Key points of PLC control system design, safety, fault prevention - Database & Sql Blog Articles

Filter 18.432M

In modern industrial production equipment, there are a large number of digital and analog control devices, such as motor start and stop, solenoid valve opening and closing, product counting, temperature, pressure, flow setting and control, etc. In the automatic control problem, if the programmable controller (plc) is used to solve the automatic control problem, it has become one of the most effective tools. This paper describes the problems that should be paid attention to when designing the PLC control system.
There are many PLC products on the market at present. In addition to domestic brands, there are foreign countries: Japan's OMRON, MITSUBISHI, FUJJ, anasonic, Germany's SIEMENS, and South Korea's LG. In recent years, the price of PLC products has dropped significantly, and its cost performance is getting higher and higher. This is an important reason for many technicians to choose PLC. So, how to choose PLC products?
1, the system scale should first determine whether the system uses PLC single machine control, or use PLC to form a network, thereby calculating PLC input and output points. The number, and when purchasing the PLC, it is necessary to leave a certain margin (10%) based on the actual number of points required.
2. Determine the load type according to whether the load on the PLC output is DC or AC, whether it is a large current or a small current, and the frequency of the PLC output point action, so as to determine whether the output terminal uses the relay output, or the transistor output, or the product. Brake output. Different loads use different output modes, which is very important for the stable operation of the system.
3, storage capacity and speed Although the PLC products of foreign manufacturers are generally the same, but there are certain differences. Products that are fully compatible between companies have not yet been discovered. The development software of each company is different, and the storage capacity of the user program and the execution speed of the instructions are two important indicators. Generally, the higher the storage capacity and the faster the PLC, the higher the price, but the PLC product should be selected according to the size of the system.
4, the programmer's optional plc programming can be used in three ways:
It is programmed with a general handheld programmer, which can only be programmed with statements in the merchant's specified statement table. This method is inefficient, but for systems with small capacity and small dosage, it is suitable, small in size, easy to debug on site, and low in cost.
It is programmed with a graphic programmer. The programmer uses ladder diagram programming, which is convenient and intuitive. The general electrician can apply it freely in a short period of time, but the programmer is expensive.
It is programmed with IBM personal computer plus PLC software package. This method is the most efficient one, but most of the company's PLC development software packages are expensive, and this method is not easy to debug on site.
Therefore, PLC products should be purchased reasonably according to the size and difficulty of the system, the length of the development cycle, and the situation of funds.
5, try to choose the products of large companies, the quality is guaranteed, and the technical support is good, the general after-sales service is also good, it is also conducive to your product expansion and software upgrade.
First, the design of the input circuit
1. Power supply loop PLC power supply is generally AC85-240V (also has DC24V), which is suitable for wide power supply range, but in order to resist interference, power purification components (such as power supply filter, 1:1 isolation transformer, etc.) should be installed.
2. The use of DC24V power supply on PLC generally has DC24V power supply on PLC products of various companies, but the power supply capacity is small, from tens of milliamperes to several hundred milliamperes. Pay attention to the capacity when using the load, and make short circuit Measures (because the overload or short circuit of the power supply will affect the operation of the PLC).
3. External DC24V power supply If the input circuit has DC24V power supply proximity switch, photoelectric switch, etc., and the DC24V power supply capacity on the PLC is not enough, the DC24V power supply should be provided from the outside; but the "-" end of the power supply should not be the DC24V of the PLC. - "End" and "COM" end are connected, otherwise it will affect the operation of the PLC.
4. Input sensitivity Each manufacturer has regulations on the input voltage and current of the PLC. For example, the input value of the Japanese Mitsubishi F7n series PLC is C24V, 7mA, the starting current is 4.5mA, and the shutdown current is less than 1.5mA. Therefore, when the input loop string has a diode or a resistor (not fully activated), or if there is a parallel resistor or a leakage current (which cannot be completely cut off), there is a malfunction and the sensitivity is lowered. Measures should be taken for this. On the other hand, when the input current of the input device is greater than the maximum input current of the PLC, it may cause malfunction. A weak current input device should be used, and the PLC with the input common-drain input should be selected, and the common point potential of the Bp input component should be selected. Relatively negative, the current is flowing out of the input of the PLC.
Second, the design of the output loop
1. Comparison between various output modes (1) Relay output:
The advantage is that different common points can have different AC and DC loads, and the voltage can be different, with load current up to 2A/point; but the relay output mode is not suitable for high frequency action load, which is the life of the relay decided. Its life span decreases with the increase of load current, generally between hundreds of thousands of times to Jl million times, some companies can reach more than 10 million times, and the response time is 10ms.
