Devices Electronics

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93

1.6 Configuring Intelligent Devices

J. BERGE (2003)

DESIGN FEATURE RECOMMENDATIONS

• Use permanently connected communications infra-structure.

• For the HART® Field Communications Protocol, use

a handheld with large, user-friendly screen.

• Use device configuration templates.

• Use a fieldbus tool that requires no proprietary filesfrom the host manufacturer.

• Be mindful of revisions when configuring fieldbus

devices.

COSTS

A HART handheld, such as Smar International’s HPC301,

costs approximately U.S. $1000.

A FOUNDATION™ fieldbus host, such as the SYSTEM302,

starts at U.S. $4000.

INTRODUCTION

Intelligent devices include “smart” instruments that have bothan analog 4- to 20-mA signal with simultaneous digital com-

munication, such as HART (Volume 3, Section 4.11), and field-

bus devices that are completely digital, such as FOUNDATION

fieldbus (Volume 3, Section 4.12). Intelligence in these devices,

along with networking and the right tools, can also be put to

good use to improve maintenance practices.

Both FOUNDATION fieldbus and HART protocols are specif-

ically designed for the configuration of field instruments andare therefore the most commonly used in the process industries.

They both have special parameters for the specific purpose of

device configuration, and both use the concept of device

description (DD) files to inform a host device how to commu-

nicate these parameters with the device. This is a unique char-acteristic of these protocols that makes them highly suitable for

instrumentation and control.

In the HART protocol, device parameter configuration is

based on commands for reading and writing. There are three

classes of commands: universal, common-practice, and spe-

cific. All HART devices support the universal commands, and

most also support several common-practice commands.

Together, the universal and common-practice commands cover

most functions needed in a device. This enables a host to per-

form most device functions without access to the DD. Essen-

tially, all devices also have a number of specific commands to

access unique functionality. Either a DD or a special driver is

required to communicate the special commands.

For FOUNDATION fieldbus device configuration parameters

are arranged in a resource block and in transducer blocks.There are also function blocks, but these are used for control

strategy building rather than device configuration. This distinc-

tion makes it easy to draw the line between device and strategy

configuration. The block parameters are accessed using sophis-

ticated communication services. A host needs to be loaded with

the DD to be able to communicate with a device. In addition,

to allow configuration of the device off-line, in advance, the

capabilities file (CF) is required.

Range setting is perhaps the most important setting of a

device based on a 4- to 20-mA signal such as used by a HART

instrument. Range sets the scale for the 4- to 20-mA signal.

Because FOUNDATION fieldbus devices communicate in floating-point mode and engineering units, there usually is no need to

set a range. Calibration shall not be confused with range set-

ting, since they are in fact different operations. For example,

for a transmitter range setting means configuring the measured

values at which the output shall be 4 and 20 mA, respectively.

Calibration, on the other hand, means adjusting the reading

from the sensor to match the correct value from a standard

(Section 1.8). Range setting does not correct the sensor read-

ing; it only affects the output scale.

To calibrate a transmitter, you must always apply a known

input. Therefore, calibration cannot be done remotely. The

expression remote calibration is often used erroneously tomean remote range setting. Some confusion exists in the mar-

ketplace as to what calibration is. This is a legacy from the

era of analog devices in which calibration and range setting

was done at the same time using the same set of potentiom-

eters. For the HART protocol, calibration is usually called

trim to distinguish it from range setting. FOUNDATION fieldbus

avoids this confusion by calling range setting scaling. Still

it is quite common in HART devices to change the range

instead of calibrating the device when the sensor reading is

wrong. The result is a correct 4- to 20-mA output, but any

digital reading in the display or host will be wrong, which

may lead to confusion.

© 2003 by Béla Lipták

http://1083ch4_11.pdf/








94 General Considerations

CONFIGURATION TOOLS

FOUNDATION fieldbus devices are permanently connected via

a network to a host in a convenient central location. HART

devices, on the other hand, normally operate only using 4 to

20 mA, and a hand-held HART communicator is connected

and communicates only when required. A FOUNDATION field-

bus host is therefore predominantly a stationary desktop com-

puter, whereas a HART communicator is usually a portable

device. However, portable interfaces for laptops exist for both

HART and FOUNDATION fieldbus operation.

The handheld can be connected at any convenient point

along the 4- to 20-mA wire as long as it is on the instrument

side of the stipulated resistor (see Figure 1.6a). This connec-

tion is often done at the device itself, giving on-line access

to information, configuration, remote monitoring, calibration

and range setting, diagnostics, and maintenance.

