A. Turo

Methodology for extracting thermoelectric module parameters

The fundamental temperature-dependent parameters of a thermoelectric (TE) module: coefficient, electrical resistance, thermal conductance, and figure-of-merit are necessary, among other factors, for the conception of models for this type of devices. In this work we present a methodology for calculating these parameters from observable variables, which are obtained from temperature, voltage, and electric current measurements made on a working TE devices. The extracted parameters are plotted against the average temperature between the hot and cold side of the module. Results are verified by inserting average-value parameters into an electro-thermal SPICE model of a thermoelectric module and comparing simulation and measured (using a TE module characterization system) results for a given test configuration.

One-dimensional modeling of TE devices considering temperature-dependent parameters using SPICE

Based on simplified one-dimensional steady-state analysis of thermoelectric phenomena and on analogies between thermal and electrical domains, we propose both lumped and distributed parameter electrical models for thermoelectric devices. A couple of important advantages of the presented models are that the temperature dependence of material properties is considered and that they can be easily simulated using an electronic simulation tool such as SPICE. For a single free-standing pellet, comparisons are made between SPICE simulations using the proposed models and with numerical simulations carried out with Mathematica software. Results illustrate accuracy of the distributed parameter models and show how inappropriate it is to assume, in some cases, constant material parameters for an entire thermoelectric element.

Ultra-low noise front-end electronics for air-coupled ultrasonic non-destructive evaluation

Abstract Air-coupled ultrasonic inspection has been demonstrated to be a non-contact method of great interest in non-destructive evaluation (NDE) applications. The absence of direct contact or a liquid couplant provides this technique very attractive benefits in front of the well-known and well-developed liquid-coupled ultrasonic inspection systems. A wide range of defects can be detected by means of ultrasound coupled to and harvested from the specimen in absence of contact when using appropriate transducers designed for their operation in air. This paper presents an easy way to integrate air-coupled piezoelectric transducers in conventional ultrasonic NDE equipment. The design of a specific front-end electronics by using an ultra-low noise amplifier enables existing inspection systems to be used for the dry-coupled ultrasonic test of materials and structures. The amplifier provides the receiver with a signal-to-noise ratio large enough for good quality signal processing and imaging. System dynamic ranges of more than 100xa0dB are achieved.

Characterization of capacitive micromachined ultrasonic transducers in air using optical measurements

Capacitive micromachined ultrasonic transducers (CMUTs) are efficient transmitters and receivers for air-coupled nondestructive evaluation applications. In this paper, we present optical measurements on CMUTs with circular and rectangular membranes. Use of laser interferometer permits accurate measurement of individual membrane displacements as well as characterization of mode shapes on the membrane. When optical displacement measurements are combined with electrical impedance measurements, all elements of the equivalent circuit model can be evaluated. In particular, an estimate of the loss in the transducer is possible. Loss mechanisms include structural losses and squeeze-film effects of air behind the membrane, the latter of which has both frequency-shifting and dissipative effects. Recently fabricated circular and rectangular membrane designs typically exhibit a real loss on the order of 500 rayls/spl plusmn/200 rayls. The degree of resonant frequency shift for unsealed membranes in air depends strongly on the particular geometry, but can be as much as 30% of the transducers resonant frequency in vacuum. Measurements of CMUT loss due to air are compared to squeeze-film theory and are included in an equivalent circuit model. This model is compared against the measurement results.

Transient distributed parameter electrical analogous model of TE devices

This paper describes a one-dimensional distributed parameter transient model for thermoelectric devices implemented using analogies between the thermal and electrical domains, where thermal variables are described by their electrical analogues. The resulting electrical network can be tested by means of an electrical simulation tool such as SPICE. This approach facilitates simulation of a thermoelectric module and its interconnections with electronic control circuits and other thermal elements under varying boundary and initial conditions. Capabilities of the model are illustrated from simulations carried out for a free-standing thermoelectric element during the pulse cooling operation. Simulation results fit well with those obtained using other models reported in the literature as well as with numerical solutions.

