Imperial College London

ProfessorRichardSyms

Faculty of EngineeringDepartment of Electrical and Electronic Engineering

Professor
 
 
 
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Contact

 

+44 (0)20 7594 6203r.syms

 
 
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Location

 

702Electrical EngineeringSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

362 results found

Syms R, Noorwali A, 2022, Polyphase codes for multiplexed acoustic signalling and sensing on pipes, Smart Materials and Structures, Vol: 31, ISSN: 0964-1726

Transmission of acoustic signals between distributed sensor nodes may be useful for status monitoring of elongated structures such as pipelines. In principle, coded signals can be used in an asynchronous multiplexed system, provided the signals are distinguishable. However, multimode effects complicate signal propagation, so any such codes should be short. A search for polyphase code families with properties suitable for acoustic code division multiple access is presented. Algorithms for reduction of search space to allow use of a laptop for code discovery are described. Short codes of base 6 are shown to outperform codes of bases 2, 3, 4 and sets suitable for systems with 2 and 3 users are identified. The codes have similar properties to Barker codes but larger sidelobes. Their use is demonstrated by simulation and experiment at kHz frequencies using an air-filled copper pipe, an electromagnetic acoustic transducer (EMAT) and a microphone designed to excite and detect the $L\left( {0,1} \right)$ mode. Low loss propagation over 25 m is achieved with a 20 kHz carrier. Excellent agreement between experiment and theory is demonstrated, with performance limited by transducer bandwidth.

Journal article

Voronov A, Syms RRA, Sydoruk O, 2022, High-performance magnetoinductive directional filters, Electronics, Vol: 11, Pages: 1-16, ISSN: 1450-5843

Multiport magnetoinductive (MI) devices with directional filter properties are presented. Design equations are developed and solved using wave analysis and dispersion theory, and it is shown that high-performance directional filters can be realised for use both in MI systems with complex, frequency-dependent impedance and in conventional systems with real impedance. Wave analysis is used to reduce the complexity of circuit equations. High-performance MI structures combining directional and infinite rejection filtering are demonstrated, as well as multiple-passband high-rejection filtering. A new method for improving filtering performance through multipath loss compensation is described. Methods for constructing tuneable devices using toroidal ferrite-cored transformers are proposed and demonstrated, and experimental results for tuneable MI directional filters are shown to agree with theoretical models. Limitations are explored, and power handling sufficient for HF RFID applications is demonstrated, despite the use of ferrite materials.

Journal article

Syms R, Liu D, 2022, Buckling Electrothermal NEMS Actuators: Analytic Design for Very Slender Beams, Micro, Vol: 2, Pages: 54-67

<jats:p>Analytic approximations are presented for the response of buckling-mode electrothermal actuators with very slender beams with a width-to-length ratio of W/L≤0.001 of the type found in nanoelectromechanical systems (NEMS). The results are found as closed-form solutions to the Euler beam bending theory rather than by an iterative numerical solution or a time-consuming finite element analysis. Expressions for transverse deflections and stiffness are presented for actuators with the common raised cosine and chevron pre-buckled shapes. The approximations are valid when the effects of bending dominate over those of axial compression. A few higher-order approximations are also presented for less slender beams with 0.001≤W/L≤0.01.</jats:p>

Journal article

Syms RRA, Voronov A, Sydoruk O, 2022, HF RFID Tag Location Using Magneto-Inductive Waves, IEEE JOURNAL OF RADIO FREQUENCY IDENTIFICATION, Vol: 6, Pages: 347-354

Journal article

Syms R, Bouchaala A, 2021, Mechanical Synchronization of MEMS Electrostatically Driven Coupled Beam Filters, MICROMACHINES, Vol: 12

Journal article

Syms R, Sydoruk O, Wiltshire M, 2021, Magneto-inductive HF RFID system, International Journal of Radio Frequency Identification Technology and Applications, Vol: 5, Pages: 148-153, ISSN: 1745-3216

