to hold its accuracy. As Accurpress’ most advanced press brake, the Accell machine uses closed-loop servo hydraulics and PC-based controls to generate parallelism and repeatability +/- 0.0004 inch.  

For more information, search keyword “Ethernet” at www.automationworld.com.

Ethernet a Determined communication technology

Ethernet has come a long way since its early days in the late 1970s. Back then, it used a shared medium, one in which all of the nodes use the same physical wires for communications. As one might imagine, packets of data from devices might bump into each other whenever they attempted to travel the data highway at the same time. Whenever that happened, the devices would wait for a random amount of time before trying to transmit their data again. If enough devices were competing, the volume generated by the continual re-sending of data could have overloaded

About 10 years ago, researchers solved the problem for most applications by using switches. Instead of connecting devices to a common medium, most vendors now connect them to packet switches, which are specialized, high-speed processors that regulate the flow of data and determine when they will travel the highway. “A device on a switch thinks it’s sharing a wire but finds it always available,” explains Harry Forbes, senior analyst at the ARC Advisory Group Inc., in Dedham, Mass. “So, there are no longer any collisions between devices competing for access to the medium.”

The only applications for which the speed of data transfer might still be an issue are those involving the precise synchronization of motion. When it comes to using Ethernet as a fieldbus for
controller-to-input/output (I/O) communications, today’s switching technology is more than adequate.

In other words, many industrial applications do not need some of the higher performance technologies that exist today. “A lot of variables in an oil refinery, for example, aren’t going to change extremely rapidly,” offers Forbes. A run-of-the-mill Ethernet technology might work fine there. “If you’re just monitoring tank levels, you can use any number of protocols, including IP (Internet protocol) communications, for your I/O.”

Invensys Process Systems, of Foxboro, Mass., has a different approach. “We address the major concern of Ethernet, the lack of determinism (specifying when data will arrive at its destination), through the use of a protocol that makes the controller the only master, the communications initiator, on the network,” says Alex Johnson, Invensys’ director of systems architecture.

A range of Ethernet technologies exists. Switching is the norm, but vendors have a variety of variations on that theme. Architectures such as Profinet IRT (Siemens) and EtherCAT (Beckhoff Automation), for example, have no switches in the conventional sense. Rather, “they have put the switching into chips that implement their interfaces,” says Forbes. “So they have essentially built two-point switches into their devices.”

Another architecture, called the Ethernet hub, seems to be one that can work in the most demanding motion-control applications. PowerLink, from B&R Automation, is an example of an Ethernet technology that uses this architecture instead of switches. “It’s really a machine control architecture for very precise synchronization,” says Forbes. “It uses a hub and an enhanced medium access protocol for allocating access to a shared medium.”