Fiber Optic Accelerometer Research Articles

Identification of a Dynamic System Using Ambient Vibration Measurements

This paper demonstrates how ambient vibration measurements at a limited number of locations can be effectively utilized to estimate parameters of a finite element model of a large-scale structural system involving a large number of elements. System identification using ambient vibration measurements presents a challenge requiring the use of special identification techniques, which can deal with very small magnitudes of ambient vibration contaminated by noise without the knowledge of input forces. In the present study, the modal parameters such as natural frequencies, damping ratios, and mode shapes of the structural system were estimated by means of appropriate system identification techniques including the random decrement method. Moreover, estimation of parameters such as the stiffness matrix of the finite element model from the system response measured by a limited number of sensors is another challenge. In this study, the system stiffness matrix was estimated by using the quadratic optimization involving the computed and measured modal strain energy of the system, with the aid of a sensitivity relationship between each element stiffness and the modal parameters established by the second-order inverse modal perturbation theory. The finite element models thus identified represent the actual structural system very well, as their calculated dynamic characteristics satisfactorily matched the observed ones from the ambient vibration test performed on a large-scale structural system subjected primarily to ambient wind excitations. It is noted that newly developed optical fiber accelerometers were used for this ambient vibration test. The dynamic models identified by this study will be used for design of an active mass damper system to be installed on this structure for suppressing its wind vibration.

Michelson fiberoptic accelerometer

An all fiberoptic accelerometer, based on Michelson interferometry, has been developed theoretically and experimentally. It makes use of an oscillating mass suspended by a transduction fiber in a mass support. The interferometric configuration is formed by a 3 dB single-mode fiberoptic beam splitter, on two distal endfaces of which high-reflectance aluminum films were directly coated and served as reflecting mirrors. An ac phase tracking modulation–demodulation scheme has been employed to process the signal. The performance of a prototype of the accelerometer has been investigated, and the results indicated that the measured frequency spectrum is qualitatively consistent with the theoretical prediction, and the detected output wave forms are also in good agreement with that of the applied simulation signals. In addition, the constraint on the lateral movement of the acceleration-sensitive mass with two specific diaphragms has been experimentally verified, nevertheless, it needs to be optimized further.

Optimization of fiber optic acoustic transduction for neutrally buoyant accelerometers

It is desirable to maximize both the sensitivity and bandwidth of fiber optic accelerometers used for acoustic motion, accelerometer, and pressure-gradient sensor applications. Designs and figures-of-merit for centrally and peripherally supported flexural disks and compressional type accelerometers will be presented and compared. The optimization of optical fiber coils, coating diameter, materials properties, and packaging will also be discussed. [Work supported by ONR, NUWC, and the UMass Dartmouth Ph.D. Program.]

Simple low-frequency optical fiber accelerometer with large rotating machine monitoring applications

A simple fiber optic accelerometer with large rotating equipment monitoring applications is presented. The sensor is optimized for detection of mechanical vibrations in the frequency range 0.2-140 Hz. The minimum detectable acceleration is 0.025 m/s/sup 2/ root mean square (rms), and it has low transverse sensitivity. The transducer operating principle, based on a fiber optic cantilever beam without any selamic mass on it, is simple and easy to construct, and the intensity-modulation and compensation techniques developed require simple electronic processing and provide very good stability and accuracy. Other advantages of the system are its robustness, compact size, and cost-effectiveness, which make it appropriate for industrial applications, specially for those where harsh environments are involved. The complete process of research and development of the sensor has been carried out, from the theoretical study of its operation principles, through the design, fabrication, experimental characterization and enhancement, to its starting up in a high-power electric station for hydrogenerator monitoring.

High performance fibre optic accelerometers

The authors report the design and performance of fully packaged, field ruggedised, high performance fibre optic (FO) accelerometers. Appropriate design considerations have resulted in FO accelerometers with many desirable engineering features which include: (i) high sensitivity (>20 dB re rad/g), (ii) low noise (<1 micro-g√Hz at 1 kHz) (iii) broad frequency response (100 Hz to 14 kHz), (iv) low sensitivity to extraneous measurands (e.g. static pressure sensitivity < –200 dB re rad/µPa), (v) small size (<35 gms total), and (vi) low cross-axis sensitivity (<–35 dB). Single device or arrays of such devices can be incorporated in many structural and vibration sensing applications at low cost.

