The task of supervision of machinery and industrial processes on a routine basis can be an excruciatingly tiresome job. Always being by the side a machine or being on a 24×7 patrol duty around the assembly line equipment checking the temperature levels, water levels, oil level and performing other checks would be considered a wastage of the expertise of the technician on trivial tasks. But, to get rid of this burdensome task, the engineers devised equipments and sensors that would prevent or at least reduce the frequency of these routine checks. As a result of that, control systems and it’s various off springs like SCADA systems were formed. Supervisory Control and Data Acquisition (SCADA) offers the ease of monitoring of sensors placed at distances, from one central location.
Fig. 1: Representational Image of SCADA system
Supervisory control first evolved in electric utility systems when a need to operate remote substation equipment without sending in personnel or line crew at the remote site was felt. In 1940s, a pair of wires for every unique equipment was used between sites. The potential of multiplexing on one pair of lines was soon put to use taking ideas from the Magnetic Stepping Switch developed by telephone companies in the 30s. Security being an issue, a select-check-operate procedure was adopted where the operator waited for acknowledgement from device before finally operating it. Taking further cues from the telephone relay systems and its coding schemes, Westinghouse and North Electric Company developed the Visicode supervisory control.
General Electric and Control Corporation too developed their own independent supervisory control programs. These were used in pipelines, gas companies and even airports for runway landing lights. These systems became popular during 1950 and 1965. By that time, i.e. in 1960s Telemetry was developed for monitoring purposes. Before 1970’s equipment was generally hard wired as solid state devices were in birth and infancy stages. But with the advent of low cost computer technology, software and computers enabled the performing of the functions previously done by technicians and operators sitting besides panel instruments and tone telemetry.
The first push was given by the 8 and 16 bit computers called minicomputers. The second was the microprocessors, several years later. Computers offered flexibility in programming and communicating with field data acquisition units that was previously being done by hard wired equipments. This was the dawn of SCADA. Many organizations have been involved with the standardization of SCADA systems since then, including the IEEE, American National Standards Institute, Electric Power Research Institute, International Electrotechnical Commission, DNP3 Users group etc.
Elements of SCADA Systems
SCADA monitors, controls and alarms the plant and/or regional facilities’ operating systems from a centralized location. It includes the communication of information between a SCADA central host computer, many scattered units and/or Programmable Logic Controllers. For example, in a water filtration plant, the remote units measure the pressure in pipes and report the readings to the central computer located somewhere in the control tower. In case of any anomaly, the SCADA system would alert the main station of the problem apprising it of other details like the severity of the anomaly and measurement values in an organized fashion. The systems may vary from simple, like temperature reporting in a building to complex like monitoring the traffic on many traffic lights. The system consists of the following elements:
1. SCADA Master Station Computer Systems: It is the repository of the real-time or near real-time reported data collected from the remote terminal units connected to it. It is generally standard computer hardware equipment and very few SCADA system suppliers have ventured out to make their own computer equipment. A few companies like IBM and CDC did try making hardware for it, but the effort was short lived and commercial off-the-shelf computer products continue to be the main stay. The back end SCADA software must be able to repeatedly poll the RTUs for data values, should have software for their retrieval, storage and processing. The processing may include unit conversion, cataloguing into tables etc.
2. Human-Machine Interface: This is the eye candy part on the host station. The values that have been stored in the host computers are presented to the human operator in an understandable and comprehensible form using HMIs. These may provide trending, diagnostic or management information and detailed schematics and animations representing the current states of the machines under its control. Pictorial representation being more understandable to humans is the preferred form in SCADA HMIs.
Elements of SCADA Contd.
3. Remote Terminal Units (RTUs): An RTU is a normally a transducer or a sensor which allows the electrical circuitry to interface with the process instrumentation and control equipment. The physical parameter like pressure, temperature etc. are measured through a change in electrical property of some component in the transducer which is indicative of the physical change. A single RTU may measure many different types of parameters. Depending on the values of the measurements, the Input/Output circuitry of a RTU can be analog or digital. Analog corresponds to measurements with a numeric range of continuous values which are later converted using an ADC, like a temperature scale, while digital have limited number of states (generally two) mainly used for flagging. Specific signals can be generated to control process equipment. These days, RTUs are microprocessor based devices and these conversions are primarily internal to them.
4. Programmable Logic Controllers: The use of microprocessors on RTUs has helped RTUs become smarter with increased functionality. PLCs have been built around the philosophy of automation. Reprogrammability being the biggest asset, PLC based RTUs can be debugged and fixed on the field itself along with adding new features like support for multiple polling, exception reporting, time-tagging etc. This also enables them to execute simple logical processes without involving the master station. Vendors using different type of communication and coding on these equipment has led to standardization of protocols and languages for RTUs too, for example the standardized control programming language, IEC 61131-3. These languages require very less training and are based on intuitive approach unlike procedural languages like C and FORTRAN.
