Wireless final control elements, which by their nature require some form of power to actuate, may not be considered a natural fit for wireless communications. However, just as wireless adoption is growing, so too are options to incorporate end actuation devices into the control mix.
As discussed in my column featured in the November 2016 issue, "Refining determinism," wireless sensor networks (WSN) can be used for closed-loop control with appropriate compensation in the control algorithms to compensate for the inherent lag times associated with signal collection (AI) and output publication (AO) in addition to the inherent response time characteristics of the field devices themselves.
A less arduous application for WSN and final control elements are for on/off valves where a discrete (digital) output (DO) either opens or closes the valve. WSNs have the advantage over plain old relay outputs (DO) in that being intelligent, they can also report back if the device actually opened or closed, without the need for additional hardware such as limit switches, associated cable and discrete input (DI) interface. They accomplish this with only tag assignment and some configuration of the wireless actuator and host system.
Though they use their own proprietary networks, some manufacturers have been selling systems similar to this since at least 2009 and continue to offer them on the market for niche applications. However, if you already have an installed WSN such as WirelessHART or ISA100.11a, it means the infrastructure is in place to connect your devices to an access point able to confirm the status of on/off valves that are only controlled by local switches.
Installing an actuator indicator with WSN support on the valve provides local indication of open or closed, and can also connect to the abovementioned infrastructure to convey status. An alternative, if necessary to confirm the physical position, would be to link a WirelessHART or ISA100.11a field adapter with one or two limit switches and associated contact(s) to provide positive identification of the actual valve stem position.
Being able to confirm status of isolation valves would be beneficial to verify procedures during normal operations and plant outages, especially for those valves that should always be open or closed, then change during the shutdown or startup. Of course, these valves normally have locks to confirm their status, but this could be a backup system for relatively low cost.
This can be done using the WSN access point as an open-protocol-based signal multiplexer. The same concept can and has been used to combine signals from isolated pieces of equipment, then transmit them farther than the conventional mesh network distance to additional repeater points. For example, collected signals can go from barges in tailings ponds or settling basins to the nearest onshore pump station connected to the control system as either a remote node or extension of the network via more conventional means such as copper or fiber.
Building systems such as this requires using design tools to confirm the network will be able to effectively update the number of signals at the required rate and, equally important, that the signal will be strong enough to be transmitted between the points. The necessary inputs to the program for signal calculations are obviously the distance between each point as well as the terrain. Ideally, you will have, as in the example above, either a nice, flat or gradually sloping beach, while across water should inherently mean the terrain is flat. Despite the Fresnel effect, radio, especially at the power levels involved with WSN, remains a line-of-sight application. Program outputs will include the number and location of the repeater points as well as information on antenna requirements to increase signal gain.
Wireless sensor networks are approaching the final frontier with more applications pushing the envelope beyond being used as a replacement for wires. This will lead to increased adoption. Though not yet the “killer app,” they're enabling unique applications that can't be done with conventional systems at close to the same cost/benefit ratio.
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