The modeling and evaluation of a prototype rotary micromotor where the annular rotor is supported electrostatically in five degrees of freedom is presented in order to study the behavior of this levitated micromotor a...The modeling and evaluation of a prototype rotary micromotor where the annular rotor is supported electrostatically in five degrees of freedom is presented in order to study the behavior of this levitated micromotor and further optimize the device geometry. The analytical torque model is obtained based on the principle of a planar variable-capacitance electrostatic motor while the viscous damping caused by air film between the stator and rotor is derived using laminar Couette flow model. Simulation results of the closed-loop drive motor, based on the developed dynamic model after eliminating mechanical friction torque via electrostatic suspension, are presented. The effects of the high-voltage drive, required for rotation of the rotor, on overload capacity and suspension stiffness of the electrostatic bearing system are also analytically evaluated in an effort to determine allowable drive voltage and attainable rotor speed in operation. The analytical results show that maximum speed of the micromotor is limited mainly by viscous drag torque and stiffness of the bearing system. Therefore, it is expected to operate the device in vacuum so as to increase the rotor speed significantly, especially for those electrostatically levitated micromotors to be used as an angular rate micro-gyroscope.展开更多
Due to large workspace,heavy-duty and over-constrained mechanism,a small deformation is caused and the precision of the 2-DOF planar parallel manipulator is affected.The kinematic calibration cannot compensate the end...Due to large workspace,heavy-duty and over-constrained mechanism,a small deformation is caused and the precision of the 2-DOF planar parallel manipulator is affected.The kinematic calibration cannot compensate the end-effector errors caused by the small deformation.This paper presents a method combined step kinematic calibration and linear forecast real-time error compensation in order to enhance the precision of a two degree-of-freedom(DOF) planar parallel manipulator of a hybrid machine tool.In the step kinematic calibration phase of the method,the end-effector errors caused by the errors of major constant geometrical parameters is compensated.The step kinematic calibration is based on the minimal linear combinations(MLCs) of the error parameters.All simple and feasible measurements in practice are given,and identification analysis of the set of the MLCs for each measurement is carried out.According to identification analysis results,both measurement costs and observability are considered,and a step calibration including step measurement,step identification and step error compensation is determined.The linear forecast real-time error compensation is used to compensate the end-effector errors caused by other parameters after the step kinematic calibration.Taking the advantages of the step kinematic calibration and the linear forecast real-time error compensation,a method for improving the precision of the 2-DOF planar parallel manipulator is developed.Experiment results show that the proposed method is robust and effective,so that the position errors are kept to the same order of the measurement noise.The presented method is attractive for the 2-DOF planar parallel manipulator and can be also applied to other parallel manipulators with fewer than six DOFs.展开更多
The dynamic dexterity is an important issue for manipulator design, some indices were proposed for analyzing dynamic dexterity, but they can evaluate the dynamic performance just at one pose in the workspaee of the ma...The dynamic dexterity is an important issue for manipulator design, some indices were proposed for analyzing dynamic dexterity, but they can evaluate the dynamic performance just at one pose in the workspaee of the manipulator, and can't be applied to dynamic design expediently. Much work has been done in the kinematic optimization, but the work in the dynamic optimization is much less. A global dynamic condition number index is proposed and applied to the dynamic optimization design the parallel manipulator. This paper deals with the dynamic manipulability and dynamic optimization of a two degree-of-freedom (DOF) parallel manipulator. The particular velocity and particular angular velocity matrices of each moving part about the part's pivot point are derived fi'om the kinematic formulation of the manipulator, and the inertial force and inertial movement are obtained utilizing Newton-Euler formulation, then the inverse dynamic model of the parallel manipulator is proposed based on the virtual work principle. The general inertial ellipsoid and dynamic manipulability ellipsoid are applied to evaluate the dynamic performance of the manipulator, a global dynamic condition number index based on the condition number of general inertial matrix in the workspace is proposed, and then the link lengths of the manipulator is redesigned to optimize the dynamic manipulability by this index. The dynamic manipulability of the origin mechanism and the optimized mechanism are compared, the result shows that the optimized one is much better. The global dynamic condition number index has good effect in evaluating the dynamic dexterity of the whole workspace, and is efficient in the dynamic optimal design of the parallel manipulator.展开更多