High-power high-resolution pulser for air-coupled ultrasonic NDE applications

This paper describes the design and development of a high-power high-resolution pulser designed especially for air-coupled ultrasonic NDE applications requiring high material penetration capability. The pulser can deliver up to 800 V in a spike excitation type pulse to appropriate low/medium frequency transducers. Moreover, the pulser operates in three different modes: single pulse, double pulse (pulse cancellation technique) and external reference signal, where the last two operating modes are intended to enhance the resolution of ultrasonic inspections.

Ultrasonic monitoring of malolactic fermentation in red wines

The progress of malolactic fermentation in red wines has been monitored by using ultrasonic techniques. The evolution of ultrasonic velocity of a tone burst 1MHz longitudinal wave was measured, analyzed and compared to those parameters of oenological interest obtained simultaneously by analytical methods. Semi-industrial tanks were used during measurements pretending to be in real industrial conditions. Results showed that the ultrasonic velocity mainly changes as a result of the conversion by lactic acid bacteria of malic acid into lactic acid and CO2. Overall, the present study has demonstrated the potential of the ultrasonic technique in monitoring the malolactic fermentation process.

Ultrasound measurements for determining rheological properties of flour-water systems

Automation of the food industry requires fast and reliable measurements of the physical properties of materials during processing. The mixture of wheat flour, water, yeast and other ingredients produces a dough with specific viscoelastic characteristics capable of retaining gas and producing aerated goods. Within the baking industry, the control of dough properties is required to achieve final product quality and consistency. Traditional methods for dough testing are slow and off-line and do not provide fundamental rheological information. There is therefore a need for the development of fast and on-line instruments capable of providing relevant data for baking. Ultrasonics provides a nondestructive, rapid and low cost technique for the measurement of physical food characteristics. In this work, the rheological properties of dough are investigated using ultrasonic techniques. The measurements are correlated with dough quality tests. A common protocol for dough preparation is used for each type of measurement. Experimental results on different flour quality, water content and dough processing are presented. The relationship between measured ultrasonic parameters and final product quality is discussed.

Ultrasonic Lamb wave NDE system using an air coupled concave array transducer

Rapid and non contact ultrasonic NDE techniques are of great industrial interest. This paper describes an air-coupled ultrasonic inspection system based on concave linear arrays working on pitch-catch configuration, The system has been designed for real-time characterisation of sheet and plate manufactured materials such as paper and resin-fibre composites. The proposed system is based on air-coupled Lamb wave excitation and reception that performs a rapid measurement of the optimum input angle of the incident beam impinging the material surface. No mechanical parts are used for tuning the plate wave excitation with the angle, doing that electronically by steering the acoustic beam. This solution increases the exploration velocity and the measurement repeatability and system reliability. The main contributions are related to the utilization of a 0.8 MHz ultrasonic air-coupled concave array transducer. This transducer, using only 32 elements, is able to generate a 2.5 square cm size flat wavefront, steering up to /spl plusmn/26 degrees with 1.6 degree resolution, keeping the distance of wave flight and the impact point constant. The angular resolution can be improved up to 0.2 degrees using a novel micro-deflection technique, without any increment of the system complexity.

Wide dynamic range electronics for air-coupled Lamb-waves ultrasonic NDT using piezoelectric transducer arrays

The excitation and front-end electronics for an ultrasonic transducer array used in an air-coupled NDT system is presented. The main characteristic of the whole system is its wide dynamic range, making possible the inspection of thin and porous materials. The system is composed of two 32 elements concave array with a resonant frequency of 780 kHz. To excite the transducers, an array of pulsers has been designed. The transducers are excited by 500 ns and 500 V electrical pulses. The receivers are ultra low noise amplifiers with a 10 MHz bandwidth and 80 dB of gain.