Browsing by Author "Bell, J F W"

Now showing 1 - 7 of 7
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    An automated monitoring system for the production and measurement of metal fatigue
    (1988) Upton, David Mark; Bell, J F W
    New equipment for producing and following the fatigue process, in metals, has been developed. The specimens, which must be in the shape of tuning forks, are resonated at high amplitudes. This is achieved by attaching small, powerful, samarium-cobalt magnets to the ends of the tynes, enabling them to be driven efficiently by a "U"-core electromagnet. A small, piezoceramic strain gauge provides a method of picking up the vibrations. To maintain resonance, the signal is used in a positive feedback loop, which incorporates an analogue multiplier to provide AGC. This also keeps the amplitude constant at any desired level, throughout the duration of an experiment.
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    The classification of slurries and other suspensions using ultrasonic techniques
    (1985) Davies, Andrew Lewis; Bell, J F W
    The attenuation of ultrasonic waves travelling through a liquid containing a suspension of solids is highly dependent on frequency. At low frequencies, where the wavelength is long compared to the particle size, the particles move with the wave with some phase lag which gives rise to a loss component. At high frequencies, where the wavelength can be made comparable to, and ultimately less than the particle diameter, there is still some movement but most of the energy is lost to the wave by scattering. In this work, is attenuation used to characterize the suspension, and is treated in terms of these two components. Absorption which is the conversion of energy into heat and scattering from the actual particles. The former is expressed as loss (dB) per wavelength and increases with frequency, while the latter, in the short wavelength scattering zone simply obstructs the wave.
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    High resolution techniques for the measurement of component and device performance
    (1996) April, Kurt; Bell, J F W
    This aim of this thesis was to conduct an investigation into the performance of components and devices, whilst focusing on specific parameters and characteristics, by making use of high precision measurement techniques and incorporating a PC-based data acquisition system. An accurate and versatile measurement-card, employing a 12-bit analogue-to-digital converter, was designed and developed. The card, which plugs directly into the I/O bus of any computer, has an accuracy of one least significant bit (approximately 0.02), which is remarkably better than most A/D cards available on the market at present. Executable software was written in Pascal and Turbo Vision to facilitate the proper operation of the card, and to provide a platform for the user to define certain parameters during experimentation. The measurement-card has been used, with success, during all of the investigative experiments conducted for this thesis. An experiment using a computer, the measurement-card, and a pressure/temperature unit, employing a MPXIOOAP pressure sensor and LM35 temperature sensor, was set up to act as a ''weather station" over short periods (minutes, hours) and long periods (days, weeks). Analogue device, viz., a barometer and thermometer, were used as control instruments in order to verify the readings taken.
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    Liquid level measurement using a coplanar transmission line
    (1988) Pink, Clive Desmond; Bell, J F W
    The coplanar line has been used very successfully as an element in microwave circuits. Small size, high Q-factor, and accurate reproduction, are some of its many advantages. The coplanar transmission lines discussed in this report, are targeted at liquid level measurement, and are typically 30 cm. long. Their operating frequencies are consequently much lower than those of microwave coplanar waveguides, but they have common advantages. The factor which separates the coplanar line from similar liquid level sensors, is that it makes use of the electrical component of the electromagnetic fringe field, setup between its inner conductor, and the surrounding ground plane. The line is effectively a sharply tuned resonator, incorporated as the frequency controlling element of an electronic oscillator. The output frequency falls as a dielectric material penetrates the fringe field. An impressive sensitivity is accomplished by using very thin conductors, thereby ensuring that the fringe field energy is maximised. The most important feature of this sensor is its ability to operate non-intrusively when used with non-conducting vessels, or if employed in a metal tank, the unit can be encased in a dielectric material where the line is non-contacting (the liquid does not penetrate the unit). This combined with its excellent mechanical and electrical stability, and an accuracy better than 1 percent, makes the coplanar line a strong competitor in the field of liquid level measurement. The research began with a theoretical approach, and used lines machined from an Aluminium plate for characteristic impedance measurement. An empirical relation between the gap width, the line thickness, and the characteristic impedance of the line is presented. To assist with the design of the sensor, a lumped capacitance model of the line was developed. Various geometries were tested, and modified until a near linear response to water level was achieved. An advanced engineering model of the level sensor has been developed, which incorporates a stable digital output display, user calibration from the line's end points, and temperature compensation. A T-shaped line, which concentrates the field around its open end, was used for other applications such as, evaporation monitoring, measurement of slurry settlement, and to observe the effect of acids, bases, and salts in water. Various applications of the different coplanar line designs are proposed.
