Determining the Scan Interval SCADA systems

One factor that determines the scan interval is the number of RTUs to be scanned. An estimate can easily be done during the design phase of the Scada system.

A second factor to consider is the amount of information that will be sent in every conversation. Depending on the controls to have in each RTU can determine the number of bits to send in information.

For signals of ON / OFF solenoid valves used in motors, switches, lamps, etc.. Will require a bit for each of the teams. In the case of pressure gauges, flow meters, rate-modulating valve opening, etc.. Will require a word of 16 bits. In some cases it will require two words of 16 bits when the resolution of field teams so require.

To simplify and get an insurance factor, you can select the highest number of RTU control points and multiply by the number of RTUs to be installed in the system.

Remember that a conversation is usually a data transfer in both directions. It is important to include the time taken by the master station to talk to each RTU. This can include both the time required for the master station will ask for the data to the RTU and the time to assign new commands to the RTU. At this point you should also analyze the RTU with the highest number of transactions in conversation and multiply by the total number of RTU for a sure value of the number of transactions.

The third factor is the speed of communication. The number of bits per second that can be transmitted over a communication medium. Depending on the communication protocol and the physical environment can change the communication speed of the system. If we use a phone line, we can work on the range of 300 to 2 400 bps. By working with an industrial protocol, we could be reaching 19 200 bps. However, if we use a protocol such as TCP / IP can achieve speeds up to 10 Mbps latter can be useful for systems where feasible economic time is in the order of seconds.

A fourth criterion is the efficiency of communication that should be taken as the ratio of time spent moving the data of interest by the total time lost in communication. For example, part of the message includes the address of the RTU is not a point of interest, so we check the errors and the algorithms used for correction. Sometimes data that does not take longer interested in the data of interest.

Some data have been calculated for certain media, for example, the efficiency for dedicated telephone line is 70% for a 40% radio modem to a telephone line dial up rate less than 1%.

Example:

Calculate the range of scan for the next SCADA system:

Initially RTU is 7, but may increase eventually to 20. Thus the number of RTU to consider is 20.

The RTU with the largest number of control points is:

POINTS	TOTAL
140 Points of state.	140
30 Points for alarm.	30
10 Indicators (1 word of 16 bits).	160
An 10 Points to logos (1 word of 16 bits).	160

The master station can send information to RTU:

POINTS	TOTAL
Discrete controls 150 (V to valves and motors).	150
6 Engine position or n (1 word of 16 bits).	96
10 V to modulating valves (1 word of 16 bits).	160

Total points: 896

The communications team is a UHF radio that can reach speeds of 1200 bps communications.

The efficiency of communication based on a UHF radio is 40%.

Now, 20 x 896 = 17 920 bits moved at a rate of 1200 bps can take: (17 920 / 1 200) = 14.93 second to a 100% efficiency.

A 40% efficiency, the scan interval is 14.93 / 0.4 = 37.33 seconds.

The system would be designed to operate in the range of 40 to 60 seconds to update each RTU.

This value is necessary whether we are able to give a minute of delay in the response time of our SCADA system for our process. That is if there is a problem in our process we are able to wait a minute to take corrective action. In our case it would be advisable to double the speed of communication of our system, ie to work to increase 2 400 bps. This does not guarantee that halved the scan of our Scada. This is because the efficiency is not a linear function.

CRITERIA FOR THE SELECTION OF SYSTEM COMPONENTS

To select the components of the SCADA system should follow the following procedure:

Choose a telemetry network.
Choosing the data communications equipment (DCE).
Choose a station master.
Choosing a remote station to local control location.

All criteria for the implementation of a Scada system will be touched in the next articles.

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