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First, preparation before departure

Those who are engaged in technical support of plc, if they want to start the system debugging, whether it is for OEM customers or for system integration of process control, they must do some preparatory work in advance. Just like a "friend" before going out, you have to change your own clothing, especially shoes, bring water, food, and some self-defense supplies in the wild, and if so, have tents, flashlights, etc., as a professional PLC technical support engineers, there are also many things to be prepared.

First of all, you have to arrange your own itinerary plan and food, clothing and accommodation. If you need accommodation, then how to set the hotel (hotel, guest house), not much to say here. If the time is long (such as more than one month), in order to save money, you should consider renting a house in the local area, especially when starting a group rather than a person. These problems are the same as ordinary travel, although these issues have the same impact on your mission as your professional level. Imagine that you are on the scene, because food poisoning causes a week of dysentery to affect the work, you know that you should prepare those things. Usually, the administrative department of an experienced automation company will consider these situations for the engineers, but as an engineer, you must confirm it to see if everything has been arranged.

After you have settled in the food, clothing, and housing, you must prepare your own tools. As a field commissioning engineer for PLC, it does not require a lot of debugging tools. However, a screwdriver and a multimeter are still necessary. In addition, if you want to coordinate the system with the on-site instrumentation sensor, there is also a signal generator to simulate the signal of the field instrument to determine that the signal on the site is intact when a problem occurs. Also, a rugged laptop is a must-have tool for programming and debugging. Although these tools you can ask the user to prepare, but as a professional PLC engineer, if you do not have these tools, you will be suspected of your professional level. Some other instruments, if you need, you can ask the company or customer to prepare them in advance, such as oscilloscopes, regulated power supplies, and so on. After all, these less versatile instruments are usually not portable or spare.

There are some small things that are often overlooked, and you must be useful after you have prepared them. For example, electrical tape, heat shrink tubing, lighter (naturally, if you smoke, no one else will have it), cable tie, and if the field wiring is completed, prepare some cables with the same signal cable. , and some screws that fix the base. You may suspect that you need to do this. Believe me, bring these, although some are worth only a few cents, but when you get to the scene, you may save you a day or even a few days, especially if you go to a remote place. The time of the mountains. Although the manufacturers have screws at the time of delivery, anything can happen at the scene. If the screws are not enough (and this is inevitable every time), it will cause many problems.

In addition to the above, it is the spare parts to be taken for this commissioning. Although the on-site cargo list may have considered the issue of spare parts, in order to cope with the situation, you may want to carry some spare parts yourself. The first is the PLC substrate (some become a rack), power supply, CPU module, because these are the basis of a PLC can work, if there is only one system in the field, then there is only one of these modules, if there is no backup, once In the event of a breakdown, all your work must stop. So it is best to bring a set.

Other I/O modules and communication modules, if there is only one on site, you should consider taking one more. For those who already have more than two modules on site, you don't have to think about bringing them.

Second, the design of the work

If the system design is not your own, then before the departure, you must fully communicate with the system's designers. For those who are unclear and do not understand, be sure to make sure that you have a clear understanding of the work to be done. Goals, and the intent of the designer. Don't easily deny the designer's plan, and don't easily say that others can't design it.

However, because you are an engineer at the site, you don't have to be too superstitious about the designer. You can make your own suggestions. For example, PLC rack and CPU module and I / O module matching principle, sometimes, ***** do not know the PLC rack you use has 4 slots, 6 slots and 8 slots, or even 12 slots Therefore, maybe all 8 slots are selected, which leads to the vacancies of several slots. You can suggest to modify it to 4 slots or 6 slots.

In the design of the scheme, the most troublesome place is communication. The most troublesome place for on-site debugging is communication. Therefore, for the communication part, you must clearly understand the framework of the system, and all the things that need to be communicated before starting. Perform the debugging again, and make sure that all the modules that need communication are communicable. For example, console computers, touch screens, PLCs, inverters, other PLCs, some smart meters and instruments, if these things need to communicate, you must confirm that they can communicate with each other, if you do not confirm, then Contact the manufacturer and try again. If the PLC has a large number of nodes, consider the distance and the manufacturer's CPU limit.

If the system is large, the limitations of the remote unit, the number of local racks, and the number of modules per rack are not due to system functionality and technical limitations, but due to the use of normative restrictions. When configuring the system hardware, consider the combination of these modules.

For the configuration of the power module, a senior engineer should be able to make a reasonable match. For example, power modules usually have 5A and 10A respectively. If there are fewer modules, you can use power modules with low power. If there are many modules, you should use high-power power modules. In general, if the field instrument requires the PLC to supply 24V DC power instead of the external power supply (such as RTU), it is usually reasonable to use a high-power power supply in the rack where the CPU is located.

