Broadband Measurement Techniques for Impedance Spectroscopy- and Time Domain Reflectometry Applications
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Beschreibung
The following thesis focuses on the application of broadband impedance measurements, often referred to as impedance spectroscopy, as well as similar measurement methods such as Time Domain Reflectometry (TDR). Broadband complex impedance measurements of a sample have been successfully conducted since about a century for obtaining characteristic frequency dependent electric and dielectric material properties. The condition of a sample can be determined based on measured data as well as suitable data processing methods and models. Potential practical applications for impedance spectroscopy are widely found in almost all areas of the daily life and include medical, biological, chemical, automotive, industrial, environmental, pharmaceutical and food quality applications. Although impedance spectroscopy is a well known scientific field with a huge amount of existing publications, there are only few real products based on this technology available today. Most scientific publications elaborate on the theoretical background and demonstrate the applicability of the method within many different applications. In most cases it can be observed that presented research results are based on measurements which have been conducted with expensive state-of-the-art laboratory equipment. The suitability of the impedance spectroscopy method is proven in many cases but mostly there is no resulting product available due to the high cost of the measurement equipment. In addition to the already studied applications there are still a lot of applications and problems where impedance spectroscopy could be used but which are not or not fully investigated yet. Many of these applications can be found in market segments where additional requirements exist such as miniaturization, low-power and battery operation and last but not least low cost.
The content of the following thesis is subdivided into two parts. The first part is about the detailed analysis of three practical measurement applications. Two applications belong to the field of biomedical engineering and the third application belongs to the field of moisture measurement. Within the second part of this thesis new developed circuits and measurement concepts are presented. Based on these new concepts it is possible to implement cost sensitive real products for the investigated applications.
Within the first medical application a capacitive sensor is developed which is able to determine the hematocrit value (HCT) of a blood sample based on impedance measurements. This sensor can be attached to standard plastic tubing in existing machines with an extracorporeal blood circulation system without the need for a direct contact with the blood. Laboratory experiments with the new developed sensor show an accuracy of approximately 4 % HCT and a resolution of approximately 0,1 %. Within the second investigated medical application the biological tissue which is close to the tip of a needle is continuously analysed during the dynamic insertion process and based on the obtained impedance data the tissue type is classified. This allows for positioning the tip of a needle or cannula within a well defined target tissue type with a minimum technical effort. By using a coaxial cannula design the achieved high spatial resolution is in the range of the diameter of the cannula. Short chirp signals are used as measurement signals and allow for a short measurement and processing time of below 1 ms for recognizing a tissue type. The third investigated application is in the field of moisture measurement. Here a water detection system with spatial resolution along a transmission line is implemented which allows for groundwater level monitoring and detection of penetrating water in buildings.
The second part of the thesis deals with the development of suitable electronic measurement equipment and circuits which cover the requirements given by the previously studied applications. Goal of the development is to dramatically reduce the size, the power consumption and the cost compared to existing standard measurement devices. In combination with the flexible design of the developed circuits and systems this allows for implementing real products in similar other applications as well. In spite of the fact that broadband impedance measurement is not a new field of work, there is still a lack of available miniaturized and cheap measurement equipment. However, in the recent past there was a lot of progress in the field of programmable and reconfigurable digital hardware. Today there are very cheap but powerful logic components available. The measurement circuits developed within this thesis are based on such programmable logic components. The technical benefits of these components are used in conjunction with suitable measurement signals and sampling methods. As a result of this thesis there are two independent platforms available. One platform is optimized for static applications where the total acquisition time is of minor importance but the requirements for a (virtual) very high temporal resolutions are present. In this case the employed sampling concept is conventional undersampling of a periodic measurement signal. The sampling system itself is based on a digital variation of a delta modulator circuit. The second developed measurement platform is optimized for dynamic applications where the required acquisition time for obtaining an impedance spectrum is a critical parameter. Chirp signals are used due to the excellent scalability with respect to signal duration, signal amplitude and signal bandwidth as well as the option for fast digital hardware signal processing.
Prototype circuits have been constructed and successfully tested based on the two developed measurement concepts. In addition the first platform which employs the undersampling scheme is used within a research project in cooperation with the University of Queensland, Brisbane, Australia. The platform is modified and used for soil moisture measurements based on the Time-Domain-Reflectometry (TDR) principle. Currently 20 TDR-meter devices are installed near Brisbane, Australia within a ground water monitoring experiment. Another 20 TDR-meter prototype devices have been built in cooperation with the University of Darmstadt and are used for water detection and moisture measurement in buildings.
Autor*in
Dennis Trebbels
Themen in »Broadband Measurement Techniques for Impedance Spectroscopy- and Time Domain Reflectometry Applications«
Impedanzspektroskopie Meß- und Regelungstechnik Zeitbereichsreflektometrie