In the fields of earth observation,deep space detection,laser communication,and directional energyweapon,the target needs to be observed and pointed at accurately.Acquisition,tracking,and pointing(ATP)systems are usua...In the fields of earth observation,deep space detection,laser communication,and directional energyweapon,the target needs to be observed and pointed at accurately.Acquisition,tracking,and pointing(ATP)systems are usually designed to stabilize the line of sight(LOS)within sub-micro radian levels.In the case of an ATP system mounted on a mobile platform,angular disturbances experienced by the mobile platform will seriously affect the LOS.To overcome the problemthat the sampling frequency of detectors is usually limited and achieving several hundreds of hertz is difficult,thewide-bandwidth inertial reference system(WBIRS)and fast steeringmirror are usually integrated into ATP systems to mitigate these angular disturbances.To reduce the structural stress,a flexible support providing two rotational degrees of freedomis usually adopted for the system.However,the occurrence of resonant points within the bandwidthwill be inevitable.Measurements have to be taken to compensate these low-frequency resonant points to realize a wide bandwidth and high precision.In this paper,the lowfrequency resonant points of a systemwere simulated using finite element analysis and tested by a systemidentification method.The results show that the first-order resonance happened at 34.5 Hz with a gain of 28 dB.An improved double-T notch filter was designed and applied in a real-time system to suppress the resonance at 34.5 Hz.The experimental results show that the resonance was significantly suppressed.In particular,the resonance peak was reduced by 79.37%.In addition,the closed-loop system settling time was reduced by 36.2%.展开更多
To reveal the resonance suppression mechanism of the blood circulation in dragonfly wings,a numerical modeling method of dragonfly wings based on Voronoi diagrams is proposed,and the changes in mass,aerodynamic dampin...To reveal the resonance suppression mechanism of the blood circulation in dragonfly wings,a numerical modeling method of dragonfly wings based on Voronoi diagrams is proposed,and the changes in mass,aerodynamic damping,and natural frequencies caused by blood circulation in veins are investigated.The equivalent mass of blood,boundary conditions,and aerodynamic damping are calculated theoretically.Modal analysis and harmonic response analysis of wing models with different blood circulation paths are performed using the finite-element method(FEM).The vibration reduction ratioδis introduced to compare the damping efficiency of different mass regions.Finally,a natural frequency testing device is constructed to measure the natural frequencies of dragonfly wings.The results indicate that the shape,mass,and natural frequencies of the dragonfly wing model constructed by numerical method agree well with reality.The mass distribution on the wing can be altered by blood circulation,thereby adjusting the natural frequencies and achieving resonance suppression.The highestδof 1.013 is observed in the C region when blood circulates solely in secondary veins,but it is still lower than theδof 1.017 when blood circulates in complete veins.The aerodynamic damping ratio(1.19–1.79%)should not be neglected in the vibration analysis of the beating wing.展开更多
Photovoltaic(PV)inverters have been widely used in large-scale PV power generation systems to reduce the system mismatch and increase the output power.With the increasing installed capacity of PV power stations,the nu...Photovoltaic(PV)inverters have been widely used in large-scale PV power generation systems to reduce the system mismatch and increase the output power.With the increasing installed capacity of PV power stations,the number of string PV inverters increases,which brings about risk of inverter resonance and influences the quality and stability of multi-parallel inverters grid-connected system.Thus,the problems of multi-parallel inverters resonance have become a hot research topic.This paper is focused on the case of grid-connected string PV inverter systems,and the grid-connected PV inverter resonance where resonance suppression strategy will be analyzed and discussed.Firstly,the structure of string PV inverter and multi-parallel inverters system are presented.Secondly,discussions are made about the model of multi-parallel inverters system and its resonance characteristics based on the multi-input-multi-output(MIMO)system,Norton equivalent circuit and carrier synchronization.Thirdly,two strategies of^inverter level"and"system level"are provided to summarize the multi-parallel inverters resonance suppression strategy.Finally,the development tendency of resonance suppression strategy and conclusions are prospected.展开更多
Nuclear magnetic resonance gyroscopes （NMRGs） are a kind of rotation-speed sensor that senses the angular velocity by measuring a frequency shift in the Larmor pre- cession of the nuclear spin in a constant magnetic...Nuclear magnetic resonance gyroscopes （NMRGs） are a kind of rotation-speed sensor that senses the angular velocity by measuring a frequency shift in the Larmor pre- cession of the nuclear spin in a constant magnetic field.展开更多
