Motion network buses:
It’s a jungle out there

Now that we have looked at some of the characteristics we should be aware of in our machine application, let’s look at the networks that are actually available, and discuss some overall concepts related to network bus selection.

First of all it is important to distinguish between dedicated motion buses, such as SERCOS, which are often proprietary and generally expensive, from buses which can be used in motion applications but which are also used in other industries such as CANbus and Ethernet. In this article we will mention dedicated motion buses, but focus mainly on the general-purpose buses. This distinction is important, because if you are going to use a dedicated motion bus, unless you happen to work for a motion control vendor, you will most likely just purchase a complete system and interact with it as a user. Buses such as SERCOS are not easily extended to handle non-motion sensors or actuators, and thus make poor general-purpose networks.

While we are looking at networks for motion control, what about the protocol that we will execute on these networks? Well, if network buses are many and varied, motion protocols are few

Networks

and far between. What higher-level protocols do exist typically do not cater to motion. Most popular are DeviceNet and CanOpen, which are protocols that are hosted on the CANbus network. Both of these higher-level networks are well defined, so that today it is possible to buy ready-made sensors and components for Can- Open or DeviceNet. A fully accepted, real-time standard for motion control over these buses does not yet exist however, so many users who choose DeviceNet build custom motion extensions, or accept the performance reduction inherent in supporting these higher level network layers.