Efforts to increase read range in passive HF RFID systems are hampered by the poor range scaling law of inductive coupling. An alternative approach to enlarging capture volume—increasing the lateral extent of the antenna—is proposed, using a magneto-inductive (MI) travelling wave arrangement to allow larger antenna sizes. A theory of load modulation in MI systems is first presented, together with field simulations in the capture volume. A 2.3 metre-long MI antenna is then constructed, and an active tag emulator is used to demonstrate load modulation. RFID is then demonstrated, with the antenna in both reflection and transmission modes, using a custom reader constructed from laboratory equipment. A transverse read range of 0.5 m is obtained using commercial off-the-shelf RFID cards with 12 W RF power, with high uniformity along the length of the antenna.

Journal article

Voronov A, Sydoruk O, Syms RRA, 2021, Power waves and scattering parameters in magneto-inductive systems, AIP ADVANCES, Vol: 11

Journal article

Wright S, Syms R, 2021, Shock-free ion transmission in a skimmer-based MEMS mass spectrometer vacuum interface, Journal of Micromechanics and Microengineering, Vol: 31, ISSN: 0960-1317

Shock-free ion transmission from atmospheric pressure to a MEMS-based mass spectrometer has been achieved using micro-engineered nickel skimmers. The signal level has increased 70-fold compared with a previous configuration in which the skimmer did not sample the supersonic flow. The skimmers are formed by electroplating internal surfaces of anisotropically etched, pyramidal holes in (100) silicon. Etching from the reverse of the wafer exposes free- standing, open-ended skimmers supported by remaining silicon. High-resolution schlieren imaging has been used to visualise gas flow within the interface. Signal enhancement and increased gas throughput are observed when the skimmer attaches to the supersonic gas expansion via oblique shocks. The silicon back wall interacts with the flow field, causing the free jet Mach disc to evolve into a bowl-shaped surface shock whose position asymptotically approaches a stand-off separation as the interface pressure decreases. Ideally, the skimmer entrance should be located approximately midway between the inlet and the back wall. This development should allow a sensitivity increase in MEMS mass spectrometers using pumps of moderate capacity.

Journal article

Bouchaala A, Syms R, 2020, New architectures for micromechanical coupled beam array filters, Microsystem Technologies: micro and nanosystems information storage and processing systems, Vol: 27, Pages: 3377-3387, ISSN: 0946-7076

Coupled resonator filters implemented as microelectromechanical systems (MEMS) offer performance advantages as band-pass filters at MHz frequencies. Here new designs based on resonant cavities for acoustic slow waves are developed to allow alternative frequency responses. Derivation of the lumped element model for coupled beam systems with in-plane motion from Rayleigh–Ritz perturbation theory is first reviewed. Departures from ideal behaviour caused by mechanical and electrostatic detuning are resolved. Slow wave theory is then used to develop linear array topologies with novel responses including band-stop and comb filtering with controlled filter roll-off. A systematic procedure is developed to allow rapid identification of design parameters without the need for lengthy numerical simulation, using the lumped element, stiffness matrix and finite element methods to investigate the layout parameters of initial design concepts, detailed mechanical effects and detailed electrostatic effects, respectively. High performance is demonstrated, with good agreement between the models.

Journal article

Syms R, Khuntikeo N, Titapun A, Chamadol N, Boonphongsathien W, Sa-Ngiamwibool P, Taylor-Robinson S, Wadsworth C, Zhang S, Kardoulaki Eet al., 2020, In vitro intraductal MRI and T2 mapping of cholangiocarcinoma using catheter coils, Hepatic Medicine : Evidence and Research, Vol: 2020, Pages: 107-114, ISSN: 1179-1535

