Broadband Measurement Techniques for Impedance Spectroscopy- and Time Domain Reflectometry Applications (English shop)

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Brief description

The present work deals with the application of broadband electrical impedance measurement, often also called impedance spectroscopy, as well as related broadband measurement methods such as time domain reflectometry. Broadband measurements of the complex electrical impedance of a sample have been successfully employed for about a century in order to determine characteristic frequency-dependent electrical and dielectric material properties. Based on the acquired measurement data and by means of suitable evaluation methods and models, a statement about the condition of the sample can be made in many cases. The potential field of application of impedance spectroscopy and the related time domain reflectometry covers almost all areas of daily life and ranges from medical, biological and chemical applications through automotive, industrial and environmental measurement technology to pharmacy and food analysis. Although this field of research and work has been known for a long time and countless publications exist, there are to date comparatively few technical implementations in the form of products. A large part of the published research and development work addresses theoretical fundamentals and demonstrates the basic feasibility in a multitude of different applications. In many cases it can be observed that the already published results are based on laboratory experiments using expensive and elaborate laboratory measurement equipment. Although proof of the basic applicability of the measurement method is thus usually provided, an implementation in the form of a genuinely usable product does not come about in many cases due to the high costs. Besides the numerous application cases already investigated, there are also still many tasks and problems which can in principle be solved with the aid of broadband measurement methods but which have not yet, or not sufficiently, been investigated. Many of these as yet uninvestigated fields of application are located in market segments that impose additional or special requirements on corresponding products, such as a miniaturized design, energy-efficient and battery-operated measurement and, not least, low costs.

In terms of content, the present work is divided into two parts. In the first part it deals with the detailed investigation of three practical tasks. Two of them are from the field of medical engineering and a further one from the field of moisture measurement technology. In the second part of the work, newly developed universal electronic circuit concepts and measurement concepts are explained with which it is possible to transfer the investigated applications into real products at low cost.

In the first medical engineering application, a capacitive sensor is developed which, based on impedance measurements, is able to determine the hematocrit value (HCT) of blood. A special feature of this sensor is that it can be attached from the outside to a tube in existing systems with extracorporeal blood circuits, without having direct contact with the blood. With the newly developed sensor, a measurement accuracy of 4 % HCT with a resolution of about 0.1 % HCT was achieved in the laboratory. In the second investigated medical engineering application, biological tissue at the tip of a cannula is analysed and classified by impedance spectroscopy during the dynamic insertion process. This enables precise positioning of the cannula tip in a particular target tissue with minimal technical effort. Through the use of a coaxial cannula design, a high spatial resolution is achieved which corresponds approximately to the diameter of the cannula. The use of short chirp signals as measurement signals enables tissue recognition within a measurement and evaluation time of less than one millisecond. The third investigated application comes from the field of moisture measurement technology. Here, with the aid of time domain reflectometry, a spatially resolved detection of the groundwater level, or the detection of extraneous water penetrating into buildings, is realized. Through the use of suitable sampling and evaluation methods, the developed measurement electronics enable detection of water along the measurement line used with a spatial resolution in the range of a few millimetres and an accuracy of about +/- 3 cm.

Due to the universal and modular design and character of the developed electronics, these are furthermore also excellently suited to solving further measurement tasks in related subject areas. The aim of the circuit developments is to significantly reduce the space requirement, the current consumption and the costs compared with the current state of the art in measurement technology, so that application-specific products can easily be realized based on the developed prototypes. Although broadband impedance measurement technology is not a new scientific field of work, there is currently still a great lack of miniaturized and low-cost measurement equipment. In recent years, however, there has been considerable technical progress, particularly in the field of programmable and configurable digital logic circuits, so that today extremely powerful components are available at very low cost. The circuit concepts developed in this work are based on the use of such programmable logic circuits. The technical possibilities of the components used are exploited in conjunction with suitable measurement signals and sampling principles. As a result of this work, two different platforms are available. The first platform is optimized for static applications in which the measurement duration is an uncritical parameter, but high demands are placed on the (virtual) temporal resolution of the measured signal. The technical sampling principle of this platform is based on the method of undersampling a periodic measurement signal, whereby a digital variant of a delta modulator is used for sampling. The second developed platform is optimized for dynamic applications in which the measurement duration required to record a complex impedance spectrum is a critical parameter. Due to the excellent scalability with respect to signal duration, signal amplitude and signal bandwidth, as well as the possibility of very fast digital processing of the sampled signals in hardware, chirp signals are used here as measurement signals.

Based on the metrological considerations, prototype circuits were constructed and successfully tested in each case. The platform for measurement with virtually very high temporal resolution was additionally further developed into a miniaturized high-resolution time domain reflectometer within the framework of a project with the University of Queensland, Brisbane, Australia. Currently, 20 of these devices are being used in pilot studies near Brisbane for spatially resolved measurement of the groundwater level. A further 20 devices were modified in cooperation with TU Darmstadt and are being successfully used in building services engineering for moisture measurement.

Description

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.