Papers by Keyword: Laser Doppler Vibrometer

Abstract: The density of green compact in powder metallurgy (PM) is a critical characteristic to determine the strength and tenacity of the product. Due to the fragility and sensitivity to the humidity of green compact, detection method triggers by knocking or immersion cannot be applied. Current inspection method for green compact is to use a densitometer with Archimedes Principle, which has to immerse green compact into water. This action considers a destructive test due to the porosity of the PM product, and the oxidation reaction of metal particles inside products cannot be controlled. Since the current detection method is destructive, most of the inspection on forming process of PM only complies after setup of compaction machine. The next non-destructive test is executed after sintering, such as Acoustic emission testing (AE) and Magnetic Testing (MT), which require a solid object for knocking or liquid immersion. To reduce defective compaction flow into the manufacturing process, a Non-destructive test in forming process is a prerequisite. Laser Doppler Vibrometer (LDV) takes advantage of measuring without damaging the test object and obtaining characteristic spectrum signal from the sample for further analysis, which is preferable for green compact of PM. The LDV test setup included a Polytec PSV-400 scanning laser vibrometer with a crystal resonator attached to the test product to generate reference vibration. Integrate spectrum in the frequency range 0 - 5000 Hz was recorded from 30 points of an annular pattern. Comparison of the spectrum and statistical analysis from defective specimens demonstrate that velocity increases within a specific frequency, which is different from with normal samples. Increases in velocity refer to uneven density distribution with an absence of particles inside product decline. The study approves the possibility of density detection in green compact using an LDV. Further studies aim to construct a relational model of specimen and compaction machine and determine a fundamental database for Advance Process Control (APC) of forming process in PM.

Abstract: This paper presents an investigation of spacing fluctuation characterization (SFC) of Hard Disk Drive (HDD) using thermal asperity (TA) sensor. Firstly, the spacing fluctuation monitored by TA sensor is correlated with that monitored by a Laser Doppler Vibrometer (LDV). Secondly, a series of experiments are conducted to study the factors affecting the spacing fluctuation, including disk distortion, head crown sensitivity, ZH sensitivity. Good disk assembly and small crown sensitivity and ZH sensitivity will be helpful to reduce spacing fluctuation.

Abstract: XY motion platform in flip chip machine plays a key module to achieve precise positioning in the process of flip chip packaging. The paper presents an XY motion platform structure. The static and modal characteristics of the XY motion platform were calculated by finite element method, and then some important conclusions were obtained. The displacement and stress distribution of XY motion platform were disclosed. The relevant change trends of displacement and stress under the different load were investigated and the first four natural vibration modes of the XY motion platform were obtained. In the experiment, the vibration behavior was measured by a non-contact laser measuring device. The experiments verify the correctness of the simulation of the XY motion platform. These results can help improve the reliability of the XY motion platform.

Abstract: The laser Doppler vibrometer is a non-contact sensing technique developed based on the Doppler effect of a laser beam emerging from a subject surface. As a vibration transducer, the laser Doppler vibrometer offers many advantages over the conventional contact vibration sensors. It allows remote, non-intrusive measurement of structural vibration and it is very useful in scenarios when traditional contacting measurement is inconvenient. In this paper, four laser-based structural dynamic studies were presented and some results were briefly reported, which include laboratory dynamic testing of a bolted steel beam, a scaled-down high-rise building model, and a prestressed concrete reaction wall, and field vibration measurement of a viaduct bridge. Through these demonstrating cases, it is anticipated to help civil engineers get familiar with the laser-based sensing technology and to extend their selections for effective measurement approaches during experimental research.

