There are many PLC manufacturers, such as Siemens and Schneider Electric; virtually every company involved in the field of industrial automation offers its own PLC products. The sheer number of manufacturers underscores the importance of PLCs. To help you better understand PLCs, this article will explain how to select control functions for a PLC. If you're interested in PLCs, feel free to keep reading.
1. Computational Functions
The computational functions of simple programmable logic controllers include logical operations, timing, and counting; those of standard programmable logic controllers also include data shifting, comparisons, and other operations; more complex functions include algebraic operations and data transfer; Large-scale PLCs also feature PID operations for analog signals and other advanced computational functions. With the emergence of open systems, communication capabilities have become standard in PLCs; some models support communication with lower-level devices, others with devices at the same level or upper-level systems, and some even enable data communication with factory or enterprise networks. When designing and selecting a PLC, one should base the decision on the actual application requirements and choose the necessary computational functions appropriately. In most applications, only logic operations and timing/counting functions are required. Some applications require data transmission and comparison, while algebraic operations, value conversion, and PID operations are used only when the PLC is employed for analog signal detection and control. Operations such as decoding and encoding are required when displaying data.
2. Control Functions
Control functions include PID control operations, feedforward compensation control operations, ratio control operations, and others; these should be determined based on control requirements. Programmable logic controllers are primarily used for sequential logic control; therefore, in most cases, single-loop or multi-loop controllers are employed to handle analog control. Sometimes, dedicated intelligent input/output units are used to perform the required control functions, thereby improving the processing speed of the PLC and conserving memory capacity. Examples include PID control units, high-speed counters, analog units with speed compensation, and ASC code conversion units.
3. Communication Functions
Large and medium-sized PLC systems should support multiple fieldbus protocols and standard communication protocols (such as TCP/IP) and, when necessary, be capable of connecting to a plant management network (TCP/IP). Communication protocols should comply with ISO/IEEE communication standards and form an open communication network.
The communication interfaces of a PLC system should include serial and parallel communication ports, RIO ports, and common DCS interfaces; communication buses for large and medium-sized PLCs (including interface devices and cables) should be configured with 1:1 redundancy. The communication buses should comply with international standards, and the communication distance should meet the actual requirements of the facility.
In the PLC system's communication network, the communication rate of the upper-level network should exceed 1 Mbps, and the communication load should not exceed 60%. The PLC system's communication network primarily takes the following forms:
1) A PC serves as the master station, with multiple PLCs of the same model acting as slave stations, forming a simple PLC network;
2) A single PLC serves as the master station, with other PLCs of the same model acting as slave stations, forming a master-slave PLC network;
3) A PLC network connects to a large-scale DCS via a specific network interface, functioning as a subnetwork of the DCS;
4) Dedicated PLC networks (manufacturer-specific PLC communication networks).
To reduce the CPU's communication load, communication processors with different communication capabilities (such as point-to-point or fieldbus) should be selected based on the actual requirements of the network configuration.
4. Programming Functions
Offline Programming: The PLC and the programmer share a single CPU. When the programmer is in programming mode, the CPU serves only the programmer and does not control field devices. After programming is complete, the programmer switches to run mode, and the CPU controls the field devices while programming is disabled. Offline programming reduces system costs but is inconvenient for use and debugging. Online programming: The CPU and the programmer each have their own CPU. The host CPU handles field control and exchanges data with the programmer within a single scan cycle. The programmer sends the program or data created online to the host, and in the next scan cycle, the host operates according to the newly received program. Although this method is more expensive, it facilitates system debugging and operation and is commonly used in medium- to large-scale programmable logic controllers.
There are five standardized programming languages: 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 must comply with the relevant standard (IEC 61131-23) and should also support multiple programming languages, such as C and BASIC, to meet the control requirements of specific applications.
5. Diagnostic Functions
The diagnostic functions of a programmable logic controller include both hardware and software diagnostics. Hardware diagnostics determine the location of hardware faults through logical analysis of the hardware, while software diagnostics are divided into internal and external diagnostics. Internal diagnostics involve using software to assess the performance and functionality of the PLC's internal components, whereas external diagnostics involve using software to evaluate the information exchange functions between the PLC's CPU and external input/output components.
The strength of a PLC's diagnostic capabilities directly affects the technical proficiency required of operators and maintenance personnel and influences the average repair time.
6. Processing Speed
Programmable logic controllers operate using a scanning method. From the perspective of real-time performance requirements, the faster the processing speed, the better. If a signal's duration is shorter than the scan time, the PLC will fail to detect that 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. PLCs feature fast response times and high speed; the execution time for each binary instruction is approximately 0.2 to 0.4 Ls, making them suitable for applications with high control demands and fast response requirements. The scan cycle (processor scan cycle) should meet the following criteria: for small PLCs, the scan time should not exceed 0.5 ms/K; for medium and large PLCs, the scan time should not exceed 0.2 ms/K.
Controller Types
PLCs are classified into two categories based on structure: monolithic and modular; into two categories based on application environment: field-mounted and control room-mounted; and into various categories based on CPU word length: 1-bit, 4-bit, 8-bit, 16-bit, 32-bit, and 64-bit. From an application perspective, selection is typically based on control functions or the number of I/O points.
Monolithic programmable logic controllers have a fixed number of I/O points, offering users limited flexibility in selection; they are used in small-scale control systems. Modular programmable logic controllers provide a variety of I/O cards or plug-in modules, allowing users to select and configure the number of I/O points in the control system more effectively. They offer convenient and flexible functional expansion and are generally used in large and medium-sized control systems.