Abstract: This paper introduces an automatic control device configured for a typical sewage treatment system in a residential community. It consists of a touch screen and a PLC, replacing the traditional relay contactor control device. It features simplicity, convenience, and reliability. Based on a brief description of a small-scale sewage treatment system, the paper discusses the structure and working principle of this automatic control device. It focuses on the screen creation function, application, and design considerations of the F940WGOT touch screen. Keywords: Sewage treatment control device, touch screen, PLC With economic development and population growth, the demand for water is increasing, while wastewater discharged from human production and daily life is causing increasingly serious pollution to the water environment, making the already limited water resources even more strained. Water scarcity has constrained socio-economic development. To reduce environmental pollution, conserve water, and make full use of limited water resources, many cities in China have begun to construct urban residential sewage treatment facilities. Wastewater from washing, washing, kitchens, and toilets is treated and then used for miscellaneous water supply, such as urban greening, car washing, cleaning, landscaping, and fire fighting. This type of small-scale water treatment system is being increasingly widely adopted. This article introduces the control device配套 (matched) with this type of sewage treatment facility in urban residential communities. This device is characterized by reliability, advanced technology, and simplicity. I. Process Flow of a Small-Scale Sewage Treatment System Figure 1 shows the process flow diagram of a typical residential community sewage treatment system. [align=center]Figure 1[/align] It consists of a screen, equalization tank, biological treatment tank, sedimentation and filtration tank, and disinfection tank. The functions of each part are as follows: 1. Screen: Sewage generally contains a large amount of impurities and floating matter. The screen is mainly used to remove larger debris and floating matter from the sewage. When the sewage reaches a certain level, the system requires the screen to be activated to automatically remove these impurities. 2. Equalization Tank: Because the sewage inflow and water quality are usually unstable, an equalization tank is set up to buffer the process and ensure the stability and efficiency of the subsequent water treatment process. The inlet valve is activated when the liquid level in the equalization tank is low, and closed when the liquid level is high, so that the tank always maintains a certain water volume. 3. Biological Treatment Tank: This is the most important part of the entire wastewater treatment process. It uses the SBR (Sequencing Batch Reactor) process, employing oxidation and aeration to separate water and sludge in the wastewater and remove small particulate impurities. The start/stop of the water pump, air compressor, and sludge return pump is controlled based on the wastewater level in the tank. When the biological treatment tank has a low level, the inlet pump is activated, and related equipment is timed. Several drain valves are sequentially opened and closed based on the corresponding water level. The air intake solenoid valve can also be controlled based on the data displayed by the electromagnetic flow meter (gas) to control the air intake flow. 4. Sedimentation and Filtration Tank: Wastewater treated in the biological treatment tank still contains a certain amount of suspended solids and a small amount of impurities. The function of the sedimentation and filtration tank is to remove suspended solids and impurities, ensuring that the effluent quality meets certain discharge standards. During the sedimentation and filtration process, the filtration pump stops when the water level in the tank falls below the specified height. 5. Disinfection Tank: The main function of the disinfection tank is to kill bacteria and viruses in the water by adding a certain amount of acid and alkali. The frequency converter is controlled based on the pH, ammonia nitrogen, dissolved oxygen, and overall influent flow rate displayed by online analytical instruments, thereby controlling the speed of the acid and alkali dosing pumps to control the dosage. When water quality changes reach their limits and exceed the frequency converter's adjustment range, the concentration of the chemicals can be manually adjusted to change the dosage. II. Composition of the Control Device Currently, large-scale wastewater treatment control systems in China mainly consist of a two-level distributed monitoring system composed of a central management unit and field PLC control units. For small-scale wastewater treatment systems, such as residential wastewater treatment, traditional relay control devices are still mostly used. Control is achieved through buttons, switches, and