Aim: Diagnostic imaging of early-stage cholangiocarcinoma is challenging. A previous in vitro study of fixed-tissue liver resection specimens investigated T2 mapping as a method of exploiting the locally increased signal-to-noise ratio (SNR) of duodenoscope coils for improved quantitative magnetic resonance imaging (MRI), despite their non-uniform sensitivity. This work applies similar methods to unfixed liver specimens using catheter-based receivers.Methods: Ex vivo intraductal MRI and T2 mapping were carried out at 3T on unfixed resection specimens obtained from cholangiocarcinoma patients immediately after surgery using a catheter coil based on a thin-film magneto-inductive waveguide, inserted directly into an intrahepatic duct.Results: Polypoid intraductal cholangiocarcinoma was imaged using fast spin echo sequences. High resolution T2 maps were extracted by fitting of data obtained at different echo times to mono-exponential models, and disease-induced changes were correlated with histopathology. An increase in T2 was found compared with fixed specimens and differences in T2 allowed the resolution of tumour tissue and malignant features such as polypoid morphology.Conclusions: Despite their limited field of view, useful data can be obtained using catheter coils, and T2 mapping offers an effective method of exploiting their local SNR advantage without the need for image correction.

Journal article

Khuntikeo N, Titapun A, Chamadol N, Boonphongsathien W, Sa-Ngiamwibool P, Taylor-Robinson SD, Wadsworth CA, Zhang S, Kardoulaki EM, Young IR, Syms RRet al., 2020, Improving the detection of cholangiocarcinoma: in vitro MRI-based study using local coils and T2 mapping, Hepatic Medicine : Evidence and Research, Vol: 12, Pages: 29-39, ISSN: 1179-1535

Aim: Cholangiocarcinoma is endemic in southeast Asia, generally developing from liver fluke infestation. However, diagnostic imaging of early-stage disease is challenging. The aim of this work is to investigate relaxometry (specifically, T2 mapping) as a method of exploiting the higher signal-to-noise ratio (SNR) of internal coils for improved reception of magnetic resonance signals, despite their non-uniform sensitivity.Methods: Ex vivo T2 mapping was carried out at 3T on fixed resection specimens from Thai cholangiocarcinoma patients using an mGRASE sequence and an endoscope coil based on a thin-film magneto-inductive waveguide and designed ultimately for internal use.Results: Disease-induced changes including granulomatous inflammation, intraepithelial neoplasia and intraductal tumours were correlated with histopathology, and relaxation data were compared with mono- and bi-exponential models of T2 relaxation. An approximately 10-fold local advantage in SNR compared to a 16-element torso coil was demonstrated using the endoscope coil, and improved tissue differentiation was obtained without contrast agents.Conclusion: The performance advantage above follows directly from the inverse relation between the component of the standard deviation of T2 due to thermal noise and the SNR, and offers an effective method of exploiting the SNR advantage of internal coils. No correction is required, avoiding the need for tracking, relaxing constraints on coil and slice orientation and providing rapid visualization.

Journal article

Alsaleh M, Barbera TA, Andrews RH, Sithithaworn P, Khuntikeo N, Loilome W, Yongvanit P, Cox IJ, Syms RRA, Holmes E, Taylor-Robinson SDet al., 2019, Mass spectrometry: A guide for the clinician, Journal of Clinical and Experimental Hepatology, Vol: 9, Pages: 597-606, ISSN: 0973-6883

Metabolic profiling, metabonomics and metabolomics are terms coined in the late 1990s as they emerged as the newest ‘omics’ technology at the time. This line of research enquiry uses spectroscopic analytical platforms, which are mainly nuclear magnetic resonance spectroscopy and mass spectrometry (MS), to acquire a snapshot of metabolites, the end products of a complex biological system. Metabolic profiling enables the detection, quantification and characterisation of metabolites in biofluids, cells and tissues. The source of these compounds can be of endogenous, microbial or exogenous origin, such as dietary or xenobiotic. This results in generating extensive, multivariate spectroscopic data that require specific statistical manipulation, typically performed using chemometric and pattern recognition techniques to reduce its dimensions, facilitate its biological interpretation and allow sample classification and biomarker discovery. Consequently, it is possible to study the dynamic metabolic changes in response to disease, intervention or environmental conditions. In this review, we describe the fundamentals of MS so that clinicians can be literate in the field and are able to interrogate the right scientific questions.

