Of the major fieldbus networks, ControlNet, DeviceNet, Foundation Fieldbus, and Profibus are all moving to support Ethernet connectivity through their protocols.
Using this network usually associated with office applications on the other side of the wall in the often harsh and demanding environment of manufacturing calls for special handling.
Factory strategies
Mike Rothwell, director of engineering, Advantech Automation, the Cincinnati-based control and data acquisition supplier (www.advantech.com), offers several suggestions for a successful experience. “Isolate control and input/output subnets from information and plant wide subnets,” states Rothwell. “Many new controllers provide dual Ethernet ports to accommodate this. Use switched networks technology. Switches are inexpensive, so use them. When selecting components, use those that support 100 megabits/second (Mbps) or faster. Cutting corners with less expensive 10 Mbps parts will seriously inhibit your current throughput and limit future growth.”
Ethernet has been used in office environments for many years and a plethora of products have been developed to expedite setting up networks there. Can these products be used in manufacturing automation applications?
Rockwell Automation’s Dave VanGompel, technology consultant for the Milwaukee supplier of automation products and services, says, “Equally important to the issue of commercial vs. industrial grade hardware is the issue of which switch features are important to industrial automation applications. There are three transmission control protocol/Internet protocol (TCP/IP) and Ethernet features in particular that are key to successful industrial applications: Full Duplex, IGMP Snooping and Port Mirroring.”
VanGompel describes the three features as follows:
Full Duplex provides independent transmit and receive circuits for the point-to-point connections between the devices and the switch, and between the switches themselves. For all practical purposes, this eliminates data collisions, which is critical in control (I/O) applications.
IGMP Snooping (for Internet Group Management Protocol) enables switches to properly manage multicast traffic, distributing each packet only to the port(s) that needs to receive it. Several of the industrial automation application layers (e.g. EtherNet/IP from ODVA and ControlNet International, HSE from Fieldbus Foundation) support producer/consumer protocols (a.k.a. publisher/subscribe) and take full advantage of universal datagram protocol/Internet protocol (UDP/IP) multicast and the associated Ethernet multicast.
Port Mirroring allows connection of a traffic analyzer to perform switch and system diagnostics without affecting the control system operation.
So, can commercial grade switches be used in industrial applications? VanGompel hedges his response just a little. “Commercial grade switches may be suitable for some industrial applications,” he states. “It all depends on the environmental conditions to which the switch (or router) will be subjected. If commercial grade equipment can be protected on the factory floor from temperatures that exceed their ratings (perhaps by air conditioned control cabinets) and isolated from vibration, they should perform adequately.
Larry Komarek, industrial automation business development manager for Phoenix Contact (www.phoenixcon.com), a Harrisburg, Pa., supplier of automation components, concurs that the use in manufacturing of IGMP snooping for multicast message control and port monitoring diagnostics is having a positive impact on uptime—a critical performance measure. Komarek notes, “We’re seeing the expansion of the managed switch in manufacturing. A second generation of industrial switches is being introduced. Some differences from the first generation are on the management side, that is, modularity and routing functions for performance. A key performance enhancement is giving real-time control traffic a higher priority than simple data flow.”
Weighing in on the commercial vs. industrial switch issue, Komarek says, “Industrialization covers not just environmental issues like electrical noise, dust and shock protection and the like. It is also taking the software functions and making them easier for plant floor people to use. This needs to be an ‘out-of-the-box’ solution without requiring a lot of adjustments.”
Gary Mintchell, [email protected]
