Introduction: When designing a PLC system, the first step is to determine the system scheme, followed by the design and selection of the PLC. Choosing a PLC mainly involves determining the manufacturer and specific model. For system schemes requiring distributed systems, remote I/O systems, and network communication capabilities, it's also necessary to consider these factors. So, how should one specifically choose a PLC? The author believes the following aspects should be considered.
I. PLC Manufacturers
When selecting a PLC manufacturer, key considerations should include the equipment user's requirements, the designer's familiarity with different manufacturers' PLCs and their design practices, the consistency of supporting products, and technical support. From a reliability perspective, products from major international companies should, in principle, be reliable.
Generally speaking, for applications controlling independent equipment or relatively simple control systems, Japanese PLC products offer a certain cost-performance advantage. However, for larger systems with high network communication requirements, open distributed control systems, and remote I/O systems, European and American PLCs have an advantage in network communication capabilities. Furthermore, for certain specialized industries (such as metallurgy and tobacco), PLC systems with proven operational track records and proven reliability should be selected.
II. Number of Input/Output (I/O) Points
The number of input/output points (I/O points) is one of the basic parameters of a PLC. The determination of the I/O point count should be based on the total number of I/O points required by the controlled equipment. Generally, the PLC's I/O points should have an appropriate margin. Typically, the estimated number of I/O points is calculated by adding 10% to 20% for expansion. When actually ordering, the number of I/O points needs to be adjusted according to the manufacturer's product characteristics.
III. Storage Capacity
Memory capacity refers to the size of the hardware storage units that the programmable logic controller (PLC) itself can provide. Program capacity refers to the size of the storage units used by the user application within the memory; therefore, program capacity is smaller than memory capacity. During the design phase, since the user application program has not yet been written, the program capacity is unknown and can only be determined after program debugging. To allow for a certain estimation of program capacity during design and selection, memory capacity estimation is usually used as a substitute. There is no fixed formula for estimating PLC memory capacity; many documents provide different formulas. Generally, it involves multiplying the number of digital I/O points by 10 to 15, adding 100 times the number of analog I/O points, and using this number as the total number of words in memory (16 bits per word), then considering a 25% margin.
IV. Control Functions
This selection includes options for features such as computing capabilities, control capabilities, communication capabilities, programming capabilities, diagnostic capabilities, and processing speed.
(I) Computational Functions
Simple PLCs offer basic logical operations, timing, and counting functions. Ordinary PLCs also include data shifting and comparison functions. More complex PLCs offer algebraic operations and data transfer. Large PLCs also include analog PID control and other advanced functions. With the advent of open systems, most PLCs now have communication capabilities. Some communicate with lower-level computers, some with peer or upper-level computers, and some even with factory or enterprise networks. When designing and selecting a PLC, the required functions should be chosen based on the specific application requirements. Most applications only require logical operations and timing/counting functions. Some applications require data transfer and comparison. Algebraic operations, numerical conversions, and PID control are used for analog signal detection and control. Data display may require decoding and encoding.
(II) Control Functions
Control functions include PID control calculations, feedforward compensation control calculations, ratio control calculations, etc., and should be determined according to control requirements. PLCs are mainly used for sequential logic control; therefore, in most cases, single-loop or multi-loop controllers are used to handle analog quantity control. Sometimes, dedicated intelligent input/output (AI) units are used to complete the required control functions, improving the PLC's processing speed and saving memory capacity. Examples include PID control units, high-speed counters, analog units with speed compensation, and ASCII code conversion units.
(III) Communication Function
Large and medium-sized PLC systems should support multiple fieldbuses and standard communication protocols (such as TCP/IP), and should be able to connect to the factory management network (TCP/IP) when needed. The communication protocol should conform to ISO/IEEE communication standards and should be an open communication network.
The communication interfaces of a PLC system should include serial and parallel communication interfaces (RS2232C/422A/423/485), RIO communication ports, industrial Ethernet, and common DCS interfaces. The communication bus (including interface devices and cables) of large and medium-sized PLCs should be configured with 1:1 redundancy. The communication bus should conform to international standards, and the communication distance should meet the actual requirements of the device.
