Abstract
Résumé / Abstract This paper describes the effect of gas pressure on the sensitivity of a micromachined thermal accelerometer. The sensor principle is as follows: a heating resistor creates a symmetrical temperature profile; two temperature detectors, placed symmetrically on both sides of the heater, measure a differential temperature. When an acceleration is applied on the sensitive axis x of the sensor, the convection heat transfer and the temperature profile become asymmetric and the differential temperature was shown to be proportional to the acceleration. Since this differential temperature is due to free convection, a simple model has been developed suggesting that the response of thermal accelerometers is linearly proportional to the Grashof number. In this case, the sensor sensitivity should be proportional to the square of gas pressure. Therefore, a thermal accelerometer with three pairs of detectors placed at 100, 300 and 500 μm from the heater was manufactured using the techniques of silicon micromachining and was used for the study of the temperature profile: it was shown that the thickness of the thermal boundary layer decreases and the power consumption increases when the pressure increases. The study of the sensitivity according to the gas pressure has shown a square dependence of the sensor response in low pressure range. For higher pressures, different optimum sensitivities were obtained according to the distance heater-detector and to the gas pressure. Finally, a displacement of this optimum from 500 to 300 μm is observed when the pressure increases from I to 30 bars.