Transactions of Nanjing University of Aeronautics & Astronautics
Volume 34,Issue 4,2017 Table of Contents
2017, 34(4):341-348.
DOI: 10.16356/j.1005-1120.2017.04.341
Abstract:
Chatter vibrations are a major limitation for rough milling operations, leading to productivity reduction, low tool life and poor surface finish. It has been shown recently that the machine tool's own drives can be used to increase the stability limit related to structural modes of the machine. To damp the low frequency modes, a new feedback loop is added to the classical position, velocity and current cascaded control. The objective of this study is to analyse the limitations of this new chatter suppression technique. Constraints related to the non-collocated control are first studied on a simplified three-mass model and then experimentally demonstrated on a three-axis horizontal milling centre. The industrial integration of the new control loop with sampling time constraints and limited drive's bandwidth is analysed. After determining the appropriate conditions to use this chatter suppression technique, a cutting test demonstrates that the stability limit can be doubled in the low regions of the stability lobes.
Chatter vibrations are a major limitation for rough milling operations, leading to productivity reduction, low tool life and poor surface finish. It has been shown recently that the machine tool's own drives can be used to increase the stability limit related to structural modes of the machine. To damp the low frequency modes, a new feedback loop is added to the classical position, velocity and current cascaded control. The objective of this study is to analyse the limitations of this new chatter suppression technique. Constraints related to the non-collocated control are first studied on a simplified three-mass model and then experimentally demonstrated on a three-axis horizontal milling centre. The industrial integration of the new control loop with sampling time constraints and limited drive's bandwidth is analysed. After determining the appropriate conditions to use this chatter suppression technique, a cutting test demonstrates that the stability limit can be doubled in the low regions of the stability lobes.
2017, 34(4):349-356.
DOI: 10.16356/j.1005-1120.2017.04.349
Abstract:
The effects of milling parameters on the surface quality, microstructures and mechanical properties of machined parts with ultrafine grained (UFG) gradient microstructures are investigated. The effects of the cutting speed, feed per tooth, cutting tool geometry and cooling strategy are demonstrated. It has been found that the surface quality of machined grooves can be improved by increasing the cutting speed. However, cryogenic cooling with CO2 exhibits no significant improvement of surface quality. Microstructure and hardness investigations revealed similar microstructure and hardness variations near the machined groove walls for both utilized tool geometries. Therefore, cryogenic cooling can decrease more far-ranging hardness reductions due to high process temperatures, especially in the UFG regions of the machined parts, whilst it cannot prevent the drop in hardness directly at the groove walls.
The effects of milling parameters on the surface quality, microstructures and mechanical properties of machined parts with ultrafine grained (UFG) gradient microstructures are investigated. The effects of the cutting speed, feed per tooth, cutting tool geometry and cooling strategy are demonstrated. It has been found that the surface quality of machined grooves can be improved by increasing the cutting speed. However, cryogenic cooling with CO2 exhibits no significant improvement of surface quality. Microstructure and hardness investigations revealed similar microstructure and hardness variations near the machined groove walls for both utilized tool geometries. Therefore, cryogenic cooling can decrease more far-ranging hardness reductions due to high process temperatures, especially in the UFG regions of the machined parts, whilst it cannot prevent the drop in hardness directly at the groove walls.
2017, 34(4):357-362.
DOI: 10.16356/j.1005-1120.2017.04.357
Abstract:
Surface modification, as a promising approach to improve biocompatibility of biomaterials, has captured extensive close attention among many researchers. Here, micro-milling technology was used in constructing pyramid micro-structures on the surface of Ti-6Al-4V implant. Cutting parameters, including spindle speed, feed rate and depth of cut, were optimized to control the generation of burrs. In addition, low melting point alloy was selected to extend the boundary of the workpiece as supporting material to prevent the generation of top burrs. The surface topographies were characterized using scanning electron microscope and laser scanning microscope. Results showed that the dimension of burrs decreased with the decrease of depth of cut, and the size of burrs decreased with the increase of feed rate. Moreover, burrs nearly not appeared on both sides of the micro-grooves machined with low melting point alloy (LMPA) coating. Pyramid micro-structure on the workpiece surface was built successfully by combining optimized cutting parameters (S=35 kr/min, Vf=60 mm/min, ap=5 μm) and LMPA coating.
