Erik A. Moro

A mobile-agent-based wireless sensing network for structural monitoring applications

A new wireless sensing network paradigm is presented for structural monitoring applications. In this approach, both power and data interrogation commands are conveyed via a mobile agent that is sent to sensor nodes to perform intended interrogations, which can alleviate several limitations of the traditional sensing networks. Furthermore, the mobile agent provides computational power to make near real-time assessments on the structural conditions. This paper will discuss such prototype systems, which are used to interrogate impedance-based sensors for structural health monitoring applications. Our wireless sensor node is specifically designed to accept various energy sources, including wireless energy transmission, and to be wirelessly triggered on an as-needed basis by the mobile agent or other sensor nodes. The capabilities of this proposed sensing network paradigm are demonstrated in the laboratory and the field.

Dynamics of a noncontacting, white light Fabry–Perot interferometric displacement sensor

A white light extrinsic Fabry-Perot interferometer is implemented as a noncontacting displacement sensor, providing robust, absolute displacement measurements with micrometer accuracy at a sampling rate of 10 Hz. This paper presents a dynamic model of the sensing cavity between the sensor probe and the nearby target surface using a Fabry-Perot etalon approach obtained from straightforward electromagnetic field formulations. Such a model is important for system characterization, as the dynamically changing cavity length imparts a Doppler shift on any signals circulating within the sensing cavity. Contrary to previously published results, Doppler-induced shifting within the low-finesse sensing cavity is shown to significantly distort the measurement signal as recorded by the sensor. Experimental and simulation results are compared, and the direct effects of cavity dynamics on the measurement signal are analyzed along with their indirect impact on sensor performance. This document has been approved by Los Alamos National Laboratory for unlimited public release (LA-UR 12-00301).

Note: Simultaneous measurement of transverse speed and axial velocity from a single optical beam

A method is introduced for simultaneously measuring transverse speed and axial velocity using a single optical beam and a standard photon Doppler velocimetry (PDV) sensing architecture. This result is of particular interest given the recent, widespread use of PDV and the fact that optical velocimetry has thus far been limited to measuring motion in one dimension per probe. Further, this result demonstrates that both axial velocity data and transverse speed data (at least qualitative) may be obtained entirely through signal analysis; not requiring hardware modification. This result is immediately relevant to analyses of existing PDV data and to future efforts in high-speed optical velocimetry.

Experimental Validation and Uncertainty Quantification of a Single-Mode Optical Fiber Transmission Model

The radial intensity distribution of a transmission from a single-mode optical fiber is often approximated using Gaussian-shaped spatial field distributions. While such approximations are useful for some applications, they do not accurately describe optical transmission intensity off of the axis of propagation. A recent paper was presented that more accurately describes the intensity distribution, and this paper presents a simple experimental setup that verifies the models accuracy through formal uncertainty quantification procedures. Agreement between the model and the experiment is established both on and off of the axis of propagation. These results are then discussed in the context of displacement sensor designs based on the optical lever architecture. Transmission behavior off of the axis of propagation controls the sensor performance when large lateral offsets (25-1500 μ m) exist between transmitting and receiving fibers. The practical implications of modeling accuracy over this lateral offset region are discussed as they relate to the development of high-performance, intensity-modulated optical displacement sensors. Specifically, the sensitivity, linearity, resolution, and displacement range of a sensor are functions of the relative positioning of the sensors transmitting and receiving fibers. It is concluded that the predictive capability of the model presented in this paper could enable an improved methodology for high-performance sensor design.

A performance comparison of transducer designs for interferometric fiber optic accelerometers

Interferometric fiber optic accelerometers constitute a high-responsivity, high-resolution sensing architecture, with achievable sensitivities of several rad/g and resolutions in the micro-g range, depending on the specific configuration. Fiber Bragg grating (FBG) optical accelerometers offer ease of multiplexing but are inherently less sensitive than their interferometric counterparts. Fiber-based accelerometers have the usual optical advantages of being lightweight, electromagnetically immune, and non-spark emitting over traditional (piezo-electric) accelerometer architectures. Among fiber optic sensing methodologies, both interferometric and FBG accelerometers can be interrogated using phase-based demodulation, which offers advantages over intensity-based sensing schemes such as increased linearity, repeatability, and insensitivity to extraneous measurands. The performance of an accelerometer is often characterized in terms of its bandwidth, sensitivity, and resolution, all of which depend on the specific transducer design (the mechanical architecture) as well as the optical interrogation architecture. For a given optical interrogation architecture, a fundamental tradeoff exists in accelerometer transducer design between bandwidth and sensitivity; attempts to increase bandwidth will generally result in a decrease in sensitivity. This paper investigates the frequency and displacement characteristics that govern this tradeoff for several transducer configurations, in order to determine a pair of configurations that offer the greatest sensitivity for a given optical interrogation methodology (interferometric or FBG), at a prescribed bandwidth. The feasibility of several mechanical architectures is assessed based on the physical dimensions required for a given configuration to achieve a primary resonance of at least 15 kHz. The deflection of those configurations under their own self-weight is then considered a measure of accelerometer sensitivity in the measurement band below primary resonance. This paper has been reviewed by Los Alamos National Laboratory and received the following release number: LA-UR 10-00671.

