Multifunctional Optical Sensor Module

Integrated Optical Micro Displacement Sensor and Its Application to a

Photoplethysmographic Sensor with Measuring Contact Force

Hirofumi Nogami

, Ryo Inoue

, Yuma Hayashida

, Hideyuki Ando

Takahiro Ueno

and Renshi Sawada

1,2

Faculty of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, Japan

Graduate School of Systems Life Sciences, Kyushu University, Motooka 744, Nishi-ku, Fukuoka, Japan

Fuzzy Logic Systems Institute, Kitakyushu-City, Fukuoka, Japan

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, Parkinson’s

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

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

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

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

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

REFERENCES

Allen, J. (2007). Photoplethysmography and its application

in clinical physiological measurement. Physiological

measurement, 28(3), R1-R39. doi:10.1088/0967-

3334/28/3/R01

Andersson, L. M., Okada, H., Miura, R., Zhang, Y.,

Yoshioka, K., Aso, H., and Itoh, T. (2016). Wearable

wireless estrus detection sensor for cows. Computers

and Electronics in Agriculture, 127, 101-108. doi:

10.1016/j.compag.2016.06.007

Asada, H. H., Shaltis, P., Reisner, A., Rhee, S., and

Hutchinson, R. C. (2003). Mobile monitoring with

wearable photoplethysmographic biosensors. IEEE

engineering in medicine and biology magazine, 22(3),

28-40. doi: 10.1109/MEMB.2003.1213624

Díaz, S. E., Pérez, J. C., Mateos, A. C., Marinescu, M. C.,

and Guerra, B. B. (2011). A novel methodology for the

monitoring of the agricultural production process based

on wireless sensor networks. Computers and

Electronics in Agriculture, 76(2), 252-265. doi:

10.1016/j.compag.2011.02.004

Ishikawa, I., Sawada, R., Higurashi, E., Sanada, S., and

Chino, D. (2007). Integrated micro-displacement sen-

sor that measures tilting angle and linear movement of

an external mirror. Sensors and Actuators A: Physical,

138(2), 269-275. doi: 10.1016/j.sna.2007.03.027

Iwasaki, T., Takeshita, T., Arinaga, Y., Uemura, K., Ando,

H., Takeuchi, S., ... and Sawada, R. (2015). Shearing

force measurement device with a built-in integrated

micro displacement sensor. Sensors and Actuators A:

Physical, 221, 1-8. doi: 10.1016/j.sna.2014.09.029

Iwasaki, W., Sathuluri, R. R., Niwa, O., & Miyazaki, M.

(2015). Influence of Contact Force on Electrochemical

Responses of Redox Species Flowing in Nitrocellulose

Membrane at Micropyramid Array Electrode.

Analytical Sciences, 31(7), 729-732. doi: 10.2116/

analsci.31.729

Nishihara, K., Iwasaki, W., Nakamura, M., Higurashi, E.,

Soh, T., Itoh, T., ... and Sawada, R. (2013).

Development of a wireless sensor for the measurement

of chicken blood flow using the laser Doppler blood

flow meter technique. IEEE Transactions on

Biomedical Engineering, 60(6), 1645-1653. doi:

10.1109/TBME.2013.2241062

Nogami, H., Okada, H., Takamatsu, S., Kobayashi, T.,

Maeda, R., & Itoh, T. (2013). Unique activity-meter

with piezoelectric poly (vinylidene difluoride) films

and self weight of the sensor nodes. Japanese Journal of

Applied Physics, 52(9S1), 09KD15. doi: 10.7567/

JJAP.52.09KD15

Okada, H., Itoh, T., Suzuki, K., and Tsukamoto, K. (2009,

October). Wireless sensor system for detection of avian

influenza outbreak farms at an early stage. In Sensors,

2009 IEEE (pp. 1374-1377). IEEE. doi: 10.1109/

ICSENS.2009.5398422

Olivares, A., Olivares, G., Mula, F., Górriz, J. M., and

Ramírez, J. (2011). Wagyromag: Wireless sensor

network for monitoring and processing human body

movement in healthcare applications. Journal of

systems architecture, 57(10), 905-915. doi: 10.1016/

j.sysarc.2011.04.001

Spencer, B. F., Park, J. W., Mechitov, K. A., Jo, H., and

Agha, G. (2017). Next Generation Wireless Smart

Sensors Toward Sustainable Civil Infrastructure.

Procedia Engineering, 171, 5-13. doi: 10.1016/

j.proeng.2017.01.304

Suzuki, K., Okada, H., Itoh, T., Tada, T., Mase, M.,

Nakamura, K., ... and Tsukamoto, K. (2009).

Association of increased pathogenicity of Asian H5N1

highly pathogenic avian influenza viruses in chickens

with highly efficient viral replication accompanied by

early destruction of innate immune responses. Journal

of virology, 83(15), 7475-7486. doi: 10.1128/

JVI.01434-08

Takeshita, T., Harisaki, K., Ando, H., Higurashi, E.,

Nogami, H., and Sawada, R. (2016). Development and

evaluation of a two-axial shearing force sensor

consisting of an optical sensor chip and elastic gum

frame. Precision Engineering, 45, 136-142. doi:

10.1016/j.precisioneng.2016.02.004

Yamashita, T., Okada, H., Kobayashi, T., Togashi, K.,

Zymelka, D., Takamatsu, S., and Itoh, T. (2016). Ultra-

thin piezoelectric strain sensor array integrated on

flexible printed circuit for structural health monitoring.

In SENSORS, 2016 IEEE (pp. 1-3). IEEE. doi:

10.1109/ICSENS.2016.7808972

BIODEVICES 2018 - 11th International Conference on Biomedical Electronics and Devices