Reasonable selection method for PLC programmable logic device - Database & Sql Blog Articles

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With the popularization of plc, the variety and quantity of PLC products are increasing, and the functions are becoming more and more perfect. In recent years, there have been dozens of series and hundreds of models of PLC products imported from the United States, Japan, Germany and other countries and domestically assembled or self-developed products. There are many kinds of PLCs, and their structure, performance, capacity, command system, programming method, price, etc. are different, and the applicable occasions also have different emphasis. Therefore, the reasonable choice of PLC plays an important role in improving the application of PLC in the control system.

1 model selection

The basic principle of PLC model selection is to select the most reliable model with the most reliable, most convenient maintenance and cost performance under the premise that the function meets the requirements.

In the case where the process is relatively fixed and the environmental conditions are good (the maintenance amount is small), it is recommended to use a PLC with a monolithic structure; in other cases, a PLC with a modular structure is preferred.

For the switch quantity control and the engineering project with the switch quantity control as the main and with a small amount of analog quantity control, the control speed generally does not need to be considered. Therefore, the A/D conversion, D/A conversion, addition and subtraction calculation, and data transmission function are selected. The low-end machine can meet the requirements.

In engineering projects with relatively complicated control and high control function requirements (such as PID computing, closed-loop control, communication networking, etc.), the size and complexity of visual control are used to select mid-range or high-end machines. Among them, high-end machines are mainly used for large-scale process control, distributed control systems for all PLCs, and automation of the entire plant. Table 1 lists several functional options for the PLC, depending on the application.

Table 1 PLC functions and applications

For a large enterprise system, the model should be unified as much as possible. In this way, the PLC modules of the same model can be spared each other, which facilitates the procurement and management of spare parts. At the same time, its unified functions and programming methods are also conducive to the training of technical strength, the improvement of technical level and the development of functions; Its external equipment is common, resources can be shared. Therefore, with the above computer, multiple PLCs controlling each independent system can be combined into one multi-level distributed control system, which is convenient for mutual communication and centralized management.

2 input / output selection

PLC is an industrial control system. Its control object is industrial production equipment or industrial production process, and the working environment is industrial production site. Its connection to the industrial production process is achieved through the I/O interface module.

The I/O interface module can detect various parameters of the controlled production process, and use the field data as control information to control the controlled object. At the same time, the processing result of the controller is sent to the controlled device or the industrial production process through the I/O interface module, thereby driving various actuators to realize the control. The information collected by the PLC from the field and the control signals output to the external device must pass a certain distance. In order to ensure the correctness of these information, the PLC I/O interface module has better anti-interference ability. According to actual needs, in general, PLC has many I/O interface modules, including digital input modules, digital output modules, analog input modules, analog output modules and other special modules, which should be used according to their Features to choose from.

2.1 Determine the number of I / O points

When determining the number of I/O points required according to the requirements of the control system, an additional 10% to 20% of the spare capacity should be added to increase the control function at any time. For a control object, the number of I/O points should be different due to different control methods or different programming levels.

Table 2 lists the number of I/O points required for typical transmission equipment and common electrical components.

Table 2 I/O points of the switch required for typical transmission equipment and common electrical components

2.2 digital input / output

Signals can be received from sensors and switches (such as buttons, limit switches, etc.) and control (on/off) devices (such as indicators, alarms, motor starters, etc.) via standard input/output interfaces. Typical AC input/output signals are 24-240V, and DC input/output signals are 5-240V.

Although the input circuit varies from manufacturer to manufacturer, some features are the same. Such as a jitter circuit for eliminating false signals; a surge protection circuit for avoiding large transient overvoltages. In addition, most input circuits have optional isolation circuitry between the high voltage power input and the control logic of the interface circuitry.

When evaluating discrete outputs, consider fuses, transient surge protection, and isolation circuits between power and logic. The fuse circuit may cost more at the beginning, but may cost less than installing the fuse externally.

2.3 Analog Input/Output

Analog input/output interfaces are typically used to sense the signals produced by the sensor. These interfaces can be used to measure flow, temperature and pressure and can be used to control voltage or current output devices. Typical ranges for these interfaces are -10 to +10V, 0 to +10V, 4 to 20mA, or 10 to 50mA.

Some manufacturers have special analog interfaces designed on the PLC to receive low-level signals such as RTDs and thermocouples. In general, such interface modules can be used to receive different types of thermocouple or RTD mixed signals on the same module.

