Pyroelectric cells based on fabricated screen-printed PZT and commercial PVDF films are proposed as thermal energy harvesting sources in order to supply low-power autonomous sensors. The cells are electrically modelled as a current source in parallel with output impedance. Heating and cooling temperature fluctuations generated by air currents were applied to the pyroelectric converters. The generated currents and charges were respectively in the order of 10 −7  A and 10 −5  C for temperature fluctuations from 300 K to 360 K in a time period of the order of 100 s, which agrees with the theoretical model. Parallel association of cells increased the generated current. The dependence of the generated current on relevant technological parameters has been also characterized. Finally, current from cyclic temperature fluctuations was rectified and stored in a 1 μF load capacitor. Energies up to 0.5 mJ have been achieved, enough to power typical autonomous sensor nodes during a measurement and transmission cycle.

Thermal energy harvesting through pyroelectricity

This paper is a review of the remarkable progress that has been made during the past few decades in design, modeling, and fabrication of micromachined resonators. Although micro-resonators have come a long way since their early days of development, they are yet to fulfill the rightful vision of their pervasive use across a wide variety of applications. This is partially due to the complexities associated with the physics that limit their performance, the intricacies involved in the processes that are used in their manufacturing, and the trade-offs in using different transduction mechanisms for their implementation. This work is intended to offer a brief introduction to all such details with references to the most influential contributions in the field for those interested in a deeper understanding of the material.

Micromachined Resonators: A Review

The characterization of three commercial thermoelectric modules, which are designed for cooling/heating applications, is presented to employ the devices for power conversion, i.e., as thermoelectric generators (TEGs). The thermoelectric theory is briefly summarized at first, taking into account the relationship between the effective temperature difference across the TEG junctions and the temperature difference applied externally, considering insulating ceramic plates having finite thermal conductance. The thermoelectric modules have been characterized in terms of open-circuit output voltage and output power density for different temperature gradients and load conditions. Measurement techniques and experimental data are reported, which show the possibility of using thermoelectric devices for energy-harvesting applications. A comparison of the results from the tested devices with the performance data of commercial TEGs that are specifically designed for power harvesting is then presented, and the main characteristics of the two device typologies are discussed. A TEG was then used to supply an autonomous system that interfaces with a temperature sensor and periodically transmits the measurement information via an RF link. Experimental data show that the system works correctly and sends the RF signal when the temperature difference that is applied across the TEG is higher than 34 K.

Characterization of Thermoelectric Modules for Powering Autonomous Sensors

A silicon resonant cantilever sensor is developed for detection of airborne nanoparticles (NPs) by monitoring the change in resonant frequency induced by an additional mass of trapped NPs. A piezoelectric stack actuator and a piezoresistive strain gauge are involved in the sensor system in order to actuate and detect the oscillation of cantilever sensor, respectively. An electrostatic precipitator is employed to trap the NPs on the cantilever surface. The proposed sensor reveals a mass sensitivity of 10 Hz/ng and a quality factor of 1206 while operated in the fundamental flexural mode. As necessary for an application under workplace conditions the limitations of the sensor sensitivity imposed by the environment are investigated, i.e., the influences of temperature, relative humidity, and pressure on the sensor are measured.

Airborne engineered nanoparticle mass sensor based on a silicon resonant cantilever

A new micromachined uniaxial silicon resonant accelerometer characterized by a high sensitivity and very small dimensions is presented. The device's working principle is based on the frequency variations of two resonating beams coupled to a proof mass. Under an external acceleration, the movement of the proof mass causes an axial load on the beams, generating opposite stiffness variations, which, in turn, result in a differential separation of their resonance frequencies. A high level of sensitivity is obtained, owing to an innovative and optimized geometrical design of the device that guarantees a great amplification of the axial loads. The acceleration measure is obtained, owing to a properly designed oscillating circuit. In agreement with the theoretical prediction, the experimental results show a sensitivity of 455 Hz/ ( g being the gravity acceleration) with a resonant frequency of about 58 kHz and a good linearity in the range of interest.

A Resonant Microaccelerometer With High Sensitivity Operating in an Oscillating Circuit

A resonant accelerometer manufactured in silicon bulk micromachining with electrothermal excitation and piezoresistive detection is presented. The structure is a seismic mass supported by two parallel flexure hinges as a doubly-sustained cantilever, with a resonating microbeam located between the hinges. The acceleration normal to the chip plane induces an axial stress in the microbeam and, in turn, a proportional change in the microbeam resonant frequency. Beam resonant frequency of around 70 kHz and acceleration sensitivity of 35 Hz/g over the range 0–3 kHz were measured on prototypes, in accordance with analytical calculations and simulations. The microbeam operates unsealed at atmospheric pressure, therefore a comparatively low quality factor results due to air damping. In this condition, the effect of the input-output cross-talk was found to be significant. The cross-talk is analyzed and modeled, and an electronic active compensation is proposed. The compensated sensor was inserted into a phase-locked loop oscillator and tested. Reported experimental results show that the sensor performs in excellent agreement with the theoretical predictions.

Silicon resonant accelerometer with electronic compensation of input-output cross-talk

PZT pyroelectric elements were designed as thermal energy harvested sources, describing the best technological parameters to achieve large currents. It was demonstrated that currents followed the pyroelectric model for hot air heating. Large voltages were achieved in capacitor electrodes, what allowed charging and storaging energy in 1 ?F capacitors. Moreover, it was also demonstrated cell parallel association to provide larger powers to a load.

Energy Harvesting from PZT Pyroelectric Cells

An application specific integrated circuit (ASIC) front-end interface in 0.7-μm CMOS for resistive-bridge sensors is proposed. The circuit is based on a relaxation oscillator where the frequency of the rectangular-wave output is related to the fractional bridge unbalance, and the duty cycle depends on the overall bridge resistance, which typically is related to temperature. In this way, two independent pieces of information are simultaneously and cost-effectively carried on the same output signal. The bridge is driven at constant current, this avoids accuracy degradation with remotely placed sensors and enables a first-order thermal compensation for piezoresistive semiconductor sensors. The circuit has been characterized by means of a 1-kΩ reference bridge showing frequency and duty cycle sensitivities of 60.4 Hz/(1000 ppm) and 0.276%/(mΩ/Ω), respectively, at a central frequency of about 6.4 kHz. The circuit has also been tested with a piezoresistive SiC sensor operated at temperatures up to 150 °C, showing results in agreement with theoretical predictions.

ASIC front-end interface with frequency and duty cycle output for resistive-bridge sensors

This paper describes the design, realization and experimental characterization of a capacitive angular position sensor The sensor is composed of two parts, namely the sensing element and the dedicated signal conditioning electronic circuit The sensor is based on a low-cost Printed Circuit Board technology for the electrodes, and the use of a thick conductive rotor that is electrically floating and does not require any electrical contact to the electronic circuit The electronic circuit is based on a relaxation oscillator and the output is a DC voltage proportional to the angular position of the rotor The sensor and the electronic circuit are electrically equivalent to a voltage-excited resistive potentiometer The fabricated device provides redundancy by including two identical and independent sensors and electronic circuits in the same housing that work as separate measuring sections using the same rotor Measurement results on the prototype over a measurement range of ±11o gave a best-fit line nonlinearity within ±1% of voltage supply

A. Ghisla

Papers

Silicon resonant accelerometer with electronic compensation of input-output cross-talk

Energy Harvesting from PZT Pyroelectric Cells

ASIC front-end interface with frequency and duty cycle output for resistive-bridge sensors

Capacitive Angular-Position Sensor