Contactless Liquid-Level Measurement With Frequency-Modulated Millimeter Wave Through Opaque Container

A novel contactless method for measuring liquid level through an opaque container is proposed. A millimeter-wave Doppler sensor is developed to "see" (i.e., sense) through a target container and measure the liquid level on the basis of the absorption of millimeter waves in liquid. One of the challenges is to accurately measure liquid level (within sub-millimeter error) despite the inherently large beam diameter of the millimeter wave due to diffraction. A piezoelectric vibrator enables accurate measurement by reflecting a limited portion of the spread beam and modulating it in frequency to distinguish it from the other portion of the beam. A prototype measurement system is fabricated and evaluated. The feasibility of our proposed method for clearly detecting an air-liquid interface concealed in an opaque container is confirmed experimentally. The nonlinearity error of the measured liquid level is within ±0.5 mm .