Instrumentation & Measurement Magazine 24-2 - 17

Electromechanical Film
Sensors
An electromechanical film sensor
is composed of two external layers
that sandwich a third layer that is
made of polypropylene layers with
air pockets dispersed within it [32]
(Fig. 3c). When external force is applied, the sensor creates an electric
charge, meaning that they are typically passive in nature [32]. The
charge generation mechanism is
based on the mutual movement of
static charges that were originally
injected into the sensor during its
creation [32]. This causes the air
pockets to act as electrical dipoles
that change with applied perpendicular pressure [32].
This sensing approach is used in
the Emfit mattress [29], a commercially available product that has also
been used for research purposes. Joshi
et al. used the Emfit sensors, placed
between the mattress and bedding
(Fig. 2b) in an infant bed to monitor
body movement, which was reported
to be robust to noise, and performed
well in clinical settings. Perez-Macias
et al. used the Emfit mattress sensor
placed under the thoracic area of a
sleeping patient (presumably placed
on top of the mattress) (Fig. 2b) to detect snoring events in a group of 33
adult subjects with an accuracy of
80%.

Capacitive Pressure Sensors
Capacitive pressure sensors are composed of two plates that can store
electric charge separated by a material [45] (Fig. 3d). The charge between
the two plates creates a potential
difference that is dependent on the
overlapping area (A) of the two plates,
the permittivity of the material between the two plates, and the size of
the gap between the two plates (d)
[45]. Varying any of the factors can be
used as the sensing foundation [45].
The potential difference is maintained
by using an external voltage, meaning that capacitive PS are typically
active in nature. The capacitance (C)
of simple two-plate parallel sensor is
given by:
April 2021	

Fig. 3. Sensor Types. (a) Wheatstone bridge with strain gauges [46]; (b) Diagram of flexiforce A401 sensor
based on [47]; (c) Electromechanical film sensor based on [48]; (d) Parallel plate capacitor based on [49];
(e) Piezoresistive sensor based on [50]; (f) Fiber-optic pressure sensor; (g) Inductive pressure sensor based
on [51]; (h) Proximity pressure sensors [52]; (i) Air pressure sensor based on [53]. (Photo credits: Wikipedia
Commons.)

IEEE Instrumentation & Measurement Magazine	17



Instrumentation & Measurement Magazine 24-2

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