Accurate estimation of airborne ultrasonic time-of-flight for overlapping echoes

In this work, an analysis of the transmission of ultrasonic signals generated by piezoelectric sensors for air applications is presented. Based on this analysis, an ultrasonic response model is obtained for its application to the recognition of objects and structured environments for navigation by autonomous mobile robots. This model enables the analysis of the ultrasonic response that is generated using a pair of sensors in transmitter-receiver configuration using the pulse-echo technique. This is very interesting for recognizing surfaces that simultaneously generate a multiple echo response. This model takes into account the effect of the radiation pattern, the resonant frequency of the sensor, the number of cycles of the excitation pulse, the dynamics of the sensor and the attenuation with distance in the medium. This model has been developed, programmed and verified through a battery of experimental tests. Using this model a new procedure for obtaining accurate time of flight is proposed. This new method is compared with traditional ones, such as threshold or correlation, to highlight its advantages and drawbacks. Finally the advantages of this method are demonstrated for calculating multiple times of flight when the echo is formed by several overlapping echoes.This work has been supported by the Spanish Government projects DPI2006-15313 and DPI2012-36959

A Novel Methodology to Estimate Single-Tree Biophysical Parameters from 3D Digital Imagery Compared to Aerial Laser Scanner Data

Airborne laser scanner (ALS) data provide an enhanced capability to remotely map two key variables in forestry: leaf area index (LAI) and tree height (H). Nevertheless, the cost, complexity and accessibility of this technology are not yet suited for meeting the broad demands required for estimating and frequently updating forest data. Here we demonstrate the capability of alternative solutions based on the use of low-cost color infrared (CIR) cameras to estimate tree-level parameters, providing a cost-effective solution for forest inventories. ALS data were acquired with a Leica ALS60 laser scanner and digital aerial imagery (DAI) was acquired with a consumer-grade camera modified for color infrared detection and synchronized with a GPS unit. In this paper we evaluate the generation of a DAI-based canopy height model (CHM) from imagery obtained with low-cost CIR cameras using structure from motion (SfM) and spatial interpolation methods in the context of a complex canopy, as in forestry. Metrics were calculated from the DAI-based CHM and the DAI-based Normalized Difference Vegetation Index (NDVI) for the estimation of tree height and LAI, respectively. Results were compared with the models estimated from ALS point cloud metrics. Field measurements of tree height and effective leaf area index (LAIe) were acquired from a total of 200 and 26 trees, respectively. Comparable accuracies were obtained in the tree height and LAI estimations using ALS and DAI data independently. Tree height estimated from DAI-based metrics (Percentile 90 (P90) and minimum height (MinH)) yielded a coefficient of determination (R2) = 0.71 and a root mean square error (RMSE) = 0.71 m while models derived from ALS-based metrics (P90) yielded an R2 = 0.80 and an RMSE = 0.55 m. The estimation of LAI from DAI-based NDVI using Percentile 99 (P99) yielded an R2 = 0.62 and an RMSE = 0.17 m2/m−2. A comparative analysis of LAI estimation using ALS-based metrics (laser penetration index (LPI), interquartile distance (IQ), and Percentile 30 (P30)) yielded an R2 = 0.75 and an RMSE = 0.14 m2/m−2. The results provide insight on the appropriateness of using cost-effective 3D photo-reconstruction methods for targeting single trees with irregular and heterogeneous tree crowns in complex open-canopy forests. It quantitatively demonstrates that low-cost CIR cameras can be used to estimate both single-tree height and LAI in forest inventories

