Design of remote monitoring system for small sewage treatment plants

With the continuous increase of the country's investment in environmental protection and the deepening of the reform of the sewage treatment industry, the operation and management of urban sewage treatment plants are developing towards marketization, specialization, scale and groupization. Many professional operating companies have to operate and manage multiple sewage treatment plants in the same city. Therefore, remote monitoring of the production of sewage treatment plants is of great significance to reducing operating costs, strengthening production management, ensuring the efficient operation of sewage treatment processes and meeting water quality standards, and improving the efficiency of enterprise operation and management. In this paper, a sewage treatment remote monitoring system based on ARM is designed for small sewage treatment plants in combination with GPRS technology.

1 Overall design of monitoring system
According to the performance requirements of sewage treatment equipment, the monitoring point system consists of data acquisition system, image acquisition system and wireless communication network. The data acquisition system consists of analog sensors, transmitters, A/D converters and ARM systems. Each analog sensor sends analog signals to the signal transmitter to amplify them into standard signals, and then sends them to the AD/converter one by one to convert the analog signals into digital signals. The ARM processor then collects data. The image acquisition part consists of a serial camera and an ARM controller. The camera first collects the scene picture and then transmits it to the ARM controller through the serial port. The ARM controller transmits the collected data and images to the monitoring center through the GPRS wireless communication network. The overall structure of the system is shown in Figure 1.

Use AT commands to dial through GPRS Modem. After the correct answer, a physical channel, that is, the GPRS channel, is established between the GPRS Modem and the GPRS network in the system. The PPP protocol transforms the original GPRS physical layer connection into an error-free data link, and the system will remotely log in to the Internet. And get the IP address assigned by the GPRS gateway. The IP protocol is used as the network layer protocol. The IP protocol connects all terminals with different IP addresses connected to the Internet. After IP routing selection, the system can interact with any IP terminal connected to the Internet. TCP is selected as the transport layer protocol to provide connection-oriented, end-to-end reliable services for data transmission. The data packet is sent to the monitoring center server port with a fixed IP address through the wireless network so that the monitoring center can monitor the parameters of the sewage in real time. The monitoring center processes, displays, stores, and prints the collected data.
The following article briefly introduces the hardware design and software parts of the monitoring terminal.
2 System hardware design
According to the performance requirements of the sewage treatment device, the system of the monitoring point consists of analog sensors, transmitters, analog-to-digital (A/D) converters, and ARM processors. Each analog sensor sends the analog signal to the signal transmitter to amplify it into a standard signal, and then the multiplexer sends it to the A/D converter one by one to convert the analog signal into a digital signal. The ARM processor then collects, processes and displays the data. At the same time, the ARM processing system sends images and data to the remote monitoring center via the GPRS network. The system hardware block diagram is shown in Figure 2.

The system terminal uses LPC2148 as the core to control the communication module TR800. The ARM CPU uses Philips' LPC2148, and the core is a general-purpose 32-bit high-speed microprocessor based on the reduced instruction set of ARM7DMI-S. The operating voltage is 3.3 V, and the core operating voltage is 1.8 V; the built-in PLL phase-locked loop can set the CPU operating frequency to 60 MHz, and the dual UART ports provide asynchronous serial transmission and reception of data: 16 kB RAM and 128 kB Flash program memory on the chip. LPC2148 provides 2 serial ports, and a total of 6 I/O ports are available. Its interface circuit part includes 8 analog input channels and RS232 serial port communication interface, among which the digital-to-analog conversion is used to control the built-in module with an accuracy of 12 bits, which can basically meet the requirements of general control objects. The terminal application is compiled with IAR on the PC and downloaded to FLASH through the JTAG interface. The commonly used parameters of the system are placed in the information memory of the main controller through the program settings. [page]

