KEYWORDS: Sensors, Liquids, Temperature sensors, Prototyping, Microfluidics, Computing systems, Signal detection, Resistors, Physics, Temperature metrology
We report a novel prototype of a two-dimensional micromachined accelerometer based on thermocapillary heat transfer. The only moving element in this accelerometer is a thermal bubble generated by vaporizing the working liquid in the microchamber. The accelerometer consists of two key components, a heating resistor for creating symmetrical temperature profiles and two pairs of temperature detectors placed symmetrically and orthogonally to each other. The prototype devices demonstrate that a sensitivity of 1.5°C/g for an operating power of 60 mW can be realized. The frequency response of the accelerometer containing deionized water is 200 Hz, and the corresponding noise equivalent acceleration is ~1 mg/Hz1/2.
We demonstrate a closed-loop adaptive control scheme for achieving accurate positioning and trajectory tracking of an electrostatically driven torsional micromirror. Compared to the conventional proportional-integral-derivative (PID) controller, the proposed adaptive self-tuning controller has advantages of on-line compensating parameter variations and model uncertainty of the torsional micromirror, resulting from fabrication imperfections. Numerical simulation results utilizing MATLAB indicate that the proposed adaptive controller has a better transient response and can more precisely follow the reference trajectory, compared to the PID control scheme. The torsional micromirror is designed and successfully fabricated using surface micromachining processes. Real-time experimental results demonstrate that the proposed adaptive control is feasible and can improve the performance of the micromirror.
An adaptive control scheme to achieve accurate positioning and trajectory tracking of torsional micromirror is presented in this study. The torsional micromirror is fabricated by using surface micromachining processes, in which phosphorusdoped polysilicon is employed as the structure layer as well as the bottom electrode. Generally, every fabrication step contributes to imperfections in micromirror. The proposed adaptive self-tuning controller has advantages of on-line compensating parameter variations or model uncertainty of the torsional micromirror, resulting from fabrication
imperfections that produce asymmetric structures, misalignment of actuation mechanism, and deviations of the center of mass from the geometric center. In our design, the amount of detection of differential capacitance between the left and right electrodes at the femtofarad (fF) level is utilized as feedback signals. Simulation results show that the designed controller has better transient response compared to the PID control scheme. The micromirror can follow the reference trajectory (5 kHz) with acceptable error in several microseconds, thus the convergence of the controller is confirmed. Furthermore, the unknown model parameters can be identified correctly while the so-called persistent excitation
condition is satisfied.
A surface-micromachined focusing mirror with variable focal length, which is controlled by adjusting the mirror’s curvature, is fabricated and characterized. The bowl-shaped micromirror, which is fabricated from the micro bilayer circular plate, focuses light beam through thermal actuation of the external heat source. Both the initial and operational curvatures are manipulated by the residual stresses in two layers of the mirror. Improper stresses would lead to the failure of the bowl-shaped structure. We analyze and design geometrical dimensions for simultaneously avoiding the structure failure and increasing the tuning range of the focal length. The interferometer has been used to measure the focal length and the focusing ability. Mirrors with nominal focal lengths approximately 730 μm, and tuning ranges of about 50 microns were demonstrated. The measurement directly through optical approach has also been tried, but requires further investigation, because the laser beam affects the focusing of the micromirror seriously.
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