Surface modification, as a promising approach to improve biocompatibility of biomaterials, has captured extensive close attention among many researchers. Here, micro-milling technology was used in constructing pyramid micro-structures on the surface of Ti-6Al-4V implant. Cutting parameters, including spindle speed, feed rate and depth of cut, were optimized to control the generation of burrs. In addition, low melting point alloy was selected to extend the boundary of the workpiece as supporting material to prevent the generation of top burrs. The surface topographies were characterized using scanning electron microscope and laser scanning microscope. Results showed that the dimension of burrs decreased with the decrease of depth of cut, and the size of burrs decreased with the increase of feed rate. Moreover, burrs nearly not appeared on both sides of the micro-grooves machined with low melting point alloy (LMPA) coating. Pyramid micro-structure on the workpiece surface was built successfully by combining optimized cutting parameters (S=35 kr/min, Vf=60 mm/min, ap=5 μm) and LMPA coating.
4 Milling Machinability of TiC Particle and TiB Whisker Hybrid Reinforced Titanium Matrix Composites
2017, 34(4):363-371.
DOI: 10.16356/j.1005-1120.2017.04.363
Abstract:
The milling machinabilities of titanium matrix composites were comprehensively evaluated to provide a theoretical basis for cutting parameter determination. Polycrystalline diamond (PCD) tools with different grain sizes and geometries, and carbide tools with and without coatings were used in the experiments. Milling forces, milling temperatures, tool lifetimes, tool wear, and machined surface integrities were investigated. The PCD tool required a primary cutting force 15% smaller than that of the carbide tool, while the uncoated carbide tool required a primary cutting force 10% higher than that of the TiAlN-coated tool. A cutting force of 300 N per millimeter of the cutting edge (300 N/mm) was measured. This caused excessive tool chipping. The cutting temperature of the PCD tool was 20%-30% lower than that of the carbide tool, while that of the TiAlN-coated tool was 12% lower than that of the uncoated carbide tool. The cutting temperatures produced when using water-based cooling and minimal quantity lubrication (MQL) were reduced by 100℃ and 200℃, compared with those recorded with dry cutting, respectively. In general, the PCD tool lifetimes were 2-3 times longer than the carbide tool lifetimes. The roughness Ra of the machined surface was less than 0.6 μm, and the depth of the machined surface hardened layer was in the range of 0.15-0.25 mm for all of the PCD tools before a flank wear land of 0.2 mm was reached. The PCD tool with a 0.8 mm tool nose radius, 0° rake angle, 10° flank angle, and grain size of (30+2) μm exhibited the best cutting performance. For this specific tool, a lifetime of 16 min can be expected.
The milling machinabilities of titanium matrix composites were comprehensively evaluated to provide a theoretical basis for cutting parameter determination. Polycrystalline diamond (PCD) tools with different grain sizes and geometries, and carbide tools with and without coatings were used in the experiments. Milling forces, milling temperatures, tool lifetimes, tool wear, and machined surface integrities were investigated. The PCD tool required a primary cutting force 15% smaller than that of the carbide tool, while the uncoated carbide tool required a primary cutting force 10% higher than that of the TiAlN-coated tool. A cutting force of 300 N per millimeter of the cutting edge (300 N/mm) was measured. This caused excessive tool chipping. The cutting temperature of the PCD tool was 20%-30% lower than that of the carbide tool, while that of the TiAlN-coated tool was 12% lower than that of the uncoated carbide tool. The cutting temperatures produced when using water-based cooling and minimal quantity lubrication (MQL) were reduced by 100℃ and 200℃, compared with those recorded with dry cutting, respectively. In general, the PCD tool lifetimes were 2-3 times longer than the carbide tool lifetimes. The roughness Ra of the machined surface was less than 0.6 μm, and the depth of the machined surface hardened layer was in the range of 0.15-0.25 mm for all of the PCD tools before a flank wear land of 0.2 mm was reached. The PCD tool with a 0.8 mm tool nose radius, 0° rake angle, 10° flank angle, and grain size of (30+2) μm exhibited the best cutting performance. For this specific tool, a lifetime of 16 min can be expected.
5 Critical Thrust Force Forecasting for Drilling Exit Delamination of Carbon Fibre Reinforced Plastics
2017, 34(4):372-379.
