Instrumentation & Measurement Magazine 24-6 - 49

Table 2 - Comparison between device-free sensors (continued)
Type
Microwave Doppler Radar
* wide range of
detection
* no adjustment
needed in different
environment (is
not sensitive to
static objects)
* only dynamic
actions can be
measured by CW
radar
* computationally
complex; feature
extraction needed
Advantages
Disadvantages
Usage in well-being
assessment
* comprehensive
human motion
recognition,
including falling
vertically or
crawling forward
[16] cognitive
assessment by
classifying gait
patterns [17]
widely used in security systems. For human motion detection,
a special circuit with two amplification and filter stages to analyze
PIR analog output is needed. We built such a circuit as
a PCB board, shown in Fig. 1, based on the circuit schematic
in [20]. To evaluate it, we had two people move in front of it
at a distance of 2 meters in two different directions and paces.
The result is illustrated in Fig. 2, which shows both the direction
and the pace of movement can be detected in the phase
and magnitude of the output voltage, respectively. There is a
limit of detecting only two directions with respect to the orientation
of the sensor; however, this limit could be overcome
by implementing an array of PIR sensors in different positions
and orientations.
Such sensors have been used for early detection of dementia,
which is a mental disability that affects many people,
especially elderly people. Fortunately, its development can be
delayed if the person can be properly treated at the early stages
of dementia. In [8], a system is proposed for early detection
of dementia. This approach uses the Martino Saltzman (MS)
model to classify a person's travel patterns into direct, pacing,
lapping and random,
which can then be used
to detect early stages of
dementia. The approach
uses PIR sensors and uses
machine learning on the binary
images of the sensors'
logs to detect dementia.
Infrared Array Sensor
Infrared array sensor is
another device-free solution
which has a
2-dimensional array of
infrared detectors. Every
object radiates IR rays; the
higher the temperature
of an object, the higher
its amount of radiation.
September 2021
Typical Price ($US)
* low cost of CW
radar (HB100
microwave motion
sensor is about
$10)
* middle cost of
FMCW radar
(FM24-NP100
microwave
ranging radar is
about $100)
Fig. 1. Our circuit with the PIR sensor.
Fig. 2. Output of our PIR sensor for movement in two different directions and paces.
IEEE Instrumentation & Measurement Magazine
49

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