indicator lights on the control panel, and some use PCs for screen monitoring. These relay control devices have complex wiring, large size, poor reliability, and are inconvenient to operate, install, and maintain, hindering their widespread application. This wastewater treatment control device uses a touch screen and PLC, employing a Mitsubishi FX1N-60MR PLC and an F940WGOT-TWD-C touch screen. The FX1N-60MR is a 60-point (36 inputs, 24 outputs) programmable controller. The F940WGOT touchscreen boasts comprehensive display functions; it features a 256-color TFT high-definition LCD display; and three communication interfaces for communication with a PC or printer (COM2: RS-232C) and PLC (COM1: RS-232C, COM0: RS-422). The device's structural block diagram is shown in Figure 2. Except for "emergency stop," all operations and monitoring are performed on the touchscreen. Depending on the water treatment process requirements, it can operate in three modes: single-step, running, and maintenance. This device can monitor parameters such as liquid level, flow rate, and pH value online. Furthermore, according to the water treatment process requirements, it achieves closed-loop regulation of these parameters through the control of pumps, valves, and other drive components (motors), completing the automatic control function of the entire wastewater treatment system. Replacing the traditional operating console, PC, or industrial control computer with a touchscreen (GOT), the touchscreen serves as a human-machine interface, allowing on-site operators to monitor, manage, and operate the entire wastewater treatment system in real time through system displays, user-created process flow diagrams, touch keys, and display commands. [align=center]Figure 2[/align] Simultaneously, this water treatment system is connected to the upper-level network as an independent component, enabling remote monitoring on the host computer within the network. Compared with traditional relay control devices, this control device has advantages such as high reliability, strong anti-interference capability, simple and flexible programming, short design and debugging cycle, and ease of operation and maintenance. III. The F940WGOT touchscreen GOT has rich screen functions and is simple to create. It includes two main parts: the system screen and the user-created screen. The main functions are as follows: 1. System screen: The system screen is the original GOT screen, which mainly includes monitoring functions, data sampling functions, alarm functions, and output printing functions. l Monitoring Function: Allows reading, writing, inserting, and deleting PLC programs in command list mode. It also monitors changes to the ON/OFF status of various soft components of the programmable controller, as well as the set or current values ​​of timers, counters, and data registers. Element numbers can be selected via touch keys for on-screen display. Specific PLC bit elements can be forcibly ON/OFF. The monitoring function also includes a PC diagnostic function, displaying I/O errors, PC hardware errors, and communication errors of the programmable controller. l Data Sampling Function: Allows specifying the data register to be sampled, the sampling time (trigger condition or specific period), and the start and end times of sampling. The sampling time (hours, minutes, seconds) and data are displayed on the touchscreen in list or chart format. Sampling data can also be displayed on user-created screens. l Alarm Function: This function simplifies fault diagnosis for the entire system. It allows up to 256 consecutive bit elements of the programmable controller to correspond to alarm information. Upper and lower limits for relevant data, such as water level and flow rate, can be preset. When these limits are exceeded, a preset alarm sound or related screen will automatically alert the operator. 1. Output Printing Function: Sampled data, alarm records, and other data can be printed, read, and saved to a floppy disk, and can be converted into text (.TXT) format for easy editing in commonly used software such as Excel and Word. 2. User-Created Screens: User-created screens are a series of screens created by users using screen creation software (SWOPC-FXDU/WIN-C) according to process and site requirements. As long as the same screen creation software is used on the computers in the upper-level network, the screens on the touchscreen can be displayed, allowing the screen resources of the field GOT to be used by personal computers at different levels. Therefore, when the entire process is connected to the upper-level network, the computer operation screens in the upper-level network do not need to be redesigned and can be displayed and operated synchronously with the field. This feature not only maintains operational consistency but also greatly shortens the design process. User-created screens mainly have the following functions: l Screen Protection Function: User-created protected screens