that is developed and simulated in MSC.ADAMS simulation environment. As a result, we obtained feasible and dynamically balanced 3-D walking motion, in which no oblique foot landing or exaggerated torso orientation variations were observed, despite the underactuated nature of the robot legs.
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3B kamera takibi için eylemsizlik algılayıcılarının birleştirilmesi
(IEEE, 2012) Özer, N.; Erdem, Tanju; Ercan, Ali Özer; Eroğlu Erdem, Ç.; Electrical & Electronics Engineering; Computer Science; ERDEM, Arif Tanju; ERCAN, Ali Özer
It is well known in a Bayesian filtering framework, the use of inertial sensors such as accelerometers and gyroscopes improves 3D tracking performance compared to using camera measurements only. The performance improvement is more evident when the camera undergoes a high degree of motion. However, it is not well known whether the inertial sensors should be used as control inputs or as measurements. In this paper, we present the results of an extensive set of simulations comparing different combinations of using inertial sensors as control inputs or as measurements. We show that it is better use a gyroscope as a control input while an accelerometer can be used as a measurement or control input. We also derive and present the extended Kalman filter (EKF) equations for a specific case of fusing accelerometer and gyroscope data that has not been reported before.
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3D simulation of droplet impact on static and moving walls
(Begell House Inc., 2022) Yılmaz, Anıl; Kayansalçik, Gökhan; Ertunç, Özgür; Mechanical Engineering; ERTUNÇ, Özgür; Yılmaz, Anıl; Kayansalçik, Gökhan
In the present study, the contact angle model and the origin of the parasitic current, precisely, the relation of the parasitic current with grid distribution, have been studied to accurately predict droplet impact on static and moving walls in the volume of fluid (VOF) framework. The authors have quantitatively shown that the number of neighboring cells of the central cell influences the gradient calculations regarding the generation and spatial distribution of parasitic current. Accordingly, the polyhedral cell structure provides smoother interface gradient distribution than the Cartesian grid structure. After implementing a modified Kistler contact angle model in OpenFOAM and using the polyhedral grid for the simulations, we could accurately validate transient droplet shapes formed upon impact with those obtained from experiments. Droplet outcomes obtained, such as deposition, partial rebound, and split deposition on stationary and moving smooth surfaces, are consistent with experimental results.
Open Access
Accuracy limits of pair distribution function analysis in structural characterization of nanocrystalline powders by X-ray diffraction
(Turkish Chemical Society, 2022) Baloochiyan, Abolfazl; Batyrow, Merdan; Öztürk, Hande; Mechanical Engineering; KAYMAKSÜT, Hande Öztürk; Baloochiyan, Abolfazl; Batyrow, Merdan
We report the minimum errors of structural parameters, namely lattice parameter, crystallite size, and atomic displacement parameters, expected from Pair Distribution Function (PDF) analysis of nanocrystalline gold powders for the first time by a self-consistent computational methodology. Although PDF analysis has been increasingly used to characterize nanocrystalline powders by X-rays, the current literature includes no established error bounds to be expected from the resulting structural parameters. For accurate interpretation of X-ray diffraction data, these error bounds must be determined, and the obtained structural parameters must be cleared from them. Our novel methodology includes: 1) simulation of ideal powder diffraction experiments with the use of the Debye scattering equation, 2) pair distribution function analysis of the diffraction data with the Diffpy-CMI analysis software, and 3) determination of the errors from PDF analysis of the simulated diffraction data by comparing them with real-space analysis of spherical gold nanocrystals that are 30 nm size and smaller. Our results show that except for the lattice parameters and even with an ideal crystalline powder sample and ideal diffraction data, the extracted structural parameters from PDF analysis diverge from their true values for the studied nanopowder. These deviations are dependent on the average size of the nanocrystals and the energy of the X-rays selected for the diffraction experiments, where lower X-ray energies and small-sized nanocrystalline powders lead to greater errors.
