Before selecting a PLC, it is important to determine the system layout. Once you have determined the system layout, you can choose the manufacturer and model that best suits your needs.
This article provides detailed information about manufacturers, models, input/output (I/O) point counts, control functions, and more to help you select the PLC that best suits your needs.
1. PLC manufacturers
When selecting a manufacturer for your PLC, it is important to consider factors such as equipment user requirements, designer familiarity with PLCs and design habits of different manufacturers, consistency of compatible products, and technical services.
From the point of view of the reliability of the PLC itself, in principle, there should be no problems with the reliability of products from large foreign companies. Generally speaking, for controlling independent equipment or simpler control systems, Japanese PLC products have certain cost advantages.
For large-scale distributed control systems with high communication requirements and network openness, European and American PLCs have advantages in network communication functionality.
Furthermore, for some special industries (such as metallurgy and tobacco), PLC systems with mature and reliable operational performance in the relevant industrial field should be selected.
2. Count of input/output (I/O) points
The I/O point count of a PLC is one of its basic parameters. Determination of I/O point count should be based on the total number of I/O points required for the control equipment.
In general, PLCs should have appropriate margins for I/O points. Typically, after adding 10% to 20% expandable margins based on calculated entry/exit point statistics, this data can then be used as an estimated entry/exit point count.
When placing actual orders, the entry/exit point count needs to be adjusted based on the specific characteristics of the manufacturer's PLC product.
3. Storage Capacity
Storage capacity refers to the size of the hardware storage unit that the programmable logic controller can provide, while program capacity refers to the size of the storage unit used by the user's application project in memory.
Therefore, the program capacity is less than the storage capacity. During the design phase, because the user's application program has not yet been compiled, the capability of the program is unknown and can only be determined after debugging the program.
To estimate program capacity during the project selection process, estimated storage capacity is typically used as a proxy. There is no fixed formula for estimating PLC memory capacity and many literature sources provide different formulas.
Generally, the total number of words in memory is estimated at 10-15 times the digital I/O point count plus 100 times the analog I/O point count (each word is 16 bits), and an additional margin of 25% should also be considered.
4. Control function
This selection includes the choice of operational, control, communication, programming, diagnostic and processing speed capabilities.
1. Operational Function
The operational functions of a simple PLC include logical operations, timing and counting functions. The operational functions of a normal PLC also include data changing, comparison and other operational functions.
More complex operational functions include algebraic operations, data transfer, etc. Large PLCs also have advanced operational functions, such as PID operation for analog signals.
With the emergence of open systems, most PLCs now have communication functions, some products have communication with lower-level machines, some have communication with peer or higher computers, and some even have data communication functions with factories or enterprise networks.
When selecting PLC based on actual requirements, it is important to reasonably select the required operating functions.
In most application scenarios, only logical operations and timing/counting functions are required.
Some applications require data transfer and comparison, and algebraic operations, numerical conversion and PID operations are only used for detection and control of analog signals. Some applications also require decoding and encoding operations to display data.
2. Control function
Control functions include PID control operations, feed-forward compensation control operations, ratio control operations, etc., which must be determined based on control requirements. Since PLC is mainly used for sequential logic control, single-loop or multi-loop controllers are often used to solve analog control in most scenarios.
Sometimes dedicated intelligent input/output units are used to complete the necessary control functions, improving the processing speed of the PLC and saving storage capacity. For example, using PID control units, high-speed counters, analog units with speed compensation, ASCII conversion units, etc.
3. Communication function
Medium to large PLC systems must support multiple fieldbuses and standard communications protocols (such as TCP/IP) and must be able to connect to factory management networks (TCP/IP) when necessary.
The communication protocol must comply with ISO/IEEE communication standards and must be an open communication network.
The communication interface of the PLC system should include serial and parallel communication interfaces (RS2232C/422A/423/485), RIO communication ports, industrial Ethernet, commonly used DCS interfaces, etc.
The communication bus of medium to large PLCs (including interface devices and cables) must consider redundant configuration, and the communication bus must comply with international standards. The communication distance must meet the actual requirements of the device.
In the PLC system communication network, the communication rate of the upper-level network should be greater than 1Mbps, and the communication load should not exceed 60%.
The PLC system communication network has several forms:
- The PC is the main station and several PLCs of the same model are the substations, forming a simple network of PLCs;
- A PLC is the main station and the other PLCs of the same model are substations, forming a master-slave PLC network;
- The PLC network is connected to a large-scale DCS through a specific network interface as a subnet of the DCS;
- A dedicated PLC network (dedicated PLC communication network from various manufacturers).
To reduce the communication task of the CPU, different communication processors with different communication functions (such as point-to-point, fieldbus, industrial Ethernet) should be selected based on the actual needs of the network composition.
4. Programming function
Offline Programming:
The PLC and programmer share a CPU. In programming mode, the CPU only serves the programmer and does not control the field device. After programming is completed, the programmer switches to run mode and the CPU controls the field device but cannot perform programming.
Offline programming reduces system costs, but is inconvenient to use and debug.
Online programming:
The CPU and programmer have their own CPUs. The main CPU is responsible for field control and exchanges data with the programmer in a scan cycle. The programmer sends the programmed program or data online to the host, and the host runs according to the new program received in the next scanning cycle.
This method has a higher cost, but the system debugging and operation is convenient and is commonly used in medium to large PLCs.
