A Beacon for Wireless Signal Integrity

Until Now, the Measurements Brought in Wirelessly Have Not Been Safety-Related or Mission-Critical, at Least With Respect to Time. If a Signal Went Offline, the Plant Manager Wasn't Going to Appear in Anyone's Office.

By John Rezabek

Our site maintenance contractor requested a beacon in the shop as a backup to walkie-talkies to help alert craftsmen to possible plant emergencies. The shop is a few hundred feet from the control house, and there's no existing signal wiring to the building—not even the business network LAN has cable or fiber going there. This seemed like an application tailor-made for wireless.

We've been using WirelessHART for over a year now, and I've been impressed when somewhat whimsical installations by curious operators, nested at some distance in jungles of steel with no obvious line-of-sight path, still manage to join the network and communicate reliably. We think we've repeatedly demonstrated that the practical capability of low-power radios far exceeds the conservative specifications. There are a few WirelessHART devices in the mesh that are a reasonable distance from the shop, and at least one is line-of-sight. So thanks to being priced below the per-transaction limit on our company MasterCard, a Rosemount model 702 wireless discrete transmitter arrived last week.

Until now, the measurements brought in wirelessly have not been safety-related or mission-critical, at least with respect to time. If a signal went offline, the plant manager wasn't going to appear in my office. While views of the criticality of our new beacon vary, it would certainly be ignored if it became subject to spurious activation, or if it came on a minute or two after being commanded from the control house. Is the robustness and signal integrity of a WirelessHART network sufficient for this challenge?

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Concerns about signal integrity of radio networks in process plant jungles of steel were anticipated by the designers of both WirelessHART and its non-interoperable cousin, ISA 100.11a. Both employ a mesh topology, in which devices don't need a direct path to the root access point or gateway. They can identify and use an alternate route if their primary pathway becomes unavailable. But it's a little different from your Moxa or Cisco wireless mesh network for Ethernet and its kin—it's quite possible the gateway is the only line-powered device in the system. The majority of devices, including routers, run on batteries, expensive ones designed for use in hazardous atmospheres. We want these batteries to last a few years if possible, so we tend to specify the longest update rate we can tolerate. A device in the mesh might only become active and transmit every 30 seconds or longer.

WirelessHART and ISA 100.11a go beyond using mesh technology to ensure signal integrity. Both use time division multiple access (TDMA), which enforces specific time slots for device communication. For operating in the increasingly crowded 2.4-GHz band, both also employ channel-hopping, so messages can avoid transients and interferences on individual frequencies. When I asked Emerson Process Management about my 702, I heard from Ted Schnaare, director of engineering-wireless. He said, "Multiple transmit slots are available within the update period of each generated data packet, and retransmissions of unacknowledged packets occur on different RF channels and via different paths."

When we come back to a wireless device a few weeks after commissioning, we usually see its "nearest neighbors" have changed. That's because both WiHART and ISA 100.11a employ self-optimizing, self-healing mesh networks that constantly probe for more efficient and reliable paths back to the gateway.

What about bad guys trying to hack the wireless network? It seems like we hear a new caveat every day about the vulnerability of wireless transmissions emanating from our cell phones and laptops. There's a lot of security built into WiHART and ISA 100.11a at the network and the message level, using 128-bit AES encryption and a message integrity code (MIC) added to every message. Joining the network is closely managed, making it highly unlikely a rogue device might provide a foothold for an attacker.

The 702 manual suggests I could see a contact closure communicated in 15 seconds or less. But it also cautions about "other latencies." Some of those latencies can arise when you're mapping the gateway points using Modbus or OPC. If your gateway is on an RS-485 network with a dozen other devices with sizable gobs of data to poll, you might be compelled to find a "dedicated" port for your critical wireless "control."

Will our beacon application light the way for future wireless control? Aside from the vagaries of Modbus and OPC mappings, the industrial wireless infrastructure is looking pretty solid.