• Title/Summary/Keyword: Stationary Grid

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Stationary and Moving Computed Radiography Grids : Comparative Observer's Perception (Computed Radiography에서 고정형 그리드와 이동형 그리드 영상의 인식률 비교)

  • Lee, Kiho;Lee, Changhoon;Jin, Gyehwan
    • Journal of the Korean Society of Radiology
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    • v.9 no.7
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    • pp.515-521
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    • 2015
  • This study assessed the degradation of image quality caused by grid artifacts and $moir{\acute{e}}$ pattern artifacts in a stationary grid, and the degradation of image quality caused by cut off artifacts in a moving grid. X-ray images were acquired in a stationary grid and a moving grid with X-ray exposure conditions of 100 cm, 80 kVp, and 30 mA using a CDRAD phantom and a 24 cm thickness acrylic phantom. Observer's perception of X-ray imaging using CDRAD Analyzer was mean 49.36, standard deviation 3.76, maximum 55.56, and minimum 38.67 in the stationary grid, and 47.04, 12.69, 55.56, and 20.89, respectively, in the moving grid. The stationary grid was superior to the moving grid in terms of the mean and standard deviation of observer's perception.

Design of stationary reference frame current and disturbance rejection control algorithms for a grid connected inverter (계통 연계형 인버터의 정지좌표전류제어 및 외란제거 제어알고리즘 설계)

  • Kim, Seonghyeon;Lee, Jae Suk
    • Journal of IKEEE
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    • v.24 no.1
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    • pp.154-160
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    • 2020
  • This paper presents a grid current control algorithm for a grid connected inverter (GCI) system in a stationary reference frame. When a Proportional Integral (PI) controller at a stationary reference frame is used in a GCI system, steady state error and phase lags are presented because AC signals are controlled at a stationary reference frame. In this paper, a feedforward controller is applied to the PI controller to compensate the steady state error and phase lags by improving command tracking performance. In addition, disturbance rejection control is applied to the PI controller to protect the GCI system by eliminating disturbance, grid voltage in a GCI system, when a grid fault such as line-to-line fault, happens. The proposed GCI current control algorithm is analyzed in a frequency domain and a simulation model of the proposed GCI current control system is developed for verification of the performance.

Stationary Frame Current Control Evaluations for Three-Phase Grid-Connected Inverters with PVR-based Active Damped LCL Filters

  • Han, Yang;Shen, Pan;Guerrero, Josep M.
    • Journal of Power Electronics
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    • v.16 no.1
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    • pp.297-309
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    • 2016
  • Grid-connected inverters (GCIs) with an LCL output filter have the ability of attenuating high-frequency (HF) switching ripples. However, by using only grid-current control, the system is prone to resonances if it is not properly damped, and the current distortion is amplified significantly under highly distorted grid conditions. This paper proposes a synchronous reference frame equivalent proportional-integral (SRF-EPI) controller in the αβ stationary frame using the parallel virtual resistance-based active damping (PVR-AD) strategy for grid-interfaced distributed generation (DG) systems to suppress LCL resonance. Although both a proportional-resonant (PR) controller in the αβ stationary frame and a PI controller in the dq synchronous frame achieve zero steady-state error, the amplitude- and phase-frequency characteristics differ greatly from each other except for the reference tracking at the fundamental frequency. Therefore, an accurate SRF-EPI controller in the αβ stationary frame is established to achieve precise tracking accuracy. Moreover, the robustness, the harmonic rejection capability, and the influence of the control delay are investigated by the Nyquist stability criterion when the PVR-based AD method is adopted. Furthermore, grid voltage feed-forward and multiple PR controllers are integrated into the current loop to mitigate the current distortion introduced by the grid background distortion. In addition, the parameters design guidelines are presented to show the effectiveness of the proposed strategy. Finally, simulation and experimental results are provided to validate the feasibility of the proposed control approach.

A Diamond Web-grid Search Algorithm Combined with Efficient Stationary Block Skip Method for H.264/AVC Motion Estimation (H.264/AVC 움직임 추정을 위한 효율적인 정적 블록 스킵 방법과 결합된 다이아몬드 웹 격자 탐색 알고리즘)

  • Jeong, Chang-Uk;Choi, Jin-Ku;Ikenaga, Takeshi;Goto, Satoshi
    • Journal of Internet Computing and Services
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    • v.11 no.2
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    • pp.49-60
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    • 2010
  • H.264/AVC offers a better encoding efficiency than conventional video standards by adopting many new encoding techniques. However, the advanced coding techniques also add to the overall complexity for H.264/AVC encoder. Accordingly, it is necessary to perform optimization to alleviate the level of complexity for the video encoder. The amount of computation for motion estimation is of particular importance. In this paper, we propose a diamond web-grid search algorithm combined with efficient stationary block skip method which employs full diamond and dodecagon search patterns, and the variable thresholds are used for performing an effective skip of stationary blocks. The experimental results indicate that the proposed technique reduces the computations of the unsymmetrical-cross multi-hexagon-grid search algorithm by up to 12% while maintaining a similar PSNR performance.

CFD PARAMETRIC STUDY FOR 2D WATER ENTRY

  • Lee, H.H.;Rhee, S.H.
    • 한국전산유체공학회:학술대회논문집
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    • 2009.04a
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    • pp.191-195
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    • 2009
  • A parametric study for for the water entry of a two dimensional symmetric wedge with deadrise angle of 10 degrees was carried out to find out the most dominant parameter. Water entry problem with constant velocity is simplified as the stationary wedge in the way of the upcoming water surface. The calculated impact loads showed that the effect of the viscosity was not so important in this problem. For a given grid system a suitable time step size can be found. The most sensitive parameter was found to be the grid size.

