smart vs conventional

8/17/2019 SMART vs Conventional

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SMART TRANSMITTERS

-

DIGITAL VS ANALOG

SMART TRANSMITTERS

-

THE FIELD SOLUTION

OBERT L. WILSON

Honeywell Inc.

Industrial Controls Division

1100

Virginia Drive

Fort Washington, PA

19034

Abstract-This paper will define “Smart” , review the advantages

of smart transmitters vs. c onve ntiona l transmitters, and

look

at

key differences between competitive offerings. It

will

also

demonstrate the superiority and benefits of transmitters

operating in the digital commu nicatio ns mod e and why all the

excitement about the coming International Field Bus Standard

(SP50).

In 1983, higher levels of microprocessor integration permitted the

introduction of ST 3000 he worlds first smart transmitter

with digital electronics and the standard 4-2OmA transmission.

Since their introduction, smart transmitters lthough selling at

a significant premium, ave been the fastest growing process

control segment. In the last 2 3 years, a number of smart sensors

and transmitters have been introduced worldwide and it now

seems assured that all future sensorkransmitter designs will be

“smart.”

WHY TH E TREND TO SMART TRANSMITTERS?

Simply because

a

truly smart transmitter offers:

EVOLUTION OF TH E PROCESS CONTROL PROBLEM

Throughout the history of industrial instrumentation prior to the

1960’s, recorders and controllers were considered to be the

limiting factor in improving process control. In 1958,

introduction of the first complete control system utilizing 2-wire

4-20mA transmission signals improved the situation significantly

and spurred the growth of control rooms covering more and

more process loops.

This was a major advancement but , through the early 1970’s, the

biggest “problem” in large processing industries was still

considered to be the control room equipment. The stand-alone

miniature electric indicators, recorders and controllers prevalent

at the time were inaccurate and unreliable and required huge

control rooms. Equally important, the many operators required

to man this equipment still could not effectively control the

complete process articulary during upsets.

Subsequent attempts at centralized control of the process with

large computers was equally unrewarding since computer failures

(frequent at the time) could shut down large portions of the

plant.

SMART CONTROLLERS

-

THE CONTROL ROOM SOLUTION

In 1975, the emerging field of microprocessor technology

permitted the introduction of TDC 2000 fully digital system

with distributed controllers, a greatly improved man-machine

interface that permitted relatively few operators to control the

entire process much more effectively, and a high level of control

security. The distributed controllers handled no more than eight

loops, and automatic-switchover back-up controllers provided

uninterruptable automatic control in the event of a failure.

Since that time, TDC 2000 and similar Distributed Control

Systems have essentially eliminated the earlier control room

problems of inaccuracy, unreliability and poor operator control.

But the success of such modern control rooms refocused

attention on what then became the weak link in the process

the field sensors and transmitters. These devices were equally

inaccurate and unreliable, but were also inaccessible,

inconvenient and costly to operate and maintain.

Accuracy improvement of at least 2: 1.

Stability improvement under varying operating conditions

(temperature, static pressure, etc.) of 3:l to 15: 1.

Much greater rangeability he range of spans over

which the transmitter will maintain specified accuracy and

stability performance.

Status information (validity)

Greater ease of specification and use.

Greater inherent reliability.

Significantly lower costs of ownership, operation and

maintenance.

In its simplest terms, there are three reasons why a user would

justify selecting one product over another:

Greater probability of improving the quality of his end

Greater personnel or plant safety.

Lower total end-product costs.

product,

Except for the more technically innovative users, smart

transmitters were originally applied to the more demanding or

critical applications where the higher up-front costs were not an

important consideration. In almost every case, early users have

since upgraded other applications, or standardized on smart

transmitters entirely ncluding applications where high

accuracy is not a major factor.

In some areas of hazardous use, this was simply because the

remote diagnostics eliminated the need to send a crew into the

hazardous area to check out the transmitter. In most cases,

however, this was due to a recognition that smart transmitters

were much more cost effective than conventional transmitters.

In one published study, a major petrochemical company reported

that they had reduced the contractors installation and start-up

allowance for smart transmitters (vs conventional transmitters)

by

50 ,

and that more than

80

of all transmitter maintenance

work orders could be eliminated or significantly reduced by the

use of smart transmitters.

A Honeywell survey of major users concluded that total start-up,

operating and maintenance reductions averaged about 2,000.00

CH2764-9/89/0000-0061 1

OO

@ 1989

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per transmitter over a five-year period. That’s 5 10 times the cost

premium for a smart transmitter.