DOI: 10.16356/j.1005-1120.2017.04.372
Abstract:
Exit delamination is excessive drilling thrust force. Therefore, it is necessary to investigate the critical thrust force which cause exit delamination when carbon fibre reinforced plastics (CRFP) is drilled. According to the linear elastic fracture mechanics, the mechanics of composite material and the classical thin plate bending theory, a common theoretical model of the critical drilling thrust force for CFRP plates is established. Compared with the experimental data of previous studies, the results show that the theoretical values agree well with the experimental values. This model can be used to forecast the critical thrust force for the drilling-induced delamination of CFRP.
Exit delamination is excessive drilling thrust force. Therefore, it is necessary to investigate the critical thrust force which cause exit delamination when carbon fibre reinforced plastics (CRFP) is drilled. According to the linear elastic fracture mechanics, the mechanics of composite material and the classical thin plate bending theory, a common theoretical model of the critical drilling thrust force for CFRP plates is established. Compared with the experimental data of previous studies, the results show that the theoretical values agree well with the experimental values. This model can be used to forecast the critical thrust force for the drilling-induced delamination of CFRP.
2017, 34(4):380-387.
DOI: 10.16356/j.1005-1120.2017.04.380
Abstract:
In high-speed machining, hardened steel materials are subjected to high temperatures and high strain rates. Under these conditions, the composition and microstructure of the material may change, and phenomena, such as thermal softening, emerge. These effects are difficult to detect by only observing the chip morphology. Here, using a microscopic detection method, the dynamic mechanical behavior and microstructure of SDK11 hardened steel (62 HRC) is investigated at high temperature and high strain-rate, and the relationship between strain hardening, thermal softening, and strain-rate strengthening is determined. The metallographic phases of specimens treated using a split-Hopkinson pressure bar, and "chips" generated during high-speed machining at high temperature and high strain rate state are compared. The results indicate that the phase composition at low temperature and low strain rate differs from that at high temperature and high strain rate. It is further concluded that shear slip occurs at high temperature and high strain rate, and the shear behavior is more pronounced at higher strain rates.
In high-speed machining, hardened steel materials are subjected to high temperatures and high strain rates. Under these conditions, the composition and microstructure of the material may change, and phenomena, such as thermal softening, emerge. These effects are difficult to detect by only observing the chip morphology. Here, using a microscopic detection method, the dynamic mechanical behavior and microstructure of SDK11 hardened steel (62 HRC) is investigated at high temperature and high strain-rate, and the relationship between strain hardening, thermal softening, and strain-rate strengthening is determined. The metallographic phases of specimens treated using a split-Hopkinson pressure bar, and "chips" generated during high-speed machining at high temperature and high strain rate state are compared. The results indicate that the phase composition at low temperature and low strain rate differs from that at high temperature and high strain rate. It is further concluded that shear slip occurs at high temperature and high strain rate, and the shear behavior is more pronounced at higher strain rates.
2017, 34(4):388-392.
DOI: 10.16356/j.1005-1120.2017.04.388
Abstract:
The controllable synthesis of uniform silver nanocubes with high purity is pivotal for the fundamental study of self-assembly and further research on the hollow nanostructures, gold nanocages for instance. Here, Ag nanocubes of different sizes were synthesized by an improved polyol method. With addition of HCl solution, Ag nanocubes with size about 100 nm were obtained under an air atmosphere. And Ag nanocubes with size around 50 nm can be produced in a short time under Argon atmosphere with the presence of NaHS instead of HCl. Meanwhile, uniform Ag nanocubes with size larger than 100 nm were also synthesized successfully via adjusting experiment parameters. Results of transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED) show that the Ag nanocubes are single crystalline with six (200) surface plane. In the UV-Vis-NIR optical absorption spectrum, the diple moment resonance absorption peak is changed in the range of 420-500 nm with the increase of Ag nanocubes size.
