PLC Programmable Logic Controller
A programmable logic controller (PLC) is a digital electronic system specifically designed for industrial applications. It employs a programmable memory that stores instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations, controlling various types of machinery or production processes through digital or analog inputs and outputs.
Working principle
Once a programmable logic controller (PLC) is put into operation, its operation generally consists of three stages: input sampling, user program execution, and output refresh. The completion of these three stages constitutes one scan cycle. Throughout the entire operation, the PLC's CPU repeatedly executes these three stages at a certain scan rate.
Input sampling phase
During the input sampling phase, the programmable logic controller (PLC) sequentially reads all input states and data in a scanning manner and stores them in the corresponding cells of the I/O image area. After input sampling is completed, the process transitions to the user program execution and output refresh phases. During these two phases, even if the input states and data change, the states and data of the corresponding cells in the I/O image area will not change. Therefore, if the input is a pulse signal, the width of the pulse signal must be greater than one scan cycle to ensure that the input can be read under any circumstances.
Programmable Logic Controller
User program execution phase
During the user program execution phase, the programmable logic controller (PLC) always scans the user program (ladder diagram) sequentially from top to bottom. When scanning each ladder diagram, it always first scans the control circuit composed of contacts on the left side of the ladder diagram, and performs logical operations on the control circuit composed of contacts in the order of left to right and top to bottom. Then, based on the result of the logical operation, it refreshes the state of the corresponding bit in the system RAM storage area of the logic coil; or refreshes the state of the corresponding bit in the I/O image area of the output coil; or determines whether to execute the special function instruction specified by the ladder diagram.
That is, during the execution of the user program, only the state and data of the input points in the I/O image area remain unchanged, while the state and data of other output points and soft devices in the I/O image area or system RAM storage area may change. Moreover, the program execution result of the ladder diagram above will affect the ladder diagram below that uses these coils or data; conversely, the state or data of the logic coils refreshed in the ladder diagram below can only affect the program above it in the next scan cycle.
During program execution, if immediate I/O instructions are used, I/O points can be accessed directly. Even when using I/O instructions, the value of the input process image register will not be updated; the program directly retrieves the value from the I/O module. The output process image register will be updated immediately, which is somewhat different from immediate input.
Output refresh phase
After the user program scan is complete, the programmable logic controller (PLC) enters the output refresh phase. During this period, the CPU refreshes all output latch circuits according to the corresponding states and data in the I/O image area, and then drives the corresponding peripherals through the output circuits. This is when the PLC actually outputs.
RTU
RTU (Remote Control Unit) is a fundamental component of a SCADA (Supervisory Control and Data Acquisition) system. An RTU is an electronic device installed remotely in the field to monitor and measure sensors and equipment located there, responsible for monitoring and controlling field signals and industrial equipment. The RTU converts the measured status or signals into a data format that can be transmitted over communication media. It also converts data from a central computer into commands to achieve functional control of the equipment.
RTUs can generally be divided into two basic types: "single-board RTUs" and "modular RTUs". A "single-board RTU" integrates all I/O interfaces on a single board. A "modular RTU" has a separate CPU module, and can also have other additional modules, usually implemented by adding a general-purpose "backplane" (like inserting an add-on card into a PC motherboard).
An RTU is a durable field intelligent processor that supports communication between the SCADA control center and field devices. It is an independent data acquisition and control unit. Its function is to remotely control field devices, acquire device data, and transmit the data to the SCADA system's dispatch center. It should at least possess the following functions:
Monitoring, control, and alarm functions for field signals and industrial equipment. Examples include data acquisition and processing, field control, data transmission (network communication), field and remote alarms, etc.; some RTUs also possess metering functions tailored to specific application areas, such as flow accumulation.
Because RTUs are dedicated devices, there is currently no unified standard, especially in communications. Typically, RTUs from one manufacturer cannot be used interchangeably with those from another. Protocol conversion and competition have created an industry. The Expert Advisory Committee of the Chinese Association of Automation will soon release general standards for RTUs to the market. These standards generally include the following:
Communication standards DNPs and IEC 870.
RTU programming standard IEC1131-3.
The central processing unit can contain a built-in or standalone modem. These modems can be connected via unconditionally leased audio-grade telephone lines or similar channels such as microwave, radio, or fiber optics. Alternatively, asynchronous serial data ports can be used instead of modems to extend communication equipment up to 36.6 Kbaud.
The central processing unit (CPU) continuously selects input channels, comparing the current state or analog quantity with previous states. If the change in the analog quantity exceeds the dead zone limit, a notification of a state change is sent to the monitoring center; otherwise, a brief acknowledgment signal is sent back to the monitoring center. All other information is interleaved during continuous selection, achieving optimal response time while ignoring selection scan time conflicts.
application
Compared to commonly used programmable logic controllers (PLCs) , Real-Time Units (RTUs) typically offer superior communication capabilities and larger storage capacity, are suitable for harsher temperature and humidity environments, and provide more computational functions. RTU products are widely used in SCADA systems in industries such as oil and gas, water conservancy, power dispatching, and municipal dispatching.
