Design of Portable ECG Monitoring System Based on LM4F120H5QR

www.ijape.org International Journal of Automation and Power Engineering (IJAPE) Volume 4, 2015 doi: 10.14355/ijape.2015.04.002

Design of Portable ECG Monitoring System Based on LM4F120H5QR Shengqian Ma*1, Xiaoyong Ni2, Mei Qin3 College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China *1

s.q.ma@163.com; 2nxy3662362@126.com; 3qinmei_23@163.com

Abstract The ECG acquisition circuit in the market now exists such defects as having complex structure , large size, high power consumption and overpriced, so to design a portable ECG monitor without the above mentioned disadvantages is urgently needed. The monitor designed by this paper uses ARM Cortex‐M4 core processor LM4F120H5QR, and selects ADAS1000‐3 as signal acquisition unit to reduce the amplification processing of ECG signals, in order to have a long period of ECG monitoring, the SD card is used as the storage medium. To realize the multi task processing and application management, the RT‐Thread operating system is adopted. The digital signal processing technology is implemented by programming to replace the hardware filtering processing, which can save the circuit board size and reduce the power consumption. At the same time,the use of GPRS module for remote data transmission with PC monitoring interface can help doctors a remote diagnosis. Slave computer displays the collected ECG signal waveform and information on Colour LCD screen. After the comprehensive simulations and prototype tests, the results show that the system can achieve the success of the ECG signal acquisition, processing, storage, display and remote transmission. That’s the instrument has a good prospect.

Keywords ECG; ADAS1000‐3; Digital filter; GPRS; RT‐Thread

Introduction In recent years, with the aging society’s coming, the cardiovascular disease has become one of the main diseases which threats human’s health and life, what’s more serious is that the heart disease causes the highest mortality rate among other illnesses[1]. There are two kinds of monitoring equipment: one is Electrocardiogram Machine, the other is Dynamic Electrocardiogram(Holter)[2‐3]. ECG machine is usually used in hospitals due to its deficiency of power consumption ,size and weight; while common patient cannot afford the high cost of Holter. Thus, there is a need to design a ECG which can overcome these shortcomings, get the balance on performance, power consumption, portability and price, at the same time it can be remotely monitored, convenient for doctors to diagnosis, finally, promote the m‐health and e‐health development[4‐5]. Currently on the market there are the corresponding products, but which generally have the following disadvantages: the circuit structure is complex, the size is large, so that the patient it is not convenient for the patient to carry; the majority of Holter can not be real‐time remote transmission of ECG signal, delays diagnosis and treatment[6‐7]; given this, an implementation of ECG monitor with low power consumption, miniaturization, portable and low cost is presented by this paper.

At present, there are many ECG monitors with microprocessor [8‐9], although they gains the low power consumption, but the signal processing, data processing and storage, communication ability are insufficient. In order to overcome the shortcomings, this paper selects the microprocessor LM4F120H5QR as CPU, it has an ARM Cortex‐M4 architecture core, FPU unit and a variety of different sleep models. In addition, high performance ADC is used as the core to build measurement system, GPRS module is used to realize the remote communication, and the SD card is applied to store the massive data. The traditional signal filtering is completed by hardware circuit [10‐ 11], the paper adopts digital filtering method, which can achieve a simplified system, and also is very good to filter out interference signals and improve the quality of the ECG signal, more than that ,it can reduce the board size and lower the power consumption.

Hardware Design The traditional ECG acquisition system includes the input protection circuit, the preamplifier and the right leg

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driving circuit, band‐pass filter circuit, trap circuit, the main amplifier circuit and voltage lifting circuit, AD conversion circuit. The design of ECG acquisition system is made up of two parts, lead interface circuit and A/D acquisition unit, high precision A/D conversion chip and digital filter technology are used to replace the traditional method for ECG signal processing by hardware filtering and amplification circuit , so that the circuit is simpler, the board volume is smaller and the power consumption is lower, the cost is more economic.

The overall structure of the portable ECG monitor is shown in Figure 1, Slave computer is composed of a power supply circuit, leads interface circuit, analog to digital conversion circuit, SD card circuit, main control unit, GPRS module, keyboard and display circuit etc.. While the main circuit includes LM4F120H5QR microprocessor and peripheral circuits, the processor uses a Harvard architecture to enable the separation of data and instruction bus, whose advantages are as follows: efficient processor cores, system and memory; Hardware division and fast digital signal processing oriented multiply operation; inherited a good performance and power efficiency from ARM7 processor; a variety of selectable sleep modes, with lower power consumption.

FIG. 1 OVERALL STRUCTURE

Working Principle Electrodes extract ECG signal from the surface of the body, and then through a shielded cable connects to lead interface circuit , after that, the ECG signal is sent to the analog digital conversion circuit, where the high performance analog‐to‐digital conversion chip ADAS1000‐3 converts the analog signal to digital signal, and delivers the ECG data through the SPI bus into the control unit. In clinical diagnosis the useful frequency of ECG signal among 0.05‐100Hz, and it needs filtering the power frequency interference. So a digital filtering of ECG data is designed in main control unit including the low‐pass filter, high pass filter, the notch filter. After filtering processing of ECG signal we get a cleaner signal which can be used for diagnosis and treatment, SD card storage and GPRS remote transmission or a real‐time display. PC‐monitoring software via the Internet gets the ECG data from lower computer, and displays the ECG waveform for doctors to diagnose.

Leads Interface Circuit

FIG. 2 LEADS INTERFACE CIRCUIT

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Leads interface and defibrillator protection circuit are shown in Figure 2. Which mainly consists of lead clips, leads interface and discrete components, the main function of leads interface unit is transfer the weak ECG signal from the human’s epidermal. Defibrillation protection circuit is constituted together with SP724，the electrostatic and surge suppressors, and bidirectional TVS transient suppression diodes D1.

