MN101EF32D single chip microcomputer realizes oscillometric method to measure blood pressure

introduction

Mercury sphygmomanometers have some inherent shortcomings: First, the speed of deflation has a direct impact on the reading, the international standard deflation rate is 3 ~ 5mmHg per second, and the speed of deflation by different doctors will affect the measurement accuracy The second is that this method is based on human vision, hearing and coordination, which is difficult to standardize. To this end, this design starts with the blood pressure detection method, using the high-speed, low-power MN101EF32D single-chip microcomputer of Panasonic Corporation of Japan as the core of blood pressure measurement, control, data reading and writing, data display, and can accurately adopt the oscillometric method (oscillation Method) Measure blood pressure.

working principle

The oscillometric method (oscillation method) determines blood pressure based on the envelope of the amplitude change of the pressure oscillation wave of the cuff during decompression. The current consensus is that when the amplitude of the cuff pressure oscillation wave is at its maximum, the cuff pressure is the average arterial pressure. Arterial systolic pressure corresponds to the first inflection point of the amplitude envelope, and diastolic pressure corresponds to the second inflection point of the envelope.

hardware design

The basic working principle of the system is shown in Figure 1. The voltage signal output by the pressure sensor is first filtered by a low-pass filter, then the signal is converted into an input signal suitable for the single-chip microcomputer by the op amp circuit, and finally the analog sampling signal is converted into a digital quantity through the MN101EF32D single-chip microcomputer. The program performs digital filtering on the collected data and analyzes it to calculate the two key indicators of human blood pressure, "diastolic blood pressure" and "systolic blood pressure", and then the microcontroller immediately stores the data in external memory and displays these important data on the LCD .

Principle of oscillometric blood pressure measurement using MN101EF32D single chip microcomputer

Introduction of sensors and design of peripheral circuits

The sensor used by the sphygmomanometer is MPS-3100-006G piezoresistive pressure sensor, which is a Hui type bridge composed of four equivalent resistors. Its output voltage is proportional to the input pressure. Under ideal conditions, when the pressure is input, the resistance The value changes accordingly, but in fact the change of temperature will also affect the output value of its resistance. In addition, due to errors in crystal and circuit design and manufacturing, plus the impact of the packaging process, the zero offset is not zero. Therefore, individual components must be used to correct individual temperature compensation circuits. Its important indicators are as follows:

a. Sensor measuring range: 5.8 ~ 15 PSIG

b. Operating temperature range:? 40 ~ 85 ℃

c. Drive current: 1.5 ~ 3mA

d. Drive voltage: 5 ~ 15V

e. Zero drift:? 25 ~ 25mV

f. The temperature coefficient of resistance is: 0.2% / ℃

Because the blood pressure signal is taken from the arm, the measured signal is easily interfered by the position of the cuff and the movement of the arm. According to these professional characteristics, the system is required to have high input impedance, high gain, high common mode rejection ratio, low noise and low drift. As shown in Figure 2, T1 in the figure is the MPS-3100-006G piezoresistive pressure sensor. The entire circuit first converts the pressure signal into a voltage signal, and then performs amplification and filtering. U1, U2 in the picture are active op amp LM324, its input impedance is very high. After the signal of the pressure sensor is amplified, the closed-loop gain of the op amp is changed by adjusting the size of VR1 to adjust to the voltage input range suitable for A / D. The U1 op amp circuit is used to measure the pressure in the cuff. The measured data is used by the MCU to analyze and control the speed of inflation and deflation of the cuff. In addition, the U2 op amp circuit amplifies the AC signal separated by the C11 capacitor. This circuit measures the pulse wave of the human body. The collected data of the two circuits constitute the important calculation parameters of each index of the sphygmomanometer.

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