The scientific steps of PLC programming are actually quite simple, but most engineers often overlook many details because they think it's easy. Ignoring details will inevitably lead to problems later. To avoid future problems, you must strictly follow the rules. Without rules, there can be no order; PLC programming also has its own set of rules.
Step 1: Read the product instruction manual
The first step seems incredibly simple, but many engineers fail to do it.
Carefully reading the instruction manual is the first step in programming. You must first read the safety guidelines to know which actuators may cause personal injury, which mechanisms are most likely to collide, and how to deal with danger. These most critical issues are all in the safety guidelines. Reading the product manual is an essential step.
Furthermore, the characteristics, usage methods, and debugging methods for each component of the equipment are also in the instruction manual. Without reading it, even if the program is correct, the equipment will still not work if the components are not properly debugged. Moreover, all the circuit diagrams, pneumatic and hydraulic circuit diagrams, and assembly drawings are also in the instruction manual; you need to read them to understand what modifications can be made to each component.
Step 2: Check the I/O according to the instruction manual.
Checking I/O, commonly known as "point checking," involves many methods, but it's crucial to check each address sequentially according to the manual, and only when absolutely safe to do so.
When inspecting input points, the input signals are generally nothing more than various sensors, such as capacitive, inductive, photoelectric, piezoresistive, ultrasonic, magnetic, and limit switch sensors. Inspecting these components is relatively simple; just place the workpiece on the workstation according to the component's specifications, or move the actuator to check if the sensor has a signal. Of course, different equipment may use different testing methods, depending on the specific circumstances.
However, extra caution is required when checking output signals. For electrically driven products, the actuator's drive must be energized under safe conditions, especially ensuring the equipment will not be impacted, to check if the actuator can move. For hydraulic or pneumatic actuators, similarly, manually energize the directional valve under safe conditions to control the actuator. When checking output signals, regardless of the actuator's drive method, always refer to the component's instruction manual. Prioritize equipment and personnel safety. Note that not all actuators can be tested by energizing them; therefore, sometimes individual output signals may not be able to be tested manually.
Whether it's an input or output device, when the sensor receives a signal or the actuator's drive is powered on, it's essential to simultaneously check if the I/O module indicator lights on the PLC are also illuminated. In many devices, input and output signals are connected to the PLC via terminal blocks. Sometimes, the terminal block indicator lights may show a signal, but this doesn't guarantee that the corresponding address on the PLC won't be connected due to an internal break in the connecting wire. This point requires special attention.
After measuring the input and output signals, record the measured addresses to ensure they match those in the instruction manual. If discrepancies exist, measure the device address again. If multiple measurements still yield inconsistencies, contact the device manufacturer first, as the manufacturer's address cannot be guaranteed to be correct at this point.
Step 3: Open the programming software, configure the hardware, and write the I/O addresses into the symbol table.
Different PLCs use different programming software. However, for any software, the first step before programming is hardware configuration, which involves establishing the hardware configuration and corresponding communication configuration based on the actual PLC type. After hardware configuration is complete, the I/O addresses previously recorded on paper are written into the software's symbol table. The definition of the symbol table may differ between software programs, but most software has this function, and this step is crucial. When writing the symbol table, not only should the absolute addresses of the device's inputs and outputs be written correctly, but it's also best to name each address and add comments. This will greatly facilitate subsequent programming. It eliminates the need to look up the absolute address every time during programming; simply enter the named address. Of course, this also depends on whether the software has this functionality.
Step 4: Write the program flowchart
Before programming, always write out the program flowchart on a draft. A complete program should include a main program, a stop program, an emergency stop program, a reset program, etc. If the software allows, each program should be written in a "block" format, that is, one program is one block, and each block can be called as needed.
PLCs excel at handling sequential control, where the main flow is crucial. It's essential to ensure the established flow is correct and to carefully check it on the draft. If the main flow has problems, when the PLC executes the program, collisions may occur, damaging equipment or posing a danger to personnel.
Step 5: Write the program in the software.
Once the main process is confirmed to be working correctly, you can begin writing the program in the software. Furthermore, pay close attention to the correctness of the stop, emergency stop, and reset procedures, especially the stop and emergency stop procedures. These are the most critical procedures related to personal and equipment safety and should never be underestimated. Ensure that under any circumstances, executing the stop or emergency stop procedure will absolutely not cause personal injury. We recommend you follow our WeChat account, TechCheng Training.
Step 6: Debug the program
The debugging process can be divided into two aspects.
1. If conditions permit, or if you have exceptional logical abilities, you can first use the software's simulation function to test, but many complex programs are difficult to verify through software simulation to determine if they are correct.
2. Download the program to the PLC for online debugging. If the equipment does not move or malfunctions during operation, do not modify the program immediately. It is likely that the sensors are not properly calibrated. Only modify the program after confirming that the sensors are working correctly.
Step 7: After debugging, edit the program again.
In the previous debugging step, since the program was modified, it is necessary to check or edit the program again before downloading the final program to the PLC.
Step 8: Save the program
In this step, it's crucial to consider where to save the program. PC hard drive? Flash memory? External hard drive? None of these are suitable, as all such storage devices are susceptible to virus infection. Therefore, the program must be burned onto a CD. Another question arises: which program should be burned? We've already downloaded the final, debugged, and modified program to the PLC. If the PLC executes the program flawlessly, we upload that program to the PC and burn it onto a CD. All of the above is for safety.
Step 9: Complete the report
After completing the programming, you should fill out a final debugging report, recording all the problems encountered and any difficulties the program faced. This is because you may forget some of the programming techniques over time, and it also helps other colleagues understand the program you wrote.
