Frequency encoding of resonant mass sensors for chemical vapor detection

Chemical vapors can be detected by a resonant mass sensor array with selective absorption coatings implementing a frequency encoding method. The sensor array consists of sensor elements with different frequencies for their identifications in the frequency response obtained with a pulse Fourier transform detection scheme. Zero-loading resonance frequencies are chosen so that frequency shift due to absorption is bounded within a predefined region so that there is no overlap of peaks and all peaks can be assigned to the correct elements at any operation conditions. Mechanical oscillations of all or selected numbers of the sensor elements are excited by application of an excitation signal. Free oscillation decay signals from all or selectively excited sensor elements are detected and digitized. The free oscillation decay signal is subjected to a spectral analysis routine converting into a frequency spectrum, in which frequency shifts due to absorption of chemical vapors can be obtained. The implementation of the frequency encoding method with pulse Fourier transform detection to resonant mass sensors allows simultaneous multisensor detection, fast data acquisition speed, high signal-to-noise ratio by coaddition of raw data, flexible excitation, reduced complexity of electronic hardware, application of advanced data/spectral analysis algorithms, and realization of many other advantages by the introduction of the pulse Fourier transform method. A practical chemical vapor sensing system is demonstrated experimentally by use of nine frequency-encoded and polymer-coated sensors.