Abstract: Flow-induced vibration (FIV) of a head gimbals assembly (HGA) in hard disk drives (HDDs) limits the positioning accuracy of magnetic head in the HGA for higher HDD magnetic recording density. The pressure fluctuations characterize turbulent fluctuations exciting the HGA off-track vibration (HGA-OTV). In this paper, experimental studies have been carried out to investigate the spectrum characteristics correspondence between pressure fluctuations around an HGA and the simultaneous HGA-OTV under different HDD rotation speeds. A practical and effective experimental setup has been implemented to enable the simultaneous measurements on both the pressure fluctuation and HGA-OTV signals, where pressure fluctuations are measured with a pressure transducer through a small hole on the HDD top cover and the direct HGA-OTV signals are detected through a laser Doppler vibrometer. Results under conditions of three different HDD rotation speeds of 7200, 9000, 10800 rpm have been investigated and compared. It is shown that the HGA off-track vibration spectra are highly associated with those of the pressure fluctuations in terms of principal peaks in four frequency bands around 1.8 kHz, 2.5-3.5 kHz, 7-7.5 kHz and 11.5-12.5 kHz. With increasing HDD rotation speed, it is shown that the spectrum magnitudes of both the pressure fluctuation and the HGA off-track vibration increase correspondingly, while the principal peak positions in spectra of either pressure fluctuation or the HGA off-track vibration always hold the line. This study demonstrates a causal nexus from the pressure fluctuation to the HGA off-track vibration and suggests the feasibility of controlling the HGA-OTV through suppression of pressure fluctuations around the HGA.

Abstract: In this study we attempted to suppress chatter vibration in end-milling of austenitic stainless steels with an end-mill having different helix angles. We showed there were two types of chatter vibration in end-milling with a conventional end-mill with regular helix angle. The two types of chatter vibration depend on the cutting speed. We detected these two types of chatter vibration by the developed monitoring method using laser Doppler vibrometers. We classified the two types of chatter vibration according to chatter generating tendency, frequency and vibration mode. We showed that chatter vibration generated at lower cutting speeds was regenerative chatter vibration. On the other hand, chatter vibration generated at higher cutting speeds was another type. We showed that a end-mill with different helix angles could prevent generative chatter vibration at lower cutting speeds and was also effective in suppressing chatter vibration at higher cutting speeds.

Abstract: A new synthetic heterodyne laser Doppler interferometer based on homodyne interferometer is addressed in this work. Interference signal includes the DC offset that is light intensity. The DC offset must be got rid because it does not need to obtain a velocity. The high pass filter is used instead of the subtractor. DC offset having random variation is difficult to get rid as using the subtractor itself. When the vibration amplitude is smaller than at least 1/ 2 of the wavelength of He-Ne laser, a serious problem of incorrect velocity measurement can be caused since there is non-zero crossing interference signal whose DC value can be eliminated in using the HPF. In order to solve this problem of using the HPF in the homodyne interferometer, a synthetic interferometer using a mechanical modulation method is proposed in this work by exciting a reference mirror with the displacement larger than the 1/2 of the wavelength. In this work, the analytical work is presented to show how the synthetic interferometer solves the problem of incorrect velocity measurement using the Fourier-Bessel function description of the interference signals. Simulation and Experimental works are also presented to validate the synthetic heterodyne interferometer proposed in the work.

Abstract: Electrostatic microswitch and its fabrication technology have been developed for the first time in Lithuania, at Kaunas University of Technology (KTU). The microdevices were produced by using a nickel surface micromachining process. The microswitch consists of a cantilevered nickel structure suspended over the bottom electrodes. The width of the structure is 30 μm, thickness - 2 μm and length ranges from 67 to 150 μm. Implementation of microswitches as a substitute for the present solid-state switching devices poses many problems. In particular higher actuation voltages, lower switching speed and a reduced lifetime are considered to be among the most significant ones. With the aim of improving these parameters a finite element model is currently developed that takes into account not only microscale-specific electrostatic actuation and air damping effects but also includes the bouncing phenomena. Experimental studies of electrical and dynamic characteristics were also carried out with the purpose of model validation and correction. The paper presents initial results of theoretical modal and air damping analysis as well it shows the first attempts to measure vibration modes of the cantilever structure of the microswitch using Laser Doppler vibrometer.