A novel fibre-optic vibration sensor

Analysis of the vibrational characteristics of materials or mechanical systems had the promise of providing useful information of their integrity. Acoustic monitoring of heavy machine bearings is a well-established technique for detecting the onset of wear [1]. Damage to materials [2] or structures [3] results in local stiffness reductions which may be detected by measuring their dynamic response. Vibration sensors could also be used to monitor the passage of traffic over bridges and roadways or for security applications such as detecting forced entry into buildings. Vibration sensing is most commonly performed by electromechanical devices such as piezoelectric, piezoresistive or capacitative accelerometers. Whilst this is reliable and tested technology it has a potential drawback. Susceptibility to electromagnetic interference limits application around strong electric fields and the need for either strongly shielded leads or a power supply for a built-in amplifier results in relatively heavy, bulky cables. If an extensive network of sensors is to be incorporated into a structure, this may prove to be a considerable problem, even more so if there is a premium on weight, as in aerospace structures. Optical fibre sensors have been shown to be suited for embedment in composites [4] and cementitious materials [5]. They have largely unaffected electromagnetic interference and in some cases they may be multiplexed to give a distributed sensing system. Optical fibre sensors tend to be more expensive than their conventional counterparts, mainly because the equipment needed to interrogate the sensor is often delicate and complex. To measure vibration, either a strain gauge-type device is attached intimately to the structure to measure surface acoustic waves, or a fibre-optic accelerometer is employed in place of a conventional one. The device presented in this latter modulates the intensity of light transmitted through it and thus requires only simple and cheap instrumentation. It is illustrated schematically in Fig. 1. The details of its construction and operation are as follows. Two 50=125 multimode optical fibres are cleaved and the protective polymer jacket of one of the fibres is stripped back a short length. The prepared fibres are then inserted into a precisiondrawn glass capillary, their end-faces separated by a few tens of microns, and fixed with epoxy resin adhesive. When the sensor is subjected to an acceleration prependicular to the axes of the optical fibres, the motion of the unsupported fibre lags behind. As it moves out of alignment, relative to the fixed fibre, the transmitted optical power decreases. The power variations are detected with a fastresponse photodiode and amplifier and sent to a personal computer for further processing. The response of the sensor to sinusoidal and transient excitation was investigated. First, the sensor was mounted on an electromagnetic coil which was driven by a signal generator and amplifier and excited over a range of frequencies from 100 to 10000 Hz. The sensors which provided the results presented here had resonant frequencies of 3 Hz and 6 Hz, but this value can be varied by changing the length of the cantilevered fibre or by choosing fibres of different diameters. To test the response to transients, sensors of 3 Hz resonant frequency were bonded to the surface of 70 3 70 mm 16-ply crossply carbon-fibre reinforced plastic (CFRP) with a cyanoacrylate adhesive. An impulse was provided by either dropping a 6.3 mm diameter steel ball onto the panel from a height of 25 cm, giving an impact energy of 2.5 mJ, or a standard pencil break test [6]. Examples of sensor signals from these events are given in Fig. 2. The impact of the steel ball, considerably the more energetic of the two events, gave a greater signal amplitude, as well as a characteristic shape different from that of the pencil break. Results were consistent within both series, with the pencil break showing a little more variability.

General-purpose photoelastic fiber optic accelerometer

This paper describes a general-purpose photoelastic fiber optic accelerometer having a sensitivity of 2 V/g and a resolution of 2.5 mg over a frequency range extending from 0 to 600 Hz. The unit has a linear amplitude range of 0 to 5g and is sealed in an aluminum case having outside dimensions of 41 mm335 mm320 mm. It relies on a force transducer made from an optically birefringent material and measurements are made based on the principles of photoelasticity. A low-cost LED is employed as an incoherent light source; multimode optical fibers with a hard plastic cladding are used to transmit signals between the transducer and the conditioning electronics.

Optical fiber accelerometer based on a silicon micromachined cantilever

An intensity-modulated fiber-optic accelerometer based on backreflection effects has been manufactured and tested. It uses a multimode fiber placed at a spherical mirror center, and the beam intensity is modulated by a micromachined silicon cantilever. This device has applications as an accelerometer and vibrometer for rotating machines. It exhibits an amplitude linearity of ±1.2% in the range of 0.1-22 m s(-2), a frequency linearity of ±1% in the range of 0-100 Hz, and a temperature sensitivity of less than 0.03%/°C. The sensor is devoted to a wavelength-division multiplexing network.