In addition, when I/O modules are installed, modules of the same model are usually placed together according to different systems. However, if the output module and the input module form a loop, it is reasonable to place the modules involved in the parameters of one loop on a rack (or a rack controlled by a CPU). Some ***** will not consider this when designing, you can adjust it.

In the selection, because various PLCs have large, medium and small, these are mainly caused by the different performance of the CPU module. In fact, not necessarily a large function is appropriate. In addition to the price considerations, the main reason is that if the number of control points is small and the small CPU module is fully capable, the small CPU will be more reliable than the large CPU module. Because small CPUs involve less resources, and large CPUs have more resources. In the execution of the program, in order to process resources that are not needed by those programs, time is also allocated to deal with them, which not only causes waste of resources, but may In the program processing, the small CPU can work more simply and stably.

In the process of designing the bottom, it is important to note that for any changes in the design, you can only make suggestions, not arbitrarily. Because your job is to follow the design, not the design, so you can give advice on anything that you find unreasonable, but you must wait until the design change confirmation is in your hands before you can follow the changes. After the design work, although this change may be your opinion. Also, even if the original design is what you did, you should notify the customer after the change and obtain the written consent of the customer.

Third, the installation steps

After arriving at the site, before installing the system, you need to consider whether the installation environment meets the requirements of the PLC environment. You can refer to the manuals of various products. However, no matter what kind of PLC, it can not be installed in the following places: places with corrosive gases, places where sunlight directly shines, places where the temperature changes up and down in a short time, where oil, water, and chemicals are easily invaded. Where there is a lot of dust, the vibration is large and the mounting parts are displaced.

If it is necessary to use it in the above environment, it is necessary to make a suitable control box for the PLC, using the specifications and necessary protective measures. If you need to use it in the wild at very low temperatures, you can use a control box with heating function. How to make these protective boxes or control boxes, manufacturers and qualified system integrators will provide customers with the appropriate supply and design.

When using the control box, pay attention to the following items in the position where the OpenPLC is installed in the control box: whether the air circulation in the control box is smooth (the appropriate distance must be maintained between the devices), and whether the transformer, motor controller, inverter, etc. are kept with the PLC. Appropriate distance, whether the power line and the signal control line are separated and configured, whether the position of the component is suitable for future maintenance, whether space is reserved for future expansion of the system.

In addition to the above considerations, there are other considerations to be aware of.

The first important thing is the isolation of static electricity. Static electricity is an invisible killer, but it may be overlooked because many people are not at risk. In the northern, dry places of China, static electricity on the human body is a factor that causes static electricity to damage electronic components. Although you are hit by static electricity, it is only a slight numbness, but it is fatal to the PLC and any other electronic devices.

There are three ways to avoid static shock: when repairing or replacing components, touch the grounded metal to remove static electricity from the body; do not touch the connector or IC pin on the circuit board; electronic components are not When using, use the package with isolated static electricity to place the components inside. Imagine that the components in the PLC are a delicate baby, and those static electricity will cause the baby to die, and it will be easier for you to treat the problem with the right attitude.

When the base is installed (RACK), after determining the various control components and slot positions in the control box, the hole position should be calibrated according to the dimensions shown in the drawing. After drilling, tighten the fixing screws until the base is secure. Before installing the power supply module, you must also pay attention to whether the grounding end of the power cable is connected to the metal chassis. If not, it must be connected. If the grounding is not good, it will lead to a series of problems, such as static electricity, surge, external interference, and so on. Because the PLC is not grounded, the PLC can work. Therefore, many engineers with less experience mistakenly believe that grounding is not so important. It's like climbing a cable without a protective cable. Although you don't have any effect when you move forward normally, once you lose it, without the rope, your life is over. The grounding of the PLC is equivalent to protecting the cable from the PLC.

When installing an I/O module, pay attention to the following items: Before inserting the I/O module into the slot on the rack, make sure that the module is a pre-designed module; the I/O module is inserted into the rack. When inserting the groove, be sure to insert it all the way to ensure that the contact points are tightly coupled; the module fixing screws must be locked; after the terminal block is inserted, the upper and lower screws must be tightened. Due to the influence of transformers and motors on site, there will be some vibration. If these screws are loose, the module will be released from the rack.

Fourth, the configuration and confirmation of the hardware module

For on-site hardware configuration and software debugging of various PLCs, usually experienced engineers should spend some time on a simple plan for their own field work, usually the following steps should be taken:

(1) System planning

First, you must have an in-depth understanding of the functions required by the system, and investigate possible control methods. At the same time, discuss the best operating procedures with the user or the design institute, formulate the system plan based on the conclusions summarized, and determine the PLC system architecture adopted. , the required number of I / O points and I / O module type.

(2) I/O module selection and address setting

When the I/O module is selected, the CPU system of the PLC automatically sets the I/O address according to the planned I/O point usage, or the user can customize the address of the I/O module.