The geometry of an inductively coupled plasma (ICP) etcher is usually considered to be an important factor for determining both plasma and process uniformity over a large wafer. During the past few decades, these pa...The geometry of an inductively coupled plasma (ICP) etcher is usually considered to be an important factor for determining both plasma and process uniformity over a large wafer. During the past few decades, these parameters were determined by the "trial and error" method, resulting in wastes of time and funds. In this paper, a new approach of regression orthogonal design with plasma simulation experiments is proposed to investigate the sensitivity of the structural parameters on the uniformity of plasma characteristics. The tool for simulating plasma is CFD-ACE+, which is commercial multi-physical modeling software that has been proven to be accurate for plasma simulation. The simulated experimental results are analyzed to get a regression equation on three structural parameters. Through this equation, engineers can compute the uniformity of the electron number density rapidly without modeling by CFD-ACE+. An optimization performed at the end produces good results.展开更多
Deuterated potassium dihydrogen phosphate damage performance at 351 nm is studied on a large-aperture laser system. Bulk and rear-surface damage are initiated under the 3ω fluences of 6.T J/cm2 and 33/cm2, and show d...Deuterated potassium dihydrogen phosphate damage performance at 351 nm is studied on a large-aperture laser system. Bulk and rear-surface damage are initiated under the 3ω fluences of 6.T J/cm2 and 33/cm2, and show different growth characteristics under multiple laser irradiations with the fluence of 6 J/cm2. The size and number of bulk damage keep unchanged once initiated. However, surface damage size also does not grow, while surface damage number increases linearly with laser shots. Different damage thresholds and growth behaviors suggest different formations of bulk and surface damage precursors. The cause of surface damage is supposed to be near-surface absorbing particles buried under the sol-gel coating.展开更多
To improve the diagnosis accuracy and self-adaptability of fatigue crack in ulterior place of the supporting shaft, time series and neural network are attempted to be applied in research on diag-nosing the fatigue cr...To improve the diagnosis accuracy and self-adaptability of fatigue crack in ulterior place of the supporting shaft, time series and neural network are attempted to be applied in research on diag-nosing the fatigue crack’s degree based on analyzing the vibration characteristics of the supporting shaft. By analyzing the characteristic parameter which is easy to be detected from the supporting shaft’s exterior, the time series model parameter which is hypersensitive to the situation of fatigue crack in ulterior place of the supporting shaft is the target input of neural network, and the fatigue crack’s degree value of supporting shaft is the output. The BP network model can be built and net-work can be trained after the structural parameters of network are selected. Furthermore, choosing the other two different group data can test the network. The test result will verify the validity of the BP network model. The result of experiment shows that the method of time series and neural network are effective to diagnose the occurrence and the development of the fatigue crack’s degree in ulterior place of the supporting shaft.展开更多
During dynamic walking of biped robots, the underactuated rotating degree of freedom (DOF) emerges between the support foot and the ground, which makes the biped model hybrid and dimension-variant. This paper addres...During dynamic walking of biped robots, the underactuated rotating degree of freedom (DOF) emerges between the support foot and the ground, which makes the biped model hybrid and dimension-variant. This paper addresses the asymptotic orbit stability for dimension-variant hybrid systems (DVHS). Based on the generalized Poincare map, the stability criterion for DVHS is also presented, and the result is then used to study dynamic walking for a five-link planar biped robot with feet. Time-invariant gait planning and nonlinear control strategy for dynamic walking with fiat feet is also introduced. Simulation results indicate that an asymptotically stable limit cycle of dynamic walking is achieved by the proposed method.展开更多
Scratch test and friction test were performed to evaluate the internal and external interface behaviors of TiAlSiN coating, respectively. The critical compressive and shearing stress of coating failure during scratch ...Scratch test and friction test were performed to evaluate the internal and external interface behaviors of TiAlSiN coating, respectively. The critical compressive and shearing stress of coating failure during scratch test were calculated and the values are 30.84 MPa and 4.98 MPa respectively. The average friction coefficients of TiAlSiN coating against 2Cr12Ni4Mo3VNbN steel are 0.70 (sliding speed 50 m/min), 0.63 (sliding speed 100 m/min), and 0.81 (sliding speed 150 m/min). The elements diffusion was analyzed by EDS. Al and Si element of coating material diffuse to the steel disc, except Ti element. The oxidation decreases with the increase of sliding speed, but the adhesion increases with the increase of sliding speed. More Al element diffuses to the steel disc at the high sliding speed, but the diffusion of Si element keeps almost constant at different sliding speeds.展开更多