This system uses the GXT-M201 serial camera module to complete the acquisition and compression of on-site images. The production process of GXT-M201 is very strict, it can maintain normal and stable operation in a variety of environments, and its performance is quite reliable. It has been widely used in wireless image transmission, industrial image acquisition monitoring, and civil monitoring equipment. From the perspective of usage function, GXT-M201 is an image acquisition module that can integrate video acquisition and image compression. It controls the capture and acquisition of images through relatively simple serial port instructions, and completes the JPEG format compression of the acquired images, and then outputs them through the serial port. Therefore, the module and the microcontroller transmit data and communicate completely through the serial port. Its cost and power consumption are relatively low, the image resolution obtained is also relatively high, and it supports variable JPEG encoding quality settings with different resolutions. The internal components of GXT-M201 mainly include the CMOS photosensitive chip OV76 40 produced by Omni Vision, memory, serial interface, and hardware JPEG compression chip OV528. The core component of the entire GXT-M201 is the embedded JPEG compression control chip OV528, which is mainly responsible for compressing the image data captured by OV7640 and transmitting it to external devices. The relatively high compression rate and good image quality are all achieved by the compression chip OV528. For EEP ROM, 11 fixed-length 6-byte user command interfaces can be provided. Users can use these commands to set a variety of different attributes, such as the color mode of the image, the size of the image, the size of the image data packet, whether to compress the image into JPEG, static mode or preview mode, and the baud rate of the serial port. It also has an error return command that can detect up to 20 types of errors. The GXT-M201 module can obtain a maximum of 300,000-pixel JPEG compressed images, which can fully meet the image resolution requirements of this system.
The GPRS module uses the TR800 of iwow. The TR800 module provides a complete GPRS and GSM wireless connection port, and can be directly used as a terminal product to complete the transmission of images and data. This is because its integrated RF circuit and GPRS baseband processor are very complete. The GSM baseband processor is the core component, which is equivalent to a protocol processor. It is used to process AT commands sent by the external system through the serial port. The RF antenna part mainly realizes the modulation and demodulation of the signal and the signal conversion between the external RF signal and the internal baseband processor. The matching power supply provides the required power for the processor baseband RF part. The GPRS module provides a standard TTL level interface, which is connected to the UART1 port of LPC2148 through the TTL level for full-duplex communication. The operating voltage of TR800 is 3.4~5.5 V. After the module is powered on, a low level of at least 30 ms is given to the module RESET pin or PWON pin (the former is recommended), and the module is started.

3 System software design
The program of this system is mainly the single-chip microcomputer control program. The main function is to cooperate with the hardware function of the system to ensure the stable and reliable operation of the entire system and meet the design requirements. The software structure of the system is shown in Table 1.

The main program is responsible for the entire process from power-on to the start of normal operation of the system. The task of this process is to initialize each submodule, including the initialization of the system clock, serial port initialization, and input and output port initialization. The system initialization subroutine set is composed of these subroutines. The main program flow chart of the microcontroller of the monitoring terminal is shown in Figure 3.

For the monitoring terminal module, the system clock of the main controller LPC2148 uses an 8M high-speed clock source. The function of input and output port initialization is to initialize the input and output modules, clear the flag bit and enable the interrupt. For the serial port USART0 receiving terminal of LPC2148 responsible for communicating with the camera module, its transmission baud rate should be set to 38400 bps during the serial port initialization process, and the serial port receiving interrupt should be enabled. The serial port USART1 is responsible for the communication between the receiving terminal and the GPRS module, and the baud rate is set to 9600 bps. Use the command AT+IPR to change the baud rate.
TCP/IP adopts a protocol layered structure: application layer, transport layer, network layer and link layer. Since the system transmits sewage parameters and images, the TCP/IP protocol can be greatly reduced. The application layer only needs to use the HTYP protocol. Using the TCP transmission control protocol in the transport layer helps to improve the reliability of data communication. The network layer needs to use IP, APP/RARP, and ICMP. The role of IP is to provide highly reliable and connectionless data packet transmission services. The ARP protocol completes the mapping of IP addresses and physical addresses. ICMP is used to monitor network communication conditions. Since wireless communication is used, it is easy to be disconnected, so GPRS sends ICMP echo requests through the timing monitoring center and monitors in real time. Once an abnormality occurs, the GPRS module automatically re-establishes the link to keep the system online in real time. The link layer uses the IP protocol supported by GPRS. As long as the socket functions 80(2ket, connect, close, bind, listen, accept, send, etc. provided by TCP/UDP are used, the TCP connection between the client and the server can be realized. By reducing the TCP/IP protocol, the functional needs can be met and the memory resources of ARM can be saved.

4
Conclusion The wireless monitoring system designed and implemented in this paper for small sewage treatment plants uses a mature third-party network to build a remote wireless monitoring communication network for real-time data, which is a new idea for wireless monitoring systems. The remote wireless monitoring system based on the GPRS network is an important attempt at enterprise informatization in the information society. Starting from the needs of small water plants, this paper designs a remote monitoring system for small water plants based on the GPRS network, combining factors such as cost, function, and transmission speed.
A useful attempt has been made on the basic theories and basic methods of system implementation. Due to the limitations of level and time, there are still many areas that need to be improved in the software and hardware of the real-time wireless monitoring system. Further research can be carried out from the following aspects: the main station software of the system can be further developed to display the collected data in real time in the form of graphics or curves, and multi-threading technology can be used to develop multi-terminal main station software.
In short, the wireless monitoring system plays a huge role in the informatization of social life and industrial automation. How to make good use of the third-party network is an important task in the development of wireless monitoring. Moreover, with the construction and use of 3G networks, the demand for application development research in this area will flourish.