Introduction to the application of single chip microcomputer in micro oxygen machine

    Micro oxygen generators using PSA (pressure swing adsorption) air separation oxygen production technology are widely used in sports health care, medical rehabilitation, and health air conditioning, etc., because they use physical methods to produce oxygen, which is convenient, safe, and inexpensive. The single-chip microcomputer plays an important role in the automatic control system of micro oxygen generators with its low price and reliable performance.

    The micro oxygen generator discussed in this article adopts the world's advanced pressure swing adsorption PSA technology, uses zeolite molecular sieve as an adsorbent, and separates the gas mixture through the difference in the adsorbent's adsorption capacity for oxygen and nitrogen in the air, thereby achieving the separation of oxygen and nitrogen. The outside air is first compressed and pressurized to maintain a certain pressure, and then sent to the pressure swing adsorption separation bed. Under the action of the separation bed, the nitrogen in the air in the bed is adsorbed, while the oxygen is not adsorbed, so that high-purity oxygen can be obtained at the outlet of the separation bed. The output oxygen flow rate is adjusted by a mechanical flow control valve. When the output flow rate increases, the oxygen purity will decrease accordingly. In order to ensure the output oxygen concentration, a gas sensor is used to detect the oxygen concentration, and the user is informed through a real-time display device. When the indicator is lower than the set value, an alarm reminder is issued.

    1 Electrical control requirements

    1.1 Working process of pressure swing adsorption separation bed

    The working principle of the pressure swing adsorption separation bed is shown in Figure 1. The separation bed consists of two adsorption devices A and B equipped with zeolite molecular sieves for oxygen production and switching valves 1 to 8, among which 1 and 3 are discharge valves, 2 and 4 are air intake valves, 5 and 7 are pressure equalizing valves, and 6 and 8 are output valves. Under the action of the automatic control system, the adsorption devices A and B of the separation bed work alternately, and the process of "A works → A exhausts and pressure equalizes → B works → B exhausts and pressure equalizes → A works..." is continuously circulated. The function of the gas storage tank is to balance the pressure of the output oxygen.

    When adsorption device A is working, valves 2 and 6 are opened (the rest are closed), compressed air enters A through valve 2, nitrogen is filtered out, and the remaining oxygen is output through valve 6. Before the molecular sieve in adsorption device A is almost saturated with adsorption, it enters the exhaust and pressure equalization state of A, valves 1, 5, and 7 are opened (the rest are closed), and device A desorbs the previously adsorbed nitrogen to regenerate the molecular sieve adsorbent. At the same time, A and B are pressure-equalized to improve the gas production efficiency. After pressure equalization, it automatically switches to adsorption device B to continue adsorption and oxygen production in B. Similarly, before the molecular sieve in adsorption device B is almost saturated with adsorption, it automatically enters the exhaust and pressure equalization state of B, valves 3, 5, and 7 are opened (the rest are closed), and device B realizes decompression, desorption and regeneration. The four working states are cycled in sequence, and the two adsorption devices alternately repeat adsorption and regeneration, thereby achieving the purpose of continuous oxygen production.

    1.2 Basic control requirements
    (1) Complete the system's four working state cycles.
    (2) Output oxygen concentration detection.
    (3) User parameter setting.
　(4) Display and alarm.