This paper reports on a method of assembling semiconducting ZnO nanowires onto a pair of Au electrodes to construct a metal--semiconductor metal (MSM) structure by dieleetrophoresis and studying on its electrical ch...This paper reports on a method of assembling semiconducting ZnO nanowires onto a pair of Au electrodes to construct a metal--semiconductor metal (MSM) structure by dieleetrophoresis and studying on its electrical characteristics by using current-voltage (Ⅰ - Ⅴ) measurements. An electronic model with two back to back Sehottky diodes in series with a semiconductor of nanowires was established to study the electrical transport of the MSM structures. By fitting the measured Ⅰ - Ⅴ characteristics using the proposed model, the parameters of the Schottky contacts and the resistance of nanowires could be acquired. The photoelectric properties of the MSM structures were also investigated by analysing the measurements of the electrical transports under various light intensities. The deduced results demonstrate that ZnO nanowires and their Schottky contacts with Au electrodes both contribute to photosensitivity and the MSM structures with ZnO nanowires are potentially applicable for photonic devices.展开更多
In order to reduce the cost,3-PRS mechanism is introduced into the application of supporting theactive reflector unit of large radio telescope.The kinematic model of 3-PRS mechanism with rotationaljoint errors is deri...In order to reduce the cost,3-PRS mechanism is introduced into the application of supporting theactive reflector unit of large radio telescope.The kinematic model of 3-PRS mechanism with rotationaljoint errors is derived to solve the error problem in actual engineering application.Then based on the errormodel,inverse and forward kinematics are analyzed.Because the solutions can not be analytically ex-pressed,a numerical method is applied.Afterwards,the parasitic motion errors are analyzed using searchmethod and empirical formulas of the maximum parasitic motion error are put forward.Finally,the toler-ance is distributed using empirical formulas to avoid interference between adjacent reflector units.Theanalyses provide a theoretical basis for the design and installation of large radio telescope active reflector.展开更多
The recording density of multilevel photochromic memory is limited by the signal-to-noise ratio (SNR) of the readout signal. In this paper, shot noise and material noise are investigated through theoretical analysis...The recording density of multilevel photochromic memory is limited by the signal-to-noise ratio (SNR) of the readout signal. In this paper, shot noise and material noise are investigated through theoretical analysis of SNR. When the bandwidth of a system is less than 1MHz, the material noise takes a prominent position; when the bandwidth of the system is more than 10MHz, the shot noise becomes dominant. The thickness of recording layer can be optimized to maximize the SNR and reduce the influence of the bandwidth of the system on SNR.展开更多
We report recent experimental progress toward a trapped ^(113)Cd+ion microwave frequency standard.With laser cooling,a large number of trapped cadmium ions are cooled to crystallization and the temperature of the ion ...We report recent experimental progress toward a trapped ^(113)Cd+ion microwave frequency standard.With laser cooling,a large number of trapped cadmium ions are cooled to crystallization and the temperature of the ion crystal is measured to be 16±3 mK.The second order Doppler frequency shift,a main limiting factor of the atomic clock's performance,is estimated to be less than 1×10^(-16).展开更多
We present a high-efficiency 532 nm green light conversion from an external cavity-enhanced second harmonic generation(SHG)with a periodically poled KTP crystal(PPKTP).The cavity is a bow-tie ring configuration with a...We present a high-efficiency 532 nm green light conversion from an external cavity-enhanced second harmonic generation(SHG)with a periodically poled KTP crystal(PPKTP).The cavity is a bow-tie ring configuration with a unitized structure.When the impedance matching is optimized,the coupling efficiency of the fundamental is as high as 95%.Taking into account both the high power output of the second harmonic and the stability of the system,we obtain over 500 mW green passing through the output cavity mirror,corresponding to a net conversion efficiency higher than 75.2%.Under these operating conditions,the power stability is better than±0.25%during 5 h.It is the highest conversion efficiency and power stability ever produced in the bow-tie ring cavity with PPKTP for 532 nm generation.展开更多
A Stewart platform is introduced in thc 500 m aperture spherical radio telescope(FAST) as an accuracy adjustable mechanism for teed receivers. Accuracy analysis is the basis of accuracy design. However, a rapid and ...A Stewart platform is introduced in thc 500 m aperture spherical radio telescope(FAST) as an accuracy adjustable mechanism for teed receivers. Accuracy analysis is the basis of accuracy design. However, a rapid and effective accuracy analysis method for parallel manipulator is still needed. In order to enhance solution efficiency, an interval analysis method(lA method) is introduced to solve the terminal error bound of the Stewart platform with detailed solution path. Taking a terminal pose of the Stewart platform in FAST as an example, the terminal error is solved by the Monte Carlo method(MC method) by 4 980 s, the stochastic mathematical method(SM method) by 0.078 s, and the IA method by 2.203 s. Compared with MC method, the terminal error by SM method leads a 20% underestimate while the IA method can envelop the real error bound of the Stewart platform. This indicates that the IA method outperforms the other two methods by providing quick calculations and enveloping the real error bound of the Stewart platform. According to the given structural error of the dimension parameters of the Stewart platform, the IA method gives a maximum position error of 19.91 mm and maximum orientation error of 0.534°, which suggests that the IA method can be used for accuracy design of the Stewart platfbnn in FAST. The 1A method presented is a rapid and effective accuracy analysis method for Stewart platform.展开更多
