Networks and Control

Network technology has changed dramatically. Speeds that would have seemed unimaginable ten years ago are off-the-shelf these days. And with the speed a lot of bottlenecks have disappeared. With the possible exception of motion control applications.

Networks for motion are still a in category by themselves. Sercos has gone through major evolution to its current level, Sercos III, to continue to hold its position as the top performing motion network. Other implementations of Ethernet, EtherCat, Powerlink and others bring the high bandwidth available in Ethernet and add features to the network to insure its performance for motion control applications.

But the network technology momentum continues. There is a specification from IEEE, 1588 Protocol, which adds hard real time to Ethernet by clocking to make sure that messages get where they are supposed to be WHEN they are supposed to be there. This feature creates a level of determinism that has long been a stumbling block to more broad acceptance of Ethernet in the industrial community, possibly eliminating any serious impediment to using Ethernet for motion.

The controversy is usually around the question of “What is Real Time?” How fast is fast? Well, its usually whatever is fast enough for your specific application. But that doesn’t really help control system manufacturers when developing solutions for a broad audience. So its nice to find that the technology migration is starting to resolve some of the basic issues with respect to motion control, with something more broad than a vendor specific solution.

Even the Open DeviceNet Vendors Association seems to be exploring the potential of IEEE 1588 as their Common Industrial Protocol as a platform for bringing the legacy networks of manufacturers together as an overall solution. This is a very significant effort, one that has been difficult to achieve, that many users need help with. Operating a manufacturing or process plant is hard enough without having 3 or 4 different networks to maintain, and worse still, exchange information across different platforms.

My guess is the cost pressure of inexpensive Ethernet components will continue to push manufacturers toward finding similar solutions. But is sure would bring everyone along more quickly if the competition among control system providers were balanced with an option that everyone can find acceptable. IEEE 1588 is certainly a possibility worth considering.

Motion and Communication

I used advise people working on motion control never to make a communications layer part of the application. There were too many things that could adversely impact the motion. But communications have gotten a lot faster, and there is a tendency to think of speed as the issue. Not really.

Motion Networks are a breed apart, Sercos, Firewire, USB and many proprietary solutions are offered in today’s marketplace. Claims are made about, not surprisingly, speed. And determinism. That’s the tricky part. Guarantee the message got there when it was supposed to.

And its kind of the same problem for PLC’s doing motion control. Yes, its true that most applications do not require tight coordination between two or more axes. So its perfectly reasonable to put the motion in the PLC and let it referee when independent motion axes start. It leads to the assumption that everything is fine no matter how many axes and if any are coordinated, synchronized or registered to a moving target. Read more