Jung-Ju Lee

Tactile sensors using the distributed optical fiber sensors

This paper describes two kinds of the tactile sensors using the optical fiber sensors. One is the tactile sensor using fiber Bragg grating (FBG) sensor. Bragg grating inscribed in an optical fiber causes reflect of a spectral component at the Bragg wavelength. This sensor can detect external force using change of the reflective wavelength. The other is the tactile sensor using the microbending optical fiber (MBOF) sensors. The microbending of the optical fiber drives the light transmission loss from the optical fiber. Using the light loss, this sensor can also detect the external force. The structures of these type tactile sensors are very simple. The tactile sensor using FBG sensor consists of an optical fiber which has a Bragg grating and bridge type transducers, and using MBOF sensors is composed of crossed fibers in the silicone rubber. In this paper, we fabricated and evaluated both tactile sensors. The performances of both tactile sensors were verified.

Strain event detection using a double-pulse technique of a Brillouin scattering-based distributed optical fiber sensor

Stimulated Brillouin scattering in optical fibers can be used to measure strain or temperature in a distributed manner. Brillouin optical time domain analysis (BOTDA) is the most common sensor system based on the Brillouin scattering. To improve the spatial resolution of these measurements, shorter pulses must be used, resulting in reduced signal powers causing a decrease of the dynamic range. In this paper, a doublepulse technique was proposed to enhance the spatial resolution of BOTDA. Experimental results showed that the ability to resolve two adjacent events could be enhanced, about twice, by using a double-pulsed pump light without decreases in the dynamic range.

Structural monitoring of a bending beam using Brillouin distributed optical fiber sensors

Due to the large measurement range, distributed optical fiber sensors are very effective in the monitoring of large structures. Brillouin distributed optical fiber sensors, which measure strain by using Brillouin frequency shift, show some measurement error for non-uniformly distributed strain because of their spatial resolution characteristics. They require compensation of measured values and positions in the end-parts of the measurement range because measured values are not distributed over all the positions of the spatial resolution range. In this paper, we present the strain measurement characteristics of Brillouin distributed optical fiber sensors and the compensation method in the end-parts of the measurement range. A deflection measurement method for a beam subjected to a bending load using Brillouin distributed optical fiber sensors is also presented. This is verified by a three-point bending experiment employing an 8 m aluminum beam. The experimental results showed that the measured value agrees with the one expected by Brillouin measurement theory.

Pulse width dependence of Brillouin frequency in single mode optical fibers

Stimulated Brillouin scattering in optical fibers can be used to measure strain or temperature in a distributed manner. Brillouin optical time domain analysis (BOTDA) is the most common sensor system based on the Brillouin scattering. This paper presents the experimental analysis of the characteristics of Brillouin gain spectrum (BGS) influenced by the width of launched pulse. Brillouin strain coefficient is also examined for the different pulse widths, which is important to apply a Brillouin scattering-based sensor to a structural health monitoring. Experimental results showed that not only the Brillouin linewidth and gain but also the Brillouin frequency were dependent on the pulse widths.

Temperature Compensation of a Strain Sensing Signal from a Fiber Optic Brillouin Optical Time Domain Analysis Sensor

In order to do continuous health monitoring of large structures, it is necessary that the distributed sensing of strain and temperature of the structures be measured. So, we present the temperature compensation of a signal from a fiber optic BOTDA (Brillouin Optical Time Domain Analysis) sensor. A fiber optic BOTDA sensor has good performance of strain measurement. However, the signal of a fiber optic BOTDA sensor is influenced by strain and temperature. Therefore, we applied an optical fiber on the beam as follows: one part of the fiber, which is sensitive to the strain and the temperature, is bonded on the surface of the beam and another part of the fiber, which is only sensitive to the temperature, is located nearby the strain sensing fiber. Therefore, the strains can be determined from the strain sensing fiber while compensating for the temperature from the temperature sensing fiber. These measured strains were compared with the strains from electrical strain gages. After temperature compensation, it was concluded that the strains from the fiber optic BOTDA sensor had good coincidence with those values of the conventional electrical strain gages.

