Multifunctional electronic sphygmomanometer based on photoelectric detection pulse wave

Introduction

In recent years, due to the rapid development of China's social economy and changes in people's lifestyles, the incidence of cardiovascular disease and related risk factors have been rising, and people are paying more and more attention to their physical and mental health. In the past, blood pressure measurement had to go to the hospital, which was tested by a doctor with a mercury sphygmomanometer, but with the development of technological level, sphygmomanometers are ushering in a new era of electronic sphygmomanometers. As long as you have a home electronic sphygmomanometer, people can sit at home and monitor changes in blood pressure at any time. If you find abnormal blood pressure, you can go to the hospital in time.

How the system works

The intermittent ejection of the heart produces pulse waves that propagate along the arterial canal from the root of the aorta. Where the pulse wave arrives, the mechanical parameters (such as flow, pressure, blood flow velocity, blood vessel cross-sectional area, etc.) in the arterial tube will change. In each cardiac cycle, the intermittent relaxation and contraction of the heart will cause the vibration of the expansion and contraction of the aortic wall and the change of high and low pressure of the aortic blood pressure. This vibration of the arterial wall is called the arterial pulse. When the arterial pulse wave is formed at the proximal end of the aorta, the blood pressure, blood flow and the periodic vibration of the blood vessel wall will be spread in a fluctuating manner along the artery tree all the way to the entire arterial system, thereby forming the entire arterial system. These three types of pulse waves are generated by each other when the blood pressure is low and high, the blood flow is slow and fast, and the blood vessel wall is contracted and expanded. Therefore, they can be collectively referred to as arterial pulse waves.

Figure 1 Extinction curve of hemoglobin

Figure 2 Block diagram of the overall structure of the system

This multifunctional electronic sphygmomanometer uses photoelectric volume method for measurement. The principle is: the difference in the signal transmitted by the light beam to the photoelectric receiver through transmission. The hemoglobin (Hb) in human blood erythrocytes is reversibly combined with oxygen molecules. The combined Hb is called oxyhemoglobin (HbO2), and the two have different absorption curves: the light absorbed by hemoglobin Hb in the red spectral region 600nm ~ 700nm Less, and HbO2 absorbs more light; hemoglobin Hb absorbs more light in the infrared spectral region 800nm ​​~ 1000nm, while HbO2 absorbs less light. In the optical path, in addition to arterial hemoglobin, there are many other absorbers, which will seriously affect the measurement accuracy. Therefore, the red and infrared LEDs are as close as possible to each other, transmitting light through a single tissue position in the human body, and using time multiplexing Process to transmit light. At the same time, at the detection end, the transmitted light intensity through the finger is converted into a voltage signal, so the photoelectric volume pulse wave signal is obtained. Finally, based on the detection principle of the photoelectric sensor and Lambert-Beer law, the transmitted light intensity can be correlated with the output voltage of the photoelectric sensor to establish a calculation method for the contraction and diastolic blood pressure of the cardiovascular system and design a non-invasive continuous blood pressure measurement test device. Its advantages are: simple and convenient to use, high measurement accuracy, easy to record the measured value, light and easy to carry, etc.

hardware design

Overall system structure

The overall system block diagram of this multifunctional electronic sphygmomanometer is shown in Figure 2. It mainly includes LPC1788 main control module, power supply and reset module, LCD touch screen module, signal conditioning module, photoelectric pulse wave sensor module and optional Zigbee module. Big module.

Master LPC1788

The main control adopts NXP's newly launched LPC1788 microprocessor based on ARMCortex-M3, which has the characteristics of high performance, high integration, low power consumption, etc. It is widely used in embedded applications with low power consumption and processing requirements of high integration, Very suitable for the design requirements of this program. It includes up to 512kB Flash memory, up to 96kB data memory, 4kB EEPROM memory, an external memory controller for SDRAM and static memory access, an LCD panel controller, an 8-channel 12-bit ADC and so on. Only a few chips can be added to realize the system functions, which fully meets the needs of this design, and can reduce the size of the entire system, reduce power consumption, control costs, and improve stability.

Photoelectric Sensors

This system adopts the transmission dual-wavelength measurement method, and the sensor uses the S0012A type finger sleeve transmission reusable blood oxygen probe produced by Shenzhen Midian Electronics Co., Ltd. The transmissive photoelectric pulse sensor includes two parts: light emitting and photoelectric receiving: for the light emitting circuit, the infrared light-emitting diode at 940nm and the red light-emitting diode with a wavelength of 660nm are selected in the probe as the light source of the pulse detection circuit in parallel ; The core component of the receiving circuit is a photodiode. In order to obtain the best signal-to-noise improvement ratio, it is required that the design of the preamplifier adopts the design principle of low-noise amplifier to reduce the noise of the input stage as much as possible, so the low-noise photosensitive device is selected to obtain the minimum noise figure.

Signal conditioning (amplification and filtering)

The pulse wave is based on the heart pulse as a power source, and is a weak low-frequency physiological signal with the characteristics of low frequency, weak signal and high impedance. Therefore, the analog circuit applied to the measurement of human physiological information can effectively improve the input impedance and common mode rejection ratio, suppress temperature drift and random noise. In this system, the low-power and high-precision instrumentation amplifier AD620 is selected, which is widely used in biomedical measurement and those occasions where the common mode interference is large when it provides a weak signal.

The main frequency component of the pulse wave is generally between 0.1 Hz and 15 Hz. Both the frequency band is appropriate to obtain the maximum fidelity when the signal is detected, and various interferences must be filtered out, such as 50 Hz power frequency interference, various external Medium and high frequency noise; in order to adapt to the data acquisition circuit, the output impedance of the output terminal should be low, the polarity of the analog signal, the level of the status signal and the dynamic range of the amplitude must all comply with the regulations of the connection. This design uses Maxim's MAX267, which is a simpler of Maxim's many switched capacitor filter (SCF) chips. It contains two second-order SCFs and one op amp that have been fixed into a band-pass type and use the same Q parameters and frequency conversion ratio. By choosing appropriate feedback resistors and Q parameters, Butterworth or Chebyshev filters with different ripple rates can be formed. In this way, the peripheral circuits are greatly reduced, and the use is flexible, and the performance is far superior to the filter circuit composed of integrated operational amplifiers, which is very suitable for this design.

Voltage range of the AC power supply is: 100~240V

The wide range makes the charger compatiable with the voltage standard in all countries. With only 15kgs, it is light weight and easy to carry.

Certificate/Conformity to standards:IEC/IECEX/CE/ROSH/IP68/ISO/EX 

The combination of layers and modules is convenient and is unnecessary to use a crane for movement." 

User safety, electro Enameled covered with paint and electrical protection according to their performance.