Dr. Johan Schoeman Profile

Dr. Johan Schoeman

at University of Pretoria

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Publications (6)

Proceedings Article | 3 February 2017 Paper

Recent advances in the modelling of the thermal conductance of uncooled microbolometers

Johan Schoeman, Monuko du Plessis

Proceedings Volume 10036, 100360B (2017) https://doi.org/10.1117/12.2245720

KEYWORDS: Thin films, Microbolometers, Modeling, Sensors, Microelectromechanical systems, Thermal modeling, Thermal effects, Resistance, Atmospheric modeling, Thin film devices

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Proceedings Article | 3 February 2017 Paper

The characterisation and design improvement of a paper-based E.coli impedimetric sensor

P. Bezuidenhout, S. Kumar, M. Wiederoder, J. Schoeman, K. Land, T-H. Joubert

Proceedings Volume 10036, 100360L (2017) https://doi.org/10.1117/12.2245754

KEYWORDS: Sensors, Biosensors, Electrodes, Gold, Silver, Manufacturing, Adsorption, Ranging, Printing, Sensor performance

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Proceedings Article | 23 June 2014 Paper

The modelling of a capacitive microsensor for biosensing applications

P. Bezuidenhout, J. Schoeman, T. Joubert

Proceedings Volume 9257, 92570P (2014) https://doi.org/10.1117/12.2066384

KEYWORDS: Capacitance, Microsensors, Sensors, Glasses, Electrodes, Gold, Biosensors, Capacitors, Dielectrics, Instrument modeling

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Microsensing is a leading field in technology due to its wide application potential, not only in bio-engineering, but in other fields as well. Microsensors have potentially low-cost manufacturing processes, while a single device type can have various uses, and this consequently helps with the ever-growing need to provide better health conditions in rural parts of the world. Capacitive biosensors detect a change in permittivity (or dielectric constant) of a biological material, usually within a parallel plate capacitor structure which is often implemented with integrated electrodes of an inert metal such as gold or platinum on a microfluidic substrate typically with high dielectric constant. There exist parasitic capacitance components in these capacitive sensors, which have large influence on the capacitive measurement. Therefore, they should be considered for the development of sensitive and accurate sensing devices. An analytical model of a capacitive sensor device is discussed, which accounts for these parasitic factors. The model is validated with a laboratory device of fixed geometry, consisting of two parallel gold electrodes on an alumina (Al₂O₃) substrate mounted on a glass microscope slide, and with a windowed cover layer of poly-dimethyl-siloxane (PDMS). The thickness of the gold layer is 1μm and the electrode spacing is 300μm. The alumina substrate has a thickness of 200μm, and the high relative permittivity of 11.5 is expected to be a significantly contributing factor to the total device capacitance. The 155μm thick PDMS layer is also expected to contribute substantially to the total device capacitance since the relative permittivity for PDMS is 2.7. The wideband impedance analyser evaluation of the laboratory device gives a measurement result of 2pF, which coincides with the model results; while the handheld RLC meter readout of 4pF at a frequency of 10kHz is acceptable within the measurement accuracy of the instrument. This validated model will now be used for the geometric design and simulation of efficient capacitive sensors in specific biological detection applications.

Proceedings Article | 23 June 2014 Paper

An instrumentation amplifier based readout circuit for a dual element microbolometer infrared detector

D. de Waal, J. Schoeman

Proceedings Volume 9257, 92570L (2014) https://doi.org/10.1117/12.2066301

KEYWORDS: Sensors, Resistance, Infrared sensors, Microbolometers, Amplifiers, Infrared radiation, Thermography, Wheatstone bridges, Bolometers, Infrared detectors

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Proceedings Article | 23 June 2014 Paper

Design issues of a low cost lock-in amplifier readout circuit for an infrared detector

L. Scheepers, J. Schoeman

Proceedings Volume 9257, 92570O (2014) https://doi.org/10.1117/12.2066294

KEYWORDS: Microbolometers, Signal to noise ratio, Interference (communication), Amplifiers, Optical amplifiers, Modulation, Infrared radiation, Bandpass filters, Sensors, Thermography

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In the past, high resolution thermal sensors required expensive cooling techniques making the early thermal imagers expensive to operate and cumbersome to transport, limiting them mainly to military applications. However, the introduction of uncooled microbolometers has overcome many of earlier problems and now shows great potential for commercial optoelectric applications. The structure of uncooled microbolometer sensors, especially their smaller size, makes them attractive in low cost commercial applications requiring high production numbers with relatively low performance requirements. However, the biasing requirements of these microbolometers cause these sensors to generate a substantial amount of noise on the output measurements due to self-heating. Different techniques to reduce this noise component have been attempted, such as pulsed biasing currents and the use of blind bolometers as common mode reference. These techniques proved to either limit the performance of the microbolometer or increase the cost of their implementation. The development of a low cost lock-in amplifier provides a readout technique to potentially overcome these challenges. High performance commercial lock-in amplifiers are very expensive. Using this as a readout circuit for a microbolometer will take away from the low manufacturing cost of the detector array. Thus, the purpose of this work was to develop a low cost readout circuit using the technique of phase sensitive detection and customizing this as a readout circuit for microbolometers. The hardware and software of the readout circuit was designed and tested for improvement of the signal-to-noise ratio (SNR) of the microbolometer signal. An optical modulation system was also developed in order to effectively identify the desired signal from the noise with the use of the readout circuit. A data acquisition and graphical user interface sub system was added in order to display the signal recovered by the readout circuit. The readout circuit was able to enhance the SNR of the microbolometer signal significantly. It was shown that the quality of the phase sensitive detector plays a significant role in the effectiveness of the readout circuit to improve the SNR.

Proceedings Article | 23 June 2014 Paper

Employing a microbolometer as a low pressure sensor

J. Schoeman

Proceedings Volume 9257, 92570K (2014) https://doi.org/10.1117/12.2066287

KEYWORDS: Microbolometers, Resistance, Sensors, Manufacturing, Titanium, Thermal effects, Instrument modeling, Metals, Protactinium, Data modeling

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Showing 5 of 6 publications

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