(3) Ladder program writing and system wiring

After determining the actual I/O address, start writing the ladder program according to the functions required by the system. At the same time, the address of the I/O has been set properly, so the wiring of the system can also be carried out.

(4) Simulation and modification of ladder program

After the ladder program is written, the program is written to the PLC, and the PC can be connected to the OpenPLC system online to perform the online simulation. If the program execution function is incorrect, you must debug and modify the ladder program.

(5) System test and actual operation

Under the online program simulation operation, if the ladder program execution function is correct and the system wiring is completed, the system can be put into actual operation and the project plan is completed.

(6) Program comments and archives

In order to ensure the convenience of future maintenance, it is necessary to endorse the ladder program that can be used for the actual operation of the test, and organize and archive it in order to shorten the time for future maintenance and inspection procedures. This is a good habit of professional engineers. Whether it is to maintain yourself or hand over to users in the future, this will bring great convenience and is a reflection of your professional standards.

In the above work, complex system planning may take several days or even longer, but a simple system planning in the hands of a programming engineer with good professional habits may only take a few hours.

Here to emphasize a problem, it is very simple but almost every project will happen, that is, the wiring of the PLC. This is often a problem that is often overlooked by inexperienced engineers. In fact, most of the problems and workloads of on-site commissioning are in terms of wiring. An experienced engineer should first check the wiring at the site. Usually, if the field wiring is done by the user or other construction personnel, the quality of the wiring can be roughly judged by looking at the wiring diagram and the appearance of the wiring. Then perform a complete and careful inspection of all wiring. The situation where the PLC is burnt out due to wiring errors has occurred repeatedly, and must be carefully checked before the actual commissioning. Even if the wiring is not your job, checking the wiring is your responsibility and responsibility, and you can save a lot of time behind you.

Fifth, the communication settings

Most of the current PLCs need to be connected to the human-machine interface, and there are often inverters that need to communicate below. In systems that require multiple CPU modules, there may be a need for different I/O modules connected to the CPU. Where the collaborative processing, or even without the need for collaborative control, may be sent to a central control room for centralized display or data storage. Even if there is only one CPU module, if there is a remote unit, it involves communication between the local CPU module and the remote unit module. In addition, even if there is only a local unit, the CPU module needs to communicate with the programmer through the communication port. Therefore, PLC communication is very important. Moreover, communication is often a headache because of the products of different manufacturers.

PLC communication has several methods such as RS232, RS485, and Ethernet. The communication protocols are MODBUS, PROFIBUS, LONWORKS, DEVICENET, etc., which are usually the most widely used by the MODBUS protocol, while other protocols are related to the brand of the product. In the future, industrial Ethernet protocols will become more and more common.

Most communication between PLC and programmer or laptop computer uses RS232 protocol serial communication. This is the way users do program downloads and diagnostics, but the way this communication is done is by no means limited. In a large number of mechanical equipment control systems, PLCs communicate with human-machine interfaces in this way. The man-machine interface usually uses a serial port, and the protocol is mainly MODBUS or a special communication protocol. The interface is designed by the manufacturer of hmi.

Now PANELPC also uses this method to communicate. It runs some configuration software on PANELPC and accesses OpenPLC data through the serial port. Due to the gradual lightening and price reduction of PANELPC, this method is also more and more. The ground is used.

When it is necessary to network multiple PLCs, if the number of PLCs is not many (within 15 nodes) and the system with a small amount of data transmission, the commonly used method is to connect through a simple serial communication port composed of RS485. Communication network. Because this communication method is simple to program, the program runs reliably, and the structure is reasonable, it is very popular among factory engineers in the discrete manufacturing industry. In systems where the total number of I/O points does not exceed 10,000 and the switch I/O points account for more than 80%, this communication method can be used to operate stably and reliably.

If the communication speed is high, you can use point-to-point Ethernet communication. Using the controller's point-to-point communication commands, the standard Ethernet port allows the user to exchange data between the controllers or between the memory of the expansion controller. This PLC is widely used in a multi-CPU module communication method. Compared with the point-to-point network formed by the RS485 of the serial port, the speed of the Ethernet is greatly accelerated, and the advantages of simple connection and convenient programming are more convenient. Yes, it can communicate directly with the host computer via Ethernet, so it is very popular with users. Even in some systems with a single PLC and a PANELPC human-machine interface, since PANELPC usually has a built-in Ethernet port, users also use this communication method. At present, OpenPLC basically adopts such a method for some SCADA systems and remote monitoring systems and control systems in the continuous process industry.

There is also a distributed network that is the most widely considered structure in large PLC systems. The data of the object controller can be obtained by using a human machine interface (HMI) and a DDE, and the data of the controller can be obtained remotely through the Internet. Each CPU runs independently, and the data is accessed by the C/S mode through the Ethernet structure. The data acquisition and control functions are implemented in the CPU module of OpenPLC, and the data is saved in the ***** of the host computer. Data display and printing are implemented via the HMI interface and configuration software.