2.4 Special function input/output

When selecting a PLC, users may face some special types of I/O limitations such as positioning, fast input, frequency, etc. that cannot be implemented with standard I/O. At this point, the user should consider whether the vendor has a special module that helps minimize control. Some special interface modules can handle a portion of the field data themselves, freeing the CPU from time-consuming task processing.

2.5 Intelligent Input/Output

Currently, PLC manufacturers have introduced some intelligent input/output modules. The general intelligent input/output module itself has a processor that can pre-specify the input or output signals and send the processing results to the CPU or directly output, which can improve the processing speed of the PLC and save the memory capacity.

Intelligent I/O modules include high-speed counters (for addition or subtraction counting), cam simulators (for absolute encoder input), cam simulators with speed compensation, single or multi-loop PID regulators, ASCII /BASIC processor, RS-232C/422 interface module, etc. Table 3 summarizes the general rules for selecting I/O modules.

Table 3 General rules for selecting the I/O interface module of the PLC

3 PLC memory type and capacity selection

The memory used in the PLC system is basically composed of three types: PROM, E-PROM and PAM. The storage capacity varies with the size of the randomizer. Generally, the maximum storage capacity of the minicomputer is lower than 6kB, and the maximum storage capacity of the medium-sized machine is 64kB. The maximum storage capacity of the mainframe can be in megabytes. When using, you can select the appropriate model according to the storage requirements of the program and data. If necessary, you can also specifically expand the memory design.

The first method of PLC memory capacity selection and calculation is to accurately calculate the actual use capacity of the memory according to the number of nodes used for programming. The second method is the estimation method. The user can estimate according to the formula of Table 4 according to the control scale and application purpose. In order to be convenient to use, generally should have a margin of 25% to 30%, the best way to obtain storage capacity is to generate the program, that is, how many words are used. Knowing the number of words used for each instruction, the user can determine the exact storage capacity. Table 4 also shows the estimation method of the memory capacity.

Table 4 Method for estimating memory capacity for control purposes

4 software selection

In the implementation of the system, PLC programming problems are very important. The user should have an understanding of the software features of the selected PLC product. Usually, the software of a system is always used to process the control hardware that the controller has. However, some applications also need to control software functions other than hardware components. For example, an application system may include special control or data acquisition functions that require complex mathematical calculations and data processing operations. The choice of instruction set will determine how easy it is to implement a software task. The set of instructions available will directly affect the time required to implement the control program and the time the program is executed.

5 Support technical considerations

When using PLC, the presence or absence of supporting technical conditions is also an important basis for selection. Support technical conditions include the following:

(1) Programming means

• The portable simple programmer is mainly used for small PLCs, and its control scale is small, the program is simple, and a simple programmer can be used;
• The CRT programmer is suitable for large and medium-sized PLCs. In addition to programming and inputting programs, it can also edit and print program texts.
• Because IBM-PC has been popularized, IBM-PC and its compatible programming software package are PLC's good programming tools. At present, PLC manufacturers are working hard to develop IBM-PC and its compatible programming software packages for their own models, and have achieved success.

(2) Perform program text processing

• Simple program text processing and processing of graphs, parameter states and locations, including printing ladder logic;
• Program annotations, including assignment names for contacts and coils, network annotations, etc., which are useful for reading or debugging programs by users or software engineers;
• Processing of graphics and text.

(3) Program storage method

For technical data files and backup materials, the program storage method includes tape, floppy disk or EEPROM storage program box, etc., depending on the technical conditions of the selected model.

(4) Communication software package

For the network control structure or the control system that needs to be managed by the host computer, whether or not the communication software package is the main basis for selecting the PLC. Communication software packages are often used with communication hardware such as modems.

6 PLC's environmental adaptability

Since PLCs are often used directly in industrial control, the manufacturer has designed it to work reliably under harsh environmental conditions. Despite this, each PLC has its own environmental technical conditions, and the user should give full consideration to the environmental conditions when selecting the control system, especially when designing the control system.

The general PLC and its external circuits (including I/O modules, auxiliary power supplies, etc.) can work reliably under the environmental conditions listed in Table 5.

Table 5 PLC working environment

7 Conclusion

With the continuous advancement of technology, the variety of PLCs has become more and more numerous, and the functions have gradually increased. Although some methods of selecting PLC are summarized in the article, in the actual work, it is necessary to make appropriate adjustments according to the actual situation in order to design a control system that meets the expectations.