Математичне моделювання пристрою з частотним виходом для вимірювання вологості

Abstract

The device for measuring humidity with a moisture-sensitive resistive element HR202 has been developed. The self-generating transducer is designed as a hybrid integrated circuit based on a bipolar transistor VT1 and a field-effect two-gate transistor VT2. The negative differential resistance, which is formed by the parallel connection of the impedance with a capacitive component at the collector electrodes of the bipolar transistor VT1, the drain of the field-effect transistor VT2 and inductance L1, leads to the occurrence of electrical oscillations in the circuit. When exposed to moisture on the sensitive resistive element RW, the capacitive component of the impedance at the electrodes of the transistor structure changes, which causes an effective change in the frequency of the oscillatory circuit. On the basis of mathematical modeling of electrical characteristics, analytical expressions for the transformation function and the sensitivity equation are obtained. It has been experimentally established that an increase in the ambient temperature in the range of relative humidity W = 30 ÷ 85% leads to an expansion of the generation range of the autogenerating humidity transducer, as well as to an increase in the sensitivity of the device to the measured value. The generation range of the autogenerating humidity transducer at a temperature of T = 20 °C acquires a value of 823 kHz (the average value of the sensitivity is 16.18 kHz /%), and at a temperature of T = 50 °C - 1323 kHz (the average value of the sensitivity is 29.10 kHz / %). To confirm the theoretical results of circuit solutions developed device in the computer modeling of LTSpice modeling environment. The studies were carried out at different temperatures (20°C, 30°C, 40°C, 50°C) in the range of change in the resistance of the moisture-sensitive resistive element from 1750 kOhm to 2.1 kOhm, which corresponds to an increase in the value of the relative air humidity from 30 % to 90 %. The results of theoretical and experimental studies have shown that at the output there are periodic oscillations device for measuring the humidity rate which increases with increasing values of relative humidity. The obtained theoretical and experimental studies are in good agreement, the relative error does not exceed 2.5%.