Design of carbon monoxide monitoring alarm based on single chip microcomputer

Introduction

With the rapid development of industry, there are more and more occasions where humans are exposed to harmful gases, which causes more and more harm to humans. In steel mills and iron mills, blast furnace gas (mainly CO), a byproduct of blast furnace production, is sent to the gas pipeline network or to the gas tank for storage after cooling, dust removal and pressurization. Workers in the workshops near the blast furnace or nearby and workers in the pressurization station often suffer from carbon monoxide poisoning, which is very harmful; and during the maintenance of gas pipelines and storage cabinets, leaked carbon monoxide gas often explodes when it encounters open flames. Therefore, the development of a carbon monoxide gas monitor has very important practical significance. This article uses AD's data acquisition chip ADuC834 as the main control chip, and introduces a carbon monoxide monitoring alarm with high sensitivity, reliable performance and simple operation.

1 System Hardware Composition

1.1 System Block Diagram

The hardware block diagram of the entire carbon monoxide monitoring alarm is shown in Figure 1.

The working principle of this system is as follows: first, the carbon monoxide data information on the scene is collected through the carbon monoxide sensor. The sensor selected is the electrochemical carbon monoxide gas sensor 7E/F from CityTechnology Ltd. in the UK; when the carbon monoxide gas diffuses through the pores on the sensor shell to the surface of the working electrode, a chemical change occurs, and the working electrode outputs a changing current, the current size of which is proportional to the gas concentration. The current signal then outputs a 0-2.5 V voltage signal through the operational amplifier circuit. This voltage signal is linearly related to the gas concentration. The ADuC834 microcontroller can obtain the carbon monoxide gas concentration value by collecting the voltage signal through its own integrated 24-bit A/D converter, and then use the LCD to display the current carbon monoxide concentration on the scene. In addition, the primary and secondary gas concentration alarm points of carbon monoxide can be set by pressing buttons. When the carbon monoxide concentration is greater than a certain alarm point, there are three alarm modes: sound, light, and vibration.

1.2 Introduction to ADuC834 MCU

The ADuC834 microcontroller integrates two independent ∑-△ ADCs, of which the main channel ADC is 24 bits and the auxiliary channel ADC is 16 bits. The main channel AD input range is ±20 mV to ±2.56 V, divided into 8 levels, and any level can be selected when used. Due to the use of ∑-△ conversion technology, up to 24 bits of no missing code performance can be achieved; in addition to the basic A/D conversion function, the auxiliary channel can also be used as the input interface of the internal temperature sensor.

The ADuC834 microcontroller uses a 32 kHz crystal to drive the on-chip phase-locked loop (PLL) and generates the required internal operating frequency through the setting of internal registers. Its microcontroller core is compatible with 8051. The on-chip peripherals include a serial port compatible with SPI and I2C, multiple digital input/output ports, watchdog timer, power monitor and time interval counter. The chip also provides 62 KB flash/electrically erasable program memory and 2 304 B on-chip RAM.

The ADuC834 microcontroller is provided with a boot program by the manufacturer, so the user program code can be easily loaded into the ADuC834 microcontroller through the standard UART serial interface, which is very convenient for program development and design.

2 Software Programming

2.1 Data Collection Procedure

When collecting data, the external reference voltage Vref of the ADuC834 microcontroller is set to 2.5 V, and different input ranges are set through the RN2, RN1, and RN0 bits of the AD0CON register to sample the input signal of the main channel.

2.2 Programming of User Flash/Electrically Erasable Data Registers

The ADuC834 microcontroller provides developers with a 4 KB EEPROM flash memory that can be used to save system configuration information when power is lost.

3 Instrument calibration and inspection

3.1 Instrument calibration

Since the linearity of the sensor itself is relatively good, the instrument is calibrated using a two-point calibration method. First, place the instrument in pure air. After the displayed data stabilizes, use this point as the first point, i.e., the zero point, and adjust the LCD indication to zero; then adjust the gas flow rate of the standard gas cylinder to 200 m1/min, and keep the gas flow steadily through the sensor for about 1 minute; after the display reading stabilizes, use the concentration of this point as the second point and adjust the LCD indication to be consistent with the gas concentration value of the standard gas cylinder, then close the gas cylinder, thus completing the entire calibration process.

3.2 Test results

3.2.1 Indication error

After various standard concentrations of gas pass through the sensor at a flow rate of 200 ml/min, the measurement results are recorded after a delay of 35 seconds, and the indication error is calculated according to the formula:

In the formula: △e is the repeatability error; A is the arithmetic mean of the readings; As is the standard gas concentration value; R is the range.

The indication errors at different concentrations were calculated and shown in Table 1.

Through the analysis of the above test data, it can be seen that the indication error of the carbon monoxide monitoring alarm is less than ±3% FS, which meets the requirements of the indication error verification regulations.

3.2.2 Repeatability error

After the same concentration standard gas passes through the sensor multiple times at a flow rate of 200 m1/min, the measurement results are recorded after a delay of 35 s, and the repeatability error is calculated according to the formula:

Where: Sr is the repeatability error; A is the arithmetic mean of the readings; Ai is the instrument reading value; n is the number of measurements, n=6.

The calculated repeatability error is shown in Table 2.

From the above test data, it can be seen that the repeatability error of the carbon monoxide monitoring alarm is less than ±2%, which meets the requirements of the repeatability error verification procedure.

4 Conclusion

The carbon monoxide monitoring alarm introduced in this article has the characteristics of simple hardware structure, small size, simple software programming and short development time. In addition, the carbon monoxide alarm designed by the author has been actually used in steel mills and iron mills, and has achieved good practical results.