An Introduction to SCADA
The term SCADA stands for Supervisory Control And Data Acquisition. A SCADA system is a common process automation system which is used to gather data from sensors and instruments located at remote sites and to transmit and display this data at a central site for control or monitoring purposes. The collected data is usually viewed on one or more SCADA Host computers located at the central or master site.
A real world SCADA system can monitor and control hundreds to hundreds of thousands of I/O points. A typical Water SCADA application would be to monitor water levels at various water sources like reservoirs and tanks and when the water level exceeds a preset threshold, activate the system of pumps to move water to tanks with low tank levels.
Common analog signals that SCADA systems monitor and control are levels, temperatures, pressures, flow rate and motor speed. Typical digital signals to monitor and control are level switches, pressure switches, generator status, relays & motors.
There is typically another layer of equipment between the remote sensors and instruments and the central computer. This intermediate equipment exists on the remote side and connects to the sensors and field instruments. Sensors typically have digital or analog I/O and these signals are not in a form that can be easily communicated over long distances. The intermediate equipment is used to digitize then packetize the sensor signals so that they can be digitally transmitted via an industrial communications protocol over long distances to the central site.
Field Devices for SCADA – RTU, PLC, IEDs, and Smartdevices
Typical equipment that handles this function are PLC’s (Programmable Logic Controllers) and RTU’s (Remote Terminal Units). These devices employ de- facto standard industrial data communication protocols such as Modbus, AB-DF1, and DNP3.0 to transmit the sensor data. Typical physical interface standards are Bell 202 modem, RS-485 & RS-232.
The SCADA Host is usually an industrial PC running sophisticated SCADA MMI (Man Machine Interface) or HMI (Human Machine Interface) software. This software is used to poll the remote sites and store the collected data in its centralized SQL or Oracle database. Logic can be configured in the SCADA Host software which then monitors and controls plant or equipment. The control may be automatic, or initiated by operator commands.
Data acquisition is accomplished firstly by the RTU’s or PLC’s scanning the field inputs connected to the RTU / PLC. This data is usually collected at a polling rate configured by the operator. The polling rate is determined by the number of sites, the amount of data at each site, the maximum bandwidth of the communication channel and the minimum required display and control time.
Once the data has been acquired and sent to the SCADA Host, the MMI software will scan the acquired data (usually at a slower rate.) The data is then processed to detect present alarm conditions, and if an alarm is present, an alarm message will flash on the operator screen and added to an alarm list. The operator must then acknowledge this alarm. There are 3 common types of data collected:
- Analog – used for trending
- Digital (on/off) – used for alarming
- Pulse (i.e. revolutions of some kind of meter) – accumulated /counted
The primary operator interface is a set of graphical screens which show a representation of the equipment being monitored. Real-time data is displayed numerically or graphically as changing bars, circles, lines or other shapes over a static background.
As the acquired data changes in real-time, the bar, circle, line or other representative shape is updated. For instance, an analog level increase may be displayed as a lengthening of the representative vertical bar or a valve graphic may look open to represent that it is open. A typical MMI will have a nested tree structure of many such screens, usually with the many overview screen on the first page with the most relevant data displayed. There are then links that go to other pages. Users can easily configure the type of I/O point, communication protocol driver, polling rate, alarm thresholds and notifications, trend process data as well as configure the User and Operator screens.
Next generation SCADA MMI software such as NetSCADA include all these functions in one convenient easy to use and cost effective package and also have seamless internet integration enabling many clients to securely view the collected data anywhere on the internet. A flexible SCADA Host can easily expand to handle additional future remote sites and I/O points.
SCADA communications can employ a diverse range of both wired (lease line, dial-up line, fiber, ADSL, cable) and wireless media (licensed radio, spread spectrum, cellular, WLAN or satellite). The choice depends on a number of factors that characterize the clients existing communication infrastructure.
Factors such as existing communications infrastructure, available communications at the remote sites, data rates and polling frequency, remoteness of site, installation budget and ability to accommodate future needs all impact the final decision. In complex SCADA architectures, there can be a variety of both wired and wireless media and protocols involved to get data back to the central monitoring site.
SCADA systems differ from DCS’s (Distributed Control Systems) which are generally found in plant sites. While DCS’s cover the plantsite, SCADA systems cover much larger geographic areas. Often SCADA Systems are required to interface to a plantsite DCS if there are remote sensors, instruments or motors and pumps that must be controlled/monitored by the plantsite DCS.
Certain types of applications like those in Oil & Gas, Electrical & Water Utilities, Water & Wastewater and Environmental Monitoring inherently require SCADA communications because of the remoteness of the assets (i.e. Oil wells, water wells, generator stations). Furthermore, due to the remoteness many of these often require the use of wireless communications. In these cases, the traditional solution is to add a radio modem stage to the standard SCADA architecture shown above.
Next generation Wireless SCADA equipment such as Bentek Systems SMX-900 or UNICON IP offers another level of integration by placing the wireless communications and RTU functionality together in the same package.
In addition to this higher level of integration, next-generation SCADA equipment such as the IP100 also support TCP/IP, UDP or other IP based communications protocols as well as strictly industrial protocols such as Modbus TCP, Modbus over TCP or Modbus over UDP all working over private radio, cellular or satellite networks. The IP100 can act as an Ethernet Serial gateway to enable older legacy serial equipment to connect to TCP/IP networks. Coupled with Host software such as NETSCADA, this enables implementation of powerful IP based SCADA networks over mixed cellular, satellite systems and land line systems.