SMART vs Conventional
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Transcript of SMART vs Conventional
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SMART TRANSMITTERS
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DIGITAL VS ANALOG
SMART TRANSMITTERS
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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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