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H1 FOUNDATION Fieldbus Device Configuration and Commissioning

Fieldbus devices require far more attention in their initial setup and commissioning than their analog counterparts. Unlike an analog transmitter, for example, where the only “configuration” settings are its zero and span calibration adjustments, a FF transmitter has a substantial number of parameters describing its behavior. Some of these parameters must be set by the end-user, while others are configured automatically by the host system during the start-up process, which we generally refer to as commissioning.


Configuration files

In order for a FF device to work together with a host system (which may be manufactured by a different company), the device must have its capabilities explicitly described so the host system “knows what to do with it.” This is analogous to the need for driver files when interfacing a personal computer with a new peripheral device such as a printer, scanner, or modem.

A standardized language exists for digital instrumentation called the Device Description Language, or DDL. All FF instrument manufacturers are required to document their devices’ capabilities in this standard-format language, which is then compiled by a computer into a set of files known as the Device Description (DD) files for that instrument. DDL itself is a text-based language, much like C or Java, written by a human programmer. The DD files are generated from the DDL source file by a computer, output in a form intended for another computer’s read-only access. For FF instruments, the DD files end in the filename extensions .sym and .ffo, and may be obtained freely from the manufacturer or from the Fieldbus Foundation1 The .ffo DD file is in a binary format readable only by a computer with the appropriate “DD services” software active. The .sym DD file is ASCII-encoded, making it viewable by a human by using a text editor program (although you should not attempt to edit the contents of a .sym file).

Other device-specific files maintained by the host system of a FF segment are the Capability and Value files, both referred to as Common Format Files, or .cff files. These are text-readable (ASCII encoded) digital files describing device capability and specific configuration values for the device, respectively. The Capability file for a FF device is typically downloaded from either the manufacturer’s or the Fieldbus Foundation website along with the two DD files, as a three-file set (filename extensions being .cff, .sym, and .ffo, respectively). The Value file is generated by the host system during the device’s configuration, storing the specific configuration values for that specific device and system tag number. The data stored in a Value file may be used to duplicate the exact configuration of a failed FF device, ensuring the new device replacing it will contain all the same parameters.

A screenshot of a .cff Capability file opened in a text editor program appears here, showing the first few lines of code describing the capabilities of a Yokogawa model DYF vortex flowmeter:

A screenshot of a .cff Capability file opened in a text editor program describing the capabilities of a Yokogawa model DYF vortex flowmeter

As with “driver” files needed to make a personal computer peripheral device function, it is important to have the correct versions of the Capability and DD files installed on the host system computer before attempting to commission the device. It is permissible to have Capability and DD files installed that are newer than the physical device, but not visa-versa (a newer physical device than the Capability and DD files). This requirement of proper configuration file management is a new task for the instrument technician and engineer to manage in their jobs. With every new FF device installed in a control system, the proper configuration files must be obtained, installed, and archived for safe keeping in the event of data loss (a “crash”) in the host system.

 

Device commissioning

This section illustrates the commissioning of a Fieldbus device on a real segment, showing screenshots of a host system’s configuration menus. The particular device happens to be a Fisher DVC5000f valve positioner, and the host system is a DeltaV distributed control system manufactured by Emerson. All configuration files were updated in this system prior to the commissioning exercise. Keep in mind that the particular steps taken to commission any FF device will vary from one host system to another, and may not follow the sequence of steps shown here.

If an unconfigured FF device is connected to an H1 network, it appears as a “decommissioned” device. On the Emerson DeltaV host system, all decommissioned FF devices appear within a designated folder on the “container” hierarchy. Here, my Fisher DVC5000 device is shown highlighted in blue. A commissioned FF device appears just below it (PT 501), showing all available function blocks within that instrument:

 

A Commissioned FF Device

Before any FF device may be recognized by the DeltaV host system, a “placeholder” and tag name must be created for it within the segment hierarchy. To do this, a “New Fieldbus Device” must be added to the H1 port. Once this option is selected2, a window opens up to allow naming of this new device:

Fieldbus Device Properties

Here, the tag name “PV 501” has been chosen for the Fisher valve positioner, since it will work in conjunction with the pressure transmitter PT 501 to form a complete pressure control loop. In addition to a tag name (PV 501), I have also added a text description (“Pressure control valve (positioner)”), and specified the device type (Fisher DVC5000f with AO, PID, and IS function block capability). The DeltaV host system chose a free address for this device (35), although it is possible to manually select the desired device address at this point. Note the “Backup Link Master” check box in this configuration window, which is grey in color (indicating the option is not available with this device).

