932 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 52, NO. 3, JUNE 2003 High-Resolution Multichannel Instrument for Resonant Sensor Array Paolo Ferrari, Alessandra Flammini, Daniele Marioli, and Andrea Taroni Abstract—In this paper, the problem of measuring instruments for resonant sensor arrays is considered. A new multichannel in- strument for frequency measurements has been developed using a numerical approach that improves noise immunity and decreases measuring time with respect to standard counting techniques. A smart least squares method for pre-processing of measurement data and an algorithm to achieve compression, are also presented. Finally some experimental results are reported to demonstrate the performance of this instrument. Index Terms—Digital measurements, frequency measurement, sensor array, signal processing. I. INTRODUCTION G RAVIMETRIC resonant sensors [1] are a good example of low-cost, noise-tolerant, and versatile devices. A wide variety of applications are guaranteed by different choices of coating layers that can improve selectivity regarding a specific compound. The mechanical impedance of an array element is changed by the absorbed mass; this way the measurand variation modifies the sensor structure characteristics as well as its reso- nant frequency. If the sensor is inserted in a phase-locked-loop (PLL), the frequency of the output signal changes accordingly. Generally, it is reasonable to consider sensor output signal fre- quency as indicated in (1) (1) where is the quiescent resonant frequency, is the mea- surand, and is the affecting quantity. The value is usu- ally of megahertz order. Due to these frequencies, the sensor sensitivity ( ) can be increased to a good level. The fre- quency estimation requires a resolution, because the signal can change a few hertz around the quiescent point. Real sensors present values not far from , which is why sensors are assembled in arrays and their signals are processed simulta- neously with multivariate techniques, such as neural networks. The dynamics are less critical since the signals come from slow sensors ( kHz s). The reference sensor array, usually ap- plied for chemical detection, is composed of a number of ele- ments that vary from 4 to 8. A whole array measuring time less than 100 ms can be considered satisfactory. The need to faithfully reconstruct the sensor dynamic response regarding variation of affecting quantity, yields a Manuscript received May 27, 2001; revised February 5, 2003. The authors are with the Department of Electronics for Automation and INFM, University of Brescia, Brescia, Italy (e-mail: flammini@ing.unibs.it). Digital Object Identifier 10.1109/TIM.2003.814681 reduction of measuring time or, generally speaking, of the time that elapses between two consecutive readouts. Moreover, to compensate the influence of affecting quantities, all sensors should be sampled at the same time; if cost reduction reason re- quires to use a multiplexed approach, the measuring time must be even smaller. Critical also is the resolution of the adopted measurement instrument, which must be high to increment the ability to appreciate small variations of the sensor signals. The application field of this instrument (the chemical sensors arrays characterization) often requires an observation time of several hours to follow slow affecting quantity dynamics (like temperature cycles or long period drift-instabilities) and, hence, the problem of storing, treating and possibly compressing the data is important. The signal incoming from the sensor is fre- quently affected by noise and any method to process measure- ment data must filter samples. Instruments currently used for experimental characterization of resonant sensors are composed by laboratory counters, based on high-resolution counting techniques, arranged in a multiplexed architecture with external synchronization circuitry. Generally, the laboratory instrumentation only gives the mean value and the standard deviation of a measurement set, sacrificing the study of the dynamics and increasing the possibility to misunderstand the signal transients. In a multiplexed system, assuming a matrix of sen- sors, in order to contain the total measuring time under imposed threshold of 100 ms, it is necessary to have a single channel measurement time of ms. As the desired resolu- tion is equal to , where is the smallest time interval that can be detected by the instrument, ps ( GHz) results. Such a solu- tion is very expensive and critical since, for example, labora- tory instruments with an elevated resolution like the HP-Agilent HP53132A and the Fluke-Philips PM6680 have a limit of 350 ps and 500 ps, respectively. In addition, a counting technique presents low-immunity with respect to impulsive noise due to the presence of high-frequency counters. The purpose of this work is to realize a multichannel fre- quency meter to manage a resonant sensor array. Our goal can be reached in two steps: 1) choosing an algorithm to estimate the frequency [2] that has a high immunity with respect to im- pulsive noise and that has low computational requirements to be implemented in a low-cost DSP system for multiple input chan- nels; 2) designing a method to reduce data size that increases signal-to-noise ratio (SNR) without loosing information about signal dynamics (i.e., transient information). 0018-9456/03$17.00 © 2003 IEEE