The handheld can be either a dedicated text-based HART

device or a HART interface for an organizer with a graphicaluser interface (GUI). Rapid development in the area of per-

sonal organizers has brought about tools that incorporate

displays larger than three inches across and are capable of

sophisticated graphics with a Windows-like appearance. They

are also are very fast (see Figure 1.6b).

Fieldbus instruments are connected on the H1 field-level

nework that ties in with a linking device on the HSE host-

level network where the host computers with the configura-

tion tool is (Figure 1.6c). This is a permanent connection

where the network used for control, monitoring, and opera-

tion is also used for configuration and diagnostics, etc.

Linking device is typically one of many functions inte-

grated in a single device. Usually the same device is respon-

sible for powering the field instruments, etc. A linking device

typically has several H1 ports and can be connected in aredundant scheme for high availability (Figure 1.6d).

For FOUNDATION fieldbus interoperability with the host is

achieved through DD. By installing the device support files

for a device, the host computer has the information required

to communicate with the device and to allow the user to prepare

a configuration in advance, even without being connected to

the device. The files usually can be obtained from the man-

ufacturer’s site on the Internet. To guarantee interoperability,

an open host uses only standard files to support any device.

However, some tools may require proprietary files for each

device to work, in which case a short list of approved devices

applies.

FIG. 1.6a

HART handheld connection.

FIG. 1.6b

HART pocket Configurator. (Courtesy of Smar International.)

Resistor

+

−

Power

Supply Al Handheld

Terminal

FIG. 1.6c

F OUNDATION ™ fieldbus system architecture.

FIG. 1.6d

F OUNDATION ™ fieldbus linking device. (Courtesy of Smar Interna-

tional.)

H1 Fieldbus

Devices

Operation Engineering Maintenance Business

HSE Fieldbus

Router

H1 Fieldbus

Devices




1.6 Configuring Intelligent Devices 95

DEVICE CONFIGURATION

HART devices are typically configured on line only, using ahandheld unit communicating directly with the device. How-

ever, some handhelds also support off-line configuration for

later download.

Although HART transmitters can operate in a purely

digital mode, the 4- to 20-mA output is almost always usedto deliver the process variable to the central controller. Setting

the range of the transmitter is therefore required. The primary

variable output settings are the most important part of the

configuration. The lower range value (LRV) is the measured

value at which the transmitter output will be 4 mA, and theupper range value (URV) is the input value at which the

output will be 20 mA; i.e., 0 and 100% of range, respectively.

Although LRV and URV are the proper terms, most call them

zero and span instead. But span is really the difference

between the URV and the LRV. The engineering unit can alsobe selected.

Range setting can be accomplished from the handheldby simply keying in the desired range values, regardless of

input. This can even be done remotely and stored as an off-

line configuration. Another method is to apply an input and

(by pressing a button on the handheld or on the transmitteritself) informing the device that the applied input is to be the

LRV or URV, thus setting the range (see Figure 1.6e). The

latter method is often used for pressure transmitters that are

installed with impulse lines that add hydrostatic pressure.

Pressing the button elevates or suppresses the zero, ensuringthat the output is 4 mA when appropriate. If the transmitter

has a noninteractive zero and span, the URV will be pushed

by the equivalent amount, leaving the span unaffected. Forexample, if the range of a pressure transmitter in a level

application starts off as 0 to 5.48 kPa, and applied rerange isdone with 1.86 kPa input, the new range becomes 1.86 to

7.34 kPa. However, when a rerange is applied for the URV,

this does not affect the zero; i.e., the span is changed instead.

The damping is a first-order lag filter time constant.

The transfer function is used to select linear or square rootextraction for differential-pressure flowmeters, and possibly

for other options such as a freely configurable lookup table or

square root of third or fifth power for open-channel flow

measurements. Square root extraction is often done in the

central control system, but it is in fact better to do it in the

transmitter, as this results in less A/D and D/A conversion error.

Fail-safe mode can be set as upscale or downscale as per

the NAMUR NE-43 standard. Downscale means that the

output current will be set to 3.6 mA in case the internal

diagnostics detect a fault. Upscale means the output will be

set to 21 mA. Ideally, the receiving controller should havean input module that can interpret the failure signal and use

this to shut down the control loop.

It is also possible to review the write protection status of

the device—if the device has been write protected using a

jumper or other solution.

