Remote Terminal Unit (RTU) in SCADA.pptx

SCADA - Remote Terminal Unit

1. Remote Terminal Unit (RTU) in SCADA

2. Definition: R emote  T erminal  U nit ( RTU ) in SCADA systems is a microprocessor-based electronic device present at geographically distributed remote locations in order to facilitate communication of various devices within the distributed SCADA system. It consists of input-output hardware and communication interface for remote sensing and controlling of ongoing processes.

3. The sole purpose of an RTU present at the remote site of the SCADA system is to send all the collected data and information to the central station with the help of sensors, monitors, production processes, etc. so that it can be stored and monitored . The remote terminal unit is sometimes known as the  remote telemetry unit  or  remote telecontrol unit .

4. Evolution of RTU In the initial phases, even before the arrival of the SCADA system, RTUs were used for remote telemetry purposes with integration to the central computer. So, at that time, RTUs were hardwired devices that were electronically designed for fixed functions such as responding to the serially received numeric codes. So , the early remote terminal units were functioned to read and transmit the current input and status values and to generate contact outputs according to the control message .

5. But at that time, the developed systems only supported accurate monitoring and not fulfilled the purpose of effective controlling. However, further innovations in that field made possible the conveying of change of status of a process to remote monitoring station as well as printing it with reported date and time. Further, with the advent of microprocessor-based technology, the manufacturers incorporated the microprocessor and microcomputer logic within the RTUs. And this further enhanced the flexibility of industrial control systems by providing better operational characteristics and performance. The reason behind this was that microprocessor chips offered fast computing features and reduced the overall cost , this improvised the performance of the system.

6. RTU Architecture The figure here shows the architectural representation of the Remote Terminal Unit :

7. The major components of RTU are CPU, Memory (volatile and non-volatile), Power supply module with battery, Real-time clock, Watchdog Timer, Communication Interface (serial port or onboard modem with I/O interface). In the figure , a hardware module is shown where the I/O modules are placed within the RTU panel and communication is established with the master station via the communication port. The I/O modules are connected to the central processor in order to communicate with the master station.

8. CPU The inbuilt central processing unit nowadays uses 16-bit or 32-bits microprocessor with the memory capacity of 256 kilobytes. It consists of 3 communication ports with ethernet links and the real-time clock is used for the overall controlling of the system. The use of watchdog timer provides real-time checking of regular execution of processes. This watchdog timer gets reset on regular basis and in case the resetting is delayed then the timer flags error.

9. As it is clear from the figure that dual CPUs are incorporated here with dual power supply. And among the two CPUs, one serves the purpose of primary and other as standby. So, in case the primary one fails, the system is automatically triggered to the standby CPU.

10. Power Supply Module For the uninterrupted operation, continuous supply of power is required by the RTU, but the unit is separated with a large distance from the point where supply is actually present. So , to facilitate uninterrupted operation in case of power failure, an alternate power source with battery backup is provided to the unit. For low power RTUs, solar panels are mostly used while sometimes thermoelectric generators are used according to the availability of gas pipelines. Generally,  lead-acid  or  nickel-cadmium batteries  are used. The typically expected value for operation is around  110/40 V AC ± 10%  with a frequency of operation  50 Hz  or  11/14/48 V DC ± 10 % .

11. Analog Input Module The applied analog input is generally a variable voltage or current, varying in direct proportion to the process under measurement. The different analog inputs are 0-1 mA , 0-10 V, ± 1.5 V, 5 V, etc. The analog inputs must specifically include a multiplexer, signal conditioner, sample and hold circuit, ADC and bus interface. The features of analog input modules are: There are 8, 16 or 32 analog inputs, It usually offers 8 to 12 bits of resolution, Generally, 4 to 10 mA range, The input resistance for the input module is 140 Kilo ohms to 1 Mega ohm, The offered conversion rate is about 10 microseconds to 30 milliseconds.

12. Analog Output Module These are used to deal with variable quantities such as graphic recording instruments or strip charts. It basically transforms the digital value received from the CPU into analog form using DAC. Further, this analog representation controls the actuators. The basic features of analog output modules are: It has 8, 16, 32 analog outputs, It offers a resolution of 8 or 12 bits, The provided conversion rate is between 10 microseconds to 30 milliseconds, The output ranges between 4-10 milliampere or 0 to 10 V.

