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Dielectric sensors
The research effort from INASCO on the development of dielectric sensors for a wide range of material transformation processes spans across the last 10 years. The areas where dielectric sensing is applied include all the processes where the change in conductivity of the materials undergoing transformation can provide quantitative information of the materials’ properties. A characteristic example is the monitoring of thermoset cure, while more exotic applications include the diagnosis of the operational capacity of nanomembranes (fouling monitoring) or the measurement of the flow speed of liquids or gases in closed cavities. In this discipline INASCO possesses leading position in the Europe and in World, having applied dielectric sensing across multi-sectoral work areas, such as Aeronautics, Transport and Energy.
 

The development of dielectric sensors in polymeric materials monitoring over the course of EC funded projects (directed by INASCO) is shown in the following scheme.

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Sensor design evolution and relevant EC funded projects during the period 1988 - 2008

 

The key model involved in the use of interdigital sensors is the electric field model. The correspondence between electrical properties of the material within the field and the measurement (i.e. capacitance) is fundamental in interpreting the readings from the dielectric sensor. The pioneering work of Mamishev et al (University of Washington, Seattle) has laid the bases for the expansion of the use of these sensing structures.

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A typical distribution of the electric potential within the electric field of an interdigital pattern.

 

Robust and reusable dielectric sensors have been designed by INASCO to measure the electrical response of thermoset resins and matrices of composite materials at specified depth over the sensor surface. The temperature (a conditional parameter) is also measured at the vicinity of the sensor surface. Through the synthesis of dielectric measurement to the temperature, the material state (a conditional parameter) is derived. To realise this potential, the material characterisation is performed in the laboratory (analytical models of material properties evolution are also used) and the mapping is used in real-time (embedded in the measurement routine) for the on-line estimation.


To involve the sensor in the process to be monitored, intelligent sensor adaptations have been realised by INASCO as shown in the following pictures.

 

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 Durable re-usable dielectric sensors have been adapted to process tooling, such as moulds or dies.
 
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Several design schemes of flat interdigital sensor elements.
 
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Dielectric sensor for measurement of gas velocity in turboshaft engines.