Infrared transmission single-channel ECG telemetry system design and working principle

ECG is an important means to diagnose heart disease, however, for various reasons, use normal ECG it is difficult to capture the abnormal electrocardiogram. Wireless ECG monitor or 24-hour ambulatory ECG monitor while effectively resolve this problem, but the cost to in-depth family. Infrared biotelemetry with anti-electromagnetic interference ability, transmitting and receiving equipment is simple, inexpensive, debugging easy; you do not need to make frequency register; easy to use, secure advantage [1]. With the rapid development of computer technology, computer has gradually developed into the family, to have the analysis, processing of family medical monitoring equipment to provide the Foundation. To this end, we developed a single-channel infrared ECG telemeter, it to the computer as the analysis, processing, display, storage, communication means, using infrared light as the transport medium, the testee for real-time monitoring. Via modem, you can put the telemetry data transfer to the medical centre for medical expert analysis, diagnosis, is an ideal front-end for telemedicine and home medical care equipment.

1 system design and working principle

Native to the computer as a terminal for receiving, processing, complete data of receiving, processing, storage, and communications capabilities, the testee for real-time monitoring.

The system adopts PCM-ASK modulation.

The first time modulation using pulse code modulation, second modulation amplitude shift keying modulator used. PCM modulation with precision, higher, stronger anti-interference characteristics. ASK modulation can improve resistance to ambient light, electromagnetic interference capability, and you can select different sub-carrier frequency, multiple sets of telemetry system work simultaneously in the same area without disturbing each other. Furthermore, because infrared emitting tube work to switch status, not only to reduce the emission of energy consumption, reduce the volume and weight of transmitter and increase the role of the distance, the long working hours, at the same time, ASK modulation modulation circuit is simple, easy, convenient for crystal frequency stabilization and improve system reliability. The receiving end, after demodulation data directly into the computer's serial port, enabling hardware structure greatly simplified and easy to use.

PCM modulator microchip that complete signal sampling, A/D conversion, data serial output.

By comparison, we used a MICROCHIP PIC16C71. production company It is a low-cost, low power, high performance, with on-chip A/D converter and program memory for 8-bit microcontroller. Its directive structure refined, high speed, small size, to 18 feet package, has four analog input channels, 13 I/O ports, four interrupt sources, 5V power supply, to 4MHz, operating current of less than 2mA.

The receiver selects the integrated infrared receiver module TEMS5380 as part of the infrared receiver.

It sets the infrared reception, conversion, filtering, zoom, sizing, demodulation, is of small size, high sensitivity, low power consumption, reliability, ease of debugging features [2]. It is the center frequency, you can transfer data 38kHz rate up to 3000 Porter, System block diagram shown in Figure 1.

Figure 1 system block diagram

System works as follows: ECG amplifier and level adjustment circuit will make amplification ECG and adjust to the A/D converter to the desired level.

SCM to 200Hz sampling rate on ECG for sampling, A/D conversion and serial communication format A/D conversion result additional start and stop bits to transfer output. Among them, 1 start bit, 8 data bits, 1 stop bit. Code rate of 2400bit/s. In order to identify the start bit, transfer the 10-bit code, followed by transfer stop bit, until the next time A/D conversion. Serial output data control door control circuit. Oscillation frequency divider produces subcarrier frequency, when serial data code for high level, door control circuit to open, sub-carrier signals through door control circuit, driven infrared emitting diode. Driver circuit for VMOS, it has the characteristics of the switch, the output current is great, the circuit is simple.

Infrared receiver module receives a signal, after conversion, filtering, amplification, demodulation, the shaping, output serial data stream.

In order to increase system, telemetry transmitter series 6 infrared emitting tube, and at the receiving end parallel four infrared receiver module, its azimuth are staggering. Infrared receiver module output from or added to, and then the door via level translation, serial communications required level, enter the computer from the computer serial port, for analysis.

Computer work in two modes: real-time monitoring and data playback mode, before you can enter the testee.

In the live monitoring mode. The monitor displays the ECG waveforms in real time and heart rate. Abnormal ECG detected, the computer emits a sound alarm. At any time by storage keys can be pressed before 50 seconds of data storage to the hard drive, in a monitoring process can save up to 21 minutes of data. If you press the freeze key, ECG waveform is frozen on the screen, you can print the ECG waveform, then you can press the continue button to continue custody or guardianship according to the exit key to exit. Exit monitoring mode, the analysis results can be displayed.

In playback mode, enter the file name, you can freely choose to store in the hard disk as an ECG data waveform playback, or lost data through the network to a rescue centre for expert analysis and diagnosis of further.

2 software design

Software design is modular in design, menu operations.

In fact, when monitoring workflow shown in Figure 2.

Figure 2 system software main flowchart

3 experimental results

The transmitter ECG amplifier ± 5V power supply, infrared emission tubes + 9V power supply, and the remainder with a 5V power supply.

The receiving end using the + 5V power supply. Infrared emitting tube work current 34mA. System of remote sensing distance up to 41m, 8m×6m×3 m Hall, regardless of the emission and reception section is where bits are received properly. In strong light and strong electromagnetic interference situations are reliable operation, signal waveforms, description system has the very strong ambient light and electromagnetic interference capability.

Figure 3 results of experiment

4 closing

The experiments showed that the present system of precision high anti-interference, simple structure, convenient operation, use the security function of the distance.

By choosing different frequencies of sub-carrier, can achieve in the same room at the same time, several sets of telemetry system; by improving the receiving part of the rate, improves signal sampling rates, multi-channel Telemetry, physiological signal can serve as an ideal, practical family medical monitoring devices and remote medical front-end that is infrared remote sensing techniques applied to clinical usefully try and explore.

This system the transmitter with A/D converters microcontroller, infrared receiving part adopts integrated infrared receiver module which enables the system structure is simplified.

However, the infrared receiver module of transfer rate is not high enough, therefore, the signal of the sampling rate is restricted. As the device development, higher transmission rate of integrated components, the problem can be solved. Or the system of infrared receiving partial use of discrete components that can effectively address the problem.

Reference documents

［1］　Takahasi M, Pollak V. Ear infra-red telemetry system.

Medical & Biological Enignering & computing, 1985,7：387—392

[2] Tang Xiao Springs, etc.

Integrated IR receiver and data communications applications. Application of electronic technology, 1996, 6: 44: 45