Design of a compliant-cylinder-type fiber-optic accelerometer: theory and experiment

Experimental and theoretical research was carried out in order to establish the dependence of the performance of a compliant-cylinder-based fiber-optic accelerometer on the geometry and elastic properties of the transducer cylinders. The sensitivity and the natural frequency of the sensor were measured as a function of the ratio ε = (inner cylinder diameter)/(outer cylinder diameter). Two transducer materials with different elastic properties, a silicone rubber (Ecosil) and a polyetheretherketone polymer (PEEK 450G), were examined. It was found that with decreasing ε the sensitivity increases in the case of Ecosil and decreases in the case of PEEK. In both cases the natural frequency increases with decreasing ε. A simple analytical model was developed in order to explain this behavior qualitatively. The model takes into account the contributions to the effective stiffness from both the cylinder material and the fiber wrapped around the cylinder. The model is useful for the design of such types of accelerometer.

Performance analysis of a fiber optic accelerometer based on a compliant cylinder design

The performance of an all fiber accelerometer, based on a compliant cylinder design and operating at λ0=1.3 μm is described. Sub-μg/√Hz resolution in a frequency range from 10 to 250 Hz has been demonstrated. The measured dynamic range exceeds six orders of magnitude. Faraday rotation mirrors have been incorporated into the accelerometer to prevent signal fading due to random changes in the state of polarization of the guided beams. Test results at elevated temperatures are presented.

A Fiber Optic Non- Contact Accelerometer

A novel non-contact fiber optic accelerometer is presented. Two laser beams that have passed through two different lengths of feed fibers are launched to the same moving target. The frequency of each beam is Doppler shifted by movement of the target. The frequency shift of the laser beam corresponds to the velocity of the target at the time of launch. The difference in launching time due to the difference in feed fiber lengths samples the velocities of the target at times spaced by the differential arrival times of the laser beams. Two return passes of the reflected light are also made different so that the light passed through the shorter feed fiber is steered to the longer return fiber by means of a combination of A/4 plate and polarization beam splitter (PBS). This arrangement of equalizing the total paths relieved a stringent requirement of the coherency of the laser. Two reflected light beams are mixed and the beat frequency corresponds to the difference in the Doppler shift frequency at two different times of arrival of the launcher. An accelerometer based upon this principle was constructed and its performance was examined using a sinusoidally vibrating hydrophone as a target. Features of such an accelerometer are reduction in the coherency requirements of the light source and simpler amplification of the signal using heterodyne detection.

Large range coherence tuned fiber optic interferometric system for application in accelerometers

A low coherence interferometric system with large tracking range and self-initialization to be used for remote signal processing of fiber optic accelerometers is investigated. The tracking range of the low coherence system is 1.1 mm at low frequency. Phase resolution better than 1 mrad/√Hz at frequencies below 600 Hz is obtained, with a dynamic range of ≊90 dB. Data are presented which show that if the system were used to process the output from a compliant cylinder-type accelerometer, resolutions of ∼10−7 g/√Hz could be achieved.

A ruggedized fibre-optic accelerometer array for low-frequency condition monitoring

This paper describes the design and performance characteristics of a passive optical fibre linked accelerometer array for the measurement of the orthogonal deflections of large structures. The system has been demonstrated to operate successfully in an offshore environment.

A fiber-optic interferometric flex disk accelerometer

A push-pull fiber-optic accelerometer, using an all-fiber Michelson interferometer will be described. The accelerometer consists of two “clamped” disks (4.6 cm in diameter, 1.0 mm thick, aluminum 6061-T6) that are joined at the outer boundary to a rigid cylinder and at the disk center by a rigid “spindle” that acts as a seismic mass. Flat spiral-wound coils attached to the inner surface of each disk detect the acceleration-induced strain in a manner similar to flexural disk hydrophone designs [Brown et al., Fiber &amp; Int. Opt. 8(3), 169–191 (1989)]. The sensitivity below the disk's fundamental resonance of 2.8 kHz is 49 rad/g. The fractional phase change per unit force (1/φ) (δφ/δF) below the resonance is 56 × 10−12/N. A noteworthy feature of this sensor is that the sensing coils comprising the legs of the interferometer and the fiber coupler are totally internal to the sensor structure and thus are not exposed, making the sensor robust for industrial or shipboard applications. [Work supported by the Naval Postgraduate School Direct Funded Research Program and the Naval Sea Systems Command.]