Six, software debugging

The PLC internally solidifies a set of system software that allows you to start the initialization work and configure the hardware. The PLC startup settings, watchdog, interrupt settings, communication settings, and I/O module address identification are all performed in the system software of the PLC.

Each PLC has its own programming software as a programming tool for the application. The common programming language is the ladder language, as well as ST, IL and other languages. How to program in a programming language is not covered here.

However, programming a very optimized program in a programming language is an expression of the level of engineer programming. Each PLC programming language has its own characteristics, and the design and layout of instructions are different. If you are familiar with the instructions of a PLC, you can program very simple, beautiful, and smooth. For example, for the same program design of the same PLC, if the programming engineer is unfamiliar with the instructions and the programming skills are poor, 1000 statements are required; but a highly skilled engineer may only need 200 statements. Achieve the same function. The simplicity of the program not only saves memory, but also has a much lower probability of error. The execution speed of the program is much faster, and it is much easier to modify and upgrade the program in the future.

Therefore, although all the ladder logic of PLC is similar, an engineer can familiarize himself with the programming of a PLC, and then learn the PLC of the second brand can quickly get started. However, when engineers use a new PLC, they should carefully read the new PLC programming manual carefully, look at the special features of the instructions, especially the instructions that they may use, and consider how to use these special The way to optimize your own programs.

The instruction design and interface design of the programming language of each PLC are different. There are no problems that are superior or inferior, mainly due to different styles. We can't arbitrarily say that Mitsubishi plc's programming language is not as good as Siemens' STEP7, nor can it say that STEP7 is better than ROCKWELL's RSLOGIX. The so-called good and bad, most of them are the programming habits formed by engineers and the design style of programming languages. .

It is often necessary to modify the programmed program on site. The reason for the modification may be that the user's needs have changed. It may be that the original programming error was found, or a power interruption occurred while the PLC was running. Some status data will be lost. If the non-retentive timer is reset, the input mapping area will be Refresh, the output map area may be cleared, but all configuration data of the status file and occasional events such as the counter's accumulated value will be saved.

Engineers may need to program the PLC at this time so that some memory can be restored to its default state. When the program does not need to be modified, you can design the application default path to restart, or use the function of the first scan bit.

All intelligent I/O modules, including analog I/O modules, lose their configuration data after entering programming mode or after power interruption. The user program must confirm that the configuration data can be Rewrite the smart I/O module.

One problem that is often overlooked when the site has been modified is that the engineer forgot to switch the PLC to programming mode. Although this error is not difficult to find, the engineer often mistakes the PLC for failure during the negligence, thus delaying a lot of time.

In addition, many PLCs are not allowed to interrupt the power supply when the PLC is downloading the program. At this time, the old program has been partially rewritten, but the new program is not completely written. Therefore, if the power supply is interrupted, the PLC will be caused. Unable to run, at this time, it may be necessary to reload the underlying software of the PLC, and many manufacturers are not allowed to do this in the field. Most new PLCs have separated the user program from the PLC's system program to avoid this problem.

Conclusion

The work of field engineers is very important. To be a good field engineer, you must first have a passionate and even passionate work on the spot, and then have a rigorous and patient attitude. The work environment on site is not as comfortable as it is in your own company, often dirty and messy, and because of the time and time constraints, the pressure is great. Moreover, to meet the requirements and accusations of users and other professional (such as electrical, instrumentation, process, pipeline construction, etc.) on-site personnel, the ability to coordinate and flexible needs to be improved. In addition, there is a big difference between field engineers and R&D engineers. At the scene, your most important task is to ensure that the system can run normally on time. If a plan does not work temporarily, you should immediately consider the second or even the third plan, even if it is first, let the system work normally. R & D work can not be modified, you must use this method to achieve this function, other methods can be achieved, but you can not use. Many field engineers often do not understand this difference, often drilled into the corner of the horn, and one road went to death, resulting in delays in on-site commissioning. This is why a good R&D engineer is often not a good field engineer. Engineers on site must have an open mind and a flexible mindset, which is based on a rigorous work attitude.

In addition, although the work pressure on the site is very large, it is necessary to pay attention to the combination of work and rest. Sometimes it seems necessary to leave the scene for three days and three nights. But for a programming and debugging engineer, it might be a good idea to go back to sleep, and the work will definitely be done faster. If the commissioning time on site is long, it is necessary to take a day or two to relax. Sometimes, swimming in the mountains is also a job.

In addition, an excellent field engineer relies mainly on the accumulation of work experience, and has little to do with personal talent. As long as he is diligent and has done enough projects, most people can become an excellent site. Engineer's. What time is it? It may be three years and five years, or it may be half a year.