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With the continuous improvement of living standards and the increasing proportion of urban aging, the familyization of medical electronic devices has gradually become a trend. Among them, home electronic blood pressure monitor is one of the typical home medical testing equipment. At present, the sphygmomanometer can be roughly divided into two types: one is a mercury-type sphygmomanometer, and its advantage is that the numerical value is stable. The shortcoming is that it cannot be operated by one person, and must be operated by a professional medical staff, and the error of the naked eye observation is extremely large and subjective. Larger size is not easy to carry. The second is the electronic sphygmomanometer, which has the advantages of simple use and single operation by one person; the measurement value is convenient for recording, and the volume is light and portable. Electronic sphygmomanometers have many advantages and are increasingly popular with ordinary families. This paper introduces the design of the home electronic sphygmomanometer with MSP430F449 as the control core, supplemented by analog signal processing circuit between BP01 pressure sensor and MCU, LCD display and voice prompt.
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 to have thrust to bypass the body for a week. The heart is pushing and pushing forward by constantly shrinking and relaxing. There are two kinds of blood pressure. One is systolic blood pressure: the blood pressure measured when the heart contracts to hit the blood vessels, and the other is the diastolic blood pressure: the pressure measured by the heart without contracting. When the pressure of the cuff is equal to the blood pressure, the blood begins to circulate and produces a so-called cuff. This is the systolic pressure. Recording starts from this moment until the last cuff sound disappears. Recording this point is diastolic. Pressure.
How does an electronic blood pressure monitor work?
The system block diagram is shown in Figure 1. The system consists of a constant current source, a pressure sensor, an amplifying circuit, a band pass filter, a secondary amplification, a blood pressure pulse trigger, a liquid crystal driver, a keyboard voice circuit, and a single chip microcomputer.
Figure 1 Schematic diagram of electronic blood pressure monitor
The main principle of the single-chip microcomputer is: PWM output control air pump air leakage to adjust the air pressure in the cuff; one ADC samples the air pressure DC component in the cuff to obtain systolic pressure and diastolic pressure; one channel ADC sampling cuff inner pressure AC component is analyzed and calculated to determine the contraction The transient time position of the pressure and diastolic pressure; receiving the blood pressure pulse signal triggers the ADC operation; outputs the calculated systolic pressure and diastolic blood pressure results to the LCD display and performs a numerical voice prompt.
hardware design
1 MSP430FF449D microcontroller main control circuit
The main control circuit of this system is shown in Figure 2. It 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. The JTAG is used to download and debug the program, and the PWM wave output circuit is used to control the air pump. When measuring blood pressure, first inflate to a height of 200 mmHg, and then slowly deflate at a rate of about 5 mmHg per second. Achieve automatic measurement of blood pressure.
Figure 2 Electronic blood pressure monitor main control circuit
2 blood pressure sensing circuit
As shown in Figure 3, this circuit uses a BP01 type pressure sensor and an operational amplifier MAX4472. The BP01 pressure sensor is specially designed for detecting blood pressure and is mainly used in portable electronic blood pressure monitors. It is made of precision thick film ceramic chip and nylon plastic package. It features high linearity, low noise and low external stress. It adopts internal calibration and temperature compensation to improve measurement accuracy, stability and repeatability. The accuracy is ±1% and the zero offset is no more than ±300μV. The MAX4472 is a low-power amplifier chip from MAXIM that integrates four operational amplifiers. In this system, the internal integrated operational amplifier A is connected to a constant current source to provide a constant current for the pressure sensor. The operational amplifier B and the operational amplifier C form an differential input, single-ended output amplification circuit, and directly input ADC0 to monitor the blood pressure DC component.
Figure 3 blood pressure sensing circuit
3 filtering and amplifying circuit
As shown in Figure 4, the circuit consists of two parts: filtering and amplification. The MAX267 is an integrated filter produced by MAXIM. It can be used in low-pass, band-pass, high-pass, and other modes. It is flexible and performs much better than the filter circuit composed of integrated op amps.
Figure 4 Filter and Amplifier Circuit
The MAX4471 is a low-power amplifier from MAXIM. The MAX9028 is a low-power comparator from MAXIM. The filter circuit uses the MAX267 to form a bandpass filter (allowing a signal of 0.8 to 38 Hz to pass), filtering out the DC component and power supply in the signal, and the high frequency noise and power frequency interference of the skin and cuff friction, and then further amplified by the MAX4471. Get the voltage signal matched by the microcontroller, enter ADC2, monitor the AC component of blood pressure. At the same time, the signal is converted into a pulse signal by the low-power comparator MAX9028, which triggers the operation of ADC1.
4 calendar clock and storage circuit
As shown in FIG. 5, it is composed of an EEPROM 24C256 and a calendar clock chip PCF8563. The 24C256 is a low voltage, serial interface with a 256K memory for storing measured blood pressure values. The PCF8563 is an industrial grade I2C bus with a very low power consumption multifunction clock/calendar chip from PHILIPS. It is used to provide the time and date when blood pressure is measured for later use. It is convenient for users to have a clear memory of their blood pressure for a period of time. Practicality, overcoming the shortcomings of some electronic sphygmomanometers.
Figure 5 Schematic diagram of the calendar clock and storage circuit
5 keyboard and display circuit
As shown in FIG. 6, it is composed of a keyboard circuit and a 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 with MPU. The keyboard circuit adopts a separate button, and there are 7 buttons, which are measurement, mmHg/kPa conversion, memory, setting, up, down, and delete. It is possible to set the calendar clock, set alarm parameters, and access and delete blood pressure measurement values. The LCD can display systolic pressure, diastolic pressure, current time and calendar; in the query state, it can realize the date, time and measurement value of blood pressure measurement in the past, and can realize the graphical display of historical data 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
This circuit is shown in Figure 7, and is mainly composed of integrated voice chip ISD2560. ISD2560 is a voice recording and playback chip with strong functions produced by Winbond. It is a permanent memory type voice recording and playback circuit with a recording time of 60s and can be repeatedly recorded and played 100,000 times. The chip uses a patented multi-level direct analog memory technology that reproduces speech very realistically and naturally. Through the pre-recorded sound, the automatic voice 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 given to remind the user to seek medical treatment.
Figure 7 voice report blood pressure, alarm circuit
4 Conclusion
The electronic sphygmomanometer has the advantages of miniaturization, low power consumption, and high degree of intelligence, and is portable and easy to operate in use, thereby showing a trend of householdization. This paper presents the complete hardware design of the portable electronic sphygmomanometer, and based on the MSP430F449, the design of the electronic sphygmomanometer is realized by the control core with the pressure sensor and the peripheral analog circuit and the LCD driver chip. Most of the chips used in this design are low-power chips that are easy to use on battery power. At the same time, it is designed to realize the requirements of humanization and intelligence. It is like a family nurse. It is very convenient for patients with hypertension and middle-aged and elderly people. It can be converted into actual products, so it has high practical value.
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