Important Principles for Selecting PLCs

Before selecting a PLC, we must first determine the system plan. Once that is established, we can choose the manufacturer and model. Do you know how to make these selections? This article provides detailed introductions from manufacturers and models to input/output (I/O) points, control functions, etc., helping you choose the most suitable PLC from different aspects.

When determining the PLC manufacturer, factors such as the requirements of the equipment user, the designer’s familiarity with different manufacturers’ PLCs, consistency of supporting products, and technical services should be considered. From the perspective of the PLC’s reliability, as long as it is a product from a large foreign company, there should not be significant reliability issues. Generally, for controlling independent devices or simpler control systems, using Japanese PLC products tends to offer a certain cost-performance advantage. For larger systems requiring high network communication functionality, open distributed control systems, and remote I/O systems, PLCs produced in Europe and America have advantages in network communication capabilities.

Additionally, for certain special industries (e.g., metallurgy, tobacco, etc.), it is advisable to choose PLC systems that have operational performance and reliability in the relevant industry field.

The number of I/O points is one of the basic parameters of a PLC. The determination of I/O points should be based on the total number of I/O points required by the control equipment. Generally, there should be an appropriate surplus in the number of I/O points. Typically, the estimated number of I/O points is calculated by taking the statistical number of input and output points and adding a 10% to 20% expansion margin. When placing actual orders, adjustments should be made based on the product characteristics of the manufacturer’s PLC.

The memory capacity is the size of the hardware storage units that the programmable controller can provide. The program capacity is the size of the storage units used by the user’s application projects in the memory, thus the program capacity is less than the memory capacity. During the design phase, since the user application program has not yet been compiled, the program capacity is unknown and can only be determined after program debugging. To estimate the program capacity during the design selection phase, the memory capacity is usually used as a substitute. There is no fixed formula for estimating the PLC memory capacity, and many documents provide different formulas. Generally, it is about 10 to 15 times the number of digital I/O points, plus 100 times the number of analog I/O points, with this figure being the total number of words in memory (16 bits per word), and then considering a 25% margin based on this number.

This selection includes choices for operational functions, control functions, communication functions, programming functions, diagnostic functions, and processing speed.

Simple PLC operational functions include logic operations, timing, and counting functions; ordinary PLCs also include data shifting, comparisons, etc.; more complex operational functions include algebraic operations and data transfer; large PLCs also have analog PID operations and other advanced operational functions. With the emergence of open systems, communication functions are now included in PLCs. Some products feature communication with lower-level devices, others with peer or upper-level devices, and some even have data communication capabilities with factory or enterprise networks. When designing and selecting, the required operational functions should be reasonably chosen based on actual application requirements. In most applications, only logic operations and timing/counting functions are needed, while some applications require data transfer and comparisons, and algebraic operations and PID operations are used only for analog detection and control. Additionally, some operations like decoding and encoding may be required when displaying data.

Control functions include PID control operations, feedforward compensation control operations, ratio control operations, etc., which should be determined based on control requirements. PLCs are mainly used for sequential logic control; thus, single-loop or multi-loop controllers are commonly used to solve analog control in most cases. Sometimes, dedicated intelligent input/output units are used to complete the required control functions, enhancing the processing speed of the PLC and saving memory capacity. For example, PID control units, high-speed counters, analog units with speed compensation, ASCII code conversion units, etc.

Medium and large PLC systems should support multiple field buses and standard communication protocols (such as TCP/IP) and should be able to connect to factory management networks (TCP/IP) when needed. Communication protocols should comply with ISO/IEEE communication standards and should be open communication networks.

The communication interfaces of the PLC system should include serial and parallel communication interfaces (RS232C/422A/423/485), RIO communication ports, industrial Ethernet, common DCS interfaces, etc.; medium and large PLC communication buses (including interface devices and cables) should consider redundancy configurations, and the communication bus should comply with international standards. The communication distance should meet the actual requirements of the device. The communication rate of the upper-level network in the PLC system should be greater than 1Mbps, and the communication load should not exceed 60%. The main forms of the communication network in the PLC system include the following:

1) PC as the master station, multiple PLCs of the same model as slave stations, forming a simple PLC network;

2) 1 PLC as the master station, other PLCs of the same model as slave stations, forming a master-slave PLC network;

3) The PLC network connects to a large DCS as a subnet through a specific network interface;

4) Dedicated PLC networks (each manufacturer’s dedicated PLC communication network).

To reduce the CPU communication task, communication processors with different communication functions (such as point-to-point, field bus, industrial Ethernet) should be selected based on the actual needs of the network composition.

Offline programming mode: The PLC and the programmer share a CPU. In programming mode, the CPU only serves the programmer and does not control the field devices. After completing the programming, the programmer switches to run mode, and the CPU controls the field devices, not allowing programming. This offline programming mode can reduce system costs but is inconvenient for use and debugging.

Online programming mode: The CPU and the programmer have their own CPUs. 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, and in the next scan cycle, the host runs based on the newly received program. This mode has a higher cost but is convenient for system debugging and operation, commonly used in medium and large PLCs.

Five standardized programming languages: Sequential Function Chart (SFC), Ladder Diagram (LD), Function Block Diagram (FBD) are three graphical languages, while Instruction List (IL) and Structured Text (ST) are two textual languages. The selected programming language should comply with its standards (IEC61131-3) and should also support multiple programming forms such as C, Basic, Pascal, etc., to meet the control requirements of special control occasions.

