Human body impedance measurement system based on single chip microcomputer

introduction

Medicine Impedance measurement is a detection technology that uses the electrical properties and changes of biological tissues and organs to extract biomedical information related to the physiological and pathological conditions of organisms. It usually uses driving electrodes to send a small alternating current (or voltage) signal to the detection object, and simultaneously measures the voltage (or current) signals at both poles to calculate the corresponding impedance, which is then applied to different purposes.

This design uses the serial port of MSP430F149 to perform long-distance real-time transmission via RS485. The host computer can process the real-time data by drawing curves and saving data.

1 System Structure

The system uses TI's MSP430F149 microcontroller. The microcontroller has 60 KB Flash and 2 KB RAM, and has powerful data processing capabilities. The microcontroller controls the frequency and amplitude of the sine wave by sending frequency words and amplitude words to AD9852. After the sine wave is processed by circuits such as current-to-voltage and power amplification, it is isolated and acts on the human body through coil T1. At the same time, the relative amount of current and voltage induced by coils TF1 and T2 is converted by the process control amplifier and true effective value, and then enters the microcontroller for A/D conversion. The microcontroller calculates the resistance based on the current and voltage values, and then transmits it to the PC through the serial port. The system structure block diagram is shown in Figure 1.

2 Hardware Circuit

2.1 MCU Control AD9852 Circuit

As shown in Figure 2, the MCU P1.0~P1.7 are connected to the data and address pins of AD9852 through multiplexing, and P2.3~P2.7 are connected to the five more important control pins of AD9852. The MCU mainly controls the frequency and amplitude of the sine wave generated by AD9852 by outputting amplitude and frequency words to DDS.

2.2 Current and voltage measurement circuit

Figure 3 shows the principle of the current measurement circuit. In order to accurately measure the current at different amplitudes, a programmable amplifier is added before the peak-to-peak value to effective value circuit. The microcontroller controls the output gain so that even if the front-end voltage is very small, it can be increased to a certain level to achieve accurate measurement. Similarly, the same schematic diagram is used in the current measurement circuit.

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2.3 RS485 Circuit

In the MCU and RS485 communication conversion circuit shown in Figure 4, three optocouplers are used to isolate the MCU circuit and the RS485 bus to improve the anti-interference ability of the system. SP485E is used to support the RS485 serial protocol, but because the working logic of SP485E is TTL level, and the logic current of RS232 communication is not TTL level, it is necessary to use SP232 device for level conversion, as shown in Figure 5. RS485 works in half-duplex communication state, and the input/output direction of data is controlled by CTR485.

3 Software Design

The software of this system is written in C language and is written and debugged in the IAR EmbeddedWorkbench environment. The software flow is shown in Figure 6. The microcontroller software is mainly divided into two parts. One part controls AD9852 to generate a sine wave, and performs A/D conversion on the voltage and current, and then performs calculations. The other part transmits the collected data to the PC. The software flow is shown in Figure 8.

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4 Experimental Results

Practice has shown that the amplitude of the sine wave has little effect on the impedance amplitude value, while the frequency has a greater effect on the impedance amplitude value. When using egg white for the experiment, it was found that as the frequency increases, the impedance value decreases, and the higher the egg white concentration, the greater the impedance. When the sine wave frequency is 440 kHz, the current and voltage are adjusted to 0.7 ~ 2.5 through the programmable amplifier, and the relationship between the non-inductive resistance and the voltage and current ratio is shown in Figure 9.

In the figure, the horizontal axis is U/I and the vertical axis is R/10. It can be seen from the figure that the two are approximately linearly related within the allowable error range.

Conclusion

This design provides a method for measuring human body impedance, which can be used as an auxiliary function of medical instruments, provide a certain reference for tissue diagnosis, etc., and can be transplanted to different applications.