Chau Fook Siong
Email Address
engcfs@nus.edu.sg
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180 results
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Publication Improved white-light reconstruction of shearograms for simultaneous measurement of displacement derivatives(1994) Ng, T.W.; Leow, C.Y.; Chau, F.S.; Toh, S.L.; MECHANICAL & PRODUCTION ENGINEERINGA white-light set-up using fibre-optic waveguides has recently been designed for improved reconstruction of shearograms. In this design note, an adapted version of the set-up is demonstrated to facilitate reconstruction of shearograms for simultaneous measurement of displacement derivatives.Publication Macro-modelling of a double-gimballed electrostatic torsional micromirror(2003-09) Zhou, G.; Tay, F.E.H.; Chau, F.S.; MECHANICAL ENGINEERINGThis paper presents the development of a reduced-order macro-model for the double-gimballed electrostatic torsional micromirror using the hierarchical circuit-based approach. The proposed macro-model permits extremely fast simulation while providing nearly FEM accuracy. The macro-model is coded in MAST analog hardware description language (AHDL), and the simulations are implemented in the SABER™ simulator. Both the static and dynamic behaviour of the double-gimballed electrostatic torsional micromirror have been investigated. The dc and frequency analysis results obtained by the proposed macro-model are in good agreement with CoventorWare™ finite element analysis results. Based on the macro-model we developed, system-level simulation of a closed-loop controlled double-gimballed torsional micromirror is also performed. Decentralized PID controllers are proposed for the control of the micromirror. A sequential-loop-closing method is used for tuning the multiple control loops during the simulation. After tuning, the closed-loop controlled double-gimballed torsional micromirror demonstrates an improved transient performance and satisfactory disturbance rejection ability.Publication A 50kHz micromachined electrostatic driven vibratory grating laser scanner(2011) Du, Y.; Zhou, G.; Chau, F.S.; Cheo, K.L.; Zhang, Q.; Feng, H.; MECHANICAL ENGINEERINGA 50kHz micromachined electrostatic driven vibratory grating laser scanner has been successfully developed for high-speed laser scanning applications. Unlike the conventional grating platform having uniform thickness across the entire diffraction surface, the grating platform in this study is thinned and reinforced by a circular frame, which is used to reduce its rotational inertia and keep its rigidity. The scanner is fabricated using a simple silicon-on-insulator (SOI) technology based micromachining process with only 4 photo masks used. This paper presents the design, simulation, fabrication process and experimental measurement results of the high-speed vibratory grating scanner. The prototype scanner with a 1mm diameter diffraction grating is capable of scanning at 50.192kHz with and optical scan angle of 14.1° when illuminated with a 635nm wavelength incident laser beam. © 2011 Published by Elsevier B.V.Publication Double-layered vibratory grating scanners for high-speed high-resolution laser scanning(2010-10) Du, Y.; Zhou, G.; Cheo, K.L.; Zhang, Q.; Feng, H.; Chau, F.S.; MECHANICAL ENGINEERINGA novel micromachined electrostatic double-layered vibratory grating scanner has been successfully developed for high-speed high-resolution laser scanning applications. This paper presents its design, modeling, fabrication, and measurement results. A comprehensive dynamic model considering the geometric nonlinearity of the platform suspension flexures is also proposed to predict the dynamic performance of the device at large scanning amplitudes. Compared with previously reported single-layered vibratory grating scanners, double-layered scannersin which the diffraction grating and its driving actuator are located in different layershave the potential to scan at large amplitudes and at high scanning speeds with large aperture sizes. We have demonstrated a prototype with a 2-mm-diameter diffraction grating which is capable of scanning at 23.391 kHz with an optical scan angle of around 33 ° and a resulting θoptical D product (product of the optical scan angle and diameter of the diffraction grating) of 66 ° ̇mm. © 2006 IEEE.Publication Flaw detection and characterization using shearography(1997) Toh, Siew-Lok; Chau, Fook-Siong; MECHANICAL & PRODUCTION ENGINEERINGShearography is a laser interferometric method developed originally for full field observation of surface strains of components. Since flaws usually induce strain concentrations around them, shearography can be employed to detect the flaws. Conventional shearography involves exposing high resolution films before and after the components are loaded. The exposed films are developed and then viewed via a high-pass filtering optical setup. Though the images obtained are good, this method is time-consuming. With the advent of high-speed computers, associated sophisticated imaging hardware and software, the Digital Speckle