I. Three key factors for selecting a PLC
1. The number and nature of input/output devices
After determining the quantity and nature of the PLC's inputs and outputs, we can further determine the type and quantity of various I/O modules. Standard switching I/O modules have 4, 8, 16, 32, and 64 points; modules with more points have a relatively lower average price per point. The external connection methods for switching I/O modules can be divided into isolated and connected types, with isolated types having a higher average price per point. If isolation between signals is not required, input/output modules with connected points should be selected. In integrated PLCs, each I/O terminal can also be divided into isolated and connected types to meet the needs of input/output devices with different voltage levels.
2. Functions of Programmable Logic Controllers
Based on the system's control process and rules, determine the functions of the programmable logic controller (PLC). Different PLC models do not have entirely identical functions. For example, some small PLCs only have on/off logic control functions and lack data processing and analog processing capabilities. When a system also requires closed-loop control such as position control, temperature control, or PID control, a template PLC should be considered, selecting I/O modules with corresponding special functions. Otherwise, these algorithms are designed using the PLC's ladder diagrams, which is both difficult to program and consumes a large amount of program space.
In addition, the operating speed of the PLC should also be considered, especially when using functions such as analog control and high-speed counters. It should be determined whether the maximum operating frequency of the PLC meets the requirements.
3. User program memory capacity
Determining the appropriate capacity of the user program memory for a programmable logic controller (PLC) is an essential step in the design and selection of PLC applications. Generally, the storage capacity of the user program memory is related to memory utilization, the total number of switch I/Os, the number of analog I/O points, and the programmer's skill level.
II. PLC Installation Precautions
While PLC control is far more reliable than relay control, it still won't work if the installation is not done correctly. Therefore, the PLC unit must be installed properly externally.
(a) The installation environment should be considered.
The temperature must not be lower than 0℃ or higher than 55℃. For example, do not install it near a large stove or in a freezer. Also, pay attention to moisture, corrosion, and shock protection, and keep it away from high-voltage lines, especially those above 6KV.
(II) The power supply and ground wire should not be underestimated.
On the PLC's wiring terminals, especially the power supply, never connect it to other terminals; otherwise, the consequences can be severe. If there are high-powered welding machines or other electrical appliances that easily generate significant interference waves on site, an isolation transformer must be added. The transformer core must be reliably grounded. When routing wiring, do not bundle power lines, control lines, and control lines together. In particular, control lines must be kept at least 0.5 meters away from control signal lines.
(III) Wiring of output terminals
The output terminals output voltage, typically used to drive contactor coils, etc. The other end of the load is connected to the public terminal COM. Especially when the PLC uses transistor outputs, a snubber diode must be connected to prevent the high voltage generated when the load contactor coil disconnects from damaging the PLC transistor. Similarly, it should be kept away from high-voltage lines and protected against strong interference.
(iv) Wiring of input terminals
Each input terminal must be connected to the COM public segment for the input to be effective. It is particularly important that the input COM public segment not be connected to the output COM public segment. Input cables should not be too long, generally not exceeding 0.5 meters. Input and output cables must be separate and kept away from high-voltage lines.
(V) The PLC needs to be debugged after installation.
Debugging is essential, especially for complex and expensive equipment. If the PLC doesn't operate as preset, it can lead to anything from production halts to very serious consequences. How do you debug it?
1. When the power is switched on, the power indicator light on the PLC should light up;
2. Set the PLC to "Monitor". If there are no program errors, the RUN indicator light will illuminate. You can manually input a signal (such as moving a limit switch) to see if the corresponding indicator light illuminates as expected (note that the output must be disconnected at this time, such as the motor). If there is a program error, the RUN indicator light will flash.
3. First, simulate the operation, or run it without a load. Only after it meets the requirements can a load be added and the test run conducted. During this process, close observation should be maintained for a period of time to prevent any mishaps.
4. Record the debugging process in a file for future reference.
