Cho Jui Tay

Piezoelectric MEMS Energy Harvester for Low-Frequency Vibrations With Wideband Operation Range and Steadily Increased Output Power

A piezoelectric MEMS energy harvester (EH) with low resonant frequency and wide operation bandwidth was designed, microfabricated, and characterized. The MEMS piezoelectric energy harvesting cantilever consists of a silicon beam integrated with piezoelectric thin film (PZT) elements parallel-arranged on top and a silicon proof mass resulting in a low resonant frequency of 36 Hz. The whole chip was assembled onto a metal carrier with a limited spacer such that the operation frequency bandwidth can be widened to 17 Hz at the input acceleration of 1.0 g during frequency up-sweep. Load voltage and power generation for different numbers of PZT elements in series and in parallel connections were compared and discussed based on experimental and simulation results. Moreover, the EH device has a wideband and steadily increased power generation from 19.4 nW to 51.3 nW within the operation frequency bandwidth ranging from 30 Hz to 47 Hz at 1.0 g. Based on theoretical estimation, a potential output power of 0.53 μW could be harvested from low and irregular frequency vibrations by adjusting the PZT pattern and spacer thickness to achieve an optimal design.

Investigation of a MEMS piezoelectric energy harvester system with a frequency-widened-bandwidth mechanism introduced by mechanical stoppers

This paper presents the design, microfabrication, modeling and characterization of a piezoelectric energy harvester (PEH) system with a wide operating bandwidth introduced by mechanical stoppers. The wideband frequency responses of the PEH system with stoppers on one side and two sides are investigated thoroughly. The experimental results show that the operating bandwidth is broadened to 18?Hz (30?48?Hz) and the corresponding optimal power ranges from 34 to 100?nW at the base acceleration of 0.6g and under top-?and bottom-stopper distances of 0.75?mm and 1.1?mm, respectively. By adjusting the mechanical stopper distance, the output power and frequency bandwidth can be optimized accordingly.

Feasibility study of a 3D vibration-driven electromagnetic MEMS energy harvester with multiple vibration modes

A novel electromagnetic energy harvester (EH) with multiple vibration modes has been developed and characterized using three-dimensional (3D) excitation at different frequencies. The device consists of a movable circular-mass patterned with three sets of double-layer aluminum (Al) coils, a circular-ring system incorporating a magnet and a supporting beam. The 3D dynamic behavior and performance analysis of the device shows that the first vibration mode of 1285 Hz is an out-of-plane motion, while the second and third modes of 1470 and 1550 Hz, respectively, are in-plane at angles of 60 ◦ (240 ◦ ) and 150 ◦ (330 ◦ ) to the horizontal (x-) axis. For an excitation acceleration of 1 g, the maximum power density achieved are 0.444, 0.242 and 0.125 μ Wc m −3 at vibration modes of I, II and III, respectively. The experimental results are in good agreement with the simulation and indicate a good potential in the development of a 3D EH device. (Some figures may appear in colour only in the online journal)

Shape measurement by use of liquid-crystal display fringe projection with two-step phase shifting

The use of an optical fringe projection method with two-step phase shifting for three-dimensional (3-D) shape measurement of small objects is described. In this method, sinusoidal linear fringes are projected onto an objects surface by a programmable liquid-crystal display (LCD) projector and a long-working-distance microscope (LWDM). The image of the fringe pattern is captured by another LWDM and a CCD camera and processed by a phase-shifting technique. Usually a minimum of three phase-shifted fringe patterns is necessary for extraction of the object shape. In this method, a new algorithm based on a two-step phase-shifting technique produces the 3-D object shape. Unlike in the conventional method, phase unwrapping is performed directly by use of an arccosine function without the need for a wrapped phase map. Hence, shape measurement can be speeded up greatly with this approach. A small coin is evaluated to demonstrate the validity of the proposed measurement method, and the experimental results are compared with those of the four-step phase-shifting method and the conventional mechanical stylus method.

