Mitsubishi's approach to safety relays closely integrates the stand alone protection and performance of safety circuits and the diagnostic capability of the PLC either by incorporating its new QS safety relay in the PLC rack, or via high speed CC-Link for the stand alone networked version. This configuration ensures that 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, each with its own stand alone 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.
The 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 reduce 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.
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 stand alone 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 status.
The QS is powered independently from the PLC, so if the PLC fails it does not affect the safety circuit.
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.
The CC Link version also supports up to three extension relays and multiple CC 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.