We propose a new method for vibration measurement in non-rigid test environment with electronic speckle pattern
interferometry (ESPI). The ESPI is useful for non-contact, real-time analysis of vibration. This method needs rigid test
environment, however. When the interferometer and a vibration surface are on a non-rigid table or their environments
are separated, especially at manufacturing areas, high-amplitude, low-frequency noise fluctuation overwhelms a
vibration signal and the amplitude fringes disappear. We use electronic shutter function of a TV camera and reduce
exposure time of an image sensor. With the time reduction, we may extract an image from many input images, during
whose acquisition time noise fluctuation turns back and its magnitude is so small that the vibration signal goes to be
included in the image. We accumulate the images and increase the contrast of the amplitude fringe map.
We evaluated usefulness of this method with circular saw vibration. The interferometer and the saw are fixed on a rigid
board and the noise fluctuation is electronically superposed on the vibration signal with sine wave. This method is
successful for a fluctuation amplitude of 60μm.
In order to extract accurate phase changes due to deformation, the phase-shifting methods have been extensively investigated and applied to many practical measurements because of its simplicity and high resolution. But it requires that several specklegrams be captured in the static, non-deforming, condition of an object, making it unsuitable for measuring continuously deforming objects. Then we developed the method which does not require specklegram in static condition and can measure a large continuous-deformation with high accuracy. But this method was applied to deformation measurement of only one direction. We have improved this method and have obtained the new method which can measure deformation along two directions by using two lasers and one color camera. In addition, the method has potential to measure deformation along three directions: two in-planes and one out-of-plane.
This paper proposes a simultaneous measurement technique of 2 displacement components by using a laser beam and
one high-speed line CCD camera. The laser beam is divided to two beams. One beam is refl ected by a corner refl ector
attached on an object and the refl ected beam is superimposed with the other beam. The superimposed beam is expanded
by a microscope objective lens and then passed through both a wedge-shape birefringent plate and a polarizer making
a fringe pattern. This pattern has a light intensity distribution like a sinusoidal shape whose envelope curve has one
peak. The pattern is captured by the line CCD camera and is used to extract nanometric longitudinal displacement and
micrometric lateral displacement measurement.
White-light interferometry combined with phase-shift technology becomes a widely used 3-D shape measurement tool in precision engineering. In white-light interference 2π phase ambiguity can be avoided in measurement of optical path difference by searching a modulation peak of light intensity change. This interference surely involves a change in modulation intensity against optical path difference, which is an envelope curve of sinusoidal variations. Then, the phase-shift algorithm by which phase is accurately measured even under the modulation change is required. There is often another requirement that phase shift between captured interferograms would be not restricted to π/2 in the use of the algorithm. Computer simulation has been carried out to estimate phase errors which are retained by applying well-known algorithms to a white-light interference. They are Carre, 4-frame, 5-frame Hariharan, and 7-frame Groot algorithms. All the algorithms have non-negligible errors under the both requirements. Therefore, I extract individual terms (Ii±Ij) in an algorithm equation by considering symmetry of light intensity against phase, where Ij is light intensity just after the j-th shift. Using computer simulation again, I then search for appropriate coefficients by which the terms are multiplied in the equation. I finally have found an algorithm satisfying both the requirements.
I propose a high-speed vertical scanning profilometry which has nanometric height resolution. The proposed profilometry is equipped with two short-coherent-light sources, which are made of extremely-high-power light emitting diodes ( LED) and whose center wavelengths are 503 and 591 nm. In 3-D profile measurements, this profilometry acquires many interferograms while vertically-scanning a Mirau-type microscope objective with 0.415-mm step/interferogram and alternately-flashing LED. Odd-numbered interferograms are acquired with 503-nm LED and even-numbered interferograms are with 591-nm LED. Regarding the acquired interferograms, a computer calculates phase and modulation contrast using a phase-shifting technique. As two step movements are repeated between acquirements of interferograms flashed with the same LED, phase step corresponds to approximately 6 p + p/2 with 503 nm and approximately 6 p - p/2 with 591 nm, respectively. After searching the interferogram having a contrast peak, the computer extracts optical path difference of the searched interferogram with nanometric resolution from the phase information. From the vertical step length of 0.415 mm and a frame rate of 110 Hz, a vertical scanning speed is given as 46 mm/s. Height resolution of the profilometry is confirmed from measured data of a step height standard.