New shielding material has become an alternative to traditional metal to shield boxes from electromagnetic interferences. This article introduces the theory of transmission line method to study the shield boxes made o...New shielding material has become an alternative to traditional metal to shield boxes from electromagnetic interferences. This article introduces the theory of transmission line method to study the shield boxes made of a new sort of material, and then expands the fundamental formulas to deal with the cases of multiple holes and polarization with arbitrary angle. By means of genetic algorithms with the aid of a three dimensional simulation tool, the damping of electromagnetic resonances in enclosures is researched. The computation indicates that under resonant frequency, electromagnetic resonance results in low, even negative shielding coefficient; whereas, for the same areas, shielding effectiveness of a single hole is worse than that of multiple holes. Shielding coefficient varies when polarization angle increases, and the coupled field through the rectangular aperture with the long side parallel to the thin wire is much weaker than that with the long side vertical to the thin wire. By using the metallic-loss dielectric layer of optimized calculation on the internal surface of the cavity, the best result of resonance suppression has been realized with the same thickness of coating. Finally, according to the calculation result, suggestions for shielding are proposed.展开更多
In this paper,a fault-tolerant control method for an input-series output-parallel modular grid-tied pulse-width modulation(PWM)current source inverter is proposed to address the most commonly seen single symmetrical g...In this paper,a fault-tolerant control method for an input-series output-parallel modular grid-tied pulse-width modulation(PWM)current source inverter is proposed to address the most commonly seen single symmetrical gate-commutated thyristor(SGCT)open-circuit fault problems.This method actively offsets the neutral point of the current space vector to ensure a sinusoidal output of the grid current,and it can achieve the upper limit power of the inverter under the condition of a single SGCT open-circuit fault.In addition,an active damping control method based on grid harmonic current feedback is proposed after analyzing the influence of the transformer ferromagnetic resonance caused by the neutral point offset on the power quality of the grid current.It has been demonstrated that the proposed method effectively suppresses the resonance caused by the transformer and the modified modulation,improving the grid current’s power quality.展开更多
基金supported by the Research on Key Problems of Wide-band Inertial Reference Based on Magnetohydrodynamics (Grant number 61733012)National Natural Science Foundation of China Youth Project (Grant number 61703303)+2 种基金Tianjin Natural Science Foundation Youth Project (Grant number No. 17JCQNJCo4100)State Key Laboratory of Precision Testing Technology and Instruments Open Project (Grant number No. PILAB1705)2017 Tianjin Education Commission Research Project (Grant number 2017KJ086)
文摘In the fields of earth observation,deep space detection,laser communication,and directional energyweapon,the target needs to be observed and pointed at accurately.Acquisition,tracking,and pointing(ATP)systems are usually designed to stabilize the line of sight(LOS)within sub-micro radian levels.In the case of an ATP system mounted on a mobile platform,angular disturbances experienced by the mobile platform will seriously affect the LOS.To overcome the problemthat the sampling frequency of detectors is usually limited and achieving several hundreds of hertz is difficult,thewide-bandwidth inertial reference system(WBIRS)and fast steeringmirror are usually integrated into ATP systems to mitigate these angular disturbances.To reduce the structural stress,a flexible support providing two rotational degrees of freedomis usually adopted for the system.However,the occurrence of resonant points within the bandwidthwill be inevitable.Measurements have to be taken to compensate these low-frequency resonant points to realize a wide bandwidth and high precision.In this paper,the lowfrequency resonant points of a systemwere simulated using finite element analysis and tested by a systemidentification method.The results show that the first-order resonance happened at 34.5 Hz with a gain of 28 dB.An improved double-T notch filter was designed and applied in a real-time system to suppress the resonance at 34.5 Hz.The experimental results show that the resonance was significantly suppressed.In particular,the resonance peak was reduced by 79.37%.In addition,the closed-loop system settling time was reduced by 36.2%.
基金sponsored by the Shandong Natural Science Foundation of the People's Republic of China(No.ZR2022ME213,ZR2023ME081).