can set login passwords and different protection levels, ensuring that operators, programmers, and staff can only operate and modify screens within their authorized scope, guaranteeing the security of the system screens. l Display Function: Users can create up to 500 display screens according to process requirements; several screens can be displayed simultaneously on GOT; and screens can be freely switched between according to preset switching conditions and time. This system has created over a dozen screens, displaying the power-on screen, login screen, operating condition selection screen, operating screens for each pool under various operating conditions, and alarm screens displaying important parameters, etc. Figure 3 shows the first three screens. In addition to calling graphics from the software's graphics library, bitmap files can also be imported from external files when creating screens. [align=center] Figure 3[/align] l Monitoring Function: User-created monitoring screens monitor and display the set or current values ​​of PLC word elements through numerical values ​​or (circular or bar) graphs. The proportion of the current values ​​of word elements is displayed by drawing (bar or circular) statistical graphs, such as displaying the current water level proportion in each pool. l Data Change Function: On the touchscreen screen, the data on the monitored values ​​or graphs can be changed using touch keys. For example, during debugging, the timing constant of the PLC timer can be changed, thereby altering the running time of the motor or pump. l Switch Function: Setting the touch keys on the display screen as a switch function allows you to ON/OFF the programmable controller's bit elements. This device uses this method to control the opening and closing of various pumps and valves in the field. This simplifies the device's hardware and makes operation more convenient and clear. 3. Design Considerations: l Common Setting vs. Individual Setting: When switching screens via touch keys, pay attention to the difference between the two and apply them appropriately. For example, if several screens share a key to return to the "menu screen," use the common setting for that key. When the screen switching is set to a common setting for the entire screen, switching will return to the menu screen regardless of which display screen is on. When the screen switching is set to an individual setting, it will switch to the corresponding screen based on the switching conditions. l Communication Data Format: When specifying the display of Chinese characters and letters through the programmable controller's word elements (T, C, D, V, Z), pay attention to the storage format of the word elements. Example: When 'AB' is specified using D0 (A=41H, B=42H), the storage state of AB in D0 is: B in the high four bits, A in the low four bits, as shown in Figure 4a; when the text '科' (89C8H) is specified using D0, the storage state is that the low four bits of '科' are stored in the high four bits of D0, and the high four bits of '科' are stored in the low four bits of D0, as shown in Figure 4b; when 'A科B' is specified using D0 and D1, the storage state in D0 and D1 is shown in Figure 4c. [align=center] Figure 4[/align] l Using screen storage to accurately reflect the required screen: For example, screen 1 and screen 2 can both switch to screen 3, as shown in Figure 5. If the screen number stored before the switch is "1", then screen 3 can return to screen 1 but not screen 2. Similarly, if the stored screen number is set to 2, then screen 2 can be returned to. Therefore, when designing screens, it is essential to pay attention to the relationship between the preceding and following screens and the switching conditions, and to store the corresponding screen numbers. [align=center]Figure 5[/align] IV. Software Design This wastewater treatment system has three operating modes: running, single-step, and maintenance. Running refers to the normal operating mode; single-step is mainly used for debugging and testing equipment; and maintenance is the operating mode when any equipment malfunctions on-site. Depending on the operating mode, the corresponding screen is selected on the touchscreen, and the PLC controls the corresponding equipment according to the program using the keys on the screen. Therefore, the PLC also has three workflows. Figure 6 shows the flowchart for the running mode. (The other two flowcharts are omitted) [align=center]Figure 6 Operating Condition Flowchart[/align] References 1. F940GOT Operation Manual, Mitsubishi Corporation 2. Cheng Jianfang, Chen Hanmin. Urban Sewage Treatment Control System Automation Instrumentation, 2000, Vol. 21, No. 7: 33-35 3. Wang Xiaocheng, Xiao Zhihuai. Application of PLC in Primary Sewage Treatment Plant, Microcomputer Information, 2001, Vol. 17, No. 1: 11-12 4. Jin Xuhua. Application of Programmable Controller in Small Sewage Treatment Station, Electronic Components, 2000, Vol. 23, No. 3: 224-226