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Inductively Coupled Passive Resonance Sensors: Readout Methods and Applications

Research output: Collection of articlesDoctoral Thesis

Details

Original languageEnglish
PublisherTampere University of Technology
Number of pages73
ISBN (Electronic)978-952-15-4096-7
ISBN (Print)978-952-15-4079-0
StatePublished - 1 Mar 2018
Publication typeG5 Doctoral dissertation (article)

Publication series

NameTampere University of Technology. Publication
Volume1524
ISSN (Print)1459-2045

Abstract

Measurement systems are used to acquire information from the surrounding world. The requirements of the measurement system depend on the application, and the acquired information is used in different ways. For example, measurements are taken as part of the control systems of industrial processes. Alternatively, the information obtained from the measurements can be used to provide answers to scientific questions. Each measurement has a case-specific importance for the user and a certain cost in terms of time and money. Therefore, the same measurement approach is not optimal in every case. The design process of the measurement systems always includes a compromise between performance, viability, and cost. These factors are, in turn, strongly dependent on the implementation of the measurement system in each separate case. Inductively coupled passive resonance sensors provide a measurement method that has two notable benefits: the simple structure of the sensors and the possibility to take short-range wireless measurements. However, the limitations of the available readout devices have often impeded the use and development of these sensors in many demanding applications. In addition, uncertainty in the measurement results due to inductive coupling hinders the use of this method.

This work concerns the development and implementation of a measurement system based on inductively coupled passive resonance sensors. A custom-made readout device to improve the feasibility of the readout in applications where continuous field measurements are performed was both specified and produced. The readout device was implemented using a simplified version of the method used in conventional impedance analyzers. In addition, signal processing methods were developed which can extract resonance characteristics from the measured data. A special algorithm was developed to compensate for the effects of the changes in the inductive coupling when the measurement distance varies. The operation of the developed readout methods was studied using simulations, and several realistic measurement configurations were tested. Competing readout methods published in the literature were also simulated. The accuracy of all the studied methods depended on the configuration of the measurement system. The inductive coupling coefficient also had a significant influence on the accuracy of the tested methods.

The newly-developed readout methods and the inductively coupled passive resonance sensor were then utilized in a medical application to monitor the pressure between the skin and compression garments. These garments are used, for example, to improve the healing of burns and reduce swelling in the legs. Effective medical treatment of such conditions requires that the appropriate pressure is applied. With this system, the pressure reading under the compression garment can be obtained by using simple disposable sensors that can be read wirelessly through a thin fabric. Using our inductive coupling compensation method, the sensor enabled the monitoring of the pressure with the required level of precision.

Inductively coupled resonance sensors can also be used to monitor the properties of the materials around the sensor. This monitoring is possible because the permittivity of the environment near to the sensor affects the sensor’s resonance characteristics. This method was tested in two applications. In the first application, the manufacturing process of ceramic slurry was monitored by a sensor that was installed inside the container where the slurry was mixed. The resonance characteristics of the sensor were measured as the manufacturing process was incrementally carried out. The results indicated that the method could be used to control the composition of the slurry. In the second application, the inductively coupled sensors were tested in monitoring the degradation processes of two different polymers during hydrolysis. In this application, the sensors were encapsulated into the tested polymers. The polymer samples were kept inside containers filled with buffer solution and the resonance characteristics of the encapsulated sensors were then measured wirelessly from outside. The results showed a clear difference in degradation profiles between the tested polymers. The method may provide a novel way to continuously monitor the degradation processes of certain materials.

In summary, the developed readout methods improved the applicability of inductive coupled passive resonance sensors in the tested applications and created novel ways to acquire information. This new technology provides a good starting point for the development of a new generation of inductively coupled passive resonance sensors.

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