Direct current pulsed metal inert-gas （DC-PMIG） welding was carried out on AZ31B magnesium alloy to obtain continuous welding joints of 3 mm and 8 mm thickness with 1.6 mm diameter of filler wire. The behavior and m...Direct current pulsed metal inert-gas （DC-PMIG） welding was carried out on AZ31B magnesium alloy to obtain continuous welding joints of 3 mm and 8 mm thickness with 1.6 mm diameter of filler wire. The behavior and mechanism of metal transfer, the parameter ranges of stable welding process, the microstructure and mechanical properties of magnesium alloy were investigated. The results show that the metal transfer form of magnesium alloy using DC-PMIG welding is realized in modes of globular transfer, projected transfer and spray transfer. Welding spatter with a large size will be produced in the mode of globular transfer, and high-quality joints with few spatters can be obtained in the mode of projected transfer and spray transfer when the linear energy of filler wire is 242-27l J/cm, droplet diameter is 1.6-0.9 mm tensile strength of weld beads is 94.2% of that of base metals. and transition frequency is 30-69 Hz. The average ultimate展开更多
A novel nonlinear adaptive control method is presented for a near-space hypersonic vehicle （NHV） in the presence of strong uncertainties and disturbances. The control law consists of the optimal generalized predicti...A novel nonlinear adaptive control method is presented for a near-space hypersonic vehicle （NHV） in the presence of strong uncertainties and disturbances. The control law consists of the optimal generalized predictive controller （OGPC） and the functional link network （FLN） direct adaptive law. OGPC is a continuous-time nonlinear predictive control law. The FLN adaptive law is used to offset the unknown uncertainties and disturbances in a flight through the online learning. The learning process does not need any offline training phase. The stability analyses of the NHV close-loop system are provided and it is proved that the system error and the weight learning error are uniformly ultimately hounded. Simulation results show the satisfactory performance of the con- troller for the attitude tracking.展开更多
In the current microwelding process using femtosecond(fs) laser between dissimilar materials,surface polishing and pressure assistance,so-called optical contact,are believed necessary.In this paper,direct welding of s...In the current microwelding process using femtosecond(fs) laser between dissimilar materials,surface polishing and pressure assistance,so-called optical contact,are believed necessary.In this paper,direct welding of soda lime glass and Kovar alloy using a fs laser is investigated to overcome the limit of optical contact.The processing of fs laser welding is comprehensively studied by varying the laser power,welding velocity and the number of welding.The shear joining strength is as high as 2 MPa.The cross-section of glass-Kovar alloy joints,the elemental diffusion and the fracture behavior of welded joints were studied.The results show that the fs laser irradiates the surface of Kovar alloy,micron/nanometer-sized metal particles are generated.These particles perform the role as an adhesive part in the welding process.It is believed that the Si atoms diffuses to Kovar alloy from the glass and partially replaces the Fe^(2+) ions on the surface of Kovar alloy,indicating that the mixing and interdiffusion of materials have occurred during the welding process.Finally,the welded sample was tested and has excellent water resistance and sealing property.Furthermore,to justify that this method can be applied to other stack ups,the glass-copper,the glass-Al6063 and sapphire-ceramic are also welded together.This work greatly simplifies the fs laser microwelding process and promotes its industrial applications,such as optoelectronic devices,medical devices and MEMS.展开更多
Cellular vehicle-to-everything(C-V2X) communications is regarded as a promising and feasible solution for 5G-enabled vehicular communications and networking. In this paper, we investigate the pilot design and channel ...Cellular vehicle-to-everything(C-V2X) communications is regarded as a promising and feasible solution for 5G-enabled vehicular communications and networking. In this paper, we investigate the pilot design and channel estimation problem in MIMO-OFDM-based C-V2X systems with severe co-channel interference due to spectrum reusing among different V2X communication links. By using zero-correlation zone(ZCZ) sequences, we provide an interference-free pilot design scheme and a corresponding time-domain(TD) correlation-based channel estimation(TD-CCE) method. We employ the ZCZ sequences from the same family set to be designed as the TD pilot symbols and guarantee the pilot sequeneces for neighboring V2X communication links are code-division multiplexing(CDM). The co-channel pilot interference of the deisgned pilot symbols can be effectively eliminated by exploiting the provided TD-CCE method. Simulation results indicate that the accuracy of channel estimation can be effectively improved by the proposed scheme, whose performance is close to that of the non-interference situation.展开更多
