From the perspective of 2013, it seems unlikely that back in the 1980s the architects of the Highway Addressable Remote Transducer (HART) protocol would have envisioned that three decades later HART technology would still dominate the process instrumentation communication landscape, much less that the majority of process automation devices and systems sold today would still speak HART.
The original wired HART specification, which superimposed a frequency shift-keyed (FSK) communication scheme atop the industry standard 4-20mA electronic transmitter signal, was to all appearances a bridge to the fieldbus standard that was soon to debut. But a funny thing happened on the way to an all-digital field.
For users, HART technology offered a compelling, "out-of-the-box" value proposition for streamlining instrument configuration and commissioning tasks via a handheld communicator or PC while maintaining the familiar and reliable 4-20mA signal. Many users of the time didn't see much farther than this. And for instrument suppliers, it was relatively easy -- and economical -- to meet this relatively limited use case by bundling the HART protocol into an analog transmitter that already packed a board's worth of integrated circuits. As a result, many instrument-makers settled on HART as a de facto base level offering. Meanwhile, by the time industry eventually settled on Foundation and Profibus-PA as its primary pure digital approaches to field instrument networking, millions of HART-capable devices already were installed, bolstering familiarity and acceptance of the HART standard among users.
This virtuous cycle of low cost, low complexity, familiarity, backward compatibility and reliability -- combined with the responsive support of a broad supplier base united through the HART Communication Foundation that turns 20 (and counting) this year -- add up to an enormous installed base and vigorous ecosystem that to this day continue to find new ways to leverage HART technology for the business benefit of the process industries.
The Case for Simplicity
A big part of the HART standard's continued appeal is that end users really don't want to think about communication protocols, says Bob Karschnia, vice president, wireless, Emerson Process Management. "Users want to keep their plant up and running, maximize productivity, ensure safety and environmental protection. They're looking for technology that is extremely reliable, proven and easy to use."
"With HART, it's simple," adds Frank Fengler, head of device integration for ABB, noting that some 95% of the company's current transmitter shipments are HART-capable. Only 5% are full digital Foundation fieldbus or Profibus-PA, and most of those are associated with large grassroots projects, Fengler says. "With Profibus-PA or Foundation fieldbus you need a separate mechanism to get the process variable into the host. Systems people have to interact with device people. But with HART, no configuration file is needed for simple applications," Fengler says.
All other network technologies since HART have added complexities, Emerson's Karshnia agrees. For example, while Foundation fieldbus is a powerful tool, its potential to facilitate control-in-the-field blurred the functional lines between instrumentation and control departments within user organizations. "HART stayed true to its calling. The supplier members of the HART Communication Foundation effectively said, ‘We're going to take communications off the table in terms of competitiveness.' They've also continued to invest in and evolve the protocol, which is why they were first with wireless," Karschnia says.
Digital networks also bring with them the need for a new set of skills and training regimens. "But any maintenance tech that can do 4-20mA can deal with HART," says John Yingst of Honeywell Process Solutions, product manager for the company's Field Device Manager (FDM) plant asset management solution. "You can get diagnostics right away and have very low risk."
Further, the need for additional training presented by digital networks will persist, even with the next generation of a "digital native" workforce, notes Yuri Zelenkov, product manager, process networks, for Rockwell Automation. Millennials may relate more intuitively to handheld apps and collaboration platforms, Zelenkov contends, "but not to the physics of what happens on a bus. The concept of point-to-point wiring is easy to understand, and mistakes are seldom made."
"HART is a relatively inexpensive protocol, and it's more than adequate for most process measurements," adds Thorsten Szczepanski, CEO of ifak system, a developer of communication interfaces for a range of industrial protocols. "HART does what's needed for device communications, and with industrial Ethernet as a backbone, it's a powerful combination," Szczepanski says.
Continued backward compatibility with the same set of tools with which users have three decades of familiarity contributes mightily to the staying power and low risk of HART communication, according to Jim Shields, Fluke product marketing manager for process calibration tools. "Our industry is necessarily conservative. It's safety first -- for technicians and for the public," Shields notes. "HART speaks to all that. Meanwhile it's evolving without leaving its legacy behind. WirelessHART is new, for example, but it doesn't change any of the existing infrastructure."{pb}
2013 CONTROL READERS' SURVEY RESULTS
Usage of Field Network Protocols
Within the installed base of process instrumentation, "analog only" field instruments remain the most common and are in use at the facilities of 83% of respondents to a recent survey of Control readers. Further, among those with analog-only instruments in use, these devices account for a median 40-60% of the installed base. Among "smart" instrument protocols, HART Communication is used by nearly as many survey respondents (72%). Other digital protocols, such as Foundation and Profibus-PA, are used at fewer plants and typically to connect with smaller numbers of instruments. (See figures.)
83% use analog-only to connect with, on average, 40-60% of their field instruments{pb}
WirelessHART Opens New Horizons
The latest version of the HART standard is most notable for its extension to wireless field network communications. WirelessHART, which has achieved global standard status as IEC 62591, leverages the time-synchronized mesh protocol (TSMP) developed by Dust Networks, now part of instrumentation subsystem supplier Linear Technology, to communicate process variables as well as instrument diagnostics over a robust but low power communication network.
"In the early 2000s, our vision was that you should be able to put a sensor anywhere," says Joy Weiss, Dust Networks president. "We knew that in order for our vision to take off, customers had to trust that [our wireless network] was as good as the wires they had before." Coincidently, a number of suppliers in the process control industry began actively seeking a wireless solution that would provide multiple years of battery life together with high network reliability in industrial environments. The resulting standard effectively grafted the long established wired HART taxonomy onto Dust's wireless mesh network, adding a wireless communication option to the HART portfolio and making it relatively straightforward to integrate wireless monitoring points and wireless device management into existing control systems and wired HART device management methodologies.
