In view of an entire dynamic model of tilting-pad journal bearing(TPJB) in which the pads swing and vibrate along geometric direction of preload, a TPJB of elastic and damped pivots was designed and manufactured. Vibr...In view of an entire dynamic model of tilting-pad journal bearing(TPJB) in which the pads swing and vibrate along geometric direction of preload, a TPJB of elastic and damped pivots was designed and manufactured. Vibration experiments were carried out under the conditions of different rotor bending stiffness and oil supply pressure to find out the relationship between the new bearing's vibration depression effect and other dynamic parameters of the rotor. The result shows that critical amplitudes can be efficaciously reduced while system's stability can be remarkably improved by this bearing. Besides, the bearing's effect of vibration depression weakens as the rotor bending stiffness increases, but heightens it as the oil supply pressure increases.展开更多
Local susceptibility variations result in <em>B</em><sub>0</sub> field inhomogeneities, causing distortions and signal losses in MR imaging. Susceptibility variations become stronger with incre...Local susceptibility variations result in <em>B</em><sub>0</sub> field inhomogeneities, causing distortions and signal losses in MR imaging. Susceptibility variations become stronger with increasing <em>B</em><sub>0</sub> magnetic field strength. Active shimming is used to generate corrective magnetic fields, which can be used to improve <em>B</em><sub>0</sub> field homogeneity. FASTMAP is an effective shimming technique for computing optimal coil currents, which uses data from six projection directions (or columns): this technique is routinely used for shimming cubic volumes of interest (VOIs). In this paper, we propose several improvements to FASTMAP at 4T. For each shim coil, using a modified 3D gradient-echo pulse sequence, we compute <em>B</em><sub>0</sub> inhomogeneity maps and project them onto eight 1<sup>st</sup> and 2<sup>nd</sup> order spherical harmonic functions. This process is repeated for shim currents between -15,000 to 15,000 with increments of 5000 Digital to Analog Converter (DAC) units, and is used to compute the gradient between spherical harmonic coefficients and DAC values for all 8 shim coils—along with the R<sup>2</sup> values of linear fits. A method is proposed (based on R<sup>2</sup> values) to further refine optimal shim currents in respective coils. We present an analysis that is numerically robust and completely flexible in the selection of the VOIs for shimming. Performance analyses, phantom results, and <em>in vivo</em> results of a human brain are presented, comparing our methods with the FASTMAP method.展开更多
基金Project(2012CB026000)supported by the National Basic Research Program of China(973 Program)
文摘In view of an entire dynamic model of tilting-pad journal bearing(TPJB) in which the pads swing and vibrate along geometric direction of preload, a TPJB of elastic and damped pivots was designed and manufactured. Vibration experiments were carried out under the conditions of different rotor bending stiffness and oil supply pressure to find out the relationship between the new bearing's vibration depression effect and other dynamic parameters of the rotor. The result shows that critical amplitudes can be efficaciously reduced while system's stability can be remarkably improved by this bearing. Besides, the bearing's effect of vibration depression weakens as the rotor bending stiffness increases, but heightens it as the oil supply pressure increases.
文摘Local susceptibility variations result in <em>B</em><sub>0</sub> field inhomogeneities, causing distortions and signal losses in MR imaging. Susceptibility variations become stronger with increasing <em>B</em><sub>0</sub> magnetic field strength. Active shimming is used to generate corrective magnetic fields, which can be used to improve <em>B</em><sub>0</sub> field homogeneity. FASTMAP is an effective shimming technique for computing optimal coil currents, which uses data from six projection directions (or columns): this technique is routinely used for shimming cubic volumes of interest (VOIs). In this paper, we propose several improvements to FASTMAP at 4T. For each shim coil, using a modified 3D gradient-echo pulse sequence, we compute <em>B</em><sub>0</sub> inhomogeneity maps and project them onto eight 1<sup>st</sup> and 2<sup>nd</sup> order spherical harmonic functions. This process is repeated for shim currents between -15,000 to 15,000 with increments of 5000 Digital to Analog Converter (DAC) units, and is used to compute the gradient between spherical harmonic coefficients and DAC values for all 8 shim coils—along with the R<sup>2</sup> values of linear fits. A method is proposed (based on R<sup>2</sup> values) to further refine optimal shim currents in respective coils. We present an analysis that is numerically robust and completely flexible in the selection of the VOIs for shimming. Performance analyses, phantom results, and <em>in vivo</em> results of a human brain are presented, comparing our methods with the FASTMAP method.
