By John Rezabek, Contributing Editor
When the ISA SP100 committee categorized measurement and control applications, the two most critical classifications—closed-loop control and safety—were deemed unlikely candidates for early wireless applications. But within a couple years of widely available WirelessHART devices, papers and presentations abound on the subject of using wireless for control. Control using intermittent signals is not a terribly new topic, but applying these principles to familiar measurements like temperature and level is new. The enthusiasm for applying "control over wireless" appears to be widespread, and studies show wireless availability approaching that of wired devices. So why not use it for control?
Measurements are one thing. Maybe you can live with updates once a minute, or even once every 10 minutes or more in an effort to optimize battery life. But control valves pose a different problem. How many of the control valves you've specified in your career, didn't require a defined fail position?
Fail position means the actuator has a powerful spring that relentlessly drives the valve either fully open or fully closed on loss of motive power. The upshot is that wireless strategies that worked for measurements; i.e., going to sleep for 59 of 60 seconds, or for 599 of 600, doesn't work so well for control valves. Most loops won't tolerate the final element going to its fail position 99% of the time.
An always-on battery scheme would require frequent, perhaps daily battery changes, and/or an enormous and expensive battery. Be surprised if a wireless, battery-powered throttling control valve appears any time soon.
But this doesn't mean that there's no place for a locally powered wireless actuator/positioner. "WirelessHART-based products for valves in on-off services will be introduced as soon as the next six to 12 months," says Kurt Jensen of Emerson Process Management. These devices would need wired, local, 24-V DC power and would be "on" all the time.
A wired-power, wireless-communication device can be a good thing for a WirelessHART network, saving nearby battery-only devices from having to "wake up" only to relay transmissions from their neighbors, thereby extending their battery life. Being continuously powered also means the wireless actuator can have a "fail" position. One use case could be a tank filling or emptying, monitored over wireless, and tripping a "wireless" on-off valve open or closed to protect a pump, or prevent an overfill condition.
Doing control or interlocks in wireless gateways, or even in the positioners themselves, has some appeal for some of the same reasons control in the field (CIF) makes sense for fieldbus. Wireless gateways are by nature a "mux" (multiplexer), and control using multiplexed I/O has been a subject of debate for years. If CIF could be exploited, control could continue even if mux-to-host or device-to-mux communications were interrupted.
WirelessHART even has provisions for PID control. Ed Ladd of the HART Communications Foundation says, "PID capability has been built into HART devices for some time...[It] opens the door to many new possibilities including new opportunities [for] CIF." While suppliers such as Emerson, Samson and Smar have tried PID in HART devices, users will have to exert their influence with suppliers to get it implemented in WirelessHART.
Powered gateways are good candidates for PID and logic functions, and those being pondered for the Foundation fieldbus RIO (remote I/O) and WIO (wireless I/O) gathering points would implement the same fieldbus function blocks found in check-marked devices. Field trials of the wired RIO versions are scheduled to start within the next year.
For either Fieldbus WIO or WirelessHART control solutions to work, consistent and easy-to-use host tools need to exist that accommodate trouble-free configuration and operator interface to PID mode, setpoints and tuning.