The controllable synthesis of uniform silver nanocubes with high purity is pivotal for the fundamental study of self-assembly and further research on the hollow nanostructures, gold nanocages for instance. Here, Ag nanocubes of different sizes were synthesized by an improved polyol method. With addition of HCl solution, Ag nanocubes with size about 100 nm were obtained under an air atmosphere. And Ag nanocubes with size around 50 nm can be produced in a short time under Argon atmosphere with the presence of NaHS instead of HCl. Meanwhile, uniform Ag nanocubes with size larger than 100 nm were also synthesized successfully via adjusting experiment parameters. Results of transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED) show that the Ag nanocubes are single crystalline with six (200) surface plane. In the UV-Vis-NIR optical absorption spectrum, the diple moment resonance absorption peak is changed in the range of 420-500 nm with the increase of Ag nanocubes size.
2017, 34(4):393-397.
DOI: 10.16356/j.1005-1120.2017.04.393
Abstract:
The temperature evolution of the crystal structure for Nd0.5Sr0.5MnO has been investigated by powder XRD between 125 K and 725 K. The structure can be described with a monoclinic symmetry (space group P21/m) in the temperature range of 125-175 K, while with the increase in temperature between 175 K and 575 K the structure involves a higher orthorhombic symmetry (space group Imma). The rhombohedral structure with space group R-3c is observed at high temperature region of 575-725 K. The increase in the magnetization at low temperatures can be ascribed to the field-induced short-range magnetic order of the Nd3+ ions. The dc and ac susceptibility data show some anomalies around the FM-PM transition region which can be attributed to the glass behavior and magnetic relaxation.
The temperature evolution of the crystal structure for Nd0.5Sr0.5MnO has been investigated by powder XRD between 125 K and 725 K. The structure can be described with a monoclinic symmetry (space group P21/m) in the temperature range of 125-175 K, while with the increase in temperature between 175 K and 575 K the structure involves a higher orthorhombic symmetry (space group Imma). The rhombohedral structure with space group R-3c is observed at high temperature region of 575-725 K. The increase in the magnetization at low temperatures can be ascribed to the field-induced short-range magnetic order of the Nd3+ ions. The dc and ac susceptibility data show some anomalies around the FM-PM transition region which can be attributed to the glass behavior and magnetic relaxation.
2017, 34(4):398-404.
DOI: 10.16356/j.1005-1120.2017.04.398
Abstract:
A rapid engineering surface panel method to analyze aerodynamics and aerothermodynamics of hypersonic vehicles is developed. To obtain the surface pressure distribution of a hypersonic vehicle, the local surface inclination method is applied to calculate the pressure coefficient for each surface panel element, of which the normal vector is corrected first by using an efficient data structure and Rey-casting algorithm, local Reynolds numbers are calculated according to the geometric streamline method, then the aerodynamic heating flux is computed by both reference enthalpy relations and Reynolds analogy method. Several typical test cases are performed and the results indicate that, the developed tool is effective in predicting the aerodynamics/aerothermodynamics for complex geometry of hypersonic vehicle in a wide range of Mach numbers with a sufficient accuracy.
A rapid engineering surface panel method to analyze aerodynamics and aerothermodynamics of hypersonic vehicles is developed. To obtain the surface pressure distribution of a hypersonic vehicle, the local surface inclination method is applied to calculate the pressure coefficient for each surface panel element, of which the normal vector is corrected first by using an efficient data structure and Rey-casting algorithm, local Reynolds numbers are calculated according to the geometric streamline method, then the aerodynamic heating flux is computed by both reference enthalpy relations and Reynolds analogy method. Several typical test cases are performed and the results indicate that, the developed tool is effective in predicting the aerodynamics/aerothermodynamics for complex geometry of hypersonic vehicle in a wide range of Mach numbers with a sufficient accuracy.
2017, 34(4):405-412.
DOI: 10.16356/j.1005-1120.2017.04.405
Abstract:
Fixture locating layout has a direct and influential impact on aeronautical thin-walled component (ATWC) manufacturing quality. The purpose is to develop a topological optimization method for ATWC fixture locating layout to minimize the manufacturing deformation. Firstly, a topological optimization model that takes the stiffness of ATWC as the objective function and the volume of the locating structure as the constraint is established. Secondly, ATWC and the locating structure are regarded as an integrated entity, and the variable-density method based topological optimization approach is adopted for the optimization of the locating structure using ABAQUS topology optimization module (ATOM). Thirdly, through a subsequent model reconstruction referring to the obtained topological structure, the optimal fixture locating layout is achieved. Finally, a case study is conducted to verify the proposed method and the comparison results with firefly algorithm (FA) coupled with finite element analysis (FEA) indicate that the number and positions of the locators for ATWC can be optimized simultaneously and successfully by the proposed topological optimization model.
