Design and working principle of infrared transmission single-channel ECG telemetry system

Electrocardiogram is an important means of diagnosing heart disease. However, due to various reasons, it is difficult to capture abnormal electrocardiograms with ordinary electrocardiographs. Although radio electrocardiogram monitors or 24-hour dynamic electrocardiogram monitors can effectively solve this problem, they are expensive and difficult to be installed in homes. Infrared light biotelemetry has the advantages of strong anti-electromagnetic interference ability, simple transmitting and receiving equipment, low cost, easy debugging, no need for frequency registration, easy to use and safe [1]. With the rapid development of computer technology, computers have gradually penetrated into homes, providing a foundation for the development of home medical monitoring equipment with analysis and processing capabilities. To this end, we have developed a single-channel infrared electrocardiogram telemeter, which uses computers as analysis, processing, display, storage, and communication means, and infrared light as a transmission medium to monitor the subject in real time. Through a modem, the telemetry data can also be transmitted to the medical center for analysis and diagnosis by medical experts. It is an ideal remote medical front end and home medical monitoring equipment.

1 System design and working principle

This machine uses a computer as a receiving and processing terminal to complete functions such as data reception, processing, storage, and communication, and conducts real-time monitoring of the subjects.

The system adopts PCM-ASK modulation. That is, the first modulation adopts pulse code modulation, and the second modulation adopts amplitude keying modulation. PCM modulation has the characteristics of high precision and strong anti-interference. ASK modulation can not only improve the ability to resist ambient light and electromagnetic interference, but also realize multiple telemetry systems working simultaneously in the same area without interfering with each other by selecting different subcarrier frequencies. Moreover, since the infrared transmitting tube works in the switching state, it can not only reduce the power consumption of the transmitting tube, reduce the size and weight of the transmitter, increase the working distance, and obtain a longer working time, but also the modulation circuit of ASK modulation is simple, easy to implement, convenient for crystal frequency stabilization, and improve the reliability of the system. After demodulation at the receiving end, the data directly enters the serial port of the computer, which greatly simplifies the hardware structure and is easy to use.

The PCM modulator is implemented by a single-chip microcomputer, which completes signal sampling, A/D conversion, and data serial output. After comparison, we used the PIC16C71 produced by MICROCHIP. It is a low-cost, low-power, high-performance 8-bit single-chip microcomputer with an on-chip A/D converter and program memory. Its instruction structure is refined, fast, and small in size. It is an 18-pin package with 4 analog input channels, 13 I/O ports, and 4 interrupt sources. Under the condition of 5V power supply and 4MHz main frequency, the working current is less than 2mA.

The receiving end uses the integrated infrared receiving module TEMS5380 as the infrared receiving part. It integrates infrared receiving, conversion, filtering, amplification, shaping, and demodulation. It has the characteristics of small size, high sensitivity, low power consumption, reliable operation, and convenient debugging [2]. Its center frequency is 38kHz, and the data transmission rate can reach 3000 baud. The system block diagram is shown in Figure 1.

Figure 1 System block diagram

The working process of the system is as follows: The ECG amplifier and level adjustment circuit amplify the ECG signal and adjust it to the level required by the A/D converter. The single-chip microcomputer samples the ECG signal at a sampling rate of 200Hz, performs A/D conversion, and shifts and outputs the result of the A/D conversion with the start bit and stop bit in the format of serial communication. Among them, there is 1 start bit, 8 data bits, and 1 stop bit. The code rate is 2400bit/s. In order to facilitate the identification of the start bit, the stop bit is transmitted immediately after the above 10-bit code is transmitted until the next A/D conversion is performed. The serial output data controls the operation of the gating circuit. The oscillator divider generates the subcarrier frequency. When the serial data code is at a high level, the gating circuit is opened, and the subcarrier signal passes through the gating circuit to drive the infrared light-emitting tube to emit light. The driving circuit is a VMOS tube, which has good switching characteristics, large output current, and simple circuit.

After receiving the signal, the infrared receiving module converts, filters, amplifies, demodulates, and shapes it to output a serial data stream. In order to increase the telemetry range of the system, six infrared transmitting tubes are connected in series at the transmitting end, and four infrared receiving modules are connected in parallel at the receiving end, with their positions staggered. The output of the infrared receiving module is added by an OR gate, and then converted to the level required by serial communication through a level conversion, and then input into the computer through the computer serial port for analysis and processing.

The computer works in two modes: real-time monitoring mode and data playback mode. Before this, the subject's information can be entered. In real-time monitoring mode, the monitor displays the ECG waveform and heart rate in real time. When an abnormal ECG is detected, the computer sounds an alarm. Pressing the storage key at any time can store the data of 50 seconds before the key is pressed into the hard disk. In one monitoring process, up to 21 minutes of data can be stored. If the freeze key is pressed, the ECG waveform is frozen on the screen and can be printed out. After that, you can press the continue key to continue monitoring or press the exit key to exit the monitoring mode. When exiting the monitoring mode, the analysis results can be displayed.

In playback mode, enter the file name and freely select any segment of ECG data stored in the hard disk for waveform playback, or transmit the data to the rescue center through the network for further analysis and diagnosis by experts.

2 Software Design

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The software design adopts modular design and menu operation. Its real-time monitoring workflow is shown in Figure 2.

Figure 2 System software main flow chart

3 Experimental Results

The transmitting end ECG amplifier is powered by ±5V, the infrared transmitting tube is powered by +9V, and the rest is powered by 5V. The receiving end is powered by +5V. The working current of the infrared transmitting tube is 34mA. The telemetry distance of the system is up to 41m. In a hall of 8m×6m×3m, the transmitting part and the receiving part can be received normally regardless of their positions. It can operate reliably under strong lighting and strong electromagnetic interference, and the signal waveform is clear, indicating that the system has a strong ability to resist ambient light and electromagnetic interference.

Figure 3 Experimental results

4 Conclusion

Experiments have shown that this system has high precision, strong anti-interference, simple structure, convenient operation, safe use, and long working distance. By selecting subcarriers of different frequencies, multiple telemetry systems can be operated simultaneously in the same room; by increasing the receiving part rate, the signal sampling rate can be increased, and the telemetry of multiple physiological signals can be realized. It can be used as an ideal and practical home medical monitoring equipment and telemedicine front end, and is a beneficial attempt and exploration of the application of infrared telemetry technology in clinical practice.

The transmitting end of this system uses a single-chip microcomputer with an A/D converter, and the infrared receiving part uses an integrated infrared receiving module to simplify the system structure. However, the transmission rate of the infrared receiving module is not high enough, so the sampling rate of the signal is limited. With the development of devices, integrated components with higher transmission rates appear, and this problem can be solved. Alternatively, the infrared receiving part of the system uses discrete components, which can also effectively solve this problem.

References

［1］ Takahasi M, Pollak V. Ear infra-red telemetry system. Medical & Biological Enignering & computing, 1985,7：387-392

[2] Tang Xiaoquan, et al. Integrated infrared receiver and its application in data communication. Electronic Technology Application, 1996, 6: 44-45