Jim Montague is the executive editor for Control. Email him at [email protected].
Is wireless still wireless if it needs a wire for power? Well, if you're talking about wireless data communications, then establishing a physical power connection via a readily available local source or running a cable to a switch, transceiver or radio still can be considered wireless — though it still seems a little sneaky and impure, doesn't it?
Nevertheless, networking suppliers seek to satisfy users who need to gather information and variables from more different and widely distributed spots where they can't always get the local power that wireless components usually rely on. Fortunately, these demands often are met because many wireless and other nodes need less power than ever before, many batteries deliver more power and longer life, and energy scavenging via solar, vibration, temperature or other methods is beginning to emerge.
"The main inspiration for wireless is to replace wires and Ethernet cable for data communications, and so we have two Ethernet gateways: 758-916 establishes a wireless local area network (WLAN) and 758-915 uses point-to-point Bluetooth," says Charlie Norz, product manage for I/O systems at Wago. "They can communicate with each other or talk to other wireless access points, PLCs, machines or a control room, but power usually comes from a local source, such as the 24-V Ethernet cable at either end. We're not talking about going miles, but just a few hundred or 1,000 feet, and so power is typically right there."
Conversely, because many sensors, I/O points and wireless switches need so much less juice than in the past, more applications are securing local power via the Power over Ethernet (PoE) standard, which delivers relatively low power over existing data cables. However, when communication over longer distances is required and local power isn't so easily accessible, batteries and solar solutions become more practical and popular, and many of these are using the WirelessHART communication protocol.
"The easiest way to power wireless access points and other devices is via backhaul links to straight power connections or PoE, but if they aren't available, then batteries with wide operating ranges are available from 12 to 48 V and down to just 0.16 A. They're often used in tandem with solar components to store power," says Nick Sandoval, field applications engineer for Moxa. "Energy harvesting from machine or other equipment vibrations is probably upcoming, but we haven't seen it produce enough to power wireless access points. It might come from ZigBee because of its ultra-low power consumption, but then you're dealing with its short range of less than 15 meters."
Bob Gardner, wireless product manager for Banner Engineering Corp., confirms that today's batteries last longer and wireless components need less power, but they're still constrained by physics. "You can have two AA batteries power a device for 10 years, which is about 50 microamps at 3.6 V, but you can transmit data only about once every 1 to 5 minutes. If you need to transmit once per second, then your battery will only last for a year. Transmitting and sending data only periodically can help a lot, and so we developed our Switch Power function, which works with our photoelectric sensors, turns them on when they sense a presence or event, broadcasts their data, and then powers down.
"We've also looked at energy-harvesting methods, but so far none of them can match the 3.6 V we can get for 10 years from an AA lithium cell battery for about $3. To get 3.6 V from solar, you'd need a $20 solar panel. Batteries are still about one-third the cost of energy harvesting. However, while solar might be overkill for very low-power applications, if you need 20 mA for a wireless transmitter that will tell you how full your tank is all the time, then a $300 solar system becomes worthwhile."
Justin Bessette, electrical engineer at Lord Microstrain Sensing Systems, reports that energy harvesting isn't a problem, but the key to making power-scavenging sensors and other components succeed is intelligently managing the power they produce. "The wireless device market has heated up over the past five years, and half of this puzzle was solved by low-power microprocessors," Bessette explains. "Now, solar panels are getting smaller, and vibration-harvesting sensors can be smaller because we're moving from the milliwatt realm into the microwatt realm. Sure, batteries can handle these tasks, but shouldn't we try to use less materials, reduce waste and the cost of pollution, cut replacement costs, and use the sun when we can?"