I. PLC Model Selection
The basic principle for selecting a machine model is to ensure reliable system operation, ease of maintenance and use, and the best performance-price ratio, while meeting the control function requirements. Specific factors to consider are as follows.
1. Reasonable structure
For applications with relatively fixed processes and favorable environmental conditions (low maintenance requirements), an integrated PLC structure should be selected; otherwise, a modular PLC structure should be selected.
2. Comparable in function and scale
For engineering projects involving on/off control, control speed is not a concern; a basic low-end machine will suffice. For projects primarily using on/off control with some analog control, a low-end machine can be selected. For more complex projects with higher control function requirements, such as those requiring PID calculations, closed-loop control, and network communication, a mid-range or high-end machine can be selected depending on the scale and complexity of the control. High-end machines are mainly used for large-scale process control, fully PLC-based distributed control systems, and factory-wide automation.
3. Standardized machine models
A large enterprise should strive to standardize its PLC models. This is because PLCs of the same model have interchangeable modules, facilitating the procurement and management of spare parts; their functions and programming methods are standardized, which benefits technical personnel training, skill improvement, and functional development; their external equipment is universal, resources can be shared, and with a host computer, multiple PLCs controlling independent systems can be connected into a multi-level distributed control system, enabling mutual communication and centralized management.
II. Capacity Selection
The capacity of a PLC includes both the storage capacity (number of words) of the user memory and the number of I/O points. In addition to meeting the control requirements, the selected PLC capacity should also have an appropriate margin for backup.
Typically, a logic instruction occupies one word of memory, while timing, counting, shifting, arithmetic operations, and data transfer instructions occupy two words of memory. The number of memory words occupied by each type of instruction can be found in the PLC product user manual.
When selecting memory capacity, a margin of 25% of actual needs should generally be considered. Similarly, a margin of 10% to 15% of actual needs should typically be considered for the number of I/O points.
III. Selection of Instruction Set
Due to the wide range of applications of programmable logic controllers (PLCs), the instruction sets of various models are not entirely the same. From an engineering application perspective, some applications only require logical operations, while others require complex arithmetic operations, and some special applications also require dedicated instruction functions. From the perspective of the PLC itself, the instructions from different manufacturers vary considerably, but generally speaking, the instruction sets are a language geared towards engineers; the differences mainly lie in the way the instructions are expressed and their completeness. Some manufacturers have developed more comprehensive control instructions, some more comprehensive digital operation instructions, while most manufacturers have developed more comprehensive logic instructions. When selecting a model, the following aspects should be considered regarding the instruction set:
(1) The total number of statements in the instruction set. It reflects all the functions included in the entire instruction set.
(2) Types of instruction systems. These should mainly include logic instructions, arithmetic instructions, and control instructions. The specific requirements depend on the actual control functions to be performed.
(3) Expression methods of instruction systems. There are various ways to express instruction systems, some of which include ladder diagrams, control system flowcharts, statement lists, sequential control diagrams, high-level languages, etc.; others only include one or two of these expression methods.
(4) Application software program structure. There are modular program structures and subroutine program structures. The former is beneficial for application software writing and debugging, but the processing speed is slow. The latter has a fast response speed, but it is not conducive to writing and on-site debugging.
(5) Software development methods. When considering the performance of the instruction set, the software development methods must also be taken into account. Some manufacturers have also developed dedicated software based on this, which can utilize general-purpose microcomputers (such as IBM-PC) as development tools, thus making it more convenient for users' needs.
IV. Selection of I/O Modules
The price of the I/O section accounts for more than half of the price of a PLC. Different I/O modules, due to their different circuits and performance, directly affect the application range and price of the PLC. Therefore, a reasonable selection should be made based on the actual situation.
1. Input module selection
The function of the input module is to receive input signals from the field and convert the high-level input signals into low-level signals for the PLC. Input modules are classified by voltage as DC 5V, 12V, 24V, 48V, 60V, and AC 115V, 220V. They are also classified by circuit configuration as either point-input or segmented input.
