Design and implementation of Doppler blood flow meter system based on single chip microcomputer

The emergence of Doppler blood flowmeter marks a significant progress in microvascular perfusion. This design adopts a dual-channel device to pick up Doppler signals, effectively suppresses noise signals, and uses a single-chip microcomputer to control and process the signal, which not only simplifies the circuit, but also facilitates signal processing and reading. The use of 12-bit AD574A not only improves signal accuracy, but also uses its bipolarity to eliminate the complex exponentiation, square root or absolute value circuits in previous signal processing. Through the four-digit LED display, the instantaneous relative quantitative value of blood cell perfusion can be read intuitively and accurately, with an accuracy of two decimal places. It is also equipped with a speaker to vividly represent the strength and change of the signal. The signal can also be detected and recorded for a long time through a plotter for research and analysis.

1 System Overall Plan

The block diagram of the system composition is shown in Figure 1. When the system is working, a beam of laser is emitted from the laser probe to illuminate the tissue, and penetrates the tissue to form a hemisphere with a radius of 1mm, with the center of the hemisphere at the probe. All blood cells passing through the area will reflect back part of the light, causing Doppler shift of the light. The intensity and frequency of the shift are related to the number and speed of blood cells passing through the area, and have nothing to do with its direction (the perfusion rate is defined as: blood cell perfusion rate = the amount of blood cells in the measurement area × the average speed of the cells). Part of the reflected light is picked up by the dual-path laser probe, and the photoelectric converter converts the optical signal into an electrical signal reflecting the size of the blood cell perfusion rate. After a series of electrical and data processing, the electrical signal is controlled by a 51 single-chip microcomputer to display the relative amount of blood perfusion.

The size of the irrigation flow is driven by the plotter to record the irrigation flow rate and the speaker is controlled to emit a sound reflecting the size of the irrigation flow rate.

2. Hardware technology solution

2.1 Design of signal processing circuit

The signal processing circuit uses optical fiber to transmit low-power laser to the probe. When the probe is placed on the tissue, the red blood cells moving in the hemispherical area with a diameter of about 1 mm will cause the light to be repeatedly reflected and refracted. These reflected and refracted composite lights undergo Doppler frequency shift due to the movement of red blood cells and a part of them is scattered back to the tissue surface and enters two symmetrical receiving optical fibers. Through these two symmetrical feeding optical fibers, they are transmitted to two phototransistors for photoelectric conversion, and the Doppler signal of known frequency can be broadened and detected. After amplification, filtering, and normalization, low-frequency noise and DC components can be filtered out. Since the two Doppler signals are differential mode signals, environmental noise, power grid noise, and laser noise will be greatly suppressed after passing through the differential amplifier. After further filtering, amplification, compensation and smoothing by the signal processing unit, a voltage signal proportional to the blood cell perfusion volume can be extracted. The specific block diagram of the signal processing circuit is shown in the front part of the A/D conversion in Figure 1.

5G28 is a MCU compatible input impedance integrated operational amplifier, which has the characteristics of high input impedance and high conversion speed, and is widely used in the amplification of micro current. Therefore, the preamplifier, 2KHz high-pass filter and 7KHz low-pass filter all use 5G28.

F007 is a single-chip single-gain operational amplifier. It does not require external frequency compensation and has a high common-mode and differential-mode input voltage range, so this amplifier is selected for the integrator. The change of the integrator time constant and gain is controlled by the microcontroller using 4066.

Since the signal converted by the phototransistor is relatively weak and contains common-mode interference such as power frequency, static electricity and electromagnetic coupling, AD521 is selected to amplify this signal. AD521 has the characteristics of high input impedance, low offset current, and high common-mode rejection ratio. Its gain can be adjusted between 0.1 and 1000. Various gain parameters have been internally compensated. It has input and output protection functions and strong overload capacity. Transformer coupling is used in use, and the gain is changed by adjusting the external resistance.

2.2 Design of the hardware of the microcontroller control circuit

Considering the actual functions and requirements of the system, this system uses AT89S52 as the controller. AT89S52 is a low-power, high-performance CMOS 8-bit microcontroller with 8K in-system programmable Flash memory. It is manufactured using Atmel's high-density non-volatile memory technology and is fully compatible with the instructions and pins of industrial 80C51 products. On-chip Flash allows the program memory to be programmable in the system and is also suitable for conventional programmers. With a smart 8-bit CPU and in-system programmable Flash on a single chip, the AT89S52 provides a highly flexible and ultra-effective solution for many embedded control application systems.

According to the accuracy requirements of the design indicators, a 12-bit successive approximation fast AD574A converter is used, and its conversion accuracy is ≤0.05%, which can meet the design accuracy requirement of 0.5%. Its maximum conversion speed is 35us. Because the blood cell flow rate is about 0.1ms, the signal conversion is slow, and an integrator is added, so there is no need to add other sample holders. According to the sampling principle, ten samples are required for each signal cycle, and a medium-speed converter can meet the requirements here. Considering the speed, accuracy and performance-price ratio, the A/D converter uses AD574A to realize the conversion from analog to digital, so that the microcomputer can be used to control the display and drive the printing. The D/A converter uses DAC0832. In this system, DAC0832 constitutes a programmable gain amplifier, which changes the output of the analog quantity by changing the digital quantity, and realizes multi-level control of the speaker sound.

Since a plotter is added to the system for long-term monitoring, and the printing speed of the plotter does not match the output speed of the data to be printed, RAM 6264 is used to store this data.

74LS164 is used as the serial interface of the keyboard, and the different functions of each key are used to achieve overall control of the instrument, making the operation clear at a glance.

The display control drive interface circuit uses MC14499, which is a 20-bit shift register. It realizes control signal output and level conversion to ensure sufficient signal driving capability. Using MC14499 to dynamically scan the digital tube uses less hardware, takes up less CPU time, has a simple circuit, and consumes less power.

The PP40 plotter is used to draw the relative perfusion flow curve of the cycle. It uses 74LS373 as the data buffer register to solve the problem of asynchronism between printing and microcomputer.

In order to observe the signal changes, the signal is integrated with different time constants and different gains. In order to switch these different integration gains, the system uses a dual four-way analog switch 4066 and an external 74LS373 to latch the input data. When different switches are closed, different resistors are connected to form different integration constants and gain multiples.

3 Software Design

According to the system functions and the actual hardware situation, the software should realize the control of data acquisition and A/D conversion, digital filtering, keyboard scanning and processing of the collected data, control of analog switches for integration time and gain selection, printing control, volume control and data display control, etc. The software adopts a modular design concept, and is designed from the whole to the part, from top to bottom. The main program flow chart is shown in Figure 3.1.

4 Conclusion

The use of Doppler phenomenon to measure the blood perfusion of tissue micro-areas has high practical value in basic research and clinical applications. This paper gives a method of using single-chip microcomputer control and Doppler phenomenon to measure the blood perfusion of tissue micro-areas. The system measures the blood perfusion of tissue micro-areas, measures the state of microcirculation, and judges the changes in physiological functions of the body, which provides convenience for surgical operations and is helpful to internal medicine, medical science, anesthesiology, orthopedics, and pediatrics.