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The fetal monitoring system uses TI's MSC1210 microprocessor with an integrated 24-bit A/D converter, offering solid analog performance and digital processing capability. The microprocessor integrates input channel selection, buffering, amplification, gain adjustment, A/D conversion, and digital processing on a single chip. A single integrated circuit can handle data acquisition for fetal heart rate, uterine contraction pressure, and fetal movement counts, and control the voice module and vibrator. Fetal heart rate is the control basis for the monitoring system; accurate and timely determination of the fetal heart rate is a prerequisite for control. Because the fetal Doppler signal has a low signal-to-noise ratio and nonstationary random characteristics, heart rate calculation can produce half-rate, two-thirds-rate, and double-rate errors that lead to control mistakes. This design applies wavelet analysis combined with a dual-threshold algorithm to obtain the fetal heart rate accurately and in real time, ensuring effective control.
Computer-Based Fetal Monitoring System Architecture
The block diagram of the computer-based fetal monitoring system is shown in Figure 1. The system is composed mainly of an ultrasound Doppler fetal probe, a uterine contraction probe, a fetal movement probe, fetal heart signal conditioning circuitry (low-pass filtering, absolute-value operation, and envelope extraction), contraction pressure signal conditioning circuitry, voice control, vibrator, the MSC1210 microprocessor, and a computer processing system. The MSC1210 and the computer processing system form the system core.
The MSC1210 controls acquisition of monitoring parameters and communication, and receives computer commands to control the voice module and vibrator. The computer system implements modules for intelligent control, communication control, data processing algorithms, and monitoring display.
Signal Conditioning Circuits
Given the importance of fetal heart rate monitoring and the complexity of the fetal Doppler signal, this section focuses on the Doppler signal conditioning circuitry. The circuit preprocesses the Doppler fetal audio signal through low-pass filtering, absolute-value operation, and envelope extraction. A second-order low-pass filter with a 250 Hz cutoff frequency is used to remove high-frequency signals and interference. The absolute-value circuit doubles the signal amplitude, improving detection sensitivity. The envelope extraction circuit uses a combination of pi-shaped and T-shaped low-pass filters with a 10 Hz cutoff frequency. Parallel diodes and capacitors limit negative signals in the circuit and damp certain bands of high-frequency noise.
MSC1210 Microprocessor
The monitoring acquisition system uses Texas Instruments' MSC1210 microcontroller as the processor. The MSC1210 integrates an enhanced 8051 microcontroller core and flash memory with a high-precision sigma-delta A/D converter. The chip uses an enhanced 80C51 core that shortens the instruction execution cycle and employs low-power design. It integrates a 24-bit ADC with a conversion rate up to 1000 Hz, an 8-channel multiplexer, an analog input current source, input buffers, a programmable gain amplifier (PGA), an internal reference voltage source, an 8-bit microcontroller, program/data flash memory, and data RAM. Digital filters are available in fast, sin2, and sin3 modes. The enhanced 8051 core has two data pointers and an instruction set fully compatible with the standard 8051, while executing instructions faster, allowing operation at lower clock frequencies to reduce power consumption and noise. Analog and digital supplies are provided separately to reduce interference. The high integration level simplifies the system hardware, reduces the number of peripheral components, and shortens development time.
The MSC1210 interface circuit is shown in Figure 4. Doppler fetal signals and contraction pressure signals enter via IN0 and IN2, then pass through a multiplexer into buffers. A variable-gain amplifier amplifies the input signals. Fetal movement signals are handled by MSC1210 interrupts. The MSC1210 receives control commands from the computer over an RS485 bus, uses port P2 to control a voice chip for voice alerts, and uses P1.7 to control the vibrator for automatic fetal arousal. Using the MSC1210 as the microprocessor enables more accurate, real-time acquisition of fetal monitoring parameters, supporting the system control functions.
