On the development of a multifunction millimeter-wave sensor for displacement sensing and low-velocity measurement

A new multifunction millimeter-wave sensor operating at 35.6 GHz has been developed and demonstrated for measurement of displacement and low velocity. The sensor was realized using microwave integrated circuits and monolithic microwave integrated circuits. Measured displacement results show unprecedented resolution of only 10 μm, which is approximately equivalent to λ₀/840 in terms of free-space wavelength λ₀, and maximum error of only 27 μm. A polynomial curve-fitting method was also developed for correcting the error. Results indicate that multiple reflections dominate the displacement measurement error. The sensor was able to measure speed as low as 27.7 mm/s, corresponding to 6.6 Hz in Doppler frequency, with an estimated velocity resolution of 2.7 mm/s. A digital quadrature mixer (DQM) was configured as a phase-detecting processor, employing a quadrature sampling signal-processing technique, to overcome the nonlinear phase response problem of a conventional analog quadrature mixer. The DQM also enables low Doppler frequency to be measured with high resolution. The Doppler frequency was determined by applying linear regression on the phase sampled within only fractions of the period of the Doppler frequency. Short-term stability of the microwave signal source was also considered to predict its effect on measurement accuracy.