Marco Demori

Autonomous Sensors Powered by Energy Harvesting from von Karman Vortices in Airflow

In this paper an energy harvesting system based on a piezoelectric converter to extract energy from airflow and use it to power battery-less sensors is presented. The converter is embedded as a part of a flexure beam that is put into vibrations by von Karman vortices detached from a bluff body placed upstream. The vortex street has been investigated by Computational Fluid Dynamics (CFD) simulations, aiming at assessing the vortex shedding frequency as a function of the flow velocity. From the simulation results the preferred positioning of the beam behind the bluff body has been derived. In the experimental characterization the electrical output from the converter has been measured for different flow velocities and beam orientations. Highest conversion effectiveness is obtained by an optimal orientation of the beam, to exploit the maximum forcing, and for flow velocities where the repetition frequency of the vortices allows to excite the beam resonant frequency at its first flexural mode. The possibility to power battery-less sensors and make them autonomous has been shown by developing an energy management and signal conditioning electronic circuit plus two sensors for measuring temperature and flow velocity and transmitting their values over a RF signal. A harvested power of about 650 μW with retransmission intervals below 2 min have been obtained for the optimal flow velocity of 4 m/s.

Interrogation Techniques and Interface Circuits for Coil-Coupled Passive Sensors

Coil-coupled passive sensors can be interrogated without contact, exploiting the magnetic coupling between two coils forming a telemetric proximity link. A primary coil connected to the interface circuit forms the readout unit, while a passive sensor connected to a secondary coil forms the sensor unit. This work is focused on the interrogation of sensor units based on resonance, denoted as resonant sensor units, in which the readout signals are the resonant frequency and, possibly, the quality factor. Specifically, capacitive and electromechanical piezoelectric resonator sensor units are considered. Two interrogation techniques, namely a frequency-domain technique and a time-domain technique, have been analyzed, that are theoretically independent of the coupling between the coils which, in turn, ensure that the sensor readings are not affected by the interrogation distance. However, it is shown that the unavoidable parasitic capacitance in parallel to the readout coil introduces, for both techniques, an undesired dependence of the readings on the interrogation distance. This effect is especially marked for capacitance sensor units. A compensation circuit is innovatively proposed to counteract the effects of the parasitic input capacitance, and advantageously obtain distance-independent readings in real operating conditions. Experimental tests on a coil-coupled capacitance sensor with resonance at 5.45 MHz have shown a deviation within 1.5 kHz, i.e., 300 ppm, for interrogation distances of up to 18 mm. For the same distance range, with a coil-coupled quartz crystal resonator with a mechanical resonant frequency of 4.432 MHz, variations of less than 1.8 Hz, i.e., 0.5 ppm, have been obtained.

Microfluidic Sensor for Noncontact Detection of Cell Flow in a Microchannel

A microfluidic sensor for detection of cells flowing in a microchannel is presented. The sensor consists of a PolyDiMethylSiloxane layer with two planar microreservoirs connected by a microchannel. The bottom sides of the microreservoirs are faced to two sensing electrodes formed on a printed circuit board. A noncontact measurement is ensured by an insulator layer between the electrodes and the fluid. Particles flowing in the microchannel cause changes in the conductivity of the narrow path formed by the fluid, producing variations in the impedance between the electrodes. A tailored electronic interface based on a direct digital synthesis device is proposed to measure the impedance variations. In the experimental tests, the cell flow is detected by changes in the effective capacitance and conductance between the electrodes. These preliminary results are promising for biological measurements such as counting and sizing of cells in different matrices.

Particle Manipulation by Means of Piezoelectric Actuators for Microfluidic Applications

In this paper the possibility to generate acoustic waves such as FPW (Flexural Plate Wave) for fluid and particle manipulation by piezoelectric actuators applied on non-piezoelectric substrates is explored. A test device with two Lead Zirconate Titanate (PZT) actuators deposited on an alumina (Al2O3) substrate by screen printing technique is presented. The experimental results show that, by exciting the actuators at their resonance frequencies, FPW modes are generated in the substrate. Circular vortex rotations are obtained in a fluid drop placed on the substrate by exciting a single actuator. In addition, micrometric particles dispersed in the drop allow to demonstrate that particle accumulation along circular lines is obtained by exciting both the actuators. These results suggest the possibility to employ the proposed actuators for fluid mixing and controlled positioning of dispersed particles.

Impedance sensors embedded in culture media for early detection of bacteria growth

In this work, the ability of an impedance sensor to rapidly detect bacteria growth in a culture medium has been investigated. A test configuration with two electrodes embedded in a Petri dish has been proposed. Impedances corresponding to a sterile medium and one inoculated with bacteria have been measured and compared during the growth process. Remarkable differences have been observed in the time evolutions of the two behaviors. In particular, impedance behavior measured at 100 Hz allows such achievement after only 1 h. An equivalent circuit of the measured impedances have been proposed. Considerations regarding the effects of the bacteria growth on the components describing the electrode-medium interface have been reported. The variations of these elements have been identified as the more significant for the early detection. From these promising results, improved configurations consisting of a matrix of electrodes can be proposed for localized and rapid detection by means of automated analysis systems.

Piezoelectric Energy Harvesting from von Karman Vortices

In this paper a study of an innovative energy-harvesting system based on a piezoelectric converter to recover energy from an airflow is presented. The converter is embedded as a part of an oscillating beam, and it is used to harvest energy from the vibrations induced by von Karman vortices detached from a bluff body placed upstream. The system has been placed in a wind tunnel in order to measure the converter voltage and the harvested power for different flow velocities and beam orientations with respect to the flow direction. Experimental results confirm the von Karman vortices as the forcing for the beam oscillations. The possibility to optimize the harvesting effectiveness with the proper beam orientation and flow velocity has been demonstrated.

A Microfluidic Device with Embedded Capacitive Sensor for Fluid Discrimination and Characterization

This paper describes the development of a microfluidic device with embedded capacitive sensing that allows the discrimination of fluids on the basis of the dielectric permittivity. The device is fabricated in a hybrid technology, which innovatively combines PDMS (PolyDiMethylSiloxane) soft photolithography and screen printing techniques. A microchannel, realized in a PDMS layer, is placed in the fringe field of a sensing capacitor formed by electrodes screen-printed on a glass substrate. Fluids inside the microchannel affect the capacitance that is measured by a tailored electronic interface system. Experimental results are reported demonstrating the capability of the system to discriminate different fluids and estimate their dielectric permittivity.