1,722,461 research outputs found
Design and optimisation of user-centric visible-light networks
No full textIn order to counteract the explosive escalation of wireless tele-traffic, the communication spectrum has been gradually expanded from the conventional radio frequency (RF) band to the optical domain. By integrating the RF band relying on diverse radio techniques and optical bands, the next-generation heterogeneous networks (HetNets) are expected to be a potential solution for supporting the ever-increasing wireless tele-traffic. Owing to its abundant unlicensed spectral resources, visible light communications (VLC) combined with advanced illumination constitute a competent candidate for complementing the existing RF networks. Although the advantages of VLC are multi-fold, some challenges arise when incorporating VLC into the classic RF HetNet environments, which may require new system architectures. This motivates our research on the system design of user-centric (UC) VLC.Our investigations are focused on system-level design of VLC and it is constituted by three major aspects, namely 1) by the cooperative load balancing (LB) in hybrid VLC and wireless local area network (WLAN) as discussed in Chapter 2; 2) by the UC cluster formation and multiuser scheduling (MUS) of Chapter 3; 3) as well as by the energy-efficient scalable video streaming design example of Chapter 4. Explicitly, we first study VLC as a complementary extension of the existing WLAN. In Chapter 2 we study various conventional cell formations invoked for networks in order to tackle the significant inter-cell interference (ICI) problem, including the traditional unity/nonunity frequency reuse (FR) techniques as well as the advanced combined transmission (CT) and vectored transmission (VT) schemes. Then a distributed LB algorithm is proposed for a hybrid VLC and WLAN network, which is then evaluated from various perspectives.In order to further mitigate the ICI in VLC networks, we focus our attention on novel UC-VLC cluster formation techniques in Chapter 3 and Chapter 4. The concept of UC cluster formation is a counterpart of the conventional network-centric (NC) cell formation, which is dynamically constructed according to the users’ location. Relying on graph theory, the joint cluster formation and MUS problem is solved in Chapter 3. Furthermore, another important optimisation aspect in most wireless networks is the achievable energy efficiency (EE). Hence, we design an energy-efficient scalable video streaming scheme for our UC-VLC network, which achieves superior performance compared to the NC cells in terms of its throughput attained, EE as well as the quality of service (QoS)
Optimal configuration of sensors in active vibration control
No full textIn this paper an investigation of a new formulation for active vibration control, which based on the optimal positions of piezoelectric sensors. The free vibration and modal properties are derived from the classical plate theory by using finite element method. A criterion is proposed to optimise the location of piezoelectric patches based on the observability Gramian of the structure and applied to a simply supported plate. The genetic algorithm is used to realise the optimal fitness function for finding the optimal configuration. The numerical simulation demonstrates that by locating piezoelectric sensors in the optimal positions, the energy delivery efficiency of the structure increases effectively
Rock drilling performance of rotary ultrasonic tools incorporating PZT piezoceramic and Mn:PIN-PMN-PT piezocrystal
Full text linkHigh performance Mn:PIN-PMN-PT piezocrystal material is investigated to understand if its extraordinary properties can replace the traditional hard piezoceramic material for space exploration applications. A bolted Langevin-style ultrasonic drill tool incorporating a pair of Mn:PIN-PMN-PT piezocrystal rings is built to compare with a same configuration ultrasonic drill tool actuated with a pair of hard piezoceramic rings, which are tuned to the first longitudinal mode (L1) at around 20 kHz. From the characterisation results, it is observed that the piezocrystal material presents significantly greater values of relative permittivity, electromechanical coupling coefficient, and piezoelectric charge coefficient than the hard piezoceramic material. Despite these outstanding properties, the piezocrystal driven ultrasonic drill tool shows similar displacement amplitudes to its counterpart. Nonetheless, the impedance magnitude of the piezocrystal driven ultrasonic drill tool at resonance is a magnitude lower than the piezoceramic actuated drill tool, due to the large piezoelectric charge coefficient d33. Ultrasonic rock drilling experiments suggest that the cutting force for sandstone and marble are greatly reduced, but limestone and tuff are less affected.
In general, the piezocrystal driven ultrasonic drill tool demonstrates a marginally improved cutting performance than the piezoceramic actuated drill tool, in terms of lower cutting force and motor power consumption, however, the tool wear appears slightly poorer. The research outcome of this paper indicates that the thickness mode of the piezocrystal rings might not be the optimal form of excitation, which could be due to the piezoelectric losses at high excitation levels, so other excitation conditions and vibration modes will need to be explored to fully adopt the extraordinary material properties of the piezocrystal material
Autonomous and ultrasonically assisted drilling in a range of rocks and ice
Full text linkDrilling in extreme environments may require reductions in weight-on-bit, applied torque, or energy use, without compromising rate-of-progress. This paper examines the use of ultrasonic vibration, directly superimposed onto an augering coring bit, to achieve this goal in aircrete, limestone, marble, tuff, and ice.
