Simone Dalola

Characterization of Thermoelectric Modules for Powering Autonomous Sensors

The characterization of three commercial thermoelectric modules, designed for cooling/heating applications, is presented in order to employ the devices for power conversion, i.e as thermoelectric generators (TEG). The thermoelectric theory is briefly described at first, also taking into account the relationship between effective temperature difference across the TEG junctions and temperature difference applied externally, when insulating ceramic plates have a finite thermal conductance. Performances of thermoelectric modules are evaluated 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, showing the possibility of use thermoelectric devices for waste heat recover. A TEG was then used to supply an autonomous system that interfaces with a temperature sensor and periodically transmits the measurement information via a radiofrequency (RF) link. Experimental data show that the system works correctly and sends the RF signal when temperature difference applied across TEG is high enough.

Autonomous Sensor System With Power Harvesting for Telemetric Temperature Measurements of Pipes

In this paper, an autonomous sensor system, with low-power electronics for radio-frequency (RF) communication, incorporating a thermoelectric energy-harvesting module for unattended operation is presented. A target application is proposed for temperature measurement of walled-in pipes. When the autonomous sensor is placed on the heat source, a thermoelectric module harvests energy, powering the autonomous sensor. In this condition, no external power source is necessary, the temperature measurement is performed, and the data are saved into a nonvolatile memory. When the external readout unit is active, the electromagnetic field is used to power the autonomous sensor system and to communicate the data. An experimental setup has been arranged and characterized by measuring the temperature along the pipe, the voltage that can be generated by thermoelectric generators, and the influence of different materials on RF communication. The temperature data of the heat source, which are collected by the autonomous sensor, are compared with that of a reference thermistor. The measurement results show good agreement between the two measured temperature data sets. The experimental data demonstrate that the autonomous system works correctly for a temperature gradient that is higher than 9degC, within a readout distance of a few centimeters. The presented autonomous sensor system can be effectively used for measurements into a close environment in which a temperature difference is present.

MEMS Thermal Flow Sensor With Smart Electronic Interface Circuit

A smart system for flow measurement is presented, consisting of a micromachined thermal flow sensor combined with a smart front-end electronic interface. The flow sensor is based on a novel thermal transduction method, which combines the hot-film and calorimetric sensing principles. The sensor consists of four germanium thermistors embedded in a thin membrane and connected to form a Wheatstone bridge supplied with a constant DC current. In this configuration, both the bridge unbalance voltage and the voltage at the bridge supply terminals are functions of the flow offering high initial sensitivity, i.e., near zero flow and wide measurement range, respectively. The front-end interface is based on a CMOS relaxation oscillator circuit where the frequency and the duty cycle of a rectangular-wave output signal are related to the bridge unbalance voltage and the voltage at the bridge supply terminals, respectively. Furthermore, the amplitude of the output signal is a linear function of the operating temperature. In this way, a single output signal advantageously carries two pieces of information related to the flow velocity and provides an additional measurement of the sensor operating temperature, which enables the correction of the temperature dependence of the sensor readouts. The system has been experimentally characterized for the measurement of nitrogen gas flow velocity at different sensor temperatures. The initial sensitivities at room temperature result 13.7 kHz/(m/s) and 23.5%/(m/s), in agreement with FEM simulations, for frequency and duty cycle readouts, respectively, with an equivalent velocity resolution of about 0.5 and 1.3 cm/s.

Integration of ZnO and CuO nanowires into a thermoelectric module

Summary Zinc oxide (ZnO, n-type) and copper oxide (CuO, p-type) nanowires have been synthesized and preliminarily investigated as innovative materials for the fabrication of a proof-of-concept thermoelectric device. The Seebeck coefficients, electrical conductivity and thermoelectric power factors (TPF) of both semiconductor materials have been determined independently using a custom experimental set-up, leading to results in agreement with available literature with potential improvement. Combining bundles of ZnO and CuO nanowires in a series of five thermocouples on alumina leads to a macroscopic prototype of a planar thermoelectric generator (TEG) unit. This demonstrates the possibility of further integration of metal oxide nanostructures into efficient thermoelectric devices.

