Wireless Temperature Measurement
Developments in wireless technology and efforts to promote industry standards are bringing the long-awaited promise of wireless temperature measurement and control closer to reality.
“Radio has no future.”
– William Thomson, Lord Kelvin, 1897
The name “Kelvin” is synonymous with temperature measurement. In 1897, he was talking about the wireless telegraph, a bold invention that captured the popular imagination following experiments by Marconi and others. The wireless telegraph would have been described as “the greatest thing since sliced bread,” except for the fact that the automated bread slicer wasn’t invented for another 20 years.
On a ship crossing the Atlantic five years after making that pessimistic prediction, Kelvin did indeed see the potential for the wireless telegraph and became an enthusiastic advocate.
“Wireless” is everywhere today. Everywhere except industrial process measurement and control. Wireless quickly progressed from Morse code to voice and music, from radio to television, from terrestrial antennas to satellites.
Twenty-five years ago, we had the first car phones with transceivers the size of a small suitcase mounted in the trunk, and most conversations began with, “Guess where I’m calling from?” Today we’re watching cell-phone videos of Buddhist monks protesting against the government in Burma. And the gargantuan corporate competitors Verizon, AT&T and Sprint are pouring enormous R&D resources into inventing new methods to trick our 12 year olds into running up huge bills by downloading ring tones on cell phones that these companies gave our kids for free.
But wireless technology is still not a factor in temperature measurement and industrial process control.
The Future is Always Just Around the CornerJim Pinto is a respected analyst and prolific writer on the technology and business of industrial process control. He was the founder and CEO of Action Instruments, now part of Invensys.
Mr. Pinto predicts that “smart, wireless networked sensors will soon be all around us, collectively processing vast amounts of previously unrecorded data to help run factories, optimize farming, monitor the weather and even watch for earthquakes. Many people (including me) think that wireless sensor networks can become as important as the Internet.”
It’s likely Mr. Pinto’s expectations will eventually come to pass. He made this particular prediction, however, in September 2003. The industry has made very little progress in the years since.
The ApplicationThe subject of “wireless temperature measurement” is too broad to cover in a brief article. Therefore, this review will focus on one typical application and briefly survey some of the new developments in wireless technology that could be a solution.
The application will use thermocouples to measure temperatures of 600-1000°C (1100-1830°F) at multiple points in a process and transmit the data to a control room 50 feet away (Fig. 1). The temperatures need to be recorded once per second. In this example, it would be prohibitively expensive to run wires – either thermocouple lead wires or a 4-20mA current loop – from the equipment to the control room. For the purpose of this example, it’s not possible to supply external power to the transmitters.
The choice for this example is now limited to a battery-powered, wireless thermocouple transmitter. Finally, although the sensors are subjected to 1000°C, we have to assume that the transmitters can be located where the ambient temperature does not exceed 70°C (160°F).
Wireless EthernetWireless Ethernet networks, which didn’t exist 10 years ago, are now very popular in offices, homes, hotels, airports and coffee shops. The main application is to connect computers, PCs, laptops and PDAs to the local area network and to the Internet.
The most important reason for the success of wireless Ethernet is that it is a “de jure standard.” Like “wired” Ethernet, wireless Ethernet is a standard hammered out by members of the IEEE (Institute of Electrical and Electronics Engineers), which deserves a big share of the credit for its success. Wired Ethernet (IEEE 802.3x) has been continually evolving and improving. Every iteration has been faster, more reliable and – very importantly for its continued acceptance – backwards compatible. The same progress is evident with wireless Ethernet (IEEE 802.11x).
Because Ethernet and wireless Ethernet are de jure standards, you can easily connect a variety of devices from a wide number of manufacturers, and it’s all going to work together – most of the time (Fig. 2).
Metcalfe's AnalysisBob Metcalfe was a young engineer at the Xerox Palo Alto Research Center in 1973 when he described “Ethernet” in a company memo. He is credited today as the principal inventor. The following quote is from Dr. Metcalfe when he spoke at the Sensors Expo and Conference last year in Chicago.
