handbook of modern sensors

11.7 Breeze Sensor 375
(A)
(B)
Fig. 11.15. Piezoelectric breeze sensor. (A) a circuit diagram; (B) a packaging in a TO-5 can.
Fig. 11.16. In a breeze sensor, gas movement strips off electric charges from the surface of a
piezoelectric element.
temperature. The elements are connected in a series-opposed circuit; that is, whenever
both of them generate the same electric charge, the resulting voltage across the bias
resistor R b (Fig. 11.15A) is essentially zero. Both elements, the bias resistor and the
JFET voltage follower, are encapsulated into a TO-5 metal housing with vents for
exposing the S 1 element to the gas movement (Fig. 11.15B).
The operating principle of the sensor is illustrated in Fig. 11.16. When airflow
is either absent or is very steady, the charge across the piezoelectric element is balanced.
Element internal electric dipoles, which are oriented during the poling process
(Section 3.6 of Chapter 3), are balanced by both the free carriers inside the material
and the charged floating air molecules at the element’s surface. As a result, voltage
across the piezoelectric elements S 1 and S 2 is zero, which results in baseline output
voltage V out . When the gas flow across both S 1 surfaces changes (S 2 surfaces are protected
by resin), moving gas molecules strip off the floating charges from the element.
This results in the appearance of voltage across the element’s electrodes, because the
internally poled dipoles are no longer balanced by the outside floating charges. The
voltage is repeated by the JFET follower, which serves as an impedance converter,
and appears as a transient in the output terminal.