SUPPLEMENT FEATURE MACHINE SAFETY STAYING IN THE LOOP
A cost saving solution from SICK to cascading safety switches and sensors is addressing the challenges of the new EN ISO 14119 machine safety standard. Dr Martin Kidman, a SICK safety specialist at SICK (UK) explains
Below: a system with cascaded safety sensors in series via Flexi Loop - PLe
competitive cost. The SICK Flexi Loop permits the series connection of dual channel devices, whilst allowing high diagnostic coverage and eliminating the potential for so-called fault masking. It is a fully open system and can accommodate standard sensor/switch devices from any vendor. As a field I/O system, it allows a designer to connect any safety system in series with another without any compromise of the safety system performance to PLe integrity.
W
ith a simple single cell safety system where two doors are fitted
with safety switches that are monitored by a safety controller, it is easy to diagnose when one of the switches malfunctions, work out which one is malfunctioning and reset the controller once the fault is rectified. Unfortunately, in real life, with multiple doors/cells and safety interlocks, the widespread practice of dual channel electro- mechanical safety switches connected in series can mask potentially dangerous faults.
HIDDEN DANGERS If there is a fault on one channel of a gate switch then that fault is likely to be detected. If one closed the faulty gate, the safety controller will inhibit a reset of the safety system. However, if one opens and closes another gate, between the faulty switch and the controller, the fault will appear to have cleared and the controller will allow a reset, masking the fault on the first switch. Thus, with current cascade and series
switching practice allowed under the existing design standards (EN 1088), it’s easy to imagine a scenario where an operative, finding one door a ‘bit faulty’ or a switch a bit ‘sticky’ finds out that the reset can be overridden by opening and closing the next door. Consequently, unsafe situations could build up which compromise PLe status of safety systems. With individually wired safety guard
S1 APRIL 2014 | MACHINE SAFETY 0
arrangements requiring extensive and complex cabling to controllers from each of the guards, separately wired cables to the controller soon multiply up. As well as being bulky, installation can be difficult and expensive. That is why engineers have preferred the series option of connection until now.
NEW MACHINE SAFETY STANDARD Because of this ‘loophole’ in the system, updating of EN 1088 as applied to the Machinery Safety Directive has been under discussion and the new EN ISO 14119 published on 31st October 2013. Implementation of the new standard
will, inevitably, have significant implications. One way of identifying individual faults on safety guards to ensure PLe levels of safety is to wire the guards back individually to the safety controller. This could mean high cost and extra bulk of additional cabling, as well as its installation and the connection hardware. One major consideration is the high cost and the sheer bulk of the extra cabling, as well as its installation and the connection hardware.
THE SOLUTION The new SICK Flexi Loop provides a connectivity solution. It achieves new dimensions of scalability, diagnostic insight and I/O connection capacity within a compact space and at a very
CAPACITY AND FLEXIBILITY With a capacity to cascade up to 32 safety sensors or switches on one loop and to create up to eight separate loops, the IP67 rated SICK Flexi Loop will provide up to 256 sensors on eight dual channel inputs, reducing the clutter of traditional connections. The loop modules also offer a standard input and output which can be used to activate solenoid locks, lamps, reset buttons and access requests. The Flexi Loop is simple to install as a fully cascadable system, using standard cable with M12/5-pin connectors. No special connections or shielded cables are required. The SICK Flexi Loop provides intelligent built-in LED diagnostics without the need for a fieldbus or complex network addressing, resulting in a decentralised cost effective solution to monitoring the status of each safety sensor/switch, loop and I/O connected to it.
DESIGN, INSTALLATION AND CONTROL The operating range allows each Flexi Loop to be up to 960m and the distance between Flexi Loop modules 30m apart. Each Flexi Loop module assures Ple as long as the sensor can fulfil that performance level, and makes calculating complex SIL or PL parameters easy. The free SICK Flexi Soft Designer Software provides pre- approved safety function blocks, simulation and all safety declaration documents at a press of a button. The functionality of SICK’s existing Flexi Soft system with Flexi Loop enables gateways to be integrated for remote diagnostics information to be passed to higher level control systems. Flexi Soft supports Profinet, Profibus, CAN open, EtherCAT, SERCOS interface, Ethernet/IP, Device Net and CC-Link.
SICK
www.sick.co.uk T: 01727 831 121
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