Fixture locating layout has a direct and influential impact on aeronautical thin-walled component (ATWC) manufacturing quality. The purpose is to develop a topological optimization method for ATWC fixture locating layout to minimize the manufacturing deformation. Firstly, a topological optimization model that takes the stiffness of ATWC as the objective function and the volume of the locating structure as the constraint is established. Secondly, ATWC and the locating structure are regarded as an integrated entity, and the variable-density method based topological optimization approach is adopted for the optimization of the locating structure using ABAQUS topology optimization module (ATOM). Thirdly, through a subsequent model reconstruction referring to the obtained topological structure, the optimal fixture locating layout is achieved. Finally, a case study is conducted to verify the proposed method and the comparison results with firefly algorithm (FA) coupled with finite element analysis (FEA) indicate that the number and positions of the locators for ATWC can be optimized simultaneously and successfully by the proposed topological optimization model.
2017, 34(4):413-419.
DOI: 10.16356/j.1005-1120.2017.04.413
Abstract:
For carrier-based unmanned aerial vehicles (UAVs), one of the important problems is the design of an automatic carrier landing system (ACLS) that would enable the UAVs to accomplish autolanding on the aircraft carrier. However, due to the movements of the flight deck with six degree-of-freedom, the autolanding becomes sophisticated. To solve this problem, an accurate and effective ACLS is developed, which is composed of an optimal preview control based flight control system and a Kalman filter based deck motion predictor. The preview control fuses the future information of the reference glide slope to improve landing precision. The reference glide slope is normally a straight line. However, the deck motion will change the position of the ideal landing point, and tracking the ideal straight glide slope may cause landing failure. Therefore, the predictive deck motion information from the deck motion predictor is used to correct the reference glide slope, which decreases the dispersion around the desired landing point. Finally, simulations are carried out to verify the performance of the designed ACLS based on a nonlinear UAV model.
For carrier-based unmanned aerial vehicles (UAVs), one of the important problems is the design of an automatic carrier landing system (ACLS) that would enable the UAVs to accomplish autolanding on the aircraft carrier. However, due to the movements of the flight deck with six degree-of-freedom, the autolanding becomes sophisticated. To solve this problem, an accurate and effective ACLS is developed, which is composed of an optimal preview control based flight control system and a Kalman filter based deck motion predictor. The preview control fuses the future information of the reference glide slope to improve landing precision. The reference glide slope is normally a straight line. However, the deck motion will change the position of the ideal landing point, and tracking the ideal straight glide slope may cause landing failure. Therefore, the predictive deck motion information from the deck motion predictor is used to correct the reference glide slope, which decreases the dispersion around the desired landing point. Finally, simulations are carried out to verify the performance of the designed ACLS based on a nonlinear UAV model.
2017, 34(4):420-425.
DOI: 10.16356/j.1005-1120.2017.04.420
Abstract:
The damping-controlled deployment technology of flexible Z-folded inflatable tube is one of the key technologies of space inflatable structures. Constraint failure between couple nodes is proposed to simulate the damping-controlled deployment. And the equation of constraint failure is established. Inflation process of Z-folded tube is simulated with control volume method. Compared with uncontrolled method, it is indicated that the new method can effectively solve the disordered deployment problem of the slender Z-folded inflatable tube. By analyzing the displacement of the apical of the folded tube with different constraint forces during deployment process, the concept of effective constraint force is proposed. In the effective region of constraint force, the folded tube deploys with little retraction and fluctuation. Otherwise, The flexible folded tube would deploy disorderly or even cannot deploy. The simulation method and numerical results have a theoretical and instructive significance to the research on the space inflatable structures.
The damping-controlled deployment technology of flexible Z-folded inflatable tube is one of the key technologies of space inflatable structures. Constraint failure between couple nodes is proposed to simulate the damping-controlled deployment. And the equation of constraint failure is established. Inflation process of Z-folded tube is simulated with control volume method. Compared with uncontrolled method, it is indicated that the new method can effectively solve the disordered deployment problem of the slender Z-folded inflatable tube. By analyzing the displacement of the apical of the folded tube with different constraint forces during deployment process, the concept of effective constraint force is proposed. In the effective region of constraint force, the folded tube deploys with little retraction and fluctuation. Otherwise, The flexible folded tube would deploy disorderly or even cannot deploy. The simulation method and numerical results have a theoretical and instructive significance to the research on the space inflatable structures.
