Microcontroller metering converter of capacitance with voltage-controlled rc-generator

Abstract

In agriculture, the moisture content of seeds, air and other materials is measured using capacitive sensors. The voltage (metering voltage, MV) that varies with a certain frequency is supplied to a capacitive sensor.The MV frequency can be either fixed or variable, depending on the measured capacitance changing. In the last case, additional errors occur during the measurement.For example, for Honeywell's HCH-1000 air humidity sensors, changing the MV frequency from 50 to 100 kHz leads to a humidity measurement error within 10 %. The main parameters of these sensors are given for the frequency of 20 kHz in the datasheet. It is known that the seed permittivity depends on the MV frequency of the capacitive sensor, in the cavity of which these seeds are located. Therefore, the task is to control the MV frequency.Microcontroller-controlled process of capacitance-to-binary code conversion is considered in the paper. Capacitance-to-frequency conversion is carried out using the NE555 chip operating in the auto-generator mode. Arduino's microcontroller measures the frequency at NE555 output, then its deviation from the value set is determined and an impact in the form of voltage is formed in order to eliminate this deviation. The result of the conversion is a binary code equivalent to the effect generated to eliminate the deviation. Thus, the MV frequency is maintained at a predetermined level.Experimental studies were carried out in a capacitancechange range from 200 to 400 pF. This range was selected because, when the air humidity varies from 0 to 100 %, the range of the HCH-1000 sensors capacitance changes from 280 to 380 pF. Microcontrollers allow us to solve the tasks of converting physical quantities into binary code quite simply and effectively. Binary code allows performing any arithmetic operations, which in turn increases the measurements accuracy. It does not require bulky hardware. All devices necessary for organizing the conversion process can be placed on a single semiconductor crystal in the future.