A common cause of machine downtime is the operation and resetting of traditional standalone safety circuits. However, Mitsubishi's innovative approach to safety relays closely integrates the standalone protection and performance of safety circuits and the diagnostic capability of the PLC either by incorporating the new QS safety relay onto the PLC rack or via a high-speed CC-Link for the standalone networked version. Mitsubishi says that this means, for the first time, detailed information about the operational condition of the safety relay is available immediately, as is the status of each of the connected safety circuits, thereby enabling the status of plant safety systems to be identified rapidly.
Jeremy Shinton, Q Series Product Manager for Mitsubishi Electric, states: "This is a major step forward for plant owners and machine operators. Typically a manufacturing plant will have a number of safety circuits on it, each with its own standalone safety relay protecting one particular aspect of the process. If one of these circuits trips, the whole plant may be effectively shut down and production lost while engineers inspect the machine or process line, looking for the appropriate relay and circuit to identify its cause, and correcting the reason before restarting operations.
"With today's tight margins, any plant down time in manufacturing is bad news.
"The New Mitsubishi intelligent QS safety relay addresses this on two levels. Firstly the tripped circuit is instantly identified at the control system. This information can then be visualised by HMI, SCADA or simple panel indicators, which dramatically reduces the circuit search-and-locate time. Secondly a history of trips and their causes can be logged and analysed, leading to identification of recurring issues, which can thus be addressed."
Avoiding nuisance trips
In developing the QS Safety Relay, Mitsubishi also realised the importance of the fact that something like half of all safety shutdowns are merely nuisance trips - typically someone brushing against an emergency stop switch or a light curtain being broken by a broom, for example. Each inadvertent stoppage adversely affects productivity and profits.
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 via CC-Link to the host PLC system. Data from these relays may be linked into higher-level control systems such as MES (manufacturing execution systems), SCADA (supervisory control and data acquisition) and various systems generating management information.
For each connected safety circuit the QS monitors eight variables: 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 status.
Shinton says: "Most importantly the QS is powered independently from the PLC. So if the PLC fails, it does not affect the safety circuit; this stays independent, protecting man and machine, come what may."
However, the key advantage of either integrating the QS with the PLC rack data bus or connecting the standalone version via CC-Link is that detailed safety circuit status can be easily transmitted to the control system. This feature cannot be replicated using traditional safety relays, where basic tripped status is all that can be derived.
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 standalone groups. The CC-Link version also supports up to three extension relays and multiple CC-Link relay stations can be configured. The flexibility of this architecture allows the safety circuits to be zoned so that a safety trip only shuts down the relevant part of the plant.
The QS is certified by TUV, and covers EN 954-1 Safety Category 3 and 4 installations. It has also been certified to the new EN ISO 13849-1 standard and meets performance level e.
Mitsubishi says the QS safety relay is the first in a series of safety products it will be launching, which will take the company into several new fields. In the UK the available safety market is thought to be valued at around £20m, and Mitsubishi believes that there has been a large gap between standalone (dumb) safety relays and costly specialised safety PLCs.
The QS Safety Relay is easy to retrofit and reduces the need for panel space, so will find favour in both new builds and upgrade projects.
Shinton concludes: "We developed the QS from listening to what our customers told us, in that they wanted something with more intelligence than a standalone safety relay but that a dedicated safety PLC was too specialised and costly. The fact that installation is as simple as clipping a standard module onto a PLC rack or a DIN rail, and that no programming is involved, means there is no learning curve to slow take-up."