Optical distance measurements to recover the material approach missed by optical velocimetry

Optical velocimetry is limited to measuring the component of the target velocity along the axis of the optical beam, thereby allowing a laterally moving tilted surface to approach a probe undetected. We are not discussing the detection of the lateral motion, but rather the detection of material approaching the probe due to lateral motion of a surface that is not perpendicular to the beam. This motion is not measured in optical velocimetry, and consequentially, integrating the velocity will in general give an incorrect position. We will present three approaches to overcome this limitation: Tilted wave-front interferometry, which maps time of flight into fringe displacement; pulse bursts for which we measure the change in the average arrival time of a burst, and amplitude modulation interferometry, in which a change in path length shows up as a change in the phase of the modulation. All three of these have the potential to be integrated with existing velocimetry probes for simultaneous velocity and displacement measurements. We will also report on initial tests of these approaches.

Performance characterization of an intensity modulated fiber optic displacement sensor

A testbed simulating an intensity-modulated fiber optic displacement sensor is experimentally characterized, and the implications regarding sensor design are discussed. Of interest are the intensity distribution of the transmitted optical signal and the relationships between sensor architecture and performance. Particularly, an intensity-modulated sensors sensitivity, linearity, displacement range, and resolution are functions of the relative positioning of its transmitting and receiving fibers. In this paper, sensor architectures with various combinations of these performance metrics are discussed. A sensor capable of micrometer resolution is reported, and it is concluded that this work could lead to an improved methodology for sensor design. This paper has been approved by Los Alamos National Laboratory for unlimited public distribution (LA-UR 10-06637).

Understanding the effects of Doppler phenomena in white light Fabry–Perot interferometers for simultaneous position and velocity measurement

In static tests, low-power (<5 mW) white light extrinsic Fabry-Perot interferometric position sensors offer high-accuracy (μm) absolute measurements of a targets position over large (cm) axial-position ranges, and since position is demodulated directly from phase in the interferogram, these sensors are robust to fluctuations in measured power levels. However, target surface dynamics distort the interferogram via Doppler shifting, introducing a bias in the demodulation process. With typical commercial off-the-shelf hardware, a broadband source centered near 1550 nm, and an otherwise typical setup, the bias may be as large as 50-100 μm for target surface velocities as low as 0.1 mm/s. In this paper, the authors derive a model for this Doppler-induced position bias, relating its magnitude to three swept-filter tuning parameters. Target velocity (magnitude and direction) is calculated using this relationship in conjunction with a phase-diversity approach, and knowledge of the targets velocity is then used to compensate exactly for the position bias. The phase-diversity approach exploits side-by-side measurement signals, transmitted through separate swept filters with distinct tuning parameters, and permits simultaneous measurement of target velocity and target position, thereby mitigating the most fundamental performance limitation that exists on dynamic white light interferometric position sensors.

Laser Speckle in Dynamic Sensing Applications

Laser velocimetry is used as a measurement technique across a variety of fields including mechanical vibration and flow research, medical diagnostics and shock physics. In modern photon Doppler velocimetry (PDV) applications, the perception is that laser speckle pollutes the measured signal, sometimes destroying useful data. Recent experiments suggest the possibility of exploiting speckle to measure transverse speeds, thereby gaining information about surface dynamics across an additional degree of freedom. In this report, we examine the influence of axial displacement and probe characteristics on speckle velocimetry measurement capabilities.

A comparison of techniques for extracting transverse speed from photon Doppler velocimetry signal content

We recently demonstrated that a single optical probe is capable of simultaneously measuring a surfaces velocity along the beam axis and its speed transverse to the beam axis. Doppler shifts in the measured data are related to axial motion, while intensity fluctuations, induced by speckle dynamics, are related to transverse motion. While it is readily apparent that speckle dynamics manifest themselves in the measured data, the ability to extract transverse speed from a particular (speckle-induced) signal feature is feature-dependent. In this paper, we relate a signals coherence, variance, and frequency content to surface dynamics, in an effort to determine the suitability of each of these features for calculating transverse motion (classification release number: LA-UR 13-26315).