The system hardware mainly includes: LM3S1138 controller, air pump, filtering and amplification circuit, alarm circuit, storage module, keyboard module, and LCD display module.
The pneumatic circuit controls external pressure, collects pressure signal data, amplifies and filters the pulse signal in the mixed blood pressure signal, and amplifies the pulse signal. The storage module is mainly used to store measurement records, and the alarm circuit is used for displaying sound and light alarms in abnormal situations; Display module, used to display blood pressure measurement data and blood pressure history records; The keyboard is used to input system setting parameters.
The pneumatic circuit consists of a cuff, an air pump, a pressure sensor, and an exhaust valve, all of which form a four connected structure. The two signals from the pressure sensor are connected to the two A/D channels of the main controller to measure the static pressure signal and pulse signal, inflation and deflation to complete the measurement.
The physiological signals of the human body have the characteristics of low frequency and low signal, usually μ V~mv, with a frequency range of 0~300 Hz and a low signal-to-noise ratio, it is important to choose a sensor with good performance.
The MPXVS050GP pressure and internal control of the sensor enterprise includes a signal operational amplifier and signal regulation system function, which has well-developed linearity. The output working voltage is proportional to the applied pressure, and can effectively compensate on chip. Its temperature compensation characteristics can overcome the temperature drift problem in semiconductor pressure sensitive devices. It can directly convert the blood pressure on the arterial wall to 04.7V electrical signal, and the corresponding blood pressure value is 0375 mmHg, which is very consistent with the design requirements of sphygmomanometer, especially suitable for oscillographic pressure measurement.
Driver circuit design:
Air drive circuit. The driving currents of the Hlpm30a air pump and the hlv01a solenoid valve are 450ma and 75ma, respectively. The signals controlling the operation of the air pump and solenoid valve are sent by the lm3s1138 controller, but the digital I/O output current of the lm3s1138 controller cannot meet the requirements. Therefore, Uln2803 is provided for the air pump and solenoid valve to output a 500ma current for driving the air pump and solenoid valve. The first and second circuits of Uln2803 are used to drive electromagnetic valves and pumps, as well as filtering and amplification circuits, respectively.
The signal from the pressure sensor is a mixed signal of pulse signal and static pressure signal, with high-frequency interference and DC or low-frequency components from the outside. Static pressure signal is a low-frequency signal with a frequency less than or equal to 0.04 Hz, and the frequency of pulse signal is generally around 1 Hz. Although the selected pressure sensor has amplification power, the amplification technology for pulse oscillation frequency signals is limited, mainly due to static pressure signals. The mixed control signal can be divided into the following two parts and connected to the LM3S1138 controller. The development of one part directly affects the operation of the A/D converter for analog-to-digital conversion, and obtains static pressure signal analysis data; The other is to amplify the pulse signal data by analog-to-digital conversion after passing through the band-pass filter amplifier module circuit.
The filter consists of two RC networks that determine the cutoff frequency. The two cutoff frequencies are calculated by the following formula.
Considering the occurrence of hypotension and the fact that the patient's blood pressure and heart rate are lower than those of normal individuals, as well as the ability to filter out system interference signal components, we set the development pulse wave frequency to be 0.5-5 Hz. Therefore, R1=1 k Ω, C1=33 μ F，R3=1 MΩ，C2=0.33 μ F。
This circuit can not only extract the required pulse signal, but also amplify weak pulse signals. The strength of the pulse signal varies from person to person, but the general range is 1 to 3 millimeters of mercury. According to the transfer function mpxv5050gp of the pressure sensor, the voltage signal converted from this pressure signal is 12-36mv. Because of filtering
The oscillator attenuates by 10 dB at a 1 Hz signal, resulting in an oscillation wave signal of 3.8-11.4 mV. After testing, the amplification factor of the amplifier is set to 150, so that the signal variation range is located in the area with high A/D conversion sensitivity of LM3S1138, which can improve the accuracy of data acquisition.