Time to Move

Supporters are pushing a variety of Ethernet-based fieldbuses for motion control, but users just want to build machines. Here’s how to find and implement the most appropriate digital network for your specific application.

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By Jim Montague, executive editor

Juggling and motion control are all about keeping appointments. Manufacturing and machine building are all about keeping promises. There’s more than one way to skin a cat. Time is money. Most machine builders know these principles by heart.

As the controls engineering supervisor at CBW Automation Inc. in Ft. Collins, Colo., Steve Corwine and his colleagues continually must devise ever-faster, simpler, more-efficient, and less costly ways for CBW’s robots to pull plastic parts out of injection molding and thermoforming machines. Do more with less. Great job—now, what have you done for me lately? We all know this drill.

To make its robots simpler to build and operate, CBW recently spent 18 months redesigning and rebuilding its TS-303 side-entry robot to include PC-based controls of its servo drives via a SERCOS digital network (Figure 1). This replaced the robot’s formerly hardwired communications between its I/O points, PLC, and servo drives. The robot now uses Beckhoff Automation’s TwinCat software to control Bosch Rexroth’s Indramat servo drives. Its PC-based scan time is 2 msec, whereas its former PLC scan time had been 27 msec.

 

Pulling Plastic
Figure 1: TS-303 side-entry robot’s ability to grab and retrieve parts is based on CBW Automation’s patented end-of-arm-tooling (EOAT) methodologies.
Photo courtesy of CBW Automation

TS-303’s ability to grab and retrieve parts is based on CBW’s patented end-of-arm-tooling (EOAT) methodologies. After building its robots, CBW integrates them into the work cells of its clients’ molding and forming machines, such as those by Husky or Stackteck, and adds capabilities as needed so they can stack parts, transfer parts to packaging machines, or put parts in boxes and tape them.

“The biggest reason for incorporating a fieldbus was its high-speed communications, and the labor saved by doing away with all the hardwiring that used to go to all the I/O points,” says Corwine. “SERCOS also lets us use fiberoptics, so we can put more components nearer to their drives and other remote locations. This means they can be in smaller boxes closer to where they need to be, rather than in big centralized boxes with lots of wiring.”

Zuri Evans, Siemens E&A’s Simotion product manager, adds that motion buses’ ability to reduce wiring and locate components further afield also enables efforts by some builders to develop more modular machines. “Each module now can have its own intelligence, but they all need a common fieldbus for communications that is fast, accurate, and safe,” says Evans. “Mechatronic solutions are replacing mechanical components with electric motors, but higher axis counts and greater synchronization requires more capable fieldbuses that are flexible, have faster cycle times, and less jitter. Basically, users need controllers that can talk motion with other controllers to handle more complex solutions.”

Recent Motion Bus History

Though digital networks communicate more efficiently and with less hardware than their point-to-point predecessors, they historically begin to break down and drop the ball when faced with the higher-speed data requirements of industrial motion control applications.

“In the late ’80s and early ’90s, we had centralized motion controllers and analog interfaces, but these made it difficult to set tuning parameters for drives, and it was difficult to get errors out of the drives,” says Markus Sandhoefner, B&R Automation’s Ethernet Powerlink specialist. “The digital interfaces that came next were better, but their 30-50 msec update times were too slow for motion control. Developers got faster with CANbus and Profibus, and then began looking at Firewire and Ethernet’s 100-µsec updating to achieve the determinism required for motion control.”

Consequently, several heroic efforts were made in recent years to bring the benefits of low-power, two-wire fieldbuses to motion control, and several different methods sprung up to serve users. Just like commandos in a war movie, this usually meant synchronizing watches. The two main camps involve time slicing or time stamping of the signals that deliver messages and software data packets, and most protocols operate via some flavor of Ethernet. The main players include SERCOS, Ethernet Powerlink, Profinet IRT, EtherCat, CIP Motion via EtherNet/IP, SynqNet, and even generic Ethernet TCP/IP itself.

Unfortunately, debates still rage over which is most useful, which should be the industry standard, and which is likely to dominate future motion-related fieldbus markets. Perhaps because there still are relatively few motion bus applications, most supporters of one protocol seem to spend much of their time criticizing the others. Bickering aside, most users just want to build machines faster, integrate motion systems with more axes, drives and nodes, and help their own end users manufacture products faster and better. Some suppliers help them. Others appear to help users, but serve mainly themselves by binding users to unnecessarily proprietary networks that can potentially restrict their flexibility and long-term growth.

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