handbook of modern sensors

246 6 Occupancy and Motion Detectors
(A)
(B)
Fig. 6.14. Dual pyroelectric sensor. (A) A sensing element with a front (upper) electrode and
two bottom electrodes deposited on a common crystalline substrate. A moving thermal image
travels from left part of the sensor to the right, generating an alternate voltage across bias
resistor, R (B).
identical elements are positioned inside the sensor’s housing. The elements are connected
to the electronic circuit in such a manner as to produce the out-of-phase signals
when subjected to the same in-phase inputs. The idea is that interferences produced,
say, by the piezoelectric effect or spurious heat signals are applied to both electrodes
simultaneously (in phase) and, thus, will be canceled at the input of the circuit, whereas
the variable thermal radiation to be detected will be absorbed by only one element at
a time, thus avoiding a cancellation.
One way to fabricate a symmetrical sensor is to deposit two pairs of electrodes
on both sides of a pyroelectric element. Each pair forms a capacitor which may be
charged either by heat or by a mechanical stress. The electrodes on the upper side
of the sensor are connected together, forming one continuous electrode, whereas the
two bottom electrodes are separated, thus creating the opposite, serially connected
capacitors. Depending on the side where the electrodes are positioned, the output
signal will have either a positive or negative polarity for the thermal influx. In some
applications, a more complex pattern of the sensing electrodes may be required (e.g.,
to form predetermined detection zones), so that more than one pair of electrodes is
needed. In such a case, for better rejection of the in-phase signals (common-mode
rejection), the sensor still should have an even number of pairs, where positions of
the pairs alternate for better geometrical symmetry. Sometimes, such an alternating
connection is called an interdigitized electrode.
A symmetrical sensing element should be mounted in such a way as to assure that
both parts of the sensor generate the same signal if subjected to the same external
factors. At any moment, the optical component must focus a thermal image of an