PLCs play a vital role in automated control systems. Therefore, selecting the right PLC is extremely important.
Faced with a multitude of PLCs from various manufacturers, each with its own advantages and disadvantages, capable of satisfying diverse user needs, they are incompatible in terms of form, composition, function, network, and programming. The lack of a unified standard makes direct comparison impossible. The following are some perspectives on PLC selection in automatic control system planning, which can serve as a reference when choosing a PLC.
By comparing the following aspects, you can select the right product.
I. Workload
This is particularly important. At the initial planning stage of an automated control system, an accurate count of the number of control points (digital and analog) should be made. This is often the primary condition for selecting a PLC, and generally, a PLC with 10% to 30% more control points than the required number should be chosen. Several factors are considered in this decision:
1. It can compensate for points missed in the planning process;
2. It can ensure that if a fault occurs at a certain point during the operation process, there is a replacement point;
3. Future demand for additional points.
II. Working Environment
The operating environment is a crucial factor for PLC operation. Automation systems free people from busy work and harsh environments, requiring them to adapt to complex environments such as temperature, humidity, noise, signal shielding, and operating voltage. These factors vary between PLCs. Therefore, it is essential to choose a product that is adapted to the actual operating environment.
III. Communication Network
PLCs are no longer simply for local control; remote communication has become a necessary problem for control systems. However, the communication protocols developed by different manufacturers vary greatly, resulting in poor compatibility. The following aspects should be considered in this regard:
1. Communication between products from the same manufacturer. Each manufacturer has its own communication protocol, and often more than one. This is more pronounced in large and medium-sized PLCs, but differs in small and micro PLCs. Some manufacturers, considering capacity, price, and functionality, often lack or have simpler communication protocols that differ from others. Therefore, the primary consideration in this area is communication between different types of PLCs from the same manufacturer.
2. Communication between products from different manufacturers. If the automatic control system design involves a partial modification of an existing automatic control system, and the selected PLC is different from the original system, or the design requires two or more PLCs from different manufacturers, then the communication problem between products from different manufacturers needs to be considered.
3. Is it beneficial for the future? Because different manufacturers use different communication protocols, and there is no internationally unified standard, the selection of PLCs is greatly constrained. Therefore, it is necessary to consider communication protocols that have a wide impact, are open to development, have comprehensive functions, and are close to universal standards.
IV. Programming
The program is the "heart" of the entire automatic control system, and its quality directly affects the operation of the entire system. Some manufacturers require programmers and programming software to be purchased separately, and these can be quite expensive; this should also be taken into account.
1. Programming Method
One method is to use a dedicated programmer provided by the manufacturer. These come in various sizes and types. Large programmers are feature-rich and suitable for all types of PLCs, but they are expensive. Small programmers are compact, portable, and inexpensive, but their functions are simple and their applicability is poor. Another method is to use programming software that relies on a personal computer application platform, which is now used by most manufacturers. Because each manufacturer has different products, they often only develop programming software suitable for their own products. The style, interface, application platform, flexibility, user-friendliness, and ease of programming of this programming software can only be evaluated after the user has actually used it.
2. Programming Languages
Programming languages are incredibly diverse and numerous; while they may appear similar, they are not universally applicable. The most commonly used programming languages can be categorized into the following five types:
(1) Trapezoid diagram
This is the programming language most commonly used by PLC manufacturers. It evolved from relay control diagrams and is relatively simple. It is most useful for discrete control and interlocking logic.
(2) Sequence Function Diagram
It provides the overall structure and coordinates with state positioning processing or machine control applications;
(3) Functional block diagram
It provides an effective development environment and is particularly suitable for process control applications;
(4) Structured text
This is a programming language similar to that used in computers, suitable for handling complex algorithms and data processing;
(5) Instruction List
It provides an environment for optimizing coding functions, which is very similar to assembly language.