After the device information has been entered for the new tag name, a “placeholder” icon appears within the hierarchy for the H1 segment (connected to Port 1). You can see the new tag name (PV 501) below the last function block for the commissioned FF instrument (PT 501). The actual device is still decommissioned, and appears as such:

 

Decommissioned Device

By right-clicking on the new tag name and selecting the “Commission” option, a new window opens to allow you to select which decommissioned device should be given the new tag name. Since there is only one decommissioned device on the entire segment, only one option appears within the window:

 

Since there is only one device decommissioned on the entire segment, only one option appears within the window

 

After selecting the decommissioned device you wish to commission, the DeltaV host system prompts you to reconcile any differences between the newly created tag name placeholder and the decommissioned device. It is possible the Resource and/or Transducer block parameters set within the placeholder do not match what is currently set in the decommissioned device, if that is what you desire. Otherwise, the existing block parameters within the decommissioned device will remain unchanged.

 

FF Device Commissioning Wizard

After selecting (or not selecting) the “reconcile” option, the DeltaV system prompts you to confirm commissioning of the device, after which it goes through a series of animated3 display sequences as the device transitions from the “Standby” state to the “Commissioned” state:

 

Device Transistion from the Standby State to the Commision State

 

As you can see, the commissioning process is not very fast. After nearly one full minute of waiting, the device is still “Initializing” and not yet “Commissioned.” The network speed of 31.25 kbps and the priority of scheduled communications are limiting factors when exchanging large quantities of configuration data over a FF H1 network segment. In order for device configuration to not interrupt or slow down process-critical data transfers, all configuration data exchanges must wait for unscheduled time periods, and then transmit at the relatively slow rate of 31.25 kbps when the alloted times arrive. Any technician accustomed to the fast data transfer rates of modern Ethernet devices will feel as though he or she has taken a step back in time when computers were much slower.

After commissioning this device on the DeltaV host system, several placeholders in the hierarchy appear with blue triangles next to them. In the DeltaV system, these blue triangle icons represent the need to download database changes to the distributed nodes of the system:

 

the blue triangle icons represent the need to download database changes to the distributed nodes of the system

 

After “downloading” the data, the new FF valve positioner shows up directly below the existing pressure transmitter as a commissioned instrument, and is ready for service. The function blocks for pressure transmitter PT 501 have been “collapsed” back into the transmitter’s icon, and the function blocks for the new valve positioner (PV 501) have been “expanded” for view:

FF Valve Positioner as a Commissioned Instrument

 

As you can see, the new instrument (PV 501) does not offer nearly as many function blocks as the original FF instrument (PT 501). The number of Fieldbus function blocks offered by any FF instrument is a function of that instrument’s computational ability, internal task loading, and the whim of its designers. Obviously, this is an important factor to consider when designing a FF segment: being sure to include instruments that contain all the necessary function blocks to execute the desired control scheme. This may also become an issue if one of the FF instruments in a control scheme is replaced with one of a different manufacturer or model, having fewer available function blocks. If one or more mission-critical function blocks is not available in the replacement instrument, a different replacement must be sought.

 

Calibration and ranging

Calibration and ranging for a FF device is similar in principle to any other “smart” measurement instrument. Unlike analog instruments, where the “zero” and “span” adjustments completely define with the instrument’s calibration and range, calibration and ranging are two completely different functions in a digital instrument.

