Sensores termo-resistivos em configurações realimentadas.

Abstract

In this thesis, a detailed study of the characteristics of thermoresistive sensors and their applications for measurement of temperature, thermal radiation and fluid flow velocity is presented. Analysis in respect of the sensitivity, response time and linearity of a typical measurement system is done. In this system, an electrically heated sensor is employed. Major emphasis has been put on the configurations in which the sensor is maintained essentially at a constant temperature. Some authors have presented small signal analyses for these configurations, with some specific assumptions, like use of amplifier with constant gain and or zero input off-set voltage, and derived expressions for the response time or equivalent time constant. This is not strictly correct. In this thesis, it is clearly shown that the response time of such a hypothetical system should be zero. Differently from the existing analyses, the analysis presented in this thesis, demonstrates explicitly the dependence of the dynamic response on the initial biasing of the sensor, for this circuit configuration. In order to obtain the static characteristics of the sensor, by relatively simple experimental procedures, it is established that the thermal test normally done for this characterization can be done away with, and the sensor characteristic parameters can be determined using numerical methods and the measurement data, obtained from the static electrical test. An investigation of use of the parametric identification methods, for the choice of an appropriate discrete time model for an NTC thermistor and its dynamic characteristics, has been initiated and some preliminary results are presented. Conventional negative feedback constant temperature measurement circuit present some limitations (e.g. sensor resistance not maintained strictly constant, limited output dynamic range and non-linearity). In this thesis, alternative feedback configurations to maintain the sensor temperature constant, eliminating or attenuating the above limitations are also proposed. These structures do not employ additional resistance in series with the sensor and thus permit the use of lower supply voltage. This finally may facilitate the integration of these configurations in the form of monolitic circuits.