Investeşte în oameni! FONDUL SOCIAL EUROPEAN Programul Operaţional Sectorial Dezvoltarea Resurselor Umane 2007 2013 Axa prioritară: 1 Educaţia şi formarea profesională în sprijinul creşterii economice şi dezvoltării societăţii bazate pe cunoaştere Domeniul major de intervenţie: 1.5 Programe doctorale i postdoctorale în sprijinul cercetării Titlul proiectului: Proiect de dezvoltare a studiilor de doctorat în tehnologii avansate- PRODOC Numarul de identificare al contractului: POSDRU 6/1.5/S/5 Beneficiar: Universitatea Tehnică din Cluj-Napoca PhD Thesis RESEARCH ON CAPACITIVE MEASUREMENT PRINCIPLES FOR LIQUID LEVELS Abstract PhD. Student eng. Vlad BANDE Scientific Coordinator: Prof. Phd. eng. Dan PITICĂ Electronics, Telecommunications and Information Technology Faculty 2011
Table of Contents 1. LIQUID LEVEL MEASUREMENT PRINCIPLES 1.1 Introduction 1.2 Classical Methods for Water Level Measurement 1.3 Level Sensor Selection Criteria 1.4 Purpose and Objectives of the PhD Thesis 1.5 Parallel Plate Capacitor s Sensorial Properties 1.6 Selected References 2. LEVEL MEASUREMENT ERROR SOURCES USING THE PARALLEL PLATE CAPACITOR 2.1 Temperature Influence on the Water s Electrical Permittivity 2.2 Temperature and Humidity Influence on the Air s Electrical Permittivity 2.3 Surface Tension Influence. Capillary Phenomena 2.4 Fringing Phenomenon Influence 2.5 Non parallel Positioning Plates Influence 2.6 Parasitic Capacitances Influence 2.7 Conclusions and Personal Contributions 2.8 Selected References 3. MODELLING THE CAPACITIVE LEVEL SENSOR 3.1 Design Particularities for the Capacitive Level Sensor 3.2 Air Filled Capacitor Model Determination 3.3 Air and Hydro-insulate Varnish Dielectric Capacitor Model Determination 3.4 Water and Hydro-insulate Varnish Dielectric Capacitor Model Determination 3.5 Air, Water and Hydro-insulate Varnish Dielectric Capacitor Model 3.6 Experimental Determinations for the Proposed Models Validation 3.7 Behavior Models for the Capacitive Sensor using PSPICE Simulations 3.8 Conclusions and Personal Contributions 3.9 Selected References 4. DEDICATED SYSTEM FOR LIQUID LEVEL DETERMINATION USING CAPACITIVE SENSORS 4.1 Capacitance Level Conversion Formula Determination 4.2 Practical Application for Capacitance and Level Calculation
4.3 Differential Autonomous System for Capacitor s Charging Time Measurement 4.4 MatLAB Application for Acquiring, Calculating and Displaying the Capacitances and the Level 4.5 Experimental Results 4.6 Conclusions and Personal Contributions 4.7 Selected References 5. FINAL CONCLUSIONS. PERSONAL CONTRIBUTIONS 6. REFERENCES 7. ATTACHMENTS 2
Abstract of the Thesis Water is basically the key component in human life quality estimation from its social impact point of view and as well from its impact on the environment, in case of undesired accidents. The adjacent zones of the major water reservoirs represent critical areas, where special protection actions must be taken into consideration, especially in case of a natural undesired phenomenon (earthquake, flood, landslide, etc), which can become the source of cracks or displacements for the hydro-technical constructions. For a good prediction of those phenomena, the water level must be carefully monitored. During the last years, the water level monitoring and controlling procedure progressed because of automated measurement systems implementation capable to take sample measurements and to make truthful predictions. The immediate effects are minimizing the inherent human error from the process of data acquiring and also minimizing the total measurement process time. When measuring the water level, the first logical step is the selection of the proper measurement principle, which will have to take into consideration several constrains: the location where the level must be evaluated, the accuracy needed, the application type or the liquid composition and aspect. The future aim of the implementation revealed in the current PhD system will be the measurement of the infiltration water level inside hydrotechnical constructions. The upstream water from the reservoir or the rain water enters via the dam s wall inside the dam s internal galleries where it is retained inside special built gutters. The gutters are connected in one point at the exit point of the galleries, where the water can be eliminated from the dam. Though, the total amount of the water infiltration can be measured in the gutter which collects all the dam infiltrations. For the correct measurement principle to be selected, it is necessary to know the minimal technical specifications needed for such a sensorial system, which will have to mandatory take into consideration the needed measurement domain. Based on the analysis made in chapter 1, the proper technical solution for such an application will be