Multifunctional Optical Sensor Module
Integrated Optical Micro Displacement Sensor and Its Application to a
Photoplethysmographic Sensor with Measuring Contact Force
Hirofumi Nogami
1
, Ryo Inoue
2
, Yuma Hayashida
2
, Hideyuki Ando
3
,
Takahiro Ueno
4
and Renshi Sawada
1,2
1
Faculty of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, Japan
2
Graduate School of Systems Life Sciences, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, Japan
3
Fuzzy Logic Systems Institute, Kitakyushu-City, Fukuoka, Japan
4
Kitakyushu Foundation for the Advancement of Industry, Science and Technology, Kitakyushu-City, Fukuoka, Japan
Keywords: Integrated Optical Sensor, MEMS, Photoplethysmographic Sensor.
Abstract: Photoplethysmography (PPG) is widely and commonly used, as it produces a wide range of information, such
as stress level, heart rate interval, respiration rate, blood vessel hardness, etc. It is necessary to control the
contact force between a PPG sensor and the measurement location (the skin surface), in order to obtain an
accurate PPG signal. We propose new multifunctional sensor modules that can measure both pulse waves and
contact force. The sensor module has a micro integrated displacement sensor chip with an optical source,
photo diodes, and op-amp circuits, and a gum frame with a mirror. Some incident light penetrates into a finger,
and the scattered light, which contains a biological signal (a pulse wave), is detected by one photodiode. The
photodiode can also detect reflected light from a mirror, which is displaced by a contact force. In this paper,
we fabricate a multifunctional sensor module and attempt to simultaneously measure the pulse wave and
contact force.
1 INTRODUCTION
For the purpose of increased safety and security,
wireless sensor network systems are being increase-
ingly used in applications such as structural health
monitoring, human health monitoring, agricultural
field monitoring, and animal health monitoring
(Spencer et al., 2017). Structural health monitoring
can improve the safety and reliability of buildings,
bridges, tunnels, and express highways by detecting
damage before it reaches a critical state. This damage
is sensed by wireless sensor nodes installed on the
structure (Yamashita et al., 2016). Human health
monitoring detects sleep disorders, Parkinsons
disease, etc., by logging a person’s daily walking
movements and posture using Global Positioning
System (GPS) devices, triaxial accelerometers, and
angular velocity sensors (Olivares et al., 2011). These
technologies have also been introduced in agricultural
field monitoring, including animal health monitoring
(Díaz et al., 2011). It is believed that wireless sensor
nodes attached to animals, in conjunction with
wireless health-monitoring systems, can achieve
early detection and prevention of diseases, and thus
reduce economic losses.
In previous studies, a wireless sensor node was
attached to a chicken’s wing and it measured and
transmitted body temperature and activity data
(Nishihara et al., 2013). The collected data can be
compared with previous epidemic data for chickens
and used to monitor the health of an individual bird.
In order to build this system, we developed several
low-power technologies for the wireless sensor node,
including a custom-built LSI for an event-driven
system, a bi-metal micro-electro-mechanical
(MEMS) temperature switch (Suzuki et al., 2009), a
miniaturized 300-MHz band loop antenna (Okada et
al., 2009), and polyvinylidene difluoride (PVDF)
switches for activity sensors (Nogami et al., 2013).
Other studies have developed wearable wireless
estrus detection sensors (Anderson et al., 2016) or
portable estrus intensity detection sensors (Iwasaki et
al., 2015). In this study, we focused on a
photoplethysmographic (PPG) sensor.
Nogami, H., Inoue, R., Hayashida, Y., Ando, H., Ueno, T. and Sawada, R.
Multifunctional Optical Sensor Module - Integrated Optical Micro Displacement Sensor and Its Application to a Photoplethysmographic Sensor with Measuring Contact Force.
DOI: 10.5220/0006596900710076
In Proceedings of the 11th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2018) - Volume 1: BIODEVICES, pages 71-76
ISBN: 978-989-758-277-6
Copyright © 2018 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
71
Figure 1: Measurement principle of photoplethysmographic
sensor.
Figure 2: Experimental setup for purpose of evaluating AC
signal with measuring contact force (a), and measurement
result (b).
Photoplethysmography has been widely and
commonly used as a pulse monitor since it was
developed by Heraman in the 1930s (Allen et al.,
2007). A reflective PPG sensor can be easily attached
with low constraint, as it can detect pulse waves from
a body’s surface. It produces a wide range of
information, such as stress level calculated from heart
rate interval, respiration rate, heart rate, blood vessel
hardness, etc. The signal detected by a PPG sensor is
composed of an alternating current (AC), caused by
the heart cycle, and a direct current (DC) resulting
from veins and stationary tissue, as shown in Figure
1 (Asada et al., 2003). The AC signal is necessary for
detecting the heart rate interval, blood vessel hardness,
and cardiac rate. However, the AC signal is easily
varied with the contact force.
Figure 2 shows the experimental setup for
evaluating the AC signal while measuring the contact
force, and the measurement results. The PPG sensor
was fixed on the force sensor (USL06-H5, Tec Gihan
Co., Ltd.). Using a low-pass filter circuit, an AC
signal (pulse wave) was detected from the output
signal of the PPG sensor. When a finger was placed
on the PPG sensor, both the AC signal of the PPG
sensor and the contact force could be recorded. The
AC signal was greatly influenced by the contact force.
Thus, a contact force sensor was necessary for
measuring a stable pulse wave.
In this paper, we propose new multifunctional
sensor modules that can measure both pulse waves
and contact force. A sensor chip was designed by
applying the principle of an optical micro
displacement sensor (Ishikawa et al., 2007). The
displacement sensor utilizes light to measure the
displacement of an object. By combining the sensor
and the structure, it is possible to measure the load
and the shear force when the structure is displaced. In
this study, the pulse wave was measured using the
light that passed through a living body, and the load
was measured with other light.
