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Design considerations of sphygmomanometer
文章来源：鑫志科技 / 作者：鑫志科技 / 发表时间：2021-09-09
The blood pressure monitor or sphygmomanometer mainly uses an inflatable cuff and monitoring equipment to measure blood pressure through listening; Pressure sensors can also be used to measure blood pressure. This monitor can be completed through two ways: manually inflate the cuff, and use a stethoscope to monitor the sound from the artery wall (stethoscopy); Or insert a pressure sensor inside the sphygmomanometer to stop measuring the vibration of the artery wall (vibration measurement method).
Automatic electronic sphygmomanometer
There are two types of automatic detection sphygmomanometer: upper arm sphygmomanometer and wrist sphygmomanometer. The upper arm sphygmomanometer includes an air bag, which is tied to the upper arm during measurement. The inflatable cuff is connected to the monitor placed near the left side of the arm through a leather tube. The size of wrist sphygmomanometer is small, and the whole installation can be wrapped in tricks. Therefore, this kind of product has strict requirements for size. Some upper arm sphygmomanometer require manual inflation, but most upper arm and wrist sphygmomanometer on the market at present are fully automatic.
The automatic electronic sphygmomanometer first inflates the cuff to make it have enough pressure, so as to prevent blood from moving in part of the aorta. As the pressure gradually releases, when blood begins to flow through the arteries, the measured pressure value is the systolic blood pressure of the heart. At the same time, it can also monitor the rate of pulse beating. When the blood flow returns to normal, the measured value is equivalent to the diastolic blood pressure of the heart. This completes a complete measurement cycle, which is completed by the installed pump, inflatable cuff, electronic valve, and pressure sensor.
The signal generated by the pressure sensor is processed by an operational amplifier or instrument amplifier and sent to an analog-to-digital converter (ADC). Then, for different types of monitors and sensors, appropriate methods are used to calculate systolic blood pressure, diastolic blood pressure, and pulse rate in the digital domain, and the resulting results are displayed on a liquid crystal display (LCD). Typically, the data and corresponding time/date are also stored in non volatile memory.
Some sphygmomanometer can upload data to the computer so as to stop further analysis and later measurement tracking, and can transmit data through USB interface. You can choose a discrete USB transceiver or a microcontroller with a built-in transceiver.
The audio prompt of the sphygmomanometer can be a simple beep or a high-end voice prompt. A simple buzzer can be driven by one or two pulse width modulation (PWM) output port pins of a microcontroller; Regarding the advanced voice prompt function circuit, it is possible to select an audio digital to analog converter (DAC) and speaker amplifier to process audio signals.
The upper arm sphygmomanometer usually uses 4 AAA (1.5V) alkaline batteries for power supply, while the wrist sphygmomanometer mostly uses 2 AAA alkaline batteries for power supply. The pump and analog circuit of the sphygmomanometer request 5V or 3.3V power supply, and the digital circuit selects 3.3V or 1.8V power supply according to the measurement technology adopted in detail. Therefore, a typical upper arm sphygmomanometer needs a buck boost switching power supply to provide 5V power for the pump/analog circuit, and a low dropout linear regulator (LDO) to provide 3.3V power for the digital circuit. A typical wrist sphygmomanometer uses boost switching power supply to provide 3.3V power required by the pump/analog circuit, and uses an LDO to convert the input voltage to 1.8V to supply power for the digital circuit.
In order to extend the service life of the battery, when the sphygmomanometer is not working, the power supply can be turned off, and only the real-time clock (RTC) can be maintained effectively and the working state of the sphygmomanometer can be quickly restored.
Presentation and backlighting
Most sphygmomanometer use simple segment LCD displays, which contain 100 segments or less, and LCD drivers are usually integrated in the microcontroller. Apply one or two white light LED (WLED) or electroluminescent (EL) light sources to provide display backlight. The wrist sphygmomanometer can use a switching power topology to provide WLED drive, and the upper arm sphygmomanometer can use a linear regulator to provide WLED drive.
All sphygmomanometer monitors must pass the electrostatic discharge (ESD) test request of IEC 61000-4-2, be able to apply devices with built-in ESD maintenance, or use ESD maintenance lines to maintain the lines exposed at the interface.