Cross-Entropy Method for Design and Optimization of Pixelated Metasurfaces

© 2013 IEEE. Electromagnetic metasurfaces are planar two-dimensional metamaterials, typically of subwavelength thickness. Unit cell elements of different shapes have been widely explored, including electric and magnetic dipoles, patches, arbitrary geometries and pixelated surfaces. Although pixelated metasurfaces have a great advantage of geometric versatility, their design and analysis requires algorithmic approach. One of the techniques for their design is via evolutionary simulation-driven optimization. Since full-wave electromagnetic simulations are time-consuming, optimization methods with fast convergence properties are preferable. In this article, we demonstrate the application of the cross-entropy optimization method to design of artificial magnetic conductors (AMCs) and thin printed phase shifters. Single-frequency AMCs at 10 GHz (X band) and dual-frequency AMCs at 8 and 12 GHz (X and Ku band) were produced that are more manufacturing-friendly, and thus cost effective, than previously reported AMCs. We also show that phase-shifting unit cells with transmission magnitudes over 0.9 (linear) can be designed using the proposed optimization technique. Other potential applications of these unit cells are in phase-correcting and beam-steering metasurfaces

The Use of a Pair of 3D-Printed near Field Superstructures to Steer an Antenna Beam in Elevation and Azimuth

The paper presents a method to design beam-steering antennas using a pair of 3D printed perforated dielectric structures (PDSs) placed in the near-field region of a base antenna, which has a fixed beam. Detailed designs and quantitative comparison of two beam-steering antenna systems are presented. One antenna system has a conical horn antenna and the other uses a resonant-cavity antenna (RCA) as the base antenna. In both cases, the first PDS transforms the phase distribution of the aperture near field and hence tilts the antenna beam to an offset angle. The second PDS, placed above the first, introduces an additional linear progression to the phase of the near field. The two PDSs are rotated independently to steer the beam in both azimuth and elevation. The PDSs have been 3D-printed using acrylonitrile butadiene styrene (ABS) filaments. Each prototype was fabricated in about 16 hours, weighs 300 grams, and costs approximately 5.5 US Dollars. The measured results show that, at the operating frequency of 11 GHz, the RCA-based system has a peak gain of 17.7 dBi compared to the 16.6 dBi gain obtained with the horn-based system. In a fixed E-plane, the variation in the aperture near-field phase of the horn antenna (115°) is much less than that of the RCA (360°). This reduces the efforts required for phase correction and hence led to the former having a larger 3dB measured gain bandwidth of 1.2 GHz compared with the 0.7 GHz bandwidth of the latter, but at the cost of 35.6% increase in the total height of the antenna system

Benchtop magnetic shielding for benchmarking atomic magnetometers

Here, a benchtop hybrid magnetic shield containing four mumetal cylinders and nine internal flexible printed circuit boards (flex-PCBs) is designed, constructed, tested, and operated. The shield is designed specifically as a test-bed for building and operating ultrasensitive quantum magnetometers. The geometry and spacing of the mumetal cylinders are optimized to maximize shielding efficiency while maintaining Johnson noise <15 fT/ Hz−−−√ . Experimental measurements at the shield’s center show passive shielding efficiency of (1.0±0.1)×106 for a 0.2-Hz oscillating field applied along the shield’s axis. The nine flex-PCBs generate three uniform fields, which all deviate from perfect uniformity by ≤0.5 % along 50% of the inner shield axis, and five linear field gradients and one second-order gradient, which all deviate by ≤4 % from perfect linearity and curvature, respectively, over measured target regions. Together, the target field amplitudes are adjusted to minimize the remnant static field along 40% of the inner shield axis, as mapped using an atomic magnetometer. In this region, the active null reduces the norm of the magnitudes of the three uniform fields and six gradients by factors of 19.5 and 19.8, respectively, thereby reducing the total static field from 1.68 to 0.23 nT

A holistic approach to remote condition monitoring for the accurate evaluation of railway infrastructure and rolling stock

