handbook of modern sensors

6.4 Triboelectric Detectors 237
6.4 Triboelectric Detectors
Any object can accumulate static electricity on its surface. These naturally occurring
charges arise from the triboelectric effect (i.e., a process of charge separation due to
object movements, friction of clothing fibers, air turbulence, atmosphere electricity,
etc.) (see Section 3.1 of Chapter 3). Usually, air contains either positive or negative
ions that can be attracted to the human body, thus changing its charge. Under the
idealized static conditions, an object is not charged: Its bulk charge is equal to zero. In
reality, any object which at least temporarily is isolated from the ground can exhibit
some degree of its bulk charge imbalance. In other words, it becomes a carrier of
electric charges.
Any electronic circuit is made up of conductors and dielectrics. If a circuit is not
shielded, all of its components exhibit a certain capacitive coupling to the surrounding
objects. In practice, the coupling capacitance may be very small—on the order of 1 pF
or less. A pickup electrode can be added to the circuit’s input to increase its coupling
to the environment, very much like in the capacitive detectors discussed in Section
6.3. The electrode can be fabricated in the form of a conductive surface which is well
isolated from the ground.
An electric field is established between the surrounding objects and the electrode
whenever at least one of them carries electric charges. In other words, all distributed
capacitors formed between the electrode and the environmental objects are charged
by the static or slow-changing electric fields. Under the no-occupancy conditions, the
electric field in the electrode vicinity is either constant or changes relatively slowly.
If a charge carrier (a human or an animal) changes its position (moves away or
a new charge carrying an object enters into the vicinity of the electrode), the static
electric field is disturbed. This results in a redistribution of charges between the
coupling capacitors, including those which are formed between the input electrode
and the surroundings. The charge magnitude depends on the atmospheric conditions
and the nature of the objects. For instance, a person in dry man-made clothes walking
along a carpet carries a million times stronger charge than a wet intruder who has come
from the rain. An electronic circuit can be adapted to sense these variable charges at
its input. In other words, it can be made capable of converting the induced variable
charges into electric signals that may be amplified and further processed. Thus, static
electricity, which is a naturally occurring phenomenon, can be utilized to generate
alternating signals in the electronic circuit to indicate the movement of objects.
Figure 6.8 shows a monopolar triboelectric motion detector. It is composed of
a conductive electrode connected to an analog impedance converter made with a
MOS transistor Q 1 a bias resistor R 1 , an input capacitance C 0 , a gain stage, and a
window comparator [4]. Whereas the rest of the electronic circuit may be shielded, the
electrode is exposed to the environment and forms a coupling capacitor C p with the
surrounding objects. In Fig. 6.8, static electricity is exemplified by positive charges
distributed along the person’s body. Being a charge carrier, the person generates an
electric field, having intensity E. The field induces a charge of the opposite sign in the
electrode. Under static conditions, when the person does not move, the field intensity
is constant and the input capacitance C 0 is discharged through a bias resistor R 1 . That