Journal article

Syms RRA, Sydoruk O, Bouchaala A, 2019, Improved optical imaging of high aspect ratio nanostructures using dark-field microscopy, Nanotechnology, Vol: 30, ISSN: 0957-4484

Improvements to white light optical imaging of widely spaced, high aspect ratio nanostructures are demonstrated using dark-field field microscopy. 1D models of bright- and dark-field imaging are developed from rigorous modal diffraction theory by assuming that features are periodic. A simple model is developed to explain dark field results and simulated line images obtained using the two modalities are compared for different dimensions and materials. Increased contrast between etched features and the substrate is demonstrated in dark field, due to its reduced sensitivity to scattering from flat areas. The results are verified using silicon nanostructures fabricated by sidewall transfer lithography, and feature separation with improved tolerance to apparent substrate brightness is demonstrated during image segmentation using the Otsu method.

Journal article

Alsaleh M, Leftley Z, Barbera T, Sithithaworn P, Khuntikeo N, Loilome W, Yongvanit P, Cox IJ, Chamadol N, Syms R, Andrews R, Taylor-Robinson Set al., 2018, Cholangiocarcinoma: a guide for the nonspecialist, International Journal of General Medicine, Vol: 12, Pages: 13-23, ISSN: 1178-7074

Cholangiocarcinoma (CCA) is a tumor with increasing prevalence around the world. The prevalence of CCA is highest in East Asia and most significantly in the countries through which the Mekong River flows, owing to the presence of liver flukes, which are consumed in raw fish dishes. Outside Asia, the causes of bile duct cancers for the most part are unknown. In this review, we assess the current state of knowledge in both fluke-associated and sporadic CCA, from etiological, diagnostic, and treatment perspectives.

Journal article

Syms RRA, Bouchaala A, Sydoruk O, Liu Det al., 2018, Optical imaging and image analysis for high aspect ratio NEMS, Journal of Micromechanics and Microengineering, Vol: 29, ISSN: 0960-1317

A strategy for optical microscopy of high-aspect-ratio (HAR) nanoelectromechanical systems (NEMS) that combine large feature spacing and large height with sub-wavelength width is presented. Line images are simulated using a 2D model of incoherent imaging based on modal diffraction theory. Beyond a sufficient depth, it is shown that sub-wavelength features appear as dark lines, while wider features are visible as their edges. The results suggest NEMS and MEMS may be separated from background in images by detection of valleys in brightness. Results are confirmed by imaging of Si NEMS containing 100 nm wide features in a bright-field microscope. Algorithms for separation of NEMS, MEMS and background in microscope images based on valley detection, thresholding and masking are demonstrated.

Journal article

Wright S, Syms RRA, 2018, Supersonic jet interactions with a micro-engineered skimmer, Journal of Micromechanics and Microengineering, Vol: 28, ISSN: 0960-1317

A micro-engineered, skimmer-based vacuum interface has been demonstrated and used to investigate gas dynamics on a sub-millimeter length scale. The interface is fabricated as a stacked assembly of silicon dies, based on an anisotropically etched inlet orifice and a pyramidal skimmer cone formed in electroplated nickel. Expansion of gas into vacuum, interaction of a supersonic jet with the skimmer and transmission of a collimated beam into a second vacuum stage have all been imaged with a schlieren microscope. Using a glass-walled vacuum chamber, flow patterns upstream and fully downstream of the skimmer have been imaged together for the first time. At low first-stage pressures, the 150–200 µm tall skimmers cannot fully penetrate the shock arising from interaction of the jet with the back wall. However, as the pressure is increased, a multiple shock cell structure evolves, the jet narrows and transmission rises sharply. Eventually, a collimated beam is transmitted to the second stage. When the skimmer aperture is smaller than the source aperture, a series of distinct peaks is evident in a plot of transmission against first-stage pressure. Imaging shows that at each successive peak, the number of shock cells increases by one and the skimmer inlet is coincident with a node.