implementing the capacitive measurement principle using the sensorial properties of the parallel-plate capacitor. Chapter 1 also analyzes the classical methods for liquid level measurement starting from the hydrostatic principles revealed by Archimedes in ancient Greece. It continues with a short presentation of the most used principles for level estimation: float ball, ultrasounds and sounds, RADAR and SONAR. The last part of the first chapter reveals all the sensorial properties of the parallel plate capacitor from its geometrical parameters point of view. Aim and Objectives The aim of the current PhD thesis is to develop a capacitive level measurement system, with future implementation in one of the hydro-technical constructions in Romania, which allows data acquisition from the sensing element, the conversion into 3
capacitance and in level terms and as well allows that the final results to be displayed on a user-friendly interface. The targeted objectives are to build a low-cost capacitive level sensor based on the parallel plate capacitor s principle and to be able to implement a general mathematical algorithm for capacitance to column liquid height conversion. In addition, in this thesis the behavior models for the inhomogeneous dielectric capacitors will be revealed. Structure and Contents Chapter 2 is a bibliographic research on the errors that can occur during a potential immersion of the capacitive level sensor into the water. The phenomena can be classified in 3 categories, from the point of view of the altered physical property: hydrodynamic-like errors caused by the water position inside the capacitor armatures: temperature and humidity influence over the water s and air s electrical permittivity and the capillary phenomenon due to the small distance between the capacitor s plates. electrical errors caused by the electrical phenomena inside the sensorial system: the parasitic capacitances occurred between two superposed PCB traces or between the adjacent traces and the delay times introduced by the electronic components inside the measurement block of the sensorial system. mechanical errors caused by the geometrical design of the system s sensorial part: the fringing phenomenon and a potential non-parallel position of one plate in respect with the other. Chapter 3 presents the design of the build capacitive sensor which consists in superposing two identical PCB plates divided in 8 identical square copper zones. The resulted effect will be obtaining 8 parallel plate capacitors with known geometrical dimensions. The major objective of this chapter is to implement the behavior model of an inhomogeneous dielectric capacitor. The mathematical model proposed for the capacitive behavior model implementation takes into consideration the physical and geometrical properties of a parallel plate capacitor. Using the general capacitance formula of this capacitor, there were identified 4 different situations that must be analyzed, corresponding to different steps in the water immersion procedure. For the model validation process, there were proposed two different methods: the first one is a dedicated system for the experimental determination of the level sensor capacitances; the second one involves a PSPICE simulation of the capacitor s behavior models. Chapter 4 proposes a mathematical algorithm for converting the capacitance into water levels, using an improved measurement system. The equation determination procedure involves the capacitance ratio of the capacitor s non immersed and the capacitance of the same capacitor when it s positioned at the separation area between the water and air. The algorithm uses the copper zone position in respect with the bottom side of the PCB plate for capacitance calculation. The level formula contains the capacitance ratio, the copper zone s position in respect with the column of which depends, the electrical permittivity of 4
the dielectric at certain moments, and as well the thickness of the hydro-insulate varnish used for covering the copper zones to avoid short-circuits. The practical application, created under the MatLAB software, for the measurement and displaying the capacitances and the level, consists in a measurement system which, based on a differential method, measures every capacitor; the results are serially transmitted to a local computer where are loaded into the MatLAB program. The charging times acquired are converted in capacitances and then using an iterative formula in level terms. For data displaying, there was built a user-friendly interface which allows viewing, analyzing and controlling the results. Finally, after making all the necessary measurements, the dependence characteristic between the capacitance and the level was generated this allows identifying the level without