(2) Thyristor output:
Load capacity is 0.2A/point, only with AC load, can adapt to high frequency action, response time is 1ms.
(3) Transistor output:
The biggest advantage is that it is suitable for high frequency action, and the response time is short, generally about 0.2ms, but it can only carry DC 5-30V load, the maximum output load current is 0.5A/point, but not more than 0.8A every 4 points. .
When your system output frequency is less than 6 times per minute, the relay output should be preferred because of its simple circuit design, anti-interference and strong load capacity. When the frequency is below 10 times/min, the relay output mode can be used. The PLC output can also be used to drive the Darlington transistor (5-10A), and then the load can be greatly reduced.
2. Anti-interference and external interlock When the PLC output has an inductive load, the load will cause a surge current impact on the PLC output. For this reason, the DC inductive load should be next to it and connected to the free-flow diode. The load should be connected to the surge absorbing circuit to effectively protect the PLC. When the outputs of the two physical quantities have been interlocked within the PLC, interlocking should also be performed outside the PLC to enhance the reliability of the system.
3, "GOM" point selection of different PLC products, the number of "COM" points is not the same, some "COM" points with 8 output points, and some with 4 output points, also with 2 Or 1 output point. When there are many types of loads and the current is large, a PLC product with one or two output points of "COM" point is used. When the number of loads is large and the type is small, a "COM" point is used with 4-8 output points. PLC products. This will bring a lot of convenience to the circuit design, adding a fuse at each "COM" point, adding 2A fuses for 1-2 outputs, and 5-10A fuses for 4-8 points output, due to PLC There is generally no fuse inside.
4. PLC external drive circuit For the case where the PLC output cannot directly drive the load, the drive circuit must be used externally: it can be driven by a triode or a solid state relay or a thyristor circuit, and a protection circuit and a surge absorbing circuit should be used. And each channel has a display diode (LED) indication. The printed board should be pluggable and easy to repair.
There are also certain requirements for the input and output wiring of the PLC. Please refer to the instruction manual of each company.
Third, the expansion module is selected for small systems, such as systems within 80 points. Generally no expansion is required; when the system is large, it needs to be expanded. Different company products have restrictions on the total number of points of the system and the number of expansion modules. When the expansion still cannot meet the requirements, the network structure can be adopted. At the same time, the individual instructions of some manufacturers do not support the expansion modules. Therefore, the software is compiled. Pay attention to it. When using analog modules such as temperature, there are some regulations for each manufacturer. Please refer to the relevant technical manual.
There are many types of expansion modules for each company, such as single input modules, single output modules, input and output modules, temperature modules, high speed input modules, and so on. This modular design of the PLC provides convenience for the user's product development.
Fourth, PLC network design When using PLC for network design, its difficulty is much greater than PLC stand-alone control. First of all, you should choose a model that you are more familiar with, and have a deeper understanding of its basic and functional instructions, and the execution speed of the instructions and the storage capacity of the user program should also be carefully understood. Otherwise, you can't adapt to your real-time requirements, causing the system to crash. In addition, communication interfaces, communication protocols, data transfer speeds, etc. should also be considered.
Finally, you should also seek network design and software technical support and detailed technical information from the PLC merchants. As for the selection of several layers of workstations, depending on the size of your system.
V. Software Preparation Before programming the software, you should first familiarize yourself with the software manual of the selected PLC product, and then program it after being proficient. If you are programming with a graphics programmer or software package, you can program it directly. If you are programming with a handheld programmer, you should first draw a ladder diagram and then program it, so that you can make fewer mistakes and speed. After the programming is finished, the air conditioning program is first executed, and after each operation is normal, the device is debugged.
Sixth, safety issues When designing a programmable controller, safety is the first important. Even when an external power supply is abnormal, when the programmable controller fails, the entire system can work in a safe state. Please be sure to Outside the programming controller, setting a safety circuit, misoperation, and false output are the causes of an accident.
1. Be sure to set the following circuits outside the programmable controller.
Emergency stop circuit, protection circuit, reverse rotation, etc., the reverse operation of the interlock circuit, positioning of the upper limit / lower line, etc. to prevent damage to the chain circuit.
2. The programmable controller CPU has a self-test function such as a watchdog timer. The programmable controller CPU monitors the timer and other self-test functions. When an error is detected, the outputs are all turned off, and the CPU that is the programmable controller cannot detect the input and output. When the abnormality of the control part is abnormal, the output cannot be controlled. In this case, in order to operate the machine in a safe state, please design the external circuit and mechanism.
