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

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Application Note

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

Troubleshooting and the

need for speed

Instrument and automation

technicians are constantly chal-

lenged 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

Better ways to

troubleshoot automation

and process control loops

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.

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

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 cur-

rent 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 per-

forms as requested, the problem

lies with the transmitter, if not,

it is elsewhere.

If a final control element

(valve positioner, etc.) is sus-

pected, use the mA source/

simulate mode on the Fluke

772/3 to feed a signal into it

while watching the local indica-

tor 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 usu-

ally 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 mea-

sure 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 measure-

ment 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 pro-

duces 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.

Field checks and plant

commissioning

Start by using a clamp-on loop

current meter like the Fluke 771

to check each loop for current

in a matter of seconds, without

disconnecting anything. If a

loop is not working, a multifunc-

tion clamp meter can also make

quick work of diagnostics. If cur-

rent is not present on some loops

go on to classic troubleshooting:

check the wiring, the power

supply, and the control system’s

I/O cards (by using the meter to

inject a signal into the I/O, then

contacting the operator to ask

what he sees. If the operator

agrees with what is being sent,

then there may be something

amiss with the transmitter—

either the transmitter itself or,

if this is a new installation,

perhaps miswiring, the sensor’s

input to the transmitter.

Checking DCS and PLC

I/O cards

The mA process clamp meter

can be used as an accurate

signal source to check the oper-

ation 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 mainte-

nance programs. Accounting for

manufacturer-specific instruc-

tions, perform quick operational

checks using the Fluke 772/3 as

a signal source while observ-

ing the valve stem position,

mechanical position indica-

tors, or flow indicators as input

changes are made.

Mitch Stewart, Field Service

Manager, L2 Systems, tells of

using the 4-20 sourcing output

of a mA process clamp meter to

drive a control valve open and

closed when the process output

from the PLC wasn’t working.

“We disconnected the PLC’s

output at the control valve and

connected the [meter] up to the

control valve and ran it open

and closed to verify that the I/P

on the valve worked correctly,”

he explains.

The general method is to

set the meter to the 4-20 mA

source/simulate mode and

connect it to the input termi-

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

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

Fluke Corporation

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©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.

Fluke. Keeping your world

up and running.®

increments between 4.0 mA

and ~3.9 mA, while feeling the

valve stem with your free hand

to check for any sign of move-

ment. 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 simi-

larly, 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 volt-

age (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 accura-

cies of 0.2 %, and can be used

for quick calibration checks,

while cutting down on the

number of instruments needed.

For example, checking a pro-

cess transmitter on the bench

normally requires (aside from

a pump and separate pressure

standard) a loop power supply

and an instrument for read-

ing the transmitter’s 4-20 mA

output. But with today’s mA pro-

cess clamp meters it’s possible to

both power the transmitter and

read the output. “This tiny little

thing,” says Paul Jusak, Main-

tenance 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 instrumenta-

tion 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 min-

utes 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.”