Inertia match of the parallel manipulator means the ratio of the inertial load of the parallel manipulator converted to each actuator shaft and the moment of inertia of the actuator is kept within a reasonable range. ...Inertia match of the parallel manipulator means the ratio of the inertial load of the parallel manipulator converted to each actuator shaft and the moment of inertia of the actuator is kept within a reasonable range. Currently there are many studies on parallel manipulators, but few mention inertia parameters and inertia match of parallel manipulators. This paper focuses on the inertia characteristics of the 3-RRR reconfigurable planar parallel manipulator. On the basis of the inverse dynamic formulations deduced with the principle of virtual work, the inertia matrix of the 3-RRR planar parallel manipulator in the actuator space is obtained in algebraic form. Then, by unifying the dimension and averaging diagonal elements of the inertia matrix, the equivalent inertia of the parallel manipulator, which is the inertial load of the parallel manipulator converted to each actuator shaft, is determined. By transforming the inertia problem of the 3-RRR parallel manipulator into that of the serial multi-bar manipulator, the practicality of the equivalent inertia deduced by inverse dynamics is demonstrated. According to the physical meaning of the inertia equation, the manipulator is divided in to three parts. Further analysis is carried out on the contribution of each part to the equivalent inertia and their distributions in the required workspace, revealing that the passive links cannot ignored in calculating the equivalent inertia of the parallel manipulator. Finally, the inertia match for the 3-RRR reconfigurable parallel manipulator under three configurations is accomplished, and reducers are selected. The equivalent inertia calculation and the inertial match results illustrate that the inertia math is a necessary step to the design of the parallel manipulator, and inertia parameters dramatically affect dynamic performances of parallel manipulators. Besides, the equivalent inertia and inertial match principles, proposed in the paper, can be widely applied in the dynamic analysis and servomotors selecting for the parallel manipulator.展开更多
The servomotor drive turret punch press is attracting more attentions and being developed more intensively due to the advantages of high speed,high accuracy,high flexibility,high productivity,low noise,cleaning and en...The servomotor drive turret punch press is attracting more attentions and being developed more intensively due to the advantages of high speed,high accuracy,high flexibility,high productivity,low noise,cleaning and energy saving.To effectively improve the performance and lower the cost,it is necessary to develop new mechanisms and establish corresponding optimal design method with uniform performance indices.A new patented main driving mechanism and a new optimal design method are proposed.In the optimal design,the performance indices,i.e.,the local motion/force transmission indices ITI,OTI,good transmission workspace good transmission workspace(GTW) and the global transmission indices GTIs are defined.The non-dimensional normalization method is used to get all feasible solutions in dimensional synthesis.Thereafter,the performance atlases,which can present all possible design solutions,are depicted.As a result,the feasible solution of the mechanism with good motion/force transmission performance is obtained.And the solution can be flexibly adjusted by designer according to the practical design requirements.The proposed mechanism is original,and the presented design method provides a feasible solution to the optimal design of the main driving mechanism for servo punch press.展开更多
基金supported by National Hi-tech Research and Development Program of China (863 Program, Grant No. 2008AA04Z312)National Natural Science Foundation of China (Grant No. 50577036)
文摘The modeling and evaluation of a prototype rotary micromotor where the annular rotor is supported electrostatically in five degrees of freedom is presented in order to study the behavior of this levitated micromotor and further optimize the device geometry. The analytical torque model is obtained based on the principle of a planar variable-capacitance electrostatic motor while the viscous damping caused by air film between the stator and rotor is derived using laminar Couette flow model. Simulation results of the closed-loop drive motor, based on the developed dynamic model after eliminating mechanical friction torque via electrostatic suspension, are presented. The effects of the high-voltage drive, required for rotation of the rotor, on overload capacity and suspension stiffness of the electrostatic bearing system are also analytically evaluated in an effort to determine allowable drive voltage and attainable rotor speed in operation. The analytical results show that maximum speed of the micromotor is limited mainly by viscous drag torque and stiffness of the bearing system. Therefore, it is expected to operate the device in vacuum so as to increase the rotor speed significantly, especially for those electrostatically levitated micromotors to be used as an angular rate micro-gyroscope.