Phase extraction from a single fringe pattern based on guidance of an extreme map

A method for automatic phase extraction from a single fringe pattern based on the guidance of an extreme map is introduced. The method uses an adaptive weighted filter to reduce noise and enhance contrast and to locate the fringe extremes. Wrapped phase values are calculated by use of an arccosine function obtained from the extreme map. With this method, wrapped phase values can be efficiently demodulated from a single fringe pattern without the need for assigning fringe order or interpolating fractional fringe order. The validity of the method is demonstrated by use of closed-fringe patterns generated by digital speckle interferometry.

Development of a laser-scattering-based probe for on-line measurement of surface roughness

The design and properties of an optical probe for on-line measurement of surface roughness are discussed. Based on light scattering, a probe that consists of a laser diode, a measuring lens, and a linear photodiode array was designed to detect surface roughness, in which the light scattered from a test surface at a relatively large scattering angle phi (=28 degrees) can be collected to enhance measuring range and repeatability. A coaxial design that incorporates a dual-laser probe and compressed air makes the proposed system insensitive to the position of the test surface and to surface conditions such as the presence of debris, vibration, and lubricants that result from machining. The results from measurements of several sets of specimens have demonstrated the feasibility of measuring surface roughness by using light scattering. On-line measurement on a diamond-turning lathe has shown that the proposed technique is stable and compact enough to be applicable to on-line measurement of surface roughness of an engineering surface.

Temporal wavelet analysis for deformation and velocity measurement in speckle interferometry

When a continuously deforming object is measured by electronic speckle pattern interferometry (ESPI), the speckle pattern recorded on a camera sensor changes constantly. These time-dependent speckle patterns would provide the deformation history of the object. Various objects are applied with both linearly and nonlinearly varying loads and speckle patterns are captured using a high-speed CCD camera. The temporal intensity variation of each pixel on the recorded images is analyzed by a robust mathematical tool—Morlet wavelet transform instead of conventional Fourier transform. The transient velocity and displacement of each point can be retrieved without the necessity of the temporal or spatial phase unwrapping process. The displacements obtained are compared with those from a temporal Fourier transform, and the results show that the wavelet transform minimizes the influence of noise and provides better results for a linearly varying load. System error in the wavelet analysis for nonlinear load is also discussed.

Surface roughness measurement in the submicrometer range using laser scattering

A technique for measuring surface roughness in the submi- crometer range is developed. The principle of the method is based on laser scattering from a rough surface. A telecentric optical setup that uses a laser diode as a light source is used to record the light field scattered from the surface of a rough object. The light intensity distribu- tion of the scattered band, which is correlated to the surface roughness, is recorded by a linear photodiode array and analyzed using a single- chip microcomputer. Several sets of test surfaces prepared by different machining processes are measured and a method for the evaluation of surface roughness is proposed.

Speckle noise reduction in digital holography by multiple holograms

An effective method for reducing speckle noise in digital holography is proposed in this paper. The proposed method does not require classical filtering technique; instead it utilizes multiple holograms of an object generated by rotating an illuminating light continuously. The reconstructed hologram intensity fields possess different speckle patterns, and by properly averaging the intensity fields, speckle noise in the reconstructed images is reduced significantly. A merlion sculpture is evaluated and experimental results show that the proposed method is simple and effective in reducing speckle noise in digital holography.

3-D deformation measurement using fringe projection and digital image correlation

In this study, we propose an integrated method which combines fringe projection and digital image correlation (DIC) method into one optical system for 3-dimensional displacement measurement. Sinusoidal fringes are projected on an object using a liquid crystal display (LCD) fringe projector. Two sets of images, one containing projected fringes and the other with only white light speckle background, are captured by a CCD camera and processed using DIC to obtain in-plane displacement. Fourier transform is also employed to evaluate the images for out-of-plane displacement. Experimental results are compared with analytic solutions and the results demonstrate that both in-plane and out-of-plane displacements can be accurately measured using the proposed method.