Phase-shifted transmission/reflection-type hybrid extrinsic Fabry-Perot interferometric optical fiber sensors

Conventional interferometric optical fiber sensors, including the extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensor, have the drawback of ambiguous measurement directions and direction changes, because their signal processing using only fringe counting cannot present measurement direction information, such as tension/compression of strain or increment/decrement of temperature. An EFPI optical fiber sensor constructed with a transmission-type structure (TEFPI optical fiber sensor) can successfully compensate for this problem. However, it has low interferometric fringe visibility and requires somewhat sophisticated signal processing to detect the measurement direction changes. In this research, a hybrid EFPI optical fiber sensor is presented from which transmission-type and reflection-type sensor signals can be simultaneously acquired. The linear combination of the actual transmission-type and reflection-type signals is shown to have a shifted phase from the reflection-type signal according to measurement directions, and thus the phase behavior of lead/lag can present the measurement direction information. Because the hybrid sensor uses separated signals for the measurement quantity and direction, its signal processing is more robust than that of the TEFPI sensor. The sensor and signal processing algorithm were verified with the strain measurement experiments.

System Design and Evaluation of the Robot Tactile Sensor Using the Microbending Fiber Optic Sensors

This paper describes the system design and the structural design to evaluate the tactile sensor using the microbending fiber optic (MBFO) sensors. The small light emitted diode (LED) and charge coupled device (CCD) are used as a single light source and a light detector for the bundle of optical fibers respectively. And the structure of this type tactile sensor which is composed of crossed fibers in the silicone rubber is very simple. And the tactile sensor element using MBFO sensor is fabricated and the performance of this sensor is evaluated.

Modeling and numerical simulation of the pseudoelastic behavior of shape memory alloy circular rods under tension–torsion combined loading

Most research on the behavior of shape memory alloys (SMAs) under tension–torsion combined loading has focused on tubular materials. In contrast to tubular SMAs, SMA rods have unique characteristics. When an SMA rod is twisted, the central region remains elastic while the outer layer undergoes a martensite transformation. The nonlinear stress distribution of an SMA rod through the radial direction gives the SMA rod peculiarities, and this is the primary factor that makes analysis of the stress–strain behavior difficult. The authors suggest a material model that can represent the stress–strain behavior of an SMA rod under tension–torsion combined loading. The proposed model is based on Brinsons phase transformation kinetics and the plastic flow rules. Furthermore, the yield is assumed to be in accordance with the Von Mises criterion and the effect of thermal expansion is not considered. The authors also present some numerical solutions of the proposed model in the context of isothermal strain and stress processes (proportional and nonproportional loading) in the pseudoelastic region. According to simulation results, the stress–strain behavior of an SMA solid rod under biaxial loading differs from that of a thin-walled tube or a rod under one-dimensional loading.

Two-way shape memory effect induced by repetitive compressive loading cycles

The NiTi alloy can be trained by repetitive loading or heating cycles. As a result of the training, a two-way shape memory effect (TWSME) can be induced. Considerable research has been reported regarding the TWSME trained by tensile loading. However, the TWSME trained by compressive loading has not been investigated nearly as much. In this paper, the TWSME is induced by compressive loading cycles and the two-way shape memory strain is evaluated by using two types of specimen: a solid cylinder type and a tube type. The TWSME trained by compressive loading is different from that trained by tensile loading owing to the severe tension/compression asymmetry as described in previous research. After repetitive compressive loading cycles, strain variation upon cooling is observed, and this result proves that the TWSME is induced by compressive loading cycles. By performing compressive loading cycles, plastic deformation in NiTi alloy occurs more than for tensile loading cycles, which brings about the appearance of TWSME. It can be said that the TWSME is induced by compressive loading cycles more easily. The two-way shape memory strain increases linearly as the maximum strain of compressive loading cycles increases, regardless of the shape and the size of the NiTi alloy; this two-way shape memory strain then shows a tendency towards saturation after some repeated cycles.

Analysis on the Behavior of the Shape Memory Alloy Using Abaqus UMAT

In this paper, the algorithm of Abaqus UMAT is introduced to analyze the shape memory alloy. The SMA has two main effects which show non-linearity. Due to this, it is hard to analyze SMA using analysis tools and to describe all of two effects. Therefore, in this study, the program using Abaqus UMAT based on Modified Brinson model is used to analyze SMA. The martensite fraction, the most important factor which defines SMA motion, is also calculated by Fortran program in UMAT. In addition, the tensile test of SMA specimen is conducted. The availability of algorithm is proved by comparing analysis to experimental result.