In this paper, we study an energy efficient multi-antenna unmanned aerial vehicle(UAV)-enabled half-duplex mobile relaying system under Rician fading channels. By assuming that the UAV follows a circular trajectory at...In this paper, we study an energy efficient multi-antenna unmanned aerial vehicle(UAV)-enabled half-duplex mobile relaying system under Rician fading channels. By assuming that the UAV follows a circular trajectory at fixed altitude and applying the decode-and-forward relaying strategy, we maximize the energy efficiency by jointly designing beamforming, power allocation, circular radius and flight speed, subject to the sum transmit power constraint on source node and UAV relay node. First, we maximize the end-to-end signal-to-noise ratio by jointly designing beamforming and statistical power allocation. Based on the obtained beamforming and power allocation results, we then obtain a semi closed-form expression of energy efficiency, and finally maximize energy efficiency by optimizing flight speed and circular radius, allowing optimal circular radius to be obtained via numerical computation. Numerical results demonstrate that the proposed scheme can effectively enhance the system energy efficiency.展开更多
AIM To evaluate the effect of long haul airplane transport of donor livers on post-transplant outcomes. METHODS A retrospective cohort study of patients who received a liver transplantation was performed in Perth, Aus...AIM To evaluate the effect of long haul airplane transport of donor livers on post-transplant outcomes. METHODS A retrospective cohort study of patients who received a liver transplantation was performed in Perth, Australia from 1992 to 2012. Donor and recipient characteristics information were extracted from Western Australian liver transplantation service database. Patients were followed up for a mean of six years. Patient and graft survival were evaluated and compared between patients who received a local donor liver and those who received an airplane transported donor liver. Predictors of survival were determined by univariate and multivariate analysis using cox regression.RESULTS One hundred and ninety-three patients received alocal donor liver and 93 patients received an airplane transported donor liver. Airplane transported livers had a significantly lower alanine transaminase(mean: 45 U/L vs 84 U/L, P = 0.035), higher donor risk index(mean: 1.88 vs 1.42, P < 0.001) and longer cold ischemic time(CIT)(mean: 10.1 h vs 6.4 h, P < 0.001). There was a weak correlation between CIT and transport distance(r2 = 0.29, P < 0.001). Mean follow up was six years and 93 patients had graft failure. Multivariate analysis found only airplane transport retained significance for graft loss(HR = 1.92, 95%CI: 1.16-3.17). One year graft survival was 0.88 for those with a local liver and was 0.71 for those with an airplane transported liver. One year graft loss was due to primary graft non-function or associated with preservation injury in 20.8% of recipients of an airplane transported liver compared with 4.6% in those with a local liver(P = 0.027). CONCLUSION Airplane transport of donor livers was independently associated with reduced graft survival following liver transplantation.展开更多
This paper presents a fundamental gaskinetic study on high speed rarefied jets expanding into vacuum from a cluster of planar exits.Based on the corresponding exact expressions for one planar jet,this paper straightfo...This paper presents a fundamental gaskinetic study on high speed rarefied jets expanding into vacuum from a cluster of planar exits.Based on the corresponding exact expressions for one planar jet,this paper straightforwardly derives the combined multiple jet flowfield solutions of density and velocity components,however,for the combined temperature and pressure solutions,extra attention shall be practiced.Several direct simulation Monte Carlo simulation results are provided and they validate these analytical solutions of rarefied planar jet flows.展开更多
The sense of telepresence is known to be essential in teleoperation environments, where the operator is physically separated from the vehicle. Usually only a visual feedback is provided, but it has been shown that by ...The sense of telepresence is known to be essential in teleoperation environments, where the operator is physically separated from the vehicle. Usually only a visual feedback is provided, but it has been shown that by extending the visual interface with haptic feedback, that is complementing the visual information through the sense of touch, the teleoperator has a better perception of information from the remote environment and its constraints. This paper focuses on a novel concept of haptic cueing for an airborne obstacle avoidance task; the novel cueing algorithm was designed to appear ＂natural＂ to the operator, and to improve the human-machine interface without directly acting on the actual aircraft commands. Two different haptic aiding concepts for obstacle avoidance support are presented： an existing and widely used system, belonging to what we called the Direct Haptic Aid （DItA） approach class, and a novel one based on the Indirect Haptic Aid （IHA） approach class. Tests with human operators show that a net improvement in terms of performance （i.e., the number of collisions） is provided by employing the 1HA haptic cue as compared to both the DHA haptic cue and/or the visual cues only. The results clearly show that the IHA philosophy is a valid alternative to the other commonly used approaches, which fall in the DHA category.展开更多
