What Is SCADA?
Supervisory Control and Data Acquisition (SCADA) Systems are computer-based systems that communicate to real world devices to monitor their physical characteristics and command changes in those characteristics either autonomously or by operator interaction. SCADA systems then, require four primary elements: (1) a group of sensors and actuators that can sense physical conditions and work to change those conditions; (2) a device to multiplex the signals from the sensors and actuators and relay those signals to the control center; (3) a communications medium to connect the multiplexers to the control center; and (4) a computer system to organize the data from the field and provide visualization, alarm, and remote control capability to a user.
The sensors and actuators take many different forms. A typical water system will have instruments to measure tank level, water pressure, flow rates, chemical parameters such as pH and chlorine residual, and many other parameters that are important in the processes being monitored and controlled. The actuators in a water SCADA system include control valves with motorized, pneumatic, or hydraulic actuators; pumps; air compressors and blowers; mixers; and other machinery. These sensors and actuators are known as I/O (Input/Output) devices.
RTUs, PLCs, and IOCs
PLC: An Electronic DeviceThe multiplexers that communicate the field conditions to the control center and relay control commands to the field come in many varieties, but are generally a class of electronic equipment called a programmable logic controller (PLC). A PLC includes many different modules for receiving field information and for controlling actuators. In the early days of SCADA, the multiplexing was done by a fairly dumb device called a Remote Terminal Unit (RTU) whose function was limited to converting the measured field conditions to digital data for transmission to the control center and receiving digital data from the control center and converting them to signals to the actuators. These RTUs typically did not make any control decisions autonomously. PLCs, on the other hand, were designed from the outset to be smart devices that could make autonomous control decisions. Early PLCs could emulate the control decisions that would previously be implemented using relays connected to field devices. More recent PLCs are powerful industrial computers that can be programmed to control complex sequences.
PLC: A Complete UnitTo complicate matters, the terms RTU and PLC are also used to refer to not only an individual piece of electronic equipment (as described above), but also to a fabricated assembly housed in a cabinet that consists of the I/O multiplexer (which today is almost always a PLC) radios or fiber optic modems, network switches, batteries and power supplies, and everything else needed to connect the equipment at a remote facility to the control center. In addition to the terms “RTU” and “PLC”, these fabricated assemblies may also be referred to as “Remote Control Panel”, “SCADA Panel”, “Telemetry Panel”, etc. The context in which the terms RTU and PLC are used generally indicates whether the reference is to a piece of electronic equipment or an entire assembly in a cabinet.
In the early days of SCADA, one could not get a great deal of computing power in inexpensive industrial controllers, so all of the field data was multiplexed and communicated to the control center where the central computer made some limited automatic control decisions and allowed operators to make manual control decisions. As computing progressed, PLCs with a reasonable amount of control capability became available, and simultaneously, central computer systems migrated from expensive mini- or micro-computer systems to less expensive and less reliable hardware and software platforms including commodity PCs running Microsoft Windows.
This change led to the decentralization of control. In a decentralized system, PLCs, programmed with logic to make autonomous control decisions were placed in the field and the computers at the control center were used for data logging, reporting, trending, alarming, and visualization, but not for automatic control. The difficulty in decentralized systems is that many processes require data communications between process areas to be able to make correct control decisions. This led to very complex communications protocols between PLCs with the attendant cost and reliability issues that complexity brings.
Very often in the computing field, progress goes in circles. Now, with very smart PLCs available and high speed communications between PLCs and field sites being achievable, Owners are deploying Ethernet-connected I/O modules that act as a remote extension of a central PLC. That PLC can now see all of the data in a process and make all the control decisions without having to communicate with other controllers. This change has led back to the prevalence of centralized control systems. Many SCADA systems are now evolving toward a centralized architecture with a single Plant Master PLC communicating by fiber optic or copper cable to a set of Input/Output Cabinets that contain Ethernet-connected I/O modules. Because we love to abbreviate everything we can, we call these Input/Output Cabinets “IOCs.” The I/O modules in the IOCs are similar to early generation RTUs in that they make no control decisions, but simply relay information to the Plant Master PLC. Under this architecture, the Plant Master PLC makes all of the automatic control decisions at the plant.
