Design of Engine Exhaust Parameters Acquisition System Based on Single Chip Microcomputer

0 Introduction
In order to meet the increasingly stringent emission regulations, the exhaust temperature, pressure and other parameters of the engine need to be tested when the vehicle leaves the factory or is tested. The exhaust parameters are also an important indicator of whether the engine is operating normally, so it is very necessary to regularly test the exhaust temperature of diesel locomotives. This paper develops a data acquisition system to automatically complete the collection and storage of exhaust parameters, and collects and stores the engine exhaust temperature and pressure parameters. Due to the different performance and the quality of the diesel used, the exhaust temperature of different models of diesel engines is also different. Therefore, we use the host computer to store the exhaust temperature parameters of different models of diesel engines during normal operation, and compare the real-time collected temperature with the stored temperature. When there is a large difference, the engine performance needs to be inspected.

1 System Design
This system consists of two parts: the host computer and the slave computer. The slave computer collects the exhaust temperature and pressure in real time and transmits the collected data to the host computer. The host computer is programmed with Visual Basic6.0 to display the temperature data transmitted by the slave computer, draw a temperature curve, and compare it with the normal operating temperature of the diesel locomotive stored in the host computer. The observer can judge whether the engine is operating normally based on the difference between the two curves. Figure 1 is the schematic diagram of this system. The single-chip K-type thermocouple amplifier with cold-end compensation and the digital converter MAX6675 are used for temperature collection. The VESX-100 pressure sensor adopts a three-wire system, 10~32 V power supply, and the output signal is 1~5 V. It can be directly connected to the AD converter. The single-chip microcomputer processes the digital quantity after A/D conversion and transmits it to the host computer.

2 Lower computer design
2.1 Lower computer hardware design
In order to accurately measure the pressure, the pressure sensor VESX-100 with high linearity was selected in the bus exhaust parameter collection experiment. The VESX-100 pressure sensor adopts a three-wire system, 10~32 V power supply, and an output signal of 1~5 V, which can be directly connected to the AD converter. In order to accurately measure the temperature, the temperature sensor used in this experiment is a high-sensitivity K-type thermocouple with a diameter of 1 mm. After selecting the sensor, the diesel vehicle exhaust temperature data acquisition system is composed of the STC89C52RC microcontroller and the 12-bit serial control AD ​​converter TLC2543, the single-chip K-type thermocouple amplifier and digital converter MAX6675, and the 512 kbit serial EEPROM memory AT24C512 and MAX232. The STC89C52RC microcontroller selected in this system is one of the STC series microcontrollers. The STC microcontroller has strong anti-interference ability and low power consumption, and is suitable for use in relatively harsh occasions. TLC2543 is a 12-bit serial controlled AD converter with internal sampling and holding. Using TLC2543 in the design can not only save the I/O resources of the single-chip microcomputer, but also has a moderate price and high resolution. MAX6675 is a complex single-chip K-type thermocouple digital converter, which has a signal conditioning amplifier, a 12-bit analog/digital thermocouple converter, cold end compensation sensor and correction, a digital controller, an SPI compatible interface and an associated logic control. The single-chip microcomputer can read the temperature value of the K-type thermocouple measured by MAX-6675. This experiment uses MAX6675 to save complex linearization circuit processing and has high temperature conversion accuracy. AT24C512 is a 64 kbyte serial electrically erasable programmable memory produced by Atmel. The data is not lost when the power is off. It is suitable for data acquisition systems with large-capacity data storage requirements. This experiment uses AT24C512 as the memory. Almost all PCs have RS-232-C interfaces. Therefore, it is most convenient for the microcontroller to communicate with the PC through the RS-232-C port. This experiment uses MAX232 to achieve the conversion between logic level and RS-232-C level. [page]

The lower computer circuit is shown in Figure 2.

2.2 Lower computer software design
The lower computer software is written in assembly language. The main parts of the program are as follows:

3 Host Computer
Visual Basic 6.0 is a powerful and easy-to-learn programming language. It not only retains all the functions of the original Basic language, but also adds object-oriented programming functions. It can not only easily and quickly compile programs suitable for data processing, multimedia, etc., but also use the ActiveX control MSComm to easily develop computer communication programs using the computer serial port. This article uses the MSComm control of VB to realize serial port communication with the lower computer and read the pressure and temperature data collected by the lower computer. The specific operation process is to declare a dynamic array, and the Input property of the MSComm control is directly assigned to the GetByte array. The purpose of the following LBound and UBound functions is to obtain the minimum index value and maximum index value of the GetByte array. With the maximum and minimum index values, the content can be transferred out in a customized way of the program for other operations, thus completing the program for receiving binary data. The received data is stored in a text file named PT.txt in the root directory of the C drive in a certain order to complete the data saving work. The upper computer software process is shown in Figure 4.

The upper computer part of the program is as follows:

4 Conclusion
The engine exhaust parameter acquisition system based on STC89C52RC single chip microcomputer can be successfully used in the detection test of engine exhaust parameters. This system has low cost, reliable operation and good application value.