Vittorio Ferrari

Modeling, fabrication and performance measurements of a piezoelectric energy converter for power harvesting in autonomous microsystems

The present work deals with the energy conversion via the piezoelectric effect. A mechano-electrical energy converter based on piezoelectric thick films of lead zirconate titanate (PZT) deposited on a steel cantilever by a low-temperature process is presented. Modeling and measurement results on the performances of the piezoelectric converter are reported and their potential use in self-powered autonomous sensor devices is proposed

Improving the accuracy and operating range of quartz microbalance sensors by a purposely designed oscillator circuit

A method is presented to measure accurately the resonant frequency and to increase the functioning range of quartz microbalance sensors subject to heavy acoustic loading, such as encountered in in-liquid operation and with viscoelastic coating films. The method is based on an electronic circuit for the active cancellation of the crystal electrical capacitance, inserted in a phaselocked loop oscillator, which enables the tracking of the quartz series resonance with virtually zero error irrespective of the load. The principle is discussed, and experimental results confirming its successful application are presented.

Development and application of mass sensors based on flexural resonances in alumina beams

New mass sensors are described, which are based on the change in the flexural resonant frequencies of ceramic cantilever beams subject to piezoelectric excitation. The devices have been fabricated by screen-printing and firing a PZT-based paste on 96% alumina substrates with the methods of thick film technology and are, therefore, low cost and easy to manufacture. Inserted in an electronic feedback loop sustaining and tracking oscillations at one of the resonant frequencies, the sensors work as resonant microbalances with frequency output and can be employed for gravimetric chemical measurements. The beams have been implemented in two different sizes and, as a consequence, operated at different frequencies (about 82 and 149 kHz), in order to vary the mass sensitivity. The sensors manufacturing and theory of operation are illustrated, and experimental results on their characterization are reported. For the two sensor sizes, mass sensitivities of about -280 and -1200 Hz/mg have been measured, in agreement with theoretical predictions. The influence of temperature has been investigated showing that, for slow thermal drifts, a satisfying degree of compensation can be achieved with a differential configuration. The devices have been successfully applied as sorption sensors for the measurement of air relative humidity (RH) by sensitizing the beams surface with hydrophilic polymeric coatings. On the basis of past investigations, poly(N-vinylpyrrolidinone) and poly(ethyleneglycol) have been adopted as coating materials, obtaining respective frequency shifts of about 500 and 1400 Hz for RH ranging from 12 to 85%.

Oscillator-based interface for measurand-plus-temperature readout from resistive bridge sensors

A signal conditioning circuit based on a relaxation oscillator is proposed for use with resistive bridge sensors. The circuit provides a rectangular-wave output whose frequency is related to the bridge unbalance, and duty-cycle is a function of the overall sensor bridge resistance, hence of the sensor operating temperature. In this way, two measurement values are simultaneously and independently carried on the same output signal. The circuit makes use of a constant current bridge excitation which enables the connection of remote sensors without accuracy degradation, and moreover, for silicon piezoresistive sensors, provides a first-order temperature compensation. A frequency-doubling output stage significantly reduces the nonlinearity due to switching delay times, at the parity of output center frequency and span. Experimental results are reported on the characterization of both the circuit alone and interfaced to a silicon pressure sensor.

In-liquid sensing of chemical compounds by QCM sensors coupled with high-accuracy ACC oscillator

A novel oscillator circuit with automatic capacitance compensation (ACC) capability has been coupled with quartz-crystal-microbalance (QCM) sensors coated with quinoxaline- and pyrazine-bridged cavitands to detect aromatic and chlorinated compounds in water. With double-side immersed 10-MHz crystals coated with the quinoxaline cavitand, the detection of toluene in deionized water was possible down to concentrations of a few parts per million. The ACC oscillator advantageously provides the simultaneous measurement of the sensor resonant frequency, damping, and value of the compensated parallel capacitance. This enabled observing that the analyte sorption in the cavitand coating not only brings about a mass uptake but also an increase of losses and, apparently, a rise in the coating average permittivity

Displacement sensor based on pyroelectric thick films and contactless light-spot cursor

A sensor is presented to measure translational displacement exploiting the position-dependent signals generated by pyroelectric films when excited by a movable light-emitting diode that works as a contactless light-spot cursor. The pyroelectric films are made of lead zirconate titanate (PZT) screen-printed on alumina substrate by the thick-film technology. The sensor employs a differential configuration which minimizes the sensitivity toward mechanical vibrations and microphonics caused by the piezoelectric response of PZT. An electronic circuit based on excitation modulation and synchronous rectification rejects the interference from thermal and light background. The sensor principle, design, and manufacturing are described, and experimental results from the device characterization are reported.

ACC oscillator for in-liquid quartz microbalance sensors

An oscillator design is presented that provides accurate tracking of the series resonant frequency of liquid-loaded thickness-shear mode (TSM) quartz microbalance sensors. This is obtained by means of the automatic capacitance compensation (ACC) of the sensor parallel capacitance, without any manual adjustment required The oscillator advantageously outputs three signals respectively related to the sensor series resonant frequency, quality factor, and compensated parallel capacitance. Experimental results are reported that were obtained with 10-MHz crystals double-side immersed into several liquids with different acoustic and dielectric loading levels.

Array of PZT pyroelectric thick-film sensors for contactless measurement of XY position

The paper presents the design, fabrication and experimental characterization of a pyroelectric sensor array made by 8/spl times/8 pixels of PZT screen printed on an alumina substrate. When an amplitude-modulated light emitting diode (LED) is placed in front of the array and moved in the XY plane parallel to the array surface, the array and the LED respectively work as the reference element and the contactless cursor of a two-dimensional position sensor.

MEMS Tilt Sensor with Improved Resolution and Low Thermal Drift

The purpose of this paper is to investigate the design methodology, test results, and potential applications of a high resolution and low thermal drift calibrated digital inclinometer based on a MEMS capacitive sensor. The inclinometer provides ±10° of measurement range in a rotational plane that is perpendicular to the earth’s gravity and it uses a primary sensing element based on high aspect ratio microstructures, processed by air gap insulated micro structure (AIM) technology. A signal conditioning electronics, processing the MEMS signals, enables flexibility in terms of linearization, thermal compensation and signal calibration. Resolution of ±0.002° has been found around 0° together with a thermal zero drift lower than ±0.002°/°C from 0°C to +50°C. The overall system consumes 15 mA dc current from a 5V supply. Examples of application fields include: platform stabilization, drill orientation, event detection and telematics/GPS.

Oscillator-based signal conditioning for resistive sensors

A signal conditioning circuit for resistive sensors is presented. It is based on a relaxation oscillator in which both the frequency and the duty-cycle of the output signal simultaneously carry information from a pair of different sensors. The output frequency is linearly related to the resistive unbalance of a Wheatstone bridge, while the duty-cycle is independently controlled by a second sensor. For this purpose, a thermoresistor may be used for thermal compensation of the primary sensor. The design, analysis and experimental characterization of the circuit and its application to a thick-film pressure sensor are reported.