HART Digital Protocol

Field instrumentation in process plants is beginning to come
under more sophisticated metrological discipline. Most new
field instruments are now smart digital instruments. One
popular digital protocol is HART, which shares the
characteristics of both analogue and digital control systems.



Fluke's 744 calibrator is for
maintaining and troubleshooting
HART instrumentation

 

 

To properly service these instruments, precision analogue
source/ measure capability and digital communication are
required. Historically, this has required two separate tools,
a calibrator and a communicator. Today, these capabilities
are available in a single HART documenting process calibrator
that can help technicians quickly and effectively service a
HART instrument workload.


Process environment demands productivity
Intense competition in the process industries demands
maximum output at minimum cost. That productivity push
extends into the maintenance department. At the same
time, regulations and quality standards are demanding
more rigorous calibration plus extensive documentation.
Yet maintenance expenses are being slashed to the
minimum. Most plants have responded by replacing
analogue instrumentation with smart instruments. Smart
describes any microprocessor-based field device. The
microprocessor offers extra functionality, such as multiple
sensor types, multiple measurement variables (e.g. flow
rate and volume), digital compensation, and temperature
compensation. These smart instruments generally offer
better accuracy, long-term stability, and reliability
than conventional analogue instruments, says Fluke.

Manufacturers of field instruments have accelerated the
changeover by offering smart transmitters at prices
approaching those of analogue units. The majority of
installed instruments are now digital instruments. Most
of these use the HART protocol.

What is the HART protocol?
The HART protocol uses 1200 baud Frequency Shift
Keying (FSK) based on the Bell 202 standard to
superimpose digital information on the conventional
four to 2OmA analogue signal. Maintained by an
independent organisation, the HART Communication
Foundation, the HART protocol is an industry standard
developed to define the communications protocol
between intelligent field devices and a control system.

HART is the most widely used digital communication
protocol in the process industries, with over eight
million HART field instruments installed in over 100,000
plants worldwide, says Fluke. HART:

Is supported by all of the major vendors of process
field instruments

Preserves present control strategies by allowing
traditional four to 2OmA signals to co-exist with
digital communication on existing two-wire loops

Is compatible with traditional analogue devices

Provides important information for installation and
maintenance, such as Tag-IDs, measured values,
range and span data, product information and diagnostics

Can support cabling savings through use of multidrop
networks

Reduces operation costs, through improved
management and utilisation of smart instrument
networks

Calibration is still required
Some instrument shops have come to believe that the
accuracy and stability of HART instruments, plus the
presence of instrument diagnostics, have eliminated the
need for calibration. Others believe that calibration can
be accomplished by re-ranging field instruments using
only a HART communicator, either in the field or from
the control room. However, regular performance
verifications with a suitable reference standard, are
necessary because:

Regulations often require evidence of traceable
calibration for process instrumentation.

Quality programs, such as ISO 9000, dictate
periodic calibration to recognised standards for
all instruments that impact product quality.

Whenever weights, measures, and custody transfer
are involved, periodic calibration is required.

Even with very long-stated calibration intervals,
the performance of instruments will change over
time. This shift may be caused by long-term shifts
in the transmitter electronics, or by exposure of the
transmitter and the primary sensing element to
temperature, humidity, environmental pollutants and
vibration.

Regular performance checks will often uncover
problems not directly caused by the instrumentation.

How are HART instruments calibrated?
Calibration of an analogue transmitter is fairly straight-
forward. Following an As-Found test, the zero and span
adjustments may be used to set the correct relationship
between the input signal and the 4-2OmA output. An
As-Left test completes the calibration.

A HART instrument is more complex, having three
distinct stages. The sensor input stage sets the
relationship between an input sensor and the PV, or
primary variable. The PV is denominated in engineering
units, for example, psi or ºC. The sensor input stage
is adjusted by digitally trimming using sensor trim. The
second stage is a computational stage, establishing
the relationship between PV (Primary Variable) and
PVAO (Primary Variable Analogue Output).

Range is scaled by assigning the PV upper range limit
and lower range limit values. The PVAO is a digital
value of the 4-20 mA output signal. The final stage,
the instrument output, is set digitally with output trim.

Performing these trims and entering the URV and LRV
has typically been done using a HART configurator or
communicator. A separate calibrator was required to
provide the precision analogue source and measure
functions.


The calibration approach for a HART instrument will
depend on how the transmitter outputs are used. If
only the 4-2OmA analogue signal is used, it may be
treated much as an analogue transmitter. Using the
manual zero and span buttons on the transmitter, or
by digitally setting the PV LRV and PV URV, the correct
relationship between input sensor and 4-20mA analogue
output are set. However, in this scenario, the sensor
input stage has not been properly adjusted. If one
were to use a communicator to read the digital value
PV, it will most likely be incorrect, even though the
4-20mA output will be correct.

New tools speed callbration
It is desirable to complete required calibrations as
quickly and efficiently as possible. In the past,
maintenance of smart instrumentation was often
done in the shop. A benchtop calibrator was used
for analogue reference signals, and a HART
communicator or configurator provided the digital
communication.

Today, most instrument maintenance is moving to
the field. This reduces process interruption and
avoids the time and expense of removing the instrument
to the shop. Portable communicators and calibrators
are often used together to complete these field
calibrations. However, the desire to carry as little
equipment as possible and to speed calibrations has
created a need for a new class of calibration tool.


Today, the HART Documenting Process Calibrator
combines calibration capabilities and essential HART
communication functions in one compact, easy-to-use
tool, says Fluke. With such a tool, the user can address
a workload of both HART and conventional
instrumentation.

The Fluke 744 Documenting Process Calibrator is a
rugged, reliable instrument which is suitable for
calibrating, maintaining and troubleshooting HART
instrumentation including temperature and pressure
transmitters.

The Fluke 744 supports all HART universal and common
practice commands that will enable the user to work
with all HART devices. In addition to offering simultaneous
source and measure capabilities for all common process
parameters, the Fluke 744 includes a serial interface
for two-way communication with popular PC-based
instrumentation management applications. Handling
of fast-pulsed RTD transmitters and PLCs are also
possible with the 744.


Whether the operator is calibrating instruments, trouble-
shooting a problem or running planned maintenance, the
multifunction Fluke 744 helps complete the job faster.
Moreover, it calibrates temperature, pressure, voltage,
current, resistance and frequency in one compact,
rugged, and reliable tool, says the company.

Guidelines for selection
A technician's typical HART workload is roughly 70%
troubleshooting and maintenance, 20% commissioning,
and 10% calibration. In selecting a HART-capable
calibrator, a user should look for a tool that can not
only calibrate but can extend to most of the daily
troubleshooting and maintenance tasks.
____________________________________

For more information, contact Fluke on Tel: +44(0) 1923 216400



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