Key points in designing PLC control system for water industry

Abstract: In modern industrial production equipment, there are a large number of digital and analog control devices, such as the start and stop of motors, the opening and closing of solenoid valves, the counting of products, the setting and control of temperature, pressure, and flow, etc. For these automatic control problems in industrial sites, the use of programmable logic controllers (PLCs) to solve automatic control problems has become one of the most effective tools. This article describes the issues that should be paid attention to when designing PLC control systems.

In modern industrial production equipment, there are a large number of digital and analog control devices, such as the start and stop of motors, the opening and closing of solenoid valves, the counting of products, the setting and control of temperature, pressure, and flow, etc. These automatic control problems in industrial sites have become one of the most effective tools if programmable controllers (PLCs) are used to solve automatic control problems. This article describes the issues that should be paid attention to when designing PLC control systems.
There are many PLC products on the market at present. Domestic brands include: Yonghong, Hollysys, Kaidian, etc.; foreign brands include: Japan's OMRON, MITSUBISHI, Panasonic; Germany's SIEMENS, South Korea's LG, etc. In recent years, the price of PLC products has dropped significantly, and its cost performance has become higher and higher. This is an important reason why many technicians choose PLC. However, there are large differences in performance indicators among PLCs of various brands. Therefore, how to choose PLC products has become an important part of system design and system function requirements.
1. System scale First, we should determine whether the system is controlled by a single PLC or a network, and calculate the number of PLC input and output points. When purchasing a PLC, we should leave a certain margin (10%) based on the actual number of points needed.
2. Determine the load type. According to whether the load carried by the PLC output terminal is DC or AC, whether it is a large current or a small current, and the frequency of the PLC output point action, we can determine whether the output terminal uses a relay output, a transistor output, or
a thyristor output. It is very important for the stable operation of the system to use different output methods for different loads.
3. Storage capacity and speed. Although the PLC products of foreign manufacturers are generally the same, there are certain differences. At present, no fully compatible products between companies have been found. The development software of each company is different, and the storage capacity of the user program and the execution speed of the instruction are two important indicators. Generally, the larger the storage capacity and the faster the speed of the PLC, the higher the price, but the PLC product should be reasonably selected according to the size of the system.
4. The selection of programmers PLC programming can be done in three ways:
one is programming with a general handheld programmer, which can only program with statements in the statement table specified by the merchant. This method is inefficient, but it is more suitable for products with small system capacity and small usage, and it is small in size, easy to debug on site, and has a low cost.
The second is programming with a graphic programmer, which uses ladder diagram programming, which is convenient and intuitive. General electrical personnel can use it freely in a short period of time, but the price of this programmer is relatively high.
The third is programming with a personal computer plus a PLC software package, which is the most efficient method. Based on the popularity of notebook computers and the convenience of PLC software programming, and easy on-site debugging. This method is the most popular method for users.
Therefore, PLC products should be reasonably selected based on the size and difficulty of the system, the length of the development cycle, and the financial situation.
5. Try to choose products from large companies whose quality is guaranteed, and have good technical support. Generally, after-sales service is also good, which is also conducive to your product expansion and software upgrade.
Design of input circuit
1. The power supply of the PLC power supply circuit is generally AC85-240V (also DC24V), which is suitable for a wide power supply range, but in order to resist interference, power purification components (such as power filter, 1:1 isolation transformer, etc.) should be installed.
2. The use of DC24V power supply on PLC. PLC products of various companies generally have DC24V power supply, but the power supply capacity is small, ranging from tens of milliamperes to hundreds of milliamperes. When using it with load, pay attention to the capacity and take anti-short circuit measures (because the overload or short circuit of the power supply will affect the operation of the PLC).
3. If the input circuit of the external DC24V power supply has a DC24V-powered proximity switch, photoelectric switch, etc., and the DC24V power supply capacity on the PLC is insufficient, a DC24V power supply must be provided from the outside; but the "-" end of the power supply should not be connected to the DC24V "-" end and the "COM" end of the PLC, otherwise it will affect the operation of the PLC.
