The installation and commissioning of a PLC control system involves various tasks that must be performed sequentially, with each step crucial to the next. Even the slightest mistake can lead to commissioning failure, delaying the project and potentially damaging the equipment. This article presents technical experience in the installation and commissioning of PLC control systems gained from field practice, and discusses and proposes solutions to frequently encountered installation and commissioning issues.
Properly arranging the system installation and debugging procedures is key to ensuring efficient and high-quality completion of installation and debugging tasks.
1. Preliminary technical preparation The more thorough the technical preparation work before system installation and debugging, the smoother the installation and debugging will be. The preliminary technical preparation work includes the following: (1) Familiarize yourself with the PC's accompanying technical data and original documents, deeply understand its performance, functions and various operating requirements, and formulate operating procedures.
(2) Thoroughly understand the design data and the system process flow, especially the process control requirements of each production equipment. Based on this, draw process flow diagrams, system function diagrams, and system operation logic diagrams for each subsystem. This will help to deeply understand the system operation logic and is an important part of the early technical preparation.
(3) Be familiar with the performance, design and installation of each process equipment, especially the control and power wiring diagrams of each equipment, and compare them with the actual equipment to identify and correct errors in a timely manner.
(4) Based on a comprehensive understanding of the design scheme and PC technical data, list the PC input/output point number table (including an internal coil list, I/O location, corresponding device and function of each I/O point).
(5) Study the program provided by the design and draw timing diagrams for the input and output points of the logically complex parts. Some logical errors in the design can be found when drawing timing diagrams.
(6) The molecular system is developed and debugged, and then integrated into a whole system debugged plan based on collective discussion.
2. PLC commercial inspection should be conducted jointly by both parties. The inspection should confirm the model, quantity, and specifications of the equipment, spare parts, technical documents, and accessories. Its performance should be verified during laboratory and on-site commissioning. Both parties should sign and exchange inspection results.
3 Laboratory Debugging (1) Laboratory Installation and Commissioning of PLC: Make a metal bracket and fix the input and output modules of each workstation on it. Connect each station to the host, programmer, printer, etc. with coaxial cables according to the installation instructions. Check that the wiring is correct and that the power supply level matches the PLC voltage selection. Then, power on according to the startup procedure, load the system configuration tape, confirm the system configuration, load the programmer loading tape, programming tape, etc., and start the system according to the operating procedures. At this time, various operation tests can be carried out.
(2) Enter the working program. (3) Simulate I/O input and output, and check and modify the program. The purpose of this step is to verify the correctness of the entered working program, whether the interlocking relationship of the process equipment expressed by the program logic is consistent with the designed process control requirements, and whether the program is smooth. If it is inconsistent or cannot run the entire process, it means that the program is wrong and should be modified. In this process, the understanding of the program will be gradually deepened, which prepares for on-site debugging. At the same time, unreasonable and imperfect parts of the program can be found so that they can be further optimized.
There are two debugging methods: ① Simulation method: A debugging board is made according to the design. A toggle switch simulates the input node, and a small relay simulates the relays and contactors of the production process equipment. Its auxiliary contacts simulate the return signal nodes during equipment operation. The advantage is the realism of the simulation, reflecting whether logical malfunctions will occur when the field mechanical contacts with significantly different switching speeds are connected to the electronic contacts in the PLC. The disadvantage is the increased debugging costs and workload.
② Forced Setting Method: Utilizing the PLC's forced setting function, mechanical contacts (switches) involved in the field are forced to "on" or "off" to compel the program to run. Its advantages are minimal debugging workload, simplicity, and no additional cost. The disadvantages are incomplete logic verification; manually forcing simulated "on" and "off" on-site nodes can cause discontinuous program execution, requiring segmented execution.
Based on our on-site debugging experience, we adopt simulation for some important on-site nodes and forced implementation for the rest, combining the advantages of both methods. During the logic verification phase, it is crucial to emphasize daily completion of the debugging log, including details of debugging personnel, time, debugging content, modification records, faults and their handling, and handover and acceptance signatures. This establishes a system of accountability for debugging work and preserves firsthand debugging data. Any modifications to the design program should be noted on the design drawings, and the designer's opinions should be solicited promptly to ensure accurate reflection of the design requirements.
After the laboratory debugging of the 4PLC is completed, the equipment should be moved to the site for installation when conditions are suitable. During installation, requirements should be met: plugs should be securely inserted and tightened with bolts; communication cables must be of the same type and cannot be mixed; if necessary, use instruments to check the signal attenuation of the line, ensuring the attenuation value does not exceed the specifications stated in the technical datasheet; measure the insulation resistance to ground of the host, I/O cabinet, connecting cables, etc.; measure the grounding resistance of the system's dedicated grounding; check the power supply, etc., and record the results. Power can only be turned on after confirming that all items meet the requirements.