基金supported by National Natural Science Foundation of China(Grant No. 50805140)National Hi-tech Research and Development Program of China(863 Program,Grant No. 2007AA04Z227)
文摘Due to large workspace,heavy-duty and over-constrained mechanism,a small deformation is caused and the precision of the 2-DOF planar parallel manipulator is affected.The kinematic calibration cannot compensate the end-effector errors caused by the small deformation.This paper presents a method combined step kinematic calibration and linear forecast real-time error compensation in order to enhance the precision of a two degree-of-freedom(DOF) planar parallel manipulator of a hybrid machine tool.In the step kinematic calibration phase of the method,the end-effector errors caused by the errors of major constant geometrical parameters is compensated.The step kinematic calibration is based on the minimal linear combinations(MLCs) of the error parameters.All simple and feasible measurements in practice are given,and identification analysis of the set of the MLCs for each measurement is carried out.According to identification analysis results,both measurement costs and observability are considered,and a step calibration including step measurement,step identification and step error compensation is determined.The linear forecast real-time error compensation is used to compensate the end-effector errors caused by other parameters after the step kinematic calibration.Taking the advantages of the step kinematic calibration and the linear forecast real-time error compensation,a method for improving the precision of the 2-DOF planar parallel manipulator is developed.Experiment results show that the proposed method is robust and effective,so that the position errors are kept to the same order of the measurement noise.The presented method is attractive for the 2-DOF planar parallel manipulator and can be also applied to other parallel manipulators with fewer than six DOFs.
基金supported by National Natural Science Foundation of China (Grant No. 50605041, No. 50775125)National Basic Research Program of China (973 Program, Grant No. 2006CB705400)
文摘The dynamic dexterity is an important issue for manipulator design, some indices were proposed for analyzing dynamic dexterity, but they can evaluate the dynamic performance just at one pose in the workspaee of the manipulator, and can't be applied to dynamic design expediently. Much work has been done in the kinematic optimization, but the work in the dynamic optimization is much less. A global dynamic condition number index is proposed and applied to the dynamic optimization design the parallel manipulator. This paper deals with the dynamic manipulability and dynamic optimization of a two degree-of-freedom (DOF) parallel manipulator. The particular velocity and particular angular velocity matrices of each moving part about the part's pivot point are derived fi'om the kinematic formulation of the manipulator, and the inertial force and inertial movement are obtained utilizing Newton-Euler formulation, then the inverse dynamic model of the parallel manipulator is proposed based on the virtual work principle. The general inertial ellipsoid and dynamic manipulability ellipsoid are applied to evaluate the dynamic performance of the manipulator, a global dynamic condition number index based on the condition number of general inertial matrix in the workspace is proposed, and then the link lengths of the manipulator is redesigned to optimize the dynamic manipulability by this index. The dynamic manipulability of the origin mechanism and the optimized mechanism are compared, the result shows that the optimized one is much better. The global dynamic condition number index has good effect in evaluating the dynamic dexterity of the whole workspace, and is efficient in the dynamic optimal design of the parallel manipulator.