The rail industry needs to address a number of important operational challenges in the foreseeable future. First of all, the safety of rail transport needs to be maintained at an absolute maximum matching the achievements of the European airline industry of zero fatalities. Secondly, promote sustainable growth to support increasing demand for both passenger and freight rail transport. Thirdly, support the implementation of measurable innovations and improvements that help increase capacity of current infrastructure through enhanced availability. Finally, maximise the environmentally benign character of railway transport through exploitation of novel technologies such as hydrogen trains and advanced electrification employing renewable energy sources. This project, primarily focused on the UK Rail infrastructure, investigated the benefits arising from a holistic approach in the application of Remote Condition Monitoring (RCM) as a critical means for the accurate, efficient, reliable and cost-effective evaluation of key railway infrastructure assets and rolling stock. This work involved the use of several techniques and innovative methodologies based primarily on Acoustic Emission (AE) and vibration analysis in order to address the evaluation requirements for different components of interest. The results obtained have been very promising and present rail infrastructure managers and rolling stock operators with new opportunities for improved and more reliable operations. This work has led to the instrumentation of multiple sites across the UK rail network enabling measurements to be carried out on various assets under actual operational conditions. At Cropredy an integrated high-frequency vibro-acoustic RCM system has successfully been installed on the Chiltern railway line on the way from London to Birmingham. This customised system has been fully operational since 2015 measuring more than 200 passenger and freight trains every day moving at speeds up to 100 miles per hour (MPH). Prior to the installation of the system at Cropredy a Certificate (PA05/06524) of Acceptance was issued by Network Rail which after being renewed recently is now valid until September 2021. The system is due for an upgrade in the following stage of development, employing wireless sensors and advanced energy harvesting devices which are being developed under a collaborative Engineering and Physical Sciences Research Council (EPSRC) project between Exeter and Birmingham Universities, Network Rail, Swiss Approval UK and Quatrro. The widespread implementation of the techniques and methodologies researched will give rise to significant potential impact with respect to the effectiveness of maintenance strategies, particularly in terms of cost efficiency, improved availability of railway assets and better planning of available resources. As modern rail transport moves towards 24-hour railway, the inspection, maintenance and track renewal and upgrade regime will need to be re-thought at a fundamental level. Effective RCM will be a key factor in realistically enabling true round the clock operations. The results presented in this thesis have been part of a six-year research effort with a clear focus on addressing the true industrial need. The findings of this work have led to a re-think within Network Rail regarding the new possibilities arising from the effective use of RCM in designing and implementing more efficient and cost-effective railway operations whilst helping reduce the cost. The use of autonomous sensing systems in the future will change the inspection and maintenance strategies currently used shifting towards a truly prognostic operational strategy

Powered Wheelchair Users’ Experiences of Urban Mobility: Researching Access and Disablement through Mobile Methodologies and Digital Technologies

Ph. D. ThesisInaccessible environments and transportation exclude disabled people from full participation in society. Accessibility is affected by a wide range of socio-material factors, including the interaction between the environment and mobility devices. Yet to date, little research has investigated powered wheelchair users’ experiences of mobility, or how their knowledge might inform policymakers or service providers. In this thesis, I have worked with powered wheelchair users to document their experiences of mobility and explore how these might be conveyed to other people. An observational go-along method was used, travelling with thirteen participants as they captured their experiences using ‘JourneyCam’, a bespoke smartphone video and data collection tool. The data were used as prompts in semi-structured interviews following each journey. Two further participants were interviewed without first using JourneyCam. Finally, through a series of group workshops, nine participants explored how their collective experiences and knowledge might be used to create more accessible environments. Powered wheelchair users accumulate situated knowledge which they use to navigate disabling barriers, but this knowledge is gained through significant personal labour and experiences of psycho-emotional disablism. Participants highlighted a wide range of socio-material enablers and disablers to their mobility, including barriers associated with poorly implemented ‘accessibility’ features. However, their efforts to convey their knowledge to service providers were met with indifference, and funnelled through individualised complaints processes. In response participants sought more effective modes of engagement that might bring about systemic change, in line with their commitment to social model principles of confronting structural disablement. I conclude that not only could powered wheelchair users’ situated knowledge be a valuable resource for service providers and policymakers, but it is also a form of collective resistance to ableist societal practices. The social model of disability remains relevant for disabled people’s efforts to counter disablement, serving as an ‘oppositional device’ that fosters shared resistance.EPSR