Doctor of Philosophy

dissertationA gasifier's temperature is the primary characteristic that must be monitored to ensure its performance and the longevity of its refractory. One of the key technological challenges impacting the reliability and economics of coal and biomass gasification is the lack of temperature sensors that are capable of providing accurate, reliable, and long-life performance in an extreme gasification environment. This research has proposed, demonstrated, and validated a novel approach that uses a noninvasive ultrasound method that provides real-time temperature distribution monitoring across the refractory, especially the hot face temperature of the refractory. The essential idea of the ultrasound measurements of segmental temperature distribution is to use an ultrasound propagation waveguide across a refractory that has been engineered to contain multiple internal partial reflectors at known locations. When an ultrasound excitation pulse is introduced on the cold side of the refractory, it will be partially reflected from each scatterer in the US propagation path in the refractory wall and returned to the receiver as a train of partial echoes. The temperature in the corresponding segment can be determined based on recorded ultrasonic waveform and experimentally defined relationship between the speed of sound and temperature. The ultrasound measurement method offers a powerful solution to provide continuous real-time temperature monitoring for the occasions that conventional thermal, optical, and other sensors are infeasible, such as the impossibility of insertion of temperature sensors, harsh environment, unavailable optical path, and more. Our developed ultrasound system consists of an ultrasound engineered waveguide, ultrasound transducer/receiver, and data acquisition, logging, interpretation, and online display system, which is simple to install on the existing units with minimal modification on the gasifier or to use with new units. This system has been successfully tested with a 100 kW pilot-scale downflow oxyfuel combustor, capturing in real-time temperature changes during all relevant combustion process changes. The ultrasound measurements have excellent agreement with thermocouple measurements, and appear to be more sensitive to temperature changes before the thermocouples response, which is believed to be the first demonstration of ultrasound measurements segmental temperature distribution across refractories

Advances in Sonar Technology

The demand to explore the largest and also one of the richest parts of our planet, the advances in signal processing promoted by an exponential growth in computation power and a thorough study of sound propagation in the underwater realm, have lead to remarkable advances in sonar technology in the last years.The work on hand is a sum of knowledge of several authors who contributed in various aspects of sonar technology. This book intends to give a broad overview of the advances in sonar technology of the last years that resulted from the research effort of the authors in both sonar systems and their applications. It is intended for scientist and engineers from a variety of backgrounds and even those that never had contact with sonar technology before will find an easy introduction with the topics and principles exposed here

Analysis of the effect of leaf-on and leaf-off forest canopy conditions on LiDAR derived estimations of forest structural diversity

UK legislation aims to conserve and enhance biological diversity within the UK and so accurate measurements of forest biodiversity are important to assess efficacy of management activities in this context. Forest structural diversity metrics can be used as indicators of biodiversity and airborne LiDAR data provide a means of producing these metrics. Forest structure metrics derived from LiDAR can be significantly affected by the canopy conditions the datasets are collected under. Existing studies have combined and compared leaf-on and leaf-off LiDAR datasets in existing analyses, however the majority of these utilise field sites where climate, species and terrain are very different to those found in the UK. Additionally, studies comparing leaf-on and leaf-off LiDAR over forested areas assess the changes in pulse penetration through the canopy and how this effects forest structure metrics and not the effect on modelled forest structure diversity. The novel aim of this research is to assess and compare the accuracy of forest structural diversity modelled from two LiDAR surveys collected under leaf-on and leaf-off conditions, and do so in a UK forest environment. A robust methodology for correcting the absolute and relative accuracy between datasets was adopted and comparative analysis of ground detection and return height metrics (maximum, mean and percentiles of return height) and return height diversity (L-CV, CV, kurtosis, standard deviation, skewness and variance) was undertaken. Regression models describing the field tree size diversity measurements were constructed using diversity metrics from each LiDAR dataset in isolation and, where appropriate, a mixture of the two. Both surveys were consistently effected by growth and differing survey parameters making the isolation and assessment of the effects of seasonal change difficult. Despite this, models created using diversity variables from both LiDAR datasets were generally very similar. Both leaf-on and leaf-off LiDAR dataset models described 65% of the variance in tree height diversity (R² 0.65, RMSE 0.05, p <0.0001), however models utilising leaf-off LiDAR diversity variables described DBH diversity, crown length diversity and crown width diversity more successfully than leaf-on (leaf-on models resulted in R² values of 0.68, 0.41 and 0.19 respectively and leaf-off models 0.71, 0.62 and 0.26 respectively). When diversity variables calculated from both LiDAR datasets were combined into one model to describe tree height diversity and DBH diversity their efficacy was increased (R² of 0.77 for tree height diversity and 0.72 for DBH diversity). The results suggest strongly that tree height diversity models derived from airborne LiDAR collected (and where appropriate combined) under any seasonal conditions can be used to differentiate between single and multiple storey UK forest structure with confidence. However, leaf-off LiDAR acquisitions can generate models with the ability to better explain the diversity of crown shapes in a forest stand than leaf-on, with no improvement in model performance when the two are combined