Shearing Interferometry (DSSI) method which employs a CCD (charged-coupled device) camera and computer image processing to produce the interferometric fringe patterns has been developed. In contrast with the conventional shearography, the electronic version does not require any film and is faster. The techniques are used to detect and characterize (a) flaws simulating delaminations in composites and (b) thinning in pipes.Publication Flaw detection in composites using time-average shearography(1991-02) Toh, S.L.; Shang, H.M.; Chau, F.S.; Tay, C.J.; MECHANICAL & PRODUCTION ENGINEERINGResults on flaw detection of glass fibre reinforced plastic beams using time- average shearography are presented here. Detection and sizing of flaws such as debonds or delaminations are successfully carried out using this technique. For easy detection of flaws, the component has to be excited at the resonance frequencies of the flaws. As the flaw gets smaller, a higher frequency is required. For flaws of the same exterior size, a deeper one will also require a higher frequency. © 1991.Publication A 3mm endoscopic probe with integrated MEMS micromirror for optical coherence tomography bioimaging(2010) Mu, X.J.; Zhou, G.Y.; Feng, H.H.; Xu, Y.S.; Yu, A.B.; Tan, C.W.; Chen, K.W.S.; Xie, J.; Chau, F.S.; MECHANICAL ENGINEERINGA 3mm diameter endoscopic probe for Optical Coherence Tomography (OCT) has been developed. It consists of a 1mm diameter micromirror, a silicon optical bench (SiOB), a gradient refractive index (GRIN) lens and a single mode optical fiber. The optical probe is enclosed within a biocompatible, transparent and waterproof polycarbonate tube for in vivo diagnostic applications. The two-axis scanning micromirror is driven by electrothermal actuators with a voltage less than 2V for a mechanical deflection of up to 11° . By integrating the MEMS micromirror with a commercial Optical Coherence Tomography system and miniaturized probe, high resolution 2-D/3-D in vivo and in situ images and excellent optical sectioning for imaging multilayer microstructures of internal organs can be realized. The capability of the system is demonstrated by obtaining a 2-D image of an onion.Publication Discrete dynamic resolution considerations in digital speckle correlation interferometry(1994) Ng, T.W.; Chau, F.S.; MECHANICAL & PRODUCTION ENGINEERINGIn digital speckle correlation interferometry, the discrete dynamic resolution of the recording determines the degree of accuracy in deriving the deformation related phase. In this letter, limiting relationships between truncation errors, conditions of saturation, self-interference levels and fringe contrast factors are established in relation with the constraints of this factor.Publication An electromagnetically driven lamellar grating based Fourier transform microspectrometer(2008-05-01) Hongbin, Y.; Guangya, Z.; Siong, C.F.; Feiwen, L.; Shouhua, W.; Mingsheng, Z.; MECHANICAL ENGINEERINGA lamellar grating based Fourier transform micro-spectrometer is presented in this paper in which an electromagnetic actuator is used to drive the mobile facets of the lamellar grating to move bi-directionally. This configuration allows double-sided interferograms to be recorded, thus avoiding problems associated with zero optical path difference (OPD) identification and at the same time satisfying the requirements for large OPD scans. Based on the experimental results, a large deflection of 62.5 νm is achieved when the electromagnetic microactuator is driven by a current of 129 mA. To demonstrate the system performance, the spectrum of the combined light beam of a He-Ne laser and a diode-pumped solid-state (DPSS) laser was measured. The two different wavelength components introduced were distinguished with a full width at half maximum (FWHM) spectral resolution around 3.8 nm at 632.8 nm and 3.44 nm at 532 nm. © 2008 IOP Publishing Ltd.Publication Diffraction grating scanner using a micromachined resonator(2004) Zhou, G.; Logeeswaran, V.J.; Tay, F.E.H.; Chau, F.S.; MECHANICAL ENGINEERINGWe present a micromachined resonant diffraction grating laser scanner that utilizes in-plane angular vibration of a planar diffraction grating causing the diffracted laser beam to scan. The scanner consists of an electrostatic comb-drive microresonator and a diffraction grating plate having a grating period of 4 μm. The proposed micromachined diffraction grating laser scanner has the potential to scan at high frequencies without the optical performance degradation due to dynamic non-rigid-body deformation compared to out-of-plane micromirror scanners. A prototype grating scanner has been fabricated using the Poly-MUMPS process. It is capable of scanning optical angles of 1.31° (1 st-order), 2.13° (2 nd-order) and 3.42° (3 rd-order) at a resonant frequency of 7.67 kHz for a 635-nm-wavelength incident laser beam, when driven by the electrostatic combdrive push-pull mechanism with 20 V DC bias and 20 V p-p AC voltages. The scanning angle of the device can be significantly increased by reducing the grating period to sub-wavelength level.