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Acoustics and heat transfer characteristics of piezoelectric driven central orifice synthetic jet actuators
(Taylor & Francis, 2022-09-19) Ikhlaq, M.; Yasir, M.; Ghaffari, O.; Arık, Mehmet; Mechanical Engineering; ARIK, Mehmet
Growth in computational capacity combined with a decrease in the size of digital devices has led to increasing demand for more active and efficient cooling of electronics. In this study, an experimental investigation into two different sizes of central orifice Synthetic Jet Actuators (SJAs) is conducted to evaluate their heat transfer as well as noise generation characteristics. Two SJAs (40 mm and 20 mm) are examined, covering a distinct span of frequencies ranging from low to medium (<5500 Hz) in regards to the effect of SJA size over performance. The SJAs’ disk deflection, structural frequency, and jet exit velocity were measured to fully characterize the jet performance. The maximum Nusselt number for the largest SJA was 3 times more than the smallest SJA, where the evaluation of stroke length suggests no effective synthetic jet formation for the smallest SJA. The noise from the SJAs was measured in an anechoic chamber using three microphones, Fast Fourier Transform (FFT) of the sound pressure levels provide contributions to different tones in the resulting noise. 1/3 Octave Constant Percentage Bandwidth (CPB) analysis was performed to identify the frequency bands making the largest contribution to the noise. The largest SJA showed the highest heat transfer at acceptable noise levels during the operation below resonance frequency.
Open Access
Active-grid turbulence effect on the topology and the flame location of a lean premixed combustion
(Society of Thermal Engineers of Serbia, 2018) Alhumairi, Mohammed Khudhair Abbas; Ertunç, Özgür; Mechanical Engineering; ERTUNÇ, Özgür; Alhumairi, Mohammed Khudhair Abbas
Lean premixed combustion under the influence of active-grid turbulence was computationally investigated, and the results were compared with experimental data. The experiments were carried out to generate a premixed flame at a thermal load of 9 kW from a single jet flow combustor. Turbulent combustion models, such as the coherent flame model (CFM) and turbulent flame speed closure (TFC) model were implemented for the simulations performed under different turbulent flow conditions, which were specified by the Reynolds number based on Taylor’s microscale (Reλ), the dissipation rate of turbulence (ε) and turbulent kinetic energy (k). This study shows that the applied turbulent combustion models differently predict the flame topology and location. However, similar to the experiments, simulations with both models revealed that the flame moves toward the inlet when turbulence becomes strong at the inlet, that is, when Reλ at the inlet increases. The results indicated that the flame topology and location in the coherent flame model were more sensitive to turbulence than those in the turbulent flame speed closure model. The flame location behavior on the jet flow combustor significantly changed with the increase of Reλ.
Open Access
Actor-critic reinforcement learning for bidding in bilateral negotiation
(TÜBİTAK, 2022) Arslan, Furkan; Aydoğan, Reyhan; Computer Science; AYDOĞAN, Reyhan; Arslan, Furkan
Designing an effective and intelligent bidding strategy is one of the most compelling research challenges in automated negotiation, where software agents negotiate with each other to find a mutual agreement when there is a conflict of interests. Instead of designing a hand-crafted decision-making module, this work proposes a novel bidding strategy adopting an actor-critic reinforcement learning approach, which learns what to offer in a bilateral negotiation. An entropy reinforcement learning framework called Soft Actor-Critic (SAC) is applied to the bidding problem, and a self-play approach is employed to train the model. Our model learns to produce the target utility of the coming offer based on previous offer exchanges and remaining time. Furthermore, an imitation learning approach called behavior cloning is adopted to speed up the learning process. Also, a novel reward function is introduced that does take not only the agent’s own utility but also the opponent’s utility at the end of the negotiation. The developed agent is empirically evaluated. Thus, a large number of negotiation sessions are run against a variety of opponents selected in different domains varying in size and opposition. The agent’s performance is compared with its opponents and the performance of the baseline agents negotiating with the same opponents. The empirical results show that our agent successfully negotiates against challenging opponents in different negotiation scenarios without requiring any former information about the opponent or domain in advance. Furthermore, it achieves better results than the baseline agents regarding the received utility at the end of the successful negotiations.