Programming software supplied by manufacturers typically includes one or more programming languages. For example, TE's XTEL programming software supports three programming languages: Ladder, Logic, and Literal; Siemens' Step7 programming software supports two: Ladder and STL; Modicon's Modsoft programming software only supports Ladder (984 Ladder), while another software, Concept, supports five programming languages: Ladder (LD), Logic (SFC), Function Block Diagram (FBD), Structured Text (ST), and Instruction List (IL). Most programming languages within the same software are interchangeable; generally, users choose the language they are most familiar with.
3. Memory
PLC memory is where programs and data are stored. There are two types: standard and external. The memory capacity is between 512 and 128 MB. It is essential to select a sufficiently large memory based on the actual situation, and some spare space is required as a buffer.
PLC memory can be categorized into Random Access Memory (RAM), Read-Only Memory (ROM), and Erasable Read-Only Memory (EPROM). RAM allows for unrestricted reading and writing, and the program is only retained for a limited time after power loss, making it ideal for debugging automatic control systems. ROM is read-only; the program is pre-programmed by the manufacturer or developer and cannot be modified, even in the event of power failure. EPROM differs from ROM only in that it can be erased and rewritten using special methods (such as ultraviolet light), making it suitable for systems that operate for extended periods with minimal changes.
4. Easy to modify
Another advantage of PLC over relay control is that it can arbitrarily change the control structure (or control process) according to practical needs, which requires convenient and quick program modification.
5. Is there a dedicated module?
Some PLC manufacturers offer specialized modules such as communication modules, PID control modules, counter modules, and analog input/output modules. The software also provides corresponding program blocks, often requiring only simple parameter input for easy programming by the user.
V. Communication with the monitoring system
1. Human-Machine Interface Control Panel. This is an early product type in monitoring systems, designed by manufacturers specifically for their PLC products, and is best suited for point-to-point control. It has a simple structure, few functions, panel control, and is relatively easy to operate, and is still widely used in field control systems. Its advantage is that it can still effectively control the field even when remote control fails.
2. With the continuous development of computers, monitoring systems based on PCs (including industrial PCs) are increasingly used in automation systems. These monitoring systems fall into two categories: one is tailor-made by PLC developers specifically for their own (or specific) products; the other is a monitoring system developed by software development companies that is suitable for most PLC products. The former has strong compatibility with PLC products and can formulate corresponding control schemes based on the characteristics of the PLC product, essentially still centered on the PLC. The latter, however, abandons the PLC product and focuses on the advantages of computers in image, animation, sound, networking, and data, providing more relaxed development conditions for secondary developers. It often produces excellent monitoring systems, and as long as there is a corresponding communication protocol (currently, communication protocols from most manufacturers are available), it can connect to various types of PLCs, making it the preferred choice for modern automation systems. Therefore, in this regard, the feasibility of the communication method between the selected PLC and the monitoring system should be considered.
VI. Dependability
This includes three aspects:
1. Product lifespan. This should roughly ensure the lifespan of the selected PLC; try to purchase products with a more recent production date.
2. Product Continuity. Whether the manufacturer's continuous development and upgrading of PLC products is backward compatible determines whether it is beneficial for the current system to use newly added functions in the future.
3. Product upgrade cycle. When a certain type of PLC (or PLC module) becomes obsolete, can the manufacturer ensure a sufficient supply of spare parts? In this case, a newer PLC should be considered.
VII. After-sales service and technical support
1. Choose good company products;
2. Choose a reputable distributor;
3. Does it have strong after-sales service and technical support?
8. Cost-effectiveness
When choosing between products, the quality of the automatic control system's functions takes precedence over its price. Price should only be considered when the other factors are closely related and difficult to choose between, in which case a product with a high cost-performance ratio should be selected.
In the process of selecting a solution, there are often many limitations, and it is not necessary to consider all of the above aspects. However, some of these aspects are necessary to consider, and the problems that exist need to be solved through other alternative methods.
Generally, after comparing the first five factors, you can identify 2-3 products. Considering the remaining factors, you can then choose a more satisfactory PLC. With the continuous development of science and technology, PLC products will certainly have a unified standard. At that time, choosing a PLC will no longer be a difficult task.
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