基金supported by Important National Science & Technology Specific Projects of China (No.2) (Nos.2009ZX02001,2011ZX02403)
文摘The geometry of an inductively coupled plasma (ICP) etcher is usually considered to be an important factor for determining both plasma and process uniformity over a large wafer. During the past few decades, these parameters were determined by the "trial and error" method, resulting in wastes of time and funds. In this paper, a new approach of regression orthogonal design with plasma simulation experiments is proposed to investigate the sensitivity of the structural parameters on the uniformity of plasma characteristics. The tool for simulating plasma is CFD-ACE+, which is commercial multi-physical modeling software that has been proven to be accurate for plasma simulation. The simulated experimental results are analyzed to get a regression equation on three structural parameters. Through this equation, engineers can compute the uniformity of the electron number density rapidly without modeling by CFD-ACE+. An optimization performed at the end produces good results.
基金Supported by the National Natural Science Foundation of China under Grant No 61505187
文摘Deuterated potassium dihydrogen phosphate damage performance at 351 nm is studied on a large-aperture laser system. Bulk and rear-surface damage are initiated under the 3ω fluences of 6.T J/cm2 and 33/cm2, and show different growth characteristics under multiple laser irradiations with the fluence of 6 J/cm2. The size and number of bulk damage keep unchanged once initiated. However, surface damage size also does not grow, while surface damage number increases linearly with laser shots. Different damage thresholds and growth behaviors suggest different formations of bulk and surface damage precursors. The cause of surface damage is supposed to be near-surface absorbing particles buried under the sol-gel coating.
基金This project is supported by National Natural Science Fundation of China (No. 50675066)Provincial Key Technologies R&D of Hunan, China (No. 05FJ2001)China Postdoctoral Science Foundation (No. 2005038006).
文摘To improve the diagnosis accuracy and self-adaptability of fatigue crack in ulterior place of the supporting shaft, time series and neural network are attempted to be applied in research on diag-nosing the fatigue crack’s degree based on analyzing the vibration characteristics of the supporting shaft. By analyzing the characteristic parameter which is easy to be detected from the supporting shaft’s exterior, the time series model parameter which is hypersensitive to the situation of fatigue crack in ulterior place of the supporting shaft is the target input of neural network, and the fatigue crack’s degree value of supporting shaft is the output. The BP network model can be built and net-work can be trained after the structural parameters of network are selected. Furthermore, choosing the other two different group data can test the network. The test result will verify the validity of the BP network model. The result of experiment shows that the method of time series and neural network are effective to diagnose the occurrence and the development of the fatigue crack’s degree in ulterior place of the supporting shaft.
基金the National Natural Science Foundation of China (No. 50575119)the 863 Program(No. 2006AA04Z253)the Ph.D.Programs Foundation of Ministry of Education of China(No. 20060003026)
文摘During dynamic walking of biped robots, the underactuated rotating degree of freedom (DOF) emerges between the support foot and the ground, which makes the biped model hybrid and dimension-variant. This paper addresses the asymptotic orbit stability for dimension-variant hybrid systems (DVHS). Based on the generalized Poincare map, the stability criterion for DVHS is also presented, and the result is then used to study dynamic walking for a five-link planar biped robot with feet. Time-invariant gait planning and nonlinear control strategy for dynamic walking with fiat feet is also introduced. Simulation results indicate that an asymptotically stable limit cycle of dynamic walking is achieved by the proposed method.
基金Supported by China Postdoctoral Science Foundation(No. 20110490380 and No. 20110490383)Dongfang Turbine Co, Ltd (No. 2011GZ011)State Key Laboratory of Tribology, Tsinghua University (No. SKLT10A01)
文摘Scratch test and friction test were performed to evaluate the internal and external interface behaviors of TiAlSiN coating, respectively. The critical compressive and shearing stress of coating failure during scratch test were calculated and the values are 30.84 MPa and 4.98 MPa respectively. The average friction coefficients of TiAlSiN coating against 2Cr12Ni4Mo3VNbN steel are 0.70 (sliding speed 50 m/min), 0.63 (sliding speed 100 m/min), and 0.81 (sliding speed 150 m/min). The elements diffusion was analyzed by EDS. Al and Si element of coating material diffuse to the steel disc, except Ti element. The oxidation decreases with the increase of sliding speed, but the adhesion increases with the increase of sliding speed. More Al element diffuses to the steel disc at the high sliding speed, but the diffusion of Si element keeps almost constant at different sliding speeds.