Journal article

Wiltshire MCK, Syms RRA, 2018, Measuring noise in microwave metamaterials, JOURNAL OF APPLIED PHYSICS, Vol: 123, ISSN: 0021-8979

Electromagnetic metamaterials are artificially constructed media composed of arrays of electrical circuits that can exhibit electric and magnetic characteristics unlike those of any conventional materials. However, the materials are lossy and hence noisy, so that the signal-to-noise ratio in practical situations is greatly reduced. In particular, operating in the double negative region, where both the permittivity and the permeability are negative so that the refractive index is real but negative, incurs significant loss and noise penalties. In this work, we report noise measurements on a double negative metamaterial at microwave frequencies and compare them with the results of a simple model based on a transmission line loaded with lossy elements that mimic the split ring resonators and fine wires of the metamaterial. A noise source is associated with the resistive part of each element, and these are added incoherently to predict the total noise spectrum of the metamaterial. The theoretical results are in good agreement with the measurements. In particular, we find that the measured noise spectrum has contributions from both electric and magnetic noise, but is dominated by the magnetic noise. This limits possible applications, even with optimised materials, to functions that cannot be realised by conventional means.

Journal article

Kamel H, Syms RRA, Kardoulaki EM, Rea Met al., 2018, Surgical wound monitoring by MRI with a metamaterial-based implanted local coil, EPJ Applied Metamaterials, Vol: 5, ISSN: 2272-2394

An implantable sensor for monitoring surgical wounds after bowel reconstruction is proposed. The sensor consists of a coupled pair of 8-element magneto-inductive ring resonators, designed for mounting on a biofragmentable anastomosis ring to give a local increase in signal-to-noise ratio near an annular wound during 1H magnetic resonance imaging. Operation on an anti-symmetric spatial mode is used to avoid coupling to the B1 field during excitation, and a single wired connection is used for MRI signal output. The electrical response and field-of-view are estimated theoretically. Prototypes are constructed from flexible elements designed for operation at 1.5 T, electrical responses are characterized and local SNR enhancement is confirmed using agar gel phantoms.

Journal article

Syms RRA, Kardoulaki E, Rea M, Choonee K, Taylor-Robinson S, Wadsworth C, Young IRet al., 2017, Magneto-inductive magnetic resonance imaging duodenoscope, Progress in Electromagnetics Research (PIER), Vol: 159, Pages: 125-138, ISSN: 1070-4698

A magnetic resonance imaging (MRI) duodenoscope is demonstrated, by combining non-magnetic endoscope components with a thin-film receiver based on a magneto-inductive waveguide.The waveguide elements consist of figure-of-eight shaped inductors formed on either side of a flexiblesubstrate and parallel plate capacitors that use the substrate as a dielectric. Operation is simulatedusing equivalent circuit models and by computation of two- and three-dimensional sensitivity patterns.Circuits are fabricated for operation at 127.7 MHz by double-sided patterning of copper-clad Kaptonand assembled onto non-magnetic flexible endoscope insertion tubes. Operation is verified by benchtesting and by1H MRI at 3T using phantoms. The receiver can form a segmented coaxial image alongthe length of the endoscope, even when bent, and shows a signal-to-noise-ratio advantage over a surfacearray coil up to three times the tube diameter at the tip. Initial immersion imaging experiments havebeen carried out and confirm an encouraging lack of sensitivity to RF heating.

