And not just any old communications scheme would work. “It had to be Ethernet-based because of the high data transfer rates and the ability to go to a number of destinations without special wiring,” says Dickerson. “It also had to be safety rated.” In other words, the job called for a very particular flavor of Ethernet. Consequently, CAMotion and its customer joined the growing ranks of manufacturers that are reading the menu of available flavors and choosing one that will work well as a fieldbus for controller-to-input/output (I/O) communications.

The robot added all of the spice to the palletizing application that CAMotion just finished. While it is loading payloads as heavy as 150 pounds in one of the three work zones along the front of the machines, the operators are busy working at the machines in the other two zones. There can be no mechanical barriers to impede the free flow of people and materials. Instead, light curtains around each zone and various sensors on the bridge keep the robot away from the people and other objects.

Two other factors complicated the mix. First was that the robot uses a vision system in its work. Second was that it travels overhead in a 100-by-25-foot workspace and the sole programmable logic controller (PLC) for the palletizing operation is on the ground. Not only would stringing 100 feet of cable be costly, but the user did not want any wires hanging from the robot. So the distributed I/O scheme had to include a wireless link between the robot and the PLC, one that would not let the vision system bog it down.

Dickerson selected the Profinet local area network (LAN) from Siemens Energy & Automation Inc., Alpharetta, Ga., because this Ethernet protocol can process both safety-rated and regular I/O in a wireless manner. “We are not separating safety and standard I/O data from each other,” says Ralph Buesgen, Siemens marketing manager for Profinet in discrete automation. The safety-related data is embedded with the regular I/O data in a standard Institute of Electrical and Electronic Engineers IEEE 802.3 Ethernet telegram.

“The Profinet safety solution works without redundant data and time stamps in the telegram, which simplifies the protocol structure and speeds the flow of data,” says Buesgen. “The maximum size of user data in a Profinet package is 1,440 bytes of user data; the maximum Ethernet-frame size is 1,520 bytes.” Consequently, the PLC can poll every I/O node, run a standard and safety program, and react in less than 50 milliseconds, even when transmitting vision data.

Both the PLC and bridge have managed switches at the wireless access point to handle the communications. The managed switch allows the Ethernet communications to be given a different priority on different packages of information. The safety data have the highest priority. The regular I/O has the second highest priority, and the data from sensors such as the vision system have the lowest.

Although the vision system can transmit images to the ground without overloading the system, the operator typically looks at the vision images during setups and debugging. The rest of the time, the data usually remains on the gantry. Likewise, most motion-control data also stay on the gantry because the drive and other necessary electronics travel with the robot along the bridge. “Real-time motion-control data are not going through the wireless portion of the Ethernet, except for some safety functions,” says Dickerson. “Motion control is all up in the moving bridge robot, where wired Ethernet updates information 1,000 times a second.”

Dickerson likes the taste of fieldbus for controller-to-I/O communications because integrating safety and other I/O saved at least 30 percent of the cost of building the system. So he plans to make Ethernet a bigger portion of the job the next time around. “We’ll probably go to a serial interface based on Ethernet on the ground too,” he says.

Injecting Simplicity

The desire for simplicity also was a driver at Ferromatik Milacron, of Malterdingen, Germany, about five years ago when the injection-molding machine builder began using Powerlink from Roswell, Ga.-based B&R Industrial Automation Corp. as the communications bus on its all-electric models. “Our goal was to reduce the number of variants that we have to support in the field,” says Thorsten Hoes, manager of research and development at Ferromatik Milacron.

That meant using only one network to link the controller with the transducers that report screw position, clamping pressure and other variables for motion and process control. The network also had to carry signals ...