基金Project supported by the National Natural Science Foundation of China (Grant No 50575113)Program for NCET
文摘This paper reports on a method of assembling semiconducting ZnO nanowires onto a pair of Au electrodes to construct a metal--semiconductor metal (MSM) structure by dieleetrophoresis and studying on its electrical characteristics by using current-voltage (Ⅰ - Ⅴ) measurements. An electronic model with two back to back Sehottky diodes in series with a semiconductor of nanowires was established to study the electrical transport of the MSM structures. By fitting the measured Ⅰ - Ⅴ characteristics using the proposed model, the parameters of the Schottky contacts and the resistance of nanowires could be acquired. The photoelectric properties of the MSM structures were also investigated by analysing the measurements of the electrical transports under various light intensities. The deduced results demonstrate that ZnO nanowires and their Schottky contacts with Au electrodes both contribute to photosensitivity and the MSM structures with ZnO nanowires are potentially applicable for photonic devices.
基金the National High Technology Research and Development Programme of China(No.2006AA04Z133)the National Natural Science Foundation of China(No.50605035,10778625)
文摘In order to reduce the cost,3-PRS mechanism is introduced into the application of supporting theactive reflector unit of large radio telescope.The kinematic model of 3-PRS mechanism with rotationaljoint errors is derived to solve the error problem in actual engineering application.Then based on the errormodel,inverse and forward kinematics are analyzed.Because the solutions can not be analytically ex-pressed,a numerical method is applied.Afterwards,the parasitic motion errors are analyzed using searchmethod and empirical formulas of the maximum parasitic motion error are put forward.Finally,the toler-ance is distributed using empirical formulas to avoid interference between adjacent reflector units.Theanalyses provide a theoretical basis for the design and installation of large radio telescope active reflector.
基金Supported by the High Technology Research and Development Programme of China (No. 2006AA04Z133) and the National Natural Science Foundation of China (No. 50605035, 50510488).
基金Project supported by the State Key Development Program For Basic Research of China (Grant No 19990330) and the National Natural Science Foundation of China (Grant No 60577035).
文摘The recording density of multilevel photochromic memory is limited by the signal-to-noise ratio (SNR) of the readout signal. In this paper, shot noise and material noise are investigated through theoretical analysis of SNR. When the bandwidth of a system is less than 1MHz, the material noise takes a prominent position; when the bandwidth of the system is more than 10MHz, the shot noise becomes dominant. The thickness of recording layer can be optimized to maximize the SNR and reduce the influence of the bandwidth of the system on SNR.
基金Supported by the National Basic Research Program of China ( No. 2007CB714007) , the National Natural Science Foundation of China ( No. 50975149) , and the Important National Science & Technology Specific Projects of China (No. 2009ZX04014-.035, 2009ZX04001-042-02).
基金Supported by the National Basic Research Program of China under Grant No 2010CB922901Tsinghua University Scientific Research Initiative Program(No 20091081474).
文摘We report recent experimental progress toward a trapped ^(113)Cd+ion microwave frequency standard.With laser cooling,a large number of trapped cadmium ions are cooled to crystallization and the temperature of the ion crystal is measured to be 16±3 mK.The second order Doppler frequency shift,a main limiting factor of the atomic clock's performance,is estimated to be less than 1×10^(-16).
基金Supported by the National Basic Research Program of China(2005CB724501)the National Science&Technology Infrastructure Program of China(2006BAK03A20)the National Major Research Program of China(2010CB922902).
文摘We present a high-efficiency 532 nm green light conversion from an external cavity-enhanced second harmonic generation(SHG)with a periodically poled KTP crystal(PPKTP).The cavity is a bow-tie ring configuration with a unitized structure.When the impedance matching is optimized,the coupling efficiency of the fundamental is as high as 95%.Taking into account both the high power output of the second harmonic and the stability of the system,we obtain over 500 mW green passing through the output cavity mirror,corresponding to a net conversion efficiency higher than 75.2%.Under these operating conditions,the power stability is better than±0.25%during 5 h.It is the highest conversion efficiency and power stability ever produced in the bow-tie ring cavity with PPKTP for 532 nm generation.
