The term "real-time control" is defined as "relevant for a calculation as long as the physical process transpires," in our context refers to the control system response to changes in the process.
Rigorously, a real-time system is one that does not introduce delays or dead time between the receipt of the measurement of process variables and the control signal.
In fact, all control systems introduce small delays in the process. This makes the introduction of small amounts of delay to the process without affecting performance measurement or process is known as a real-time control.
Many process control systems are considered in real time. Figure 7 shows a control system in real time, because the only delay consideration is the processing time information in the process controller.
Fig.7 - System for real time control
Figure 8 shows a simple SCADA master unit Scanned three RTUs. The teacher asks the RTU unit 1 by the flow through the FT101, then this question to other RTUs by flow FT102 and FT103 their transmitters.
The scan interval is the time between a conversation with a RTU and the next conversation with the same RTU. It is obvious that this method used by the SCADA system is the slow speed and will introduce a time delay.
Figure 8 - Simple SCADA master unit with three RTUs Scanned
Figure 9 shows the delay. The decision on how it can affect the scan time in the process, can only be done by a person familiar with the process. During the beginning of the SCADA system design, the scan interval can be selected to minimize the effects of time delays.
Figure 9 - Delay in RTU's communication
In particular, systems or alarms to indicate states should be designed so that the time delays between alarm detection and recognition by the operator should be minimal. Many examples may help understand this issue. For example, a faulty device has been detected. The Scada system alerts the operator of the alarm condition exists. A response from the operator is made to return the process to its nominal conditions.
Example 1, a condition out of bounds:
A discharge pump, oil has stopped at the location 10-22. As the pump feeds a reservoir flow to the tank stops. An alarm signal appears "10-22 pump stopped." The expected response is that in the next field visit to the operator to lose enough time on site to determine the cause of pump failure. You can write a description of the failure and call the maintenance staff to repair the problem. The economic time is feasible in the order of hours to days.
Example 2, a condition out of bounds:
A submersible pump located in Section 6-33 has stopped.
Alarm signal is "Bomb 6-33 Stopped"
Expected response .- The submersible pumps are expensive and usually high-volume flow pump. Therefore the pump stops producing these great economic losses to the company. So the time to repair the equipment must a minimum.
Feasible economic time for this response is in the order of minutes and can be a maximum of one hour.
Example 3, an out of limit condition:
The connection between the generator G150 and the transmission line has been opened.
Alarm signal "Open connection G150 Generator"
Expected response-generator is essential for the operation of the plant. Therefore, their reconnection should be as soon as possible.
The economic time is feasible in the order of seconds. Than 5 seconds.
From these examples it is clear that the SCADA system design depends on the conditions to control the process and must be adjusted to have a shorter response time than the process for the SCADA system is recognized as a "Real Time".
