The installation and commissioning of a PLC control system involves various tasks that must be carried out in sequence, with each step closely linked to the next. The slightest mistake can lead to commissioning failure, which can not only delay the project schedule but also damage the equipment.
This article introduces the installation and debugging technical experience of PLC control systems summarized from field practice, and puts forward discussion opinions and solutions to frequently encountered installation and debugging-related problems in the field.
I. System Installation and Debugging
Properly arranging the system installation and debugging procedures is crucial to ensuring efficient and high-quality completion of installation and debugging tasks. The following section provides a detailed explanation of the on-site PLC debugging and installation steps.
1. Preliminary Technical Preparations
The more thorough the technical preparations before system installation and debugging, the smoother the installation and debugging will be. Preliminary technical preparations include the following:
(1) Be familiar with the PLC's random 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 have a comprehensive understanding of the system's technological process, especially the control requirements of the process on each production equipment. Based on this, draw process flow interlock diagrams, system function diagrams, and system operation logic diagrams according to subsystems. This will help to deeply understand the system's operating 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 PLC technical data, list the PLC input/output point number table (including an internal coil list, I/O location, corresponding equipment 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.
2PLC commercial inspection
The commodity 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 testing and on-site commissioning. Both parties should sign and exchange the inspection results.
3. Laboratory commissioning
(1) When experimenting with the PLC, a metal test bench is used to fix the input and output modules of each workstation on it. According to the installation instructions, connect each station to the host, programmer, printer, etc. with coaxial cables. After checking that the wiring is correct and that the power supply level matches the PLC voltage selection, 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 turn on the system according to the operating procedures. At this time, various operation tests can be carried out.
(2) Enter the working procedure.
(3) Simulate I/O input and output, and check the modified program. The purpose of this step is to verify the correctness of the input working program. Whether the interlocking relationship of the process equipment expressed by the logic of the program is consistent with the designed process control requirements, and whether the program is smooth.
If the program does not match the specifications or cannot complete the entire process, it indicates an error and should be modified. During this process, the understanding of the program will gradually deepen, preparing for on-site debugging. It will also help identify unreasonable and imperfect parts of the program for further optimization.
There are two debugging methods:
①Simulation method
Make a debugging board according to the design, use toggle switches to simulate input nodes, use small relays to simulate relays and contactors of production process equipment, and use auxiliary contacts to simulate return signal nodes when the equipment is running.
Its advantage is its simulation realism, reflecting whether logical malfunctions will occur when field mechanical contacts with significantly different switching speeds are connected to electronic contacts within the PLC. Its disadvantage is the increased debugging costs and workload.
② Forced placement method
By using the PLC's forced setting function, the mechanical contacts (switches) involved in the field in the program can be forced to "on" or "off", thus forcing the program to run.
Its advantages are that the debugging workload is small and simple, and no additional cost is required. The disadvantages are that the logic verification is not comprehensive, and manually setting the simulated field nodes to "connect" or "disconnect" will cause the program to run discontinuously and can only be done in segments.
Based on our experience in on-site commissioning, we adopt simulation for some important on-site nodes and forced execution for the rest, taking advantage of the strengths of both methods.
During the logic verification phase, it is crucial to maintain a daily debugging log, including details of the debugging personnel, time, debugging content, modification records, faults and their resolution, and handover and acceptance signatures. This establishes a responsibility system 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 feedback should be sought promptly to ensure accurate reflection of the design requirements.
4PLC on-site installation and inspection
After the laboratory debugging is completed and conditions are met, the equipment will be moved to the site for installation. The following points should be noted during installation.
(1) During installation, the requirements should be met, the plug should be inserted securely, and it should be tightened with bolts;
(2) Communication cables must be of the same type and cannot be mixed. If necessary, instruments should be used to check the signal attenuation of the line, and the attenuation value should not exceed the index specified in the technical data.
(3) Measure the insulation resistance to ground of the main unit, I/O cabinet, connecting cables, etc.;
(4) The grounding resistance of the dedicated grounding for the measurement system;
(5) Check the power supply, etc., and make a record. Only after confirming that all items meet the requirements can the power be turned on.