Difference between RTU and PLC
RTU stands for Remote Terminal Unit, which is installed in power plants and substations in a power grid dispatch automation system.
This is an automated device based on "telemetry," "telecommunications," "teleadjustment," and "telecontrol" technologies. It features remote control reception, output execution, and data acquisition and transmission functions for telemetry and telecommunications. With the rapid development of technology, it has been widely used in various industrial fields.
The most significant feature of RTU is its remote function, which is the monitoring function it performs by transmitting information over long distances to the dispatch center.
A PLC (Programmable Logic Controller) is used for data aggregation and instruction processing within a plant. It features various control functions, data acquisition, storage and processing capabilities, mathematical operations, input/output interface conditioning, communication networking, a human-machine interface, and editing and debugging functions.
1. RTUs are generally not limited by geographical environment and can be installed near outdoor measurement points. One RTU can control several, dozens, or hundreds of I/O measurement points locally, thus demonstrating strong adaptability to harsh environments. Their operating temperature is generally -40~85℃; they have a high protection level and can be used in submarines; a single battery pack can sometimes power them for months. PLCs are generally used for controlling industrial production lines within plants and are installed indoors. The operating environment temperature requirement is 0-55℃, and the air humidity should be less than 85%. For temperatures exceeding 55℃, fans should be installed for ventilation; for temperatures exceeding 60℃, fans or air coolers should be installed for cooling. 2. Because RTUs are not constrained by the working environment, their technical specifications must meet the requirements for harsh environments and specific requirements. They have a large data storage capacity, strong analog signal acquisition capabilities (up to 24 channels), and their analog functions are far more powerful than those of PLCs.
3. RTU has powerful communication capabilities. Since the RTU needs to transmit the collected analog, digital, and switch signals to the dispatch center, which is sometimes located thousands of miles away, it requires remote communication capabilities. While PLCs also have communication functions, they are limited to short-distance data transmission within the plant, making their communication capabilities slightly inferior to those of RTUs.
4. Traditional RTUs lack programmable computing capabilities, but most modern RTUs possess programmable computing functions, along with PID control, logic control, and flow accumulation functions. As an emerging technology, it is inevitably more advanced. It features ladder diagram and C language programming, on-screen configuration software programming, strong computing power, and can be equipped with an LCD display for local display and control. These are all features that PLCs cannot match.
5. The functions of RTUs and PLCs are not entirely the same. An RTU has remote, local, and self-testing/self-adjusting functions. It is geographically distant from the dispatch center and connects to the dispatch center's computer via a channel. Its local function refers to the RTU's ability to monitor and control the network locally through itself or connected display and recording devices. The RTU's program self-recovery capability refers to its ability to automatically resume normal operation when the program malfunctions due to interference. A PLC only has local functions. In various industrial sectors, RTUs are mainly used for telemetry, remote signaling, remote adjustment, and remote control of various production processes that significantly impact a company's economic benefits, while PLCs focus on controlling the parameters of internal work nodes. For example, the RTU in a power plant is mainly used to measure and transmit data such as generator, main transformer, 110kV, 220kV and above line current, active power, reactive power, active energy, reactive energy, total active power, total reactive power of the whole plant, and total on-grid electricity of the meter at the gate. The PLC is used to control parameters such as the current of 380V low-voltage motors and 6kV high-voltage motors, the supplied load, and the temperature and pressure of steam turbines and boiler auxiliary equipment. It mainly reflects internal economic indicators such as plant power consumption, steam consumption, heat consumption and coal consumption.
6. Many power plants (especially hydropower plants) now use computer monitoring systems, with PLCs acting as controllers. In most cases, one PLC can control multiple generating units simultaneously, with each unit receiving PLC control in a group. The AGC (Automatic Generation Control System) sends a setpoint power control signal to the PLC, providing a general setpoint power command to multiple generating units. The PLC then distributes the active power requirement to the participating unit control units (LCUs) according to local strategies (DCS in thermal power plants). The LCUs respond to the PLC's command, analyze and compare it, and send variable-width pulse signals (VPWM) to the unit governors to increase or decrease power, tracking the actual generated power until it gradually approaches the desired value. The RTU (Remote Terminal Unit) is a simpler generator control method. This device receives power generation increase/decrease pulse control commands from the dispatch center, directly controlling the governors to increase or decrease active power. The approximation of the desired power value is determined by the AGC, without intermediate closed-loop processing. Therefore, the control procedures of the two methods are different.
