Correct selection of PLC in the automatic control system design

PLC (programmable logical controller, PLC for short) has been increasingly used in industrial control systems, and plays a very important role in the automatic control system. Therefore, it is very important for the correct selection of PLC.


Numerous suppliers of various types of PLC, each of them has advantages and disadvantages, to meet user needs, but in the morphology, composition, functions, networks, programming the inconsistent, no uniform standards and cannot be compared. The following suggests some opinions on selection of PLC in the automatic control system design, you can select PLC as a reference.


You can compare by, choose suitable products.


And volume


This is especially important. Early in the design of automatic control system, it should be on the number of control points (digital or analog) have an accurate statistic, this is often a pre-requisite for select PLC, generally choose more than the number of control points of 10%~30% PLC. Here are some considerations:


1, can make up for missing points in the design process;
2, can guarantee the individual points in the process when there is a fault, there are alternative points;
3, the need for additional points in the future.


Second, work environment


Work environment is the hard targets of PLC. Automatic control system for people from busy work and out in the harsh environment, on the automatic control system to adapt to complex environments, such as temperature, humidity, noise, signal shielding, operating voltage, the PLC varies. Be sure to select the products that adapt to the actual working environment.


Third, communication network


Now PLC is not a simple control and PLC control system of remote communication have become issues that must be addressed, but the manufacturers to develop communication protocols vary widely, poor compatibility. At this point, take into consideration the following aspects:


1, the same communication between manufacturers. Each manufacturer has its own communication protocol, and more than one. This is evident in the large and medium machine, and differ in small, micro-machine, some manufacturers take into account for volume, price, features and other aspects, often without or with other protocols, and easier communication. Therefore, the main consideration in this regard is the same in various types of communication between PLC;


2, communication between products from different manufacturers. If the automatic control system design of the automatic control system currently part of the transformation, and the choice is different from the original system of PLC, or design needs more than 2 or 2 PLC, and selected products from different manufacturers, which need to consider the problem of communication between products from different manufacturers;


3, whether in the future. Due to manufacturers to develop communication protocols vary, there is no uniform international standard, so the limit on the PLC type selection. Consider impact, growth, fully functional, close to a common communication protocol.


Four, programming


Program is the whole control system of heart program directly affects the operation of the automatic control system. Some manufacturers require extra programming and programming software to buy, and expensive, it is also required into account.


1, programming methods


One is to use the manufacturer‘s dedicated programmers. Various types, large programming functions, suitable for various models of PLC, high price small programmer compact, easy to carry, low prices, but the function is simple, bad fit other programming software is relying on the personal computer platform, has now been adopted by most manufacturers. Manufacturers due to their different products, their products often only developed for programming software, and programming style, interface and application of the software platform, flexibility, adaptability, ease of programming and so can be given only after users hands-on evaluations.


2, programming language


Programming language is the most complex, diverse, and appears to be the same, but not universal. Can be divided into the following 5 classes of the most commonly used programming languages:


(1) the ladder diagram
This is the PLC manufacturers in most programming languages, was originally controlled by the relay of evolution, is relatively simple, discrete control and interlock logic is most useful;


(2) sequential function chart
It provides the overall structure and coordinate with State orientation or machine control applications;


(3) the function block diagram
It provides an effective development environment, and particularly suitable for process control applications;


(4) structured text
It is similar to a programming language for your computer, it is suitable for complex algorithms and data processing;


(5) the list of instructions
It provides an environment for optimal performance, and assembly language are very similar.


Manufacturers generally include one or more of the programming language in the programming software, such as TE XTEL programming software can use the ladder (Ladder), sequential function chart (Grafcet), structured text (Literal) 3 programming language; Siemens Step7 programming software can use the ladder (Ladder), instruction list (STL) two programming languages Modicon company Modsoft using ladder diagram programming software only (984 ladder) is a programming language, while another Concept programming software 5 programming languages can be used, followed by ladder diagram (LD), sequential function chart (SFC), function block diagram (FBD), structured text (ST), instruction list (IL). Most programming languages can be interchanged under the same programming software, usually choose familiar programming language.


3, memory


Save the PLC memory is where programs and data, both standard and extrapolation, 512~128M bytes of memory capacity, be sure to choose a large enough memory according to the actual situation, and ask for a spare part as a cache.


PLC memory according to the type of random access memory (RAM), readonly memory (ROM), erasable readonly memory (EPROM) and so on. RAM can read and write, in power only after a specific period of time, the most suitable for automatic control system for debugging purposes. ROM read only cannot be written, the program is made up of manufacturers or developers prior cure cannot be changed, even if the power is not lost. ROM and EPROM EPROM in special ways (such as ultraviolet rays) can be erased and then write, suitable for the application in the system of working long hours and the modifications are minor.


4, easy to change


PLC control relay is another advantage in that it can change control structure as needed (or control), this requires changing programs fast and easy.


5, is there a special module


Special modules for PLC products provide some of the parts suppliers, such as communication modules, PID control module, module, analog input/output module. Software is also available on the corresponding block, often simply input some parameters can achieve, user-friendly programming.


Five communications, and monitoring system


1, interactive console. This is the early type of monitoring system, is the production of PLC manufacturers specifically for their product design, best suited for point to point control. Simple structure, less features, Control Panel, operation is easy, is still widely used in control systems. When the advantages of remote control, can be well controlled.


2, with the development of computers, relying on the PC (including IPC) monitoring systems are increasingly used in automatic control systems, this control system PLC developers exclusively for own (or specific) tailored products; another is a software development company to develop products suitable for PLC control system. Qian a and PLC products of compatible sex strong, can according to PLC products of features developed corresponding of control programme, should said still to PLC for Center; Hou a is throwing open has PLC products, focused on computer in image, and animation, and voice, and network, and data, aspects of advantage, to II times development personnel provides has more loose of development conditions, often can making out excellent of monitoring system, as long as has corresponding of communications agreement (currently has has has most production manufacturers of communications agreement), Can be connected with the various types of PLC, is now the preferred automatic control system. So, in this regard should be given to the selected PLC and control system communication is possible.


Six, ductility


Meaning here consists of three aspects:


1, the product life. Can be selected to ensure the PLC life, try to buy closer to the date of manufacture;


2, product continuity. Suppliers to constantly develop PLC product upgrade is backward-compatible, which determines whether system application to new functionality in the future;


3, the product renewal cycle. When some kind of model PLC (or PLC module) after being eliminated, suppliers are able to ensure that there is sufficient spare parts (spare). At this time should consider a new PLC.


Seven, after-sales service and technical support


1, choose a good product;


2, choose a reputable agent;


3, good after-sales service and technical support.


Eight, most cost-effective


Relative to the priceperformance of automatic control system for priority selection. Just in front of some of the more close, and not easy to choose, consider the price factor, choose more cost-effective products.


In the actual selection process, often subject to many restrictions, not necessarily taking into account all of the above, but some of them must be taken into account but the problem must be resolved through alternative means.


In General through a comparison of the top 5, 2~3 products can be determined, after considering several, you can select PLC is satisfactory. With the development of science and technology, PLC products will have a unified standard. At that time, the selection of PLC is not a difficult thing.