By Mike Bacidore, Managing Editor
In Mexico City, at the 1968 Olympic Games, American Bob Beamon set a track-and-field world record when he jumped 8.90 m. It wasn't until the distance was converted to English standard units that Beamon realized he had not only broken the previous world record, but had soared past it by almost 2 ft. Surely, this new record would stand forever as an unsurpassed feat of accomplishment. A distance of 29 ft, 2.5 in., would never, could never, be reached, let alone surpassed, by another man, unless he were being carried on wires.
Well, never is a long time. For 23 years, long jumpers inched, or centimetered, their way closer and closer to the mark, and finally in 1991 Mike Powell did the unthinkable and leaped 5 cm—2 in., for those of you in the U.S., Liberia and Myanmar (it will always be Burma to me)—past that unobtainable target. Wirelessly.
The examples of human triumph are too numerous to count. From the Wright brothers' first flight to Neil Armstrong's acknowledgment of his small step on the surface of the moon, the impossible has continued to become footnote after footnote in the annals of our accomplishments.
Similarly, wireless field control now has officially become one of those reachable stars. Two recent installations have demonstrated the effectiveness of control over WirelessHART networks. At bioprocess technologies supplier Broadley James (www.broadleyjames.com) in Irvine, Calif., WirelessHART pH and temperature transmitters control a single-use disposable bioreactor. "We conducted batch runs using mammalian cell culture," says Scott Broadley, president. "The observed pH and temperature control using wireless measurements was equivalent to that achieved using wired transmitters."
The technology is available to address most control applications, with scan rates as fast as 1 sec. "Yesterday, you couldn't do PID wirelessly," says Randy Balentine, DeltaV product marketing manager at Emerson Process Management. "Today, that's no longer the case."
What makes this possible, Balentine explains, is a new control algorithm. "We knew WirelessHART devices and the way they do non-periodic updates, and we understood how we could accommodate that in the PID algorithm and function block," he says. "We've held a PID steady in some of its calculations and not made ill-advised moves in the algorithm without an update. The proof and the technology are here."
The PID is designed to work with WirelessHART devices that support period and exception reporting. However, the impact of faster updates on battery life must be considered.
A workshop at the 2009 Emerson Global Users Exchange compared performance between wired and wireless transmitters and demonstrated how to address lost communications. Similar results were seen at another installation, the J.J. Pickle Research Campus at the University of Texas where stripper and absorber control is done using WirelessHART transmitters. Column pressure control and heater stream flow control using WirelessHART transmitters provided the same dynamic response and comparable performance to that achieved using wired transmitters.
Both installations use the enhanced PID algorithm available with the DeltaV S-series. This PID algorithm for wireless devices delivers control in an exception reporting environment.
Yet, with all the success of wireless control in these installations, most engineers will argue that wireless is not yet ready for industrial control of critical applications.
"Many different devices use the same communication frequencies or create interference," says Eric Hollister, product sales engineer, Pilz Automation Safety (www.pilz.com). "For monitoring data, this interference does not pose a huge problem when 1 sec of data is lost due to a disruption in the signal. For control, the signal must always go through and must always be correct to ensure that the machine functions properly. Until the wireless control technology reaches a point where electrical interference, network load and cross-talk no longer influence the control signals, it will not be ready."
With wireless, you can verify that 95% of the data is received, but you can't go beyond that and guarantee that all 100% of the packets transmitted from the field device are received by the gateway, says Robert Schosker, product manager at Pepperl+Fuchs (www.pepperl-fuchs.com). "These lost information packets, or any delay in receiving the data packets due to rerouting or interference from moving structures such as rail cars, construction work or vehicles, is a limiting factor, so control can be done, but at this point only to a limited degree," he explains.
Wireless control of critical applications is a reachable star. It will take time and plenty of small steps, but ultimately networks will leap into the possibilities it opens.