13. Status Inputs These represent status or alarm signals for the ongoing process. Generally, remote terminal units hold input status card that has two state real-world information. It has isolated voltage or current source that is used to have a check-on over the position of remote contact (i.e., whether it is open or closed) at the RTU site. Depending on the contact position the status of various devices like electrical breakers, alarm conditions, etc. can be determined.

14. Digital Output Module The digital output module of the RTU usually drives the output voltage at each respective output channel such as high current capacity relays so as to switch the power on and off. The approaches used for driving are as follows: Triac Switching, Read Relay Switching, TTL voltage outputs.

15. Communication Interfaces A remote terminal unit can be interfaced to multiple control stations and intelligent electronic devices using different communication media like RS485, RS232, ethernet , microwave and satellite, etc. When a communication interface is maintained between two ends then either side can initiate the data transfer so as to have synchronized data transfer with the least possible traffic issues. Also, if the multiple RTUs within the SCADA system wants to share a common communication line then using multi-drop scheme this can be done. As unique addressing is provided to each unit and response will be understood depending on their polls and commands.

16. SCADA: Master Station Master Station is one of the three components of a SCADA system. The term in a small system refers to a single computer responsible for communicating with the field equipments. In a large SCADA system, master station consists of multiple servers, distributed software applications and disaster recovery sites. Master station server and software communicate with, apart from the field equipments like RTU/PLC, HMI software running in the workstation of control room or at other place.

17. Presenting The Information Information to the operating personnel is generally presented graphically by the SCADA system. This graphic representation is known as a mimic diagram . This enables the operator to have a schematic representation of the controlled plant. For example, it shows a graphic of a pump connected to a pipe from which the operator can see both running of the pipe as well as the volume of fluid being pumped through the pipe at the given moment. The operator can switch off the pump depending on the need. The flow rate of fluid in the pipe decreases over real time as shown by the HMI software. Process elements are usually represented by schematic symbols and line graphics of mimic diagrams. The process equipment is represented in animated symbols by digital photograph.

18. HMI Workstation There is a drawing program in a typical SCADA system with HMI package. The operators or system maintenance people use this drawing program to make changes in the way of representation of these points in the interface. An on-screen traffic light representing actual traffic light state in the field is an example of a simple representation. A multi-projector display to represent in a skyscraper position of all the elevators is an example of a complex representation. Platforms like LINUX were not much used earlier on account of its dynamic development environment. However now all major operating systems like LINUX, UNIX and OPENVMS are used for master station servers and also HMI workstations.

19. SCADA: Communication Infrastructure Communication infrastructure is one of the components of a SCADA system. For meeting requirements of communication, combined radio and direct serial or other modern connections have been used by SCADA systems. Railways and Power Stations which are large sites very often prefer use of Ethernet or IP over Synchronous Optical Networking (SONET). The term Telemetry is used when a SCADA system performs function of remote management and monitoring.

20. SCADA Communication Protocols While the earlier low bandwidth protocols are still persisting, customers now mostly want SCADA data to transmit through preset corporate network and sharing of the network with other applications. Protocol designs in SCADA are compact and are so designed as to send information to master station only in case the RTU is surveyed for information by the master station. Modbus , RP570 and Conitel are the traditional vendor specific SCADA communication protocols. Standard communication protocols include IEC61850, DNP3, Profibus and IEC60870-5-101 or 104. All major SCADA vendors recognize these protocols. Communication protocols with extensions can operate in internet protocol TCP/IP . Modbus TCP/IP has now become standard for lot of hardware manufacturers and is a widely accepted communication protocol. Although for safety and security of SCADA system, it is advisable not to connect it to internet and expose it to risk, Ethernet TCP/IP has found its way into industrial automation breaking the barriers in majority of SCADA/HMI markets.

21. SCADA Control Protocols Before evolution of standards for interoperability in industry, a lot of automatic control devices like RTU were already developed. This resulted in proliferation of control protocols by various developers. Besides to keep their customers hooked, the vendors made their own protocols. Some communication between devices was originally not designed to be part of industrial network. Now there are solutions gaining wide acceptance for interconnecting diverse hardware and software permitting communication even between devices not part of original network.