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Adaptive DCO-OFDM for underwater visible light communications
(IEEE, 2019-04) Elamassie, Mohammed; Karbalayghareh, Mehdi; Miramirkhani, Farshad; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Elamassie, Mohammed; Karbalayghareh, Mehdi; Miramirkhani, Farshad
Visible light communication (VLC) has been introduced as a complementary technology to acoustic communications for underwater applications. Underwater VLC can achieve much higher data rates sufficiently high for real-time image and video transmission. Such high data rates over underwater channels with frequency-selectivity necessitate the use of efficient multi-carrier techniques such as orthogonal frequency division multiplexing. In this paper, we consider an adaptive DC-biased optical OFDM (DCO-OFDM) underwater VLC system. The design of adaptive algorithm is formulated to maximize the throughput under error rate performance constraints. The receiver first calculates the signal-to-noise ratio (SNR) per each subcarrier. Then, based on SNR information, it determines which subcarrier should be loaded first and selects the maximum constellation size for each subcarrier while satisfying a predefined targeted bit error rate (BER). Our simulation results demonstrated that significant improvements in throughput can be obtained through link adaptation.
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An adaptive modulation scheme for coded free-space optical systems
(IEEE, 2014) Hariq, S. H.; Odabasioglu, N.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat
For very slowly-varying fading channels as in the case of atmospheric turbulence-induced fading, it is possible to reliably estimate the channel state information and send it back to the transmitter. The transmitter can use this information to vary the transmission parameters such as power, modulation size, code rate etc according to the channel conditions. This is known as link adaptation. In this paper, we consider a turbo-coded free-space optical (FSO) system with M-ary pulse position modulation over log-normal turbulence channels and propose an adaptive modulation scheme under peak power constraints. Our simulation results demonstrate significant performance improvements of the proposed adaptive scheme over non-adaptive counterparts.
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Adaptive relay selection method for asynchronous amplify and forward cooperative communications
(IEEE, 2016) Celik, Y.; Odabasioglu, N.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat
In this paper, we propose a new adaptive relay selection method named as Adaptive Best Relay Assessment (ABRA) for cooperative communication with amplify and forward (AF) relaying. ABRA is based on two relay selection criteria which are cascaded channel gain coefficient and cascaded time offset in source-relay-destination (S-R-D) channel. Our simulation results demonstrate that the channel gain coefficient is the best criteria at low signal-to-noise ratio (SNR) regimes and time offset at high SNR regimes. Therefore, we combine these two criteria to select best relay for all SNR values. Finally, ABRA achieves the best bit error rate (BER) performance in asynchronous AF cooperative communications for three different path gain ratios 30 dB, 0 dB, and -30 dB.
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Adaptive shared control with human intention estimation for human agent collaboration
(IEEE, 2022) Amirshirzad, Negin; Uğur, E.; Bebek, Özkan; Öztop, Erhan; Computer Science; Mechanical Engineering; BEBEK, Özkan; ÖZTOP, Erhan; Amirshirzad, Negin
In this paper an adaptive shared control frame-work for human agent collaboration is introduced. In this framework the agent predicts the human intention with a confidence factor that also serves as the control blending parameter, that is used to combine the human and agent control commands to drive a robot or a manipulator. While performing a given task, the blending parameter is dynamically updated as the result of the interplay between human and agent control. In a scenario where additional trajectories need to be taught to the agent, either new human demonstrations can be generated and given to the learning system, or alternatively the aforementioned shared control system can be used to generate new demonstrations. The simulation study conducted in this study shows that the latter approach is more beneficial. The latter approach creates improved collaboration between the human and the agent, by decreasing the human effort and increasing the compatibility of the human and agent control commands.
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Adaptive unipolar MIMO-OFDM for visible light communications
(IEEE, 2019) Al-Nahhal, Mohamed; Basar, E.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Al-Nahhal, Mohamed
Unipolar orthogonal frequency division multiplexing (U-OFDM) appears as an attractive optical OFDM solution for emerging visible light communication (VLC) systems. This paper proposes spectral efficiency improvement for U-OFDM systems by applying adaptive transmission over realistic VLC links. This adaptive transmission includes switching among a number of multiple-input multiple-output (MIMO) modes combined with appropriate modulation size selection. The considered MIMO modes are repetition coding, spatial modulation, and spatial multiplexing, where each mode supports different modulation sizes. The selection of the corresponding MIMO mode and its modulation size is based on the received signal-to-noise ratio and target bit error rate. The proposed U-OFDM system is applied over different VLC MIMO setups with realistic channel models for 8 x 8, 4 x 4 and 2 x 2 MIMO systems. Our simulation results show that the proposed adaptive system provides a significant spectral efficiency improvement over stand-alone U-OFDM MIMO modes/setups.