5. Inspection and adjustment of wiring, I/O contacts and signals of field process equipment
(1) Check and confirm the correctness of the control circuit and main circuit wiring of each process equipment on site, and conduct individual unit test runs in manual mode;
(2) Check and repeatedly operate all input points (including changeover switches, buttons, relay and contactor contacts, limit switches, instrument on/off adjustment switches, etc.) that enter the PLC system and their connections to the PLC input module to confirm their correctness;
(3) Check all relays, contactor coils and other actuators that receive PLC outputs and their connections to the output module to confirm their correctness;
(4) Measure and record its 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 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, it is necessary to check the primary detection or transmission elements that supply analog input signals to the PLC, as well as the regulating or actuating devices that receive the PLC's analog outputs, to confirm their correctness.
When necessary, analog input signals should also be sent to the detection and transmission 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 those of the PLC should be sent to the regulating or actuating elements receiving the PLC's analog output signals to check whether the regulating or actuating devices can function properly. A PLC equipped with analog input and output modules can monitor process parameters (analog signals) during production, 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 simulated joint airdrop test
The purpose of this step 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 field 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) so that it would not rotate when energized. System simulation experiments were conducted item by item for different operating modes and other control functions of the subsystem according to the 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 through 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 also 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 cannot operate during the simulated linkage airdrop experiment, such as 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 are used to simulate them as needed, in order to assist the PLC in directing the entire system to operate according to the designed logic control requirements.
7 PLC-controlled unit test run
The purpose of this test 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.
No-load linkage test run of the system under 8 PLC control
The purpose of this test step is to confirm whether the process equipment, after undergoing individual no-load test operation, can operate correctly according to process requirements after being connected to the PLC, which has been proven to have correct logic through system simulation test operation, and 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 plan for the system must be prepared, discussed and confirmed, and strictly implemented according to the plan. During the test, the interlocking of the subsystems should be performed first, with manual assistance (node short-circuiting or forced setting) for the interlocking of the subsystems. Then, the entire system should be linked. The test content should include various start-up, shutdown, and operation modes required by the design, shutdown under accident and emergency conditions, and various signals. In short, it should be fully considered as much as possible to make it more in line with the actual field conditions. Accident conditions can be simulated using forced setting methods, and the location of the accident point should be determined according to the 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.
II. Three points to note during installation and debugging
1. Discussion of signal attenuation problem
(1) The maximum signal attenuation from the PLC host to the I/O station is 35dB. Therefore, careful planning should be done before laying the cable, and a cable laying diagram should be drawn to minimize the cable length (signal attenuation is 0.8dB for every 1km increase in length); and the use of splitters (signal attenuation is 14dB per splitter) and cable joints (signal attenuation is 1dB 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.
2. Discussion of system grounding issues
(1) The grounding of the main unit and all branch stations above the main unit should be made of 10mm² braided copper wire connected together and then connected to an independent grounding grid via a separate down conductor. It must be separated from the low-voltage grounding grid 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, i.e., the power supply neutral line should be floating. When unbalanced current occurs, it will directly enter the system neutral point through the power supply neutral line, instead of forming a loop through the protective ground, which would cause interference to the PLC operation.
(5) The ground of the I/O module is connected to the power supply neutral line.
3. Issues to be aware of during debugging
(1) Before the system goes online, it must be configured, which means determining the number of I/O points managed by the system, the number of input registers, holding registers, communication ports and their parameters, I/O station matching and scheduling methods, the size of the logical area occupied by users, etc. Once the configuration is confirmed, the system will run according to certain constraint rules.
Note: When reconfiguring, programs generated according to the original configuration conventions will not run under the new configuration; otherwise, system errors will occur. Therefore, the first configuration must be carefully considered. Sufficient margin must be left for I/O stations, I/O points, registers, channel ports, and user storage space, taking into account near-term future development. However, setting the number of I/O stations, I/O points, registers, and ports will consume memory, increase scan time, and reduce running speed. Therefore, the margin should not be too large. It is particularly important to note that a running system must never be reconfigured.
(2) For large and medium-sized PLCs, since the CPU scans the program in segments, the state of the I/O points is updated once each segment is scanned, which greatly improves the real-time performance of the system. However, improper program segmentation may also cause problems such as reduced real-time performance or slower running speed.
Note: Different segmentation methods will significantly affect program execution time, especially when individual program segments are particularly long. Generally speaking, ideal program segmentation involves segments of roughly equal length.
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.
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