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    Measurement of linear position using a magnetostrictive wire
    (1986) Pennington, A J; Bell, J F W
    This thesis reports the details of construction and testing of a linear position measuring device and subsequent experimentation with the system. The design goals established were to construct a measuring device using established TTL devices, proven interfaces, ultra-reliable noise-clean, circuits, inexpensive components and to be based on transmission of magnetostrictively generated pulses in nickel wire. In addition it was to be able to operate in a harsh environment (e.g. underwater) where alternative devices would not function. While an accuracy objective was not established, it was hoped to be able to push accuracy near to the theoretical resolution. A. prototype measuring device was constructed consisting of a stretched nickel wire threaded through a transmitting coil and three receiving coils together with transmitting, receiving, timing and interface circuitry. The nickel wire was mounted on an optical bench with one receiving coil mounted to a moveable trolley. This configuration facilitated calibration and testing. The system was interfaced to a microcomputer via an IEEE 488 GPIB controller and calibration, testing and position appropriate computer programmes.
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    Transient response of temperature sensors
    (1994) Prefumo, Jean-Pierre Guito; Bell, J F W
    Transient of time constant 10 seconds will be within 2 % of its final value in 40· seconds. Cancelling this response and iritroducing one of time constant 1 second enables the 2 % band to be reached in just 5 seconds. It was also possible to correct transient responses made up of two exponentials to speed them up. Recursive estimators, which is a technique derived from successive approximations, namely the linear recursive estimator was used to estimate temperature changes in real time by basically weighing the new estimate according to how good the previous estimate was. Temperature measurement in the presence of noise is possible by using the Kalman filter which is a very powerful and impressive recursive estimator. The Kalman filter is basically a simple low-pass digital filter. The processed output is simply the weighted sum of the current input and the previous output and the results obtained were quite remarkable indeed.
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    Ultrasonic transducers for wideband attenuation measurements
    (1987) Mayer, J Bertrand; Bell, J F W
    An application of ultrasound presently being investigated at the Council for Mineral Technology {MINTEK) is the measurement of the concentration of graphite particles in suspension in an agitated pulp. The principle of the measurement is based on the attenuation characteristics of the medium over a frequency range extending from 0.1 MHz to 10 MHz. This wide range and the highly attenuating medium (typically 3 dB/cm) require therefore that the ultrasonic transducers exhibit efficient wideband characteristics. Piezoceramic transducers vibrating in thickness-mode are the most suitable to generate ultrasound at these frequencies. However, their high acoustic impedances relative to their liquid loads make them inherently narrowband and inefficient. To overcome these undesirable features, impedance matching techniques are used whereby the impedance mismatch between a transducer and its load is reduced. In this thesis, various electrical and acoustic impedance matching schemes were implemented on a number of conunercial PZTSA transducers which were then evaluated in terms of bandwidth and efficiency. At 1 MHz, the 3-dB bandwidth of a PZTSA transducer operating in pulse-echo mode in water was improved from 0. 1 MHz for the unmat.ched case to 0. 83 MHz when 2 quarter-wave matching sections were bonded to the radiating surf ace and an electrical matching section was inserted at the electrical port. The resulting 2-way Insertion Loss over the pass band was 16 dB. Similarly, the 3-dB bandwidth of a 500 kHz PZTSA transducer improved from 50 kHz to 138 kHz with an accompanying Insertion Loss of 12 dB when both electrical and acoustic impedance matching were implemented. Certain radiation characteristics of thickness-mode transducers such as beam directivity and radiation resistance were investigated as functions of the "diameter to wavelength" ratio, D/)... , of the radiating transducer. Maximum radiation resistance (and therefore maximum acoustic coupling) was observed when D/ .A =13. The same ratio also produced the best performance in terms of directivity and sensitivity to parallelism between transmitter and reflector {or receiver). A comprehensive study of the resonance spectra of flexural, in-plane and thickness-mode vibrations of thin disks was also performed using various materials commonly used in transducer construction. The elastic constants of the materials were then determined by three different methods, each method making use of a different resonance spectrum.