IIoT Product Design at the Extreme Edge

IIoT field devices often bring to mind relatively “cozy” manufacturing environments and IT cabinets, but the technology may have seen its strongest deployment to date in remote and/or harsh industrial environments. After all, MQTT’s inventors, Andy Stanford-Clark and Arlen Nipper, initially developed the IIoT protocol’s low-bandwidth predecessor to monitor Phillips 66 (then Phillips Petroleum) pipelines in the late 1990s.

Whatever the software, it’s the hardware that has to bear up to the elements for years or even decades without complaint. Repeatedly sending technicians out to troubleshoot IIoT equipment defeats the whole point. But shielding sensors and other delicate electronic components from the elements, all the while maximizing data transmission reliability and battery life, is a complex design challenge. According to Grid Connect CEO Adam Justice, the design process begins with weighing production cost against long-term survivability.

“There’s what’s the worst thing that can happen, and what’s the most likely thing to happen, and how much do we want to take that into account in our bill of materials?” says Justice, who’s engineering design firm specializes in the design of custom IIoT solutions, often for harsh industrial environments. “There could be a flood, for example, but if that’s not likely to happen very often, then you don't want the cost of making everything completely waterproof. You’re going to want to design for what's normal and likely for that environment, not for what’s the outside-the-norm.”

Ingress

Given that, he says keeping dust, dirt and moisture out is a given in IIoT field device design, with the understanding that production cost scales with each increase ingress protection (IP). Per the IEC 60529 standard, an IP rating is based on two numbers, the first corresponding to protection against foreign objects (on a scale from 0 to 6) and the second number against water/moisture (0-9).

While the commonly seen designation, IP65, suffices for consumer electronics, the lowest workable rating for ruggedized industrial equipment starts at IP67 (temporary immersion in water). However, for spray-down applications or in environments prone to water immersion or condensation due to temperature fluctuations, an airtight enclosure with an IP69K rating may be required.

Vibration and Temperature

Another strategy, he says, is potting vulnerable electronics in compounds like epoxy or silicon. In addition to protection from moisture and corrosive chemicals, potting also provides mechanical resilience—critical for mobile heavy machinery or military applications, for example, that would expose a field device to continuous vibration and physical stress. Determining which potting material to use comes down to the environmental conditions the device will likely encounter.

Epoxy cures to form a hard shell protective against impact and vibration but can crack or become brittle at extreme temperatures (below −40°C or above 150°C). In contrast, silicon potting materials excel at the extremes, holding up to wide temperature swings from −60°C to 200°C. While not as rigid or thermally conductive as epoxy, silicon does provide flexibility that allows for greater thermal expansion. Silicon can also be removed after it cures, allowing the encased electronics to be replaced, whereas removing hardened epoxy would incur component damage.

Power

Ingress, vibration and temperature swings can also play havoc with the device’s power source. Given that a tank level sensor or an ultrasonic flow meter on gas pipeline may be required to record and transmit data unattended for years, choosing the right battery type and chemistry is crucial.

Many IIoT field devices typically draw minimal current for short bursts (or pulses) and spend the majority of their operating life in hibernation mode. As a result, passive energy losses from the battery need to be minimized. Since alkaline cells suffer from high self-discharge rates, lithium chemistries are favored for their power density, longevity and resistance to extreme temperatures.

Among the most common lithium chemistries, lithium thionyl chloride (LiSOCl2) cells, especially in a bobbin-style battery construction, exhibit the highest energy density at the lowest discharge rate, typically on the order of roughly 700 Wh/Kg with a 1% discharge rate per year. Being non-aqueous, this type of  battery cell can also operate at temperatures down to −80°C and up to 125°C. As a result, some low power devices can operate continually for decades, even at temperatures that would degrade the performance of other battery types.

Signal Integrity

Of course, ruggedness and long battery life don’t add up to much if an IIoT field device fails to transmit its data back to base. Since running Ethernet cables, or even Wi-Fi signals, to remote areas is impractical if not impossible, IIoT devices designed for hard-to-reach applications often rely on low-power, wide-area network (LPWAN) protocols. Common choices include Sigfox in the unlicensed band and low-power, 3GPP defined protocols including NB-IoT and LTE CAT M.

Of these, narrowband NB-IoT is a popular solution, says Quinn Jones, senior product manager for IoT connectivity provider, Digi International, especially for low power applications that only need to send small data packets (250 kbits/s max rate) over a long distances. However, while less expensive to implement than LTE M, NB-IoT is only suitable for static installations. In situations that require higher uplink/downlink speeds, mobility or real-time communication, LTE M is the preferred solution.

Of course, the above assumes that the remote field device in question would be located somewhere with cellular network coverage. For the most remote of deployments, Jones says a mesh network, in which each node in the network connects and transmits to its neighbors, may be the better choice.

“Here, you have the options of short range mesh and long range mesh,” he says. “We have the Digi XBee DigiMesh protocol which runs on sub-gigahertz and we support Wi-SUN, which is a standards-based sub-gigahertz mesh networking protocol. So if you think of agriculture, oil and gas fields, a sub-gigahertz mesh like Wi-SUN or DigiMesh would give you the ability to cover 100s of acres.

“At the same time,” Jones continues, “it still gives you the redundancy benefits of a mesh networking protocol, like self-discovery, self-healing…so that if one node goes down, your network doesn’t go down.”

Given the wide range of conditions an IIoT device might encounter over its lifecycle, both Jones and Justice agree that validating the design of this type of ruggedized IIoT field device requires extensive field testing. The confluence of variable hazardous conditions is too complex and unpredictable, they say, to rely solely on lab testing.

“We see a lot of engineers that come up with a solution in the lab, and based on their research, they believe it should last 10 years,” he says. “But when they throw it out in the field, they quickly discover they didn't take into account the real world—the RF noise, the harsh environment and all the challenges that come with it. So you do your lab testing, but you also definitely need field testing to make sure that you've thought through all the parameters needed to have that rugged solution optimized for years of life.”