Measuring converter of capacitance for dielectric grain moisture meters

The problem of increasing the accuracy of grain moisture measurements while minimizing the hardware of a dielectric moisture meter is considered. To solve it, an algorithmic compensation of the analog path errors, implemented on an 8-bit microcontroller, is proposed. The developed algorithm for converting the capacitance of the sensing element (sensor) into a digital code provides high slew rates of the test voltage and ensures direct interfacing of the sensor with the microcontroller. The software and hardware for a capacitance measuring transducer have been developed, its main characteristics have been determined, and the possibility of its use as part of dielectric grain moisture meters has been assessed. A unipolar meander generated by the microcontroller's built-in timer/counter is used as a test signal applied through a reference resistor to the capacitive sensor. The maximum and minimum voltages on the sensor are converted into digital codes, which are used to calculate the actual capacitance value. The capacitance is also calculated using a parameter that depends on the resistance of the reference resistor and the square wave frequency. The sensor voltages are converted into digital codes by comparing them with a reference voltage generated by a pulse-width modulator integrated into the microcontroller and an external RC filter. Voltage comparison is performed by an analog comparator built into the microcontroller. Using simulation modeling, the dependence of the parameters of the converted test signal on the sensor capacitance was investigated. The model was developed in the SimInTech environment, which is designed for solving problems of mathematical modeling, synthesizing control algorithms, and programming computing devices. The experimental capacitance measurement converter is built using the Arduino Nano development board (Arduino, Italy), which contains the ATmega328PB microcontroller (Microchip Technology Inc, USA). The square wave frequency in the experiment was 500 kHz. Over a capacitance range of 25–45 pF, the resolution of the capacitance measuring converter was 0.1 pF. The deviation of the measurement results from the set capacitance value measured by the LCR-819 precision device (GW Instek, Taiwan) did not exceed ±0.2 pF. The capacitance measuring converter was tested as part of experimental moisture meters manufactured at Stavropol State Agrarian University for a range of 9–19 % wheat grain moisture at a grain temperature of 23–25 °C. The deviation of the results of wheat grain moisture measurements by experimental moisture meters did not exceed ±0.6 % of the moisture content determined according to GOST 13586.5-2015 “Grain. Method of moisture content determination”. The optimal fi eld of application for the converter is information-measuring and control systems based on capacitive sensors. systems based on capacitive sensors.

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