Is PoE technology ready for prime time?

Power over Ethernet (PoE) technology is expected to some day power all industrial sensors and controllers, but is it ready for use now? Senior Tech Editor Rich Merritt explains in this installment of Bandwith.

By Rich Merritt, Senior Technical Editor

WHY DOES YOUR TELEPHONE continue to work during a power failure? Because the phone company delivers electrical power over the phone wires. The same basic concept, using Power over Ethernet (PoE) technology, will some day power industrial sensors and controllers.

As a user, it will let you install devices in remote areas that have no AC power, such as storage tanks and hazardous areas. For OEMs and machine builders, it can eliminate all the wiring you are now installing to power devices. This dramatically reduces the installed cost of electrically-powered devices, because all you have to do is run an Ethernet cable to the device—no power cord needed. That eliminates conduit, barriers, labor costs for electricians, and all the other expenses of supplying AC power to a remote location.

Alas, PoE isn’t ready for prime time. It’s a very recent standard, only a few products are available, and none of them are suitable for industrial use. The few applications we’ve seen are in the IT world. Nevertheless, PoE promises to simplify your life in the near future, so it pays to keep an eye on it.

PoE is based on a simple concept. If you closely examine a standard CAT5 Ethernet cable, you’ll see four wire pairs. At present, two pairs are used for data in 10BaseT and 100BaseT communications, and two pairs are not used at all. In 1999, IEEE determined that those two pairs could be used to supply power to devices, so the IEEE802.3af standard was developed. It was formerly accepted as a standard in June 2003.

Under normal usage, an Ethernet network carries no power. To use PoE, you install an “injector” that inserts a DC voltage onto the cable. Devices that are to be powered use a “picker” or “splitter” to extract the voltage.

Industrial equipment to take advantage of PoE does not exist yet, but we can tell you how it could work in the future on a machine or a process unit that is attached to a company LAN. The LAN cable plugs into an injector located near the machine or unit. The injector adds 12, 24 or 48 VDC power to the Ethernet network that goes to the machine. At the machine, a picker or splitter extracts the power and routes it to the appropriate device or devices, along with the Ethernet data signal. See Figure.

A UPS at the injector access point maintains PoE power in the event of a power failure, so devices connected to the Ethernet can keep running.

Two problems prevent you from adopting this technology immediately:

  1. Power available is very limited.
  2. Industrial devices compatible with PoE are virtually nonexistent.

The power available is currently limited to 13 W at the receiving device. Any more than that interferes with the Ethernet data signals. “In the near term, it seems unlikely that PoE will make dramatic changes in how industrial devices are deployed until more power can be provided to the device,” says Ferrell Mercer, design engineer at GE Fanuc Automation. “With increases in power, this could be an applicable technology with a micro controller or a slice I/O type device.”

Karsten Loehken, product manager at Lumberg, says it will require a change in product design. “The main issue with PoE is the limitation of power that can be transmitted without interrupting the communications,” he explains. “PoE most likely will not change the way Ethernet is used today. It could change the way new devices will be designed in the future. It will make it possible to connect devices with only one cable, similar to the way networks such as DeviceNet do it today.”

To break into the industrial world with a “one cable powers all” feature, PoE will have to overcome the lead of two other networks, says Helge Hornis, intelligent systems manager at Pepperl+Fuchs. “Two currently available industrial network solutions offer the same feature: DeviceNet and AS-Interface,” he says. “Based on customer preferences in the AS-I world, powering inputs such as sensors and pushbuttons is a good idea. But most people like to have outputs on aux-power, so they can cut power for E-stop purposes without stopping network communication to the field device.”

Powering outputs requires a power source at the machine or process unit, so the need for PoE is minimized.

The lack of compatible devices also limits usage of PoE. Mike Fahrion, marketing manager at B&B Electronics, has been using PoE in some of its new products. “We're doing a pilot of an Ethernet conversion device that uses PoE from a Cisco switch to power our device,” he says. “We swipe power from the PoE to power the end device--a card scanner for a point of sale application. In this case, we're eliminating all power supplies in the system by using PoE. Very clean install, and no ugly transformers or extra cables and connections.”

Regrettably, commercial application is much easier than industrial. “In the commercial market, you start with a 110 VAC power source, so creating 48 VDC isn't a huge hurdle," says Fahrion. “But starting with the typical 24 VDC available in most industrial cabinets to create 48 VDC comes with some pain, both in the footprint of the device and the cost of goods. First, there are few or no 24 to 48-V DC to DC converters available off the shelf. Second, it takes a physically large inductor or capacitor reactive component to pull this off. Third, it probably will be 80% efficient at best. That's a significant packaging problem, not to mention a tremendous cost-adder.”


We found one PoE product suitable for industrial applications. “Lumberg sells cordsets to manufacturers of devices that use PoE technology,” says Loehken. “However, most of those applications are in a non-industrial environment.”

Philong recently announced 20 and 30-W injectors. Both are powered by 90-264 VAC. The 20-W injector complies with IEEE802.3af, but the 30-W version does not. It uses both the data and spare pair of wires to deliver 30 W. Prices start at $25.

“The source for PoE has to be isolated,” adds Keith Hopwood, president of Philong. “So you need to add an isolated DC/DC from your 24 V. We have shipped 24-V systems, but the total power is even less since you are current-limited by CAT5 cables. Long term, it is likely PoE can get to 50 W.”

Hornis says P+F is not working on PoE devices. “Experience tells us that we will find ways to make use of the technology for industrial applications,” says Mercer of GE Fanuc. Fahrion says we just have to wait. “As Ethernet continues to migrate down into devices that are not located in a cabinet or near a power source, I think we’ll start to see the ROI for PoE,” he predicts.

Meanwhile, you can keep up to date on PoE developments at www.poweroverethernet.com, which hosts white papers explaining the technology and displays new PoE products.

SPARE WIRES PUT TO WORK





















Power over Ethernet uses the two spare wire pairs in a CAT5 cable to deliver power. An “injector” adds power to the cable, and a “picker” extracts it and feeds power to the device. Source:
www.PowerOverEthernet.com

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