Compared to traditional rotary drilling processes using the same tool, the ultrasonically assisted drilling processes demonstrated improved rate-of-progress (∼ 400%) in all materials studied. In aircrete and limestone, there were also modest but consistent reductions in torque power demand and, at optimum vibration amplitudes, total energy consumption (∼25%). The other materials gave more mixed results: ultrasonically assisted drill cycles in marble were energy intensive, those in tuff were unpredictable due to the inconsistencies in that material, and those in ice led to the failure of the tooth bonding
Ultrasonically assisted coring of rocks
Full text linkDrilling in extreme environments such as extraterrestrial objects allow us to study the original geological and biological signature, and to understand the history hidden beneath. This will require reductions in weight-on-bit, torque and energy usage, without compromising the penetration rate-of-progress. This paper examines the use of ultrasonic vibration, directly superimposed onto an augering coring bit, to achieve this goal in aircrete and limestone. Experimental results show that compared to traditional rotary drilling processes using the same tool, the ultrasonically assisted drilling processes has demonstrated an improved rate-of-progress. A total energy consumption is founded at optimum vibration amplitude at the cutting teeth
Full and half-wavelength ultrasonic percussive drills
Full text linkUltrasonic-percussive drills are a leading technology for small rock drilling applications where power and weight-on-bit are at a premium. The concept uses ultrasonic vibrations to excite an oscillatory motion in a free-mass, which then delivers impulsive blows to a drilling-bit. This is a relatively complex dynamic problem involving the transducer, the free-mass, the drilling-bit and, to a certain extent, the rock surface itself. This paper examines the performance of a full-wavelength transducer compared to a half-wavelength system, which may be more attractive due to mass and dimensional drivers. To compare the two approaches, three-dimensional finite element models of the ultrasonic-percussive stacks using full and half wavelength ultrasonic transducers are created to assess delivered impulse at similar power settings. In addition, impact-induced stress levels are evaluated to optimize the design of drill tools at a range of internal spring rates before, finally, experimental drilling is conducted. The results suggest that full-wavelength systems will yield much more effective impulse but, interestingly, their actual drilling performance was only marginally better than half-wavelength equivalents
Metal-free flat horn for ultrasonic surgical devices
No full textUltrasonic surgical devices are traditionally designed as bolted Langevin transducers (BLTs), in which a bolt pre-stresses a stack of piezoelectric rings between two metallic end masses. This configuration offers high mechanical stability and acoustic power handling capability, but it suffers from increased bulk, weight, and self-heating issues, significantly restricting the ergonomic suitability for hand-held surgical procedures.Recently, planar ultrasonic scalpels have emerged as a more compact, lightweight, and thermal-efficient alternative. Unlike BLTs, which operate in the thickness mode of the piezoelectric stack, planar scalpels utilize the length-thickness mode of the piezoelectric plates bonded with epoxy near the longitudinal displacement nodes of a flat metal horn. However, their limited volume of active material reduces the dynamic performance of the devices. This work presents a novel planar design in which the flat horn is made entirely from a PZT plate, eliminating the metal counterpart. The goal is to maximize the PZT volume and minimize the epoxy usage, thereby enhancing the electromechanical performance
Meet the MOUSE (Micro-Optic Ultrasonic Exfoliator): Mars rover Rock Abrasion Tool (RAT) performance enhanced by ultrasonic technology
No full textThe Mars exploration Athena science goal is to explore areas where water may have been present on the early surface of Mars, and investigate the palaeo-environmental conditions of these areas in relation to the existence of life. The Rock Abrasion Tool (RAT) designed by Honeybee Robotics has been one of four key Athena science payload instruments mounted on the mechanical arm of the Spirit, Opportunity and Curiosity Mars Exploration Rovers. Exposed rock surfaces weather and chemically alter over time. Although such weathered rock can present geological interest in itself, there is a limit to what can be learned. If the geological history of a landing site is to be constructed, then it is important to analyse the unweathered rock interior as clearly as possible. The rock abrasion tool’s role is to substitute for a geologist’s hammer, removing the weathered and chemically altered outer surface of rocks in order to view the pristine interior. The RAT uses a diamond resin standard common grinding technique, producing a 5mm depth grind with a relatively high surface roughness, achieved over a number of hours per grind and consumes approximately 11 watts of energy. This study assesses the benefits of using ultrasonic assisted grinding to improve surface smoothness. A prototype Micro-Optic UltraSonic Exfoliator (MOUSE) is tested on a range of rock types and demonstrates a number of advantages over the RAT. In addition to a smoother grind finish, these advantages include a lower rate of tool tip wear when using a tungsten carbide tip as opposed to diamond resin, less moving parts and a power consumption of potentially <4 Watts depending on ultrasonic wave amplitude
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