Assen Shulev

New automatic FFT filtration method for phase maps and its application in speckle interferometry

In this article a new automatic phase FFT filtration method is described. It is compared with other well known filtration techniques for noise removal. This automatic FFT filter is applied on interferograms obtained through one dimensional folding shear interferometry for displacements measurement of a loaded glass flask. Different algorithms have been applied for processing of digitally synthesized phase maps and the advantages of the enhanced automatic FFT filtration are shown. The phase filtered maps obtained in this way can be demodulated even with the simplest unwrapping technique.

Threshold selection in transform-domain denoising of speckle pattern fringes

A transform-domain fringe pattern denoising technique is presented. The Discrete Cosine Transform (DCT) is applied in a sliding window manner to get an overcomplete image expansion, and then the transform coefficients are thresholded to reduce the noise. We investigate the proper size of the sliding window and the proper threshold level. The latter is determined individually for each window position using a local noise variance estimate. In order to deal with a rather inadequate but simplified noise model, a proportionality factor, related with the speckle size, is found by experiments with digitally simulated speckle fringes. Such a proportionality factor suggests that the technique could be made fully automatic. We demonstrate promising results in denoising of real speckle fringe patterns, obtained through an out-of-plane sensitive Digital Speckle Pattern Interferometry (DSPI) set-up in a process of non-destructive testing of reinforced composite materials deformation.

MEMS sensors for mm-range displacement measurements with sub nm-resolution

It is challenging to provide contact measurements of travel in mm-range with nm/sub-nm resolution. It is even more complex to perform such measurements in static regime. In order to respond to the need for a simple, reliable and costeffective tool for contact travel measurements in mm-range with nm/sub-nm resolution, test MEMS sensor with sidewall embedded piezoresistors have been developed. The sensor comprises of two outer members having thickness of 270μm and two symmetrical sets of in-plane compliant elements: differential springs and displacement detection cantilevers, having thickness of 12μm. The MEMS devices have been bonded directly on low-noise amplifier PCB. For detailed characterization of the sensors in mm-travel range, two different experimental setups have been used. Measurements of 0.6 mm travel range at 1nm resolution have been demonstrated experimentally.

Force monitoring transducers with more than 100,000 scale intervals

This paper presents the results obtained at characterization of novel, high performing force transducers to be employed into monitoring systems with very high accuracy. Each force transducer comprises of a coherently designed mechanical transducer and a position microsensor with very high accuracy. The range of operation for the mechanical transducer has been optimized to fit the 500μm travel range of the position microsensor. Respectively, the flexures’ stiffness corresponds to achieve the maximum displacement at 70N load force. The position microsensor is a MEMS device, comprising of two rigid elements: an anchored and an actuated ones connected via one monolithic micro-flexure. Additionally, the micro-flexure comprises of two strain detecting cantilevers having four sidewall embedded piezoresistors connected in a Wheatstone bridge. The particular sensor provides a voltage signal having sensitivity in the range of 240μV/μm at 1V DC voltage supply. The experimental set-up for measurement of the load curve of the force transducer has demonstrated an overall force resolution of about 0.6mN. As a result, more than 100,000 scale intervals have been experimentally assessed. The present work forms development of a common approach for accurate measurement of various physical values, when they are transduced in a multi-D displacement. Due to the demonstrated high accuracy, the force transducers with piezoresistive MEMS sensors remove most of the constraints in force monitoring with ppm-accuracy.

All-silicon microforce sensor for bio applications

It is well known that for precise biological cell injection the applied forces should be exactly controlled in any specific case. In this paper we present design, prototyping, calibration and testing of a microforce sensor, which fulfils the requirements for injection monitoring of biological cells. It is an axial all-silicon piezoresistive Micro Electro-Mechanical System (MEMS) device. The layout of the all-silicon force sensor is adapted for operation at partial dipping in biochemical solutions. A prototype of this sensor is investigated and its parameters are measured. To verify the applicability of the MEMS for bio-applications, it was used in force monitoring injection of Xenopus Oocyte cells. Force measurements during the injection process and interpretation of the force-penetration curve are presented and discussed. Thus, it has been experimentally proved that the developed all-silicon force sensor can be successfully applied for force monitoring in different bio-applications.

Force sensor for cell injection and characterization

This paper presents the basics idea and principles of a force sensor for cell injection and characterization of mechanical properties of cells. It utilizes fluidics, piezo resistive MEMS, and piezo actuated mechanics to provide sub-micronewton sensitivity in vertical injection mode. The prototyping, calibration and investigation of the proposed sensor are also presented in brief. Main advantages and drawbacks are also discussed.

Cell Membranes Under Hydrostatic Pressure Subjected to Micro‐Injection

The work is concerned with the determination of the mechanical behaviour of cell membranes under uniform hydrostatic pressure subject to micro‐injections. For that purpose, assuming that the shape of the deformed cell membrane is axisymmetric a variational statement of the problem is developed on the ground of the so‐called spontaneous curvature model. In this setting, the cell membrane is regarded as an axisymmetric surface in the three‐dimensional Euclidean space providing a stationary value of the shape energy functional under the constraint of fixed total area and fixed enclosed volume. The corresponding Euler‐Lagrange equations and natural boundary conditions are derived, analyzed and used to express the forces and moments in the membrane. Several examples of such surfaces representing possible shapes of cell membranes under pressure subjected to micro injection are determined numerically.

Projection moire measurement of glass specimens retrofitted with safety film

Protection of buildings and critical public infrastructure against blast load has been recently improved by retrofitting glass windows with a safety film. As the exact physical mechanisms of the interaction between glass and safety film are not quite well understood, intensive research is conducted on the properties of this assembly. The loadings on the glass/film assembly are typically dynamic (blast, wind pressure, impact), so the lab tests are done on a drop weight set-up, where a mass is falling on a retrofitted glass plate. In this work, the drop weight setup was combined with pattern projection (moire) technique to study the time history of the out-of-plane deformations of the glass/film assembly. The fringe pattern, projected on the back side of the specimen, was generated by means of a sinusoidal phase grating under divergent high intensity infrared illumination. The whole process was recorded with a high speed camera. Local routines based on Fast Fourier Transform were used to process the captured images, and to extract the phase. The exact out-of-plane displacements were calculated by means of calibration based on previous shape measurements of several different objects with known dimensions.

Two-wavelength and two-spacing projection interferometry for real objects contouring

The comparison between two-wavelength and two-spacing projection phase-stepping interferometry for real object contouring is presented. Single exposure reflection hologram with two-frequency generating temperature stabilized diode laser is realized. Theoretical approach, experimental results as well as the assessment of sensitivity and uncertainty of the measurements are discussed.

Roughness Measurement of Dental Materials

Abstract This paper presents a roughness measurement of zirconia ceramics, widely used for dental applications. Surface roughness variations caused by the most commonly used dental instruments for intraoral grinding and polishing are estimated. The applied technique is simple and utilizes the speckle properties of the scattered laser light. It could be easily implemented even in dental clinic environment. The main criteria for roughness estimation is the average speckle size, which varies with the roughness of zirconia. The algorithm used for the speckle size estimation is based on the normalized autocorrelation approach.