Comunicación presentada al First Internacional Workshop on Marine technology. Villanova i La Geltrú (Barcelona), 2005.The goal of this project is to design a low-cost smart accelerometer, making use of a piezoelectric
element as basic sensing material, and adding a
mixed-mode conditioning circuit

Carlosena, Alfonso

De la Cruz, Carlos A.

Gárriz, Segio

Gómez, Marisol

Lecumberri, Pablo

López, Antonio

Massarotto, Marco

Pintor, Jesús M.

Cruz, Carlos A. de la

Pintor Borobia, Jesús María

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Instrumentation ViewPoint. Autumn 200534Design of Low-Cost Smart AccelerometersAlfonso Carlosena , Antonio López, Marco Massarotto, Carlos A. De la CruzPablo Lecumberri (1), Marisol Gómez (1), Jesús.M Pintor (2), Sergio Gárriz(2)Universidad Pública de NavarraDept. Electrical and Electronic Engineering(1) Department of Mathematics and Computer Science, (2)IMACCampus de ArrosadíaE-31006 Pamplona.carlosen@unavarra.es1. IntroductionThe goal of this Project is to design a low-costsmart accelerometer, making use of a piezoelectricelement as basic sensing material, and adding amixed-mode conditioning circuit. This syntheticgoal, consist of a number of combined objectives,which must be achieved:1. Development of calibration, compensation andfrequency range extension techniques of basicsensing elements made typically of piezoelectricceramics.2. Development and implementation of novelsignal processing techniques as they apply to theabove mentioned sensors for the estimation(parametric, non-parametric, blind,..) of mechanicalsystems, and excitation signals (impact). Bothmultiple and single sensor applications will beconsidered.3. Adaptation or development of new calibrationtechniques, to make the above mentionedmethods possible4. Implementation of the preceding techniqueson a physical device consisting of a basic sensingelement (piezoelectric) and a mixed modeconditioning circuit5. Validation of the so designed sensors in asignificant and varied number of applications.The project has been active for two years now,and is carried out by three different researchteams from the Public University of Navarre, andthe Polytechnic University of Catalonia. The resultsshown in the next section correspond mainly tothe accomplishments of the UPNa teams2. Results and DiscussionFigure 1 shows a general block diagram of thedifferent systems and task that need to bedesigned within the Project frameworkThe base sensor selected, based on a previousexperience of the team, corresponds to a lowcost piezoelectric device Murata PKGS, withlow sensitivity and high bandwidth [1]. To cut itshort, the hardware-software that will processthe signal coming from the sensor, should becapable not only to correct and compensatethe sensor drifts, but also to counteract the highfrequency peaking resonance, extending theuseful  bandwidth of the device beyond it. Figure2 shows an example of the raw signal given bythe sensor+conditioner, under an impact, andthe compensated signal after a deconvolutionalgorithm.In addition, and depending on the application,the sensor should be able to provide processedmeasurement parameters. To achieve this goal,a combination of hardware techniques andsignal processing algorithms have to be used.The complete systems will be integrated on achip (ASIC), and on commercial DSPs,depending on the target application.The accomplishment of the project, as of todate, can be summarized as follows:Sensor Characterization:A procedure has been developed, and iscurrently being improved, to characterize sensorsensitivity in a wide frequency range. Acombination of impulsive and vibrationtechniques is used. The method will be usedduring the calibration process.Analog ConditioningA charge amplifier has been designed to copewith a specific limitation of the application: theneed to provide current common mode rejection,Figure1. Smart Sensor Block DiagramFigure 2. Typical Sensor Signal, before and afterdeconvolutionInstrumentation ViewPoint.Autumn 200535maximizing signal to noise ratio. This is requiredfor applications where the sensor needs to beplaced on metallic surfaces, with potentialcapacitive coupling for electro-static discharges.Some innovative designs have been proposed[2-4]. Both discrete and fully integrated versionsfor the charge amplifier have been implementedand tested.Analog to Digital Converter.An incremental architecture has been implementedon silicon, and is currently being tested. It hasbeen specifically designed for sensor applicationsuch as that described in this paper. The designspecifications are 12 bits with a samplingfrequency of about 100kHz [5]. A new version isunder development to achieve 14 bits of resolutionand a sampling frequency of more than 200kS/sec., which are more convenient for the sensorat hand.The basic building blocks are implemented witha novel design that leads to operation with very-low supply voltage and at the same time efficientpower consumption. These cells are called SuperClass-AB [6]. Moreover several strategies atalgorithm level on the incremental converter havebeen incorporated in order to fully exploiting theperformance of such cells and minimizing secondorder effects. The converter can operate with asupply voltage of ±1.35V and a static powerconsumption of 3.5mW.Signal Processing-Blind Deconvolut ionThere are two basic approaches to process thedigitized signal coming from the sensor. The firstone assumes no prior knowledge of the “systemresponse”, which accounts for sensor responseplus effects due to impedances, both mechanicaland electrical. Then, blind techniques aremandatory. First, regularization steps are appliedto compensate for singularities such as those dueto noise or resonance peaking (i.e. poles). Then,redundant information must be used to deconvolvethe incoming signal which represents theacceleration. In our case, we are mostly interestedin impacting (non-repetitive) signals and thus,such redundant information can only be obtainedthrough multichannel acquisition. Details of onethe methods developed can be found in [7]The second approach makes use of systeminformation obtained through the sensor calibrationprocess. Due to the requirements of widebandwidth a specific calibration combining impactand vibration measurements needs to bedesigned. .From those measurements, andresorting on only regularization methods, impactingsignals can be measured.Sensor PackagingA sensor casing is being designed to host thesensor and ASIC, so that the sensor can beelectrically isolated and protected, without theintroduction of interfering vibration modes.3. ConclusionWe have described in this paper the mainobjectives accomplished in a Project whose aimis to design a low-cost, wide-band accelerometerfor impact applications4. References[1] Jun Tabota, “Two-End Clamp StructureEnhances Characteristics in SMD ShockSensors”. Hi-Tech Report, MurataManufacturing Co., Ltd. EE August, 1994.[2] A. Carlosena, A. López-Martín, M.Massarotto. “Charge Amplifiers”, to bePublisher in Sensors Encyclopedia[3] A. Carlosena, J. Baztán, J. Fernández deMuniain, J. L. Pina i, “Charge-Type andCurrent-Type Sensors Amplifying Circuit”,EP 1574862A1, 2005.[4] M. Massarotto, A Carlosena, A. López-Martín, “Two Stage Differential Charge and Transresistance Amplifiers”. Submitted toIEEE Trans.  Inst. Meas.[5] A.J. López-Martín, C. A. De La Cruz-Blas,X. Ugalde, R. G. Carvajal, and J. Ramirez-Angulo, Micropower CMOS S&H circuit forambient intelligence applications, ElectronicsLetter August 2005 Vol. 41 No. 17.[6] A.J. López-Martín, S. Baswa, J. Ramirez-Angulo, and R. G. Carvajal, Low-VoltageSuper Class-AB CMOS OTA Cells with VeryHigh Slew Rate and Power Efficiency, IEEEJournal of Solid-Sate Circuits Vol. 40 No. 5May 2005 pages 1068-1077.[7] P. Lecumberri, M. Gómez, A. Carlosena,“Multichannel Blind Deconvolution ofImpulsive Signals”, EUSIPCO 2005, Antalya,Turkey, 2005

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