5. Inspection and Adjustment of Wiring, I/O Contacts, and Signals for On-Site Process Equipment: Check and confirm the correctness of the control circuits and main circuits of each on-site process equipment. Perform individual unit tests in manual mode. Inspect and repeatedly operate all input points entering the PLC system (including selector switches, buttons, relay and contactor contacts, limit switches, instrument on/off switches, etc.) and their connections to the PLC input modules to confirm their correctness. Inspect all relays, contactor coils, and other actuators receiving PLC outputs and their connections to the output modules to confirm their correctness. Measure and record their loop resistance and insulation resistance to ground. If necessary, supply power to the output circuit according to the power supply voltage level of the output node to ensure that the output circuit is not short-circuited. Otherwise, when the output point supplies power to the output circuit, the module will be burned out due to a short circuit.
Generally, large and medium-sized PLCs, if equipped with analog input/output modules, can also receive and output analog signals. In this case, the primary detection or transmitting elements that supply analog input signals to the PLC, as well as the regulating or actuating devices that receive the PLC's analog outputs, should be checked to confirm their correctness. If necessary, analog input signals should also be sent to the detection and transmitting devices to verify their correct installation and whether the output analog signals are correct and meet the standards required by the PLC; analog signals identical to the PLC's analog signals should be sent to the regulating or actuating elements that receive the PLC's analog output signals to check whether the regulating or actuating devices can work normally. PLCs equipped with analog input/output modules can monitor process parameters (analog signals) in the production process, perform calculations and adjustments according to the predetermined model in the design scheme, and implement process control of the production process.
This step is crucial, and the inspection and adjustment process is complex and troublesome, so it must be taken seriously. Because as long as all external process equipment is in good working order, all external nodes fed into the PLC are correct, reliable, and stable, all wiring connections are correct, and the program logic verification is error-free, then the linkage debugging will be successful in one go, achieving twice the result with half the effort.
6. System Simulation Linkage Airdrop Test: The purpose of this test is to place the PLC and logic program, which have been debugged in the laboratory, into the actual process flow and verify the logic of the system operation through the input and output nodes and connection lines of the on-site process equipment.
During the test, two phases of the main circuit of the PLC-controlled process equipment (mainly referring to electrically driven equipment) were disconnected (leaving only one phase as the relay control power supply) to prevent it from rotating when energized. System simulation experiments were conducted item by item for different operating modes and other control functions of the subsystem according to design requirements. First, the correct positions of each changeover switch, operating mode selection switch, and other preset switches were confirmed. Then, the system was started by the PLC, and the engagement and disengagement of the relays and contactors corresponding to each output node of the PLC were observed and recorded according to the interlocking sequence. The sequence, time intervals, and signal indications were checked to ensure they conformed to the designed process flow logic control requirements. The operation of other devices was also observed and recorded. For actuators that could not operate during the simulated linkage air-drop experiment, level switches, limit switches, instrument switch and analog input/output nodes, and interlocks with other subsystems, manual assistance, external input, and internal forced setting methods were used to simulate these, depending on the specific circumstances, to assist the PLC in directing the entire system to operate according to the designed logic control requirements.
The purpose of the 7. PLC-controlled unit test run is to confirm whether the PLC output circuit can drive the relays and contactors to operate normally, and to check whether the return signal of the operating equipment can be correctly sent to the PLC input circuit and whether the limit switches can operate normally.
The method involves, under PLC control, forcibly setting the output node of a specific process equipment (motor, actuator, etc.) within the machine to activate its relays and contactors, thus commencing equipment operation. At this time, the equipment's operation status should be observed and recorded, and the correctness of the equipment's return signal, limit switches, and actuator actions should be checked.
During testing, special attention should be paid to ensuring that any equipment subjected to forced start-up has a hazard warning sign displayed and is monitored by designated personnel. The PLC operator can only force start the equipment after the monitoring personnel issue a command. It is crucial to emphasize that, throughout the entire commissioning process, forced start-up of the equipment is strictly prohibited without adequate preparation to ensure safety.
The purpose of this step in the no-load linkage test run of the 8 PLC-controlled system is to confirm whether the process equipment, after undergoing individual no-load test runs, and the PLC, whose logic has been proven correct through system simulation tests, can operate correctly according to process requirements, whether the signal system is correct, and to verify the reliability and stability of each external node. Before the test, a no-load linkage test run plan must be prepared, discussed, and confirmed, and then strictly implemented according to the plan. During the test, the linkage of individual subsystems should be performed first, with manual assistance (node short-circuiting or forced setting) used for interlocking. Then, the linkage of the entire system should be performed. The test content should include various start-stop and operation modes required by the design, shutdown under accident and emergency conditions, and various signals. In short, it should be fully considered to better reflect the actual situation on site. Accident conditions can be simulated using forced setting methods, and the location of the accident point should be determined according to process requirements.
