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1 September 2005

HART unappreciated still

By Donald Long

Perhaps life was much simpler 40 or 50 years ago; certainly power plant instrument control systems were.

The 3-15 psi air signal was the standard, and vast networks of pneumatic tubes snaked along catwalks, up and down columns, carrying signals ever so slowly from fundamental cam actuated controllers to flapper valves, which regulated flows and activated cylinders and dampers.

Despite the slowness and inaccuracy of the system, by nature the 3-15 psi signal was inherently intrinsically safe. No danger existed of ignition from sparking electrical wires in hazardous locations. However, the inefficiency of the pneumatic signal and the advent of solid-state electronics in the 1960s and 1970s ushered in the era of the present 4-20mA current loop.

Pneumatic tubing gave way to Twisted Shielded Pair. Response times went from fractions of whole seconds to milliseconds. Accuracies increased dramatically from roughly 0.75% to .015%.

Mean-time-between-failures grew from a few years at best to decades. As a result, the 4-20mA current loop has become the backbone of the power plant instrumentation signaling system since the late 1960s, early 1970s.

But the need to know more information about exactly what was taking place at primary control elements distributed around the ever larger, more complex power plant on boiler management systems, temperatures, pressures, ammonia injection systems, pH, NOx/SO2 analyzers, led to the development of the Highway Addressable Remote Transmitter (HART) "smart" digital system.

Rosemount, now Emerson, pioneered HART and the likes of Smar, and others refined it as an open system in the 1980s.

The HART smart waveform is a 1.4-volt maximum, Frequency Shift Key data system with a 1200 Hz and 2200 Hz frequency representing digital "1" and "0" respectively, impressed on the standard 4-20mA current loop.

An entire new world of informational possibilities was ushered in with the HART system. Two hundred and fifty six individual bits of information, with regard to status, health, calibration (zero, span), configuration (type of thermocouple or RTD), and device specific parameters, in addition to basic temperature, pressure, and flow information, could be up or downloaded between the field instrument and the controller.

Despite there being over 5 million smart transmitters, valves, and controllers sold into the instrumentation world including power plants, only a small fraction of those are working at anything but simple remote calibration.

This is due in small part to the ignorance of HART's capabilities, but also largely due to the relatively slow speed of data transmission—having scan rates on the order of one per second—HART consequently is not regarded as a control system.

It has become a maintenance and calibration tool, albeit a potentially very useful one.

Behind the byline

Donald Long (dlong@mtl-inst.com) is an ISA member and has an electrical engineering degree. He was an instrumentation engineer on the Lunar Landing Team, part of NASA's Apollo Program. He works in project management and applications engineering with MTL. This piece is part of his paper, Fieldbus systems for the power industry, which he presented at the 15th Annual Joint ISA POWID/EPRI Controls and Instrumentation Conference and 48th Annual ISA POWID Symposium in June 2005. Nicholas Sheble (nsheble@isa.org) edits the Networking & Communications department.