October 2009

Data acquisition

Supervisory control and data acquisition is the technology that enables a user to collect data from one or more distant facilities and send limited control instruction to those facilities. SCADA has seen use to monitor and control very large process facilities. SCADA is not normally used to operate a small facility like factory; although, some factories are large enough to benefit from using SCADA. Control systems like programmable logic controllers or distributed control systems will do a better job for most factory control and monitoring.

SCADA allows a human operator in a location central to a widely distributed process, such as a pipeline system, oil or gas field, hydroelectric generating complex, or irrigation network, to make set point changes on distant process controllers, to open or close valves or switches, to monitor alarms, and to gather measurement information.

When the dimensions of the process become very large, hundreds or even thousands of kilometers from one end to the other, one can appreciate the benefits such a system offers in terms of reducing the cost of routine visits to monitor and control that process. The benefits grow if elements of the process are not only distant, but physically difficult to reach, perhaps requiring a helicopter or other special transportation. Once the benefits of a SCADA system are recognized, improved control methods can be utilized to operate the facility more efficiently and safely than could be done without the system.

Applicable processes

SCADA technology is best applied to processes spread over large areas, simple to control and monitor, and that require frequent, regular, or immediate intervention.

One process in which SCADA sees use is in groups of small hydroelectric generating stations that are turned on or off in response to customer demand and are usually located in remote locations. They can be controlled easily by opening or closing valves to feed water to turbines. They need to respond quickly to changes in demand and must be monitored continuously.

Another process is in oil or gas production facilities (wells, gathering pipelines, fluid measurement equipment, and pumps), which are usually spread over large areas, require simple controls such as turning motors on and off, need to gather fluid metering information regularly, and must respond quickly to conditions in the rest of the field.

Other processes include pipelines for gas, oil, chemicals, or water, which have elements located at various distances from a central control point; electric transmission systems, which may cover thousands of square kilometers; irrigation systems, which often cover hundreds of square kilometers; and at a heavy oil up-grader, which is controlled by a distributed control system.

Typical signals gathered from remote locations include status indications, alarms, analog values, and totalized meter values. This seems like a small list of simple options, but a vast range of information can be gathered with this apparently limited menu of available signal types.

Signals sent from the SCADA system's central location to the remote sites are usually limited to discrete binary bit changes or to analog values addressed to a device at the process. An example of a binary bit change could be an instruction to a motor to change from OFF to ON. An example of an analog value is an instruction to a valve controller to change the valve set point from 60% open to 70% open.

Elements of a SCADA system

At the center of a SCADA system is the operator, who interfaces with the system through some form of input/output (I/O) device. The I/O device in turn communicates with the master terminal unit (MTU). The MTU communicates with one or more remote terminal unites (RTUs), each of which communicates with sensors or actuators at the process.

The two purposes of a SCADA system are to extend the operator's ability to see what is happening in the process and to extend the operator's ability to make changes to the process. When the process is very small and simple, the operator may be able to directly see what is happening and to reach out to make the needed change with his or her own hand. When the process gets a bit larger, that becomes impossible; however, it may still be the operator and a wire from a switch near the operator to the motor or valve that the operator needs to adjust to make the process work properly. As the process increases in size even more, the signals to and from the operator start to degrade and the cost of running dedicated wires for each element becomes prohibitive. The operator must still know what is happening in the process, and he or she must still be able to make the changes to the process.

The master terminal is the system controller. Some industries use the term host computer, host controller, or server, but we can refer to it as the MTU. In modern SCADA systems, MTUs are always based on a computer. With appropriate software, the MTU can monitor and control the process even when the operator is not present. It does this by means of a built-in scheduler, programmed to repeat instructions at set intervals. The MTU, for instance, may be scheduled to require an update from each RTU every six minutes. The operator's I/O and the SCADA system, generally, must be a two-way device. Just as a SCADA system is not just a telemetry system gathering data from the field and presenting it to the operator, the I/O must not be limited to presenting data to the operator. It must also be capable of receiving instructions from the operators and passing them on.

SOURCE: SCADA: Supervisory Control and Data Acquisition, 4th Edition, by Stuart A. Boyer, ISA, 2010.