CD1609-Troubleshooting-420-mA.pdf

SPECIAL REPORT

HOW TO TROUBLESHOOT AND CALIBRATE

www.controldesign.com

How to troubleshoot and calibrate 4-20 mA Best practices for troubleshooting analog circuits to the PLC By Mike Bacidore, editor in chief

A

Control Design reader writes: We have several temperature, pressure and flow

sensors on a new medical-device cleaning skid that we are developing. These instruments are connected to a PLC as 4-20 mA inputs, and there is also a 4-20 mA

output used to control a pump motor speed. A recent failure of a flow sensor brought the process skid instrumentation to my company’s quality manager’s attention. He asked how we know that the temperatures, pressure and flow are accurate, and how do we know that we are cleaning properly. I’ve been tasked to write a procedure for troubleshooting, calibrating and testing the 4-20 mA instruments on the skid. I can probably stumble through this, but what are some best practices for troubleshooting the analog circuits to the PLC; how do I calibrate the instruments; and how should I periodically test the devices? Should I add programming to the PLC and HMI to simplify these functions?

How to troubleshoot and calibrate 4-20 mA

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ANSWERS How do I calibrate the instruments; and

BEST PRACTICES, CALIBRATION

how should I periodically test the devices?

AND PROGRAMMING

Instrument calibration should be performed

What are some best practices for trouble-

by a qualified instrument technician. If you

shooting the analog circuits to the PLC?

choose to self-perform, it may be best to

Once the skid has completed startup and

shadow a third-party technician until you

commissioning, there is a limited amount

have purchased the necessary calibration

of troubleshooting to maintain the system.

equipment and learned the skills needed to

If a short in the wiring occurs in the analog

do the calibrations. Calibration serves as the

wires, you will likely get a blown fuse indica-

testing of the devices, and the test or cali-

tion. If an open occurs in the analog wires,

bration interval is determined by the level of

you likely have a loose termination and

accuracy required for your application. Ob-

this will be found with continuity checks.

viously the higher the required accuracy is,

For failures in the transmitter, the indica-

the more often calibration will be required.

tion will either be an overdriven signal (>20

Typically annual calibrations are sufficient;

mAdc, for example, 22 mAdc) or a signal

however, this will need to be determined

significantly below zero, such as 3.5 mAdc.

from experience with this specific equip-

The direction the transmitter drives to in

ment and application.

a failure mode will depend on whether the transmitter is set up to provide an upscale

Should I add programming to the PLC and

or downscale burnout (upscale 22 mAdc

HMI to simplify these functions? No pro-

vs. downscale 3.5 mAdc). The determina-

gramming is necessary or helpful in simpli-

tion of which failure mode to use—upscale

fying the calibration work; however, good

or downscale burnout—depends on the use

alarming is always helpful in troubleshoot-

of the transmitter. This selection is made

ing and maintaining a control system. While

based on making the loop failsafe. For ex-

adequate alarming is important, also con-

ample, if it is a temperature transmitter and

sider moderation when applying alarms. If

it is used for a high-temperature interlock,

you flood the HMI with too many alarms in

we would select upscale burnout. In con-

an attempt to build in troubleshooting intel-

trast, assume a flow transmitter for control-

ligence, you will overwhelm the equipment

ling the cooling for motor bearings. If we

operator. Also, excessive alarms can quickly

have a flow transmitter failure, we may want

become a nuisance, so they begin to be ig-

to apply full cooling. If so, we would prefer

nored. A flood of redundant alarms can also

a downscale burnout so that we respond by

distract from a pertinent or critical alarm

applying full flow.

that may be missed as a result of overuse.


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In addition to a proper calibration inter-

rity, like making sure there isn’t a VFD nearby

val, quality checks of the signals can aid in

that would throw out electromagnetic inter-

detecting a deteriorating transmitter. For

ference that would distort signal.

example, as a transmitter begins to lose its accuracy, the calibrated span typically

Calibration of the instruments are fairly

shifts above or below zero (4 mAdc). For

straightforward when using standard cali-

example, in a flow transmitter, the 16 mAdc

bration tools, but will vary between instru-

span (4–20 mAdc) may still be accurate

ment manufacturers.

but, with zero flow through the sensing element, the transmitter may be sending a

Given that 4-20 mA instruments aren’t

signal of 4.3 mAdc. This means that a 20.3

smart—able to diagnose themselves or alert

mAdc output may represent full flow in-

operations to issues—I would think there is

stead of 20 mAdc. To detect this condition

definitely periodic testing that should occur.

you may consider doing off-state verifica-

In today’s day and age, I would be recom-

tion. In short, by monitoring the flow signal

mending to this individual that he consider

when the pump is not running, you will

a digital solution that provides a lot more

catch an elevated zero as an indication of

robust information about the health of the

a need for calibration. The same technique

skid. HART is a simple upgrade to an exist-

could be used to indicate an improperly

ing 4-20 mA skid and provides familiarity.

calibrated pressure transmitter. A compari-

If the individual wants to make a drastic

son of pump speed to flow and pressure is

step change and go fully digital with some

another means of qualifying the accuracy

increased horsepower, he should consider

of these transmitters. It will only detect a

Foundation Fieldbus (FF). FF actually makes

gross error, but this may be useful data

for fantastic solutions in skid mounts be-

depending on the application.

cause everything can be completed at the factory level and then, when delivered on

— Tim Green, director of field services, Maverick Technologies, www.mavtechglobal.com Columbia, Illinois

site, it’s as simple as landing a trunk line to the host system and you can rock and roll. — Talon Petty, marketing and business development

LET’S GET DIGITAL

manager, FieldComm Group, www.fieldcommgroup.org

This gentleman indicates he’s using purely 4-20 mA instrumentation on his system.

