Abstract: In modern industrial production, a large number of programmable control systems have been used. Programmable controllers can work normally under harsh working conditions, but their control systems sometimes appear due to factors such as design, installation, and interference. malfunction. Some problems are caused by ill-considered design of the system. According to the experience and lessons learned in practice, this article explains the issues that should be paid attention to when designing a programmable control system.
1. A mature technology used in a system should account for at least 75% of the "mature technology". First, it is a programmable controller product or similar design that has undergone a certain test of production practice, or it can be determined in the future production practice. The second test is that the design staff must have experience or have the ability to master it for the technologies that need to be used. It is feasible to design and configure the technology and design options for a programmable control system. Because a production process control system, once it is made, is used for a long time, it is difficult to find opportunities to repeatedly modify it. Designed hardware systems and programming software, some of which are missing, may remain hidden in the finished system. In the event of a destructive condition, the consequences are unpredictable.
2. The hardware structure and network of the system should be concise and clear. The hardware structure should not be tedious. The network configuration should not pursue too many crossover factors. It is necessary to use the networking capabilities of the programmable controller itself. When forming a template for an I/O chassis, it is recommended that the model be simple and consistent. The density of the template should not be too large. The junction point used should not be too much. From the current manufacturing and wiring process of the chassis, the input and output wiring density cannot be too high.
3. The functions of the control system and the functions of the management system should be strictly divided. Since the real-time requirements in the process control system composed of programmable controllers are high, the network communication is allowed to temporarily lose communication links, and then it can be resumed later. However, the programmable controller will be out of control during this interval before resuming. In addition, when using a plurality of programmable controller systems to form a large system, the key commands for the main control, in addition to using the programmable controller's own network communication to transmit its information, it is better to use it. O point made of hardware interlock, especially the "emergency stop" between the two; although both systems in their own communication scan with each other "stop" or "emergency stop" command, but because of one When the emergency stop failed, the operation was stopped. The other party did not receive the stopped information and operated as usual. The consequences were unpredictable. The key "emergency stop" of the programmable controller control system should cut off the power of the actuator first, and then send the signal to the programmable controller, so that the time of equipment safety protection can be obtained.
4. The program of the programmable controller should be concise and readable. The preparation of the user software is "flat and direct". The user software can be seen as an orderly "black box" series. Each "black box" is divided according to structured language. , can be divided into several typical sentences. Each sentence method and technique may be quite monotonous, but it must be clear. When designing and writing these statements, if you use too many logic relations that are not easy to reason, or if there are too many statement elements and too many special conditions, you will be very difficult to understand when reading these statements. Therefore, the readability of the user software of a programmable controller, that is, the written software can be understood by most people, and it is very important to understand what "What happened" when the programmable controller executes this statement. Each piece of the program strives for a single and smooth function, which is an important condition for software use and maintenance.
5, programmable control system in the hardware and software on the premise, there are key monitoring conditions for running the detection programmable control system is configured with a color graphics workstation / screen monitoring, but from the time of the price and reflect the status of the scene, the screen Monitoring is not convenient. Critical faults, or near critical mechanical equipment, can be equipped with indicators that can be made with digital outputs for monitoring the normal operation of the program or for debugging the program, with a function sign on the side of the indicator. Can help the operator to confirm the normal operation of the programmable control system and timely reflect the fault.
6, do not run out of its hardware and software resources in the design of large and medium-sized programmable control system For the design of the new system, the hardware must retain at least about 15% of the redundancy, when the software is compiled, it is also necessary to estimate the user software to the computer Resource needs and usage. Especially for the use of intermediate relays, counters/timers, there is room for improvement. Because after the commissioning and operation, the software will always be modified, supplemented, and even reprogrammed. The software that has been compiled cannot be modified and perfected, and it is not practical in engineering.
7. The redundant programmable logic control system that can reasonably configure the programmable logic controller system may make multiple ways of redundancy. The dual-system hot standby and cold standby redundancy of the central processor are common methods. In addition, dual-system redundancy, ie, the central processor and all input, output, and network communication are completely redundant. Although its price and practicality are difficult to be accepted in many engineering projects, it is a toxic and harmful chemical production environment. This redundancy is necessary. In the design system, it is necessary to make the configuration redundant mode more economical and practical, and strive to reduce the fault to the device. Do not cause a device to malfunction, causing the impact of the operation or state of the relevant equipment in the process flow.