In the communication network of a PLC system, the communication speed of the upstream network should be greater than 1Mbps, and the communication load should not exceed 60%. The main forms of communication networks for PLC systems are as follows:
1) A PC is the master station, and multiple PLCs of the same model are slave stations, forming a simple PLC network;
2) One PLC is the master station, and other PLCs of the same model are slave stations, forming a master-slave PLC network;
3) The PLC network is connected to a large DCS as a subnet of the DCS through a specific network interface;
4) Dedicated PLC network (dedicated PLC communication network of each manufacturer).
To reduce the communication burden on the CPU, PLCs should select communication processors with different communication functions (such as point-to-point, fieldbus, and industrial Ethernet) based on the actual needs of the network configuration.
(iv) Programming Functions
Offline programming: The PLC and programmer share a single CPU. When the programmer is in programming mode, the CPU only provides services to the programmer and does not control the field devices. After programming is complete, the programmer switches to run mode, and the CPU controls the field devices but cannot program them. Offline programming can reduce system costs, but it is inconvenient to use and debug.
Online programming: The CPU and programmer each have their own CPU. The host CPU is responsible for field control and exchanges data with the programmer within a scan cycle. The programmer sends the online-compiled program or data to the host. In the next scan cycle, the host runs according to the newly received program. This method is more expensive, but the system is easy to debug and operate, and it is commonly used in medium and large-sized PLCs.
Five standardized programming languages are required: three graphical languages—Sequential Function Chart (SFC), Ladder Diagram (LD), and Function Block Diagram (FBD)—and two text-based languages—Instruction List (IL) and Structured Text (ST). The selected programming language should comply with its standard (IEC 6113123) and should also support multiple programming language formats, such as C, Basic, and Pascal, to meet the control requirements of special control applications.
(V) Diagnostic Functions
PLC diagnostic functions include hardware and software diagnostics. Hardware diagnostics determine the location of hardware faults through logical judgments, while software diagnostics are divided into internal and external diagnostics. Internal diagnostics diagnose the PLC's internal performance and functions through software, while external diagnostics diagnose the PLC's CPU and its information exchange functions with external input/output components through software.
The strength of a PLC's diagnostic capabilities directly affects the technical skills required of operators and maintenance personnel, and also influences the mean time to repair (MTBL).
(vi) Processing speed
PLCs operate using a scanning method. From a real-time perspective, 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 the length of the user program, CPU processing speed, and software quality. Currently, PLC contacts have fast response and high speed, with each binary instruction execution time of approximately 0.2–0.4 μs, thus meeting the needs of applications with high control requirements and fast response. The scan cycle (processor scan cycle) should meet the following requirements: scan time for small PLCs should not exceed 0.5 ms/K; scan time for medium and large PLCs should not exceed 0.2 ms/K.
V. PLC Models
PLC types: PLCs are classified into two types according to their structure: integrated type and modular type.
Integrated PLCs have fewer and relatively fixed I/O points, limiting user options and typically used in small control systems. Examples of this type of PLC include Siemens' S7-200 series, Mitsubishi's FX series, and Omron's CPM1A series.
Modular PLCs offer a variety of I/O modules that can be plugged into the PLC baseboard, allowing users to easily select and configure the number of I/O points in the control system according to their needs. Therefore, modular PLCs offer greater flexibility in configuration and are generally used in medium to large-scale control systems. Examples include Siemens' S7-300 and S7-400 series, Mitsubishi's Q series, and Omron's CVM1 series.
VI. Selection of Various Modules
(I) Digital I/O Modules
The selection of digital input/output modules should take into account application requirements. For example, for input modules, application requirements such as input signal level and transmission distance should be considered. There are also many types of output modules, such as relay contact output type, AC120V/23V bidirectional thyristor output type, DC24V transistor drive type, and DC48V transistor drive type.
