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Article | May 31, 1998
Hershey 'opens' up for digital motion control (sidebar)
What SERCOS is and isn't
SERCOS which stands for SErial Realtime COmmunications System is a communications standard for connecting several drives to a motion controller. It's a type of network though typically it's self-contained within a packaging machine. Instead of hard-wiring each drive directly to the motion controller all these components are connected via a common fiber-optic link creating a loop (see illustration). For packaging equipment perhaps the most important benefit of this type of network is that it permits real-time synchronized multi-axis motion control at very high speeds. The motion controller issues a burst of several separate motion-control commands to the various drives. The transmission speed of four megabits/sec is fast enough to easily keep up with even the highest-speed packaging equipment accor-ding to SERCOS North America the organization that promulgates the standard. For example on a modern high-speed flow wrapper the film will advance the product will be conveyed and the sealing will occur all in precise synchronization-even at speeds in excess of 1 pieces/min. Other benefits of a digital multi-axis motion-control network include reduced wiring costs and easy replacement of components from the same or different manufacturers. The network can eliminate as many as 200 individual electrical connections-each a potential failure point-in a typical 8-axis packaging machine. And because the fiber-optic cable uses light instead of electricity to transmit information it is unaffected by electrical "noise" generated by other equipment. SERCOS was created in the late '80s by a consortium of companies. Today more than 27 companies incorporate this international communications standard published as IEC 1491 in their components. What SERCOS isn't is a communication network between stand-alone packaging machines. For this function Hershey uses another open standard-Ethernet. Ethernet which got its start in office automation but which is increasingly being used on the factory floor allows transmission speeds between Ethernet-capable PLCs motion controllers and PCs at speeds of 10 megabits/sec. SERCOS also isn't considered a device-level network (also known as a fieldbus). This type of network permits sensors actuators and other "devices" to be placed on a single network cable that's tied to the machine's PLC-again all within a packaging machine. However with a module that plugs into a drive a SERCOS network can accommodate communications with such devices. In some of Hershey's machines sensor and other device input/output (I/O) is handled this way.
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