在“05-1”多国海上演习中，法车海军一架“阵风”M多用途战斗机在美国海军“艾森豪威尔”号（CVN-69）航母上进行了接航复飞，一架来自法军“夏尔·戴高乐”（Charles De Gaulle)号航母（R-91）的E-2C“鹰眼”预警机在“艾森豪威...在“05-1”多国海上演习中，法车海军一架“阵风”M多用途战斗机在美国海军“艾森豪威尔”号（CVN-69）航母上进行了接航复飞，一架来自法军“夏尔·戴高乐”（Charles De Gaulle)号航母（R-91）的E-2C“鹰眼”预警机在“艾森豪威尔”（CVN-69）航母上进行了起降。展开更多
基金Project (IRT1008) supported by Changjiang Scholars and Innovative Research Team in University, ChinaProject (51005035) supported by the National Natural Science Foundation of ChinaProject (51025520) supported by the National Natural Science Funds for Distinguished Young Scholar, China
文摘Direct current pulsed metal inert-gas （DC-PMIG） welding was carried out on AZ31B magnesium alloy to obtain continuous welding joints of 3 mm and 8 mm thickness with 1.6 mm diameter of filler wire. The behavior and mechanism of metal transfer, the parameter ranges of stable welding process, the microstructure and mechanical properties of magnesium alloy were investigated. The results show that the metal transfer form of magnesium alloy using DC-PMIG welding is realized in modes of globular transfer, projected transfer and spray transfer. Welding spatter with a large size will be produced in the mode of globular transfer, and high-quality joints with few spatters can be obtained in the mode of projected transfer and spray transfer when the linear energy of filler wire is 242-27l J/cm, droplet diameter is 1.6-0.9 mm tensile strength of weld beads is 94.2% of that of base metals. and transition frequency is 30-69 Hz. The average ultimate
基金Supported by the National Nature Science Foundation of China (90716028)~~
文摘A novel nonlinear adaptive control method is presented for a near-space hypersonic vehicle （NHV） in the presence of strong uncertainties and disturbances. The control law consists of the optimal generalized predictive controller （OGPC） and the functional link network （FLN） direct adaptive law. OGPC is a continuous-time nonlinear predictive control law. The FLN adaptive law is used to offset the unknown uncertainties and disturbances in a flight through the online learning. The learning process does not need any offline training phase. The stability analyses of the NHV close-loop system are provided and it is proved that the system error and the weight learning error are uniformly ultimately hounded. Simulation results show the satisfactory performance of the con- troller for the attitude tracking.
基金Project(GKZY2119502) supported by the Special Funds for Local Scientific and Technological Development guided by the Central Government,ChinaProject(JGY2021001) supported by the Innovation Project of Guangxi Graduate Education,China。
文摘In the current microwelding process using femtosecond(fs) laser between dissimilar materials,surface polishing and pressure assistance,so-called optical contact,are believed necessary.In this paper,direct welding of soda lime glass and Kovar alloy using a fs laser is investigated to overcome the limit of optical contact.The processing of fs laser welding is comprehensively studied by varying the laser power,welding velocity and the number of welding.The shear joining strength is as high as 2 MPa.The cross-section of glass-Kovar alloy joints,the elemental diffusion and the fracture behavior of welded joints were studied.The results show that the fs laser irradiates the surface of Kovar alloy,micron/nanometer-sized metal particles are generated.These particles perform the role as an adhesive part in the welding process.It is believed that the Si atoms diffuses to Kovar alloy from the glass and partially replaces the Fe^(2+) ions on the surface of Kovar alloy,indicating that the mixing and interdiffusion of materials have occurred during the welding process.Finally,the welded sample was tested and has excellent water resistance and sealing property.Furthermore,to justify that this method can be applied to other stack ups,the glass-copper,the glass-Al6063 and sapphire-ceramic are also welded together.This work greatly simplifies the fs laser microwelding process and promotes its industrial applications,such as optoelectronic devices,medical devices and MEMS.
文摘Cellular vehicle-to-everything(C-V2X) communications is regarded as a promising and feasible solution for 5G-enabled vehicular communications and networking. In this paper, we investigate the pilot design and channel estimation problem in MIMO-OFDM-based C-V2X systems with severe co-channel interference due to spectrum reusing among different V2X communication links. By using zero-correlation zone(ZCZ) sequences, we provide an interference-free pilot design scheme and a corresponding time-domain(TD) correlation-based channel estimation(TD-CCE) method. We employ the ZCZ sequences from the same family set to be designed as the TD pilot symbols and guarantee the pilot sequeneces for neighboring V2X communication links are code-division multiplexing(CDM). The co-channel pilot interference of the deisgned pilot symbols can be effectively eliminated by exploiting the provided TD-CCE method. Simulation results indicate that the accuracy of channel estimation can be effectively improved by the proposed scheme, whose performance is close to that of the non-interference situation.