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Aerosol attenuation model for high altitude UAV-based FSO links
(IEEE, 2022) Elamassie, Mohammed; Uysal, Murat; Electrical & Electronics Engineering; ELAMASSIE, Mohammed; UYSAL, Murat
Free space optical (FSO) communication is well positioned to address connectivity needs in ground-UAV, inter-UAV and UAV-ground links. An accurate performance analysis of airborne FSO links requires the use of proper path loss models. While earlier studies have successfully modeled attenuation arising from rain, drizzle, fog, and snow, aerosols have received less attention. Aerosols are tiny particles suspended in the atmosphere. They can be found drifting in Earth's atmosphere from the stratosphere to the troposphere to the Earth's surface. In this paper, we conduct extensive simulations in MODTRAN to determine the extinction coefficient values for aerosol over the wavelength from 350 nm to 1550 nm including typical wavelengths (e.g., 690 nm, 780nm, 850nm, and 1550 nm) used in commercial FSO systems. Non-linear curve fitting is then used to obtain new closed-form expressions for extinction coefficients.
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All-optical amplify-and-forward relaying system for atmospheric channels
(IEEE, 2012-10) Aminikashani, Mohammadreza; Rad, M. M.; Safari, M.; Uysal, Murat; Electrical & Electronics Engineering; UYSAL, Murat; Aminikashani, Mohammadreza
In this letter, we investigate the performance of a dual-hop free space optical link with an all-optical amplify-and-forward relay. We employ photon counting methodology and derive closed form expressions for the end-to-end signal-to-noise ratio and the outage probability. In our derivations, we consider either full or partial channel state information (CSI) at the relay and take into account practical limitations such as amplifier noise and filtering effects. Our results indicate significant performance improvements over direct transmission and furthermore demonstrate that semi-blind relaying (which depends only on statistical CSI) provides nearly identical performance to its full-CSI counterpart.
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Analysis of asynchronous cognitive radio system with imperfect sensing and bursty primary user traffic
(Springer Science+Business Media, 2016-03) Ercan, Ali Özer; Electrical & Electronics Engineering; ERCAN, Ali Özer
This paper presents a theoretical analysis of the spectrum utilization levels in a cognitive radio system. We assume that the traffic of the primary network is bursty and asynchronous with the secondary network, which performs imperfect spectrum sensing. Collisions of the primary and the secondary packets are assumed to result in increased packet error probabilities. We present primary and secondary utilization levels under optimized secondary transmission periods for varying primary traffic characteristics and secondary sensing performance levels. The results are also validated by extensive Monte Carlo simulations. We find that an asynchronous cognitive radio network with imperfect spectrum sensing is feasible when optimized transmission periods are used. The effects of primary traffic’s burst pattern and secondary sensing performance are discussed.
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Analysis of near-field radiation transfer within nano-gaps using FDTD method
(Elsevier, 2014-10) Didari, Azadeh; Mengüç, Mustafa Pınar; Mechanical Engineering; MENGÜÇ, Mustafa Pınar; Didari, Azadeh
Enhancement of near-field radiative emission via coupling of surface plasmons in nano-gaps formed between thin films is important for understanding and implementation of energy harvesting using nano-thermophotovoltaic cells. Design and construction of such cells need to be carried out along with detailed modeling studies, necessitating accurate calculation of near-field emission within thin films. The objective of this paper is to provide a methodology based on finite difference time domain analysis for the calculation of the near-field thermal radiation emission based on local density of electromagnetic states. Near-field thermal emission is investigated within the nano-gap formed between thin silicon carbide layers where both support surface phonon polaritons. Modeling of this problem with the FDTD method is not trivial particularly for establishing the Drude–Lorentz permittivity model and the selection of the right boundary conditions. We present an effective boundary condition, for calculation of Local Density Of electromagnetic States (LDOS) via Finite Difference Time Domain Method (FDTD) for applications to nano-scale geometries. We conclude that Convolutional Perfectly Matched Layer (CPML) is the optimum boundary condition that gives the most accurate results compared against the other methodologies for parallel plates separated by nano-gaps. This boundary condition allows more streamlined simulations to be carried out when working with sub-wavelength structures. The challenges and the possible solutions to overcome these difficulties are discussed in detail.
