introduction
Currently, Programmable Logic Controllers (PLCs) have become one of the most important automation devices in the industrial control field due to their powerful functions, programmability, and intelligence. Mitsubishi PLCs are the main means of implementing current electrical programmable control technology. Replacing traditional relay control methods with Mitsubishi PLC control systems simplifies wiring, facilitates debugging, and improves system reliability.
Mitsubishi touchscreens are high-tech human-machine interface products designed specifically for Mitsubishi PLC applications. Due to their advantages such as ease of operation, beautiful interface, space-saving control panel, high cost performance, and good human-machine interaction, they have been increasingly used in industrial control and other fields in recent years.
This paper presents a water level sensor testing system developed using Mitsubishi PLC and touch screen technology. This system is mainly used for quality testing of water level sensors in washing machines. The entire system is simple to implement, has good stability, and a high degree of automation. It replaces the previous purely manual operation, better meets the requirements of actual production, and improves production efficiency.
System control principles and requirements
The working principle of a water level sensor in a washing machine is to convert changes in water level height into changes in pressure on the diaphragm inside the sensor, which in turn causes a change in the sensor's output inductance L. By combining the output inductance of the water level sensor with an external circuit to form an LC oscillation circuit, the change in inductance can be converted into a change in oscillation frequency. Different water levels can generate different oscillation frequencies through the water level sensor. Finally, by detecting the correspondence between the oscillation frequency and the water level height, the quality of the water level sensor can be detected.
Inputs X0 to X1 are the A and B phase output pulse signals of the encoder, X3 is the oscillation frequency signal, and X4 to X14 are the signals for buttons, selector switches, limit switches, and count start, respectively. Outputs Y0 to Y7 control relays, signal indicator lights, etc.
Water level measurement is primarily accomplished using an encoder. The encoder's A and B phases send pulses to the high-speed counting terminal of the programmable logic controller (PLC), and the high-speed counter C251 obtains the count value of these pulses. As the motor rotates, the high-speed counter continuously increments. Through a well-designed transmission mechanism, each pulse corresponds to a 0.25mm change in water level. By programming, the actual water level change can be calculated.
The frequency of the oscillation signal can be measured using the high-speed counter C253 of the Mitsubishi PLC. By programming, the high-speed counter C253 can be used to count the number of pulses of the oscillation signal within a specified time (such as 3s), and the count value is retrieved and placed in the data memory D0. The value obtained by dividing the value in D0 by 3 is the magnitude of the oscillation frequency to be measured.
The touchscreen uses the cost-effective PWS6600S manufactured in Taiwan, equipped with a 5.7-inch high-definition LCD screen with a resolution of 320×240. It communicates serially with the Mitsubishi PLC via an RS232 serial port. It supports static text controls, as well as dynamic objects such as on/off buttons, numerical inputs, screen buttons, numerical displays, and status indicator lights. It also supports Chinese character display.
When a variable is specified in a static text control, the touchscreen can display the variable value in the connected Mitsubishi PLC on the screen in real time, which greatly facilitates system monitoring and status detection for staff.
When the operator touches the numeric input control, the PWS6600S automatically pops up a virtual numeric keypad, including numbers from 0 to 9 and functions such as Clear, Cancel, Delete, and Confirm. After entering a number, pressing the Cancel key cancels the input value, and pressing the Confirm key confirms the input. After the virtual numeric keypad disappears, the number in the control becomes the input value, and the corresponding variable in the Mitsubishi PLC changes accordingly.
When an operator touches controls such as the on/off button, screen buttons, status indicator lights, and numerical displays, the PWS6600S can trigger events such as button press, button release, screen switching, status display, and numerical display. Operators can then perform tasks such as clearing data, changing working modes, and selecting screen displays.
System software design
The system software consists of two parts: Mitsubishi PLC control software and touch screen software.
Mitsubishi PLCs have a rich set of programming instructions and a good software design environment, supporting basic programming languages such as ladder diagrams (LD), sequential function charts (SFC), and instruction lists (IL). This system uses ladder diagram programming with FXGP as the programming software.
The computer (PC) is used for programming and debugging. After successful debugging, the control program is downloaded to the Mitsubishi PLC via an interface cable.
The Mitsubishi PLC program mainly includes a main program and subroutines for segmented rise and segmented fall. The segmented rise and segmented fall subroutines are mainly used to make the thin steel pipe rise and fall in seven segments according to the test requirements, so as to test the frequency of the sensor output at different water levels and thus determine the quality of the water level sensor.
The PWS6600S touchscreen interface is designed and configured using dedicated support software ADP6.0. First, the interface, including windows, menus, and buttons, is designed on a personal computer using this software. After design, the program is downloaded to the PWS6600S touchscreen memory via RS232 serial port. The Mitsubishi PLC then reads and writes data to the touchscreen's status control area and notification area to achieve information exchange between the two. The Mitsubishi PLC reads data from the touchscreen's status notification area to obtain the current screen number, and writes data to the touchscreen's status control area to force screen switching. After the touchscreen is powered on, it enters the design screen. Data in the Mitsubishi PLC's data memory can be displayed and modified using the touchscreen buttons, enabling communication with the Mitsubishi PLC.
The entire screen consists of two parts: one part is the display screen, which mainly includes the system screen, the operating status of the test system, the water level display, the oscillation frequency output, and the display of the total daily output, as shown in Figure 4; the other part is the parameter setting screen, which is mainly used to set the working mode, the value of segmented rise and fall of the water level, etc.
The PWS6600S touchscreen has achieved good user results due to its strong human-computer interaction capabilities, easy operation, simple interface, and high reliability.
Conclusion
Applying Mitsubishi PLC and touch screen technology to the water level sensor detection system simplifies operation. Speed and water level can be controlled according to test requirements, greatly improving system reliability and efficiency. The system offers high control precision, strong operability, and allows users to observe the internal working status of the Mitsubishi PLC and the on-site conditions through the touch screen, verifying relevant parameters. Operation is flexible and convenient.
Since its successful development, this system has been put into use by several water level sensor manufacturers that supply washing machine manufacturers. The system is stable and reliable, and its economic benefits are very obvious. At the same time, it has been widely praised by users for its simple operation, strong practicality, and real-time data monitoring.
