1. Principle of blood pressure measurement

The concept of blood pressure is the pressure of blood flowing through the walls of blood vessels. The blood coming out of the heart needs thrust to go around the body. The heart pushes the blood forward by constantly contracting and relaxing. There are two types of blood pressure. One is systolic blood pressure: the blood pressure measured when the heart contracts and pumps blood to the blood vessels, and the other is diastolic blood pressure: the pressure measured when the heart is not contracting. When the pressure of the cuff is equal to the blood pressure, blood can begin to circulate and produce the so-called cuff sound, which is expressed as systolic blood pressure. From this moment, record it until the cuff sound disappears, and record this point as diastolic Pressure.

2. Working principle of electronic sphygmomanometer

The system block diagram is shown in Figure 1. The system is composed of constant current source, pressure sensor, amplifying circuit, band-pass filter, secondary amplification, blood pressure pulse trigger, liquid crystal driver, keyboard voice circuit and single-chip microcomputer.

Figure 1 Schematic diagram of electronic sphygmomanometer

The main principles of the single-chip microcomputer are: PWM output controls air pump inflation and leakage to adjust the air pressure in the cuff; ADC samples the DC component of the cuff to obtain the systolic and diastolic blood pressure; ADC samples the AC component of the cuff to determine the contraction after analysis and calculation Transient time position of blood pressure and diastolic blood pressure; receiving blood pressure pulse signal to trigger ADC work; output the calculated systolic blood pressure and diastolic blood pressure to the LCD display and give voice prompts of the values.

Three, hardware design

1 MSP430FF449D MCU main control circuit

The main control circuit of this system is shown in Figure 2, which is mainly composed of MSP430F449 chip, JTAG interface circuit, clock generation circuit, clock output circuit, reset circuit, PWM wave output circuit, and power supply circuit. JTAG is used to download and debug programs, and the PWM wave output circuit is used to control the air pump. When measuring blood pressure, first inflate to a height of 200mmHg, and then slowly deflate at a rate of about 5mmHg per second. Realize automatic blood pressure measurement.

Figure 2 Electronic blood pressure monitor main control circuit

2 Blood pressure sensing circuit

As shown in Figure 3, this circuit uses BP01 pressure sensor and op amp MAX4472. BP01 pressure sensor is specially designed for detecting blood pressure, mainly used in portable electronic blood pressure monitor. It adopts precision thick film ceramic chip and nylon plastic package, which has the characteristics of high linearity, low noise and low external stress. It adopts internal calibration and temperature compensation methods to improve measurement accuracy, stability and repeatability. In the full range, The accuracy is ±1%, and the zero offset is not more than ±300μV. MAX4472 is a low-power amplifying chip that integrates four operational amplifiers from MAXIM. The internal integrated op amp A in this system is connected to a constant current source to provide a constant current for the pressure sensor. Op amp B, op amp C, and op amp D form a differential input and single-ended output amplifier circuit, directly input ADC0 to monitor the DC component of blood pressure.

Figure 3 Blood pressure sensing circuit

3 Filtering and amplifying circuit

As shown in Figure 4, the circuit consists of two parts: filtering and amplifying. Among them, MAX267 is an integrated filter produced by MAXIM Company, which can be formed into low-pass, band-pass, high-pass, and other modes, which is flexible in use and has far better performance than a filter circuit composed of integrated operational amplifiers.

Figure 4 Filtering and amplifying circuit

MAX4471 is a low-power amplifier of MAXIM Company. MAX9028 is a low-power comparator of MAXIM Company. The filter circuit adopts MAX267 to form a band-pass filter (allowing 0.8-38Hz signal to pass) to filter out the DC component and power supply in the signal, as well as the high frequency noise and power frequency interference caused by friction between the skin and the cuff, and then further amplify by MAX4471, Obtain the voltage signal matched by the single-chip microcomputer and enter ADC2 to monitor the AC component of blood pressure. At the same time, this signal is converted into a pulse signal by the low-power comparator MAX9028, which triggers ADC1 to work.

4 Calendar clock and storage circuit

As shown in Figure 5, it is composed of EEPROM24C256 and calendar clock chip PCF8563. 24C256 is a low-voltage, serial interface memory with a capacity of 256K, used to store the measured blood pressure value. PCF8563 is an industrial-grade multi-function clock/calendar chip with extremely low power consumption with an I2C bus built in by PHILIPS. It is used to provide the time and date when the blood pressure is measured, so that it can be used for future queries. It is convenient for users to have a clear memory of their blood pressure for a period of time. It has strong practicability and overcomes the shortcomings of some electronic blood pressure monitors.

Figure 5 Schematic diagram of calendar clock and storage circuit

5 Keyboard and display circuit

As shown in Figure 6, it consists of two parts: the keyboard circuit and the liquid crystal display circuit. The liquid crystal display circuit adopts ZJM12864BSBD, which is a low-power dot matrix LCD with a display format of 128 dots (columns) × 64 dots (rows). It has multi-function instructions and is easy to connect to the MPU. Among them, the keyboard circuit adopts independent keys, and there are 7 keys, which are measurement, mmHg/kPa conversion, memory, setting, up, down, and delete. You can set the calendar clock, set the alarm parameters, and access and delete the blood pressure measurement value. LCD can display systolic blood pressure, diastolic blood pressure, current time and calendar; when querying the status, the date, time, and measurement value of the previous blood pressure measurement can be realized, and the historical data can be graphically displayed through software programming (such as drawing blood pressure fluctuations) Curve), convenient and intuitive.

Figure 6 Keyboard and display circuit

6 Voice pressure and alarm circuit

It is mainly composed of integrated voice chip ISD2560. ISD2560 is a powerful voice recording and playback chip produced by Winbond Company. It is a permanent memory type voice recording and playback circuit. The recording time is 60s and can be repeatedly recorded and played 100,000 times. The chip uses multi-level direct analog storage patented technology, which can reproduce voice very realistically and naturally. Through the pre-recorded sound, the automatic sound prompt of the blood pressure measurement value is realized. If the blood pressure is higher than the upper and lower limits of the normal blood pressure, an alarm will be activated to remind the user to seek medical attention.

Fourth, the conclusion

Electronic sphygmomanometers have the advantages of miniaturization, low power consumption, and high degree of intelligence, and are portable and easy to operate in use, thus presenting a trend of home use. This paper presents a complete hardware design scheme of portable electronic blood pressure monitor, and realizes the design of electronic blood pressure monitor based on MSP430F449 as the control core with pressure sensor, peripheral analog circuit and LCD driver chip. Most of the chips used in this design are low-power chips, easy to use battery power. At the same time, the design fulfills the requirements of humanization and intelligence. Just like a family nurse, it is very convenient for hypertension patients and middle-aged and elderly people. It can be transformed into actual products, so it has high practical value.

Previous: EM78P447S MCU and its infrared remote control in DC motor

Next: The function and application principle of the pull-up resistor