Design and Test of Wearable Wireless ECG Recorder

ECG signal is one of the earliest human bioelectrical signals researched and applied in medicine and clinic. Compared with other bioelectrical signals, ECG signal is easier to detect and has more intuitive regularity. Due to the suddenness and long-term nature of heart disease, heart patients often need long-term treatment and monitoring. Therefore, it is extremely important for patients to record their ECG for a long time. Long-term ECG recording can record transient abnormal ECG events that are not likely to occur when the ECG is detected. This provides an important basis for disease analysis. ECG monitor can only monitor ECG signals when it was first applied in the 1960s. It is called single-parameter monitor. With the advent of large-scale integrated circuits and microprocessors, current ECG monitors have been able to monitor dozens of parameters. In view of the difficult recognition of ECG signals and the professionalism of ECG monitoring related operations, the implementation of ECG monitoring is often limited to hospitals and health institutions, and the daily ECG monitoring of patients is not easy to implement.

The wearable medical instrument has basic function modules such as physiological signal detection and processing, signal feature extraction and data transmission, and can realize non-invasive continuous monitoring, diagnosis and treatment of human body. In traditional electrocardiographs, the hardware devices are mainly connected by communication cables. The operating platform is also based on wired devices. Although it has certain applicability in certain occasions such as hospitals and communities, it is not compatible with existing ones. The integration of personal communication terminals (such as mobile phones, PDAs, laptops, etc.). In view of this situation, this study designs and implements a more convenient and comfortable wearable wireless ECG recorder under the premise of ensuring the quality of signal acquisition. It adopts the low-power ECG acquisition chip ADS1191 and low-power single-chip microcomputer. MSP430F2112 constitutes a signal acquisition circuit, and the collected ECG signals can be transmitted to the communication terminal for display and analysis through Bluetooth; and the entire device is completely sealed by using wireless transmission and wireless charging technology to achieve the waterproof function and meet the medical safety standards.

The wearable wireless ECG recorder should have features such as low power consumption, small size, and high processing speed. The size of ECG electrodes used in this study was approximately 30-75 mm. Taking into account the supporting ability of ECG electrodes, the expected height of the device in this study is about 0.5 cm and the weight is <30 g. The simulation results show that ECG electrodes can well support the volume and weight of the device, and will not fall off during exercise.

Electrocardiograph is mainly composed of electrode paste connection module, ECG front end, main control unit, Bluetooth module, wireless charging module, lithium battery, voltage regulator and power management module. Block diagram, see Figure 1.

Figure 1 Blockage of a wearable wireless ECG recorder

The design of ECG signals on the body surface has the characteristics of weak amplitude and susceptibility to interference. The ECG signal collected by this module undergoes preamplification through a differential amplifier circuit with high input impedance to suppress zero drift and reduce the interference of the common mode signal. After the signal is further amplified (about 1000 times), the interference signal is filtered out again. Level up; then sent to the core processing controller for processing. This module is implemented by TI's ADS1191 low-power ECG acquisition chip. After the ECG signal is filtered, amplified, and A/D converted, it is sent to the low-power microcontroller MSP430F2112 for further analysis and processing through the SPI transmission mode. Wireless transmission is implemented using the HM-6 Bluetooth module (produced by Huamao Technology Co., Ltd.) for pocket devices.

Wireless charging technology originates from wireless power transmission technology. The mainstream wireless charging technologies are mainly implemented in three ways (electromagnetic induction, radio wave and resonance). The wireless charging module uses Qi wireless charging technology and BQ24201 charger. Qi is the world's first standardization organization to promote wireless charging technology - Wireless Charging Alliance (WPC) launched the "wireless charging" standard, with two features of convenience and versatility. Qi's wireless charging technology uses magnetic resonance to transfer charges in the air between the charger and the device. The coils and capacitors form resonances between the charger and the device to achieve efficient transmission of electrical energy. The basic principle is to form the two coils. Resonance thus enables wireless transmission of electrical energy. BQ24201 is a charger for Li-Ion or Li-Polymer that integrates internal high-precision voltage regulation, power MOSFET, temperature monitoring, state of charge, and charge termination circuitry on a single chip; fewer external components save space and cost .

