An On-Ear Pulse Wave Monitoring System for Daily Life

Transcription

1 An On-Ear Pulse Wave Monitoring System for Daily Life 1 Hag-seong Kang, 2 Gi-hyun Hwang, 3 Do-un Jeong 1, First Author Graduate School of Ubiquitous IT, Dongseo University, Busan, Korea, @hanafos.com 2 Division of Computer & Information Engineering, Dongseo University, Busan, Korea, hwanggh@gdsu.dongseo.ac.kr *3, Corresponding Author Division of Computer & Information Engineering, Dongseo University, Busan, Korea, dujeong@gdsu.dongseo.ac.kr Abstract Photoplethymosgraphy (PPG) measurements in convention would require a patient to clip the photo sensor on the finger and thus restrict the patient movement while taking the measurement. In this paper, we developed a wireless ear hook-type PPG measurement system using Bluetooth sensor is the aim for easy and convenience pulse wave monitoring purpose. The wireless ear hook-type PPG measurement system consists of a photo sensor, a PPG signal conditioning circuit, and a Bluetooth transceiver. Phot o sensor is placed on the ear lobe while there is a mini hooker attached to hold the sensor in place. PP G signal sensed from the earlobe is amplified through cascaded amplifiers analog circuits is amplified and then filter using band-pass technique. PPG signal is 12 bits sampling at frequency of 100Hz. The s ampled pulse data will be sent to smart phone for display and monitoring purpose. An android applicat ion is developed for real time and continuous PPG signal monitoring. Keywords: Bluetooth, Wearable, Ear PPG, Cascade Amplifier, Smart Phone 1. Introduction The idea of Lorna ubiquitous society bombards the current society with its brilliant thinking of comb ining Information Technology with social life. We hereby foresee the bright future of ubiquitous healt hcare application will be the next mile stone in future healthcare services. In fact, ubiquitous healthcare technology has been heavily discussed in literature and existing researc hes. User trial experience on mobile healthcare concluded the demand of telemedicine service in future healthcare application [1]. Photoplethymosgraphy (PPG) is one of the vital signals that provide useful clinical information in determining blood pressure and heart rate [2]. Sallen-key filter with a gain control feedback resistor has seems to be a common technique used in amplifying the PPG signal [3-4]. However, PPG measurement system suggested in [3-4] does not promote wearable bio sensors. Nevertheless, various researches and studies in wearable ear PPG measurement system haven carried out to promote ubiquitous healthcare application [4-10]. In this paper, a wireless ear hook-type PPG measurement system is proposed. The wireless ear hooktype PPG measurement system consists of photo sensors, a PPG signal conditioning circuit, and an integrated Bluetooth transceiver.photo sensor is placed on the ear lobe while there is a mini hooker attached to hold the sensor in place. PPG signal sensed from the earlobe is amplified through cascaded amplifiers analog circuits is amplified and then filter using band-pass technique. PPG signal is 12 bits sampling at frequency of 100Hz. An android application is developed for real time and continuous PPG signal monitoring. 2. System architecture The reflective photo sensor consists of an ultra small CNB10112 photo sensor, compromising a photo transistor and an IR emitting diode into a single one. Substrate board is to provide a soldering platform for connecting the photo sensor. A layer of non-conductive epoxy is insulated around International Journal of Digital Content Technology and its Applications(JDCTA) Volume7,Number9,May 2013 doi: /jdcta.vol7.issue

2 CNB10112 to protect wiring connection. An extra layer of transparent thin film is applied to reduce contact impedance. Hooker is mounted on the other side of the substrate to hold the earlobe while wearing it. The signal conditioning circuit consists of an amplifier circuit and a first order active band pass filter. The gain of the amplifier is 150 and the bandwidth of the circuitry is 0.3Hz to 3.39Hz. An ultra low power wireless transceiver, Bluetooth is integrated with the signal conditioning circuitry to provide wireless transmission interface for the ear PPG system. Figure 1. Ear PPG system : (a)snap shoot on the ear PPG system (b) Appearance of wearing the Figure 2. Side view of the system architecture schematic 2.1. Hook-type photo sensor. Figure 3. Hook-type photo sensor 581

3 A mini size earlobe hook-type PPG sensor is constructed to measure the pulsation of blood flow in earlobe. A 3.2mm x 4.5mm x 1.2 mm reflective photo sensor, CNB10112 is mounted on a 1.2cm x 1cm x 0.1cm substrate board. The reflective photo sensor compromises a photo transistor and an infra red emitting diode into a single one, making a very small size ear PPG photo sensor possible. A layer of non-conductive epoxy is insulated around the reflective photo sensor to protect the wiring connection. Another layer of transparent thin film is applied on the sensor s surface to serve as a protection shield to reduce contact impedance. Lastly, an ear lobe hooker is mounted on the back side of the substrate board. Ear sensor is hold in place at the earlobe s backside while the mini hooker is hooked at the earlobe s front side. The hooker is to hold the sensor in place. Figure 4. (a) Reflective photo sensor without transparent thin film (b) Reflective photo sensor with transparent thin film 2.2. Hardware architecture. Figure 5. Hardware architecture wireless ear PPG measurement system 582

