summary:
To address the issue of numerous switch signal inputs, this paper presents a PLC switch signal acquisition method based on group scanning input, using the FX series PLC from Mitsubishi Corporation of Japan as an example. With the help of an input interface board, multiple switch input signals can be connected to a single input point of the PLC. Using this method, for control systems with a large number of input points, the number of PLC input points can be reduced, the information acquisition efficiency of the PLC can be improved, and this is of great significance for reducing the cost of the control system.
1 Introduction
PLC is a modern industrial control technology that has been developed over the past forty years. It combines the advantages of computer programming (flexible, feature-rich, and widely applicable) with the advantages of relay systems (simple control, ease of use, strong anti-interference ability, and low price). In addition, it has the characteristics of small size, low power consumption, and stable performance. Therefore, it has been widely used in industrial production process control and is known as one of the three pillars of modern industrial automation (PLC, CNC technology, and industrial robots).
Systems with a small number of input/output points may not require interface expansion; however, when the number of points is large, input/output expansion is necessary. Different companies' PLC products have limitations on the total number of system points and the number of expansion modules. When expansion still cannot meet the needs, a network structure must be used, which increases both the complexity and cost of the system.
To address the issue of numerous switch signal inputs, this paper presents a PLC switch signal acquisition method based on group scanning input, using the FX series PLC from Mitsubishi Corporation of Japan as an example. With the help of an input interface board, multiple switch input signals can be connected to a single input point of the PLC. Using this method, for control systems with a large number of input points, the number of PLC input points can be reduced, the information acquisition efficiency of the PLC can be improved, and this is of great significance for reducing the cost of the control system.
2 Hardware Design
For switch and button signals that are frequently used in industrial settings, they are usually connected to the PLC input point according to the wiring method shown in Figure 1. This method uses the COM terminal as the common terminal for all switch input signals, and each switch or button is connected to one PLC input point.
Figure 1. Common methods for connecting switch signals
To address the issue of numerous digital input signals, a PLC digital input acquisition method based on group scanning input was designed using the signal scanning principle. The hardware structure is shown in Figure 2. Taking 16 digital input signals as an example, these 16 signals are divided into 4 groups and connected to four interface boards (each interface board can connect to 4 signals, outputting via diodes). After passing through the interface boards, k1, k5, k9, and k13 are all connected to the PLC's X1 input terminal; similarly, k2, k6, k10, and k14 are connected to the PLC's X2 input terminal; k3, k7, k11, and k15 are connected to the PLC's X3 input terminal; and k4, k8, k12, and k16 are connected to the PLC's X4 input terminal. Thus, the 16 digital input signals occupy only 4 input terminals of the PLC.
The four interface boards are selected by the four outputs y1 to y4 of the PLC (shown as dashed lines). For example, when y1 is active and y2 to y4 are inactive, interface board i is selected, and signals k1 to k4 are fed into x1 to x4. When y2 is active and y1, y3, and y4 are inactive, signals k5 to k8 are fed into x1 to x4. The other two sets of signals are fed in the same way. In this structure, the output y replaces COM as the common terminal.
This method involves scanning four times per cycle, sending 16 signals to the PLC's input terminals in four separate scans. After each scan, the states of x1 to x4 are transferred to other positions in the program. These 16 input signals occupy only four input terminals and four output terminals, saving half the number of PLC input/output points. In practical applications, this can be flexibly expanded as needed to achieve higher efficiency. For example, if each interface board connects eight digital input signals, four boards will connect a total of 32 input signals, occupying eight PLC input terminals, while the output terminals remain at four.
When designing, pay close attention to the selection of diodes in the interface board. High-quality, stable diodes must be chosen. If a diode is damaged or breaks down, the input signal will not be correctly sent to the PLC input terminal, or input malfunctions will occur. Additionally, the number of input signal groups should not be too large. Figure 2 shows 4 groups. If the time interval between each scan is 100ms, the scan cycle for 4 scans is 400ms, and the maximum input signal delay could reach 400ms. If the number of groups is too large (e.g., more than 10 groups), signal delay will occur, leading to a decrease in system sensitivity.
Figure 2 Hardware structure for PLC digital input acquisition based on group scanning
Figure 3 Software Flowchart
3 Software Design
Two main issues need to be considered in the software design. First, a single scan strobe signal must be output periodically to select the corresponding interface board. Second, the scanned data must be transferred to other locations in a timely manner. In this PLC input design method, one of each group of digital signals is sequentially sent to a PLC input terminal in each scan cycle. Therefore, the state of the previously scanned digital signal must be transferred to another location for storage before the next scan data arrives.
Figure 3 shows the software flowchart, and the software program (ladder diagram) designed based on the above considerations is shown in Figure 4. Each scan interval is 100ms. After the 16 switch inputs are acquired by the PLC, they are sent to m100~m115 for storage. The instructions rol and ref represent cyclic left shift and output refresh, respectively.
Figure 4. PLC input software implementation supporting multi-point scanning input
4. Conclusion
This paper presents a PLC switch signal acquisition method based on group scanning input. Utilizing the signal scanning principle, this method can effectively solve the problem of a large number of switch signal inputs in industrial settings. It can significantly reduce the number of PLC input points, lower the design cost of the control system, and provides a stable system structure with good scalability and flexibility. It has certain practical value and promotion significance.