When performing range setting or calibration, the user needs

to know the sensor limits and is therefore usually prompted with

this information, which typically can be reviewed at any time

(see Figure 1.6f). HART device information includes lower

range limit (LRL), upper range limit (URL), and the minimumspan. The minimum span is the smallest permitted absolute

difference between the URL and LRL for the sensor.Many configuration options are device specific in HART

devices. For example, for a temperature transmitter, the sen-

sor type and wiring has to be configured (see Figure 1.6g).For a valve positioner, the actuator type has to be configured.

In most FOUNDATION system installations, the device con-

figuration is created off-line for all devices in advance and

downloaded after installation. Moreover, the device configu-

ration is typically accomplished with the network and control

FIG. 1.6e

HART transmitter range setting.

FIG. 1.6f

HART transmitter sensor limits.

FIG. 1.6g

Sensor and wiring selection is device specific.





strategy configuration, all in the same tool. Thus, when con-

figuring devices in a FOUNDATION system, the first step is to

create the devices on the networks and give the devices aphysical device tag (PD_TAG). Fieldbus devices are develop-

ing very rapidly. New versions are constantly being released,

and many users already have several versions around the plant

and in the store, this on top of a variety of brands of the same

kind of device. When inserting new devices, it is important tospecify the revisions to be used (see Figure 1.6h). Usually, the

latest revision is the default. There is a risk that you will

accidentally download a device configuration to the wrong

model or wrong version device. A good tool prevents this

and thus avoids the many headaches that could result.

Fieldbus devices do not require any range, because all

values are communicated as floating-point values in engi-

neering units. Ranges are typically used only in PID functionblocks, or possibly in AI function blocks, to cater for con-

versions in inferred measurements, such as converting differ-

ential pressure to flow within specified ranges. That is, scal-

ing is done as part of the control strategy configuration, notas part of the device configuration.

Every fieldbus device, H1 as well as HSE, needs to have

one resource block. Really, the only parameter that must be

configured is the mode. The MODE_BLK parameter target

shall be set to automatic.

A fieldbus transducer block is required, in conjunction

with every sensor and actuator, to act as an interface betweenthe device and the control strategy. By parameterizing the

transducer blocks, the device can be set up for the proper sensor

or actuator type, such as HART devices. Similarly the trans-

ducer block also contains information about sensor limits.

Indeed, there is a transducer scale range indication in thetransducer block, but it is essentially a reflection of a setting

done in the associated I/O function block. The MODE_BLK

parameter target shall be set to automatic.

IDENTIFICATION

Information for identifying the device is very helpful during

commissioning and maintenance. During the commissioning

stage, it is useful for further assurance that connection has

been made to the correct device. During maintenance, it is

helpful for retrieving serial numbers, special instructions,

important dates, and model numbers. The user can configure

some of these parameters to be pertinent to the application.

These parameters do not affect the operation of the device.

In addition to the sensor information such as range limits,HART device data such as sensor serial number, final assem-

bly number, message, device tag, descriptor, date, manufac-

turer, device type, software and hardware revisions, and cir-

cuit board serial number are provided. For HART devices,

the tag can use up to eight characters. The descriptor andmessage are 16 and 32 characters in length and can be con-

figured as annotations describing the application and remind

technicians of special precautions when servicing a device

(see Figure 1.6i).

The date has no specific purpose but may be used to store

when calibration or maintenance was last performed or is

scheduled. Manufacturer, model, and version information canalso be accessed (see Figure 1.6j).

HART devices such as pressure transmitters usually pro-

vide additional information about the materials of construction

for the parts wetted by the process, e.g., the sensor isolatingdiaphragm, O-ring, flange and remote seal parts, and so on.

The resource block found in any FOUNDATION fieldbus device

contains, among other things, identification information for the

device. The tag descriptor parameter may be used to describethe application of the device. Manufacturer, model, and version

information can also be accessed (see Figure 1.6k ).

FIG. 1.6h

Pay attention to revisions when creating device configurations.

FIG. 1.6i

Application information from a HART device.

FIG. 1.6j

Detail attributes of a HART device.




1.6 Configuring Intelligent Devices 97

Fieldbus devices such as pressure transmitters usually pro-

vide additional information about the materials of construction

for the parts wetted by the process, e.g., the sensor isolating

diaphragm, O-ring, flange and remote seal parts, and so on

(see Figure 1.6l).