4 Figure 5 shows the system architecture of the proposed ear PPG measurement system. It consists of a hook-type ear sensor, a PPG amplifier circuit and a wireless transceiver, Bluetooth. Phototransistor converts light energy into electrical current. A varying current drop across a resistor resulting a varying voltage. The resulting voltage is feed into an active high pass filter. High pass filter with low cutoff frequency of 0.3Hz is applied to reduce baseline noise. The low amplitude PPG signal is amplified through a cascade amplifier with a gain factor of 150. The cascade amplifier consists of two inverting amplifiers and one non-inverting amplifier, where is the output voltage of the first inverting amplifier and is the output of voltage of the second inverting amplifier, is the final output voltage of the cascade amplifier. Figure 6. Schematic of cascade amplifier Overall system gain: =( ( (1) Equation (1) can be expressed by a series of resistor ratio as shown in equation: 1 (2) If we let and, than it wields equation. 1 (3) Base on equation (3), we set = 100kΩ and R = 20kΩ, it yields an overall system gain of 150. One of the key to note is that, the gain can be pull up to 300 without any OPAMP saturation is seen. But in typical demonstration, gain factor of is sufficient. A low pass filter with 3.39Hz high cutoff frequency is used to reduce the size of the dominant of DC component and remove high frequency noise. Lastly, the ear PPG amplifier module is integrated with Bluetooth using 30 pins connector Wireless transmission. Bluetooth data transmission consumes low power (100mW) at an affordable price.furthermore, the interesting characteristic Bluetooth technology allows data to be transmitted through multi-frequency hopping spread spectrum up to 79 bands. Therefore, Bluetooth technology is relatively fast and transmission efficiency. In addition, the Bluetooth signal has high penetration powerin short distance. This allows the signal to be sent and received even though there is medium blockage. Nevertheless, Bluetooth comply international standard in worldwide and it can be easily made available in market in a lot of countries. Bluetooth Module HC-06,is a surface mounted module which used as wireless transceiver in this paper. Commercial Bluetooth V2.0 protocol based on CSR Bluetooth chip is used. The size of the Bluetooth module is 28mm 15mm 2.35mm. 583

5 2.4. Monitoring system. Figure 7. Bluetooth modules Android-based monitoring software is developed to monitor real time PPG signal by using Smartphone. Class name MainView MainViewThread GraphPath IState MoitoringState IntriState APPManager PPGMonitorActivity Table 1. Monitoring system class role Class role Class to perform a variety of events and is drawn on the screen, data update To view constantly updated threads The class with the path of the graph to be drawn Be drawn depending on the view implementation in each state in the sub Implementation monitoring is to be drawn when things Drawn when the intro screen, the ones implemented Information storage and set various options Activator class bits to configure the main screen of the Android Figure 8 shows the diagram shows the various classes based on the PPG signal to monitor Android. Descriptions of each class are shown in Table 1. Figure 8. Monitoring System class diagram 584

6 3. Experimental and results Figure 9 shows the real time application scenario where user is wearing the wireless Ear-PPG system to monitor his real time PPG signal by using mobile device. The convenience and comfortable way of monitoring the PPG and pulse rate at everywhere and anytime has been presented. Figure 9. (a) shows the scenario where user is monitoring his real time PPG signal while sitting and (b) shows the scenario where user is monitoring his PPG signal while walking around the room. Figure 9. PPG monitoring of daily life 3.1. Performance evaluation on photo sensor. An amplified and noise filtered PPG signal is under observation using digital oscilloscope to observe its signal quality. From the observation result, we realized that direct contact impedance would corrupt the PPG signal if the transparent thin film layer is excluded. A clean and distortion-free PPG signal is observed when applying an extra thin film layer on the ear sensor s surface. Figure 10 (a) shows the distorted PPG signal when transparent thin film is not applied. Figure 10 (b) shows a clean and noise free PPG signal when transparent thin film is applied. The reason of applying transparent thin film is to prevent direct contact of the earlobe tissue from the reflective photo sensor, thus, reducing contact impedance. Figure 10. (a) without transparent thin film layer, PPG signal is distorted due to contact noise (b) with transparent thin film layer, PPG signal is clean. 585

7 3.2. Wireless monitoring of ear PPG signal. Figure 11. Implementation of wireless ear PPG measurement system. Figure 11 shows the actual implementation of wireless ear hook-type PPG measurement system. PPG signal is first sensed from the ear lobe and amplified through the signal conditioning circuit. PPG signal is digitized at a sampling rate of 100Hz and data is sent to Smartphone for real time monitoring purpose. 4. Conclusion An ear hook-type pulse wave measurement system is designed and implemented. Applying an additional layer of transparent thin film could reduce the contact impedance and thus gives a better quality signal. Cascade amplifier is a useful technique to provide three stages analog signal amplification. In addition, real time monitoring of ear PPG signal is presented. User wearing the ear hook-type PPG measurement system is able to measure PPG signal wirelessly and at everywhere and anytime. Thus, the objective of ubiquitous monitoring of PPG signal is achieved in the end of this paper. 5. Acknowledgment This research was supported by research program of Dongseo University s Ubiquitous Appliance Regional Innovation Center supported by the grants from Ministry of Knowledge Economy of the Korean government(no. B ). And this research also supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology(No ). 6. References [1] Val Jones, Valerie Gay, Peter Leijdekkers, Body Sensor Networks for Mobile Health Monitoring: Experience in Europe and Australia, Proceedings of the 2010 Fourth International Conference on Digital Society, ICDS, pp , [2] R. Shriram, A. Wakankar, N. Daimiwal, et al, ContinousCuffless Blood Pressure Monitoring Based On PTT, Cummins College of Engineering for Women, ICBBT, pp.51-55,

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