implementing the mathematical algorithm. The final paragraphs of the chapter 4 were reserved to a comparative analysis between the capacitive sensor and another two practical methods to evaluate the water level. The analysis revealed a 4.9 mm maximum deviation between the system and the indication of a 1mm precision scale, respectively comparable precision in respect with an industrial capacitive sensor s results. Chapter 5 is reserved to point the final and general conclusion and as well the original contributions over the thesis. Original Contribution in this Thesis This section presents the author s original contributions over the capacitive measurement principles. All the obtained results were published both in national/international conference and in journals. 1. Based on the bibliographic researches, there were identified the major error sources that can alter the measurement procedure of a parallel plate capacitor. 2. For every phenomenon analyzed, there were generated dependence characteristics with the environment s parameters (humidity, temperature). 3. For the mechanical errors, there were identified practical methods for reducing or eliminating the undesired effects. 4. A low-cost capacitive sensor was build for level measurement with an original design of the sensitive part. 5. The models of the capacitor in all 4 cases were determined, based on electrical and mathematical principles. 6. A practical and experimental procedure was implementing for the capacitance measurement. 7. A PSPICE analysis was made based on the behavior models identified. 5
Final Remarks 8. A mathematical algorithm was implemented in order to obtain the capacitance level dependence equation. 9. An iterative formula derived from the above equation, which allows calculating the level knowing only the position of every copper zone in respect with the bottom side of the PCB plate. 10. An improved solution for the capacitance measurement procedure was identified, which evaluate the capacitance based on the both plate s potential variation. 11. A MatLAB application was generated for acquiring, converting and displaying the capacitances and the level. 12. An interactive interface was built for viewing, controlling and analyzing the final results. 13. A comparative analysis was made using two different external methods to evaluate the liquid level. 14. Based on a complete set of measurements, the dependence characteristic between the capacitance and level was generated. The level measurement process is a complex procedure that involves physical, mathematical and electronic knowledge. The designed capacitive sensor proposed in this thesis is an original method for level detection and measurement which will be the subject of a future implementation inside a hydro-technical construction. Publications Referenced in this Thesis [1] V. BANDE, I. Ciascai, D. Pitica - Parasitic Influences in a Capacitive Transducer Behavior - ISSE 2011 34 th International Spring Seminar on Electronics Technology May 11-15, High Tatras, Slovakia. ISBN: 978-80-553-0646-9. [2] V. BANDE - Electrical Model of a Capacitive Based Level Sensor PRODOC Conference 24 25 June, Cluj Napoca, Romania. [3] V. BANDE, I. Ciascai, D. Pitica Low-cost Capacitive Sensor for Well Level Measurement - 33 rd International Spring Seminar on Electronics Technology - ISSE 2010, 12-16 May, Warsaw, Poland. ISBN: ISBN: 978-1-4244-7849-1. [4] V. BANDE, I. Ciascai, D. Pitica MatLAB Platform for Well Level Evaluation SIITME 2010 IEEE 16 th International Symposium for Design and Technology in Electronic Packaging September 23-26, 2010 Pitesti, Romania. ISBN: 978-1- 4244-8122-4. [5] V. BANDE, I. Ciascai Differential Charging Time Measurement Method for a Multi Capacitive Level Sensor Design Idea EDN Journal. ISSN: 0012-7515. 6
Vlad Bande Contact Data: vlad.bande@ael.utcluj.ro No.2 Observatorului St., Cluj Napoca, Romania Applied Electronics Department, Electronics, Telecommunications and Information Technology Faculty Technical University of Cluj Napoca Phone: 0040-264-401809 Personal Data: Date and place of birth: 19.05.1984, Zalau, Salaj Education: 2007 graduated Industrial Electronics Engineering Electronics, Telecommunications and Information Technology Faculty, Technical University of Cluj Napoca. Professional Experience: 2007 technical specialist at TES Group SRL. 2008 PhD Student at Technical University of Cluj Napoca, Applied Electronics Department, Electronics, Telecommunications and Information Technology Faculty, Technical University of Cluj Napoca. Scientific and research activities: 13 published papers in national and international journals and conferences proceedings. 1 paper proposed for an international conference. Project member in 23 national projects. Foreign languages: English very good. French good. 7