Omron Solutions:

Building on its in-house resources and expertise to project manage a solution, Omron developed a system based around its C200Ha PLCs, each using the communications version of the CPU, providing four communications ports. Omron senior application engineer Matt Bridgman takes up the story: “In programming the PLCs, Omron had to take into account the implementation of the telemetry protocol, the differing requirements of the radio modems and the wired modems used on the leased lines around various areas of the site, as well as the use and differing functionalities of hub controllers and satellite controllers.”

“For simplicity and ease of serviceability, we developed one program that would provide both hub and satellite functionality,” he continues. “By doing this, we could load every PLC with the same program, and then simply tell the PLC what sort of controller it was.”

“Reliability was always going to be the key area of judgment in assessing the rival systems, so integrity of communications was crucial. With the appropriate skills available in house, Omron was able to implement the telemetry protocol to interact effectively with the SCADA system and proved its reliability. Just as important though, was the hub to satellite link. The radio modems proved straightforward to talk to but the wired modems were more troublesome,” says Bridgman.
 

“Their design was such that you had to send them a ‘wake up’ signal before you could transmit data. This meant building in a delay and a handshake. We also found that when we finished sending or receiving data, the modem would throw out a string of garbage as it shut down, so we had to build a level of robustness into the program to deal with this. As a matter of fact, we had to program the PLCs to report any errors to the SCADA system and to reject any messages that were totally corrupt, and in both cases take the appropriate action. By the time we had finished, the PLCs were actually providing more error handling capabilities than the original system.”

“The specification called for the use of some form of interactive operator feedback, and Omron’s answer was the NT20S human machine interface (HMI) hooked up to one of the C200Ha’s communication ports. The HMI was an important addition to the system to provide more information locally, and it proved to be an important differentiator for Omron,” explains Bridgman.

To give Bristol Water an idea of the power of the Omron HMI, Omron developed a number of demonstration screens, building in a lot of clever and useful functions, for alarm handling, analysing the I/O, trending, pump management, etc. As well as its functionality, though, a key advantage of the NT20S was its ability to operate reliably in a hostile environment - an outstation can see extremes of temperature from 0 up to 50 deg C.

The communications version of the C200H Alpha also provided a spare port, giving Bristol Water the option to implement security measures in every outstation, allowing the SCADA package to log the identity and activity of everyone interacting with the system.

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