文摘To reveal the resonance suppression mechanism of the blood circulation in dragonfly wings,a numerical modeling method of dragonfly wings based on Voronoi diagrams is proposed,and the changes in mass,aerodynamic damping,and natural frequencies caused by blood circulation in veins are investigated.The equivalent mass of blood,boundary conditions,and aerodynamic damping are calculated theoretically.Modal analysis and harmonic response analysis of wing models with different blood circulation paths are performed using the finite-element method(FEM).The vibration reduction ratioδis introduced to compare the damping efficiency of different mass regions.Finally,a natural frequency testing device is constructed to measure the natural frequencies of dragonfly wings.The results indicate that the shape,mass,and natural frequencies of the dragonfly wing model constructed by numerical method agree well with reality.The mass distribution on the wing can be altered by blood circulation,thereby adjusting the natural frequencies and achieving resonance suppression.The highestδof 1.013 is observed in the C region when blood circulates solely in secondary veins,but it is still lower than theδof 1.017 when blood circulates in complete veins.The aerodynamic damping ratio(1.19–1.79%)should not be neglected in the vibration analysis of the beating wing.
基金the key program of National Natural Science Foundation of China under Grant 51277051。
文摘Photovoltaic(PV)inverters have been widely used in large-scale PV power generation systems to reduce the system mismatch and increase the output power.With the increasing installed capacity of PV power stations,the number of string PV inverters increases,which brings about risk of inverter resonance and influences the quality and stability of multi-parallel inverters grid-connected system.Thus,the problems of multi-parallel inverters resonance have become a hot research topic.This paper is focused on the case of grid-connected string PV inverter systems,and the grid-connected PV inverter resonance where resonance suppression strategy will be analyzed and discussed.Firstly,the structure of string PV inverter and multi-parallel inverters system are presented.Secondly,discussions are made about the model of multi-parallel inverters system and its resonance characteristics based on the multi-input-multi-output(MIMO)system,Norton equivalent circuit and carrier synchronization.Thirdly,two strategies of^inverter level"and"system level"are provided to summarize the multi-parallel inverters resonance suppression strategy.Finally,the development tendency of resonance suppression strategy and conclusions are prospected.
基金supported by the National Natural Science Foundation of China(Nos.61673041,61673041,and 61227902)the National High Technology Research and Development Program 863(No.2014AA123401)
文摘Nuclear magnetic resonance gyroscopes （NMRGs） are a kind of rotation-speed sensor that senses the angular velocity by measuring a frequency shift in the Larmor pre- cession of the nuclear spin in a constant magnetic field.
基金the National Natural Science Foundation of China (50237040)
文摘New shielding material has become an alternative to traditional metal to shield boxes from electromagnetic interferences. This article introduces the theory of transmission line method to study the shield boxes made of a new sort of material, and then expands the fundamental formulas to deal with the cases of multiple holes and polarization with arbitrary angle. By means of genetic algorithms with the aid of a three dimensional simulation tool, the damping of electromagnetic resonances in enclosures is researched. The computation indicates that under resonant frequency, electromagnetic resonance results in low, even negative shielding coefficient; whereas, for the same areas, shielding effectiveness of a single hole is worse than that of multiple holes. Shielding coefficient varies when polarization angle increases, and the coupled field through the rectangular aperture with the long side parallel to the thin wire is much weaker than that with the long side vertical to the thin wire. By using the metallic-loss dielectric layer of optimized calculation on the internal surface of the cavity, the best result of resonance suppression has been realized with the same thickness of coating. Finally, according to the calculation result, suggestions for shielding are proposed.
文摘In this paper,a fault-tolerant control method for an input-series output-parallel modular grid-tied pulse-width modulation(PWM)current source inverter is proposed to address the most commonly seen single symmetrical gate-commutated thyristor(SGCT)open-circuit fault problems.This method actively offsets the neutral point of the current space vector to ensure a sinusoidal output of the grid current,and it can achieve the upper limit power of the inverter under the condition of a single SGCT open-circuit fault.In addition,an active damping control method based on grid harmonic current feedback is proposed after analyzing the influence of the transformer ferromagnetic resonance caused by the neutral point offset on the power quality of the grid current.It has been demonstrated that the proposed method effectively suppresses the resonance caused by the transformer and the modified modulation,improving the grid current’s power quality.
