What is the difference between RTU and PLC?

RTU is called remote terminal unit in English and remote terminal control system in Chinese. RTU has the following characteristics:

1, long communication distance.

2, used in all kinds of harsh industrial places

3. The module is structured and easy to expand.

4. Widely used in water conservancy, electric power dispatching, municipal dispatching and other industries with remote communication, telemetry and remote control functions.

At present, RTU products are playing an increasingly important role in combination with wireless devices and industrial TCP/IP products.

RTU (Remote Terminal Unit) is a remote measurement and control unit device, which is responsible for monitoring and controlling field signals and industrial equipment. Compared with the commonly used programmable controller PLC, RTU usually has excellent communication ability and larger storage capacity, which is suitable for worse temperature and humidity environment and provides more calculation functions. It is precisely because of the improvement of RTU function that RTU products have been widely used in SCADA system.

Remote Terminal Equipment (RTU) is an electronic device installed at a remote site, which is used to monitor and measure sensors and equipment installed at the remote site. RTU converts the measured state or signal into a data format that can be sent on the communication medium. It also converts the data sent by the central computer into commands to realize the functional control of the equipment.

Monitoring and data acquisition is a broad term, which can be applied to systems that can centrally control and monitor devices installed in remote sites. SCADA system can be designed to meet various applications (water, electricity, gas, alarm, communication, security, etc. ), and meet the customer requirements of design indicators and operating philosophy. SCADA system can be as simple as a switch connected to the remote end through a pair of wires, or as complex as a computer network. It is composed of many wireless remote terminal devices (RTU) and communicates with a powerful microcomputer installed in the central control room.

The remote terminal equipment of SCADA system can be realized by various hardware and software. This depends on the nature of the controlled site, the complexity of the system, the requirements for data communication, real-time alarm report, analog signal measurement accuracy, state monitoring, equipment adjustment control and switch control.

Summary of remote measurement and control terminal

I. Overview

In the production process automation equipment, PLC and DCS are the two most widely used control systems. Before 1980s, I/O cards of these control systems were concentrated in the control room far away from the site, and connected with the field devices (including AI/AO analog input/output devices and DI/DO switch input/output devices, etc.). ) is a direct one-to-one connection. Now, we can still see a large number of cables entering and leaving the control room and large-size cable troughs in many stations. In the late 1980s, two kinds of control systems, PLC and DCS, successively introduced remote I/O cards installed far away from the control room, which were installed on the site and could be connected with nearby field equipment. These remote I/O cards exchange information with controllers installed in PLC and DCS control rooms through a cable. Prior to this, that is, in the early 1980s, some relatively small-scale manufacturers used their advantages in data acquisition, conversion and communication to launch RTU, and used RTU to form a computer SCADA (Supervisory Control and Data Acquisition) system. Sometimes we call it a four-remote (telemetry, telemetry, remote control, remote control) system. As we know, IMP remote measurement and control terminal and its SCADA system developed by British Schelumberger Company in the early 1980s are an example of the early successful application of RTU.

So what is RTU? RTU is the abbreviation of Remote Terminal Unit and the basic unit of SCADA system. A RTU can have several, dozens or hundreds of I/O points, which can be placed in the field near the measuring points. RTU should have at least the following two functions: data acquisition and processing, data transmission (network communication), and of course, many RTUs also have PID control function or logic control function, traffic accumulation function and so on.

As a product embodying the idea of "decentralized measurement and control, centralized management", RTU, a remote measurement and control terminal, has been widely used since it was introduced into China in the 1980s. Its advantages in improving the reliability of signal transmission, reducing the burden on the host, reducing the number of signal cables and saving installation costs have also been affirmed by users.

Second, the application of remote measurement and control terminal RTU

(1) Application occasions

In addition to a large number of applications in traditional industrial production processes (such as power plants, steel mills and chemical plants), remote measurement and control terminal RTU is also widely used in the following industries: urban water supply automation control system; Urban sewage treatment system; Comprehensive dispatching system of urban gas pipeline network: automatic system of natural gas and petroleum industry; Power remote data set control system; Automatic control of heating network pipeline; Environmental monitoring, such as air/water quality; Hydrological forecasting system of water regime; Lighthouse navigation AIDS, inland navigation, port and mine dispatching system.

(2) Application examples

The data acquisition system of 2× 130t/h boiler and 25MW steam turbine in Yumen Power Plant, Gansu Province consists of 34 893-IDCB remote measurement and control terminals RTU and 5 microcomputers. It collects and controls 380 analog input signals, 30 analog output signals, 125 switch input signals and 6 switch output signals of the whole system, and realizes the operation parameters of thermal power units.