2 SENSOR
In previous work, we developed a micro optical
displacement sensor that had a VCSEL and
photodiodes in the sensor chip (Iwasaki et al., 2015).
However, that sensor required external circuits for
each photodiode in order to amplify the output
signals, which increased the size the whole sensor
system. In this study, CMOS op-amp circuits were
integrated in a micro optical displacement sensor chip
(Takeshita et al., 2016). Using these sensor chips, we
BIODEVICES 2018 - 11th International Conference on Biomedical Electronics and Devices
72
fabricated a multifunctional sensor module that could
measure not only pulse waves but also contact force.
2.1 Design
Figure 3 shows a model of an optical displacement
sensor (Iwasaki et al., 2015). A VCSEL was centered
on the sensor chip and photo diodes were
monolithically fabricated around the VCSEL. A
mirror was attached to the object. The VCSEL emits
beams to the mirror and the PD measures the intensity
of the light reflected from the mirror. This sensor can
measure the displacement or angle of the object.
Figure 3: Model of a micro optical displacement sensor.
Figure 4: Model of a multifunctional sensor module which
could both pulse wave and contact force.
Figure 4 shows a model of a multifunctional
sensor module that can measure both pulse waves and
contact force. A micro optical displacement sensor
was sealed and attached to a gum frame with a mirror.
Some incident light penetrates into a finger and the
scattered light, which contains a biological signal (a
pulse wave), is detected by a photodiode. The
photodiode can also detect light reflected from a
mirror. The displacement of the mirror varies with
the contact force between the sensor module and the
finger. In this paper, we fabricated a multifunctional
sensor module and measured both pulse waves and
contact force.
2.2 Fabrication
Figure 5 shows the sensor chip design. An LED was
set at the center, instead of a VCSEL. Four photo
diodes were monolithically fabricated around the
LED. A CMOS op-amp circuit was located near the
photodiode. The op-amp circuit was designed to
amplify PD output by around 11 times (Fig. 5(b)).
Figure 5: Design of a displacement sensor with a CMOS
op-amp circuit(a), and the op-amp circuit (b).
Figure 6 shows a photograph of the integrated
displacement sensor chip. The size of this sensor is 3
mm by 3 mm, it is 0.7 mm in thickness, and it is
Multifunctional Optical Sensor Module - Integrated Optical Micro Displacement Sensor and Its Application to a Photoplethysmographic
Sensor with Measuring Contact Force
73
Figure 6: Photograph of a micro optical displacement
sensor with a CMOS op-circuit.
Figure 7: Photograph of a previous displacement sensor
including external amplifier circuits and a new sensor chip.
fabricated using MEMS technology. We have
successfully downsized the previous displacement
sensor that had external amplifier circuits (Fig. 7).
3 EXPERIMENTAL
Figure 8 shows photographs of a fabricated sensor
chip and a multifunctional sensor module. A light-
emitting diode (LED) was mounted in the center of
the sensor chip, instead of a VCSEL. The sensor chip
was die-bonded on a printed-circuit board, and wire-
bonded on it. After sealing the sensor chip, we
attached a gum frame with a mirror to it. Output
signals could be measured throughout the printed
circuit board.
Figure 8: Photographs of a fabricated sensor chip (a) and a
multifunctional sensor module (b).
Figure 9: Experimental setup.
Figure 9 shows the experimental setup with a
fabricated sensor module. The sensor module was
used to make a measurement of a finger as
incremental force was applied step-by-step to the
finger. The force was controlled with a force gage.
The signal of PD-A, the signal of PD-C, and the force
were recorded simultaneously.
BIODEVICES 2018 - 11th International Conference on Biomedical Electronics and Devices
74
Figure 10: Measurement results of PD-A (pulse wave) and
PD-C (Contact force) (a), and PD-C and force gage (b).
4 RESULTS & DISCUSSION
Figure 10 shows the results for (a) PD-A (pulse wave)
and PD-C (contact force) and for (b) PD-C and the
force gage. The signal of PD-C increased as the force
was increased incrementally (Fig. 10(b)). The signal
of PD-A decreased as the force was increased
incrementally, and the pulse wave could be confirmed.
Figure 11 shows the signals of PD-A for different
contact forces. The signal of PD-A showed that the
pulse wave had the highest amplitude when the force
gage was at 2.3 N. These results support the idea that
our multifunctional sensor module can simultaneous-
ly measure both the pulse wave and the contact force.
On the other hand, the PD-C signals were noisy.
The amplitude of the noise was higher than the
increase in the signal when the force was increased
incrementally. One of the causes is considered to be
signals from light reflected from the skin’s surface. It
is thus necessary to have a structure that detects only
the reflected light from the mirror and to improve the
signal-to-noise ratio.
Figure 11: Relationship between PD-A (pulse wave) and
contact force.
5 CONCLUSIONS
We succeeded in fabricating an ultra-compact optical
displacement sensor chip with integrated LED, PDs,
and CMOS amplifiers. By using this sensor chip, our
new multifunctional sensor module can measure
simultaneously both the pulse wave and contact force
between the sensor module and measurement
location.
ACKNOWLEDGEMENTS
This research was partially supported by grants from
the Project of the Bio-oriented Technology Research
Advancement Institution, NARO the research
project for the future agricultural production utilizing
artificial intelligence). In addition, the device of this
work was fabricated at “The 7th Novel Device Design
& Fabrication Contest in Hibikino” held at the
Semiconductor Center in Kitakyushu Science and
Research Park.
Multifunctional Optical Sensor Module - Integrated Optical Micro Displacement Sensor and Its Application to a Photoplethysmographic
Sensor with Measuring Contact Force
75
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