Assembly model plans an important role in assembly sequence planning.Flexible assembly system (FAS) is a dis- crete event system (DES),so Petri net,as an important analytical tool for the DES,is used for modeling of a...Assembly model plans an important role in assembly sequence planning.Flexible assembly system (FAS) is a dis- crete event system (DES),so Petri net,as an important analytical tool for the DES,is used for modeling of an FAS.All of the feasible assembly sequences can be obtained by firing the Petri net model.In order to enhancing the efficiency of the assembly sequence planning,knowledge-based Petri net,combining an usual Petri net with expert’s knowledge and experiences,is proposed to con- struct the assembly model.So the feasible assembly sequences can be reduced greatly,and the optimum assembly sequence could be generated without the combinatorial explosion of solutions.展开更多
Existing approaches to automatic assembly planning often lead to combinatorial explo- sion. When the parts composing the assembly increase in number, computer-aided planning be- comes much slower than manual planning....Existing approaches to automatic assembly planning often lead to combinatorial explo- sion. When the parts composing the assembly increase in number, computer-aided planning be- comes much slower than manual planning. Efforts to reduce the computing time by taking into ac- count various constraints and criteria to guide the search for the optimal plan requires too much input information, so as to offset the convenience of automatic assembly planning. In addition, as the planner becomes more complicated, such efforts often fail to reach the objective. This paper presents a new concep── unit , asserting that the intemal structure of an assembly is hierachical. Every disassembly operation only handles several units, no matter how many parts are involved. Furthermore, the scenario of disassembly is brought to light. It relates to only two key data──the liaison type and the assembly direction. The computational cast of this approach is roughly propor. tional to the number of parts. A planner, implementing these principlcs can generate the optimal as- sembly plans dramatically faster than the known approaches.展开更多
Well-defined Si- and Ti-MCM-41 mesoporous molecular sieves were synthesized in high yields through the halogen anion mediated S(+)X(-)I(+) assembly in the presence of cetylpyridinium bromide as template. The spectrosc...Well-defined Si- and Ti-MCM-41 mesoporous molecular sieves were synthesized in high yields through the halogen anion mediated S(+)X(-)I(+) assembly in the presence of cetylpyridinium bromide as template. The spectroscopy characterization of the as-synthesized samples confirmed that Ti (IV) could be isolated in the lattice positions of the MCM-41 mesostructure by this method.展开更多
The attitude control system design and its control effect are affected considerably by the mass-property parameters of the spacecraft. In the mission of on-orbit servicing, as fuel is expended, or the payloads are add...The attitude control system design and its control effect are affected considerably by the mass-property parameters of the spacecraft. In the mission of on-orbit servicing, as fuel is expended, or the payloads are added or removed, the center of mass will be changed in certain axe; consequently, some thrusters＇ directions are deviated from the center of mass（CM） in certain plane. The CM of assembled spacecraft estimation and thruster direction control are studied. Firstly, the attitude dynamics of the assembled spacecraft is established based on the Newton-Euler method. Secondly, the estimation can be identified by the least recursive squares algorithm. Then, a scheme to control the thrusters＇ directions is proposed. By using the gimbal installed at the end of the boom, the angle of the thruster is controlled by driving the gimbal; therefore, thrusters can be directed to the CM again. Finally, numerical simulations are used to verify this scheme. Results of the numerical simulations clearly show that this control scheme is rational and feasible.展开更多
