Applied Automation May 2008 • 7

CLOSING THE LOOP

HART monitors extract datafrom smart instruments

Simple modules expand transmitter usefulness

A ccording to the HART CommunicationsFoundation (www.hartcomm.org), morethan 20 million HART-enabled instrumentsare installed in chemical and processplants worldwide, and nearly every pro-cess transmitter made today has an inter-

face with a HART-enabled device.A HART-enabled transmitter, valve positioner, flowme-

ter or other “smart device” imposes a digital signal onits 4-20 mA process signal. The HART digital signal oftencontains additional process measurements and othervariables that may include instrument status, diagnosticdata, alarms, calibration values and commands.

Many HART field transmitters are hard at work mea-suring process parameters, and producing 4-20 mAsignals that are used for process control by a DCS, PLCor some other control system. In many cases, HART

instruments were installed simply because they couldbe configured and diagnosed easily with a handheldHART communicator device.

For a variety of reasons, the rest of the HART dataoften goes unused. One reason is because of the pro-hibitive cost of installing a plant-wide HART monitoringsystem. Another reason is the lack of familiarity withalternatives.

A simple and cost effective solution for gatheringHART information is to use a HART interface deviceonly in the specific instances where it is needed most.Fortunately, HART interface devices, available from sev-eral manufacturers, make acquiring HART data a fairlysimple proposition. This HART data is then made avail-able to the control system via analog signals, discreteoutputs or serial communications.

Extracting HART dataA HART interface module can be installed across any

By Greg Feliks, Moore Industr ies

Fig. 1. A HART interface module connected to the 4-20 mA process signal can extract up to four process variables. The 4-20 mAsignal containing the primary variable continues to the control system, while the additional process variables and diagnosticinformation can be sent to the control system as a MODBUS RTU signal or as three 4-20 mA signals and two digital signals.

254.37LB/HR

Control system(DCS or PLC)

Flow

HART communicatorcan be connected

anywhere along the4-20 mA loop

(HART secondary master)

Smart HARTmultivariablemass flow transmitter

HART interface module“breaks out” data fromSmart HART instruments(HART primary master)

4-20 mA proportional to second variable (DP)4-20 mA proportional to third variable (P)4-20 mA proportional to fourth variable (T)High alarm in response to fourth variable (T)HART transmitter fault alarm

HART digital signal carrying primary, secondary, third or fourthvariable process data and instrument diagnostics information

4-20 mA representing the primary variable (mass flow)

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8 • May 2008 Applied Automation

termination point on a 4-20 mA process loop. Theinstallation does not interfere with the 4-20 mA sig-nal to the control system, nor does it interfere with ahandheld HART communicator or a plant wide HARTmonitoring system if present. It can be used with anyHART enabled field device.

In the case of a mass flow transmitter, the 4-20mA signal carries the Primary Variable (flow) whilethe HART digital signal carries up to three additionalprocess variables: typically temperature, pressure, dif-ferential pressure, density or raw flow; plus discretedata such as alarms and status (Fig. 1). Some HARTinstruments can generate a dozen or more variables,all carried by the digital HART signal.

The HART interface module is configured by theuser to extract the variables of interest. The modulein Figure 1 is configured to extract three process vari-ables, a high alarm and a fault alarm.

Essentially, the HART interface monitor “strips out”the variables of interest from the HART digital signal;converts them to 4-20 mA and discrete signals; andsends the additional data to the process control sys-tem or other intelligent devices such as a PC or PLC.

Outputs from a HART interface module can bedirectly wired into the I/O rack of the control systemas analog and digital signals, or they can be sent asa MODBUS RTU signal. The interface module canalso be connected to a wireless device, for transmis-sion via RF.

Advanced HART applicationsThe wide variety of HART interface modules on the

market today allows engineers to assemble some inter-esting and useful HART applications. These creativeapplications include:

• HART alarms – HART-enabled instruments almostalways have built-in diagnostics; the diagnostic infor-mation is contained in the HART digital signal. A HARTalarm module can be installed across the 4-20 mAsignal to monitor diagnostic data and sound an alarmwhen process conditions warrant it. A typical HARTalarm module continually reads the HART diagnosticdata, examines the field device status byte and sendsa fault alarm to an annunciator or control system ifit detects alarms such as “field device malfunction,”“analog output saturated,” “variable out of limits,” etc.