DEFINITION OF A SMART TRANSMITTER

There is no “industry standard” definition, but certainly a

truly

smart transmitter should have the following characteristicsand

capabilities:

Microprocessor-basedwith predominately digital

electronics.

Remote communication and configuration.

The ability to continuously monitor its operating

conditions and correct itself for potential errors such as

non-linearity, ambient temperature influence, static

pressure influence, etc.

High turndown and reduction or elimination of re-ranging

errors.

Continuous diagnostics of its sensing element (i.e., meter

body for pressure transmitters), and electronics as well as

the loop power supply and wiring.

VARIATIONS IN SMART TRANSMITTERS

There are several truly smart transmitters currently on the market

that meet, to a greater or lesser degree, the above criteria. As

might be expected, some offer specifications hat are

conservatively rated while others are considerably less

conservative making it incumbent on the potential user to select

carefully.

On the other hand, there are the not-so-smart or what we might

call pseudo-smart transmitters. These are simply conventional

transmitters with digital electronics and remote communication

and configuration. Therefore, they meet the first and second

criteria above, but are not self-correcting,do not meet the

turndowdre-ranging criteria, and have limited diagnostics hat

do not include the sensor/meter body. Technical literature for

those pseudo-smart transmitters typically indicate that they can

be remotely re-ranged resumably without recalibration.

However, it is left to the user to discover that, although this is

possible, very large errors will result.

Therefore, these pseudo-smart transmitters do not offer the wide

range of performance, convenience and cost advantages of their

truly smart cousins. What you get is little or no performance

improvement and a very limited set of diagnostic and

configuration capabilities.

One other significant difference in analog (4-2OmA) smart

transmitters is the digital “protocol” used when communicating

between the transmitter and its hand-held communicator. Some

transmitters (i.e., ST 3000) interrupt the PV signal during

communication and transmit a low baud rate, serial, digital pulse

stream varying between approximately 4-2OmA. This is

considered a very “secure” method, but has the disadvantage of

requiring that the loop be in manual during communication.

When communicating n the digital mode, this is not a

disadvantage since the PV signal is broadcast continuously.

signal on top of the cont inuously broadcast 4-2OmA PV signal.

This does not disrupt the process, but is considered a less

“secure” method.

ANALOG TRANSMITTER PERFORMANCE -

CONVENTIONAL VS SMART

The great majority of differential pressure measurements are

covered by the range from 1-400” of water. “Turndown” reflects

the total range over which a single transmitter is designed to

operate and is a major consideration. With conventional

transmitters, a specific model must be selected to fit a specific

application in advance. If an error is made, or if the flow rate is

different than predicted, or if the flow rate subsequently changes

significantly, he transmitter must be removed and a different

model substituted. Smart transmitters are much more universal.

You

can usually order one model, install it anywhere and re-span

(without recalibration) anytime.

Table I compares the turndown ratio for Conventional vs Smart

DP transmitters. Note that four conventional transmitters are

required to cover the 1-400 inH,O range of the ST 3000 smart

transmitter.

TABLE I

TURNDOWN RATIOS:

SMART

VS

CONVENTIONAL TRANSMITTERS

Min-Max Spans Turndown

(inH,O) Ratio

ST 3000 DP: 1-400 400: 1

Conventional: 1-6 6:1

5-30 6: 1

25-150 6:1

125-750 6:1

Almost all transmitters specify error in 070 Span. However,

what’s really significant is the error of the reading currently being

taken. Consider in Table 11, for example, a transmitter spanned

at 400 inH,O and with a Reference Accuracy of 0.1070 Span. As

a o Span, the error is always 0.1%. However, as a percent of

Reading, the error is 0.1Vo when reading at 400 inH,O, 0.2% at

200 inH,O, 0.4% at 100 inH,O, etc. For this reason, the errors

in subsequent Tables are specified in

070

Reading.

TABLE

I1

COMPARING

‘ o

SPAN VS

‘ o

READING FOR A

TRANSMITTER SPANNED AT 400 INCHES OF WATER

Error Error

Reading (070 Span) 070 Reading)

400

200

100

5

25

0.1

0.1

0.1

0.1

0.1

0.1

0.2

0.4

0.8

1.6

Alternate methods include the high frequency FSK (frequency

shift keying) method which superimposes he communication

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Table 111 compares total performance (error) for conventional vs

smart transmitters when exposed to an ambient temperature

change of 50°F and a static pressure change of 1000 psi.

