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 1ê¯ 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.
