1. Domestic situation
At present, based on domestic and foreign literature research, commonly used VOCs detection methods include gas chromatography detection technology, catalytic detection technology, spectral detection technology, and electrochemical sensor detection technology.
1) Gas chromatography detection technology: Gas chromatography is the detection of gas substances or substances that can be converted into gas at a certain temperature. Analysis first uses a chromatographic column to separate the components to be tested, and then the selected detector determines the name of the components based on the peak position, and determines the concentration size based on the peak area. Meteorological chromatography is relatively insensitive to changes in environmental conditions, has good stability, and is suitable for routine analysis of constant or trace amounts.
2) Catalytic detection technology: VOCs react with oxygen in the catalyst, causing the metal oxides in the catalyst to be reduced. Then, the reduced metal oxides are oxidized by oxygen in the gas phase, and sensors are used for sampling during the catalytic process.
3) Spectral detection technology: Spectral detection method is mainly an analytical method established based on the absorption, emission, Raman scattering, and other effects of light, which conducts qualitative and quantitative analysis through the wavelength and intensity of the spectrum. Spectral detection methods include three types: absorption spectroscopy, emission spectroscopy, and dispersion spectroscopy.
4) Electrochemical sensor detection technology: Gases generally have active chemical properties, characterized by reducing or oxidizing properties. In the process of chemical reactions, electrons are released or absorbed to form a weak current. By measuring the weak current, the concentration of the gas to be measured can be obtained. Its advantage is relatively stable performance, while its disadvantage is that electrochemical sensors are consumables with relatively short service life and relatively high maintenance costs.
Regardless of any of the above detection methods, the detection system will include electronic sensors or related amplification circuits for signal acquisition and processing. Due to the attenuation and temperature drift of electronic components over time, temperature, and humidity, in order to ensure the accuracy of the obtained data, it is necessary to regularly send engineers to the site for equipment calibration and consumables replacement. From the actual situation, this traditional operation method is inefficient, with high labor time costs and vehicle fuel costs. Based on the above practical situation, a remote control system has been designed to replace engineers in operating VOCs detection equipment to achieve the purpose of equipment calibration.
2. Development of Remote Control System
The working principle of the remote control system: The client APP simulates the keyboard operation interface of the actual device. Customers can select one of the keys to be pressed through the APP, and then transmit the instructions to the DTU through the cloud platform; The DTU and panel actuator communicate through wireless USART, and the communication protocol meets the Modbus RTU protocol specification. When the panel executing mechanism obtains the command to press one of the buttons, the MCU first searches for the coordinates of the button; This mainly includes the walking steps of the X-axis and Y-axis stepper motors. Then, the MCU calibrates the coordinates through algorithms and drives the X-axis and Y-axis motors to rotate the same step distance. Finally, the MCU drives the Z-axis motor to perform relevant actions to simulate the process of pressing the button.
2.1 Hardware design of panel actuator
According to the Functional requirement, the actuator hardware mainly includes communication module, power management module, motor drive module and MCU minimum system module. These modules are described in detail below.
1) MCU minimum system module: This control system selects STM32F103RBT6 as the main control chip, with a main frequency of 72 MHz, RAM of 20 kB, and FLASH of 128 kB; Its peripherals include 2-way USART, 1-way CAN, and 51way IO; Due to the lack of complex mathematical algorithms in the panel execution mechanism program, its performance fully meets development requirements. When designing the minimum MCU system, first connect the corresponding power pins of the MCU to the 3.3 V power supply and GND; Then connect PIN5 and PIN6 to an 8 MHz crystal oscillator and a 22 pF filter capacitor; Simultaneously pull down the PIN60 (BOOT0) pin, indicating that after resetting the microcontroller, the program will start from Flash; Finally, extend the download port pins (JTCK, JTMS, RESET) for program download. At the same time, LED circuits and buzzer circuits were designed in the minimum system module to represent the operational faults of the actuator.
