Measuring level, pressure, temperature without wires

01 October 2006

Wireless sensors are here, but the software, the infrastructure, and needed security are all lagging behind.

The consumer public, almost overnight, has embraced WiFi standards. The question now is, will industry do the same? Manufacturing processes are less fault-tolerant than the average consumer, yet according to many industry observers, who understand this reservation very well, the industrial implementation of wireless sensors is poised for large growth, much like WiFi was not that long ago in the consumer industry.

Few are probably more enthusiastic about wireless’ prospects than Honeywell’s Tom Phinney. ‘In ten years
from now 70 to 90% of new plants should be wireless,’ he predicts.

Certainly, it has already created some success stories. ‘Wireless, for the last decade, has been an easy sell in noncritical applications,’ comments Graham Moss, of the Australia-based wireless modem company Elpro Technologies. He got into the business after seeing more antennae sprouting from control room
rooftops in his continent’s mining industry. With so much wireless coming on board, he also saw the need to take the next step: a network approach to communications infrastructure.

For many in the process industries, the HART wireless specification is a catalyst to drive more expansive adoption of wireless. ‘Big vendors such as Emerson and Honeywell—by embedding Û20 wireless HART chips in all their products—will make HART the industry’s defacto standard,’ says Mr. Moss. With an eye to networks, he sees wireless mesh networking as a system driver in providing semi-self-configuration
capabilities, similar to a PC’s ‘plug and play’ facility.

What will likely drive that growth is that many recognise that the cost of installing a wired system is a lot more
than wireless. The installation cost for wired networks alone can be as much as three times the sensor and cable cost. Ease of installation with wireless can be remarkably simple, if the planning is done properly.

Wireless promoters like to cite examples where radio is the unique solution to a particular problem, such as a safety loop at Shell Oil in Texas. A public motorway interrupted the plant’s physical layout, so the only alternative was to use wireless.

At the moment, it is the big companies—Honeywell, Invensys, and Emerson— who are talking about the advantages of radio systems and discussing their ‘roadmaps.’ In reality, it is the small, nearly unheard-of companies who are actually providing solutions.

Take Sensicast, for example. It has wireless temperature sensors installed on the 1.8 mW sea water circulation pumps at San Onofre, a nuclear power station on the south California coast. The plant is worried about the effects of salt corrosion and so it has installed the sensors to monitor the temperature from each pump’s stator coil. By hopping frequencies around a mesh network, the sensors deliver the information tens of metres away to a central station. Based on their success, Sensicast has installed other wireless sensors to monitor gas emissions and serial information from equipment in remote sections of the plant.

‘RFID’ capabilities
Combining the well-known but still developing RFID (radio frequency identification) technology with sensors gives some unique capabilities in terms of mobility. Each sensor could have its own RFID transmitter, and travel where it needs to be, reporting the ambient conditions at its location or the condition of the part it’s attached to.

Wireless RFID sensors, such as offered by MicroStrain, are already doing this. They monitor the stress/strain or temperature of ‘dummy parts’ in the manufacturing process. This provides real-time in-process data, rather than data measured along the periphery of the parts production process. RFID is rather short-range, so reading stations have to be nearby.

Steve Walker of Adalet Wireless sees an increasing trend with wireless sensor hardware to interface with pre-existing thermocouples and process pressure sensors.

The radio is becoming more tightly integrated to the sensor. In fact wireless sensors may become something of a commodity with postage-stamp size sensors that can be fixed to a tank to transmit temperature levels. Again, this would likely be a short range radio solution.

Mr. Walker thinks hazardous locations are a good application for wireless sensors, but not necessarily
intrinsically safe ones. After many years of making explosionproof electrical boxes for the underground coal mining industry, he says the big advantage of these boxes is that they allow much higher RF output compared to low-power intrinsically safe radio at one watt output. ‘It’s about 50-times more powerful than intrinsically safe units—and the transmission range is proportional to the square root of power density—so you’re looking at seven times the range.’

But where’s the software?
Like many developing technologies over the past several decades, wireless sensors fall into a familiar developer’s trap: the hardware is here, but the software isn’t. Tendril Networks’ CEO Tim Enwall, in view of the technology and products now being offered to wireless adopters, says companies have two software hurdles to overcome in their wireless sensor projects:

....Months may be required to program even basic wireless-sensor networks using the software currently available; and

....There is equal difficulty in adding control capabilities to such sensor networks.

Indeed, Mr. Enwall says most companies complain about the software and are looking for ways to get around the problem. Sensor hardware is robust, but system software to manage these networks—a wireless network operating system—has been lacking.

Because of these software limitations, companies have employed specialised programming teams with hard-to-find skills to do these applications. In many cases, the programmers waste a lot of time and expertise just getting useful packets back and forth between devices; never mind developing useful higherlevel enterprise functions.

Mr. Enwall says new software, such as his company’s Service Broker, promises to cut deployments from months to days and allow end users to add complex control capabilities as a simple step in the process.

Microwave ovens, a hindrance?
The physical magnitude of a plant may be formidable, limiting the effectiveness of wireless sensors. ExxonMobil’s Baytown (Texas) refinery, for example, covers 100 square kilometres, and that doesn’t include the adjacent chemical plant. Honeywell’s Tom Phinney thinks he already has his arms around the solution: ‘A natural approach,’ he says, would be to adopt of a cellular structure overlaying a plant, with plant
compartmentalisation along the boundaries of plant upgrades.’

Radio propagation can be highly variable due to physics. Movement of cranes or other large pieces of
equipment can create reflections, fades, and such. Moreover, the favoured 2.4 GHz frequency, which is usable in most countries, is the same as microwave ovens. That means any leaky microwave oven becomes a 1-kW interference when up against the nanowatt- and picowattlevel signals that sensors are trying to send or receive. Outside the walls of the plant, there have been instances of interference from nearby local cafés and their WiFi hotspots.

Mr. Phinney says the government needs to get involved. ‘Clearly, in the longer term, the government must
designate a spectrum for critical infrastructure that has no other legal interferers.’

Others, such as the ISA’s SP100 wireless standard committee, are taking a strategic approach to coexistence—recognising that multigenerational instrument radios and instrument mesh networks will coexist simultaneously in different regions of a plant. If a small part of a plant is upgraded, existing technology—from inventory—will be employed. But if say 20% of a plant is upgraded, the then-latest technology will be deployed.

In 10 years Mr. Phinney believes that deliberate jamming (interference) is likely to be fully avoidable due to
technology such as low-power softwaredefined radios. Because a perpetrator can’t know when such a radio is on, high-power jamming won’t be feasible without continuous jamming, which indirectly identifies the source. At that point, 90% penetration of industrial markets by wireless will be achievable, he contends.

Despite challenges, industrial applications use wireless sensors to provide performance unparalleled by
hardwired systems. The technology exists and the solutions are being successfully deployed. The question is, are you ready for wireless?

--adapted from an article by Richard Phelps, Control Engineering


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