A block diagram of an analog pressure transmitter shows the zero and span adjustments:


A block diagram of an analog pressure transmitter shows the zero and span adjustments

The “zero” and “span” adjustments together define the mathematical relationship between sensed pressure and current output. Calibration of an analog transmitter consists of applying known (reference standard) input stimuli to the instrument, and adjusting the “zero” and “span” settings until the desired current output values are achieved.

A “smart” (digital) transmitter equipped with an analog 4-20 mA current output distinctly separates the calibration and range functions:

 

A Smart (Digital) transmitter equipped with an analog 4-20 mA current output distinctly Separates the calibration and range functions.

Calibration of a “smart” transmitter consists of applying known (reference standard) input stimuli to the instrument and engaging the “trim” functions until the instrument accurately registers the input stimuli. Ranging, by contrast, establishes the mathematical relationship between the registered input value and the output current value. To illustrate the difference between calibration and ranging, consider a case where a pressure transmitter is used to measure water flow through a venturi tube. Suppose the transmitter’s pressure range of 0 to 100 inches water column translates to a venturi tube flow range of 0 to 250 gallons per minute. If we desired to re-range an analog pressure transmitter to measure a lower range of flow (say, 0 to 130 gallons per minute), we would have to re-calculate the new pressure range (0 to 27.04 inches water column, abiding by the quadratic behavior of the venturi tube) and then subject the analog pressure transmitter to a new (standard) pressure of 27.04 inches water column while we re-adjusted the transmitter’s zero and span so it accurately represented the new pressure range. The only way we can re-range an analog transmitter is to completely re-calibrate it.

In a “smart” (digital) measuring instrument, however, calibration against a known (standard) source need only be done at the specified intervals to ensure accuracy despite the instrument’s inevitable drift. If our hypothetical transmitter were calibrated accurately against a known pressure standard and relied upon not to have drifted since the last calibration cycle, we could re-range it by programming it with the new LRV (lower range value) and URV (upper range value) parameters so that 27.04 inches water column now drives its current output to 20 mA instead of 100 inches water column pressure as was required before. Digital instrumentation allows us to re-range without re-calibrating, representing a tremendous savings in technician time and effort.

Fieldbus instruments, of course, are “smart” in the same way, and their internal block diagrams look much the same as the “smart” transmitters with analog current output, albeit with a far greater number of parameters within each block. The rectangle labeled “XD” in the following diagram is the Transducer block, while the rectangle labeled “AI” is the Analog Input block:


Fieldbus Instruments are smart in the same way and have more number of parameters

Calibration (trim) values are set in the transducer block along with the engineering unit, making the output of the transducer block a digital value scaled in real units of measurement (e.g. PSI, kPa, bar, mm Hg, etc.) rather than some raw ADC “count” value. The analog input function block receives this pre-scaled “Primary Value” and translates it to another scaled value based on a proportionality between transducer scale values (XD Scale high and low) and output scale values (OUT Scale high and low). The L Type parameter residing in the analog input block determines whether the ranging is direct (output value equals primary input value), indirect (proportionately scaled), or indirect with square-root characterization (useful for translating a differential pressure measurement across a flow element into an actual fluid flow rate).

To calibrate such a transmitter, the transducer block should first be placed in Out Of Service (OOS) mode using a handheld FF communicator or the Fieldbus host system. Next, a standard (calibration-grade) fluid pressure is applied to the transmitter’s sensor and the Cal Point Lo parameter is set to equal this applied pressure. After that, a greater pressure is applied to the sensor and the Cal Point Hi parameter is set to equal this applied pressure. After setting the various calibration record-keeping parameters (e.g. Sensor Cal Date, Sensor Cal Who), the transducer block’s mode may be returned to Auto and the transmitter used once again.

To range such a transmitter, a correspondence between sensed pressure and the process variable must be determined and entered into the analog input function block’s XD Scale and OUT Scale parameters. If the pressure transmitter is being used to indirectly measure something other than pressure, these range parameters will become very useful, not only proportioning the numerical values of the measurement, but also casting the final digital output value into the desired “engineering units” (units of measurement).

