Fluke 28 Ii Application Note Better Ways To Troubleshoot Automation And Process Control Loops

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Better ways to
troubleshoot automation
and process control loops
Application Note

Troubleshooting and the
need for speed
Instrument and automation
technicians are constantly challenged to keep instrumentation
loops and I/O working at peak
efficiency while using the least
possible time to do it.
When Fluke first released
the 771 mA Clamp Meter in

2007, technicians found that
measuring loop current without
breaking the circuit saved a
great deal of time. Now, the new
772 and 773 models can save
even more time.
By incorporating the functions
of a loop calibrator, these
more advanced tools allow
technicians to troubleshoot on
the spot.

Tracing control loop
problems
Often the first indication of
a control loop problem comes
from the operator: “I think we
have a bad valve” or “this loop
isn’t responding the way it
used to.” In either case, it’s the
technician’s signal to begin
troubleshooting.

From the Fluke Digital Library @ www.fluke.com/library

The first step is to measure
the 4-20 mA signal, either by
breaking the loop connecting in
series with a DMM, or by using
a mA clamp meter like the Fluke
771 and verifying the loop current value. If the loop current
measured is not as expected,
there are three likely causes:
broken/disconnected/shorted
wires, a bad loop power supply,
or faulty instrumentation.
If no problem is found in the
wires, use a DMM (or the 773
clamp meter) to check the loop
power supply. If the power
supply shows no output, use the
24 V loop power function of the
meter to substitute for it; if the
loop then works properly the
source of the problem is obvious.
If the wiring and the power
supply both check out, it’s time
to check the transmitter. If you
have a loop calibrator, process
calibrator or multi-function
clamp-on meter, use its mA
simulate mode to substitute for
the transmitter. If the loop performs as requested, the problem
lies with the transmitter, if not,
it is elsewhere.
If a final control element
(valve positioner, etc.) is suspected, use the mA source/
simulate mode on the Fluke
772/3 to feed a signal into it
while watching the local indicator for a response.

Loop malfunctions
If the problem is not a dead loop
but an inaccurate one, likely
possibilities include a bad I/O
card on the PLC or DCS, or a bad
final control element (I/P on a
valve positioner, etc.). It’s usually best to start by doing a field
check of the transmitter, local or
remote indicator or final control
element.
For a final control element,
use a clamp-on meter to measure loop current and compare
the value to the local position
indicator on the valve or other
final control element. Relay that
information to the operator to
verify findings.

In the case of a measurement loop, use the clamp meter
to measure loop current, then
check with the operator to see
how well the value indicated on
the control panel agrees with
the actual loop current. This will
give a quick check on the PLC or
DCS I/O card that handles that
particular loop. It’s also possible
to use the meter’s mA source/
simulate mode to send a known
signal to the control room; as
before, compare the value as
read by the operator to the
actual current in the loop.
Some loops show random
fluctuations or intermittent faults
that tend not to happen while
a technician is watching. The
solution here is to use a clamp
meter with a scaled mA output.
In this mode the meter measures
the current in the loop without
breaking the circuit, and produces an identical and isolated
mA output. Feed that output to
a DMM with a logging function;
by allowing the DMM to record
over time, any disturbance will
be recorded.

Checking DCS and PLC
I/O cards
The mA process clamp meter
can be used as an accurate
signal source to check the operation of input/output cards on
programmable logic controllers
(PLCs) and distributed process
control systems (DCSs). For
4-20 mA input cards, disconnect
the process loop and use the
meter’s mA source mode to feed
in a known signal value (4.0 mA
for zero, 12 mA for 50 %—using
the meter’s 25 % step function,
and 20.0 mA for 100 %) and
compare it to the value shown
on the operator’s readout.
Voltage input cards (1 V to 5 V
or 0 V to 10 V) are checked in
a similar way, using the meter’s
voltage source function.