We propose a new method for measuring vibration frequency with electronic speckle pattern interferometry. In this method, laser beam wavelength is modulated with an independent frequency. In accordance with a frequency difference between the modulation and the vibration, a maximum intensity in a fringe pattern image changes. When the modulation magnitude is reduced, the frequency difference between the vibration and the modulation, in which the fringe pattern diminishes, widens and the maximum intensity in the fringe pattern image alters gently while the modulation frequency is scanned. Then we can search the vibration frequency, in which the maximum intensity in the image has a peak when the modulation frequency equals to the vibration frequency, with a hill-climbing method. It is confirmed in an experiment that the vibration frequency can be measured in a short time sufficient for practical use.
We propose a new vertical-scanning profilometry which has high-speed in measurements. The proposed profilometry measures 3-D shape by using phase-shifting technique with large phase shifts around 6 π ± π /2. With such large shifts, 2 π phase ambiguities normally suffer precise measurements of shapes. Therefore, the profilometry is equipped with two short-coherent-light sources of different mean-wavelength and alternately acquires interferogram for each wavelength every large phase shift (one interferogram /one shift). From the interferograms acquired through vertical scanning , it searches the highest-contrast frame position regarding the each wavelength and calculates the phases with the searched frames. From the phases of the two wavelengths and a vertical-scanning step height (relating to the large phase shifting), 3-D profile is calculated with a precision as high as a well-known phase-shifting interferometry.
We propose a new vertical-scanning profilometry which has potentiality of realizing high-speed measurement. The proposed profilometry measures 3-D shape by use of phase-shifting techniques with a large phase shift 2n π + π/2. With such large shifts, 2 pai phase ambiguities normally suffer precision measurements of phases. Therefore, the profilometry was equipped with two short-coherent-light sources of different wavelengths and a double-exposure camera, and measures the phases of two different wavelengths at nearly the same optical path difference. From the phases and a vertical-scanning step height, 3-D profile is calculated with nanometer precision.
We propose a new technique of speckle interferometry which can measure a dynamic phase change in large deformation. In this method, we use a continuous tracking approach of the deformation we proposed previously, and apply a new technique, which can measure the large deformation by eliminating a noise term not correlating to the deformation, to the approach. A 450 μm in-line deformation of an aluminum plate was successfully tracked in an experiment.
In step-like deformations, discontinuous deformations, the techniques using a laser beam of single wavelength cannot measure the deformation amount. Because the deformation changes between captured specklegrams would be larger than the wavelength of the laser. We have developed the technique that can measure the large deformations having step-like discontinuities by using two laser beams of different wavelengths.
We propose a new profile measurement system with light sectioning, which is available to detect step profiles on objects. We can intercept obstructive signals due to the day light reflected from the objects, by using a modulated laser beam and lock-in demodulation. The performance of this system was confirmed by experiments, in which step profiles of objects at a distance of 500 mm could be measured.
Resolution-variable moiré topography for measuring the three-dimensional profile of an object is described. With this method, moiré fringes are formed by projecting two sets of interference fringes of laser beams on an object. The interference fringes are formed using a Mach-Zehnder interferometer and are divided into two sets by a beamsplitter. The image, including the moiré fringes, which are formed in accordance with the object depth, is detected by an image sensor. The effectiveness of this method is demonstrated by practically measuring the profiles of a small object. The intervals between adjacent moiré fringes could be experimentally changed from 0.16 to 1.6 mm. The advantage ofthis method is that the interval between moiré fringes can be easily changed continuously by a mechanical operation.
The use of the phase shifting interferometric technique is discussed to make quantitative surface profiling
using the Nomarski differential interference microscope. Lateral shift of the Nomarski prism introduces
mutual phase shift between interfering two wavefronts with small amount of shear. Since the analyzed
phase distribution corresponds to the differential of the surface profile under test, integration of the phase
distribution gives the correct surface topography. The procedure for an analysis method and experimental
results are presented.
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