If any changes are ever made to the device parts, it is

important to update this information.

CALIBRATION

When the sensor reading differs from the actual applied input,

the sensor has to be calibrated. The correct reading is entered

from the maintenance tool, and the device then performs the

necessary adjustment. A special case of sensor calibration iszeroing, which by definition is done with a zero value applied.

This is accomplished, for example, by venting in the case of

a pressure transmitter; hence, no value has to be entered (see

Figure 1.6.m).

Nonzero calibration is usually done with a precision source

applied. Calibration is usually done in two points, known as

the low and high calibration points, respectively. For most

transmitters, calibration of these two points is noninteractive.

There is a limit to how close the two calibration points can be,

and the distance is referred to as the minimum span. Output

converters also need to be calibrated. This is usually done by

first forcing an output at one end of the scale and then

comparing the actual output against a standard. The actual

reading is entered into the device that makes the necessarycorrection. Control valve positioners usually calibrate their

position-sensing sensor themselves by automatically strokingthe valve over its entire travel.

Since HART devices rely on 4 to 20 mA, this current loop

can also be calibrated. However, it is rarely done because,being totally electronic, this part of the device rarely experi-

ences any drift at all. For a HART transmitter, this means that

a fixed output current is generated. The technician checks the

current against a standard and keys it into a device that makes

the necessary correction (see Figure 1.6n). For HART outputdevices, a signal is injected, and the device is informed of the

true current.Sensor or actuation calibration for FOUNDATION fieldbus

devices is done from the associated transducer block. The

standard transducer block also includes several parameters for

storing information about the last calibration. This includes

two parameters for keeping the values of the two calibration

points. This is useful when determining if a device has beencalibrated at points that are suitable for the operating range.

Additional information that can be stored includes calibration

date, location, the method used, and who performed the cali-

bration. It is good practice to update this information at each

calibration and to display it along with useful sensor limit

information (Figure 1.6o).

FIG. 1.6k

Fieldbus device identification from resource block.

FIG. 1.6l

Pressure transmitter materials of construction.

FIG. 1.6m

Zero calibration of HART transmitter.

FIG. 1.6n

Loop current calibration.





MONITORING

Configuration tools for HART and FOUNDATION fieldbus sys-

tems are generally designed in different ways, not only in terms

of the way they look, but also the way they work. In HART

tools, menus are usually arranged according to the function

performed, whereas device configuration in fieldbus tools is

arranged according to the resource and transducer blocks. Thus,

in a HART device, there generally is a screen on which the

transmitter variables can be monitored on line (see Figure 1.6p).

SIMULATION

At the time of commissioning, it is common to check that

all indicators, recorders, and computer screens show the cor-

rect values, that alarm trips are working properly, and so on.

This is particularly important for HART devices, because

there is a chance that the range set in the device does not

match that set in the central controller, and any difference

would result in operational problems. To verify that all ranges

are consistent, it is helpful to use the simulation function

found in HART and fieldbus devices.

When performing simulation in a HART transmitter, the

output current is manipulated independently of the applied

input (see Figure 1.6q). Generally, the handheld allows the

current to be set slightly above and below the 4- to 20-mArange to simulate fault conditions.

Simulation is primarily used to test the control strategy.Therefore, simulation for FOUNDATION fieldbus devices is

done from the input and output function blocks.

DIAGNOSTICS

Many device diagnostics can be performed from a configu-ration tool, but only if communication is established. If there

is no functional communication, troubleshooting has to rely

on traditional means. When it comes to diagnostics, one of

the major differences in the application of HART and FOUN-

DATION fieldbus becomes most evident. While fieldbus devices

are constantly communicating, making it possible to monitordevice conditions continuously and instantly detect faults,

communication with HART devices is typically carried out

to confirm a problem only after the failure has already been

detected. Although applications with continuous HART com-munications do exist, they are rare. To fully benefit from

intelligence in field devices, the engineering tool should con-

tinuously communicate with the instruments.

Generic diagnostic information is communicated in every

exchanged HART message, giving the device the opportunity

to inform the user of any failure. Such error message includes

general malfunction, exceeded limits, and a message that

more detailed status report is available (see Figure 1.6r).

FIG. 1.6o

Calibration-related information.

FIG. 1.6p

Monitoring of dynamic transmitter variables from a HART tool.

FIG. 1.6q

Using loop test to simulate process variable from a handheld.

FIG. 1.6r

Basic HART diagnostics.




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