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Performance Improvement of Grid-Connected Inverter Systems under Unbalanced and Distorted Grid Voltage by Using a PR Controller

  • Lee, Jong-Hyun;Jeong, Hae-Gwang;Lee, Kyo-Beum
    • Journal of Electrical Engineering and Technology
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    • v.7 no.6
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    • pp.918-925
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    • 2012
  • This paper proposes a control method for grid-connected inverter systems under unbalanced and distorted grid voltage. The proposed method can reduce the power ripple caused by the unbalanced condition and compensate for the low-order harmonics of the output currents caused by the distortion of grid voltage. To reduce the power ripple, our method replaces the two conventional PI controllers with one PR controllers in the stationary frame. PR controllers can implement selective harmonic compensation without excessive computational requirements; the use of these controllers simplifies the method. Both the simulated and experimental results agree well with the theoretical analysis.

A grid-line suppression technique based on the nonsubsampled contourlet transform in digital radiography

  • Namwoo Kim;Taeyoung Um;Hyun Tae Leem;Bon Tack Koo;Kyuseok Kim;Kyu Bom Kim
    • Nuclear Engineering and Technology
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    • v.55 no.2
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    • pp.655-668
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    • 2023
  • In radiography, an antiscatter grid is a well-known device for eliminating unexpected x-ray scatter. We investigate a new stationary grid artifact suppression method based on a nonsubsampled contourlet transform (NSCT) incorporated with Gaussian band-pass filtering. The proposed method has an advantage that extracts the Moiré components while minimizing the loss of image information and apply the prior information of Moiré component positions in multi-decomposition sub-band images. We implemented the proposed algorithm and performed a simulation and an experiment to demonstrate its viability. We did this experiment using an x-ray tube (M-113T, Varian, focal spot size: 0.1 mm), a flat-panel detector (ROSE-M Sensor, Aspenstate, pixel dimension: 3032 × 3800 pixels, pixel size: 0.076 mm), and carbon graphite-interspaced grids (JPI Healthcare, 18 cm × 24 cm, line density: 103 LP/inch and 150 LP/inch, ratio: 5:1, focal distance: 65 cm). Our results indicate that the proposed method successfully suppressed grid artifacts by reducing them without either reducing the spatial resolution or causing negative side effects. Consequently, we anticipate that the proposed method can improve image acquisition in a stationary grid x-ray system as well as in extended x-ray imaging.

Numerical Study on Transient Aerodynamics of Moving Flap Using Conservative Chimera Grid Method (보존적 중첩격자기법을 이용한 동적 플랩의 천이적 공력거동에 관한 수치적 연구)

  • Choi S. W.;Chang K. S.;Kim I. S.
    • Journal of computational fluids engineering
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    • v.5 no.2
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    • pp.9-19
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    • 2000
  • Transient aerodynamic response of an airfoil to a moving plane-flap is numerically investigated using the two-dimensional Euler equations with conservative Chimera grid method. A body moving relative to a stationary grid is treated by an overset grid bounded by a 'Dynamic Domain Dividing Line' which has an advantage for constructing a well-defined hole-cutting boundary. A conservative Chimera grid method with the dynamic domain-dividing line technique is applied and validated by solving the flowfield around a circular cylinder moving supersonic speed. The unsteady and transient characteristics of the flow solver are also examined by computations of an oscillating airfoil and a ramp pitching airfoil respectively. The transient aerodynamic behavior of an airfoil with a moving plane-flap is analyzed for various flow conditions such as deflecting rate of flap and free stream Mach number.

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Numerical Study on Transient Aerodynamics of Moving Flap Using Conservative Chimera Grid Method (보존적 중첩격자기법을 이용한 동적 플랩의 천이적 공력거동에 관한 수치적 연구)

  • Choi S. W.;Chang K. S.
    • 한국전산유체공학회:학술대회논문집
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    • 1999.11a
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    • pp.85-94
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    • 1999
  • Transient aerodynamic response of an airfoil to a moving plane-flap is numerically investigated using two-dimensional Euler equations with conservative Chimera grid method. A body moving relative to a stationary grid is treated by an overset grid bounded by a 'dynamic domain-dividing line' the concept of which is developed in this study. A conservative Chimera grid method with a dynamic domain-dividing line technique is applied and validated by solving the flowfield around circular cylinder moving supersonic speed. The unsteady and transient characteristics of the flow solver is also examined by computations of a oscillating airfoil and a ramp pitching airfoil respectively. The transient aerodynamic behavior of an airfoil with a moving plane-flap is analyzed for various flow conditions such as deflecting rate of flap and free stream Mach number.

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Artifact Reduction in Digital Radiography Images with the Stationary Grid Based on 1-Dimensional Filters (고정 그리드를 사용한 디지털 방사선 영상에서 1차원 필터에 기초한 왜곡의 제거)

  • Kim, Dong-Sik
    • Journal of the Institute of Electronics Engineers of Korea SP
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    • v.47 no.5
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    • pp.117-126
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    • 2010
  • In order to obtain more clear x-ray images, an antiscatter grid, which can absorb the scattered rays, is employed. In the high-resolution direct digital radiography, however, the artifacts due to the grid are visible. In this paper, the grid artifacts are reduced by applying simple 1-dimensional low-pass filters in the spatial domain based on the rotated grid. Since the proposed algorithm does not use any detection scheme for the artifact frequencies and discrete Fourier transforms for 2-dimensional filters, it can simply and fast reduce the grid artifacts. The performance using the order 1 average filter is compatible to that of using 2-dimensional filters in the frequency domain.