Electronic sphygmomanometer designed with MSP430F449

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.

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