Santamu operation area in Tarim Oilfield, Xinjiang is an operation area of hundreds of square kilometers in Fiona Fang, which is located at the edge of Taklimakan desert and has a very harsh natural environment. There are 80 oil wells in the whole region, which are usually unattended, and the measurement data of oil wells can only be collected by manual inspection. However, due to the vast territory and harsh natural conditions, it often takes half a day to complete a patrol, and equipment failures cannot be found in time. Therefore, determining the operation area to transform the oilfield metering system involves 5 metering stations and 2 valve group stations. The FOPLC system of Hollysys Company is adopted for data acquisition and control, and the equipment operation status information is transmitted to the central station through digital radio. After the system is applied in Tarim Oilfield, it runs stably. The polling of seven RTU stations by the central station can be completed in just a few minutes (the longest distance between the central station and the RTU station is 13km), and all data are collected accurately, avoiding data acquisition errors caused by human factors and truly realizing metering automation.

The sewage remote monitoring system of Kunming Environmental Protection Bureau consists of control centers and remote monitoring terminals scattered in hundreds of monitoring points in Kunming. The remote monitoring terminal is a micro PLC with GSM/GPRS module. Remote monitoring terminal transmits data of monitoring points (including sewage discharge flow value, running time of sewage treatment equipment, etc.). ) regularly send it to the control center through the short message service provided by the telecommunications department, and record it in the SQL Server database, and accumulate the input sewage discharge flow value and the running time of sewage treatment equipment in real time. Report the accumulated data of the day to the data center regularly every night. In addition, the monitoring personnel of the control center can check the real-time operation data of the monitoring points scattered in the urban area of Kunming at any time, and know the actual operation of each monitoring point, such as whether the sewage treatment device is running, the actual sewage discharge and other data.

Third, the performance comparison of RTU of remote measurement and control terminal

From the user's point of view, we can compare the performance of various remote measurement and control terminal products from the following aspects:

(1) appearance

Remote measurement and control terminals are usually rectangular, and most of them are equivalent to a black box. There is no keyboard for display and operation, and the operation and display must be carried out in the upper computer. SuperE of Beijing Ankong Technology Development Co., Ltd. can be equipped with LCD screen and operation keyboard according to user's requirements. The LCD is monochrome, and the display resolution is 240× 128. The display contrast and backlight value of the display can be adjusted online. It can also be used for on-site monitoring, on-site operation, on-site diagnosis, on-site maintenance and on-site upgrade on the LCD display and operation keyboard of the remote measurement and control terminal. The maximum configuration screen is 40 pages, the maximum dynamic elements of the screen are 40, and the operation keyboard is 28, which shows that it has strong operation ability and rich display functions. The related pictures of this theme are as follows:

Figure1EP105 RTU of EPRO Company

(2) Diversity

There are only a few varieties of IMP from Schlumberger Company in Britain, such as DC analog input, resistor input, DC analog output and switch input/output, but each IMP has good versatility. For example, the analog input IMP can accept three types of input signals: thermocouple, DC current and DC voltage, and each type of signal has multiple scales or multiple ranges to choose from. Another example is the resistance input IMP, which can accept four types of input signals: resistance, DC voltage, Pt 100 and strain, and each type of signal has multiple bridges or multiple ranges to choose from. For example, switch input/output IMP can freely choose whether each point is a switch input or a switch output. This flexibility is not available in other instruments at the same time, so its universal input characteristics are generally welcomed.

893-IDCB of Nanjing Institute of Technology inherited the above advantages of IMP. At the same time, according to the needs of domestic users, many new varieties of RTU have been developed, such as power frequency AC input and intelligent regulator. To meet the requirements of domestic (especially power plants) for power signal acquisition and PID regulation circuits. The related pictures of this theme are as follows:

Figure 2 Beijing security RTU products

SuperE of Beijing Ankong Technology Development Co., Ltd. chose another way of thinking. It can configure five types of signal channels in each RTU, including analog input, analog output, switch input, switch output and pulse input, with a maximum of 35 channels. When selecting the type, the user only needs to determine whether it is necessary to select all five signal channels and the number of channels of each signal according to the type and quantity requirements of field signal acquisition. As we know, every specific production site is often not just a single signal type, such as aeration sedimentation tank for sewage treatment. It has analog input signals such as temperature, liquid level, flow rate and pH value, analog output signals for controlling the action of regulating valves, on-off and off-off signals for pumps and gates, and even some flow signals may be pulse input signals. Therefore, the production site needs to be able to accept different signal types of mixed RTU. Moreover, the number of points of various signal types varies greatly in different production sites, which requires that the number of points of various signals can be selected flexibly. In the field wiring of RTUs such as IMP and 893-IDCB, in order to connect all single signal types to the same RTU, it is necessary to lengthen the connecting cables of some signals. Therefore, the RTU and points of mixed signal input and output can be changed appropriately, such as SuperE, which can flexibly adapt to the requirements of various production sites, which not only facilitates the choice of users, but also saves the number of RTUs and reduces the number of signal cables.

SuperE-M is a miniaturized product of SuperE, which has at most 15 I/O channels and integrates the latest GPRS (General Packet Radio Service) technology. Another product is SuperE-S, which adopts modular building block structure, and the number of I/O channels ranges from a few to 600. After joining these products, users have more choices.

