Browsing by Author "Andrew, R."

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    C.V.D. annual report: January, 1967 research project ru27-1 : analogue study of semiconductor device structures
    (College of Aeronautics, 1967-01) Andrew, R.; Loeb, H. W.
    The e::tension of the resistance network analogue method to the study of a M.O.S.T. structure is described. By means of an iterative technique, data regarding channel current, field distribution, surface charge and position of pinch-off point as function of gate and drain voltagen can be obtained which do not involve the usual 'gradual' channel approximation Results for a particular device geometry are presented. A discussion of a digital computer approach to the solution of semiconductor device current flow problems is included, together with preliminary results.
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    Fault diagnosis of distributed systems : analysis, simulation and performance measurement.
    (1992-01) Mohammed, Thabit Sultan; Andrew, R.
    Fault diagnosis forms an essential component in the design of highly reliable distributed computing systems. Early models for diagnosis require a global observer, whereas the diagnosis is shared between the systems nodes in later models. These models are reviewed and their different diagnosability properties reconciled. The design of improved fault diagnosis algorithms for systems without a global observer provides the main motivation for the thesis. The modified algorithm SELF3 [Hoss88] is taken as a starting point. A number of communication architectures used in distributed systems are reviewed. The properties of diagnosis algorithms depend strongly on the testing graph. A general class of testing graphs, designated as H-graphs, (which are a generalization of Dꞩṭ graphs introduced in [Prep67]), are investigated and their diagnostic properties determined. A software simulator for distributed systems has been written as the main investigative tool for diagnosis algorithms. The design and structure of the simulator are described. The diagnosis process is measured in terms of diagnostic time and number of messages produced, and the factors upon which these quantities depend are identified. The results of simulation of a number of systems are given under various fault conditions. A modified way of routing diagnosis messages, which, especially in large system s, results in a reduction in both the number of diagnosis messages and the time required to perform diagnosis, is presented. The thesis also contains a number of specific recommendations for improving existing self-diagnosis algorithms.
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    Theory and design of an electrodeless portable conductivity meter
    (Cranfield University, 1994) Pampalos, George; Andrew, R.
    The method of electrodeless conductivity measurement using two coils wound on toroidal ferrite cores in close proximity, coupled by the induced current in the electrolyte and operating in the audio range of frequencies has been used in process control since its introduction in the 1950s. It presents several advantages over the traditional method of measuring conductivity most important of which are stability, ruggedness, and maintenance free operation. Its major disadvantage is the large sample size required for correct operation. Despite its long established use in industry with various configurations of probe design, there is no published theory of its operation. A model of the probe operation is presented which takes into account the pattern of current flow induced in the electrolyte. This allows the electrolyte to be modelled as a resistance and the effect of geometrical factors upon this resistance is described. The operation of the probe can be represented by an equivalent circuit and theoretical expressions for the cell constant are derived. The correctness of this theory is confirmed by practical measurements. Factors affecting the performance of the probe such as operating frequency, cable length and coupling have been examined. The design and development of a conductivity meter which operates with an existing electrodeless sensor is described. It comprises analog circuitry which interfaces the sensor to a microcontroller. The microcontroller provides synchronisation, analog circuit control, calculations, display update, and interfacing with a small keyboard. A requirement that the meter be portable necessitated the minimisation of component count, cost and power consumption. Addition of reference curves for various chemicals stored in the microcontroller's EEPROM allows the instrument to be used as a concentration meter.