Non-invasive capacitive sensor for the determination of revolution speed and fill level in screw conveyors

This paper presents a capacitive measurement principle for the determination of the mass and volume flow rate and the revolution speed of screw conveyors. The measurement is realized using a set of transmitter electrodes and a single receiver ring electrode on the outer pipe surface. Sequential measurement of the inter-electrode capacitances between active transmitter and receiver is performed by means of a versatile hardware platform. A laboratory setup was designed and built. Sets of sensor data were acquired for different cereals. It is shown that the fill level influences the measured capacitances due to the permittivity of the bulk solids. Furthermore, the revolution speed and its variation over time can be directly deduced from the sensor signals. The good signal to noise ratio allows for a reliable measurement of the mass and volume flow rate.