Micromachined piezoresistive inclinometer with oscillator-based integrated interface circuit and temperature readout

In this paper a dual-chip system for inclination measurement is presented. It consists of a MEMS (microelectromechanical system) piezoresistive accelerometer manufactured in silicon bulk micromachining and a CMOS (complementary metal oxide semiconductor) ASIC (application specific integrated circuit) interface designed for resistive-bridge sensors. The sensor is composed of a seismic mass symmetrically suspended by means of four flexure beams that integrate two piezoresistors each to detect the applied static acceleration, which is related to inclination with respect to the gravity vector. The ASIC interface is based on a relaxation oscillator where the frequency and the duty cycle of a rectangular-wave output signal are related to the fractional bridge imbalance and the overall bridge resistance of the sensor, respectively. The latter is a function of temperature; therefore the sensing element itself can be advantageously used to derive information for its own thermal compensation. DC current excitation of the sensor makes the configuration unaffected by wire resistances and parasitic capacitances. Therefore, a modular system results where the sensor can be placed remotely from the electronics without suffering accuracy degradation. The inclination measurement system has been characterized as a function of the applied inclination angle at different temperatures. At room temperature, the experimental sensitivity of the system results in about 148 Hz/g, which corresponds to an angular sensitivity around zero inclination angle of about 2.58 Hz deg−1. This is in agreement with finite element method simulations. The measured output fluctuations at constant temperature determine an equivalent resolution of about 0.1° at midrange. In the temperature range of 25–65 °C the system sensitivity decreases by about 10%, which is less than the variation due to the microsensor alone thanks to thermal compensation provided by the current excitation of the bridge and the positive temperature coefficient of resistance of the piezoresistors.

Autonomous Sensor System with RF Link and Thermoelectric Generator for Power Harvesting

Autonomous sensing system is a promising approach in applications that do not allow cabled solutions and the use of battery has maintenance problems. In this paper an autonomous sensor system with low power electronics for RF communication and with an energy harvesting module is presented. The sensored system is proposed for temperature measurements, but it can be used with other sensor types to measure different quantities. When the autonomous sensor is placed on a heat source, a thermoelectric module harvests energy, powering the autonomous sensor. In this condition no external electromagnetic field is necessary, the temperature measurement is performed and the data are saved in non-volatile memory. When the readout unit is active the electromagnetic field is used to power the autonomous sensor system and to communicate the data. An experimental set-up has been arranged to compare the temperature data of the heat source collected by the autonomous sensor and a referenced thermoresistance. The experimental measurements shows good agree between the two measured temperature data. The wake-up signals demonstrate that the autonomous system works correctly for a temperature gradient of less than 9degC, and with a readout distance of few centimeters. The presented autonomous sensor system can be efficiently used for measurements into close environment in which a temperature difference is present.

Smart flow sensor with combined frequency, duty-cycle, and amplitude output

A thermal flow sensor with smart electronic interface is presented. The sensor is based on four germanium thermistors embedded in a thin membrane and connected to form a Wheatstone bridge. Both the bridge unbalance voltage and the voltage at the bridge supply terminals under constant current excitation are functions of the flow offering high initial sensitivity and wide measurement range, respectively. The signal interface is based on a relaxation oscillator which provides a rectangular-wave output whose frequency is related to the bridge unbalance, whereas the duty cycle is a function of the voltage at the bridge supply terminals. Hence, both sensor signals are simultaneously and independently carried on the same output signal, featuring unambiguous characteristic over a wide flow range accompanied with a high relative sensitivity at low flow velocities. The amplitude of the output signal depends linearly on the operating temperature, which enables the correction of the temperature dependence of the sensor readouts.

Investigation of Seebeck Effect in ZnO Nanowires for Micropower Generation in Autonomous Sensor Systems

The Seebeck effect of ZnO nanowires has been investigated with the future aim to build thermoelectric devices based on nanowire arrays for energy harvesting and potential use in low-power portable electronics and autonomous sensor systems. Bundles of ZnO nanowires have been deposited on alumina substrates by a thermal evaporation process. The ZnO nanowires have been characterized by means of a purposely-developed experimental set-up, showing a negative Seebeck coefficient as for n-type semiconductors.

Investigation of Seebeck Effect in Metal Oxide Nanowires for Powering Autonomous Microsystems

The Seebeck effect in ZnO (n-type) and CuO (p-type) nanowire bundles grown on alumina substrates has been investigated. By combining n- and p-type nanostructured elements, a planar thermoelectric device has been proposed and characterized, confirming the feasibility of fabricating planar thermoelectric generators based on metal oxide nanowires with the future aim of powering autonomous sensors and microsystems.