I/O of SEAT Australia MOX original card includes 8-point AI, 8-point DI, 8-point DO and 2-point counting pulses, while MOX Unity card can be selected by users. If users need more I/O points, they can connect various AI, AO, DI and DO cards through RS485 and Ethernet. MOX Unity card can also realize the redundant configuration of I/O points and power supply, but the redundant function is rare in RTU. The MOX GATEWAY fieldbus component can provide the conversion between various standard fieldbus (such as Profibus-DP, Interbus, DeviceNet, ControlNet, CANOpen, AS-I) and industrial standard protocols (such as MODBUS and MODNET).

(3) Power supply

When the number of IMPs in Schlumberger's S network is five or less, the IMP can be directly powered by the host computer, and when the number of IMPs exceeds five, it is externally powered, with the power supply level of 12 ~ 50VDC.

According to special requirements, 893-IDCB of Nanjing Institute of Technology adopts 220VAC power supply or 12 ~ 36 VDC power supply.

SuperE of Beijing Ankong Technology Development Co., Ltd. adopts 220VAC power supply, 24VDC power supply and 12V solar battery power supply, which has the functions of power saving mode, battery charging and backup. Power saving mode means that RTU enters sleep state under the control of software, and the power consumption is reduced. Once the input is interrupted, the system will resume power supply, which is especially suitable for solar cell power supply. Another advantage of SuperE is that it can provide isolated 24V power output for field instruments (such as transmitters), thus greatly reducing the workload of special power supply, connecting wires and wiring for distribution.

ENET wireless RTU of Beijing Hua Xun Communication Electronic Technology Co., Ltd. uses 10~30VDC for power supply, which supports solar cell power supply.

The RTU of ELPRO can provide AC power supply, DC power supply, solar battery power supply, battery power supply and 24VDC ring network power supply.

(4) communication

Generally speaking, communication can be divided into two categories: wired and wireless. The wired mode adopts the private network of each company or the field bus network meeting international standards, and the wireless mode adopts remote dial-up communication, radio station, satellite communication, private line communication and GSM/GPRS communication.

IMP remote measurement and control terminal of Schlumberger Company in Britain is connected by S network, which is a special double-core twisted pair shielded cable with total length of 1000m and communication baud rate of 163k, and can scan 1000 channels per second. The connection with the host is through the S- network adapter, and the number of hosts can only be one.

The 893-IDCB remote measurement and control terminal of Nanjing Institute of Technology is connected through 893- network. 893- network is an ordinary double-core twisted-pair shielded cable, with a total length of 1200m and a communication baud rate of 187.5k, which can scan 1600 channels per second. The connection with the computer is through the 893 network card, which is a multi-host system, and the maximum number of hosts is 3 1.

SuperE of Beijing Ankong Technology Development Co., Ltd. is much more flexible in communication. It uses standard communication protocols and various communication methods to communicate, and can provide two external communication interfaces. For example, the following communication modes can be adopted: RS232, RS485, radio station, remote dial-up communication, satellite communication and private line communication. The communication distance of RS232 is 16m, RS485 is 1200m, the number of radio stations is 10km, and the dedicated line communication also exceeds 10km. Users can choose or combine from a variety of communication modes according to the use environment. It also supports the standard MODBUS RTU/ASCII communication protocol, and can also customize the communication protocol.

(5) System functions

IMP of Schlumberger Company in Britain can only constitute a single-function data acquisition system, and it can't carry out PID loop control and logic control.

Nanjing Institute of Technology 893-IDCB can form a data acquisition and control system with PID loop control because of its intelligent regulator, but the PID loop control should not be too complicated.

SuperE software of Beijing Ankong Technology Development Co., Ltd. includes logic ladder program and C program, which can complete general functions such as calculation, logic control and PID loop control. C program can complete complex functions such as calculation, logic control and PID loop control, so it can form a data acquisition and control system combining logic control and PID loop control. Such a system can also be called PLC or DCS system.

(6) Software

IMP of British Schlumberger Company only provides a few softwares such as drivers and Windows-based configuration packages.

Nanjing Institute of Technology 893-IDCB only provides software such as network driver, network demonstration debugging program and interface program with various industrial control software.

SuperE of Beijing Ankong Technology Development Co., Ltd. adopts multi-task system, and its internal program includes four parts: monitoring program, logic ladder program, C program and screen configuration program. The monitoring program controls the operation of the whole system and completes data collection, storage and communication. The logic ladder diagram program can complete general functions such as calculation, logic control and PID adjustment. C program can complete complex calculation, logic control, PID adjustment and other functions, and can also realize custom communication protocol; The screen configurator completes the configuration display of the screen. SuperE can also provide a variety of gas flow algorithms and special monitoring software packages, such as sewage treatment, water source well control, pumping unit control, oil and gas metering software packages, and users can also carry out secondary development.