The above-mentioned several aspects, is in the programmable control system overall plan design, must pay special attention to the question, only when designs the system, considers thoughtfully, after the system puts into operation, the design personnel can have less regret.
1. A mature technology used in a system should account for at least 75% of the "mature technology". First, it is a programmable controller product or similar design that has undergone a certain test of production practice, or it can be determined in the future production practice. The second test is that the design staff must have experience or have the ability to master it for the technologies that need to be used. It is feasible to design and configure the technology and design options for a programmable control system. Because a production process control system, once it is made, is used for a long time, it is difficult to find opportunities to repeatedly modify it. Designed hardware systems and programming software, some of which are missing, may remain hidden in the finished system. In the event of a destructive condition, the consequences are unpredictable.
2. The hardware structure and network of the system should be concise and clear. The hardware structure should not be tedious. The network configuration should not pursue too many crossover factors. It is necessary to use the networking capabilities of the programmable controller itself. When forming a template for an I/O chassis, it is recommended that the model be simple and consistent. The density of the template should not be too large. The junction point used should not be too much. From the current manufacturing and wiring process of the chassis, the input and output wiring density cannot be too high.
3. The functions of the control system and the functions of the management system should be strictly divided. Since the real-time requirements in the process control system composed of programmable controllers are high, the network communication is allowed to temporarily lose communication links, and then it can be resumed later. However, the programmable controller will be out of control during this interval before resuming. In addition, when using a plurality of programmable controller systems to form a large system, the key commands for the main control, in addition to using the programmable controller's own network communication to transmit its information, it is better to use it. O point made of hardware interlock, especially the "emergency stop" between the two; although both systems in their own communication scan with each other "stop" or "emergency stop" command, but because of one When the emergency stop failed, the operation was stopped. The other party did not receive the stopped information and operated as usual. The consequences were unpredictable. The key "emergency stop" of the programmable controller control system should cut off the power of the actuator first, and then send the signal to the programmable controller, so that the time of equipment safety protection can be obtained.
4. The program of the programmable controller should be concise and readable. The preparation of the user software is "flat and direct". The user software can be seen as an orderly "black box" series. Each "black box" is divided according to structured language. , can be divided into several typical sentences. Each sentence method and technique may be quite monotonous, but it must be clear. When designing and writing these statements, if you use too many logic relations that are not easy to reason, or if there are too many statement elements and too many special conditions, you will be very difficult to understand when reading these statements. Therefore, the readability of the user software of a programmable controller, that is, the written software can be understood by most people, and it is very important to understand what "What happened" when the programmable controller executes this statement. Each piece of the program strives for a single and smooth function, which is an important condition for software use and maintenance.
5, programmable control system in the hardware and software on the premise, there are key monitoring conditions for running the detection programmable control system is configured with a color graphics workstation / screen monitoring, but from the time of the price and reflect the status of the scene, the screen Monitoring is not convenient. Critical faults, or near critical mechanical equipment, can be equipped with indicators that can be made with digital outputs for monitoring the normal operation of the program or for debugging the program, with a function sign on the side of the indicator. Can help the operator to confirm the normal operation of the programmable control system and timely reflect the fault.
6, do not run out of its hardware and software resources in the design of large and medium-sized programmable control system For the design of the new system, the hardware must retain at least about 15% of the redundancy, when the software is compiled, it is also necessary to estimate the user software to the computer Resource needs and usage. Especially for the use of intermediate relays, counters/timers, there is room for improvement. Because after the commissioning and operation, the software will always be modified, supplemented, and even reprogrammed. The software that has been compiled cannot be modified and perfected, and it is not practical in engineering.
7. The redundant programmable logic control system that can reasonably configure the programmable logic controller system may make multiple ways of redundancy. The dual-system hot standby and cold standby redundancy of the central processor are common methods. In addition, dual-system redundancy, ie, the central processor and all input, output, and network communication are completely redundant. Although its price and practicality are difficult to be accepted in many engineering projects, it is a toxic and harmful chemical production environment. This redundancy is necessary. In the design system, it is necessary to make the configuration redundant mode more economical and practical, and strive to reduce the fault to the device. Do not cause a device to malfunction, causing the impact of the operation or state of the relevant equipment in the process flow.
The above-mentioned several aspects, is in the programmable control system overall plan design, must pay special attention to the question, only when designs the system, considers thoughtfully, after the system puts into operation, the design personnel can have less regret.
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