When selecting an input module, please note:
(1) Voltage selection. The distance between the field equipment and the module should be considered. Generally, 5V, 12V, and 24V are considered low voltages, and their transmission distance should not be too far. For example, the maximum distance for a 5V module should not exceed 10m. For equipment at a greater distance, a higher voltage module should be selected.
(2) Number of points connected at the same time. The number of points connected at the same time for high-density input modules (32 points, 64 points) depends on the input voltage and ambient temperature. Generally speaking, the number of points connected at the same time should not exceed 60% of the total number of input points.
(3) Threshold level. To improve the reliability of the control system, the threshold level must be considered. The higher the threshold level, the stronger the anti-interference capability and the longer the transmission distance.
2. Selection of output module
The function of the output module is to transmit the PLC's output signals to an external load and convert the PLC's internal low-level signals into the required output levels. Output modules are classified into three types based on their output method: relay output, transistor output, and bidirectional thyristor output. Furthermore, the output voltage and output current also vary among these modules.
When selecting an output module, please note:
(1) Selection of output method. Relay output is inexpensive, has a wide applicable voltage range, and low conduction voltage drop. However, it is an original contact element, with a slower operating speed and shorter lifespan, so it is suitable for loads that are not frequently switched on and off. When driving inductive loads, its maximum switching frequency should not exceed 1Hz. For inductive loads with low power factor that are frequently switched on and off, contactless switching elements should be used, i.e., transistor output (DC output) or bidirectional thyristor output (AC output).
(2) Output Current. The output current of the output module must be greater than the rated value of the load current. There are many specifications for the output current of the module, and the appropriate one should be selected according to the actual load current.
(3) Number of points simultaneously connected. The cumulative current of the points simultaneously connected to the output module must be less than the current allowed through the common terminal. Generally, the number of points simultaneously connected should not exceed 60% of the total number of output points.
V. Selection of Power Supply Module
Choosing a power supply module is simple; only the output current needs to be considered. The rated output current of the power supply module must be greater than the combined current consumed by the CPU module, I/O modules, and dedicated modules, with a certain margin. The following points should generally be considered when selecting a power supply module:
(1) Input voltage of the power supply module. The power supply module can include various input voltages, such as 220V AC, 110V AC, and 24V DC. In practical applications, the appropriate input voltage should be selected based on the specific circumstances. Once the input voltage is determined, the output voltage of the system power supply is also determined.
(2) Output power of the power supply module. When selecting a power supply module, its rated output power must be greater than the sum of the power consumption of the CPU module, all I/O modules, etc., and a margin of about 30% should be reserved. When the same power supply module is used to power both the host unit and the expansion unit, the voltage drop of the line from the host unit to the farthest expansion unit must be less than 0.25V.
(3) Power supply modules in expansion units. In some systems, because intelligent modules and some special modules are installed in the expansion units, it is required to install corresponding power supply modules in the expansion units. In this case, the output power of the corresponding power supply modules can be calculated according to their respective power supply ranges.
(4) Power Module Wiring. After selecting the power module, the wiring terminals and connection method of the power module must be determined in order to design the system power supply correctly. Generally, the input voltage of the power module is connected to the power supply through the wiring terminals, while the output signal is connected to the bus of the programmable controller CPU through the bus socket.
(5) System grounding. The power module grounding wire should be a copper wire with a diameter of not less than 10mm². The connection between the power module and the AC voltage regulator, UPS uninterruptible power supply, isolation transformer, etc., and the system grounding should be as short as possible; the system grounding wire should also be connected to the chassis.
(6) Environmental conditions. When selecting a PLC, it is necessary to consider whether the environmental conditions of the site meet its requirements. Generally, the following indicators should be considered: ambient temperature, relative humidity, allowable power supply fluctuation range, and anti-interference.
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