I. Overview
Detecting the fetal heart rate is a strong technical work. Since the fetal heart rate is very fast, between 1,600 times per minute, use traditional stethoscope even with ultrasonic Doppler, using artificial counts, it is difficult to measure accurate . The ultrasonic Doppler's fetal heart monitor with digital display function, is expensive, only for a few large hospitals, in the middle, small hospitals and the vast rural areas cannot be popularized. In addition, ultrasonic vibration waves act on the fetus, which will have a large adverse effect on the fetus. Although the detection dose is low, it is also desirable to detect, not suitable for recipient, repetitive inspection and household use.
This project is based on VTI's SCA600C13H1G MEMS acceleration sensor, proposes a non-invasive fetal heart detection method, developing a simple and easy to learn, intuitive and accurate clinical diagnosis and pregnant women between fetal heart stethoscope and Doppler fetal monitor Self-test medical auxiliary instruments.
Second, SCA600C13H1G
The SCA600C13H1G silicon capacitor acceleration sensor is made of single crystal silicon and glass. This design guarantees that with the changes in time and temperature, the product has good reliability, accuracy and stability. Its capacitance detection principle is simple and reliable, and it is measured based on two parallel plate spacing. A pair of parallel plates The capacitance and charge storage depends on the spacing and board area between the parallel plates. The sealing structure of the product has lowered the package requirements. Particles or chemicals cannot enter the sensor, thereby ensuring the reliability of the product. The dual capacitors of the product and the symmetry design improve the zero stability, linearity and lateral sensitivity of the product. Typically, the temperature coefficient is less than 0.05 fs / ~ c, and the lateral sensitivity is less than 3%. This new sensor has the following characteristics; compact structures, low power consumption, good reliability, excellent performance.
The VTI's acceleration sensor is made based on the 3D-MEMS technology that has been proved. The three-dimensional microelectronic mechanical system (3D-MEMS) is a three-dimensional combination of various technologies that can be machined into three-dimensional structures, and their packages and contacts are easy to install and assemble, and the sensors made of this technique have excellent precision. , Minimal size and extremely low power consumption. A advanced sensor can be made only by a small silicon and can measure the acceleration of three mutually vertical directions.
Third, the principle and composition of system work
The working principle of this sensor system is converted into an analog voltage signal by an acceleration sensor through an acceleration sensor, and a difference between the pre-placed instrument amplifier achieves a difference. A series of intermediate signal processing such as filtering is then performed, and the analog voltage signal is converted into a digital signal with the A / D converter. Enter the 8051 microcontroller by the optical isolation device for analysis processing, the final output processing result
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1, front amplification circuit
The preamplifier mainly considers noise, input impedance, and common mode suppression than these three effects.
2, signal processing circuit
The fetal heart rate signal in the parent is a low frequency weak signal under strong noise interference. Because of its very weak, only a micro-energy, while the interference is very large, the effective signal will tend to be overwhelmed. The interference signal typically includes high frequency electromagnetic interference, 50 Hz power frequency interference, and other interference sources in the parent. The power frequency interference is mainly present in the form of a common mode signal, and is usually averaged for a few volts. The frequency of the interference signal and the fetal heart rate signal in the mother is not overlap. It is possible to focus on the characteristics of the fetal heart rate signal in the range of 7oHz ~ L10Hz, and the fetal heart rate signal processing circuit is designed to be strong, superior, and a stable and reliable fetal heart rate signal processing circuit is designed, as shown in Figure 2.
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The intermediate signal processing circuit is divided into four parts: band passage-saving circuit, secondary amplifying circuit, 50 Hz trap and gain adjustment circuit. The state variable filter with pass saving circuit is used as a band pass filter. The maximum range of the passband is set to 50 Hz; - "140 Hz is shown in Figure 3. This filter has three advantages: appropriate selection element value. It is unrelated to the quality factor q and the center frequency; and q is very sensitive to component parameters; Q can only be high (can be comparable to high Q value with source belt filter)., Technology area
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