A SONAR-Based Water Level Monitoring System: An Experimental Design

Storage of water is common to all human for domestic and industrial utilization. Today, water tanks are the conventional and a major component in water storage systems as they have the capacity to hold large volume of water over a long period of time. They come in different shapes and sizes. However inadequate monitoring of water level in this storage facility may lead to either shortage of water supply when needed or its wastage during lifting process. Hence, there is need for proper monitoring of water level in a storage facility. This need motivates this work, which is the development of water level monitoring system that based on sonar technology. Materials employed in the development include ultrasonic sensor module, ATmega-328P microcontroller, 16MHz crystal oscillator, 12V dc water pump among others. The developed system monitors the water level and activates the pump as appropriate based on the input the ultrasonic sensor gives to the microcontroller which occupies the centre of the system. The developed system has potential of mitigating the attendant problems that usually arise from wastage or shortage of water supply with respect to water storage facility. Keywords—SONAR, water storage, monitoring syste

Joint University Program for Air Transportation Research, 1988-1989

The research conducted during 1988 to 1989 under the NASA/FAA-sponsored Joint University Program for Air Transportation Research is summarized. The Joint University Program is a coordinated set of three grants sponsored by NASA Langley Research Center and the Federal Aviation Administration, one each with the Massachusetts Institute of Technology, Ohio University, and Princeton University. Completed works, status reports, and annotated bibliographies are presented for research topics, which include computer science, guidance and control theory and practice, aircraft performance, flight dynamics, and applied experimental psychology. An overview of the year's activities for each university is also presented

Signal processing methods for defect detection in multi-wire helical waveguides using ultrasonic guided waves

This thesis was submitted for the degree of Master of Philosophy and awarded by Brunel University LondonNon-Destructive Testing of industrial components carries vital importance, both financially and safety-wise. Among all Non-Destructive techniques, Long Range Ultrasonic Testing utilizing the guided wave phenomena is a young technology proven to be commercially valid. Owing to its well-documented analytical models, Ultrasonic Guided Waves has been successfully applied to cylindrical and plate-like structures. Its applications to complex structures such as multi-wire cables are fairly immature, mainly due to the high complexity of wave propagation. Research performed by the author approaches the long range inspection of overhead transmission line cables using ultrasonic guided waves. Existing studies focusing on guided wave application on power cables are extremely limited in inspection range, which dramatically degrades its chances of commercialization. This thesis consists of three main chapters, all of which approaches different problems associated with the inspection of power cables. In the first chapter, a thorough analysis of wave propagation in ACSR (most widely used power cable) cables is conducted. It is shown that high frequency guided waves, by concentrating the energy on the surface layers, can travel much further in the form of fundamental longitudinal wave mode, than previous studies have shown. Defect detection studies proved the system’s capability of detecting defects which introduce either increase or decrease in cross sectional area of the cable. Results of the chapter indicate the detectability of defects as small as 4.5% of the cross sectional area through a 26.5 meter long cable without any post-processing. In the second chapter, several algorithms are proposed to increase the inspection range and signal quality. Well-documented wavelet-denoising algorithm is optimized for power cables and up to 24% signal-to-noise ratio improvement is achieved. By introducing an attenuation correction framework, a theoretical inspection range of 75 meters is presented. A new framework combining dispersion compensation and attenuation correction is proposed and verified, which shows an inspection range of 130 meters and SNR improvement up to 8 dBs. Last chapter addresses the accurate localization of structural defects. Having proven the optimum excitation and related wave propagation in ACSR cables, a system having a more complex wave propagation characteristics is studied. A new algorithm combining pulse compression using Maximal Length Sequences and dispersion compensation is applied to multi-modal signals obtained from a solid aluminum rod. The algorithm proved to be able to improve signal quality and extract an accurate location for defects. Maximal Length Sequences are compared to chirp signals in terms of SNR improvement and localization, which produced favourable results for MLS in terms of localization and for chirp in terms of SNR improvement