Checking a valve
positioner

Milliamp clamp meters can be
used for periodic in-field checks
of electronic valve positioners
as part of preventive maintenance programs. Accounting for
manufacturer-specific instrucField checks and plant
tions, perform quick operational
commissioning
checks using the Fluke 772/3 as
a signal source while observStart by using a clamp-on loop
current meter like the Fluke 771 ing the valve stem position,
mechanical position indicato check each loop for current
tors, or flow indicators as input
in a matter of seconds, without
changes are made.
disconnecting anything. If a
Mitch Stewart, Field Service
loop is not working, a multifunction clamp meter can also make Manager, L2 Systems, tells of
quick work of diagnostics. If cur- using the 4-20 sourcing output
rent is not present on some loops of a mA process clamp meter to
go on to classic troubleshooting: drive a control valve open and
closed when the process output
check the wiring, the power
from the PLC wasn’t working.
supply, and the control system’s
“We disconnected the PLC’s
I/O cards (by using the meter to
output at the control valve and
inject a signal into the I/O, then
connected the [meter] up to the
contacting the operator to ask
control valve and ran it open
what he sees. If the operator
and closed to verify that the I/P
agrees with what is being sent,
on the valve worked correctly,”
then there may be something
he explains.
amiss with the transmitter—
The general method is to
either the transmitter itself or,
set
the meter to the 4-20 mA
if this is a new installation,
source/simulate
mode and
perhaps miswiring, the sensor’s
connect it to the input termiinput to the transmitter.
nals of the valve positioner. Set
the meter to output 4 mA and
wait for the positioner to settle;
then vary the current in small

2 Fluke Corporation Better ways to troubleshoot automation and process control loops

increments between 4.0 mA
and ~3.9 mA, while feeling the
valve stem with your free hand
to check for any sign of movement. Adjust for zero movement
between these two current
settings by using the zero
adjustment on the positioner.
Next increase and decrease
current from 4 mA to ~4.1 mA.
Insure that the valve stem just
begins movement above the
~4.1 mA setting and fully closed
at 4 mA.
Span can be checked similarly, by setting the meter at
20 mA, ~19.9 mA and ~20.1 mA,
and linearity can be checked
by using the meter’s 25 % step
function.

Checking loop isolators
To check a loop isolator, apply
a mA input signal to the device
and measure its 4-20 mA output
using the clamp-on current
measuring function. This two
channel simultaneous source/
measure function in the 773
can also be used for valves
that report their position using
4-20 mA.

Checking VFDs
Variable frequency drives (VFDs)
are used to power motors,
blowers and fans in process
applications as well as conveyor
systems and machine tools.
Control inputs are generally voltage (1 V to 5 V or 0 V to 10 V) or
current (4 mA to 20 mA). A mA
process clamp meter can feed
in a signal to simulate a normal
input while the technician
observes the result.

Quick calibration
While not classified as loop
calibrators, today’s mA process
clamp meters boast accuracies of 0.2 %, and can be used
for quick calibration checks,
while cutting down on the
number of instruments needed.
For example, checking a process transmitter on the bench
normally requires (aside from
a pump and separate pressure
standard) a loop power supply
and an instrument for reading the transmitter’s 4-20 mA
output. But with today’s mA process clamp meters it’s possible to

both power the transmitter and
read the output. “This tiny little
thing,” says Paul Jusak, Maintenance Engineer, Puget Sound
Energy, “allows you to do that
function without having to drag
out a separate power supply.”

Summary
Today’s mA process clamp
meters can save instrumentation and automation technicians
a great deal of time in trouble
shooting, because they can
replace a number of separate
instruments. The technician no
longer has to spend 15 minutes going back to the shop to
get an instrument, because the
one instrument he takes with
him will do all the necessary
functions. And, adds Jusak,
“instead of having two tools in
your pouch you now have one
tool in your pouch for doing all
4-20 mA loop calibrations and
troubleshooting. That to me is
pretty doggone convenient.”

Fluke.	 Keeping your world
	
up and running.®
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
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Web access: http://www.fluke.com
©2009 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 6/2009 3497464A A-EN-N
Modification of this document is not permitted
without written permission from Fluke Corporation.

3 Fluke Corporation Better ways to troubleshoot automation and process control loops



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