    2 Single-chip microcomputer automatic control system

    2.1 Control system hardware design

    The hardware principle of the system is shown in Figure 2. This system uses the AT89C51 single-chip microcomputer, which has a 4 kB Flash memory on the chip, so there is no need to expand the external program memory. The display part uses a 3-digit LED digital tube. The left digit is used for status display. "P" means that the last two digits currently display the output oxygen concentration; "b" means that the last two digits currently display the set minimum oxygen concentration. When the output oxygen concentration is lower than the set concentration, an alarm is issued. The P0 port of AT89C51 is used as the segment control port of the LED. The position control line uses P2.0 P2.1, P2.2. The segment control and position control signals are inverted by ULN2003 to drive the LED. This system sets 4 buttons, namely power on, increase 1, decrease 1 and confirmation. For specific operations, see the software design part. The oxygen concentration value measured by the oxygen sensor is amplified and sent to channel 0 of ADC0809 for A/D conversion. The address of ADC0809 is 7FFFH. According to the control requirements of the oxygen machine, P1.0~P1.4 are selected as the control terminals of valves 1#~8#, and then the solid-state relay is isolated and driven to control the on and off of the corresponding valves. When P1.0~P1.4 outputs a low level, the corresponding solenoid valve is turned on, and the valve is disconnected when the level is high. P1.5 is the power indicator, P1.6 and P1.7 are alarm indicators (where P1.6 is connected to the green part of the two-color light-emitting diode, and P1.7 is connected to the red part). When the machine is just turned on, it emits a yellow light (both red and green are on) to indicate the standby state. When working, if the oxygen concentration is higher than the set value, it emits a green light to indicate normal operation. If the oxygen concentration is lower than the set value, it emits a red light to alarm.

    2.2 Control system software design

    The control system software mainly includes the main program, display subroutine, key processing subroutine, oxygen concentration detection and processing subroutine, alarm judgment and processing subroutine, timing interrupt and solenoid valve control program and other parts.

    The main program flow is shown in Figure 3. After the oxygen machine is powered on, the air compressor works and the system enters the standby state. Press the "Power On" button to enter the working state and start oxygen production. The lower limit of oxygen concentration can be set at any time during the working process. The oxygen concentration detection and processing subroutine completes the measurement and display of oxygen concentration. ADC0809 uses a delay method for conversion. Since the oxygen concentration measured by the CY-16 oxygen sensor is logarithmically related to the output current, it needs to be corrected by software and the display subroutine is called for display.

    The key processing subroutine completes the setting of oxygen concentration. When making oxygen, you can set a new concentration by pressing the key and end the setting by pressing the "Confirm" key.

    The alarm judgment and processing subroutine compares the measured oxygen concentration value with the lower limit of oxygen concentration set by the user to determine whether an alarm is needed. If an alarm is needed, the corresponding indicator light will be turned on.

    The time control principle is adopted in the timing interrupt and solenoid valve control program to control the on and off of the corresponding solenoid valve to ensure that the adsorption devices A and B work in turn. For this purpose, the timer interrupt is used to realize the cycle of the four working processes. When the timing time is reached, an interrupt request is generated. The working process is converted in the interrupt service program and the corresponding timing constant is set according to the control requirements.

    2.3 System Trial Production

    Usually, satisfactory results can be obtained with only 1 to 2 seconds of exhaust and pressure equalization time. Therefore, for the automatic control system, the key to trial production is to determine the optimal working time t of the adsorption device A (or B). t mainly affects the output oxygen flow rate Q and oxygen concentration. During the trial production process, the t value is changed, and the corresponding flow rate when the output oxygen concentration is 90% is measured through experiments, and the corresponding Qt relationship curve is obtained as shown in Figure 4. Therefore, the optimal working time of the adsorption device A (or B) of the system is 25 seconds. The entire system automatically cycles according to the process of "A works for 25 seconds → A exhausts for 2 seconds → B works for 25 seconds → B exhausts for 2 seconds". The indication of oxygen content is calibrated by standard instruments.

    3 Conclusion

    The trial-produced micro oxygen generator has an output flow rate of 5 L/min when the output oxygen concentration is 90%. The oxygen concentration can reach more than 95% when the output flow rate is small, which can meet the needs of 1 to 3 people at the same time. It has now been put into small-scale production by Suzhou Saipu Gas Co., Ltd. Its automatic control system uses single-chip microcomputer technology, which greatly reduces costs and improves operational reliability.

    References
[1] Pan Xinmin, Wang Yanfang. Practical System Design of Single-Chip Microcomputer [M]. Beijing: People's Posts and Telecommunications Press, 1992.
[2] Lu Yamin. Principle and Application of Single-Chip Microcomputer [M]. Beijing: China Light Industry Press, 2000.
[3] Feasibility Study on Production of Micro Oxygen Generator [S]. 2002.