Programmable Logic Controllers provide Bristol Water with a telemetry solution

Solution for modern demands on water quality

When Bristol Water looked to replace its telemetry outstations and plant control equipment, it looked beyond the traditional and the conventional in order to ensure it installed the best performance solution. Bringing new ideas to the water industry, Omron provided an outstation controller and telemetry solution built around programmable logic controllers.

Serving the population across 922 square miles, Bristol Water supplies some 325 million litres of treated water each day to over one million people and 40,000 Products. Controlling and monitoring the water treatment process are over 100 outstations, linked to the company's central control room via a telemetry system. The telemetry outstations operating at Bristol Water dated back to 1976. Although the central control system had been replaced a number of times to take advantage of the massive leaps in computing power over the years, the various outstations had been simply upgraded. Although operational, after 20 years they were becoming difficult to service.

Other factors, too, were driving a need for change, as James Reckhouse, Mechanical and Electrical Engineering Manager at Bristol Water, explains: Modern demands on water quality mean you need much greater feedback on plant operations from the outstations, and improved process control. Reckhouse had investigated the use of modern programmable logic controllers (PLCs) before in control of the process, and had found them to be both robust and reliable. So when Omron's water industry team offered a solution that would match and exceed Bristol Water's telemetry outstation needs, he was interested. Although traditional use for PLCs in the industry is for works automation, the specification for this new angle on control showed Omron's PLCs to be an improved solution not only for plant control, but also for providing the required telemetry functionality.

The ability to use an existing proprietary telemetry protocol to communicate via radio with our central system was crucial, says Reckhouse, since this was an aspect of the existing architecture we had determined to keep. Having put the replacement of the outstations out to tender, and having received a number of proposals, a shortlist was drawn up, including Omron's PLC solution. The shortlisted control companies were each invited to install a test system. We had the test systems installed in the most environmentally demanding pumping outstation we could find, says Reckhouse. It was noisy - both electrically and in terms of vibration - hot, and overall thoroughly unpleasant. We put each system through weeks of rigorous testing, including getting all the people who were ever likely to use or access the system to try it out, get hands on experience, and give their assessments.

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 we 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, and 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 judgement in assessing the rival systems, so integrity of communications was crucial. With the appropriate skills available in house, Omron were 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 course, 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 an NT20S human machine interface (HMI) hooked up to one of the C200Ha's communication ports. The HMI was an important addition to the system for providing 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, we 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. With the trial system proving more than capable, Omron was awarded the contract to replace some 110 systems in the field. We were impressed with their in-house expertise, their in-house resources to develop and manage the project, and the quality and capabilities of their products, says Reckhouse. As well as a rugged and robust solution, the PLC system provided all the monitoring, diagnostics and statistical information we needed to perform the essential local trending. Equally important though are the system's inherent future proofing and expandability. With the PLC solution, we can add I/O as we go along, allowing us to incorporate the latest intelligent instruments as they come onto the market.

The Omron solution provides us with a rolling upgrade path, he concludes. We now have the scope to let the system evolve.