Limits...
Temperature-frequency converter using a liquid crystal cell as a sensing element.

Marcos C, Sánchez Pena JM, Torres JC, Isidro Santos J - Sensors (Basel) (2012)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Tecnología Electrónica, Universidad Carlos III de Madrid, Madrid, Spain. cmarcos@ing.uc3m.es

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.

Show MeSH
(a) NLC cell used in the T-f converter. (b) Equivalent electric circuit of the NLC cell.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3376606&req=5

f4-sensors-12-03204: (a) NLC cell used in the T-f converter. (b) Equivalent electric circuit of the NLC cell.

Mentions: The T-f converter is obtained by a simple modification of the multivibrator circuit. When the capacitor is replaced by an NLC cell (Figure 3(b)), the frequency only depends on the NLC cell temperature because the average electric field applied to the NLC cell (Vb) is constant. Commercial off the shelf twisted nematic liquid crystal cell manufactured by LC-Tec Company [16] with an effective area of 21 × 21 mm2 was used in the experiments (Figure 4(a)). As is mentioned above, an NLC cell under an AC bias voltage can be electrically modeled as an R-C parallel circuit (Figure 4(b)). This equivalent electrical circuit is valid for an applied bias voltage with frequencies in the range from 1 Hz to 104 Hz. Therefore, to determine the capacitance and resistance values, the NLC cell is electrically analyzed applying a bias voltage of 5 kHz and a square signal with a 50% duty cycle. This bias voltage is equivalent to the electrical conditions in the multivibrator circuit.


Temperature-frequency converter using a liquid crystal cell as a sensing element.

Marcos C, Sánchez Pena JM, Torres JC, Isidro Santos J - Sensors (Basel) (2012)

(a) NLC cell used in the T-f converter. (b) Equivalent electric circuit of the NLC cell.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3376606&req=5

f4-sensors-12-03204: (a) NLC cell used in the T-f converter. (b) Equivalent electric circuit of the NLC cell.
Mentions: The T-f converter is obtained by a simple modification of the multivibrator circuit. When the capacitor is replaced by an NLC cell (Figure 3(b)), the frequency only depends on the NLC cell temperature because the average electric field applied to the NLC cell (Vb) is constant. Commercial off the shelf twisted nematic liquid crystal cell manufactured by LC-Tec Company [16] with an effective area of 21 × 21 mm2 was used in the experiments (Figure 4(a)). As is mentioned above, an NLC cell under an AC bias voltage can be electrically modeled as an R-C parallel circuit (Figure 4(b)). This equivalent electrical circuit is valid for an applied bias voltage with frequencies in the range from 1 Hz to 104 Hz. Therefore, to determine the capacitance and resistance values, the NLC cell is electrically analyzed applying a bias voltage of 5 kHz and a square signal with a 50% duty cycle. This bias voltage is equivalent to the electrical conditions in the multivibrator circuit.

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Tecnología Electrónica, Universidad Carlos III de Madrid, Madrid, Spain. cmarcos@ing.uc3m.es

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.

Show MeSH