“ARCnet was the first commercially available LAN for PCs. It was introduced in 1977 by a leading PC company called Datapoint, and it was a beautiful thing. When we were forming IEEE 802 in 1979 to standardize Ethernet, I was sent to ask Datapoint Vice President Victor Poor whether he would submit ARCnet specifications to the IEEE so that it might be considered for standardization. Two weeks later, Mr. Poor called me back to say that Datapoint had decided not to standardize ARCnet.
“ARCnet lost to Ethernet in PC networking because of the Ethernet business model. Consider that Ethernet is not just a standard, but also a networking standard. As a standard, Ethernet grows cheaper over time because of overwhelming investments, increasing volumes and fierce competition. Buyers like that. But as a networking standard, Ethernet grows valuable over time because of, well, Metcalfe’s Law – V~N2. Standards have advantages, but networking standards have those advantages squared.”
The success of Ethernet is all the evidence one needs to be convinced that de jure standards, particularly IEEE standards, are likely to prevail.
Is Wireless Ethernet (802.11x) a Solution for This Application?Ethernet is already a great success in industrial applications, winning out over a number of proprietary networking schemes that are arguably preferable for a particular narrow application. And wireless Ethernet is a great method for transmitting data between a central-computer control system and portable devices (Fig. 3). Wireless Ethernet is often referred to as “WiFi.” The Wi-Fi Alliance, however, is actually an association of companies that certifies 802.11 products for interoperability.
Unfortunately, the power requirements of wireless Ethernet make it an impractical method for transmitting data from a sensor. Wireless Ethernet transmitters would require either continuous line power or batteries that can be frequently recharged or replaced.
Alternatively, a battery-powered wireless Ethernet device could be programmed to wake up briefly every few minutes, transmit the data and go back to sleep. Under that scenario, the batteries could last for a few years. But the application in our example requires taking a temperature reading once every second – not once every few minutes.
Bluetooth is a standard for wireless “personal area networks,” and it is becoming increasingly popular as a wireless alternative to USB.
The Bluetooth Special Interest Group is a trade association led by the cell-phone manufacturers Ericson, Nokia and Motorola as well as IBM and Intel. Bluetooth operates in the same license-free “ISM” band – at 2.45GHz – as wireless Ethernet. Bluetooth has three classes for levels of power and range from 1mW with a range of 1 meter to 100mW with a range of 100 meters.
Although Bluetooth has many promising applications, a Bluetooth thermocouple transmitter is not one of them. Bluetooth uses less power than wireless Ethernet, but for this application it still requires too much. Bluetooth is intended to support a much greater data throughput than is needed for a temperature sensor. The application described here would need the Class-1 version of Bluetooth, which requires 100mW. The batteries in a Bluetooth thermocouple transmitter would need to be recharged or replaced too frequently to be practical.
IEEE 802.15.4Fortunately, a new IEEE standard has emerged to serve our temperature-measurement application – IEEE 802.15.4. As do wireless Ethernet devices, these new 802.15.4 devices operate at 2.4GHz (Fig. 4). Presumably, the 2.4GHz frequency will prevail because it is allowed worldwide.
The standard range for 802.15.4 is also the same as for wireless Ethernet and Bluetooth, approximately 100 meters outdoors without any obstructions or interference. In practical terms, the actual range is much less – an extremely important point to consider. There are high-power variations of 802.15.4 that can significantly increase range or penetrate some obstructions. Naturally, the high-power versions use considerably more battery power as well.
A temperature sensor transmits a miniscule amount of data compared to a computer or game console streaming video. A well-designed IEEE 802.15.4 temperature transmitter could run for years on two AA batteries. But unlike Ethernet and Bluetooth, the IEEE 802.15.4 standard does not cover everything that needs to be defined so that devices can communicate with each other right out of the box.
IEEE 802.15.4 defines the “PHY” and “MAC”, the “physical layer” and the “media access control” but not the higher layers that would bring about universal compatibility. In other words, IEEE 802.15.4 devices could be talking at the same frequency but not talking the same language. There are a few teams of professionals working to develop that common language.
In September 2007, the HART Communication Foundation announced a wireless communication standard for process measurement and control. The standard the HART Foundation proposes is a communication layer on top of the physical IEEE 802.15.4.