Combination of CO_(2) capture using inorganic alkali with subsequently electrochemical conversion of the resultant HCO_(3)^(-)to high-value chemicals is a promising route of low cost and high efficiency.The electroche...Combination of CO_(2) capture using inorganic alkali with subsequently electrochemical conversion of the resultant HCO_(3)^(-)to high-value chemicals is a promising route of low cost and high efficiency.The electrochemical reduction of HCO_(3)^(-)is challenging due to the inaccessible of negatively charged molecular groups to the electrode surface.Herein,we adopt a comprehensive strategy to tackle this challenge,i.e.,cascade of in situ chemical conversion of HCO_(3)^(-)to CO_(2) and CO_(2) electrochemical reduction in a flow cell.With a tailored Ni-N-S single atom catalyst(SACs),where sulfur(S)atoms located in the second shell of Ni center,the CO_(2)electroreduction(CO_(2)ER)to CO is boosted.The experimental results and density functional theory(DFT)calculations reveal that the introduction of S increases the p electron density of N atoms near Ni atom,thereby stabilizing^(*)H over N and boosting the first proton coupled electron transfer process of CO_(2)ER,i.e.,^(*)+e^(-)+^(*)H+^(*)CO_(2)→^(*)COOH.As a result,the obtained catalyst exhibits a high faradaic efficiency(FE_(CO)~98%)and a low overpotential of 425 mV for CO production as well as a superior turnover frequency(TOF)of 47397 h^(-1),outcompeting most of the reported Ni SACs.More importantly,an extremely high FECOof 90%is achieved at 50 mA cm^(-2)in the designed membrane electrode assembly(MEA)cascade electrolyzer fed with liquid bicarbonate.This work not only highlights the significant role of the second coordination on the first coordination shell of the central metal for CO_(2)ER,but also provides an alternative and feasible strategy to realize the electrochemical conversion of HCO_(3)^(-)to high-value chemicals.展开更多
The formation of self assembled CdSe quantum dots under Stranski Krastanow (S K) mode by low pressure metalorganic chemical vapor deposition (LP MOCVD) was reported for the first time. The samples were grown directly ...The formation of self assembled CdSe quantum dots under Stranski Krastanow (S K) mode by low pressure metalorganic chemical vapor deposition (LP MOCVD) was reported for the first time. The samples were grown directly on GaAs (100) surfaces by LP MOCVD. DimethylSelenide (DMSe) and DimethylCadmium (DMCd) were used as precursors. The growth pressure was kept at 2 93×10 4Pa and the growth temperature was 500℃. CdSe with the thickness of about 2 monolayers was grown directly on GaAs (100) surfaces. For the purpose of AFM observation, this uncapped sample was cooled down immediately to room temperature and was monitored under a Digital Instruments Nanoscope Ⅲa system at the same day of growth. The AFM images show that the average diameter, height and density of those self assembled CdSe quantum dots are 50±15nm, 13±4nm and 5μm -2 , respectively. And those dots’ diameter height ratio is about 4～5, just the same as those results observed in other Ⅱ Ⅵ and Ⅲ Ⅵ compounds which were grown under S K mode by MBE.展开更多
文摘Assembly model plans an important role in assembly sequence planning.Flexible assembly system (FAS) is a dis- crete event system (DES),so Petri net,as an important analytical tool for the DES,is used for modeling of an FAS.All of the feasible assembly sequences can be obtained by firing the Petri net model.In order to enhancing the efficiency of the assembly sequence planning,knowledge-based Petri net,combining an usual Petri net with expert’s knowledge and experiences,is proposed to con- struct the assembly model.So the feasible assembly sequences can be reduced greatly,and the optimum assembly sequence could be generated without the combinatorial explosion of solutions.
文摘Existing approaches to automatic assembly planning often lead to combinatorial explo- sion. When the parts composing the assembly increase in number, computer-aided planning be- comes much slower than manual planning. Efforts to reduce the computing time by taking into ac- count various constraints and criteria to guide the search for the optimal plan requires too much input information, so as to offset the convenience of automatic assembly planning. In addition, as the planner becomes more complicated, such efforts often fail to reach the objective. This paper presents a new concep── unit , asserting that the intemal structure of an assembly is hierachical. Every disassembly operation only handles several units, no matter how many parts are involved. Furthermore, the scenario of disassembly is brought to light. It relates to only two key data──the liaison type and the assembly direction. The computational cast of this approach is roughly propor. tional to the number of parts. A planner, implementing these principlcs can generate the optimal as- sembly plans dramatically faster than the known approaches.