The diagnostic functions of the PLC include hardware and software diagnostics. Hardware diagnostics determine the fault location through logical judgment of the hardware, while software diagnostics are divided into internal diagnostics and external diagnostics. Internal diagnostics involve diagnosing the performance and functionality of the PLC itself through software, while external diagnostics involve diagnosing the information exchange functionality between the PLC’s CPU and external input/output components through software.

The strength of the PLC’s diagnostic functions directly affects the technical capabilities required of operational and maintenance personnel and influences the average repair time.

PLCs operate in a scanning mode. From the perspective of real-time requirements, the faster the processing speed, the better. If the signal duration is less than the scan time, the PLC will not be able to scan the signal, resulting in data loss.

Processing speed is related to the length of the user program, CPU processing speed, software quality, etc. Currently, PLC contacts respond quickly, with execution times for binary instructions around 0.2 to 0.4μs, thus meeting high control requirements and quick response needs. The scan cycle (processor scan cycle) should meet: the scan time for small PLCs should not exceed 0.5ms/K; for medium and large PLCs, it should not exceed 0.2ms/K.

PLC types: PLCs are divided into integrated and modular types based on structure.

Integrated PLCs have fewer and relatively fixed I/O points, thus offering less flexibility for user selection, and are usually used for small control systems. Representatives of this type of PLC include Siemens S7-200 series, Mitsubishi FX series, Omron CPM1A series, etc.

Modular PLCs provide various I/O modules that can be plugged into the PLC base, allowing users to reasonably select and configure the I/O points of the control system based on their needs. Therefore, modular PLCs are more flexible in configuration and are generally used for medium and large control systems. Examples include Siemens S7-300 series and S7-400 series, Mitsubishi Q series, Omron CVM1 series, etc.

The selection of digital input/output modules should consider application requirements. For input modules, factors such as input signal levels and transmission distances should be considered. There are many types of output modules, such as relay contact output types, AC120V/230V bidirectional thyristor output types, DC24V transistor drive types, DC48V transistor drive types, etc.

Relay output modules typically have advantages such as low cost and wide voltage range, but they have a shorter lifespan and longer response times. When used with inductive loads, surge absorption circuits need to be added; bidirectional thyristor output modules have faster response times and are suitable for frequent switching with low power factor inductive loads, but they are more expensive and have poorer overload capacity.

Additionally, input/output modules can be classified by the number of input/output points, such as 8 points, 16 points, 32 points, etc., and should be reasonably equipped based on actual needs.

Analog input modules can be categorized based on the type of analog input signal, including current input types, voltage input types, thermocouple input types, etc. The typical signal levels for current input types are 4 to 20mA or 0 to 20mA; voltage input modules typically have signal levels of 0 to 10V, -5V to +5V, etc. Some analog input modules can accommodate both voltage and current input signals.

Analog output modules are similarly divided into voltage output modules and current output modules, with typical current output signals being 0 to 20mA or 4 to 20mA. Voltage output signals typically include 0 to 10V, -10V to +10V, etc.

Analog input/output modules can be classified by the number of input/output channels, such as 2-channel, 4-channel, 8-channel, etc.

Function modules include communication modules, positioning modules, pulse output modules, high-speed counting modules, PID control modules, temperature control modules, etc. When selecting a PLC, the compatibility of function modules should be considered, involving both hardware and software aspects.

a. For critical process units: CPU (including memory) and power supply should have 1B1 redundancy.

b. When needed, a hot backup redundancy system composed of PLC hardware and hot backup software, or dual or triple redundancy fault-tolerant systems can also be selected.

a. Multi-point I/O cards for control loops should be configured redundantly.

b. Multi-point I/O cards for important detection points can be configured redundantly. For important I/O signals, dual or triple I/O interface units may be selected as needed.

Once the PLC model and specifications are generally determined, the basic specifications and parameters of each component of the PLC can be determined one by one according to control requirements, and the model of each component module can be selected. When selecting module models, the following principles should be followed.

When selecting a PLC, the performance-to-price ratio should be considered. When considering cost-effectiveness, factors such as application scalability, operability, and input-output ratio should be compared and balanced to ultimately select a satisfactory product.

The number of input/output points directly affects the price. Each additional I/O card incurs additional costs. When the number of points reaches a certain value, the corresponding memory capacity, rack, motherboard, etc., also need to be increased, thus impacting the selection of the CPU, memory capacity, and the range of control functions. Sufficient consideration should be given during estimation and selection to ensure a reasonable performance-to-price ratio for the entire control system.

Generally speaking, there are often many types of modules that can meet control requirements as a PLC. When selecting, the principles of simplifying circuit design, ease of use, and minimizing external control components should be followed. For example, for input modules, priority should be given to input forms that can connect directly with external detection elements, avoiding the use of interface circuits. For output modules, priority should be given to modules that can directly drive loads, minimizing intermediary components like relays.

When selecting, the uniformity and universality of the PLC’s constituent modules should be considered to avoid an excess variety of modules. This not only facilitates procurement and reduces spare parts but also increases the interchangeability of the system’s components, providing convenience for design, debugging, and maintenance.

When selecting the constituent modules of the PLC system, compatibility should be fully considered. To avoid compatibility issues, the number of manufacturers of the main components of the PLC system should not be excessive. If possible, try to choose products from the same manufacturer.

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