The original HART (Highway Addressable Remote Transducer) Communications Protocol was a method to communicate digital information over legacy 4-20mA analog wiring. Now an open standard, HART was developed by Rosemount Inc., which is now Emerson Process Management. Emerson is a leader in the introduction of Wireless HART devices. As of this writing, however, there are no Wireless HART devices in the field except for those testing at a few beta sites.
Along with Emerson, the other foundation board members are ABB, Endress+Hauser, Siemens and Honeywell. Honeywell tried, without success, to persuade its colleagues to merge the HART wireless protocol with the ISA100 protocol.
ISA100 is another proposed standard based on the IEEE 802.15.4. The ISA (Instrumentation, Systems and Automation Society) is an outstanding nonprofit organization founded in 1945 with a membership of 30,000 professionals working in instrumentation and automation.
Backed by several instrumentation/automation manufacturers, ISA100 is intended to be a universal network with the seemingly impossible task of compatibility with the most popular industrial-networking protocols, including HART, Profibus, Modbus and Foundation Fieldbus. As of this writing, the ISA100 standard is still on the drawing board.
ZigBee is another specification for communication protocols based on the IEEE 802.15.4 standard using small, low-power transmitters.
The ZigBee Alliance describes itself as “an association of companies working together to enable reliable, cost-effective, low-power, wirelessly networked monitoring and control products based on an open global standard.”
The alliance has an ambitious objective: “For the first time, companies will have a standards-based wireless platform optimized for the unique needs of remote monitoring and control applications, including simplicity, reliability, low-cost and low-power.”
The first public standard, “ZigBee 1.0,” was published in December 2004, and “ZigBee 2006” was published in December 2006. The list of ZigBee promoters reads like a “Who’s Who” of the semiconductor industry – Philips, Freescale, Texas Instruments, Siemens, Samsung and Mitsubishi among others. As the chairman of Ember Semiconductor, Dr. Metcalfe is an outspoken evangelist for the future of ZigBee.
Despite the great intentions and a roster of heavyweight promoters, at this time less than a half dozen ZigBee-certified devices have come to market. ZigBee will likely find its initial success in building automation and energy management, where a few products have been introduced. The ZigBee standard for industrial automation is far from complete, however, and there are no ZigBee devices – such as thermocouple transmitters – on the market.
ZigBee, Wireless HART and ISA100 aren’t there yet. Bluetooth and WiFi are not practical for this application. There are, however, some solutions available today.
In June of 2007, Omega Engineering introduced a solution that would work in this application – the UWTC (Universal Wireless Thermocouple Connector). The UWTC series is a wireless-thermocouple transmitter using the IEEE 802.15.4 standard (Fig. 5). It does not use the added layers of ZigBee, Wireless HART or ISA100. The lithium-battery-powered devices transmit to a wireless receiver that connects to a PC through the USB port (Fig. 6). The USB receiver allows 12 different connectors to operate at the same time.
Alternatively, the devices can transmit to a coordinator that connects directly to the local Ethernet network. The coordinator serves active web pages that graph the temperatures and can be viewed without any special software other than a web browser (Fig. 7). Free software makes it easy to log the data to an Excel spreadsheet and send alarms by e-mail and to cell phones.
Michael Macchiarelli, the Omega design-department manager said, “One of the big benefits of a wireless-thermocouple system is a substantial savings on installation labor and overall material cost. This product provides an easy installation solution for many previously difficult applications.”
Although the Omega UWTC may be one of the first of its kind, there will likely be many similar products introduced in the next few years. The IEEE 802.15.4 standard is an ideal fit for wireless-sensor transmitters like that required in this example. Whether it comes from the ISA, ZigBee or the HART foundation, sooner or later the added layer that will enable interoperability with a wide assortment of devices will soon be available as well.
Kelvin was obviously mistaken in saying “Radio has no future.” However, if Kelvin had said in 1897, “It will take a hundred years before radio has a future in temperature measurement,” he would not have been far off the mark. IH
For more information: Steve Hollander, Newport Electronics, Inc., 2229 South Yale Street, Santa Ana, CA 92704; tel: 800-NEWPORT or 714-540-4914; e-mail: info@NewportUS.com; web: www.NewportUS.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: wireless technology, thermocouple, wireless thermocouple