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2b). Normal breathing, central sleep apnea events, and obstructive sleep apnea events were differentiable using their
methods based on sleep data from 63 adults [43].
Ren et al. used a fiber-optic PSM placed between the bedframe and mattress (Fig. 2c) to measure center of pressure
(COP) trajectories as an indication of critical health events
(e.g., falls in older adults). Measurements were taken from one
female older adult (78 years old) before and after a fall event,
which presented distinct differences in COP trajectory [16].
More recently, Azimi et al. proposed using PSM data as an
early screening tool for the detection of central sleep apnea.
Using PSM placed between the bedframe and mattress (Fig.
2c), recordings from nine older adults were taken over a period
of about one year, and they were able to unobtrusively screen
for central sleep apnea [41].

Inductive Sensors
Bakhoum et al. presented a new pressure sensor based on
inductive principles that are linear, very small, have low temperature and pressure hysteresis and have sufficient level of
sensitivity (Fig. 3g). An iron core is placed inside an inductor
coil, and when pressure is applied perpendicularly, the iron
core is displaced outside the coil. The inductance changes depend on where the iron core is relative to the coil. The change in
inductance between the two states provides a measure of pressure applied. In other words, as pressure is applied, change in
inductance increases linearly [57].
Electromagnetic sensors are also based on the electromagnetic induction principle. Sensors may use a non-magnetized
ferromagnetic material to change the magnetic resistance of
the flux path [45]. When a force is applied to a magnet that is in
an external field, the force can be computed using [45]:
	

F

1 2
B S	(3)
2 0

where F is the magnetic force, μ0 is the permeability of the free
space around the magnet, B is the magnetic field intensity and
S is the area of the ferromagnetic material that is perpendicular to the magnetic field itself [45]. Mahdavi and Rosell-Ferrer
designed a sensor system placed between the bedframe and
mattress (Fig. 2c) using this principle that was able to identify
user presence in bed, their position, breathing and heart activity [58].

Proximity-based Sensors
Selzer et al. and Wallace et al. presented a novel flexible PSM
that is able to detect breathing rate (Fig. 3h). The infrared proximity sensors were placed within a compressible material layer
[35], [59]. When external pressure is applied, the compressible
material moves closer to the sensor and causes light variations
within the cavity. The infrared light sensor measures this variation in light as a measure of applied pressure [35], [59].

Air Mattress Pressure Sensors
Some hospital mattresses contain air pockets within them.
Air pressure sensors are typically used to detect mechanical
April 2021	

deformation in response to air pressure changes caused by a
person shifting in bed (Fig. 3i). The underlying mechanical
sensing principle varies depending on the PSM in question,
and examples include the use of strain gauges or pressure sensitive resistors or capacitors that are affected by the air pressure
changes within an air cell.
Chee et al. used an air mattress sensor system, composed
of 19 air cells, to monitor breathing and heart beats in a supine posture when a person was lying on the mattress directly.
Preliminary results suggest that the air mattress may have applications in sleep analysis and remote health and neonate
monitoring.
The Sleep Tracking Mat is a commercial product that can be
implemented in a smart-home, designed to be placed between
the bedframe and mattress (Fig. 2c) [13]. It uses a similar air inflation system, so when force is applied to the inflated sensor,
sleep quality, snoring detection, and heart rate are extracted by
its built-in software.

Sensor Selection
Now that we have identified what PS can measure and the underlying sensing principles that have been used, the following
section provides a comparative overview to consider when selecting a PS for health monitoring purposes.

Sensor Configuration
As mentioned, sensor configuration is an important factor
to consider when measuring signals with pressure sensors.
A single, very sensitive sensor would perform well when
monitoring very small pressure variations (e.g., heart rate).
However, when measuring a signal that has relevant spatial
characteristics (e.g., long-term internal fluid distribution) a
sensor array would be more appropriate. Furthermore, when
using an array of sensors, the spatial distribution between each
individual sensor is also important in terms of geometry when
extracting spatial information.

Sampling Rate
As with many other sensors, the sampling rate needs to be selected strategically. Higher sampling rates lead to more data,
meaning that more storage is required for the overall system.
To measure a signal with a low frequency (e.g., long-term internal fluid monitoring that varies over period of hours), use
a reduced sampling rate. However, to monitor a signal with a
higher frequency (e.g., heart rate, which is about 60 beats per
minute), a higher sampling rate is required to fully capture the
signal.

The Surrounding Set Up
When measuring bed-based pressures, the surrounding set up
must be considered. The first consideration is the location of
the sensor itself. Some studies have investigated placing the
PSM on top of the mattress, as in [37], at the cost of user comfort. Typically lying on sensors is not as comfortable as lying
on the mattress itself. Other sensors have been placed directly
below the bed legs [4], [14], meaning that the recorded signal

IEEE Instrumentation & Measurement Magazine	19



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