Tired of getting lost in PLC selection? Follow these 8 practical principles to make the right choice!
Tired of getting lost in PLC selection? Follow these 8 practical principles to make the right choice!
Tired of getting lost in PLC selection? Follow these 8 practical principles to make the right choice!
Before selecting a PLC, the first step is to define the system requirements. Once this is clear, you can then choose the manufacturer and model. But how do you make these decisions? This article provides a detailed guide to help you choose the PLC that best fits your needs, covering manufacturers, models, I/O points, and control functions.
1. PLC Manufacturers
When determining the PLC manufacturer, key factors like the user’s requirements, the designer’s familiarity with different brands, compatibility of supporting products, and technical service support should be considered. Generally, products from major international companies are reliable. For smaller, independent equipment or simple control systems, Japanese PLCs often offer better value. For large-scale systems with high networking and communication requirements, industrial PLCs from Europe and the U.S. tend to be more advantageous due to their superior communication capabilities.
For specialized industries like metallurgy or tobacco, it’s advisable to select PLC systems with proven performance and a reliable track record in those sectors.
2. Input/Output (I/O) Points
The number of I/O points is a fundamental parameter of a PLC. To determine the required I/O points, calculate the total number of I/O points needed for your control equipment. Typically, you should include a 10% to 20% margin for scalability. When ordering, adjustments should also be made based on the manufacturer’s PLC product characteristics.
3. Storage Capacity
The storage capacity of a PLC refers to the hardware storage units it provides. Program capacity, the storage actually used for user applications, is smaller than the total storage capacity. Since the program capacity is unknown during the design phase (before the user application program is written), it’s estimated based on the storage capacity. A common estimation formula is: (number of digital I/O points × 10–15) + (number of analog I/O points × 100) = total words (16 bits per word), with an additional 25% margin.
4. Control Functions
When selecting a PLC, consider the following key characteristics: computational functions, control functions, communication capabilities, programming features, diagnostic tools, and processing speed.
Computational Functions
Basic PLCs typically support logical operations, timing, and counting. Mid-range PLCs also include data shifting and comparison functionalities. Advanced computational functions like algebraic operations and data transmission are common in larger PLCs. High-end PLCs even support PID operations for analog control and other advanced calculations. Depending on the application, most scenarios only require logic and timing/counting operations, while data transmission and comparison may be needed in others.
Control Functions
Control functions include PID control, feedforward compensation, ratio control, etc. PLCs are primarily used for sequential logic control. In many cases, single or multi-loop controllers handle analog control tasks. For complex control functions, intelligent input/output modules (e.g., PID units or high-speed counters) can enhance processing speed and save memory.
Communication Functions
Mid-to-large PLC systems should support multiple fieldbuses and standard communication protocols (e.g., TCP/IP) and should be capable of connecting to factory management networks when necessary. Communication interfaces should include serial/parallel ports, industrial Ethernet, etc. For redundancy and reliability, communication buses should comply with international standards and meet distance requirements.
Programming Functions
PLC programming can be done offline (shared CPU between PLC and programmer) or online (separate CPUs for PLC and programmer). The five standardized programming languages are Sequential Function Chart (SFC), Ladder Diagram (LD), Function Block Diagram (FBD), Instruction List (IL), and Structured Text (ST). Ideally, the PLC should also support additional languages like C or Basic for specialized applications.
Diagnostic Functions
PLC diagnostics cover both hardware and software. Hardware diagnostics identify faults via logical checks, while software diagnostics include internal (performance-related) and external (communication-related) checks. Strong diagnostic capabilities reduce maintenance time and technical requirements for operators.
Processing Speed
PLC processing speed affects real-time performance. If a signal’s duration is shorter than the PLC’s scan cycle, the signal may be missed. Processing speed depends on factors like program length and CPU capabilities. Modern PLCs handle binary instructions in 0.2–0.4 microseconds, meeting high-speed control requirements. Scan cycle times should be ≤0.5ms/K for small PLCs and ≤0.2ms/K for larger systems.
5. PLC Types
PLCs are categorized into integrated and modular types. Integrated PLCs have limited and fixed I/O points, making them suitable for small control systems (e.g., Siemens S7-200, Mitsubishi FX series). Modular PLCs offer flexible I/O configurations via interchangeable modules and are ideal for larger systems (e.g., Siemens S7-300/S7-400, Mitsubishi Q series).
6. Module Selection
Digital I/O Modules
Digital I/O modules vary in specifications (e.g., relay outputs, transistor outputs) and I/O points (8, 16, 32 points). Relay outputs are cost-effective but have shorter lifespans, while thyristor outputs are faster but more expensive. Selection should align with application requirements.
Analog I/O Modules
Analog input modules handle signals like 4–20mA current or 0–10V voltage. Analog output modules similarly provide current or voltage signals. Modules vary in channel counts (2, 4, 8 channels) and should be chosen based on specific needs.
Function Modules
Function modules include communication, positioning, pulse output, high-speed counting, PID control, and temperature control modules. When selecting, consider both hardware and software compatibility.
7. Redundancy Functions
For critical applications, redundancy can be implemented for control units (e.g., 1B1 redundancy for CPU and power supply) and I/O interfaces. Redundant configurations enhance system reliability.
8. General Rules
Once the PLC type and specifications are broadly defined, determine the basic specifications and parameters of each component based on control requirements. When selecting modules, prioritize:
- Economic efficiency: Balance cost-performance ratios, expandability, and operational ease.
- Ease of use: Simplify design and reduce external control elements.
- Standardization: Use uniform modules to ease procurement and maintenance.
- Compatibility: Ensure all components are compatible, ideally from the same manufacturer.