W. Craig Michie

Simultaneous measurement of strain and temperature: error analysis

Many optical fiber sensors designed to recover quasi-static strain fields in the presence of significant temperature changes have been reported in recent years. A general theoretical analysis of the influ- ence of systematic errors associated with the measurement process is presented and applied to a range of techniques that are of current inter- est in the literature. The performances of measurement methods based on Bragg grating sensors, polarization-maintaining Fabry-Perot interfer- ometers, combined dual-mode interference/polarimetry sensors, and dis- persive Fourier transform spectroscopy measurements are contrasted with respect to the influence of measurement error, calibration error, cross talk, and engineering practicality.

Temperature compensation for optical current sensors

An analysis of an optical current transducer (OCT) considering in particular the influence of temperature induced variations in the Verdet constant is presented. The analysis is supported by an experimental evaluation of a prototype OCT over a range of operating temperatures and current values and concludes with a laboratory demonstration of a temperature compensation scheme that improves the measurement precision to better than 0.7%.

Implementation of herd management systems with wireless sensor networks

The work summarises a study of the data communications requirements for agricultural livestock monitoring applications using wireless sensor networks (WSNs). Several design challenges are identified and analysed in depth based on actual global positioning system positioning data gathered from an actual herd of cattle. A wireless system including antennae diversity together with data downloads optimisation schemes utilising data collector and routers are developed and tested in a working farm environment. Two analysis metrics, connection availability and connection duration, are used to quantify the impact of cattle movement on network connectivity. The major contributions of this study stem from a definition of the communication issues in deploying animal monitoring platforms in free-ranging farm environments and the analysis and optimisation of the wireless data download performance using as the foundation knowledge gained from a series of working farm trials. Additionally, the data download protocols are designed particularly to treat animal movement. The results prove the viability of WSN-based solutions for livestock monitoring applications.

GEOMETRIC REPRESENTATION OF ERRORS IN MEASUREMENTS OF STRAIN AND TEMPERATURE

Many optical fiber sensors, designed to recover quasistatic strain fields in the presence of significant temperature changes, have been reported in recent years. A number of recent publications have attempted to devise a method for assessing the relative performances of such sensing schemes. Here we report an analysis that represents the data recovery process from a geometrical standpoint and provides useful insight into the physical differences between measurement schemes. The performance of methods based on Bragg grating sensors, polarization-maintaining Fabry-Perot interferometers, combined dualmode interference-polarimetry sensors, and dispersive Fourier-transform spectroscopy measurements are contrasted.

Fiber optic sensor for simultaneous measurement of strain and temperature

The paper proposes polarimetric and two-mode differential interferometric schemes incorporated in an elliptical-core fiber to resolve strain and temperature simultaneously. Initial results indicate accuracies of +/- 10 microns/m and +/- 5 C for strain and temperature measurements, respectively. A technique, based on the evaluation of the condition number of a matrix, is shown to be useful in evaluating comparative merits of multiparameter sensing schemes. Experimental results for four fibers are presented, and cross-sensitivity issues are discussed.

Fiber optic technique for simultaneous measurement of strain and temperature variations in composite materials

A technique based upon the differential sensitivities of dual mode and polarimetric sensing schemes is shown to be capable of resolving simultaneously temperature and strain variations to within 20 micro-epsilon and 1 K over a strain and temperature excursion of 2 micro-epsilon and 45 K. The technique is evaluated experimentally over an 80 cm sensing length of unembedded optical fiber and in an 8 ply unidirectional carbon/epoxide laminate subject to temperature and strain cycling. A comparative analysis of the performance of the embedded and the unembedded fiber sensors is presented.

Vibration compensation technique for an optical current transducer

Details are presented on a technique for compensation of vibration-induced noise in an optical current transducer (OCT). The light intensity changes that result from vibration are reciprocal, i.e., their magnitude does not depend on the direction of light propagation. However, intensity changes that result from the Faraday rotation induced as the light travels through the OCT are dependent on the direction of propagation and hence, by using two counter-propagating beams, vibration- induced intensity noise can be canceled by applying simple signal processing. This solution also enables true dc current measurement to be made. The principles of operation of the compensation method an experimental evaluation of the system are presented and the limitations associated with the compensation scheme are discussed.

Auction-Based Network Selection in a Market-Based Framework for Trading Wireless Communication Services

A digital marketplace (DMP) is a market-based framework in which a network selection mechanism is facilitated through a variant of procurement first-price sealed-bid auction, i.e., wireless network operators (NOs) bid for the right to transport the subscribers requested service over their infrastructure. In this paper, we create an economic model of this mechanism, and we characterize the equilibrium under generic assumptions about the cost distributions of the NOs. Furthermore, the equilibrium is explicitly derived under more specific assumptions about the model, i.e., two NOs and costs drawn from uniform distributions. In this case, we also characterize the expected prices that the subscriber has to pay depending on their preferences about the service, e.g., trading off quality for a lower price. Finally, we provide a numerical analysis of the case with more than two NOs.

Hydrogel/fiber optic sensor for distributed measurement of humidity and pH value

The combination of chemically sensitive, swellable polymer materials with novel optical fiber cable designs to transduce the swelling activity into microbend loss enables a simple yet powerful sensor to be produced. Interrogating such cables with standard optical time domain reflectoctrometry (OTDR) instruments allows particular chemicals of interest to be detected and located along a cable which may extend to several kilometers. We report here on a sensor cable which uses a water swellable material, a hydrogel, to detect positions of water ingress, relative humidity level or pH value. In direct water ingress tests, wet sensor lengths as small as 5 cm in several hundreds of meters have been detected using conventional OTDRs. Following a review of the sensor design, we present the results of an investigation of the mechanical interaction between the hydrogel polymer and the optical fiber within the sensor. The behavior of the sensor is then characterized within environments of different relative humidity levels from 70 percent to 100 percent at temperatures ranging from 0 to 60 degrees C. The sensor was initially designed for applications within civil engineering but can be applied to a much broader range of measurement requirements, for example soil moisture measurement. We will report details on experimental observations on concrete cure within reinforcing tendon ducts and soil humidity measurements within different soil types.

Optical sensors for temperature and strain measurement

The measurement of quasi-static strain field using optical fibers presents a considerable challenge due to the inherent sensitivity of optical fibers to temperature. This paper summarizes recent work we have carried out on two approaches to this problem. Dual mode polarimetric measurements were investigated as a means of implementing distributed temperature measurements and radio frequency subcarrier sensors have been used to perform the same measurement on an integrated basis. These techniques are contrasted and assessed against other technologies such as Bragg gratings and dispersive Fourier transform spectroscopy on the basis of measurement capability, ease of implementation and technological maturity.