As evidence of the uptake of WirelessHART, a just completed survey of Control readers indicates that 28% of readers already have wireless field networks up and running in their plants (see figure). Further, WirelessHART leads the pack among readers with active wireless networks or plans to deploy them: more than a third indicated they have settled on WirelessHART. Another measure of WirelessHART acceptance is the number of instrument manufacturers that are being told by their users that they need something that will fit into their wireless infrastructure, Weiss notes. "Sometimes it's their first HART implementation -- they're going straight to WirelessHART," she says.
WirelessHART mesh networks also can be used effectively in combination with other wireless technologies, notes Garrett Schmidt, product manager, wireless I/O and networks, for Phoenix Contact in the Americas. The company offers a longer range, powered Wi-Fi device to backhaul local sub-networks of WirelessHART devices. The number of devices on each WirelessHART sub-network depends on topology and desired measurement update rate, adds Dave Burrell, a wireless product specialist, also with Phoenix Contact. "The range is 25-100 devices per [backhaul] node, with 25-50 being typical," Burrell says.
A key benefit of WirelessHART for users is that it's low cost and easy to add new process measurements. When implementing a WirelessHART solution there's no need to disturb crumbling or capacity-limited wired infrastructure.
HART Leading Wireless Adoption
A just completed survey of Control subscribers shows that the implementation of wireless instrument networks continues to gather momentum. A full 27% say they currently are using wireless field instrument networks (first pie chart) and another 21% intend to test one in the future. The other half of respondents are equally split among the undecided and those who do not intend to implement wireless field instrument networks. WirelessHART also leads the pack (second pie chart) among respondents who have decided on a wireless instrument network protocol.
Which of the following statements best characterizes your current or planned usage of wireless instrument networks?
{pb}Additional User-Requested HART Protocol Features
Because Wi-Fi, in turn, is a wireless network that can accommodate standard TCP/IP communications, Phoenix Contact's backhaul approach will soon benefit from another new HART protocol enhancement: HART IP. Phoenix Contact's first WirelessHART backhaul offering relied on Modbus/TCP to bring monitoring points back to the host system. Soon, they'll be changing over to HART IP over Wi-Fi in new implementations, which will make it easier to communicate all the richness of HART devices' diagnostic data as well as up to eight process variables.
While communication of HART data over other Ethernet-based protocols certainly was doable in the past, with HART IP "all that mapping goes away," explains Emerson's Bob Karschnia. "HART IP also allows for a standardized flow of data into historians, control systems and other plant applications," he says. "Compared with using OPC, Modbus/TCIP, EtherNetIP -- the end user doesn't need to worry about it." All information in a HART device is available to the host system is available using standard HART commands. HART IP is expected to find a home as well in the direct communications of more complex, non-loop-powered instruments that deal with a lot of data, such gas analyzers and four-wire transmitters.
Part of the continued appeal of HART is the creative ways in which it can be applied to solve unique measurement problems. Here, a series of VEGA Americas HART-based radar level gauges operating in digital-only mode are multi-dropped to a Moore Industries HCS (HART Concentrator System) that translates the gauges' HART-speak into Modbus registers for communication to a supervisory RTU. Importantly for this solar-powered application, the gauges only consume 4mA of power in digital-only mode. Further, both top level and interface level measurements are of interest, and HART allows communication of both process variables, notes Dave Bigalke, vice president of TechStar, a distributor servicing users in the Texas oilpatch. "We need easy, we need safety, and we need low power," Bigalke says. "And we don't need data every second. Every couple of minutes is light speed [compared with clipboard rounds]."
The HART Protocol now also includes a number of other enhancements such as new, larger command strings, more process variables (up to eight now), longer tag-names, and headroom for additional supplier names and model numbers (the number of HART devices and manufacturers having exceeded the original authors' envisioned upper limit). Other new capabilities defined in the specification focus primarily on patterns in the communication signals themselves: for example, alarms by exception, rate-of-change alarms, time-stamped data, and triggered bursts.The current HART Protocol also defines for the first time test parameters and tools for ensuring the interoperability of devices and host systems. This is a non-trivial process for the vendor community, especially suppliers of host systems that must demonstrate the deft handling of even "worst-case" implementations by device manufacturers, but should ultimately pay dividends for the end user community. Device manufacturers, meanwhile have a somewhat easier time of it, as they are free to pick and choose what advanced features they will implement in order to differentiate their products.
The Future Is Bright
As the HART Protocol approaches its third decade of development and the HART Communication Foundation turns 20, it's clear that the movement's best years may well lie ahead. HART underpins the connectivity of a wide range of current and legacy field devices and host systems, and more are set to debut in the coming months. "We're looking at HART communication as being here long term; we're adding it to every new product we build," says Scott Saunders, chief operating office for Moore Industries-International, maker of a broad range of specialized signal conversion devices.
"There's just an enormous base of knowledge," adds Jeff Dobos, president of ProComSol, maker of PC-based HART communications and diagnostics software. "People know how to use it, and they want to use what they know; wiring is simpler, and wireless is an option," Dobos says. "Further, there are more and more suppliers using HART," he says. "Competition is strong, prices are down, and innovation continues to drive forward."
Gary Cusick, vice president of MACTek Corp., a manufacturer of interface devices for both wired and WirelessHART, is a bit less reserved about the technology's continued potential. "We believe HART technology is the best available field communication protocol for process automation applications – for new and existing plants," he says. "Users have spoken and have overwhelmingly selected HART communication as the world's leading field protocol."