TABLE I11

PERFORMANCE OF CONVENTIONAL VS

SMART ANALOG TRANSMITTERS

Conditions: Span at 2 inH,O; Reading at I nH20

Error in Reading)

ST 3000

Conventional Analog

Reference Accuracy 0.40 0.20

Ambient Temperature 3.20 0.35

Influence (50°F)

Static Pressure Influence 2.19 0.40

(1K psi)

Total (Worst Case) 5.79 0.95

Total (RSS) 3.90 0.57

Note in this example (far from worst case) that the RSS (root

sum square) errors for conventional transmitters are seven times

those for the analog ST 3000.

DIGITAL VS ANALO G TRANSMITTER PERFOR MANCE

Significant additional performance improvements can be realized

when operating a smart transmitter in the digital communication

mode rather than the analog (4-2OmA) mode. Since all smart

transmitters utilize digital electronics, hey must pass their digital

signal through a D/A converter to transmit the 4-2OmA analog

signal. This D/A converter is a major source of error and it is

by-passed when communicating in the digital mode.

Table IV is an expansion of Table

111

with a column added to

show the relative performance of ST 3000 in the digi tal mode as

well. When reading at 50% of span, the digital performance (for

the specified conditions) is twice s good as performance of

ST 3000 in the analog mode and 14 times as good as a

conventional transmitter.

TABLE IV

PERFORMANCE OF DIGITAL VS

ANALOG TRANSMITTERS

Conditions: Span at

200

inH20;Reading at

100

inH,O

Error in Reading)

Conven- ST3000 ST3000

tional Analog

Digital

Reference Accuracy 0.40 0.20 0.15

Ambient Temperature

3.20

0.35 0.13

Influence (50'F)

Static Pressure Influence 2.19

0.40

0.20

(1K psi)

Total (Worst Case) 5.79 0.95 0.48

Total (RSS) 3.90 0.57 0.28

However, this does not demonstrate the major performance

advantage of ST 3000 in the digital mode. Since analog

transmitters specify performance errors in terms of

Vo

of Span, it

is important to set the span or upper range value of an analog

transmitter as low

s

possible. But when ST 3000 is operated in

the digi tal mode, it becomes a Yo of Reading device similar to

magnetic flow meters, positive displacement meters and most

precision laboratory instruments.

For this reason, a digital ST 3000 can be spanned at its upper

range limit and left there regardless of the range of

measurements o be made. The impact of this on performance is

shown by Table V where, under similar operating conditions and

reading at 2.5% of span, a digital ST 3000 is 10 times

s

accurate

as an analog ST 3000 and 70 times as accurate as a conventional

transmitter.

TABLE V

PERFORMANCE OF DIGITAL VS ANALOG TRANSMITTERS AT A FIXED SPAN

Conditions: Span at 400 inH20;Reading at 400 100

O

inH20

Error in Yo Reading)

Reading at 400 inH,O Reading at 100 inH,O Reading at 10 inH,O

Conven- ST3000 ST3000 Conven-

ST3000 ST3000

Conven- ST3000

ST3000

tional

Analog Digital tional Analog

Digital

tional Analog

Digital

Reference Accuracy 0.20 0.10 0.075 0.80 0.40 0.15 8.00 4.00 0.30

Ambient Temperature (Influence 1.20 0.175 0.125

4.80

0.70 0.125 48.00 7.00 0.33

(50 F)

Static Pressure Influence (1K psi) 1.48 0.20 0.20 5.92 0.80 0.20 59.20 8.00 1.00

Total (Worst Case) 2.68 0.475 0.40 11.52 1.90 0.475 115.20 19.00 1.63

Total (RSS) 1.92 0.28 0.25 7.66 1.10 0.28 76.60 11.36 1.09

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This effect is shown graphically n Fig.

1

which compares

Reference Accuracy for both a conventional transmitter and a

typical analog smart transmitter with ST 3000 operating in the

digital mode. With all three transmitters spanned at

400

nH,O

and reading at

25

inHIO, the Reference Accuracy errors for

conventional, analog smart, and digitalST 3000 transmitters are

3.2, 1.6

and

0.15 of

Reading respectively.

Compar ison

Of

Reading Error

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0

\

\

\

\

\

\

0.075 Span

or

0.15Oh Reading

wh icheve r i s

smaller

I

I I I I

I

I

25

5

100 200

3CO

4

Reading (Inches of Water)

Fig.

1.

Comparison of reading error

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