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Welder takes safety to next level

18 December 2008

Architecture for safety systems that centralises the safety relays into the PLC (programmable logic controller) instead of using standalone devices spread around the plant has had its first UK use.
Lamba Welding Systems in Richmond, North Yorkshire has used the QS safety relay in a Mitsubishi Q series PLC as the basis of a new integrated control and safety system for a refurbished resistance welding machine.

Welder takes safety to next level
Welder takes safety to next level

The machine is a used for manufacturing wire mesh shelves for fridges and freezers and is about 25 years old. John Horn, Lamba’s service manager explains his decision to go with such an innovative approach: “Mitsubishi has been our preferred control and automation equipment supplier since our founding in 1989. We always go for Mitsubishi unless the customer specifically requests something else.
“Its only a matter of weeks since I heard of the QS integrated safety idea, but I’ve been itching to use it because it seems so sensible. It promises to be easy to set up, easy to install and easy to commission and maintain – this is all music to my ears because basically time saved on any project translates directly into profit on our bottom line. Plus I can also tell the end user that they will get fewer extraneous trips and rapid restarting after trips; reduced downtime is improved profit for them.”

The resistance welding machine being refurbished still had its original control system, which was looking very old fashioned, so it was not a difficult decision to strip it out and replace it with up to the minute technology.

Horn calculated that the QS saved about half-to-one day in the design stages of the project because it brings a rationality that cannot be achieved with field-deployed devices. A day or more was saved in the wiring up of the system, then there were yet more time savings during commissioning.

“QS’s plug and play functionality, made the whole process clear and simple. Commissioning was a pleasure and we could be confident we had tried and proven every possible trip-out mode.
“It was also a joy to demonstrate it to the customer. Safety is so important these days that clients like every ‘i’ dotted and ‘t’ crossed. With QS this was a breeze!”

The QS can also be used as a field device, connected not by masses of conventional wiring but by a single CC-Link fieldbus cable. John feels that between these two options every situation can be addressed - except when the end user specifies something else.

Mitsubishi's innovative approach to safety relays integrates the standalone protection and performance of safety circuits and the diagnostic capability of the PLC. Detailed intelligence is instantly available on the operational condition of the safety relay as well as the status of each of the connected safety circuits, thereby allowing rapid identification of safety conditions on the plant.

“Typically a manufacturing plant will have a number of safety circuits on it, each with its own stand alone safety relay protecting one particular aspect of the process,” said Ian Paterson, the Mitsubishi engineer who introduced the QS to Lamba. “If a single circuit trips, the whole plant may be effectively shut down and production lost for some considerable time."
“With today’s tight margins any plant downtime in manufacturing is bad news."

The intelligent QS Safety Relay addresses this on two levels. Firstly the tripped circuit is instantly identified at the control system. Secondly trips are logged and analysed.

The QS Safety relay sits on the rack of Mitsubishi’s Q series PLC and integrates fully with the plant or machine’s functional control system. The standalone version is connected over CC-Link to the host PLC system. Data from these relays may be linked into higher level control systems such as MES (manufacturing enterprise systems), SCADA (supervisory control and data acquisition) and various management information generating systems.

The QS monitors eight variables per connected safety circuit, safety input status (on/off), safety output status (on/off), safety relay coil status (on/off), and safety relay contact status (on/off). The various possible combinations of these are the basis of a strong diagnostic capability developed from the control system’s intelligent assessment of the overall stat.

Multiple QS relays can be included in a single PLC rack, each of which can support up to three extension relays, connecting directly to individual field devices such as drives, switches, light curtains, interlocks and temperature monitors. Networking of PLCs effectively allows large safety systems to be configured. The networked CC-Link version further enhances system adaptability with the ability to create small stand alone groups.

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