The power management module uses the BQ24312 as a front-end protection scheme for Li-Ion chargers and provides 4.25 V overvoltage protection. The ADuM5000 is used for power isolation. The ADuM5000 is an isolated DC/DC converter based on Analog Devices technology. The circuit provides 5 V power isolation. According to product requirements, the ECG recorder needs to perform the collection and transmission of an uninterrupted ECG signal for > 4 h. After estimating the overall circuit, this study intends to use a 90 mAH rechargeable lithium battery. Power supply part of the charging circuit, see Figure 2. The lithium battery provides a stable 3.3 V digital voltage and analog voltage for the ADS1191, MSP430F2112, and Bluetooth modules via the MIC5205LB regulator chip.

Figure 2 power supply part of the charging circuit

The ECG recorder software development environment uses the IAR Embedded Workbench, a cross-compiler designed by IAR for MSP430 microcontrollers, and is written in C language. System software flow chart, see Figure 3. In order to meet the low-power design requirements, this study uses the following methods to control power consumption in software design:

1 According to the function, the software is divided into several relatively independent modules, which are triggered by interrupts; 2 The software is used to control the chips that are not working at the moment to enter hibernation or idle state; 3 The program with short machine cycles is used to optimize each module program and reduce the actual system. Runtime, which reduces system power consumption.

Figure 3 ECG recorder software flow chart

The physical map of the ECG recorder designed and implemented is shown in Figure 4. As can be seen from FIG. 4 , the recorder is mainly divided into two parts, each having a diameter of about 30 mm, a thickness of about 5 mm, an overall length of about 10 cm, and a weight of 25 g; the connecting line between each other includes a ground wire and a power supply wire; the electrode is attached to the back Embedded electrode buckle, directly buckled on the electrode when used. The position of the electrode patch is about 3 cm below the midline of the left clavicle, as shown in Figure 5.

The power consumption of the ADS1191 electrocardiograph ECG acquisition front-end processor and the MSP430F2112 processor is 0.98 mA, and the power consumption of the Bluetooth real-time transmission data is 16 mA. Figure 6 shows the ECG waveform when a male subject was tested. It can be seen that the ECG waveform after preprocessing such as denoising and smoothing is stable, the baseline drift is not obvious, and the characteristics of P, QRS and T waves are obvious. Can be used for arrhythmia analysis.

Figure 4 Electrocardiograph physical map, where (a) is a front view, (b) is a rear view.

Figure 5 Electrocardiograph usage diagram left Figure 6 Receiver ECG waveform right

(1) Input impedance. Generate a “100 mV 10 Hz” sine wave from the signal generator and connect a 620 kΩ resistor to the input of the ECG recorder. The measured signal generator has a sine signal amplitude of U=98.8 mV and the voltage at the input of the recorder. At 95.84 mV, the input impedance of the ECG recorder R=20.07 MΩ was calculated. According to the National ECG standardization document, the input impedance of the ECG machine must be ≥ 2.5 MΩ. Therefore, the input impedance of the ECG recorder complies with industry standards.

(2) Frequency response. In the test circuit, the signal generator's output signal frequency is continuously changed from 0.1 Hz to 150 Hz (commonly referred to as "sweeping") and maintained at a constant amplitude of 100 mV at the output via an oscilloscope or other recorder. The amplifier records the corresponding output level of this continuous change, and the frequency response curve of the device can be obtained. In the frequency range of 0.1 to 150 Hz, the fluctuation amplitude of the signal amplification of the electrocardiograph is 2.37 dB, which is less than the nationally specified 3 dB and meets the requirements.

(3) Common mode rejection ratio. The common mode rejection ratio refers to the ratio of the amplification factor at the differential mode input and the amplification factor at the common mode input, which reflects the anti-interference ability of the electrocardiograph. First measure the amplification of the common-mode input, introduce the "1.5 V / 50 Hz" signal from the signal source, input it to the short-circuited input, connect the signal source ground to the right-hand drive, and record the output signal amplitude; The input signal amplification was compared and the calculated common-mode rejection ratio was 106.4 dB, which was in line with the national ECG standard.

The portable ECG recorder designed and implemented in this study has low power consumption and small size and has the following features: 1 It is easy to operate, simple to measure, inexpensive to use, easy to use; 2 non-invasive, safe, accurate, and repeatable; 3 Real-time waveform display; 4 with wireless charging. The ECG recorder can be widely used in the long-term real-time monitoring of ECG signals.