Application of MEMS acceleration sensor in fetal heart rate detector

I. Overview

At present, the international maternal and child health indicators have been used as sensitive indicators for measuring social production and economic development. Most of the health indicators, average life expectancy, infant mortality rate, maternal mortality rate and other indicators stipulated in China that reflect the well-off level need to be achieved through maternal and child health care. The routine prenatal examinations in hospitals now include fetal heart rate, fetal position, blood pressure measurement, weight measurement, abdominal circumference measurement and uterine floor height. Among them, detecting fetal heart rate is a very technical task. Because the fetal heart rate is very fast, between l20 and 160 times per minute, it is difficult to measure by manual counting with a traditional stethoscope or even an ultrasound Doppler with magnifying effect. accurate. The ultrasound Doppler fetal heart rate monitor with digital display function is expensive and is only used by a few large hospitals. It cannot be popularized in small and medium-sized hospitals and large rural areas. In addition, the ultrasonic vibration wave acts on the fetus, which 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 repetitive inspections and home use.

This project is based on the SCA600C13H1G MEMS acceleration sensor of VTI, and proposes a non-invasive fetal heart rate detection method. A simple and easy to learn, intuitive and accurate clinical diagnosis between pregnant woman fetal heart stethoscope and Doppler fetal monitor and pregnant women Auxiliary medical instrument for self-inspection.

Second, SCA600C13H1G

SCA600C13H1G silicon capacitive acceleration sensor is made of single crystal silicon and glass. This design can ensure that with time and temperature changes, the product has good reliability, accuracy and stability. 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 depends on the spacing 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, thus ensuring product reliability. The product's dual capacitor structure and symmetrical design improve the product's zero stability, linearity, and lateral sensitivity. 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 acceleration sensor is based on the proven 3D MEMS technology. The three-dimensional microelectronic mechanical 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 by an acceleration sensor, and the difference amplifier is realized by an instrument amplifier 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 with an A / D converter. Through the optical isolation device input to the 8051 microcontroller for analysis and processing, and finally output the processing results.

System block diagram

1. Preamplifier circuit

The preamplifier mainly considers the effects of noise, input impedance and common mode rejection ratio.

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