Talking about the design points of PLC control sys

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Talking about the design points of PLC control system and its problems in use

1 introduction

plc is the basic platform of industrial automation. The primary problem in the design of PLC application system is the project selection and the architecture design of programming platform. Secondly, when solving automation engineering problems, PLC first faces the problem of the engineering design correctness of the input and output system. For a large number of digital and analog control devices, such as the opening and closing of solenoid valves, the start and stop of motors, the setting of temperature, pressure and flow, the counting and control of products, etc., these automatic control nodes in the industrial field need to be classified in advance for engineering analysis and correct design

2 type selection design of PLC and programming system

there are many kinds of PLC products in today's industry market. In addition to domestic brands, foreign brands include Omron, Hitachi, Fuji, Idec, Panasonic, m, which can reduce (increase) the temperature of the experimental container outside the tank itsubishi, Siemens in Germany, LG in South Korea, etc. How to reasonably select PLC products

2.1 communication requirements and i/o points margin

first, it should be determined whether the system is controlled by a single PLC or formed by a PLC network, so as to calculate the input and output (i/o) points, and when purchasing a PLC, a 10% margin should be reserved on the basis of the actual number of points required

2.2i/o load analysis

determine the load type. According to whether the load on the PLC output end is DC or AC, whether it is high current or low current, and the frequency of PLC output point action, it is determined whether the output end adopts relay output, transistor output, or thyristor output. It is very important for the stable operation of the system to choose different output modes for different loads

2.3 running speed

execution speed. Storage capacity and instruction execution speed are important indicators of PLC selection. Generally, the larger the storage capacity and the faster the speed of PLC, the higher the price. Although the products of foreign manufacturers are generally the same, there are also certain differences

2.4 independent i/o common port (COM) grouping size

"com" point selection. Different PLC products have different numbers of "com" port points. Some have 8 output points, some have 4 output points, and some have 1 or 2 output points. When there are many kinds of loads and the current is large, use a "com" point with 1-2 output points. When there are many kinds of loads, use a "com" point with 4-8 output points

2.5 system compatibility

system compatibility. Because the development software of each manufacturer is different, the compatibility of the system is also the focus of purchase. At present, no fully compatible products have been found, so PLC products should be selected reasonably according to the system

2.6 programming platform

selection of programmers. PLC can be programmed in three ways: one is to use a general hand-held programmer. It can only be programmed with the statements in the statement table specified by the manufacturer. Its advantages are easy on-site debugging, small size and low cost, but its efficiency is low, and it is suitable for systems with small system capacity and consumption. The second is graphic programmer programming. This method adopts graphic programming, which is convenient and intuitive. General electrical personnel can use it freely in a short time, but the programmer price is high. Third, programming with IBM and its compatible personal computer +plc software package is the most efficient way and the most commonly used way, but most software packages are expensive

2.7plc manufacturers choose

try to choose products from large companies. Because the products of large companies have guaranteed quality, good technical support and general after-sales service, which is conducive to the expansion of products and the upgrading of software and hardware in the future

3 design of input and output circuits

3.1 main body power circuit

plc power supply is generally ACV (also DC24V), which is suitable for a wide range of power supply, but in order to resist interference, power purification elements (such as power filter, 1:1 isolation transformer, etc.) should be added

3.2 external i/o drive power supply

if the input circuit has proximity switches and photoelectric switches with DC24V power supply, and the DC24V power supply capacity on the PLC is not enough, the DC24V power supply should be provided from the outside, but the "one" end of the power supply should not be connected with the "one" and "com" ends of the DC24V power supply of the PLC, otherwise the operation of the PLC will be affected

3.3 self provided i/o drive power supply

PLC products of all companies generally have DC24V power supply, but the capacity of the power supply is small, ranging from tens of milliamps to hundreds of milliamps. When using it with load, pay attention to the capacity and take measures to prevent short circuit (because the overload or short circuit of the power supply will affect the operation of PLC)

3.4 input sensitivity

the manufacturer has regulations on the input voltage and current of PLC. When the input current of the input element is greater than the maximum input current of PLC or there is leakage current, there will be misoperation and reduce the sensitivity. Therefore, weak current input should be applied and protective measures should be taken for the leakage current, and PLC with common leakage input should be selected. When the two-wire sensor (photoelectric switch, contactless switch) has led limit switch, the input leakage current will produce wrong input or light up. The countermeasure is to connect the discharge resistance to reduce the input impedance

for the output terminal of transistor or bidirectional thyristor, if it is connected to the equipment with a large impulse current, load protection must be considered

3.5 formula for calculating paperboard strength inductive load treatment

when the input and output ends are connected with inductive load, an impact suppressor or freewheeling diode should be connected in parallel at both ends of the load

3.6 PLC external drive circuit

when the PLC output cannot directly drive the load, the drive circuit must be used externally, which can be driven by solid-state relay or thyristor circuit. At the same time, the protection circuit and surge absorption circuit should be used. In addition, the input and output wiring of PLC also has certain requirements. Please refer to the operating instructions of each company. In particular, for common AC220V inductive small loads, such as AC contactors, AC coil relays, solenoid valves, etc., although the technical nominal may meet the direct drive specification, the simple design of PLC direct drive is absolutely not advocated in the project, and it should be driven indirectly through the intermediate relay

