PLC selection considerations

First, PLC model selection

The basic principle of model selection is to ensure reliable system operation, convenient maintenance and best performance-price ratio under the premise of meeting the requirements of control functions. The specific factors to be considered are as follows.

1. Reasonable structure

For the case where the process is relatively fixed and the environmental conditions are good (the maintenance amount is small), the integral structure PLC is selected, and in other cases, the modular structure PLC is selected.

2, the function, the scale is quite

For the engineering project of switching quantity control, the control speed is not considered, and the general low-end machine can meet the requirements. For engineering projects with a large amount of analog control and a small amount of analog control, a low-speed machine can be selected. For engineering projects with more complicated control and higher control functions, such as PID computing, closed-loop control, communication networking, etc., the medium-sized or high-end machines are selected for visual control scale and complexity. 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.

3, model uniform

A large enterprise should try to achieve uniformity of models. Because the PLC of the same model, the modules are interchangeable, it is convenient for the procurement and management of spare parts and spare parts; its function and programming method are unified, which is conducive to the training of technical strength, the improvement of technical level and the development of functions; Resources can be shared. With the above computer, multiple PLCs that control each independent system can be connected into a multi-level distributed control system to communicate with each other and centrally manage.

Second, the choice of capacity

The capacity of the PLC includes the meaning of the storage capacity (word count) of the user memory and the number of I/O points. In addition to meeting the control requirements, the PLC capacity should be left with an appropriate margin for backup.

Usually, a logic instruction occupies one word of memory, and instructions such as timing, counting, shifting, arithmetic operation, and data transfer occupy two words of memory. The number of words of various instructions in the memory can be found in the PLC product manual.

When selecting the memory capacity, the margin can generally be considered at 25% of the actual need. Usually, the number of I/O points can be considered as the margin of 10% to 15% of the actual need.

Third, the choice of command system

Due to the wide range of programmable controller applications, the command systems of various models are not identical. From the perspective of engineering applications, in some cases only logical operations are required, in some cases complex arithmetic operations are required, and in some special cases special instruction functions are required. From the point of view of the programmable controller itself, the instructions of various manufacturers are quite different, but on the whole, the instruction system is a language for engineering technicians, and the difference is mainly reflected in the expression of instructions and the integrity of instructions. Some manufacturers have developed more in terms of control commands. Some manufacturers have developed more in terms of digital arithmetic instructions, and most manufacturers have developed more in terms of logic instructions. When selecting a model, pay attention to the following from the command system:

(1) The total number of statements in the command system. It reflects all the functions included in the entire instruction.

(2) The type of command system. It should mainly include logic instructions, arithmetic instructions and control instructions, and the specific requirements are related to the actual control functions to be completed.

(3) The expression of the instruction system. There are many ways to express the instruction system, including ladder diagrams, control system flowcharts, statement tables, sequence diagrams, and high-level languages. Some include only one or two expressions.

(4) The program structure of the application software. The program structure has a modular program structure and a subroutine program structure. The former one is advantageous for application software writing and debugging, but the processing speed is slow, and the latter one is fast, but it is not conducive to writing and on-site debugging.

(5) Software development tools. When considering the performance of the command system, we must also consider the software development tools. Some manufacturers have also developed special software on this basis, which can be used as a development tool by using a general-purpose microcomputer (such as IBM-PC), which makes it more convenient for users.

Fourth, the choice of I / O module

The price of the I/O part accounts for more than half of the PLC price. Different I/O modules, because of their different circuits and performances, directly affect the application range and price of the PLC, and should be reasonably selected according to the actual situation.

1, the choice of input module

The function of the input module is to receive the input signal from the field and convert the input high level signal into a low level signal inside the PLC. The types of input modules are classified into DC 5V, 12V, 24V, 48V, 60V, and AC 115V and 220V. According to different circuit forms, it is divided into two types: sink input type and split input type.

Note when selecting an input module:

(1) Selection of voltage. It should be considered according to the distance between the field device and the module. Generally, 5V, 12V, and 24V are low voltages, and the transmission distance should not be too far. For example, the 5V module should not exceed 10m at the farthest, and devices with higher distances should use higher voltage modules.

(2) The number of points that are simultaneously turned on. The number of points that the high-density input module (32 points, 64 points) are simultaneously turned on depends on the input voltage and the ambient temperature. Generally speaking, the number of points that are simultaneously turned on should not exceed 60% of the input points.

(3) Threshold level. In order to improve the reliability of the control system, the threshold level must be considered. The higher the threshold level, the stronger the anti-interference ability and the farther the transmission distance.

2, the choice of output module

The function of the output module is to transfer the output signal of the PLC to the external load and convert the low-level signal inside the PLC to the output signal of the external desired level. The output module is divided into three types: relay output, transistor output and bidirectional thyristor output according to the output mode. In addition, the output voltage and output current are also different.

Note when selecting an output module:

(1) Selection of output method. The output of the relay is cheap, the applicable voltage range is wide, and the conduction voltage drop is small. However, it is an original contact element, which has a slower speed and a shorter life, and is therefore suitable for loads that are not frequently turned on and off. When driving an inductive load, its maximum on/off frequency must not exceed 1 Hz. For low-power factor inductive loads that are frequently switched on and off, non-contact switching components should be used, either transistor output (DC output) or triac output (AC output).

(2) Output current. The output current of the output module must be greater than the rated value of the load current. The module output current has many specifications and should be selected according to the actual load current.

(3) The number of points that are simultaneously turned on. The current accumulated value of the output module simultaneously turning on the number of points must be smaller than the current value allowed by the common terminal. Usually the number of points that are simultaneously turned on should not exceed 60% of the number of output points.

Five, the choice of power module

The choice of power module is simple, just consider the output current. The rated output current of the power module must be greater than the sum of the current consumption of the CPU module, I/O module, and dedicated module, and leave a certain margin. The following points should generally be considered when selecting a power module:

(1) Input voltage of the power module. The power module can include a variety of input voltages, including 220V AC, 110V AC, and 24V DC. In the actual application, it is necessary to select according to the specific situation, and after determining the input voltage, the output voltage of the system power supply is also determined.

(2) The output power of the power module. When selecting a power module, its rated output power must be greater than the sum of the total power consumption of the CPU module and all I/O modules, and leave a margin of about 30%. When the same power module is to supply power to both the host unit and the expansion unit, the line voltage drop from the host unit to the farthest expansion unit must be less than 0.25V.

(3) The power module in the expansion unit. In some systems, because the intelligent module and some special modules are installed in the expansion unit, it is required to install the corresponding power module in the expansion unit. At this time, the corresponding power module output power can be calculated according to the respective power supply range.

(4) Power module wiring. After selecting the power module, determine the terminal block and connection method of the power module to properly design the system power supply. The input voltage of the general power module is connected to the power supply through the terminal, and the output signal is connected to the bus of the programmable controller CPU through the bus socket.

(5) Grounding of the system. The grounding wire of the power module is selected to be a copper wire of not less than 10 mm2. Connect with the AC voltage regulator, UPS uninterruptible power supply, isolation transformer, etc. and the grounding of the system as short as possible; the ground wire of the system should also be connected to the chassis.

(6) Use environmental conditions. When selecting a PLC, consider whether the environmental conditions at the site are in compliance with its regulations. Generally considered are: ambient temperature, relative humidity, power supply allowable fluctuation range and anti-interference indicators.