基金supported by National Natural Science Foundation of China (Grant Nos. 10973023,11103046,11203048)
文摘A Stewart platform is introduced in thc 500 m aperture spherical radio telescope(FAST) as an accuracy adjustable mechanism for teed receivers. Accuracy analysis is the basis of accuracy design. However, a rapid and effective accuracy analysis method for parallel manipulator is still needed. In order to enhance solution efficiency, an interval analysis method(lA method) is introduced to solve the terminal error bound of the Stewart platform with detailed solution path. Taking a terminal pose of the Stewart platform in FAST as an example, the terminal error is solved by the Monte Carlo method(MC method) by 4 980 s, the stochastic mathematical method(SM method) by 0.078 s, and the IA method by 2.203 s. Compared with MC method, the terminal error by SM method leads a 20% underestimate while the IA method can envelop the real error bound of the Stewart platform. This indicates that the IA method outperforms the other two methods by providing quick calculations and enveloping the real error bound of the Stewart platform. According to the given structural error of the dimension parameters of the Stewart platform, the IA method gives a maximum position error of 19.91 mm and maximum orientation error of 0.534°, which suggests that the IA method can be used for accuracy design of the Stewart platfbnn in FAST. The 1A method presented is a rapid and effective accuracy analysis method for Stewart platform.
基金supported by National Hi-tech Research and Development Program of China (863 Program, Grant No. 2006AA04Z133)National Natural Science Foundation of China (Grant No. 50605035, 10778625)
文摘Inertia match of the parallel manipulator means the ratio of the inertial load of the parallel manipulator converted to each actuator shaft and the moment of inertia of the actuator is kept within a reasonable range. Currently there are many studies on parallel manipulators, but few mention inertia parameters and inertia match of parallel manipulators. This paper focuses on the inertia characteristics of the 3-RRR reconfigurable planar parallel manipulator. On the basis of the inverse dynamic formulations deduced with the principle of virtual work, the inertia matrix of the 3-RRR planar parallel manipulator in the actuator space is obtained in algebraic form. Then, by unifying the dimension and averaging diagonal elements of the inertia matrix, the equivalent inertia of the parallel manipulator, which is the inertial load of the parallel manipulator converted to each actuator shaft, is determined. By transforming the inertia problem of the 3-RRR parallel manipulator into that of the serial multi-bar manipulator, the practicality of the equivalent inertia deduced by inverse dynamics is demonstrated. According to the physical meaning of the inertia equation, the manipulator is divided in to three parts. Further analysis is carried out on the contribution of each part to the equivalent inertia and their distributions in the required workspace, revealing that the passive links cannot ignored in calculating the equivalent inertia of the parallel manipulator. Finally, the inertia match for the 3-RRR reconfigurable parallel manipulator under three configurations is accomplished, and reducers are selected. The equivalent inertia calculation and the inertial match results illustrate that the inertia math is a necessary step to the design of the parallel manipulator, and inertia parameters dramatically affect dynamic performances of parallel manipulators. Besides, the equivalent inertia and inertial match principles, proposed in the paper, can be widely applied in the dynamic analysis and servomotors selecting for the parallel manipulator.
基金supported by National Natural Science Foundation of China(Grant No.51021064)National Key Scientific and Technological Program of China(Grant No.2010ZX04004-116)
文摘The servomotor drive turret punch press is attracting more attentions and being developed more intensively due to the advantages of high speed,high accuracy,high flexibility,high productivity,low noise,cleaning and energy saving.To effectively improve the performance and lower the cost,it is necessary to develop new mechanisms and establish corresponding optimal design method with uniform performance indices.A new patented main driving mechanism and a new optimal design method are proposed.In the optimal design,the performance indices,i.e.,the local motion/force transmission indices ITI,OTI,good transmission workspace good transmission workspace(GTW) and the global transmission indices GTIs are defined.The non-dimensional normalization method is used to get all feasible solutions in dimensional synthesis.Thereafter,the performance atlases,which can present all possible design solutions,are depicted.As a result,the feasible solution of the mechanism with good motion/force transmission performance is obtained.And the solution can be flexibly adjusted by designer according to the practical design requirements.The proposed mechanism is original,and the presented design method provides a feasible solution to the optimal design of the main driving mechanism for servo punch press.