Relay output modules typically have advantages such as low price and wide operating voltage range, but they have a shorter lifespan, longer response time, and require surge absorption circuits when used with inductive loads. Bidirectional thyristor output modules have a faster response time and are suitable for frequent switching and inductive low power factor loads, but they are more expensive and have poor overload capacity.
In addition, input/output modules can be divided into specifications such as 8 points, 16 points, and 32 points according to the number of input/output points. When selecting, you should choose the appropriate configuration based on your actual needs.
(II) Analog I/O Module
Analog input modules can be categorized based on the type of analog input signal: current input, voltage input, thermocouple input, etc. Current input modules typically have signal levels of 4–20mA or 0–20mA; voltage input modules typically have signal levels of 0–10V, -5V to +5V, etc. Some analog input modules are compatible with both voltage and current input signals.
Analog output modules are also divided into voltage output modules and current output modules. Current output signals are typically 0~20mA or 4~20mA. Voltage output signals are typically 0~0V or -10V~+10V.
Analog input/output modules can be classified into specifications such as 2-channel, 4-channel, and 8-channel according to the number of input/output channels.
(III) Functional Modules
Functional modules include communication modules, positioning modules, pulse output modules, high-speed counting modules, PID control modules, and temperature control modules. When selecting a PLC, the compatibility of these functional modules should be considered; the selection of functional modules involves both hardware and software aspects.
In terms of hardware, the first consideration should be the ease of connection between the functional modules and the PLC. The PLC should have relevant connections, installation locations and interfaces, connecting cables, and other accessories. In terms of software, the PLC should have corresponding control functions, allowing for easy programming of the functional modules. For example, Mitsubishi's FX series PLCs can easily control corresponding functional modules using the "FROM" and "TO" instructions.
VII. Redundancy Functions
(I) Control Unit Redundancy
1. Important process units: CPU (including memory) and power supply should both be 1B1 redundant.
2. When necessary, a hot standby redundant system composed of PLC hardware and hot standby software, a dual or triple redundant fault-tolerant system, etc., can also be selected.
(ii) Redundancy of I/O interface units
1. The multi-point I/O cards of the control loop should be redundantly configured.
2. Redundant configuration of multi-point I/O cards for important detection points. 3) For important I/O signals, dual or triple I/O interface units can be selected as needed.
General principles
After the PLC model and specifications are roughly determined, the basic specifications and parameters of each component of the PLC can be determined one by one according to the control requirements, and the model of each component module can be selected. The following principles should be followed when selecting module models.
(I) Economic efficiency
When selecting a PLC, the performance-price ratio should be considered. When considering economic factors, factors such as application scalability, operability, and return on investment should also be taken into account. A comparison and balance should be made to ultimately select the most satisfactory product.
The number of input/output points (I/O points) directly impacts the price. Each additional I/O card increases the cost. As the number of points increases to a certain level, the corresponding memory capacity, rack space, motherboard, etc., also need to be increased. Therefore, increasing the number of points affects the selection of the CPU, memory capacity, and control function range. These factors should be fully considered during estimation and selection to ensure a reasonable performance-price ratio for the entire control system.
(ii) Convenience
Generally speaking, there are many types of modules that can meet the control requirements of a PLC. When selecting a module, the principles should be simplified circuit design, ease of use, and minimizing the use of external control devices. For example, for input modules, priority should be given to input types that can be directly connected to external sensing elements, avoiding the use of interface circuits. For output modules, priority should be given to output modules that can directly drive the load, minimizing intermediate relays and other components.
(III) Universality
When selecting a PLC, it's important to consider the uniformity and commonality of its various modules to avoid an excessive number of module types. This not only facilitates procurement and reduces the need for spare parts, but also increases the interchangeability of the system's components, making design, debugging, and maintenance easier.
(iv) Compatibility
When selecting the components of a PLC system, compatibility should be fully considered. To avoid compatibility issues, the number of manufacturers producing the main components of the PLC system should be limited. If possible, choose products from the same manufacturer.
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