2017, 34(4):426-437.
DOI: 10.16356/j.1005-1120.2017.04.426
Abstract:
To investigate the effect of soil-pile-structure interaction (SPSI effect) on the dynamic response of a base-isolated structure with buried footings on a pile foundation, certain shake table tests are previously conducted. Based on the test results and the existing related studies, an efficient simplified model and a corresponding calculation method are verified for estimating the dynamic characteristics of a base-isolated structure with buried footings on a pile foundation with the SSI effect. In this method, the solutions by Veletsos and co-workers for a non-isolated structure with the SSI effect are verified and advanced for a base-isolated structure, and the solutions by Maravas and co-workers for a non-isolated structure on a pile foundation are introduced to consider the effect of the piles. By comparison with the shake table test, this work proves that the simplified method can efficiently estimate the dynamic responses of a base-isolated structure with buried footings on a pile foundation. Using parameter analysis, this work also shows that the dynamic characteristics of a non-isolated structure are quite similar to those of the base-isolated structure when the soil foundation is sufficiently soft, which means that the isolation layer gradually loses its isolation function as the soil foundation softens.
To investigate the effect of soil-pile-structure interaction (SPSI effect) on the dynamic response of a base-isolated structure with buried footings on a pile foundation, certain shake table tests are previously conducted. Based on the test results and the existing related studies, an efficient simplified model and a corresponding calculation method are verified for estimating the dynamic characteristics of a base-isolated structure with buried footings on a pile foundation with the SSI effect. In this method, the solutions by Veletsos and co-workers for a non-isolated structure with the SSI effect are verified and advanced for a base-isolated structure, and the solutions by Maravas and co-workers for a non-isolated structure on a pile foundation are introduced to consider the effect of the piles. By comparison with the shake table test, this work proves that the simplified method can efficiently estimate the dynamic responses of a base-isolated structure with buried footings on a pile foundation. Using parameter analysis, this work also shows that the dynamic characteristics of a non-isolated structure are quite similar to those of the base-isolated structure when the soil foundation is sufficiently soft, which means that the isolation layer gradually loses its isolation function as the soil foundation softens.
2017, 34(4):438-448.
DOI: 10.16356/j.1005-1120.2017.04.438
Abstract:
Sector capacity estimation plays an important role in applied research of airspace management. Previous researches manifest that sector capacity should be influenced by its standard flow, or routes in that sector. However, if air traffic controller (ATCO) workload busy levels (level of proactivity of an ATCO) are ignored, the estimated sector capacity may not be accurate. There is a need to compare the estimated sector capacity with and without busy levels consideration, both with differentiated routes consideration. This paper proposes a method for sector capacity estimation based on ATCO workload considering differentiated routes and busy levels. Firstly, the main routes in the sector are identified, and for each route, the ATCO workload per flight is determined. Secondly, the workload for each route at three busy levels is determined. Regression analysis is then applied to determine the relationship between workload and the number of flights (with and without considering busy levels) in 15 min and 1 h time slices. Sector capacity is then determined on the basis of a specified workload threshold, for the two cases with and without considering busy levels. Comparing the two scenarios and following validation by ATCO survey, it is found that capacity estimation considering busy levels is a more realistic and accurate approach. The validated capacity values for the Zhengzhou approach (ZHCC AP) airspace sector accounting for the busy levels were determined accurately as 10 and 33 flights for the 15 min and 1 h slices, respectively. The corresponding results without considering busy levels were 12 and 41 flights for the 15 min and 1 h time slices, respectively.
