Understanding Smart Sensors - Nomads.usp

60 Understanding Smart SensorsExtrusionStretch5:1 ratioPolarize100 V/mMetallizeα Phaseβ Phase++++++++++----------Becomespiezoelectric(a)Metallizationgrabs chargecreated byfilm elementFabricateintosensorsVolts0.80.60.40.20−0.2−0.4−0.6−0.850 mS /Divpassenger car Small truck Large 2-axle truck(b)Figure 3.5 (a) A Kynar piezoelectric film is polarized in an intense electric field of approximately100 V/mm and metallized to create a transducer. (b) The output of a vehiclesensor in a voltage mode is a voltage proportional to weight and speed ofthe vehicle. (After: [10].)3.3.3 Hall EffectA vertical Hall-effect structure has been designed for the detection of magneticfields oriented parallel to the plane of the chip. Bulk micromachining wasused to achieve higher sensitivity than a device without micromachining.The micromachined unit had an output of 70 mV for an applied magneticfield of 400 mT, which was almost five times the sensitivity of the unetchedunit [12].3.3.4 Chemical SensorsA metal oxide chemical sensor’s resistivity changes, depending on the reducingor oxidizing nature of the gaseous environment around the sensor. Figure 3.6shows sensitivity data for hydrogen (H 2 ) and carbon monoxide (CO) in air forone design [13]. Conductances of over 100(10 −7 )Ω −1 for H 2 and almost32(10 −7 )Ω −1 for CO were measured with the sensor exposed to concentration of