5. SCADA Communication: The conveying of data from an RTU to the master station and commands from the host to the RTU need to be done over a communication system. Also, since a SCADA system might not be localized to just a single plant, the vastness of the network also has to be catered to along with speed, accuracy, security and performance being among other important issues. Before the computer networking solutions were made available, most systems for communication were voice communication based. SCADA communication systems were also built using the same infrastructure and had the same bandwidth limitations. But, with the corporate now wanting to include the SCADA information network into their core networks over security concerns, SCADA systems have also embraced LANs and WANs for seamless integration with everyday office computer networks. This has an advantage for the corporate users that they would not need a separate parallel network for SCADA systems.
Generations of SCADA Systems
SCADA systems have grown from simple to sophisticated with rapidly changing technology and the time line can be divided into three generations:
1. Monolithic SCADA Systems:
Fig. 2: Diagram Showing Monolithic SCADA System
Owing to their origins in times when computing revolved around standalone ‘Mainframe computers’ with networks being virtually non-existent. The communication between RTU and the central computer was a dedicated line solely for that purpose. The protocols developed by vendor were to suit their own market and offered neither flexibility of functionality nor inter-market operatibility. Redundancy was provided by connecting a similar mainframe at the bus level which continuously monitored and took over the main computer in case of failure.
2. Distributed SCADA Systems:
Fig. 3: Distributed SCADA Systems
Using the LAN networks to its advantage, the computing load was distributed across multiple systems, each system being given a specific function like communication processor, calculation processor, database server etc. and sharing information in real time. This had a limitation of geographical extent and could not be used for widely distributed systems. The parts where LAN protocols were proprietary, vendors developed their own protocols optimized for SCADA systems. The use of WAN to provide communication between the RTUs and the main distributed system remained unchanged.
3. Networked SCADA Systems:
Fig. 4: Networked SCADA Systems
Based on the second generation, it follows open system architecture than being vendor controlled environment. Using Open standards mitigates many limitations allowing cross vendor compatibility and the use of any off-the-shelf standard product. This made vendors move out of hardware manufacturing and put companies like HP, Compaq and Sun Microsystems in the game of hardware manufacturing. The use of WAN networks like Internet Protocol for communication has separated the Main master station from the network by the use if an intervening communications server, thus adding another layer of security to the data and improved disaster survivability.
Protocols & Layers
Protocols and Layers
Fig. 5: SCADA Protocols and Layers
In a SCADA system, an RTU generally does not know what it is measuring. It is just following orders and reporting back. It is the master station that must know what the data is, and whose data it is. For this, there are protocols to be followed. Each protocol has two divisions: The Master Protocol, containing statements from master to RTU and the RTU Protocol, containing instructions from RTU to the main computer. Communication between master and RTU forms a model for RTU to Intelligent Electronic Device IED Communications, the most popular being International Electrotechnical Commission (IEC) 60870-5 series and the Distributed Network Protocol Version 3 (DPNP3).
With the drifting towards open standards, on one hand the SCADA systems have been easily integrated with diverse industrial systems, it also has increased the risk of people with lesser knowledge or lesser integrity gaining access and control from the TCP/IP based systems. This exposes them to variety of threats like the Denial of Service attacks, System Downtime, Trojans, keyloggers for password stealing, defamation etc. Hence, dedicated security layers for SCADA systems are needed. In the aftermath of the 9/11 attacks, Departments of Homeland Security of many countries have identified the importance of SCADA systems.
Corporate and SCADA
Merging Corporate and SCADA Systems
Fig. 6: Figure Showing Merging of Corporate Networks and SCADA Systems
SCADA has had a long journey from the obscurity of research labs into the industrial mainstream and the everyday talk of people. From monitoring and controlling the quality of water that we feed on to the air we breathe, from power generation plants to automobile factories, SCADA systems have grown roots everywhere. It significantly reduces operating labor costs while improving upon the plants or regional systems reliability and performance. Personnel no longer need to waste time wandering all over the site and since SCADA systems also amply show the level of threat, the urgency and need for a site visit can be a more judiciously prioritized decision. SCADA is used by Power companies, Utility companies like water and sewage services, Government bodies like municipalities, Physical sites like refrigeration plants, manufacturing companies for monitoring inventories, mass transportation companies and traffic managements among many others.
With every enterprise aiming to maximize profits, the human factor can be believed to be the weakest link in the whole production chain. The minimization of human factor can be achieved by increasing the level of automation, to which SCADA systems actively cater to. With the introduction of PLC devices, these systems have become more intelligent and capable of taking decisions at local levels also. Thus as the companies aim to squeeze more profits out of their plants, and governments increase the plant capacities of municipal structures for the general good, SCADA systems would thrive and help deliver accurate data and exercise precision control over processes.