4 selection of expansion module

for small systems, such as systems within 80 points, expansion is generally not required; When the system is large, it needs to be expanded. The products of different companies have restrictions on the total number of system points and the number of expansion modules. When the expansion still cannot meet the needs, the network structure can be used. At the same time, some manufacturers' individual instructions do not support expansion modules, so pay attention when programming software. When analog modules such as temperature are used, manufacturers also have some regulations. Please refer to the relevant technical manuals

5plc communication design

when using PLC for network design, it is much more difficult than PLC single machine control. First of all, you should choose a familiar model, have a deeper understanding of its basic instructions and functional instructions, and carefully understand the execution speed of instructions and the storage capacity of user programs. Otherwise, it cannot meet the real-time requirements, resulting in system crash. In addition, the communication interface, communication protocol and data transmission speed should also be considered

finally, Chen Bo has a doctor's degree in advanced molecular materials, and seeks network design, software support and detailed technical information from the PLC manufacturer. As for the selection of several layers of workstations, it depends on the size of the system

6 software programming

before compiling the software, you should first be familiar with the selected PLC product manual, and then program after being familiar with it. If the graphic programmer or software package is used for programming, it can be programmed directly. If the hand-held programmer is used for programming, the ladder diagram should be drawn first, and then the programming, so that errors can be reduced and the speed is fast. After the programming is completed, it runs empty first, and then it can be debugged on the equipment after all actions are normal

7problems needing attention in PLC application

although PLC is a basic equipment of front-end automation technology used in industrial sites, it can be directly used in most industrial environments. However, when the production environment is too bad, the electromagnetic interference is particularly strong, or the installation and use are improper, it may cause program errors or calculation errors, resulting in wrong input and wrong output, which will cause out of control and misoperation of the equipment, and thus cannot guarantee the normal operation of PLC. To improve the reliability of PLC control system, on the one hand, PLC manufacturers are required to improve the anti-interference ability of the equipment; On the other hand, it is required to attach great importance to design, installation, use and maintenance, and multi-party cooperation can improve the problem and effectively enhance the anti-interference performance of the system. Therefore, the following problems should be paid attention to in use

7.1 ensure the PLC working environment

(1) temperature. PLC requires that the ambient temperature is 0-55 degrees Celsius. During installation, it cannot be placed under components with high calorific value, and the surrounding ventilation and heat dissipation space should be large enough

(2) humidity. In order to ensure the insulation performance of PLC, the relative humidity of the air should be less than 85% (no condensation)

(3) air. Avoid corrosive and flammable gases, such as hydrogen chloride, hydrogen sulfide, etc. For the environment with more dust or corrosive gas in the air, the PLC can be installed in the control room or control cabinet with good sealing

(4) vibration. PLC shall be kept away from strong vibration source to prevent frequent or continuous vibration with vibration frequency of 10~55hz. When vibration is unavoidable in the use environment, damping measures must be taken, such as the use of damping glue

(5) power supply. PLC has certain resistance to the interference brought by the power line. In the environment with high reliability requirements or particularly serious power interference, an isolation transformer with a shielding layer can be installed to reduce the interference between the equipment and the ground. Generally, PLC has DC 24V output to the input end. When the input end uses external DC power supply, DC regulated power supply should be selected. Because the common rectifier filter power supply, due to the influence of ripple, it is easy for PLC to receive error information

7.2 interference in the design of control system and its source

on site electromagnetic interference is one of the most common factors in PLC control system and is most likely to affect the reliability of the system. The so-called "addressing the symptoms first, addressing the root causes, finding out the problem, and then putting forward solutions to the problem. Therefore, the source of on-site interference must be known

(1) interference sources and general classification. Most of the interference sources that affect the PLC control system are generated in the parts where the current or voltage changes violently. The reason is that the current changes to produce a magnetic field, which produces electromagnetic radiation to the equipment; The magnetic field changes to generate current, and the electromagnetic wave generated by electromagnetic high-speed decreases by about 45% year-on-year. Generally, electromagnetic interference is divided into common mode interference and differential mode interference according to different interference modes

(2) main sources and ways of interference in PLC system

for strong current interference, the normal power supply of PLC system is powered by electricity. Due to the wide electrical coverage, it will be subject to all space electromagnetic interference and induced voltage on the road. In particular, the internal changes of electricity, knife switch operation surges, start and stop of large power equipment, harmonics caused by AC and DC drives, electrical short-circuit transient shocks, etc., are transmitted to the original side of the power supply through the transmission line. Interference in the cabinet, high-voltage electrical appliances in the control cabinet, large inductive loads, and chaotic wiring are easy to cause a certain degree of interference to PLC

interference from signal line. All kinds of signal transmission lines connected with PLC control system, in addition to transmitting all kinds of effective information, there will always be external interference signal intrusion. There are two main ways of this interference: one is to supply power through the transmitter

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