HART HAS BEEN
a mainstay of process automation for more than a decade, but don’t mistakenly equate its maturity for stagnation. The HART Communications Foundation (HCF) staff and its members have been working for the past several years on enhancements that will increase the HART protocol’s value to users. The fruit of those efforts will become evident during the coming months with the release of the new Enhanced Device Description Language (EDDL) and a wireless version of HART.
EDDL represents an “upgrade” to the DDL that has been an important component of HART Communications since 1992. Instrument vendors use DDL, a text-based language, to write device descriptions (DDs), which are binary files that identify their products to a host system and provide the system with their operating parameters. (DDs are analogous to driver files provided to PC users by the makers of printers, scanners and other peripherals. The driver allows the PC to recognize the device, configure it and control its operation.) Organizations responsible for other digital field communications protocols, including Fieldbus Foundation and Profibus Nutzerorganisation (PNO), have adopted the use of DDL as well. In 2004, the International Electrotechnical Commission designated DDL as an international standard.
As “smart” field instrument technology progresses, suppliers embed more capabilities in them, including the ability to transmit information on additional variables, perform increasingly complex math functions, provide more sophisticated diagnostics, etc. As a result, configuring asset management and control systems to use this abundance of information has become a more complex task. Complicating things further is the fact that vendors must develop and maintain separate DDLs for every different host system.
“Some vendors already have added EDDL capabilities to their products and major systems vendors will incorporate it into control systems starting early next year.”
To ease the burden on users and vendors alike, HCF, the Fieldbus Foundation (FF) and PNO have been working together for the past three years to develop an enhanced version of DDL that gives vendors the ability to add new capabilities to DDs while improving cross-platform compatibility and facilitating device set-up. More recently, the OPC Foundation joined the development effort.
Ed Ladd, HCF’s director of Technology Programs, says some vendors already have added EDDL capabilities to their products and adds that major systems vendors will incorporate it into control systems starting early next year. To make their systems EDDL-capable, systems vendors must add an EDDL Host Service Module, which will be available from HCF, FF and PNO.
Among the most noteworthy improvements enabled by EDDL is instrument vendors’ ability to use DDs to dictate the look and feel of graphical interfaces to instrument information and set up, regardless of host platform.
“Using the current DDs, device attributes show up in a hierarchical tree,” says Ladd. “But with the new interface options and graphical capabilities, device suppliers create actual windows that enable users to see all of the pertinent information about intelligent instruments. And, with the exception of things like colors, fonts, etc., that data will appear the same way on System X, System Y or System Z.”
Because vendors will not have to develop or maintain DDs for each host system that communicates with their field devices, development and maintenance of device description files should be far less complex, he adds.
EDDL also will enable the incorporation of graphical elements, such as valve signatures, x-y graphs and bar charts, directly into the displays, eliminating the need for separate applications to display the data, says Ladd.
The enhanced DDL also allows persistent storage of data related to field devices. For example, users will be able to store multiple valve signature files generated by DDs and compare them to their valves’ current states, enabling users to more easily and quickly evaluate each device’s performance and whether maintenance is necessary.
EDDL also allows vendors to include in DDs instructions to host systems to execute advanced math functions that provide users with important information about the status and operation of their instruments.
Wireless HART represents the other imminent improvement to the HART protocol. Today, HART enables communication of a device primary value over standard twisted-pair wiring, using a 4-20 mA signal, while secondary values are digitally “piggybacked” on the analog signal via Frequency Shift Keying or Phase Shift Keying. With wireless HART, the primary value may still be carried via wiring, but users will be able to use a wireless signal either to supplement or replace the wired transmission of digital data.
|“Wireless HART will improve their ability to establish full-time digital connectivity to field devices. It will lower the cost to add points to a HART network, reduce wiring and make sensing more ubiquitous throughout installations.”|
HCF’s Wireless HART Working Group, comprising representatives of numerous instrument and systems providers, began development of the wireless HART specification late last year and set March 2006 as the goal for completion of the specification, says Kelly Orth, distinguished technologist with Emerson Process Management’s Rosemount Division and leader of the working group.
“HART is a very lightweight protocol, so it is relatively simple to ‘tunnel’ HART messages inside wireless data packets,” says Orth. “At this point, we’ve generated a list of almost 50 requirements of how we want a wireless HART system to behave.”
Orth estimates that only 5 to 10 percent of users who have installed nearly 20 million HART-enabled devices use anything more than their 4-20 mA analog capabilities to communicate with higher level systems. As a result, most users are missing the opportunity to capture data that can vastly improve their operations and maintenance capabilities.
“Wireless HART will improve their ability to establish full-time digital connectivity to field devices. It will lower the cost to add points to a HART network, reduce wiring and make sensing more ubiquitous throughout installations,” he says.
The Wireless HART Working Group is considering a “mesh network” architecture for the protocol to deliver reliable communications. Unlike point-to-point networks, in which all nodes communicate directly with a line-of-sight “base” station, mesh networks allow nodes to communicate with one another, establishing ad hoc, redundant paths to the base.
“This improves reliability,” says Orth. “If a particular path is blocked, there are other ways for the message to get through. Mesh networks also scale well. You just add more repeaters or routers without having to add much power. Also, the bigger a mesh network is, the more reliable it is, because more redundant pathways are automatically created.”
Point-to-point networks are really just a subset of mesh networks, so the wireless HART solution will cover these applications as well.
As part of the development of wireless HART, the working group is coordinating its activities with other industry wireless organizations, including the ISA SP100 Wireless Committee, to ensure continuity and uniformity with wireless standardization efforts currently under way.