Journal article

Syms RRA, Kardoulaki E, Rea M, Taylor-Robinson S, Wadsworth C, Young IRet al., 2017, Metamaterial Magnetic Resonance Imaging Endoscope, 2017 11th International Congress on Engineered Material Platforms for Novel Wave Phenomena (METAMATERIALS), Publisher: IEEE, Pages: 337-339

Conference paper

Kamel H, Syms R, Kardoulaki EM, Rea Met al., 2017, Metamaterial MRI-based Surgical Wound Monitor, 2017 11th International Congress on Engineered Material Platforms for Novel Wave Phenomena (METAMATERIALS), Publisher: IEEE, Pages: 334-336

Conference paper

Syms R, 2017, Rapid evaporation-driven chemical pre-concentration and separation on paper., Biomicrofluidics, Vol: 11, ISSN: 1932-1058

Airflow-enhanced evaporation is investigated as a method for rapid chemical preconcentration on a thin porous substrate. The mechanism is described by combining 1D models of capillary rise, chromatography, and pervaporation concentration. It is shown that the effective length of the column can be shorter than its actual length, allowing concentrate to be held at a stagnation point and then released for separation, and that the Péclet number, which determines the concentration performance, is determined only by the substrate properties. The differential equations are solved dynamically, and it is shown that faster concentration can be achieved during capillary filling. Experiments are carried out using chromatography paper in a ducted airflow, and concentration is quantified by optical imaging of water-soluble food dyes. Good agreement with the model is obtained, and concentration factors of ≈100 are achieved in 10 min using Brilliant Blue FCF. Partial separation of Brilliant Blue from Tartrazine is demonstrated immediately following concentration, on a single unpatterned substrate. The mechanism may provide a method for improving the sensitivity of lab-on-paper devices.

Journal article

Syms RRA, Liu D, Ahmad MM, 2017, Nanostructured 2D cellular materials in silicon by sidewall transfer lithography NEMS, Journal of Micromechanics and Microengineering, Vol: 27, ISSN: 0960-1317

Sidewall transfer lithography (STL) is demonstrated as a method for parallel fabrication of 2D nanostructured cellular solids in single-crystal silicon. The linear mechanical properties of four lattices (perfect and defected diamond; singly and doubly periodic honeycomb) with low effective Young's moduli and effective Poisson's ratio ranging from positive to negative are modelled using analytic theory and the matrix stiffness method with an emphasis on boundary effects. The lattices are fabricated with a minimum feature size of 100 nm and an aspect ratio of 40:1 using single- and double-level STL and deep reactive ion etching of bonded silicon-on-insulator. Nanoelectromechanical systems (NEMS) containing cellular materials are used to demonstrate stretching, bending and brittle fracture. Predicted edge effects are observed, theoretical values of Poisson's ratio are verified and failure patterns are described.

Journal article

Syms RRA, Floume T, 2017, Parasitic coupling in magneto-inductive cable, Journal of Physics D: Applied Physics, Vol: 50, ISSN: 0022-3727

Magneto-inductive (MI) waveguides are linear arrangements of magnetically coupled L–C resonators that propagate electrical energy at radio frequency without direct connection. To achieve the strong magnetic coupling needed for low-loss propagation, adjacent elements must be in such close proximity that electric coupling arises. In contrast to electric coupling in split ring resonators, the coupling occurs between the inductive tracks of adjacent resonant loops. Parasitic capacitance is demonstrated in flexible magneto-inductive cable, and shown to introduce additional propagation bands above the MI band. Simple models are developed to predict this effect, and strategies discussed to improve high-frequency isolation.

