Instrumentation & Measurement Magazine 24-6 - 50

Therefore, an infrared array sensor can produce a low-resolution
thermal image of the environment. Consequently, an
infrared array sensor acts as a low-resolution camera which
is independent of the ambient lighting, enabling detection
in darkness, and protects privacy because the resulting image
does not reveal enough details to the human eye to
infringe privacy.
An example is the AMG8831 sensor with an 8×8 array of
IR detectors which is used in [9] by installing it on the ceiling
to detect no event, stopping, walking, sitting and falling,
the latter useful for fall detection. Another device-free realtime
fall detection system is proposed in [10] which utilizes
the temperature of the IR array sensor as the indicator. In
this research, performed experiments show that the elderly
person stays motionless for a while after falling. This sign of
unconsciousness leads to a stationary point which provides
suspicious fall points to recognize whether the person has
fallen or not. A double-layer model is proposed to prevent
false detection because other activities like jogging could
be incorrectly identified as falling due to the similarities between
their features. The system achieves an accuracy as
high as 94%.
Acoustic Sensor
An acoustic sensor can record and process sound, which is useful
for healthcare audio event classification. For example, [11]
proposes an acoustic system that can cover the large variations
often faced in healthcare audio events and can recognize activities
such as falling downstairs, screaming, walking, opening
and closing doors, and phone ringing.
Acoustic sensors can also detect inaudible sound, such as
ultrasound. An ultrasonic sensor is an electronic device containing
a transmitter and a receiver which emits ultrasonic
waves and subsequently waits until waves are reflected back
after hitting objects. These waves are in the frequency range
of 40 to 70 KHz which is beyond human hearing of approximately
20 Hz to 20 KHz. They can measure the distance D to
the object in meters using (1):
D 
VT

2
(1)
where T is the time from wave emission to reflection reception
in seconds, and V is the velocity of the sound at approximately
343 meters per second. To test how well human activity can be
recognized, we built a circuit with the HY-SRF05 ultrasonic
sensor and an AVR atmega32 microcontroller, shown in Fig. 3a.
For each distance measurement, the trig pin of SRF05 should
be trigged with a pulse for at least 10 microseconds. After that,
8 pulses at 40 KHz are sent. When receiving the reflection, the
echo pin of SRF05 becomes high and the duration of it staying
high is measured by the microcontroller. This measured
time is the same as T in (1). As we can see from the results in
Fig. 3b, we can easily distinguish between walking, standing
and running.
An example can be seen in [12], which has the ability to
recognize three different activities using HC-SR04 ultrasonic
sensors: standing, sitting and falling, all with movement direction.
The challenge with such ultrasonic sensors is their
reduced coverage. Therefore, four HC-SR04 sensors are placed
in the corners of a square board and another sensor called LVMaxSonar-EZ0
is placed at the center point to reduce the dead
zone, as shown in Fig. 4.
Radio-Frequency Identification (RFID)
RFID uses electromagnetic fields to transmit digital encoded
data from the RFID tags to the readers. If these tags are attached
to objects and appliances of daily use, then RFID is
considered an object-tagged sensor. But if the tags are installed
in the infrastructure (walls, ceiling, floor, etc.), then RFID can
be considered device free. RFID tags could be passive or active.
Passive tags, which are more common, are powered by energy
from the reader and send encoded data to the reader. In contrast,
active tags are powered by a power source. Therefore, in
the active mode, the tag and the reader could be farther apart.
Even with passive tags it is possible to localize objects within 4
cm accuracy [21], so RFID's potential for HAR is high.
RFID technology can be utilized for HAR using the signal
strength which is propagated between tags and readers. The
strength could be measured by a parameter called Received
Signal Strength Indicator (RSSI). In an indoor environment,
a transmitted signal propagates to the receiver through
Fig. 3. Our ultrasonic sensor. (a) The circuit; (b) Activity recognition results.
50
IEEE Instrumentation & Measurement Magazine
September 2021

Instrumentation & Measurement Magazine 24-6

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