文摘Well-defined Si- and Ti-MCM-41 mesoporous molecular sieves were synthesized in high yields through the halogen anion mediated S(+)X(-)I(+) assembly in the presence of cetylpyridinium bromide as template. The spectroscopy characterization of the as-synthesized samples confirmed that Ti (IV) could be isolated in the lattice positions of the MCM-41 mesostructure by this method.
基金supported by the National Natural Science Foundation of China(11302010)
文摘The attitude control system design and its control effect are affected considerably by the mass-property parameters of the spacecraft. In the mission of on-orbit servicing, as fuel is expended, or the payloads are added or removed, the center of mass will be changed in certain axe; consequently, some thrusters＇ directions are deviated from the center of mass（CM） in certain plane. The CM of assembled spacecraft estimation and thruster direction control are studied. Firstly, the attitude dynamics of the assembled spacecraft is established based on the Newton-Euler method. Secondly, the estimation can be identified by the least recursive squares algorithm. Then, a scheme to control the thrusters＇ directions is proposed. By using the gimbal installed at the end of the boom, the angle of the thruster is controlled by driving the gimbal; therefore, thrusters can be directed to the CM again. Finally, numerical simulations are used to verify this scheme. Results of the numerical simulations clearly show that this control scheme is rational and feasible.
基金financially supported by the Natural Science Foundation of Shandong Province (ZR2020QB132,ZR2020MB025)the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL202108SIC)the Taishan Scholar Program of Shandong Province (ts201712046)。
文摘Combination of CO_(2) capture using inorganic alkali with subsequently electrochemical conversion of the resultant HCO_(3)^(-)to high-value chemicals is a promising route of low cost and high efficiency.The electrochemical reduction of HCO_(3)^(-)is challenging due to the inaccessible of negatively charged molecular groups to the electrode surface.Herein,we adopt a comprehensive strategy to tackle this challenge,i.e.,cascade of in situ chemical conversion of HCO_(3)^(-)to CO_(2) and CO_(2) electrochemical reduction in a flow cell.With a tailored Ni-N-S single atom catalyst(SACs),where sulfur(S)atoms located in the second shell of Ni center,the CO_(2)electroreduction(CO_(2)ER)to CO is boosted.The experimental results and density functional theory(DFT)calculations reveal that the introduction of S increases the p electron density of N atoms near Ni atom,thereby stabilizing^(*)H over N and boosting the first proton coupled electron transfer process of CO_(2)ER,i.e.,^(*)+e^(-)+^(*)H+^(*)CO_(2)→^(*)COOH.As a result,the obtained catalyst exhibits a high faradaic efficiency(FE_(CO)~98%)and a low overpotential of 425 mV for CO production as well as a superior turnover frequency(TOF)of 47397 h^(-1),outcompeting most of the reported Ni SACs.More importantly,an extremely high FECOof 90%is achieved at 50 mA cm^(-2)in the designed membrane electrode assembly(MEA)cascade electrolyzer fed with liquid bicarbonate.This work not only highlights the significant role of the second coordination on the first coordination shell of the central metal for CO_(2)ER,but also provides an alternative and feasible strategy to realize the electrochemical conversion of HCO_(3)^(-)to high-value chemicals.
文摘The formation of self assembled CdSe quantum dots under Stranski Krastanow (S K) mode by low pressure metalorganic chemical vapor deposition (LP MOCVD) was reported for the first time. The samples were grown directly on GaAs (100) surfaces by LP MOCVD. DimethylSelenide (DMSe) and DimethylCadmium (DMCd) were used as precursors. The growth pressure was kept at 2 93×10 4Pa and the growth temperature was 500℃. CdSe with the thickness of about 2 monolayers was grown directly on GaAs (100) surfaces. For the purpose of AFM observation, this uncapped sample was cooled down immediately to room temperature and was monitored under a Digital Instruments Nanoscope Ⅲa system at the same day of growth. The AFM images show that the average diameter, height and density of those self assembled CdSe quantum dots are 50±15nm, 13±4nm and 5μm -2 , respectively. And those dots’ diameter height ratio is about 4～5, just the same as those results observed in other Ⅱ Ⅵ and Ⅲ Ⅵ compounds which were grown under S K mode by MBE.