Fiber-optic accelerometer having cantilevered acceleration-sensitive mass

An optical fiber accelerometer with a rectilinear actuator, such as a push rod, acting on a middle region of an optical fiber pass tautly strung between two supports so that linear actuator motion during sensed acceleration causes lateral displacement of the pass's middle region causing nonlinear (i.e., quadratic) strain in the fiber. This nonlinear response allows the accelerometer to detect acceleration signals in a noisy, low-frequency signal environment through use of a high frequency signal carrier whose amplitude is determined by a low-frequency measured acceleration signal using an optical fiber interferometer.

A microbend horizontal accelerometer for borehole deployment

A highly sensitive microbend horizontal fiber-optic accelerometer has been designed, built, and tested. This particular model is aimed at deployment in seismological research where minute accelerations must be detected. The device uses a cantilever beam and the compliance of an optical fiber mounted between deforming teeth to act as the springs in the accelerometer's spring-mass system. Acceleration is detected by sensing the movement of the mass relative to the case through the changes in the intensity of light propagating through the deformed fiber. Accelerations as small as 5 μg at 1 Hz have been detected with a dynamic range in excess of 90 dB.

DC fibre-optic accelerometer with sub-μg sensitivity

We describe a fibre-optic accelerometer, based on the technique of nonlinear displacement-to-strain conversion (NDSC), suitable for use at DC and low frequencies. The sensor exhibits a minimum detectable acceleration of less than 1 μg RMS (g = 9.8 m/s 2 ) at 0.5 Hz and a linear dynamic range greater than 10 6 .

Fiber Optic Sensors: An Overview

Fiber optic sensor technology is explained in terms of theory and applications. Various components common to all fiber optic sensors are compared. Two classes of sensing devices are emphasized: amplitude-modu-lated sensors and phase-modulated sensors. Specific examples of amplitude-modulated devices-one a pressure sensor using an optical reflection technique and another an accelerometer using a microbend technique-are described. Four types of fiber optic interferometers used in phase-modulated sensors are discussed. The effect of the optical fiber jacket on the sensitivity of phase-modulated sensors is considered. A miniature pressure sensor and a highly sensitive fiber optic accelerometer, both employing phase modula-tion, are described.

Fiber-optic systems for temperature and vibration measurements in industrial applications

Fiber-optic systems for temperature and vibration measurements have been developed. It is shown that using the principle of wavelength conversion in the sensor, measurement systems, in which only one fiber is used for the connection of the sensor to the instrument, can be realized. The measuring accuracy is practically independent of the properties of the fiber-optic system such as fiber length, fiber diameter and type of connectors used. These properties are very important, reducing installation problems and costs. The adoption of semiconductor technology allows batch fabrication of sensor elements. For the prototype temperature instrument an accuracy of 0·5°C was obtained in the temperature range 0 to +200°C. Fiber-optic accelerometers for vibration measurements with a resolution of 0·05 g and a dynamic range of 70 dB have been realized. The systems have been tested in different industrial environments and have properties that make them suitable for many industrial applications. Advantages of fiber-optic systems are their noise immunity, galvanic isolation and inherent intrinsic safety.

Batch fabrication of micromechanical elements in GaAs–Al xGa 1– xAs

If III-V semiconductors are used in micromechanical elements rather than silicon, a number of new possibilities arise, such as efficient energy conversions from electrical to optical or mechanical energy due to direct band gaps and non-zero piezoelectric coefficients. A fabrication process is described for micromechanical elements in GaAs–Al x Ga 1- x As, and fabricated structures have been analysed with a number of methods. It is possible to fabricate thin structures with remarkably flat surfaces. It is found that the built-in curvature of thin structures, quantified by a bending radius, is dependent on thickness ratios, though not uniquely. The vibration characteristics of cantilever beams can be understood from classifical theory. The primary application for the fabricated structures is as fibre-optic accelerometers for vibration monitoring.