Browsing by Author "Shih, YT"

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A novel cross-coupling control design for Bi-axis motion
(2002-11-01)
Shih, YT
;
Chen, CS
;
Lee, AC
;
機械工程學系
;
Department of Mechanical Engineering
A new structure of cross-coupling controller for precise tracking in motion control is proposed in this paper. When compared with the conventional cross-coupling system, this new structure has the advantage that the compensators in CCC have a simpler design process than conventional ones and so does its stability analysis. The proposed compensator (or controller) is evaluated and compared experimentally with a traditional uncoupled controller on a microcomputer controlled dual-axis positioning system. The experimental results show that the new structure of cross-coupling controller remarkably reduces contour error. In addition, this new controller can be implemented easily on a majority of motion systems in use today via reprogramming the reference position command subroutine. (C) 2002 Elsevier Science Ltd. All rights reserved.
Formation of self-assembled ZnTe quantum dots on ZnSe buffer layer grown on GaAs substrate by molecular beam epitaxy
(2002-07-01)
Kuo, MC
;
Yang, CS
;
Tseng, PY
;
Lee, J
;
Shen, JL
Self-assembled ZnTe quantum dot structures were grown by molecular beam epitaxy on GaAs substrates with a 200 nm ZnSe buffer layer. Surface morphology was studied by atomic force microscopy. A three-dimensional Volmer-Weber growth mode was identified. Two types of dots were observed. Strong photoluminescence observed at 1.9-2.2 eV was attributed to emission from the large type II ZnTe quantum dots. Emission from the smaller ZnTe quantum dots was observed at an energy of around 2.26 eV. The density of the larger and smaller dots was approximately 10(8)/cm(2) and 10(9)/cm(2), respectively. (C) 2002 Elsevier Science B.V. All rights reserved.
Hindered and modulated rotational states and spectra of adsorbed diatomic molecules
(1996-10-15)
Shih, YT
;
Chuu, DS
;
Mei, WN
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電子物理學系
;
光電工程研究所
Both vertical and horizontal adsorption configurations of a diatomic molecule were modeled as the rigid rotor with which thr spatial motion was confined by a finite conical well. In addition to the polar hindering potential, a sinusoidal azimuthal modulation. which bears the local symmetry of the adsorption site, was incorporated. Eigenfunctions for different models were expressed analytically in terms of the hypergeometric functions, and eigenvalues were solved numerically. We found that the rotational energy levels exhibit oscillatory behavior when plotted as functions of the hindrance angle. This particular phenomenon was interpreted as the occurrence of resonance transmission of the rotor wave function at certain hindrance condition. We also found that the rotational levels were grouped into bands when the azimuthal modulation strength was increased. The solutions were used to calculate the rotational-state distribution of desorbed molecules, and agreement with the previous experiment was obtained.
Hindered rotation and external electric field effect of adsorbed diatomic molecules
(1996-09-01)
Shih, YT
;
Chunu, DS
;
Mei, WN
;
電子物理學系
;
光電工程學系
A finite conical well was proposed to model the surface hindering potential to which a horizontally adsorbed molecule is subjected. Eigenfunctions and eigenvalues for this model were presented. In addition to the hindering rotation, the external electric field effect on the rotational states of a horizontally adsorbed dipole molecule was also investigated by variational calculation. We found that, as with our previous result of the vertical configuration, the rotational energy Levels of the horizontally adsorbed molecule exhibit oscillatory behavior when plotted as functions of the hindrance angle. On the other hand, our result showed that the Stark shifts of the rotational energies were suppressed by the conical well if the field strength is not comparable with the hindering potential. However, when the applied held is strong, large negative shifts were found. Furthermore, the plotted angular distributions of wave function showed that a very strong electric field will enduce a change of adsorption configuration from horizontal to vertical. Copyright (C) 1996 Published by Elsevier Science Ltd
Optical properties of Zn1-xCdxSe epilayers grown on (100)GaAs by molecular beam epitaxy
(2004-08-01)
Kuo, MC
;
Chiu, KC
;
Shih, TH
;
Lai, YJ
;
Yang, CS
Zn1-xCdxSe epilayers were grown on (100) GaAs substrates by molecular beam epitaxy. Lattice constants of the epilayers were measured by (004) rocking curve X-ray diffraction. A full width at half maximum of 475 to 2100 arcsec was obtained. The dependence of the energy gap on temperature, measured by the photoluminescence (PL) spectra, was fitted by Varshni's [Y. P. Varshni: Physica 34 149 (1967)] and O'Donnell's [R. P. O'Donnell and X. Chen: Appl. Phys. Lett. 58 2924 (1991)] models. The fitting parameters beta (161 K to 368K) and (hv) (13 meV to 24 meV), related to phonon energy, were obtained from Varshni and O'Donnell fits, respectively. The activation energies calculated from the integrated PL intensity versus inverse temperature decrease as the Cd content increases. The broadening of the PL linewidth with temperature was fitted by Gamma(T) = Gamma(0) + Gamma(a)T + Gamma(LOI)/[exp(homega(LOI)/kT) - 1] + Gamma(LO2)/[exp(homega(LO2)/kT) - 1] + Gamma(i)exp(-/kT). The impurity binding energy, (E-b), was found to decrease as the Cd composition increases.