Analog to Digital Conversion Circuit To achieve a portable, compact, Low power consumption design, we use high precision analog‐to‐digital conversion chip ADAS1000‐3, which has 3 ECG channels and 1 drive lead, it also can regulate the noise and power, when it is in shutdown mode, the total power consumption only has 15mW, and the interior has a programmable gain and an optional low‐pass filter. In addition, its data frame rate are available to 2 kHz, 16 kHz and 128 kHz ,and also have AC/DC lead off detect function. Analog to digital conversion circuit’s clock signal is provided by an 8.192MHz crystal, meanwhile, its power is provided by 3.3V analog and digital power, which through the SPI bus connects the microprocessor, then its ECG channel and a driving lead is connected with leads interface circuit.

Software Design Block Diagram of System Software Program design uses a modular design thought, the software function block diagram shows in Figure 3. It is made up of four modules, human‐computer interaction, data processing, data transmission and leads detection. The main function of human‐computer interaction is setting and displaying during ECG monitoring; data processing module is used to software filtering and QRS wave analysis, the function of data transmission module is storage ECG data in SD card and remote transmission ECG data by GPRS; leads detection module is used to DC‐lead off detect.

FIG. 3 SOFTWARE FUNCTION BLOCK DIAGRAM

Lower computer software design is based on the embedded real‐time operating system RT‐Thread, it is an open source embedded real‐time operating system (under the GPLv2 license agreement). In order to make the application management easy, multiple threads are used to simplify the program complexity, which improves the real‐time ECG signal processing.

Digital Filter Because of the high or low frequency existing in external may interfere with the ECG signal, in addition, the electromagnetic interference, which is generated by the power supply network and whose frequency is 50/60Hz[12‐ 13],or generated by ECG acquisition device. Therefore, the system designs a equiripple FIR low‐pass filter whose cutoff frequency is set to 100Hz; a zero phase shift IIR high‐pass filter whose cutoff frequency is set to 0.05Hz; and a design of 50Hz work frequency IIR notch filter.

Zero phase digital filtering method is: first, to filter the input signal X(n) in sequential, then to get the result Y1(n), next do mirroring and get the result Y2(n). Second, to use inverse filter to deal with the Y2(n), after the that get result Y3(n), then do mirroring of Y3(n). Finally, to get the output without phase distortion signal Y(n). It’s described in the frequency domain as fallows:

 

   

Y1 e j  X e j H e j (3.1)

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 

Y2 e j  e

 j  N 1

 







Y1 e  j (3.2)

  

Y3 e j  Y2 e  j H e j (3.3)

 

Y e j  e

 j  N 1

 

Y3 e j (3.4)

By (3.1)‐ (3.4)：

 

   

Y e j  X e j H e j

2

(3.5)

By formula (3.5) shows that the output signal without phase distortion, is only amplitude decreased. Zero phase shift filter can eliminate the baseline drift.

Prototype Testing Figure 4 shows the collected ECG signal lead I; figure 5 shows the lead I signal after filtering; Figure 6 shows the lead I information displays on LCD; Figure 7 shows the lead I waveform displays on PC. Where in the y‐axis in figure 4,5,7, V represents amplitude of the ECG. The test results show that the ADAS1000‐3 can acquire ECG signal under the interference, the digital filter can eliminate the power frequency, high frequency interference and baseline drift, after that we get a cleaner ECG signal. DC‐lead off detection and the calculation results of heart rate are accurate, in addition, the host computer receives data packet with accurate results and low loss rate.

5 4

1.5

3 1

2 V/mV

V/mV

1 0 -1

0.5

0

-2 -0.5

-3 -4 -5

-1 0

500

1000

1500

2000

t/ms

0

500

1000

1500

2000

t/ms

FIG. 4 COLLECTED ECG SIGNAL LEAD I

FIG. 5 LEAD I SIGNAL AFTER FILTERING

FIG. 6 LEAD I INFORMATION DISPLAYS ON LCD FIG. 7 LEAD I WAVEFORM DISPLAYS ON PC

Conclusions This paper designed an ECG monitoring system based on ARM, which has the advantages of a simple circuit structure, small size, low power consumption, portable, high reliability. While using RT‐Thread embedded real‐ time operating system to manager application, it can display a long time span ECG signal in real time, also can realize the filtering, storage, remote communication of ECG signal and other functions. The test results show that the ADAS1000‐3 can capture ECG signal with interference, the digital filter can eliminate the interference and baseline drift, after that we get a cleaner ECG signal, DC‐lead off detection and the calculation results of heart rate are accurate, in addition, the host computer receives data packet whose loss rate is low, and the results are accurate. It can solve the existing problems in a better way, and promote the realization of universal health care and family‐oriented ECG monitoring.

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ACKNOWLEDGMENT

Authors would like to thank and acknowledge the support of the National Natural Science Foundation of China under grant NO. 61162017， and the Postgraduate Tutor Project of Universities of Gansu Province: NO. 1101‐03.

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Shengqian Ma was born in Gangu, China in 1954. He received his masterʹs degree in Automatic Control from Lanzhou Jiaoton University, Lanzhou, China, in 1986. His research interests include computer measurement and control, adaptive signal processing.

Xiaoyong Ni was born in Gutian, China in 1989. He received his BS. Degrees in Electrical Engineering and Automation from Fuzhou University, Fuzhou, China, in 2013. His research interests include computer measurement and control, digital signal processing.

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Mei Qin was born in Zhenyuan,China in 1990. She received her BS. Degrees in Physics And Electronic Engineering from Northwest Normal University, Lanzhou, China, in 2013. Her research interests include Image Processing.

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