R.T. Rakowski

The Effect of Aluminium Inclusions on the Dielectric Breakdown of Polyethylene

A narrow size distribution of irregular aluminium particles was blended into power cable insulation grade polyethylene. Some batches of the resulting material were then melt-filtered to reduce the size of particles present and narrow the distributions further. The failure statistics of the loaded polymers were then determined under AC ramped stress. The data have been fitted to extreme value statistics, and trends with size and density of defect can clearly be identified. In addition, for the filtered material, a minimum breakdown field can be associated with a given filter size: a result of commercial importance. Some indications exist to suggest that different modes of failure operate at high and low fields. Candidates for these modes are analysed and discussed in terms of the distributions of defects present. Local field enhancement due to the included flaws were calculated using finite-element techniques. The results are compared with a percolation model of breakdown. Predictions are found to quantify accurately the reduction in the characteristic strength of the material over the narrow range of defect concentrations examined.

Continuous wavelet transform and neural network for condition monitoring of rotodynamic machinery

This paper describes a novel method of rotodynamic machine condition monitoring using a wavelet transform and a neural network. A continuous wavelet transform is applied to the signals collected from accelerometer. The transformed images are then extracted as unique characteristic features relating to the various types of machine conditions. In the experiment, four types of machine operating conditions have been investigated: a balanced shaft; an unbalanced shaft, a misaligned shaft and a defective bearing. The back propagation neural network (BPNN) is used as a tool to evaluate the performance of the proposed method. The experimental results result in a recognition rate of 90 percent.

Serpentine magnetoresistive elements for tactile sensor applications

Performance data on a new tactile sensor configuration are presented. The sensor is based on magnetoresistive (MR) technology but incorporates a patented serpentine path element structure which offers a wide range of advantages over previously considered designs. Previous work on MR tactile sensors did not approach the problem of sensor optimization at the transducer level. By placing the emphasis on the bottom-up method of design, a wide range of sensors for different applications could be developed. Sensitivity is a key feature in tactile sensor design. This is a measure of the minimum detectable force change that can be accurately detected. The requirement for high signal-to-noise ratios indicates the need for high element resistances, the use of AC magnetic fields, and small geometric size (pixel resolution). >

Magnetoresistive force sensors for use in an instrumented pen

Abstract The project involves the design of a magnetoresistive force sensor to provide a feedback element in an instrumented pen. The pen, together with a functional electrical stimulation system, would allow writing to be achieved by tetraplegic patients. Experiments have been carried out to define the force range and force sensor design required for successful writing. Considering that tangential and normal forces are to be measured, magnetoresistive sensors are selected as the transduction technique. Experiments with current-carrying conductors and permanent magnets have been carried out to define the magnetic source for the sensors, in an attempt to optimize the system for low power consumption. Cast and sintered alloy magnets are finally chosen. If this type of permanent magnet is put in an orthogonal position with respect to the sense element plane, a lateral displacement sensitivity of −40 mV/mm in a range of 1.5 mm can be achieved, with low sensitivity to the separation between the magnet and the sensor (−5 mV/mm).

Development of Metallic Digital Strain Gauges

A joint Brunel-Southampton Universities’ research team has developed digital strain gauges based on a metallic triple-beam resonator structure with thick-film piezoelectric sensor elements. The resonator, an oscillating structure vibrating at resonance, is designed such that its resonant frequency is a function of the measurand. The resonator substrate was fabricated by a double-sided photochemical etching technique and the thick-film piezoelectric elements were deposited by a standard screen-printing process. The new metallic digital strain gauges can be used on stiff structures, have high overload capacities, low power consumption, frequency output for digital processing, and offer prospects for wireless-batteryless operation. The device can be easily mass-produced at low cost for use in a wide range of measuring systems, e.g. load cells, weighing machines, torque transducers and pressure sensors.

Reference source for the calibration of acoustic emission measurement

Acoustic Emission (AE) is a widely used technique for the integrity testing of many structures and is still growing with its use in process, condition and health monitoring. Its use is however limited by the current inability to gain any useful quantitative information from the measurements made. This is due to the fact that the quantitative information gathered is mostly a function of the structure of about which little is known, and is in most applications complex. Without reliable quantitative measurement, transfer of information between measurement systems will always be relative. This paper proposes a technique that aims to make the calibration of AE measurements possible with the use of a reference source. The use of the energy descriptor and a suitable calibrated reference source will allow in-situ calibrations of complete AE measurement systems, allowing quantitative measurements to be made on the calibrated system. The reference source discussed is a conical shaped piezoelectric transducer with a point contact tip. Its linearity with energy has been shown to be very good and its calibration has been demonstrated using an elastic ball impact.

Dielectric breakdown in metal-loaded polyethylene

The failure statistics of polyethylene plaques loaded with a know volume fraction of metallic particles of fixed size distribution have been determined. The results are compared with the predictions of the percolation breakdown model. It is shown that the large decrease observed in characteristic strengths at low defect loadings obeys the theoretical expression derived for the model. However, the type of breakdown statistic could not be determined unambiguously.

An optical current transducer for residual current sensing in the milliampere range

Abstract Current-sensing devices play an important role in electrical safety engineering. Optical current transducers developed so far have been applied to the measurement of hundreds of amperes. This paper presents the design considerations for a magneto-optic sensor suitable for use in a residual current device (RCD) where the requirement is to measure differential currents in the milliampere range. The principles of operation, electronic design and experimental results for such a device are presented in this paper. In high-power electrical networks, electrical isolation, saturation and frequency-response characteristics of the magneto-optic sensors can provide a cost-effective and versatile sensing method. Results show that the device has an independent linearity of 99.91 % and a sensitivity of 40 mV A −1 .

U Rod linear refractometer for on-line compositional sensing

A novel refractometer has been developed for use in on-line compositional sensing in the food and drink industry. The device is designed to measure total amounts of soluble solids in liquid foods such as jams, syrups and pulps. A system is required that is simple and cheap, yet optimised for accuracy of refractive index measurement and linearity. A U Rod is presented as the principal component, the multiple reflections within the U Rod provide a key design feature to reduce the influence of non-total internal reflection and significantly improve the measurement linearity.

A virtual instrument for the characterisation of magneto-optical materials

The scarce information relating to the magneto-optical properties of materials precludes an optimal solution in the design of new sensors and applications. A number of these properties must be accurately determined before further decisions in system design are made. Such properties must be characterized in order to select a material that enables a sensor with required sensitivity. Virtual instruments are suitable for automating such a test and measurement process, as already done in similar situations. A characterization system using a virtual instrument was developed for measuring the relevant properties of candidate materials under controlled laboratory conditions. Results indicate that the system is capable of providing measurements of Verdet constant with accuracy better than 2.5% and a resolution of 402 /spl times/ 10/sup -6/ rad.