Path planning for CNC contouring around a corner
(2004-03-01)
Shih, YT
;
Chen, CS
;
Lee, AC
;
機械工程學系
;
Department of Mechanical Engineering
Corner motion is a common and important case in contouring applications. If the corner error is well-controlled, a machine can own maximal throughput while maintaining the machining accuracy. In this paper, we investigated corner error yielded by different Acc/Dec processing in the command generation phase. At first, we derive some analytical forms of corner error induced by Acc/Dec processing; both linear and FIR Acc/Dec types are studied. Then a simple method of controlling the corner error without changing the Acc/Dec structure is presented. The method is simply to add some delay time to retard the execution of the second linear command. Although some results here do not explicitly present their final exact analytic expressions, their numerical solutions can be easily solved.
Photoluminescence of ZnSexTe1-x/ZnTe multiple-quantum-well structures grown by molecular-beam epitaxy
(2004-12-15)
Shih, YT
;
Tsai, YL
;
Yuan, CT
;
Chen, CY
;
Yang, CS
This work investigates photoluminescence (PL) spectra from ZnSexTe1-x/ZnTe multiple-quantum-well structures grown on GaAs(001) substrates by molecular-beam epitaxy. The PL data reveal that the band alignment of the ZnSexTe1-x/ZnTe system is type II. The thermal activation energy for quenching the PL intensity was determined from the temperature-dependent PL spectra. The activation energy was found to increase initially and then decrease as the thickness of the ZnSexTe1-x layers decreases from 7 to 3 nm. The temperature-dependent broadening of the PL linewidth was also investigated. The LO-phonon scattering was found to be the dominant broadening mechanism. (C) 2004 American Institute of Physics.
Rotational states of an adsorbed dipole molecule in an external electric field
(2003-08-15)
Shih, YT
;
Liao, YY
;
Chuu, DS
;
電子物理學系
;
Department of Electrophysics
The rotational states of an adsorbed dipole molecule in an external electric field were investigated. The surface hindering potential was modeled as a finite conical well and a dipole-field interaction was added to the hindering potential. The molecular wave functions were expressed in terms of the eigenfunctions of molecular hindered rotation in the absence of electric field. Eigenenergies were determined by the matrix diagonalization procedures. Our results showed that, for both vertically and horizontally adsorbed molecules, there is avoided crossing between two adjacent rotational energy levels, as the field strength is increased, and finally all state energies decrease rapidly as the field strength is strong enough. The avoided crossing is due to the redistribution of wave function between different potential well regions. By employing the sudden unhindrance approximation, the rotational-state distributions of molecules desorbing from a solid surface in the presence of external electric field were calculated. Our results showed that the rotational-state distributions are significantly influenced by the external electric field. Since the electric field increases the ground-state energy of adsorbed molecule, the distribution shifts towards the high-J region if the electric field is applied to orient the molecular axis against the molecular preferred orientation. On the contrary, the distribution shifts towards the low-J region if the electric field is applied to orient the molecular axis towards molecular preferred orientation because the electric field decreases the ground-state energy of adsorbed molecule. The solutions to the finite conical well were also used to calculate the rotational alignment in the photodesorption of CO from Cr2O3(0001). Our results showed that at low-J values the CO molecules desorb like a helicopter, while at high-J values a cartwheel-like motion is preferred. This result is in qualitative agreement with the experimental observation.
Sensor and actuator placement for modal identification
(1998-09-01)
Shih, YT
;
Lee, AC
;
Chen, JH
;
機械工程學系
;
Department of Mechanical Engineering
In modal identification, the use of different actuator and sensor locations results in different frequency response functions (FRFs) and consequently affects the modes shown on the FRF plots. This paper presents a method of placing sensors and actuators to obtain reliable measured data for estimating the modal parameters. The method is based on proposed controllability and observability measures for second-order linear systems, which are suitable for identification purposes and have a physical interpretation on the FRF plots. These measures, combined with an objective criterion, provide a method for ranking the effectiveness of alternative actuator and sensor distributions and hence a rational basis for choosing their locations. The method is applied successfully to the selection of actuator and sensor locations for a set of target modes for the structural characterisation of a simulated system. Finally, an iterative procedure for performing modal surveys is proposed. (C) 1998 Academic Press.