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
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
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
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.
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