Successful diagnostics show value of proper process 28 June 2013

Plant control problems at an unnamed West Midlands dairy are being used by Mitsubishi Electric to highlights the importance of effective program management, properly commented documentation, regular system backups and a reliable service contract.

According to Mitsubishi, site engineers had noted that the blending plant was exhibiting unexplained operation of the valves and indicating valve position feedback alarms.

Downtime from any operational failure is never good news and a return to full production was critical, especially given the £3,000 per hour cost associated with any breakdown.

However, monitoring the system controlling the process didn't reveal anything wrong so engineers put in a call to the Mitsubishi technical support hotline. They were put through to Mitsubishi Electric's service agent NISCAM, and service engineer Carl Chadwick set off on the 33 mile journey to the plant.

On arrival, he found nothing wrong with the PLC itself, so Chadwick investigated one of the problematic valves and started a code trace of the valve control logic. He soon found that there was a request to open the valve but no output to the open solenoid.

He checked the logic against a backup taken three months earlier during a routine health check, as part of the plant's Mitsubishi 3 Diamond Service Contract, and immediately saw differences.

Examining the site laptop revealed two versions of the program – one fully commented, looking like the original post-commissioning work, and a second, later version.

Comparing these two with the one loaded in the PLC, Chadwick quickly identified differences surrounding the valve control logic.

It transpired that modifications had been made by one of the dairy's site engineers the previous Friday, as part of ongoing efforts to improve productivity.

Chadwick's suspicion was that the valve control modification had been implemented but with an error, so he noted the required changes, re-loaded the program and, after testing, found that the valves were now operating correctly, with no positional errors.

After 30 minutes further observation, while the process was restarted, and with the process sequence stable, he saved the program as the latest version, and retained a copy for the Mitsubishi archive system.

The moral: such a problem would have been difficult and time consuming to diagnose but for the archived backup against which the incorrect PLC program could be compared.

Brian Tinham

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