Journal article

Kardoulaki EM, Syms RRA, Young IR, 2016, MRI for noninvasive thermometry, eMagRes, Vol: 5, Pages: 1203-1217, ISSN: 2055-6101

MRI was recognized for its potential use as a noninvasive in vivo thermometer 30 years ago. Today, the most popular application of MR thermometry is the guidance of thermal therapies for the treatment of cancer and other pathologies. These minimally invasive operations are routinely performed on patients who are not eligible for surgery in approximately 40 medical centers globally. The aim is to deliver or abduct thermal energy in order to cause local tissue necrosis or to sensitize a lesion to chemotherapy or radiotherapy without causing harm to the surrounding healthy tissue. Here we explain the principles of operation of MR thermometry and provide a critical review of the proposed methods, highlighting remaining fundamental and technical issues as well as recent progress. Emphasis is placed on hardware advances (RF receivers) for improved signal-to-noise ratio (SNR) which would lead to better accuracy, spatiotemporal resolution, and precise calibration. We conclude with a general outlook for the field.

Journal article

Kardoulaki EM, Syms RRA, Young IR, Rea Met al., 2016, SNR in MI catheter receivers for MRI, IEEE Sensors Journal, Vol: 16, Pages: 1700-1707, ISSN: 1530-437X

Internal coils have a signal-to-noise ratio (SNR) advantage during magnetic resonance imaging. However, coils with continuous cables are generally unsafe, due to the risk of RF heating. Segmented cables, such as magneto-inductive waveguides, should introduce inherent safety at the price of increased noise, from both the cable and the body. Here, we derive analytical SNR expressions for both types of noise, develop a model to compare the SNR of different types of receiver, and validate the model with data from imaging experiments at 3T. Experiments and theory confirm that body noise does not prevent an SNR gain compared with an eight-element external coil, even when a long section of waveguide is loaded with tissue.

Journal article

Syms R, Wright S, 2016, MEMS Mass Spectrometers: the Next Wave of Miniaturization, Journal of Micromechanics and Microengineering, Vol: 26, ISSN: 1361-6439

This paper reviews mass spectrometers based on micro-electro-mechanical systems (MEMS) technology. The MEMS approach to integration is first briefly described, and the difficulties of miniaturizing mass spectrometers are outlined. MEMS components for ionization and mass filtering are then reviewed, together with additional components for ion detection, vacuum pressure measurement and pumping. Mass spectrometer systems containing MEMS sub-components are then described, applications for miniaturized and portable systems are discussed, and challenges and opportunities are presented.

Journal article

Syms R, Solymar L, 2015, A dynamic competition model of regime change, Journal of the Operational Research Society, Vol: 66, Pages: 1939-1947, ISSN: 1476-9360

A dynamic competition model for an oppressive government opposed by rebels is proposed, based on coupled differential equations with constant coefficients. Depending on their values, the model allows scenarios representing a stable, oppressive government and violent regime change. With constant coefficients, there can be no limit cycles. However, cycles emerge if rebels and governments switch characteristics after a revolution, if resources change hands and rebel motivations switch from grievance to greed. This mechanism is proposed as an explanation for the establishment of a new repressive regime after the overthrow of a similar regime.

Journal article

Mokhtar MHH, Syms RRA, 2015, Tailored fibre waveguides for precise two-axis Lissajous scanning, Optics Express, Vol: 23, Pages: 20804-20811, ISSN: 1094-4087

A two-axis optical imaging system using a Lissajous scan pattern with non-integer frequency ratio is presented. A waveguide with precisely tuned mechanical resonant frequencies is constructed by dip coating two fibres with a transparent polymer. Motion is achieved by mounting a waveguide cantilever at 45° on a single piezoelectric actuator with a dual-frequency drive. Confocal signal collection is achieved using a mode-stripping detector, and feedback signals needed for frequency and phase locking are derived from intermittent reflection from an apertured mirror. The first scan axis is locked to the resonance of one of the modes, while the second scan axis is locked to the correct phase at the desired frequency ratio. Accurate acquisition of two-dimensional images is demonstrated.

Journal article

Kardoulaki EM, Syms RRA, Young IR, Rea M, Gedroyc WMWet al., 2015, Thin-film micro-coil detectors: Application in MR-thermometry, Sensors and Actuators A: Physical, Vol: 226, Pages: 48-58, ISSN: 1873-3069

Journal article

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