• Valve monitoring – A HART alarm module can beconnected to a HART valve positioner to monitor valveperformance. Some HART alarm modules have multiplerelay outputs that can be tied to specific valve diag-

nostic and position data. For example, themonitor can provide relay outputs to indicateopen or closed valve position, low actuatorpressure, positioner temperature or any othervalue that indicates valve problems. A HARTmonitor is a simple and effective way to getposition feedback from a smart positionerwithout the expense of running additionalsignal wires to the positioner in the field.

• Emergency shutdown valve test-ing – Using the capabilities of HART andan appropriate HART monitor, partial stroketesting of emergency shutdown valves can beperformed without the need for a technicianstanding by or the need to install limit switch-es. Instead, use a HART monitor to extractstem position from the HART data and sendit to a PLC or DCS (Fig. 2). The controllerthen commands the valve to close, monitorsdata from the HART monitor to determinethat the valve is closing and then opens itagain, thus verifying that the valve works.

• Monitoring multiple instruments– HART instruments can be set up eitherindividually – where a single HART transmit-

Fig. 2. By using a HART interface monitor, a control system can conduct apartial stroke test on an emergency shutdown valve without the need forlimit switches or a technician.

4-20 mA valve control signalwith superimposedHART digital signal

Valvestem

Controlvalve

Valve full open(Relay #1)

Valve 90% open(Relay #2)

Valve output pressure(4-20 mA)

Valve travel(4-20 mA)

HARTinterfacemodule

facilitatesESD valve

testing

Actuator

Smart HARTpositioner

Controlsystem

HARTdatastring

18.40

Fig. 5. A HART concentratorsystem converts a HART digitalsignal to a serial communicationprotocol, permitting multiple smart HART-capable transmitters and valves to interfacedirectly with MODBUS-based monitoring andcontrol systems.

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ter connects via a twistedwire pair to a control sys-tem; or in a multi-dropHART network – whereup to 16 HART instru-ments communicate digi-tally over a single wirepair. For a multi-dropsystem, a HART “concen-trator” can extract HARTdigital data from any orall 16 of the field instru-ments, and transmit thedata to a control systemvia a MODBUS RTU con-nection (Fig. 3). Thisallows the control systemto treat the 16 instru-ments almost like a field-bus system. Specifically, itcan obtain primary pro-cess variables from theinstruments on one wirepair and status, diagnos-tic and additional processvariable data from theinstruments on a field-bus-like “segment.”

• Protecting processsignals – Sometimes 4-20 mA HART signals aresubject to problems fromvoltage surges, spikes,transients and groundingproblems. Although 90%of HART instruments areisolated, problems stilloccur. Also, 10% are notisolated; a simple groundloop can cause problems.A HART isolator breaksthe common galvanic path that can pass dangerousoverloads from a DCS or PLC to a transmitter, or viceversa.

• Sharing HART signals – In some cases, it is desir-able to share the output of a HART transmitter withtwo different systems. For example, a refinery maywant to share signals from transmitters with a DCS andan Emergency Shutdown System. A simple HART isola-tor (Fig. 4) makes this possible. An isolator also protectsone system from the other: both 4-20 mA process sig-nals are fully isolated.

A HART monitor really is a simple solution to expandthe usefulness of a HART-enabled transmitter, save onwiring costs and maximize the availability of data need-ed by the control system to operate efficiently withoutadding new transmitters.

Greg Feliks is a senior application engineer at MooreIndustries. He has been in the industrial and processindustries for more than 20 years, involved with fieldinstrumentation, recorders, controllers, control systemsand various other types of industrial controls. Feliksowned a manufacturer’s representative firm in theAtlanta area, and worked for Dynisco Instruments andEurotherm before joining Moore Industries. He has heldpositions in sales, marketing and product management.For the past three years, he has specialized in signalconditioning products including isolators, signal con-verters, alarm trips, remote I/O and related devices.

502.78 502.78502.78

Control system

MODBUS RTU data link

HART digitalsignal carryingtransmitter process,status anddiagnosticinformation

Up to 16 HART devicesin multi-drop mode

Fig. 3. Data from up to 16 HART instruments can be extracted by a single HART “Concentrator”module and transmitted to the control system via MODBUS RTU.

Fig. 4. A HART isolator makes it possible to share the same 4-20 mA process signal with two systems, such as a DCS and anEmergency Shutdown System.

Power

+PS

-PS

+IN

++

- -

+

-

+ -

-IN

Optoisolation

HIX

4-20 mA + HART

4-20 mA + HART

SmartHART

transmitter

Primary master(PLC or DCS)

Secondary master(Emergency

shutdown system)

As wireless HART gains greateracceptance, the use of hand-held devices continues togrow as well.

10 • May 2008 Applied Automation

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