Sector capacity estimation plays an important role in applied research of airspace management. Previous researches manifest that sector capacity should be influenced by its standard flow, or routes in that sector. However, if air traffic controller (ATCO) workload busy levels (level of proactivity of an ATCO) are ignored, the estimated sector capacity may not be accurate. There is a need to compare the estimated sector capacity with and without busy levels consideration, both with differentiated routes consideration. This paper proposes a method for sector capacity estimation based on ATCO workload considering differentiated routes and busy levels. Firstly, the main routes in the sector are identified, and for each route, the ATCO workload per flight is determined. Secondly, the workload for each route at three busy levels is determined. Regression analysis is then applied to determine the relationship between workload and the number of flights (with and without considering busy levels) in 15 min and 1 h time slices. Sector capacity is then determined on the basis of a specified workload threshold, for the two cases with and without considering busy levels. Comparing the two scenarios and following validation by ATCO survey, it is found that capacity estimation considering busy levels is a more realistic and accurate approach. The validated capacity values for the Zhengzhou approach (ZHCC AP) airspace sector accounting for the busy levels were determined accurately as 10 and 33 flights for the 15 min and 1 h slices, respectively. The corresponding results without considering busy levels were 12 and 41 flights for the 15 min and 1 h time slices, respectively.
2017, 34(4):449-455.
DOI: 10.16356/j.1005-1120.2017.04.449
Abstract:
Modeling experiences of traditional grey-Markov show that the prediction results are not accurate when analyzed data are rare and fluctuated. So it is necessary to revise or improve the original modeling procedure of the grey-Markov (GM) model. Therefore, a new idea is brought forward that the Markov theory is used twice, where the first time is to extend the original data and the second to calculate and estimate the residual errors. Then by comparing the original data sequence from a fault prediction case with the simulation sequence produced by the use of GM(1,1) and the new GM method, results are conforming to the original data. Finally, an assumption of GM model is put forward as the future work.
Modeling experiences of traditional grey-Markov show that the prediction results are not accurate when analyzed data are rare and fluctuated. So it is necessary to revise or improve the original modeling procedure of the grey-Markov (GM) model. Therefore, a new idea is brought forward that the Markov theory is used twice, where the first time is to extend the original data and the second to calculate and estimate the residual errors. Then by comparing the original data sequence from a fault prediction case with the simulation sequence produced by the use of GM(1,1) and the new GM method, results are conforming to the original data. Finally, an assumption of GM model is put forward as the future work.
2017, 34(4):456-464.
DOI: 10.16356/j.1005-1120.2017.04.456
Abstract:
The sparse unmixing problem of greedy algorithms still remains a great challenge at finding an optimal subset of endmembers for the observed data from the spectral library, due to the usually high correlation of the spectral library. Under such circumstances, a novel greedy algorithm for sparse unmixing of hyperspectral data is presented, termed the recursive dictionary-based simultaneous orthogonal matching pursuit (RD-SOMP). The algorithm adopts a block-processing strategy to divide the whole hyperspectral image into several blocks. At each iteration of the block, the spectral library is projected into the orthogonal subspace and renormalized, which can reduce the correlation of the spectral library. Then RD-SOMP selects a new endmember with the maximum correlation between the current residual and the orthogonal subspace of the spectral library. The endmembers picked in all the blocks are associated as the endmember sets of the whole hyperspectral data. Finally, the abundances are estimated using the whole hyperspectral data with the obtained endmember sets. It can be proved that RD-SOMP can recover the optimal endmembers from the spectral library under certain conditions. Experimental results demonstrate that the RD-SOMP algorithm outperforms the other algorithms, with a better spectral unmixing accuracy.
The sparse unmixing problem of greedy algorithms still remains a great challenge at finding an optimal subset of endmembers for the observed data from the spectral library, due to the usually high correlation of the spectral library. Under such circumstances, a novel greedy algorithm for sparse unmixing of hyperspectral data is presented, termed the recursive dictionary-based simultaneous orthogonal matching pursuit (RD-SOMP). The algorithm adopts a block-processing strategy to divide the whole hyperspectral image into several blocks. At each iteration of the block, the spectral library is projected into the orthogonal subspace and renormalized, which can reduce the correlation of the spectral library. Then RD-SOMP selects a new endmember with the maximum correlation between the current residual and the orthogonal subspace of the spectral library. The endmembers picked in all the blocks are associated as the endmember sets of the whole hyperspectral data. Finally, the abundances are estimated using the whole hyperspectral data with the obtained endmember sets. It can be proved that RD-SOMP can recover the optimal endmembers from the spectral library under certain conditions. Experimental results demonstrate that the RD-SOMP algorithm outperforms the other algorithms, with a better spectral unmixing accuracy.
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