Since 2002, NFPA 79, the Electrical Standard for Industrial Machinery, has permitted the software control of machine safeguards in theU.S., a privilege European builders have enjoyed for a while under IEC61508. U.S. machine manufacturers no longer have to rely on point-to-point relay logic, according to Michael Bryant, executive director of the AS-i Trade Organization (pronounced "ozzy"), the manufacturing consortium responsible for the Safety at Work (SaW) system. Engineers can fit their machines with software configured e-stops, position switches, cable switches, light curtains, and scanners. These then follow a single yellow 2-wire interface cable back to a main control cabinet. SaW could mean the end of long, complex wire bundles running between main panels and individual devices, says Jochen Bihl, CEO of consortium-member Bihl+Wiedemann GmbH. Here's a look at some of the newest products.
The Rotoscan RS4-4E scanner from Lueze Lumiflex connects directly to the AS-i system. It can scan an area both vertically and horizontally through an angle of 190 degrees out to several meters. Up to four separate work zones within this region can be selected and programmed independently. For more information , visit http://rbi.ims.ca/4394-520.
The heartbeat of the AS-i system, the safety monitor, sits beside standard I/O modules in a main control cabinet. It holds a redundant set of safety relays, known as OSSDs, or Output Signal Switching Devices. Instead of reporting a system as being safe, the monitor assumes it's not, commanding the safety nodes that report to it to prove otherwise. It compares incoming code tables from all of the slave devices with code tables it stores for each of them. If the monitor finds disagreement between code tables, it activates relays to shut out part, or all, of the machine. Response time for the Siemens safety monitor shown here is 40 msec or less. For more information, visit http://rbi.ims.ca/4394-521.
Field devices connect to the AS-i network via safety modules. The Pepperl+Fuchs VAA-2E-G4-SN module has two safe inputs, allowing safety systems implementation up to a Safety Category 4 (two redundant contacts connected to two inputs on the safety node) depending on the rating of the safety device. The module replaces the safety relays of a conventional, hardwired system. The Pepperl+Fuchs VAA-2E-F85-V1 e-stop can connect directly to the AS-i system without a module. For more information, visit http://rbi.ims.ca/4394-522.
Engineers use Pepperl+Fuchs VAZ-SW-SIMON or the equivalent configuration software to assign slaves to the Safety Module's OSSDs by way of drag-and-drop commands. If a slave malfunctions, the identical component from any manufacturer can be substituted and the monitor will adjust to the new code table. That means plenty of spares exist in the U.S. for these systems, according to Philip Marshall of Hilscher North America, a provider of fieldbus products and services. For more information, visit http://rbi.ims.ca/4394-523.