2) Power management module: There are three types of internal power supply voltage for panel actuators: 12, 5, and 3.3 V. 12 V mainly supplies power to the stepper motor to convert it into mechanical energy; 5 V is mainly used to supply power to driver chips and communication related chips; 3.3 V is mainly used to power the microcontroller. For 12 V power supply, we directly choose AC-DC power supply products (AC220 V input, DC12 V output). When the 12 V power supply is connected to the circuit board, first connect a slow break fuse: when the load or motor is short circuited, the power input can be quickly cut off; Then connect one NTC to prevent the driver module from aging too quickly due to excessive load in the future. The 12 V power supply is converted into a 5 V power output after passing through the LM2596SX-5.0 chip; The maximum output current of LM2596SX-5.0 is 3 A, fully meeting the load requirements of the circuit board. The 5 V power supply is converted into 3.3 V output through the AMS117-3.3 chip, mainly supplying power to the smallest system module.
3) Communication module: The panel actuator and DTU communicate through a wireless USART module, and the communication protocol meets the Modbus RTU specification. At the same time, we have extended one CAN channel for online upgrade of the execution mechanism program, and extended two USART channels for offline data upload and debugging with the computer. Select the MAX232D chip here to convert the TTL level into an RS232 signal; Select the TLE8250 chip to convert the TTL level into an RS485 signal.
4) Motor drive module: The panel execution mechanism mainly uses three stepper motors on the X, Y, and Z axes to replace human hands to operate the equipment panel. The reason for choosing a stepper motor is mainly because it has the advantages of simple driving and controllable accuracy. First of all, we select two SN74LVC4245DW level conversion chips for the conversion of 3.3V and 5V level signals. After the level conversion, it is connected to the A4988 module to control the stepper motor. Here, instead of selecting the H-bridge circuit, we choose the integrated IC to drive the motor, mainly because the integrated IC has the MOS overheating shutdown function, bus Undervoltage-lockout, load short circuit protection and other functions, which can protect the motor in a timely and effective manner in case of motor failure.
2.2 Panel actuator software design
This design uses the A4988 integrated module to drive the stepper motor. The microcontroller only needs two IO ports to control the driving direction and pulse of the motor; The software design of the executing mechanism is divided into bottom driver design and application layer software design. The bottom driver design is based on C language programming code, which mainly includes 2-way USART driver configuration, 1-way CAN configuration, and 3-way A4988 driver module IO port configuration. The application layer mainly includes LED and buzzer fault alarm processing, Modbus RTU protocol analysis, and stepper motor drive control; The application layer adopts the model based design (MBD) development mode, and we write M Scripting language to achieve:
1) One click import of model data parameters into Simulink engineering;
2) Define and import relevant environmental parameters;
3) Generative model C code;
4) Generate Keil engineering related interface codes;
5) Copy the generated. h and. c files to the designated folder of the Keil project;
6) Compile the Keil project and generate relevant Hex files.
2.3 Forming physical debugging
Firstly, we install the panel actuator onto the device panel and communicate with the DTU through a wireless serial port module. Then we configure the DTU to connect to the designated Wi Fi through the relevant client upper computer, ensuring that the DTU can be connected to the remote server; Finally, we operate the corresponding virtual buttons through the client app to remotely control the pressing of the corresponding buttons on the VOCs device panel. The experimental results show that we can freely manipulate any button to perform corresponding actions through the client app.
3 Conclusion
The system can realize remote control of VOCs detection equipment. Engineers can calibrate and maintain the equipment remotely, reduce the attendance rate of operation and maintenance personnel, save labor and fuel costs for the company, and improve the operation and maintenance efficiency. This system has undergone durability testing at the relevant air super workstation and is currently working normally. During the debugging process, we also found other issues: when the Ethernet network is poor, there will be a significant delay from the client to the panel execution mechanism, which reduces the experience; When the Ethernet network is interrupted due to other reasons, it is impossible to remotely control the VOCs detection equipment. We will continue to optimize these issues in the future.