3. Designed to ensure the safety of the sensor's backup power supply when overloaded:
When the sensor's backup power supply is overloaded, the voltage will automatically drop. Except that the programmable controller input does not work, all outputs are turned off. In this case, design the external circuit and mechanism to work under safe conditions.
4. Designed to ensure machine safety when there is an output signal associated with a major fault:
Due to the output relay, the failure of the transistor, the output is turned ON, and sometimes it is OFF. When there is an output signal related to a major fault, please design the external circuit and mechanism to make the machine operate under safe conditions.
VII. Fault prevention when designing programmable control system In modern industrial production, a large number of programmable control systems are used. The programmable controller can work normally under harsh working conditions, but the control system composed of it is designed. Factors such as installation, interference, etc. sometimes fail. Some problems are caused by poorly designed systems. According to the experience and lessons in practice, this paper describes the problems that should be paid attention to when designing programmable control systems.
1. The mature technology used in a system should account for at least 75% of “mature technologies”. One is a programmable controller product or similar design that has undergone certain production practices, or it can be determined in future production practices. The second test is that the design staff must have experience or master the skills needed to use the technology. It is practical to design and configure a technology and design solution for a programmable control system. Because a production process control system, once made, will be used for a long time, it is difficult to find opportunities to repeatedly modify. Designed hardware systems and programming software, some of which are missing, may remain hidden in the completed system. If the conditions of the destructive effect are encountered, the consequences are difficult to predict.
2. The hardware structure and network of the system should be concise and clear. The hardware structure should not be cumbersome. 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 the template for the I/O chassis, it is recommended that the model is simple and consistent, and the template density should not be too large. The junction points used should not be too much. From the current manufacturing and wiring process of the chassis, the input and output wiring density should not be too high.
3. The function of the control system and the function of the management system should be strictly divided. Because the real-time requirements in the process control system composed of the programmable controller are very high, the network communication allows the communication link to be temporarily lost, and can be restored afterwards. However, the programmable controller will be out of control at this interval before resuming. In addition, when using a plurality of programmable controller systems to form a large system, it is preferable to use the I/ for the key commands of the main control in addition to using the programmable controller's own network communication to transmit its information. The hardware interlock made by point O, especially the "emergency stop" between the two; although both systems change the "stop" or "emergency stop" commands in their own communication scans, When the emergency stop is faulty, the operation has stopped. The other party has not received the stopped information and runs as usual. The consequences are unpredictable. The key "emergency stop" of the programmable controller control system should first remove the power of the actuator and then send its signal to the programmable controller, so that the safety time of the device can be obtained.
4, programmable controller program should be concise and readable user software is written "tiling straight", user software can be seen as an ordered "black box" series, each "black box" according to structured language Can be divided into several typical statements. Each statement method and technique may be very monotonous, but it must be clear. When designing and writing these statements, if you use too many logical relationships that are difficult to reason, or if there are too many statement factors and too many special conditions, it 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 read by most people, 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 the software to be used and maintained.
5, programmable control system in hardware and software presets, there are key monitoring conditions for running detection programmable control system configured with color graphics workstation / screen monitoring, but from the price and time to reflect the status of the scene, the screen Monitoring is not convenient. Critical faults, or near critical mechanical equipment, can be configured with indicators that can be made with digital outputs to monitor the normal operation of the program, or to debug the program, with a functional sign next to the indicator light. It can help the operator to confirm the normal operation of the programmable control system and reflect the fault in time.
6. Don't exhaust its hardware and software resources when designing a large and medium-sized programmable control system. For the new system designed, at least 15% of the redundancy should be reserved on the hardware. In software compilation, the user software should also be estimated. The needs and usage of resources. Especially for the use of intermediate relays, counters/timers, there is room for it. Because after debugging and running, the software will always be modified, supplemented, or even re-programmed. The software that has been compiled cannot be modified or improved, and it is not practical in engineering.
7. The redundant programmable control system that properly configures the programmable 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, the dual system redundancy, that is, the central processing unit and all the input, output, and networking communication are completely redundant, and its price and practicability are difficult to be accepted in many engineering projects, but in a toxic and harmful chemical production environment. This redundancy is necessary. In the design system, it is economical and practical to make the configuration redundancy method, and strive to make the fault shrink on the device. Do not cause the equipment to operate or the state of the process is affected by the failure of a certain equipment.
The above-mentioned aspects are the issues that should be paid special attention to when designing the overall control system of the programmable control system. Only when designing the system, consider the thoughtfulness, the designer can be less regretful after the system is put into operation.