The concept of ranging a FF transmitter makes more sense viewed in the context of a real application. Consider this example, where a pressure transmitter is being used to measure the level of ethanol (ethyl alcohol) stored in a 40 foot high tank. The transmitter connects to the bottom of the tank by a tube, and is situated 10 feet below the tank bottom:

 

Concept of ranging a FF transmitter make more sense viewed in the context of a real application

Hydrostatic pressure exerted on the transmitter’s sensing element is the product of liquid density (γ) and vertical liquid column height (h). When the tank is empty, there will still be a vertical column of ethanol 10 feet high applying pressure to the transmitter’s “high” pressure port. Therefore, the pressure seen by the transmitter in an “empty” condition is equal to:

 Pemptyγhemptyu = (49.3 lb/ft3)(10 ft)
Pempty493 lb/ft2 = 3.424 PSI

When the tank is completely full (40 feet), the transmitter sees a vertical column of ethanol 50 feet high (the tank’s 40 foot height plus the suppression height of 10 feet created by the transmitter’s location below the tank bottom). Therefore, the pressure seen by the transmitter in a “full” condition is equal to:

Pfull = γhfull = (49.3 lb/ft3)(50 ft)
Pfull2465 lb/ft2 = 17.12 PSI 

 

The control system does not “care” about the transmitter’s 10-foot suppression, though. All it needs to know is where the ethanol level is in relation to the tank bottom (relative to an “empty” condition). Therefore, when we range this transmitter for the application, we will set the analog input block’s range parameters as follows4:

AI block parameter
Range values
XD_Scale

3.424 PSI to 17.12 PSI

OUT_Scale

 0 feet to 40 feet

L_Type

 Indirect

 

Now, the ethanol tank’s level will be accurately represented by the FF transmitter’s output, both in numeric value and measurement unit. An empty tank generating a pressure of 3.424 PSI causes the transmitter to output a “0 feet” digital signal value, while a full tank generating 17.12 PSI of pressure causes the transmitter to output a “40 feet” digital signal value. Any ethanol levels between 0 and 40 feet will likewise be represented proportionally by the transmitter. I

f at some later time the decision is made to re-locate the transmitter so it no longer has a 10 foot “suppression” with regard to the tank bottom, the XD Scale parameters may be adjusted to reflect the corresponding shift in pressure range, and the transmitter will still accurately represent ethanol level from 0 feet to 40 feet, without adjusting or re-calibrating anything else in the transmitter.

 

1One of the tasks of the Fieldbus Foundation is to maintain approved listings of FF devices in current manufacture. The concept is that whenever a manufacturer introduces a new FF device, it must be approved by the Fieldbus Foundation in order to receive the Fieldbus “badge” (a logo with a stylized letter “F”). Approved devices are cataloged by the Fieldbus Foundation, complete with their DD file sets. This process of approval is necessary for operational compatibility (called interoperability) between FF devices of different manufacture. Without some form of centralized standardization and approval, different manufacturers would invariably produce devices that were mutually incompatible with each other.

2On the Emerson DeltaV system, most options are available as drop-down menu selections following a right-mouse-button click on the appropriate icon.

3Animated graphics on the Emerson DeltaV control system prominently feature an anthropomorphized globe valve named Duncan. There’s nothing like a computer programmer with a sense of humor . . .

4When configuring the XD Scale high and low range values, be sure to maintain consistency with the transducer block’s Primary Value Range parameter unit. Errors may result from mis-matched measurement units between the transducer block’s measurement channel and the analog input block’s XD Scale parameter.

 

Click here to go to the next page, H1 FOUNDATION Fieldbus Segment Troubleshooting

Click here to go back to the previous page, FOUNDATION Fieldbus function blocks

 

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Comments (1)Add Comment
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Mabuhay
written by Eduardo, April 20, 2012
Keep it up Sir,Very clear intuitive topic....using as much as possible layman's terminology while going in depth...congrats and more power tnx

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