Contactless capacitive approach sensor design with semi-differential based synchronous demodulation method

Capacitive sensors are used in different applications in the food industry, especially in the grain industry. These usage areas are areas such as level detection of grain products, presence or absence of product, moisture determination in order to monitor and control their processes in real-time production systems. Capacitive sensors are more preferred in the food industry due to their advantages such as being less affected by environmental conditions, high sensitivity, easy applicability and low cost compared to other sensor (optical, inductive, etc.) groups. There are many sensing methods and circuit topologies in the literature for capacitive sensing. With the new methods developed in capacitive sensing or the improvement of existing reading circuits, its applicability in different sectors is increasing day by day. Thus, in this thesis, non-contact capacitive measurement method was used for the detection of grain products such as bran, wheat and corn. Non-contact capacitive measurement is the measurement of the capacitance between the electrode and the product without physical contact between the product and the electrode. The fact that the product does not contact the electrode and the dielectric coefficient of products such as bran is low increases the measurement difficulty of the product. For this reason, semi-differential synchronous demodulation-based measurement method induced by high frequency (250kHz) signal is used for more accurate and sensitive measurement. As a result of the thesis, a non-contact capacitive sensor design that detects products with low dielectric values such as bran and razmol has been realized. Before starting the design processes, preliminary studies on AC/DC simulation analyzes of the front-end circuit were made. With the simulation studies, suitable value ranges for the frequency value applied to the electrode, transimpedance (TIA) circuit, analog switch and Sallen-Key low-pass filter were determined. In addition, using the obtained AC simulation data, the input frequency and resistance value affecting the gain parameter of the TIA circuit in the front face circuit are optimized in order to reduce the sensitivity of the sensor to temperature. Electrode design and mechanical housing designs, which are other important parts of the sensor design, were designed simultaneously. Various test processes and optimization studies were carried out to verify the design. In the tests performed, gain performance test in the presence or absence of products (wheat, bran, corn), temperature tests and fill-empty (presence-absence) tests were carried out by connecting to the real-time factory environment. In the presence or absence of products, as a result of gain tests, bran with the smallest change (37mV) and the largest change (100mV) is wheat. These change rates are at a level that can be read by the ADC peripherals of the microcontrollers (STM32F103C8T6). In another test, temperature tests were carried out in 5°C steps between -10°C and 60°C to test the sensitivity of the sensor to the temperature change in the environment it is connected to. In these tests, the sensor directly detected the ambient temperature without any material in front of the sensor. The obtained data show a parabolic decrease in the sensor output. In total, approximately 280 mV change was observed at 70°C temperature change, and this rate of change was associated with an average of 4 mV change at 1°C. At the sensor output, voltage value below a certain set value is considered "absent", while a voltage value above the set value is considered "available". In this case, changes caused by temperature changes may affect the sensitivity of the sensor to temperature and its performance. In environments where temperature change is low, values as low as 4 mV caused by temperature can be ignored for the set value even in products with lower gain performance such as bran. However, temperature compensation studies have been carried out in order for the sensor to perform well in environments with high temperature changes. As a result of these studies, the temperature tests were repeated and an average of 15 mV change was observed at the sensor output at a temperature change of 70°C. In this way, the need to set a separate set value for each temperature environment is eliminated. These compensation studies provided a more stable performance by reducing the sensitivity of the sensor to temperature. Finally, in the tests performed in the real-time system, the sensor's ability to detect the presence and absence of wheat was tested by connecting the sensor to the upper pan of the yield scale and filling and emptying the wheat in the pan. In the tests carried out, it has been observed that the PNP and NPN outputs are active when the sensor reaches the determined set value, and the sensor works synchronously with the system. The synchronous operation of the sensor with the system shows that the sensor performance is not affected by the effects caused by the temperature, even if there is a temperature change in the environment. Thus, as a result of the tests carried out, the design and optimization studies of the sensor were confirmed and the test results were found sufficient for this sensor at the prototype stage. As a result, with the non-contact capacitive sensor designed as a result of the thesis study, the grain product was detected without contact. Thus, the sensor design, which will reduce negative problems such as deterioration of the products formed by the probe capacitive sensors used in the grain industry, and mechanical diffraction, has been optimized and tested.