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Analysis of perlite and pumice based building insulation materials
(Elsevier, 2016-06) Çelik, S.; Family, Roxana; Mengüç, Mustafa Pınar; Mechanical Engineering; MENGÜÇ, Mustafa Pınar; Family, Roxana
Insulation panels composed of perlite, pumice, and cement were fabricated. Perlite and pumice were used to achieve high thermal resistance, and low density. Experimentation was conducted for determining R-values of perlite-pumice-cement (PPC) panels at different moisture content values. Dry and moist sample panels were tested with and without soil and moss at the top where the one with the soil and plant coupling represents a green roof. Reverse heat leak method was used in determination of the R-values of the samples. Verification of theory was achieved via experimentation with an acceptable error range. Thermal diffusivity values of the developed samples were measured using flash method.
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Analysis of vibratory gyroscopes: drive and sense mode resonance shift by coriolis force
(IEEE, 2017) Cetin, Hakan; Yaralıoğlu, Göksen Göksenin; Electrical & Electronics Engineering; YARALIOĞLU, Göksen Göksenin; Cetin, Hakan
In this paper, we demonstrate the analysis of coupling between drive and sense systems of vibratory gyroscopes. Vibratory gyroscopes have attracted a lot of interest recently with the development of MEMS gyroscopes. These gyroscopes made their way through portable devices and smart phones. Novel gyroscope architectures have been proposed and analyzed in detail. However, in most of these analyses, coupling between the sense and drive systems was ignored. We analytically show that drive and sense systems are coupled together via Coriolis and centrifugal force. As a result, system resonances shift as the rotation rate increase for linear and torsional gyroscope systems. Starting from a simple gyro system, we calculated the sense and drive resonant frequency shifts in various configurations. Then, for more complex systems where analytical solution is difficult to obtain, we used commercially available FEM tools to determine corresponding frequency shift. In general, we found that the shift is small and can be ignored for linear vibratory gyroscopes where Q of the sense system is less than 2500 for mode matched gyros. But for higher Q systems, the frequency shift may affect the linearity of these gyroscopes. This sets a fundamental limit for the linearity of vibratory gyroscopes. Based on our calculations the non-linearity is above 1% for linear 2-DOF mode-matched vibratory gyroscopes where Q is above 3000 and for torsional 2-DOF vibratory gyroscopes where Q is above 600. Multi-DOF and ring vibratory gyroscopes are also examined. We find that the effect is less pronounced for Multi-DOF gyros, whereas there is a large effect on the linearity of ring gyroscopes.
Open Access
Analysis of X(4140) like states and their radial excitations in QCD
(Sissa Medialab Srl, 2017) Türkan, Arzu; Dağ, Hüseyin; Natural and Mathematical Sciences; TÜRKAN, Arzu; DAĞ, Hüseyin
In this work, we investigated the X(4140) and like states and their radial excitations by using molecular and diquark-antidiquark currents which couple to scalar, axial vector and tensor states via QCD sum rules. In operator product expansion, we considered quark, gluon and mixed vacuum condansates up to dimension eight. For the ground states coupling to these currents, we found that masses are almost degenerate with X(4140). For the excited states, we found that scalar and tensor currents are coupling to D∗ sD∗ s threshold. However for the axial vector currents, the mass of the first excited state is compatible with X(4274). Thus we conclude that, X(4274) might be the first radial excitation of X(4140).
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Angular analysis and branching fraction measurement of the decay B-0 -> K*(0)mu(+)mu(-)
(Elsevier, 2013-11-25) Chatrchyan, S.; Işıldak, Bora; The CMS Collaboration; Natural and Mathematical Sciences; IŞILDAK, Bora
The angular distributions and the differential branching fraction of the decay B0→K⁎(892)0μ+μ− are studied using a data sample corresponding to an integrated luminosity of 5.2 fb−1 collected with the CMS detector at the LHC in pp collisions at View the MathML source. From more than 400 signal decays, the forward–backward asymmetry of the muons, the K⁎(892)0 longitudinal polarization fraction, and the differential branching fraction are determined as a function of the square of the dimuon invariant mass. The measurements are in good agreement with standard model predictions.

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