Non-invasive liquid recognition based on interdigital capacitor
Introduction
Dielectric properties of materials can be determined with the use of electromagnetic (EM) waves (radar/microwave) in the investigations of material and structural assessment. Permittivity of a material may be used to determine various properties usable for research and application in food science, medicine, biology, agriculture, chemistry, electrical devices, defense industry (security), and engineering. Many techniques have been applied for the measurement of dielectric properties of a material [1], [2], [3], [4], [5].
Identification of liquids is of great importance in security, biology and beverage industry. Some of the measurable physical properties like melting point, boiling point, refractive index, density, solubility, and viscosity can discriminate between two possible compounds [6].
In the development of basic elements for sensing applications, the most important features of every sensor are suitability for integration of the sensor element and signal-processing electronics, and also an inexpensive way for manufacturing. Compared to other sensor technologies [7], [8], [9], [10], [11] interdigital capacitors (IDC) have been extensively researched [12], [13], [14], [15] and successfully used as sensors for large variety of applications [16], [17], [18], [19], [20], [21]. Non-destructive testing and evaluation based on electromagnetic principles become widely used due to simplicity, fast response and low cost [22].
The purpose of this paper was to investigate the possibility to distinguish between liquids without a direct contact between a sample and an IDC sensor. Conditioning circuit measures the change in capacitance of the IDCs as a change in a charge/discharge time converted into frequency. Data were processed by a microcontroller with results available on 2 × 16 character display as well as PC. The system has been experimentally tested against benzene, olive oil, acetone, alcohol, methanol, purified water and formaldehyde. Container type and sample volume dependency was examined and results reported.
Section snippets
Interdigital capacitor operation principle and design
An interdigital capacitive sensor is a coplanar structure consisting of multiple comb electrodes. Compared with parallel plate capacitor structure, IDC electrodes open up thus providing planar structure (Fig. 1). By applying different potential on electrodes [23] electromagnetic field generates in between. Electrode and geometry as well as dielectric properties of material under test (MUT) affect the capacitance and conductance between electrodes.
For sensing applications two electrode
Interface circuit
As used in our prior work, capacitance was measured using timer TLC555 [27]. This timer operates at frequencies up to 2 MHz. Because of its high input impedance, this device is suitable for small timing capacitors, with more accurate time delays and oscillations possible. As referred to timer basic astable configuration taken from [27], resistors used for PCB IDC were RA = 5.1 kΩ and RB = 100 kΩ, while for paper-based IDC RA = 100 kΩ and RB = 200 kΩ were chosen. Changes in IDC's capacitance reflect through
Conclusion
Unique dielectric constant of every material makes it possible to distinguish materials based on this parameter. As some of the substances deteriorate and can be dangerous to handle due to possibility of intoxication of the operator, the purpose of this paper was to investigate a non-invasive recognition of samples packed in predefined containers.
Glass and polypropylene containers were examined and the effect of sample volume was investigated as well. Special attention was dedicated to
Acknowledgment
This work has been supported by Ministry of Education, Science and Technical Development, Republic of Serbia, through scholarship gained on project III 43008.
Aleksandra Vuković Rukavina received the professional degree of bachelor with honors in electrical and computer engineering (2010), as well as the academic degree of master in electrical engineering in (2011) from Faculty of Technical Sciences, University of Novi Sad, Serbia. She is currently on her PhD studies at Faculty of Technical Sciences, University of Novi Sad, Serbia as a researcher through scholarship gained on project III 43008. Her research interests include sensors, microcontrollers
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Aleksandra Vuković Rukavina received the professional degree of bachelor with honors in electrical and computer engineering (2010), as well as the academic degree of master in electrical engineering in (2011) from Faculty of Technical Sciences, University of Novi Sad, Serbia. She is currently on her PhD studies at Faculty of Technical Sciences, University of Novi Sad, Serbia as a researcher through scholarship gained on project III 43008. Her research interests include sensors, microcontrollers and applied electronics.