Abstract
A new temperature-frequency converter based on the variation of the dielectric permittivity of the Liquid Crystal (LC) material with temperature has been demonstrated. Unlike other temperature sensors based on liquid crystal processing optical signals for determining the temperature, this work presents a system that is able to sense temperature by using only electrical signals. The variation of the dielectric permittivity with temperature is used to modify the capacitance of a plain capacitor using a LC material as non-ideal dielectric. An electric oscillator with an output frequency depending on variable capacitance made of a twisted-nematic (TN) liquid crystal (LC) cell has been built. The output frequency is related to the temperature of LC cell through the equations associated to the oscillator circuit. The experimental results show excellent temperature sensitivity, with a variation of 0.40% of the initial frequency per degree Celsius in the temperature range from −6 °C to 110 °C.
Highlights
Flat panel displays for electro-optical visual applications have been the core market of Liquid Crystal (LC) for the last 40 years
Different applied voltages to the nematic LC (NLC) cell were tested in order to obtain the optimal value in terms of the sensitivity and linearity of the system
The results demonstrate the right operation of a T-f converter developed in a wide temperature range using an NLC cell working as an electric transducer
Summary
Flat panel displays for electro-optical visual applications have been the core market of LC for the last 40 years. In the last decade new LC-based applications are focusing the interest of the research community [1,2]. New LC materials with ad-hoc anisotropic properties have been synthesized. Sensors 2012, 12 and innovative non-optic applications such as passive tunable radio frequency (RF) and microwave devices have been fabricated and reported [3–6]. Concerning sensor applications, different types of LC-based sensors such as biological, shear force or pressure have been previously developed [7–10]. Because of the dependence of optical properties on temperature, measuring devices based on LC materials have been widely reported as either optical or electrooptical temperature sensors [11–13]. This research presents a novel temperature-frequency (T-f) converter based on an LC cell which behaves as pure electric transducer
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