Principle and Design of Infrared Transmission Single Channel ECG Telemetry System
Infrared light biological telemetry has the advantages of strong anti-electromagnetic interference ability, simple transmitting and receiving equipment, low cost, and easy debugging; no frequency registration is required; easy to use and safe [1]. With the rapid development of computer technology, computers have gradually penetrated into families, providing a basis for the development of home medical monitoring equipment with analysis and processing capabilities. To this end, we have developed a single-channel infrared electrocardiograph, which uses a computer as a means of analysis, processing, display, storage, and communication, and uses infrared light as a transmission medium to monitor the subjects in real time. Through the modem, it can also transmit telemetry data to the medical center for medical experts to analyze and diagnose. It is an ideal remote medical front-end and home medical monitoring equipment.
Electrocardiogram is an important method for diagnosing heart disease. However, due to various reasons, it is difficult to capture an abnormal electrocardiogram with an ordinary electrocardiograph. Although radio ECG monitors or 24-hour dynamic ECG monitors can effectively solve this problem, they are expensive and difficult to penetrate into the home.
1 System design and working principle
This machine uses the computer as a receiving and processing terminal to complete the functions of data reception, processing, storage, communication, etc., and to monitor the subjects in real time.
The system uses PCM-ASK modulation. That is, the first modulation uses pulse code modulation, and the second modulation uses 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 select multiple sub-carrier frequencies to enable multiple sets of telemetry systems to work simultaneously in the same area without interfering with each other. Moreover, because the infrared emitting tube works in the switching state, it can not only reduce the power consumption of the emitting tube, reduce the volume and weight of the transmitter, increase the working distance, and obtain a longer working time. At the same time, the ASK modulation mode modulation circuit is simple and realize It is easy to stabilize the frequency of the crystal 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 realized by a single chip microcomputer, which completes signal sampling, A / D conversion, and data serial output. After comparison, we adopted the PIC16C71 produced by MICROCHIP. It is a low-cost, low-power, high-performance, 8-bit microcontroller with on-chip A / D converter and program memory. Its command structure is refined, fast, small size, 18-pin package, with 4 analog input channels, 13 I / O ports, 4 interrupt sources, under the condition of 5V power supply and main frequency of 4MHz, the operating current is less than 2mA.
The receiving end selects the integrated infrared receiving module TEMS5380 as the infrared receiving part. It integrates infrared receiving, conversion, filtering, amplification, shaping, and demodulation, and 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 composition block diagram is shown in Figure 1.
Figure 1 System composition block diagram
The working process of the system is as follows: ECG amplifier and level adjustment circuit amplify and adjust the ECG signal to the level required by the A / D converter. The single chip microcomputer samples the ECG signal and converts it to A / D at a sampling rate of 200 Hz, and adds the start and stop bits to the output of the A / D conversion according to the format of serial communication for shift output. Among them, 1 start bit, 8 data bits, 1 stop bit. The code rate is 2400bit / s. In order to facilitate the identification of the start bit, after transmitting the above 10-bit code, the stop bit is transmitted immediately until the next A / D conversion. The serial output data controls the operation of the gate control circuit. The oscillation frequency divider generates the subcarrier frequency. When the serial data code is at a high level, the gate control circuit is opened, and the subcarrier signal passes through the gate control 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 outputs the serial data stream through conversion, filtering, amplification, demodulation, and shaping. In order to increase the telemetry range of the system, six infrared emitting tubes are connected in series at the transmitting end, and four infrared receiving modules are connected in parallel at the receiving end, and their positions are staggered from each other. The output of the infrared receiving module is added by the OR gate, and then after level conversion, it becomes the level required for serial communication. The computer serial port is input to the computer for analysis and processing.
The computer works in two modes: real-time monitoring mode and data playback mode, before which the subject information can be entered. In real-time monitoring mode. The display shows the ECG waveform and heart rate in real time. When an abnormal ECG is detected, the computer issues an audible alarm. Press the storage key at any time to store the data of 50 seconds before the key to the hard disk, and store up to 21 minutes of data during a monitoring process. If you press the freeze button, the ECG waveform is frozen on the screen, you can print the ECG waveform, and then you can press the continue button to continue monitoring or press the exit button to exit monitoring mode. When exiting the monitoring mode, the analysis results can be displayed.
In the playback mode, enter the file name, you can freely select any piece of ECG data stored in the hard disk for waveform playback, or transmit the data to the ambulance center through the network for experts to analyze and diagnose.
2 Software design
The software design adopts modular design and menu operation. The real-time monitoring workflow is shown in Figure 2.
Figure 2 Main flow chart of system software
3 Experimental results
The transmitter ECG amplifier is powered by ± 5V, the infrared transmitter is powered by + 9V, and the rest is powered by 5V. The receiving end is powered by + 5V. The working current of infrared emitting tube is 34mA. The telemetry distance of the system is up to 41m. In the hall of 8m × 6m × 3m, it can receive normally no matter where the transmitting part and receiving part are. Reliable operation under strong lighting and strong electromagnetic interference, 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 show that the system has high precision, strong anti-interference, simple structure, convenient operation, safe use, and long working distance. By selecting subcarriers of different frequencies, multiple sets of telemetry systems in the same room can work simultaneously; by increasing the rate of the receiving section, the sampling rate of the signal can be increased, and the telemetry of multiple physiological signals can be achieved. Home medical monitoring equipment and remote medical front-end is a useful attempt and exploration of infrared telemetry technology applied in clinic.
The transmitting end of the system adopts a single chip microcomputer with an A / D converter, and the infrared receiving part adopts 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, this problem can be solved. Or the infrared receiving part of the system uses discrete components, which can also effectively solve this problem.
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