Fetal heart rate detector based on MEMS sensor

1. Overview

At present, the international community has taken maternal and child health indicators as sensitive indicators for measuring social production and economic development. Most of the health indicators, life expectancy per capita, infant mortality rate, maternal mortality rate and other indicators stipulated by China that reflect the well-off level need to be achieved through maternal and child health care. Routine prenatal examinations in hospitals now include measuring fetal heart rate, fetal position, blood pressure, weight, abdominal circumference and uterine fundus height. Among them, detecting fetal heart rate is a highly technical task. Since the fetal heart rate is very fast, between 120 and 160 times per minute, it is difficult to measure accurately with traditional stethoscopes or even ultrasonic Doppler instruments with only amplification functions and manual counting. Ultrasonic Doppler fetal heart rate monitors with digital display functions are expensive and are only used in a few large hospitals. They cannot be popularized in medium and small hospitals and large rural areas. In addition, the ultrasonic vibration wave acting on the fetus will have a great adverse effect on the fetus. Although the detection dose is very low, it also belongs to the category of destructive detection and is not suitable for regular and repeated examinations and home use.

Based on the SCA600C13H1G MEMS accelerometer of VTI Company, this project proposes a non-invasive fetal heart detection method and develops a simple, easy-to-learn, intuitive and accurate medical auxiliary instrument for clinical diagnosis and maternal self-examination between the fetal heart stethoscope and the Doppler fetal monitor.

2. SCA600C13H1G

The SCA600C13H1G silicon capacitive accelerometer is made of single crystal silicon and glass. This design can ensure that the product has good reliability, accuracy and stability over time and temperature. Its capacitance detection principle is simple and reliable, and is measured based on the change in the distance between two parallel plates. The capacitance and charge storage between a pair of parallel plates depend on the distance between the parallel plates and the plate area. The sealed structure of the product reduces the packaging requirements. Particles or chemicals cannot enter the sensor, thereby ensuring the reliability of the product. The dual capacitor structure and symmetrical design of the product improve the zero stability, linearity and lateral sensitivity of the product. Generally, the temperature coefficient is less than 0.05FS/~C and the lateral sensitivity is less than 3%. This new sensor has the following characteristics: compact structure, low power consumption, good reliability and excellent performance.

VTI's accelerometer is made based on the proven 3D-MEMS technology. Three-dimensional micro-electromechanical system (3D-MEMS) is an innovative combination of various technologies. It can process silicon into a three-dimensional structure. Its packaging and contacts are easy to install and assemble. Sensors made with this technology have excellent accuracy, extremely small size and extremely low power consumption. An advanced sensor can be made from only a small piece of silicon and can measure acceleration in three mutually perpendicular directions.

3. System working principle and composition The working principle of this sensor system is shown in Figure 1. The fetal heart rate is converted into an analog voltage signal through the accelerometer, and the difference is amplified

by the instrument amplifier used for preamplification . Then a series of intermediate signal processing such as filtering is performed , and the analog voltage signal is converted into a digital signal by an A/D converter. It is input to the 8051 microcontroller through an optical isolation device for analysis and processing, and finally the processing result is output System block diagram 1. Preamplifier circuit The preamplifier mainly considers the influence of three items: noise, input impedance and common mode rejection ratio. 2. Signal processing circuit The fetal heart rate signal in the mother is a low-frequency weak signal under strong noise interference. Because it is very weak, only at the microvolt level, and the interference is very large, the effective signal is often submerged. Interference signals generally include high-frequency electromagnetic interference, 50Hz power frequency interference, and other interference sources in the mother. Power frequency interference mainly exists in the form of common-mode signals, usually with an amplitude of several volts. The frequencies of the interference signal in the mother and the fetal heart rate signal do not overlap. According to the characteristics that the fetal heart rate signal is mainly concentrated in the range of 7OHz to l10Hz, a fetal heart rate signal processing circuit with strong pertinence, superior performance, stability and reliability is designed, as shown in Figure 2. Intermediate signal processing circuit The intermediate signal processing circuit is divided into four parts: bandpass frequency selection circuit, secondary amplifier circuit, 50Hz notch filter and gain adjustment circuit. The bandpass frequency selection circuit uses a state variable filter as a bandpass filter, and the maximum range of the passband is set to 50Hz; -140Hz as shown in Figure 3. This filter has three advantages: appropriate selection of component values. The quality factor Q can be made independent of the center frequency; Q is very insensitive to component parameters; Q can only be very high (comparable to high-Q active bandpass filters).

