Development and design of pulse blood pressure monitor

    In the past, my country mainly used mechanical mercury sphygmomanometers as medical instruments to measure patients' blood pressure. However, with the development of my country's economy and society, as well as the improvement of medical and health care, electronic sphygmomanometers have been widely used and won a considerable market share. As for traditional mechanical sphygmomanometers and electronic sphygmomanometers, although the latter has a humanized interface and is easy to operate, its stability and accuracy need to be improved, so as to provide a scientific basis for the treatment of patients.

　　my country's traditional Chinese medicine culture is extensive and profound. Pulse diagnosis is an important method of "observation, auscultation, questioning and palpation". Based on pulse diagnosis, the patient's condition is observed and roughly estimated. The pulse is measured by electronic instruments. Regarding the current research status of pulse measurement in China, although relevant scholars have conducted in-depth research and discussion, the process of purifying pulse signals is relatively complicated and susceptible to nonlinear deformation and the combined influence of multiple interferences, which generally leads to fundamental changes after the signal extraction is completed. Therefore, research in this area needs to be strengthened.

　　Based on my own work experience, the author organically integrates the measurement of pulse, heart rate and blood pressure, which is conducive to the development of clinical medicine. Based on the study of relevant academic works at home and abroad, the author adopts more advanced intelligent technology and signal processing technology to eliminate the noise interference in the nonlinear deformation and pulse extraction process to the maximum extent, so as to improve the stability and accuracy of the measurement data. The pulse and blood pressure meter in this design has been successfully developed and clinically verified. The pulse extraction can meet the design requirements and has achieved good results.

　　Measurement principle overview

　　In this design, the author uses the pressure sensor MPX50GP produced by Motorola. When the pressure range is controlled within 0 to 40KPa, it generates an output signal of 20 to 60mv. The Motorola MC68HC05SR3 microcontroller is used as the microcontroller. At the same time, Dalian Dongfang's EDM 12864B is used as the graphic dot array crystal.

　　Based on the mechanical blood pressure measurement method, the method for determining diastolic and systolic blood pressure is designed. The patient's arm is surrounded by an air pump cuff. During the pumping process, the CP cuff pressure signal and BP pulse signal are automatically tracked and monitored. The amplitude of the BP pulse signal will continue to increase as the cuff pressure signal CP increases. When it reaches the corresponding value, it will show a significant decreasing trend until the pulse signal disappears completely. When the pulse signal disappears completely, then increase the pulse signal by 5KPa.

　　For the exhaust process, the thresholds of diastolic and systolic pressures are set in advance. During the exhaust process, when the pulse signal drops to the corresponding amplitude, a pulse signal BP is generated. This moment is the moment when the blood vessels bound by the cuff are opened by the pulse. The pulse signal is tracked and monitored. When the amplitude of the cuff pressure signal CP is greater than the systolic pressure threshold, the value of the systolic pressure is the value of the cuff pressure signal. The pulse signal BP is continuously tracked. When the pulse signal amplitude reaches the corresponding value and decreases rapidly, and reaches the pre-set threshold, the measured cuff pressure signal is the patient's diastolic pressure.

　　For the exhaust process, the pulse signal is converted into a rectangular wave, and the rectangular wave is used as a trigger signal for interruption.

　　After completing the capture and positioning of the systolic pressure, the strongest pulse signal is obtained when the cuff pressure signal decreases by 2KPa. The pulse signal acquisition process must be accurate and fast. In other words, 128 data samples must be sampled within 1.5 pulse signal cycles. After relevant processing, the required static pulse graph can be obtained, making the data more intuitive.

　　Figure 1 Pulse blood pressure meter system block diagram

    The pump exhaust in the blood pressure cuff is realized by microcontroller, and the sensing conversion of the cuff air pressure is completed by the pulse sensor. After the electrical signal is generated by the pressure sensor, it is relatively weak, generally within 10uf, but has an invalid bias of more than 20uf, and also contains a large amount of high-frequency noise. Therefore, the signal needs to be differentially amplified through the preamplifier stage, and the noise removal work is completed at the same time, and then the secondary amplification and zero point adjustment are carried out.

　　The separation of the cuff pressure signal and the pulse signal is completed based on the signal separation part. The obtained pulse signal is sent to the pressure detection channel for subsequent processing after the analog/digital conversion. Since the cuff pressure signal is relatively weak compared to high-frequency interference after separation (mainly 50HZ AC interference), the post-amplification needs to achieve three-stage amplification after completing high-frequency filtering. The processed signal is divided into the following two paths: one path is shaped to convert the signal into a rectangular wave, which is then used as a trigger interrupt to facilitate the realization of the timing capture function. The other path is sent to the micro-control channel after completing the analog-to-digital conversion link to complete multiple processing tasks such as digital processing.

　　In order for the system to work efficiently and accurately, the software system function diagram is as follows:

　　Figure 2 Schematic diagram of the functional structure of the blood pressure monitor software

　　Key factors to consider in design

　　For the MPX50GP sensor, its output signal consists of the following: CP signal (frequency less than 0.4HZ) and BP signal (frequency is 1HZ), so when separating these two signals, it is necessary to pass through a two-stage high-pass filter. If the CP signal is not ideally suppressed, the BP signal baseline cannot be stabilized, and the comparison benchmark of the pulse oscillation cannot be unified. The figure below is a schematic diagram of the two-stage filter circuit.

　　Figure 3 Schematic diagram of two-stage filter circuit

　　As can be seen from the figure, the two RC networks form a filter, so that the cutoff frequency is determined, and it is necessary to ensure that the oscillation signal is not lost or distorted. In addition, the factor that determines the gain of the filter is R2/R1, and the pulse signal is amplified by this gain. The high-frequency component is filtered out by C3.

　　The separated pulse signal must also be amplified, filtered, and baseline calibrated to serve as the criterion for determining diastolic and systolic blood pressure

    .

　　The operation interface of this pulse blood pressure monitor has achieved good readability, ease of operation and simplicity as much as possible. In addition, the system has also achieved the following important breakthroughs:

　　(1) All control functions can be achieved with just three buttons;

　　(2) Safe and accurate key operation process. For unused keys, the system can temporarily shut down the keys to prevent errors caused by misoperation;

　　(3) Simple and intuitive LCD interface;

　　(4) Buzzer signal prompts and indicator lights are set;

　　In summary, this pulse blood pressure monitor has met the needs of clinical medicine in terms of stability, clarity, precision and accuracy of measured data. In addition, due to its simple operation, concise and friendly operating interface and low cost, it can occupy a large market share.