Five standardized programming languages:
Sequential Function Chart (SFC), Ladder Diagram (LD), Function Block Diagram (FBD), three graphical languages, and Instruction List (IL) and Structured Text (ST), two text languages.
The selected programming language must comply with its standard (IEC6113123) and support multiple forms of language programming, such as C, Basic, Pascal, etc., to meet the control requirements of special control scenarios.
5. Diagnostic function
The PLC diagnostic function includes hardware and software diagnostics. Hardware diagnostics determine the location of hardware faults through logical hardware judgments, while software diagnostics include internal and external diagnostics.
Diagnosing the performance and functions of the PLC internally through software is an internal diagnosis, while diagnosing the CPU and the information exchange function of external input/output components through software is an external diagnosis.
The strength of the PLC's diagnostic function directly affects the technical capabilities required of operators and maintenance personnel and affects the mean repair time.
6. Processing speed
The PLC works in scan mode. From the perspective of real-time requirements, the processing speed should be as fast as possible. If the signal duration is shorter than the scan time, the PLC will not be able to scan the signal, resulting in the loss of signal data.
Processing speed is related to user program duration, CPU processing speed, software quality, etc.
At present, the response time and speed of PLC contacts are fast, and the execution time of each binary instruction is about 0.2 ~ 0.4 μs, which can meet the requirements of high control and response applications quickly.
The scan cycle (processor scan cycle) must meet the following criteria: the scan time of the small PLC must not exceed 0.5 ms/K, and the scan time of the medium to large-scale PLC must not exceed 0 .2ms/K.
7. PLC models
PLCs can be classified into two types: integral and modular, based on their structures.
Integral PLCs have a relatively fixed and small number of I/O points, which limits user choices and is typically used in small control systems. Examples of this type include the Siemens S7-200 series, the Mitsubishi FX series and the Omron CPM1A series.
Modular PLCs provide multiple I/O modules that can be connected to the PLC baseboard, allowing users to select and configure the number of I/O points according to their needs.
This makes modular PLC configurations more flexible and is commonly used in medium to large control systems. Examples of this type include the Siemens S7-300 and S7-400 series, Mitsubishi Q series and Omron CVM1 series.
8. Selecting multiple modules
1. Digital I/O Module
The selection of digital input/output modules must consider application requirements. For example, for input modules, considerations should include input signal levels, transmission distances, etc.
There are also many types of output modules, such as relay contact output, AC120V/23V bidirectional thyristor output, DC24V transistor drive type, DC48V transistor drive type, etc.
Typically, relay output modules have the advantages of low cost and wide voltage range. However, they have shorter lifespans, longer response times, and require surge-absorbing circuits when used with inductive loads.
Bidirectional thyristor output modules have faster response time and are suitable for frequent switching and low power factor load occasions, but they are more expensive and have lower overload capacity.
In addition, input/output modules can be divided into specifications such as 8 points, 16 points, 32 points, etc. according to the number of inputs/outputs, and should be reasonably equipped according to actual needs.
2. Analog I/O Module
Analog input modules can be divided into current input type, voltage input type, thermocouple input type, etc. according to the type of analog input signal.
The signal level of a current input module is generally 4~20 mA or 0~20 mA, while that of a voltage input module is generally 0~10V, -5V~+5V, etc. current input signals.
Analog output modules also have voltage output type and current output type. The range of current output signal is generally 0~20mA, 4~20mA, while that of voltage output signals is generally 0~0V, -10V~+10V, etc.
Analog input/output modules can be divided into specifications such as 2 channels, 4 channels, 8 channels, etc. according to their input/output channel numbers.
3. Function Modules
Function modules include communication modules, positioning modules, pulse output modules, high-speed counting modules, PID control modules, temperature control modules, etc.
When choosing a PLC, one must consider the possibility of matching functional modules, which involves hardware and software aspects.
8. General Rules
After the PLC model and specifications are approximately determined, the basic specifications and parameters of each PLC component can be determined one by one according to the control requirements, and the models of each component module can be selected.
When selecting module models, the following principles must be followed:
1. Economy
When selecting a PLC, the performance-price ratio must be considered. When considering economics, factors such as application scalability, operability, input-output ratio, etc. they must be compared and balanced to choose a satisfactory product.
The number of entry/exit points has a direct impact on the price. Increasing the number of entry/exit cards requires additional costs. When the number of points increases to a certain value, the capacity of memory, rack, motherboard, etc. increases. corresponding must also be increased.
Therefore, increasing the number of points impacts CPU selection, memory capacity, and the scope of the control function. It should be fully considered in the estimation and selection to make the entire control system have a more reasonable performance-price ratio.
2. Convenience
Generally, there are many types of modules that can meet the control requirements of a PLC. When selecting, the principle of simplifying circuit design, convenience of use and minimizing external control components should be followed.
For example, for input modules, the input form that can be directly connected to external sensing elements should be prioritized to avoid the use of interface circuits.
For output modules, output modules that can drive loads directly should be prioritized, and intermediate relays and other components should be minimized.
3. Generality
When selecting, the uniformity and generality of each PLC component module should be considered to avoid too many types of modules.
This not only favors procurement by reducing spare parts, but can also increase the interchangeability of various system components, providing convenience for design, commissioning and maintenance.
4. Compatibility
When selecting each PLC system component module, compatibility must be fully considered to avoid poor compatibility issues.
The production manufacturers of the main components of the PLC system should not be many. If possible, products from the same manufacturer should be selected.