Before conducting a load test, a comprehensive inspection of the entire system must be carried out again, and operators must be trained to ensure that the load test is successful on the first attempt.
Discussion on signal attenuation problem (1) The maximum signal attenuation from the PLC host to the I/O station is 35dB. Therefore, the cable laying should be carefully planned before laying, and a cable laying diagram should be drawn to shorten the cable length as much as possible (the signal attenuation is 0.8dB for every 1km increase in length); and the use of splitters (14dB signal attenuation per splitter) and cable joints (1dB signal attenuation per cable joint) should be minimized.
(2) Communication cables should preferably be laid in a single-bus manner, that is, connected to I/O stations through a brancher from a unified communication trunk line, rather than being laid in a star-shaped radial pattern. The number of I/O stations and the transmission distance on the left and right sides of the PLC host should be as consistent as possible to ensure a better network impedance matching.
(3) The brancher should be placed as close as possible to the I/O station to reduce interference.
(4) A 75Ω BNC cable terminator should be connected to the end of the communication cable and connected to each I/O cabinet. When the cable is removed from the I/O cabinet, the terminator with the 75Ω resistor should be connected to one end of the cable network to maintain good matching.
(5) The distance between communication cables and high-voltage cables should be at least 40cm/kV; when they must cross high-voltage cables, they must cross perpendicularly.
(6) Communication cables should avoid being laid parallel to AC power lines to reduce interference from AC power. Similarly, communication cables should be kept away from large motors, welding machines, large inductors, and other equipment as much as possible.
(7) Communication cables should be laid away from areas with high temperatures and areas susceptible to chemical corrosion.
(8) When laying cables, a margin of 0.05%/℃ should be left to meet the requirements of thermal expansion and contraction.
(9) All cable joints, splitters, etc. should be tightly connected and secured with screws.
(10) When stripping the cable sheath, be careful not to damage the shielding layer. When cutting the metal platinum and the insulator, be sure to use wire strippers. Be careful not to scratch or damage the center conductor.
Discussion on system grounding issues (1) The grounding of the main unit and the parts above each branch station should be connected together with 10mm2 braided copper wire and then connected to an independent grounding network through a separate down conductor. It must be separated from the low-voltage grounding network to avoid interference. The system grounding resistance should be less than 4Ω. A 3mm thick rubber pad should be placed between the PLC main unit and each panel/cabinet and the foundation base for insulation. The bolts should also be insulated.
(2) The grounding of the I/O station equipment body shall be connected to the common grounding grid by a separate down conductor.
(3) The shielding layer of the communication cable should be connected to the dedicated grounding network of the system at the I/O processing module on the PLC host side, and should not be grounded on the I/O station side. The grounding of the cable joint should also be connected to the dedicated grounding network through the cable shielding layer. It is particularly important to note that the cable shielding layer must never have two grounding points forming a closed loop, otherwise it will easily cause interference.
(4) The power supply should be isolated, that is, the neutral line of the power supply is floating. When unbalanced current occurs, it will directly enter the neutral point of the system through the neutral line of the power supply, instead of forming a loop through the protective ground, which will cause interference to the operation of the PLC.
(5) The ground of the I/O module is connected to the power supply neutral line.
Issues to be noted during debugging: (1) Before the system goes online, it must be configured, that is, the number of I/O points managed by the system, the number of input registers, holding registers, the number of communication ports and their parameters, the matching and scheduling method of I/O stations, the size of the logical area occupied by users, etc. Once the configuration is confirmed, the system will run according to certain constraint rules. When reconfiguring, the program generated according to the original configuration will not be able to run under the new configuration, otherwise it will cause system errors. Therefore, the first configuration must be done with caution. The number of I/O stations, I/O points, registers, channel ports, user storage space, etc. must all have leeway, and the near future development must be considered. However, the setting of the number of I/O stations, I/O points, registers, ports, etc. will occupy a certain amount of memory, and at the same time extend the scanning time and reduce the running speed. Therefore, the leeway should not be too much. It is particularly important to note that the running system must not be reconfigured.
(2) For large and medium-sized PLCs, the CPU scans the program in segments, updating the I/O point state after each segment is scanned, thus greatly improving the system's real-time performance. However, improper program segmentation can lead to reduced real-time performance or slower operation. Different segmentations will significantly affect program execution time, especially for particularly long segments. Generally speaking, ideal program segmentation involves segments of roughly equal length.
In conclusion, the installation and commissioning of a PLC control system is a systematic project that requires a step-by-step approach to ensure successful commissioning. This article is merely a summary of the author's experience based on on-site verification. The PLC installation and commissioning procedures described in this article have been applied to a series of technical upgrades within enterprises, which have not only shortened the construction period but also ensured successful commissioning and trial runs on the first attempt, achieving satisfactory results.