基金supported in part by the National Science Foundation (NSFC) for Distinguished Young Scholars of China with Grant 61625106the National Natural Science Foundation of China under Grant 61531011
文摘In this paper, we study an energy efficient multi-antenna unmanned aerial vehicle(UAV)-enabled half-duplex mobile relaying system under Rician fading channels. By assuming that the UAV follows a circular trajectory at fixed altitude and applying the decode-and-forward relaying strategy, we maximize the energy efficiency by jointly designing beamforming, power allocation, circular radius and flight speed, subject to the sum transmit power constraint on source node and UAV relay node. First, we maximize the end-to-end signal-to-noise ratio by jointly designing beamforming and statistical power allocation. Based on the obtained beamforming and power allocation results, we then obtain a semi closed-form expression of energy efficiency, and finally maximize energy efficiency by optimizing flight speed and circular radius, allowing optimal circular radius to be obtained via numerical computation. Numerical results demonstrate that the proposed scheme can effectively enhance the system energy efficiency.
文摘AIM To evaluate the effect of long haul airplane transport of donor livers on post-transplant outcomes. METHODS A retrospective cohort study of patients who received a liver transplantation was performed in Perth, Australia from 1992 to 2012. Donor and recipient characteristics information were extracted from Western Australian liver transplantation service database. Patients were followed up for a mean of six years. Patient and graft survival were evaluated and compared between patients who received a local donor liver and those who received an airplane transported donor liver. Predictors of survival were determined by univariate and multivariate analysis using cox regression.RESULTS One hundred and ninety-three patients received alocal donor liver and 93 patients received an airplane transported donor liver. Airplane transported livers had a significantly lower alanine transaminase(mean: 45 U/L vs 84 U/L, P = 0.035), higher donor risk index(mean: 1.88 vs 1.42, P < 0.001) and longer cold ischemic time(CIT)(mean: 10.1 h vs 6.4 h, P < 0.001). There was a weak correlation between CIT and transport distance(r2 = 0.29, P < 0.001). Mean follow up was six years and 93 patients had graft failure. Multivariate analysis found only airplane transport retained significance for graft loss(HR = 1.92, 95%CI: 1.16-3.17). One year graft survival was 0.88 for those with a local liver and was 0.71 for those with an airplane transported liver. One year graft loss was due to primary graft non-function or associated with preservation injury in 20.8% of recipients of an airplane transported liver compared with 4.6% in those with a local liver(P = 0.027). CONCLUSION Airplane transport of donor livers was independently associated with reduced graft survival following liver transplantation.
文摘This paper presents a fundamental gaskinetic study on high speed rarefied jets expanding into vacuum from a cluster of planar exits.Based on the corresponding exact expressions for one planar jet,this paper straightforwardly derives the combined multiple jet flowfield solutions of density and velocity components,however,for the combined temperature and pressure solutions,extra attention shall be practiced.Several direct simulation Monte Carlo simulation results are provided and they validate these analytical solutions of rarefied planar jet flows.
文摘The sense of telepresence is known to be essential in teleoperation environments, where the operator is physically separated from the vehicle. Usually only a visual feedback is provided, but it has been shown that by extending the visual interface with haptic feedback, that is complementing the visual information through the sense of touch, the teleoperator has a better perception of information from the remote environment and its constraints. This paper focuses on a novel concept of haptic cueing for an airborne obstacle avoidance task; the novel cueing algorithm was designed to appear ＂natural＂ to the operator, and to improve the human-machine interface without directly acting on the actual aircraft commands. Two different haptic aiding concepts for obstacle avoidance support are presented： an existing and widely used system, belonging to what we called the Direct Haptic Aid （DItA） approach class, and a novel one based on the Indirect Haptic Aid （IHA） approach class. Tests with human operators show that a net improvement in terms of performance （i.e., the number of collisions） is provided by employing the 1HA haptic cue as compared to both the DHA haptic cue and/or the visual cues only. The results clearly show that the IHA philosophy is a valid alternative to the other commonly used approaches, which fall in the DHA category.
