[Shenzhen University Electric Competition] Air pressure control device

This solution uses STM32F103R8T6 microcontroller ^[1] as the microprocessor, which is responsible for data processing and peripheral control of the entire air pressure control system.

The MPS-3119 series pressure sensor is used to collect the air pressure data of the air bag. The weak voltage signal of the sensor first needs to be amplified before it can be input to the AD converter. This solution uses the INA128 measurement amplifier ^[2] for signal amplification. The amplifier has high precision and can gain Adjustable, has the characteristics of high common mode rejection ratio and low temperature drift, and is suitable for amplification of small signals. The amplified signal is input to the ADC in the microcontroller, converted into a digital signal, and then input to the microcontroller for further operations.

Two rotary coding switches are used as parameter adjustment inputs, one of which is used to set the target air pressure value. It is initially in coarse adjustment mode. The rotary coding switch can be pressed, and the knob switch is pressed to switch between coarse and fine adjustment modes. In the coarse adjustment mode, the encoder rotates one circle clockwise and the air pressure target value is +20, and one rotation counterclockwise is -20, and the change step is 1. In the fine adjustment mode, the encoder rotates one circle clockwise and the air pressure target value is +2 and counterclockwise. One turn -2, the change step is 0.1. The adjustment range of the target air pressure is 0.0~30.0 . Another rotary coding switch is used as an adjustment knob for the inflation/deflation speed. Turn one space clockwise to increase the filling/deflation speed by +1, and turn one space counterclockwise -1. The change range is 0 to 10. Press the button to switch to adjust the inflation speed and deflation speed. Air speed. The air bag is inflated using a 5V DC-driven air pump. The PWM signal output by the microcontroller controls the switch of an NMOS field effect transistor, and the NMOS controls the work of the air pump. The frequency of the PWM output is 5Hz. The output end of the air pump is connected to a one-way air valve to prevent air backflow causing air bag leakage. The deflation circuit uses a two-position three-way micro solenoid valve driven by 5V DC. The solenoid valve has three interfaces (as shown in the picture on the right). Hole 1 is connected to the air bag, hole 2 is used for exhaust, hole 3 is connected to the air pump, and a one-way valve is connected to prevent the gas from flowing back from the air pump. When the solenoid valve is not energized, the air pump can inflate the air bag at any time. When the solenoid valve is energized, the air pump inflation channel is closed and the air bag begins to deflate at the same time.

The status of the air pressure control system is displayed by a 0.96-inch 12864 OLED display module. The display is controlled by an IIC interface and has four pins. In addition to power and ground, there is also a clock signal pin and a data line pin. Directly driven by the 3.3V microcontroller pin. The display module has 128×64 pixels, and it takes 16×16 pixels to display one Chinese character, so the module can display 4 lines of Chinese characters at the same time, and can simultaneously display the current air pressure, target air pressure, inflation speed, deflation speed, and indicate the current target. Air pressure adjustment speed, sensor working status, charging and deflating adjustment status. The first line shows the current air pressure, the second line shows the target air pressure, the third line shows the inflation speed and deflation speed, the left side of the fourth line shows the rate of adjusting the target air pressure, the middle shows the sensor working status, and the right side shows what the right knob is adjusting now. Inflation speed or deflation speed.

The three-stage switch is used to set the working status of the main control board. When the three-stage switch is moved to the upper position, it is in the calibration state. At this time, the pressure sensor is not connected to the airbag. The microcontroller collects the voltage value in the current state and sets it to the air pressure 0 point. When the toggle is moved to the middle, it is in the pause state, and the system is on standby, without collecting data or inflating or deflating. Turn it to the lower position for normal working status. This solution uses ±5V power supply. In theory, any voltage input of 5~8V can work normally. The function of negative voltage is to provide negative power supply to the amplifier. Otherwise, the output will not reach 0V, which will cause the data to be inaccurate. Using AMS1117-3.3 low voltage dropout linear regulated power supply, the power input can be converted into a 3.3V constant voltage output to power the microcontroller system.