Details
Programmable controller (PLC) is a new type of industrial controller developed by microprocessor-based, integrated computer technology and automatic control technology, which is widely used in various fields of industrial control. In modern industrial production equipment, there are many digital and imitation control equipment, such as the start and stop of the motor, solenoid valve opening and closing, product counting, temperature, pressure and flow of the setting and control, etc., in the industrial site to use the programmable controller (PLC) to solve the problem of automatic control has become one of the most useful methods. This article describes the matters that should be noted in the planning of PLC control system.
A. Hardware purchase
Now there are many PLC products on the market, more and more cost-effective, the optional scale is getting bigger and bigger, for this reason, we should think in general from the following aspects when purchasing, reasonable purchase of PLC products: 1.
1. System scale The first thing should be determined system is to use PLC stand-alone control, or PLC network control, and thus calculate the number of PLC input and output points In determining the number of PLC input and output points usually need to leave a 10% margin on the basis of the number of points required in practice.
2. load type according to the PLC output with the load is DC type or AC type, is a large current or small current, as well as the PLC output point action frequency and other factors to determine the output selection relay output, or transistor output, or pin gate output different load selection of different output methods, which is very important for the stable operation of the system.
3. Storage capacity and speed Usually the larger the storage capacity, the faster the speed of the PLC offer is higher We should not blindly seek high capacity and high speed when selecting, but should be based on the size of the system and the reasonable choice of PLC products.
4. Operating environment The operating environment is a hard indicator of PLC operation. The automatic control system will be released from the busy work and harsh environment, it is requested that the automatic control system can use the chaotic environment, such as temperature, humidity, noise, signal shielding, operating voltage, etc., must be selected to use the practical operating environment of the product.
5. After-sales service Should try to use the products of large companies, their quality is guaranteed, and good technical support, usually better after-sales service, but also conducive to the expansion of your products and software upgrades. In after-sales service, in this regard, Coolmay company to do quite in place, our technical customer service will be the first time to help you solve your problems.
Second, the input circuit planning considerations.
1. Power supply circuit PLC power supply is usually AC85-240V (also DC24V), the use of power supply scale is wide, but in order to anti-interference, should be installed with power purification components (such as power filter 1: 1 blocking transformer, etc.).
2. The use of DC24V power supply on PLC There are usually DC24V power supplies on PLC products of various companies, but the capacity of this power supply is small, from tens of mA to hundreds of mA, so attention should be paid to the capacity when using it with load, and there should be a short-circuit maintenance method at the same time.
If there are DC24V-powered switches near the input circuit, such as optical switches, and the capacity of DC24V power supply on PLC is not available, DC24V power supply should be supplied from outside, but the - end of the power supply should not be connected with the DC24V end and COM end of PLC, otherwise it will affect the operation of PLC. Otherwise, the operation of the PLC will be affected.
4. Input sensitivity When the input voltage and current are less than the rules of each manufacturer on the voltage and current of the input terminal of PLC, such as when there are diodes or resistors in the input circuit (which cannot be completely started), or perhaps there are parallel resistors or leakage current (which cannot be completely blocked), there will be a false action and the sensitivity will be reduced, and the measures should be adopted for this. When the input current of the input device is greater than the maximum input current of the PLC, will also cause misoperation, should be selected weak current input devices, and the use of inputs for the common drain type input PLC.
Third, the output circuit planning considerations.
1. Output method selection
(1) relay output: the advantage is that different common points can be brought between different AC and DC loads, and the voltage is not the same, with load current up to 2A points; but the relay output method is not suitable for high-frequency action of the load, which is determined by the life of the relay
(2) thyristor output: with load capacity of 0.2A/ point, only with communication load, can use high-frequency action, the response time is 1ms
(3) transistor output: the biggest advantage is the use of high-frequency action, short response time, usually 0.2ms cloth, but it can only take DC5-30V load, the maximum output load current of 0.5A / point, but every 4 points shall not be greater than 0.8A. When your system output frequency of 6 times per minute or less, should be preferred to relay output, because of its circuit planning simple, anti-interference and load-carrying capacity Strong When the frequency is 10 times / min below, both the relay output method can be used, but also the PLC output drive Darlington transistor (5-10A), and then drive the load.
2. Anti-interference and external interlock
When the PLC output with a rational load, the load power failure will form an inrush current impact on the output of the PLC, for this reason, the DC rational load should be connected next to the current-continuing diode, the communication of the rational load should be connected to the surge absorption circuit, can be useful to maintain the PLC when the output of the two physical quantities in the PLC internal software interlock, the external PLC should also be interlocked to strengthen the reliability of the system. The selection of COM points
3.COM point selection
Different PLC products, the number of COM points is different, some COM points with 8 output points, some with 4 output points, but also with 2 or 1 output point When the variety of loads, and the current is large, the use of a COM point with 1-2 output points of the PLC products; when the number of loads and less variety, the use of a COM point with 4-8 output points of the PLC This will bring a lot of convenience to the circuit planning Add a fuse at each COM point, 1-2 outputs with 2A fuse, 4-8 outputs with 5-10A fuse.
4. PLC external drive circuit
On the PLC output can not be directly with the load, the external drive circuit must be selected: can be driven by triode, can also be driven by solid-state relay or thyristor circuit, together with the maintenance circuit and surge absorption circuit should be selected, and each road has a display diode (LED) indication. Printed board should be made plug-in type, easy to repair the PLC input and output wiring also has certain requirements, please see the company's operating instructions.
Fourth, the choice of expansion modules
On the small system, such as the system within 80 points. Usually do not need to expand, when the system is larger, to expand. Different companies' products have constraints on the total number of system points and the number of expansion modules, and when the expansion or cannot meet the request, the network structure can be used. Some manufacturers' products do not support expansion modules for individual instructions, therefore, when making software preparation, we should pay attention to the fact that there are also some rules for each manufacturer when selecting imitation modules such as temperature, please refer to the relevant technical manual. Many varieties of expansion modules of various companies, such as single input modules, single output modules input and output modules, temperature modules, high-speed input modules, etc..
Five, PLC network planning
When using PLC for network planning, its difficulty is much greater than PLC stand-alone control First of all, you should choose to use your own more understanding of the model, the fundamental instructions and functional instructions have a deeper understanding, and the instruction execution speed and user program storage capacity should also be carefully understood. Otherwise, you cannot use your real-time request and form a system collapse. In addition, the communication interface, communication protocol data transmission speed, etc. should also be considered. Finally, the PLC business to seek network planning and software technical support and specific technical information, as for the choice of several layers of operating stations, depending on the size of your system.
Sixth, software preparation
In the preparation of software before, should first understand the software manual of the PLC products selected, to be skilled before programming. If you use the graphic programmer or software package programming, it can be programmed directly, if you use the handheld programmer programming, should first draw a ladder diagram, and then programming, so less mistakes can be made, the speed is also fast after the completion of programming first air conditioning procedures, to be normal for each action, and then debugged on the device.