4. Input sensitivity Each manufacturer has regulations on the voltage and current of the PLC input terminal. For example, the input value of the FX series PLC of Mitsubishi Corporation is: DC24V, 7mA, the starting current is 4.5mA, and the shut-off current is less than 1.5mA. Therefore, when there is a diode or resistor in series in the input circuit (it cannot be fully started), or there is a parallel resistor or leakage current (it cannot be completely cut off), there will be false operation and the sensitivity will decrease. 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 will also cause false operation. Weak current input devices should be used, and PLCs with common drain input should be selected. The common point potential of the input element is relatively negative, and the current flows out of the input terminal of the PLC.
Design of output circuit
1. Comparison between various output methods
(1) Relay output: The advantage is that different common points can carry different AC and DC loads, and the voltage can also be different. The load current can reach 2A/point; but the relay output method is not suitable for high-frequency loads, which is determined by the life of the relay. Its lifespan decreases with the increase of load current, generally between hundreds of thousands and millions of times, and some companies' products can reach more than 10 million times, with a response time of 10ms . (
2) Thyristor output: The load capacity is 0.2A/point, it can only carry AC load, can adapt to high-frequency operation, and the response time is 1ms.
(3) Transistor output: The biggest advantage is that it is suitable for high-frequency operation and has a short response time, generally around 0.2ms, but it can only carry DC 5-30V load, and the maximum output load current is 0.5A/point, but it should not exceed 0.8A for every 4 points.
When the output frequency of your system is less than 6 times per minute, relay output should be the first choice, because its circuit design is simple, anti-interference and load-carrying capacity is strong. When the frequency is less than 10 times/min, you can use relay output; you can also use PC output to drive Darlington transistor (5-10A), and then drive the load (see Figure 2), which can greatly reduce
2. Anti-interference and external interlocking When the PLC output carries an inductive load, the load will cause a surge current impact on the PLC output
when the power is off. For this reason, a freewheeling diode should be connected next to the DC inductive load, and a surge absorption circuit should be connected to the AC inductive load, which can effectively protect the PLC. After the outputs of two physical quantities have been software interlocked inside the PLC, they should also be interlocked outside the PLC to enhance the reliability of the system.
3. Selection of "COM" points Different PLC products have different numbers of "COM" points. Some have 8 output points, some have 4 output points, and some have 2 or 1 output points. When there are many types of loads and large currents, use a PLC product with 1-2 output points on a "COM" point; when there are many loads and few types, use a PLC product with 4-8 output points on a "COM" point, which will bring a lot of convenience to circuit design. Add a fuse at each "COM" point. Add a 2A fuse for 1-2 outputs, and add a 5-10A fuse for 4-8 outputs, because there is generally no fuse inside the PLC.
4. PLC external drive circuit When the PLC output cannot directly drive the load, a drive circuit must be used externally: it can be driven by a triode, or a solid-state relay or thyristor circuit. At the same time, a protection circuit and a surge absorption circuit should be used, and each channel should have a display diode
(LED) indication. The printed circuit board should be made plug-in and easy to maintain.
There are also certain requirements for the input and output wiring of the PLC. Please refer to the user manual of each company.
Selection of expansion modules
For small systems, such as systems with less than 80 points, expansion is generally not required; when the system is larger, expansion is required. Products from different companies have restrictions on the total number of system points and the number of expansion modules. When expansion still cannot meet the requirements, a network structure can be used; at the same time, some manufacturers' individual instructions do not support expansion modules, so pay attention when compiling software. When using analog modules such as temperature, each manufacturer also has some regulations, please refer to the relevant technical manual. There are
many types of expansion modules from various companies, such as single input modules, single output modules, input and output modules, temperature modules, high-speed input modules, etc. This modular design of PLC provides convenience for user product development.
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, have a deeper understanding of its basic instructions and functional instructions, and the execution speed of instructions and the storage capacity of user programs should also be carefully understood. Otherwise, it cannot meet your real-time requirements and cause system crashes. In addition, the communication interface, communication protocol, data transmission speed, etc. should also be considered.
Finally, you should also seek network design and software technical support and detailed technical information from the PLC vendor. As for the number of workstations to choose, it depends on your control requirements and system size.
Software compilation
Before compiling the software, you should first be familiar with the software manual and programming instruction manual of the selected PLC product, and then program after you are proficient. If you use a graphic programmer and software package for programming, you can program directly. If you use a handheld programmer for programming, you should first draw a ladder diagram and then program, which can reduce errors and speed up. If you use a personal computer and software for programming, simulate the program after programming, and then debug on the equipment after each action is normal.