Zinc oxide is a promising a wide band gap material for optoelectronic applications. Alloying ZnO with MgO gives the possibility of band gap tuning and fabrication of engineered heterostructures for optoelectronic applications. In this paper we report the growth of c-axis oriented crystalline Zn<sub>1-x</sub>Mg<sub>x</sub>O thin films on amorphous silica and p-type silicon (100) substrates by pulsed laser deposition. The dependence of optical properties on Mg content in the Zn<sub>1-x</sub>Mg<sub>x</sub>O films were investigated. All the films are highly transparent in the visible region. The scanning electron microscope (SEM) images shows the films are very smooth. Heterojunction diodes were fabricated by depositing n-type Zn<sub>1-x</sub>Mg<sub>x</sub>O on p-type silicon. Rectification is observed with a ratio of forward to reverse current as high as 1000 in the range -5 to +5 V. The forward bias current-voltage characteristics indicate the current is dominated by single carrier injection in to the p-Si. The capacitance measurements show a strong frequency dispersion, which can be attributed to the traps at the interface.
Hysteresis in smart materials hinders the wider applicability of such materials in actuators. In this paper, a systematic approach for coping with hysteresis is presented. The method is illustrated through the example of controlling a commercially available magnetostrictive actuator. We utilize the low-dimensional model for the magnetostrictive actuator that was developed in earlier work. For low frequency inputs, the model approximates to a rate-independent hysteresis operator, with current as its input and magnetization as its output. Magnetostrictive strain is proportional to the square of the magnetization. In this paper, we use a classical Preisach operator for the rate-independent hysteresis operator. In this paper, we present the results of experiments conducted on a commercial magnetostrictive actuator, the purpose of which was the control of the displacement/strain output. A constrained least-squares algorithm is employed to identify a discrete approximation to the Preisach measure. We then discuss a nonlinear inversion algorithm for the resulting Preisach operator, based on the theory of strictly-increasing operators. This algorithm yields a control input signal to produce a desired magnetostrictive response. The effectiveness of the inversion scheme is demonstrated via an open-loop trajectory tracking experiment.
A conventional Zernike filter measures wavefront phase by superimposing the aberrated input beam with a phase-shifted version of its zero-order spectral component. The Fourier- domain phase-shifting is performed by a fixed phase-shifting dot on a glass slide in the focal plane of a Fourier- transforming lens. Using an optically-controlled phase spatial light modulator (SLM) instead of the fixed phase-shifting dot, we have simulated and experimentally demonstrated a nonlinear Zernike filter robust to wavefront tilt misalignments. In the experiments, a liquid-crystal light valve (LCLV) was used as the phase SLM. The terminology 'nonlinear' Zernike filter refers to the nonlinear filtering that takes place in the Fourier domain due to the phase change for field spectral components being proportional to the spectral component intensities. Because the Zernike filer output intensity is directly related to input wavefront phase, a parallel, distributed feedback system can replace the wavefront reconstruction calculations normally required in adaptive- optic phase correction systems. Applications include high- resolution phase distortion suppression for atmospheric turbulence, optical phase microscopy, and compensation of aberrations in optical system components. A factor of eight improvement in Strehl ratio was obtained experimentally, and simulation results suggest that even better performance could be obtained by replacing the LCLV with a more sophisticated optically-controlled phase SLM.
Computational micromagnetics plays an important role in design and control of magnetostrictive actuators. A systematic approach to calculate magnetic dynamics and magnetostriction is presented. A finite difference method is developed to solve the coupled Landau-Lifshitz-Gilbert (LLG) equation for dynamics of magnetization and a one dimensional elastic motion equation. The effective field in the LLG equation consists of the external field, the demagnetizing field, the exchange field, and the anisotropy field. A hierarchical algorithm using multipole approximation speeds up to the evaluation of the demagnetizing field, reducing computational cost from O(N<SUP>2</SUP>) to O(NlogN). A hybrid 3D/1D rod model is adopted to compute the magnetostriction: a 3D model is used in solving the LLG equation for the dynamics of magnetization; then assuming that the rod is along z-direction, we take all cells with same z-coordinate as a new cell. The values of the magnetization and the effective field of the new cell are obtained from averaging those of the original cells that the new cell contains. Each new cell is represented as a mass- spring in solving the motion equation. Numerical results include: (1) domain wall dynamics, including domain wall formation and motion; (2) effects of physical parameters, grid geometry, grid refinement and field step on H - M hysteresis curves; (3) magnetostriction curve.
Computational micromagnetics in three dimensions is of increasing interest with the development of magnetostrictive sensors and actuators. In solving the Landau-Lifshitz-Gilbert (LLG) equation, the governing equation of magnetic dynamics for ferromagnetic materials, we need to evaluate the effective field. The effective field consists of several terms, among which the demagnetizing field is of long-range nature. Evaluating the demagnetizing field directly requires work of O(N<SUP>2</SUP>) for a grid of N cells and thus it is the bottleneck in computational micromagnetics. A fast hierarchical algorithm using multipole approximation is developed to evaluate the demagnetizing field. We first construct a mesh hierarchy and divide the grid into boxes of different levels. The lowest level box is the whole grid while the highest level boxes are just cells. The approximate field contribution from the cells contained in a box is characterized by the box attributes, which are obtained via multipole approximation. The algorithm computes field contributions from remote cells using attributes of appropriate boxes containing those cells, and it computes contributions from adjacent cells directly. Numerical results have shown that the algorithm requires work of O(NlogN) and at the same time it achieves high accuracy. It makes micromagnetic simulation in three dimensions feasible.
In previous work we had proposed a low (6) dimensional model for a thin magnetostrictive actuator that was suitable for real-time control. One of the main results of this modeling effort was the separation of the rate-independent hysteretic effects from the rate-dependent linear effects. The hysteresis phenomenon may also be captured by a (modified) Preisach operator with the magnetic field H as the input. If one can find an inverse for the Preisach operator, then the composite system can be approximately linearized. In this paper, we complete the proof of the existence of an inverse theorem due to Brokate and Sprekels and propose a new algorithm for computation of the inverse. Previous algorithms used linearization of the operating point. As numerical differentiation is involved, this approach can cause divergence. Our algorithm does not linearize the Preisach operator, but makes use of its monotone increasing property. Convergence of the algorithm is proved using the contraction mapping principle.
In this paper, we discuss a smart motor concept, in which piezoelectric and magnetostrictive actuators forming a resonance electric circuit function together to produce bidirectional motion of a steel drum. Resonance frequency is set at 4 KHz, thereby enabling operation of the motor at reasonably high frequencies.
We investigate the prospects for intelligent control of smart composites (containing sensors, actuators, power supply, and signal conditioning) that are envisioned for applications in rotorcraft systems (rotor blades, power shafts, fuselage shell). This paper is concerned with multi-dimensional wavelets and relevant heuristic procedures for fast and parsimonious identification. We also discuss some control techniques based on the idea of a homogeneous system model.