900 MHz RadioSCADA systems are characterized by having RTUs located too far from the control center to allow direct wiring to the field I/O. However, SCADA systems are also used for process control in industrial and municipal plants like the water and waste water treatment plants. When a SCADA system is used to control a plant, the system is often referred to as a Process Control System (PCS). However, this is a semantic difference with no real meaning in practice. Generally, the same equipment and techniques are used to create both SCADA and PCS systems. The only distinction between the two is, generally, in the communications media used to communicate the field I/O to the control center.
For PCS, it is common to use fiber optic or copper cables to connect the various RTU, PLCs, IOC, or other I/O multiplexers to the control center. This is possible because the distances tend to be short and the owner generally has conduit or duct bank between the various process areas in which to install the cables. In SCADA systems with RTU located at a great distance from the control center, direct cable communications is not practical, and other communications media are needed.
In the early days of SCADA, options for communications media to connect the field equipment to the control center over long distances was limited to leased telephone lines or radio. The past 20 years have seen an explosion of communications options for telephone and data transmission, and nearly all of those options are available to a SCADA system owner to connect RTUs to control centers. In addition to leased telephone and radio, SCADA systems can use cell phone networks, satellite communications, and a variety of leased broadband services. However, radio communications remains one of the preferred methods.
Because SCADA communications tend to be between a control center and a number of RTUs, point to multipoint communications are very efficient. Using a point-to-multipoint communications channel, the control center can poll each RTU in sequence to get an update of the field status and issue control commands. The radio technology designed specifically for this application is called Multiple Address System (MAS) radio. MAS radios have a master radio, generally at the control center or at a repeater location that can see the control center and most of the remotes. Remotes that can’t communicate with the MAS master are usually connected through one or more radio repeaters.
As described above, SCADA systems are computer-based systems that communicate to real world devices to monitor their physical characteristics and command changes in those characteristics either autonomously or by operator interaction. All of the equipment mentioned above, the field devices, the smart and not-so-smart I/O multiplexers, and the communications channels serve to make all of the automatic control decisions in the SCADA system and to get data back to the central computers so that operator interaction is possible. Consequently, the purpose of the SCADA computers is to acquire data from the field, store that data for historical purposes, present the operators with graphical representations of the state of all of the real-world devices being monitored and controlled, provide the operators with controls to manually manipulate the field devices, and provide alarms when field conditions get outside of normal ranges. In addition, the computer systems provide trending and analysis tools so that operators can understand how the process is behaving over time, solve problems, and optimize operations.
Often the SCADA system will be augmented with an array of networking infrastructure including domain controllers, network switches, firewalls, and a reporting server, which provides a pathway for reporting and analysis data to be shared between the SCADA system and the owner's business network. Connecting SCADA networks to other networks is fraught with difficulty. In the early days of internetworking, clients saw there SCADA system performance decline to unacceptable levels due to competing traffic on the SCADA network. With proper network segmentation, these problems can be avoided, but network security has become vastly more complicated. When a SCADA system is connected to another network, there is always the opportunity for unauthorized users or malicious software to be introduced into the SCADA system, possibly with disastrous consequences.
Timberline Engineering has been designing SCADA systems since 1991. We have seen the evolution of SCADA from centralized, monolithic solutions to distributed commodity solutions and back again. We have developed expertise in all of the disciplines relevant to SCADA design:
- Standard and "smart" water/wastewater field instruments.
- PLC and RTU hardware.
- Plant process controls.
- Data telemetry.
- SCADA hardware and software systems.
- User interfaces.
- Supervisory control algorithms.
- Enterprise network design.
- Computer and network security.
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