State variable filter

The structure of the secondary amplifier circuit is similar to that of the gain adjustment circuit. Both are connected by an op amp in the form of voltage negative feedback. The former amplifies the signal, while the latter controls the gain of the entire circuit, which can reach up to 120dB. Its structural diagram is shown in Figure 4. Use voltage series negative feedback structure. Its advantages are simple structure and irreplaceable superior performance:



Secondary amplification and gain adjustment circuit

(1) Input equivalent impedance, j=(1, output equivalent impedance is small, Ro=Rod(I+AF), where, fd is the input impedance of the operational amplifier, and is the output impedance. It not only completes the signal amplification function, but also acts as a buffer. It effectively isolates the modules of the front and rear stages without adding additional impedance converters and matching modules; (2)

The use of capacitor C53 makes the entire module have a low-pass function, which can not only remove high-frequency interference in the signal, but also compensate the high-frequency part of the effective signal due to its advanced compensation function. Through reasonable design, the phase of the circuit frequency band will change smoothly.

Figure 5 shows a 50Hz power frequency notch filter using a typical active double-T notch


Notch circuit

3, A/D conversion and photoelectric coupling circuit

The signal from the sensor is an analog voltage signal. It needs to be converted into a digital signal through A/D conversion before it can be sent to the 805l single chip for processing. In order to ensure safety and prevent interference between analog and digital circuits, the photoelectric isolation circuit is also an indispensable module. As shown in Figure 6.



4. MCU signal processing

MCU mainly processes the signal by analyzing the data transmitted from the A/D converter and calculating the fetal heart rate value. Designing an adaptive filter based on wavelet transform can better process the signal and extract the signal we need from it.

The MCU uses atmel's AT89C205l, and the connection circuit with the LCD is shown in Figure 7. Connection


circuit diagram of MCU and LCD

This single-chip microcomputer is designed with high-performance static 80C51. It is manufactured by advanced CMOS technology and has non-volatile Flash program memory. All support l2 clock and 6 clock operations. AT89e205 contains 128 bytes of RAM, 16 I/O lines, 2 16-bit timers/counters, 5-input 2-priority nested interrupt structure, 1 serial I/O port can be used for multi-machine communication I/O expansion or full-duplex UART, and on-chip oscillator and clock circuit.

In addition, because the device adopts a static design, it can provide a wide operating frequency range (the frequency can be reduced to 0). Two power saving modes selected by software can be realized- Idle mode and power-down mode. Idle mode freezes the CPU, but the RAM, timer, serial port and interrupt system still work. Power-down mode saves the contents of RAM, but freezes the oscillator, causing all other on-chip functions to stop working. Because the design is static, the clock can be stopped without losing user data. Operation can be resumed from where the clock stops.

5. Display circuit

The design uses the YLF12232F LCD display. YLF12232F is a graphic dot matrix LCD display with a built-in 8192 16" 16-dot Chinese character library and 128 16*8-dot ASCII character set. It mainly consists of a row driver/column driver and a 128×32 full-dot LCD display. It can complete graphic display, and can also display 7.5×2 (16×16 dot matrix) Chinese characters. It is a fetal heart monitoring instrument controlled by serial mode with an external CPU interface.

Figure 8 is a design using a new MEMS acceleration sensor



V. Conclusion

Fetal heart rate detector designed based on MEMS acceleration sensor. For this type of instrument, the most important thing is how to get rid of interference and noise. Extract the signal we need from it. With the development of technology, the instrument is becoming more and more miniaturized and portable. And after appropriate improvement, it can be used as a terminal to make a remote fetal heart monitoring system. It combines the current conventional fetal heart monitoring with computers and telephone communications, and uses the above-mentioned fetal heart detector, telephone and the central signal acquisition and analysis monitoring host of the hospital to form a system. The central signal acquisition and analysis monitoring host on the hospital side gives an automatic analysis result, and the doctor diagnoses the result. If there is a problem, the pregnant woman is notified to come to the hospital in time. This technology is conducive to pregnant women to check the condition of the fetus at any time, which is beneficial to the health of the fetus and pregnant women.