CALIBRATE INPUT/OUTPUT

There are no “smarts” in analog. If he’s using

First, the calibration and verification of cali-

HART on his 4-20 mA instruments, then there

bration depends on the type of sensor used.

is hope. Without it, then he’s only going to be

I will include general calibration procedure

able to verify calibration and segment integ-

at the end. There are several things that can


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be done in the design stage to increase the reliability of a system using analog sensors.

3. Verify that the signals are changing with respect to changes in the process that affects that signal.

The use of 0-20 mA analog cards instead

4. If the normal response time for a certain

of 4-20 mA analog cards will allow the

output to be registered as an anticipated

program to detect a signal less than 4 mA,

input can be determined, then this can be

which could mean a sensor is bad or the

checked on a frequent basis to determine

signal cable has a break in it. Without this

if the system response is within expecta-

check, the PLC would just assume the ana-

tions. If it is not, then the system may have

log signal is at its lowest state.

to be checked and possibly re-calibrated. 5. As part of the maintenance procedure,

Another design method can be to use

the gauges mentioned above could be

redundant sensors—one could be assigned

checked on a regular basis.

as the primary signal, and the logic could be programmed such that if one of these

Calibration of the loops is required both at

signals varies by x%, then an error message

commissioning and also when an issue has

is sent and the program could either shut

been detected by the methods indicated

down or choose which sensor to use and

above. The loops should be verified both at

call for maintenance to resolve.

the low (zero or 4 mA) end, and then the span is calibrated to obtain the correct set-

In general, there should always be a me-

ting for the high (100% or 20 mA) end.

chanical gauge on the equipment that would allow an operator or maintenance

For input devices: The analog system is best

person to make sure that the measured

tested from the source to the PLC. Devices

signal on the HMI matches the mechanical

can be used to inject generated signals into

gauge within reason.

the PLC, but this does not test the sensor. 1. The input sensors have a procedure to set

Within the program, several checks can be made to ensure that the sensors are work-

the sensor to provide a zero (4 mA) signal. 2. By connecting a calibrated meter with

ing as required.

the current loop, the actual current flow-

1. Verify that the signal is at its resting state,

ing can be read.

(4 mA or 20 mA, depending on its function) when the system is turned off. 2. Verify that the signal changes when the part of the system related to this device is started up.

3. The PLC should have the ability in setup mode to consider this reading to be the zero point, even if it is not quite accurate. 4. The sensor should be set to provide the high (20 mA) signal or the span.


5


5. Again, the meter should verify this is producing close to 20 mA. 6. The PLC should then be set to read this value to the high setting. 7. Note that the new low setting and the new high setting will be the minimum and maximum set points of the scaling of the loop. 8. Further verification can be made by applying a known value to the sensor and

the output will increase linearly from this zero position as the signal increases. If the zero point is determined to be 4.2 mA, then the setup routine should record this value as the zero point, or the output device should be adjusted. 4. In this case, the PLC setup should allow the output loop to generate a high (20 mA) signal or the span. 5. A calibrated meter should be connected

verifying that the PLC reads the same

into the current loop so the actual current

value. For instance, if the device is mea-

flowing can be read.

suring pressure and its range is 0-100

6. Once the high signal is generated, the

psi, then the sensor could be attached

output of the controlled device needs

to a pressure source with a calibrated

to be determined. Furthermore, the out-

source. If 0 psi and 100 psi are applied,

put device needs to be manipulated by

then the PLC should read the same val-

changing the output loop to make sure

ues. This additional calibration should

the output will decrease linearly from

also be conducted after the meter used

this max position as the signal decreas-

for calibration has been taken out of the

es. If the span point is determined to be

circuit to make sure the circuit has been

19.5 mA, then the setup routine should

reconnected correctly.

record this value as the span point or the output device should be adjusted.

For output devices: Again, the whole loop

7. Note that the new low setting and the

should be tested at the same time.

new high setting will be the minimum

1. In this case the PLC setup should allow

and maximum set points of the scaling

the output loop to generate a low (4 mA) signal. 2. A calibrated meter should be connected

of the loop. — Kevin J Pitts, automation application engineer, Eaton, www.eaton.com

into the current loop so the actual current flowing can be read. 3. Once the low signal is generated, the

4 QUESTIONS How do we know that we are cleaning

output of the controlled device needs to

properly?

be determined. Furthermore, the out-

1. There are specific cleaning requirements

put device needs to be manipulated by

that you can research through the U.S.

changing the output loop to make sure

Food & Drug Administration. Piping,


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installation of instruments and cleaning methods all need to be researched. 2. Some of these methods, such as cleanin-place (CIP) or remove and clean, will depend on his application. 3. There are also specific chemicals to use, as well as a time limit for cleaning. 4. Lastly, the way the instruments are in-

1. Each Instrument has an accuracy statement and depending on how you calibrate and test will determine how accurate your instruments really are. 2. This is where you need a good instrument service company to take a role in your maintenance and calibration process. 3. Look to partner with a service company

stalled, the actual materials used in the

that is ISO and ISA quality-compliant and

process and whether they will affect the

certified.

instruments all come into play as to how you would clean the system.

4. Your relationship with that service organization will increase your confidence in the accuracy of your instruments.

How do we troubleshoot, calibrate and test the instruments?

— Antonio J. Biondo, northeast regional service account manager, measurement & analytics, ABB, www.abb.com

1. The instrument manufacturer should have a documented procedure for testing and

LOOK FOR LINEARITY

calibrating each instrument.

First and foremost, the questions you are

2. There are specific pieces of test equipment used for each device in-place. 3. The International Society of Automation

asking about are handled differently between companies and even between lines with different requirements. So, the short

(ISA) has a number of good manuals that

answer is there is no real “standard” to go

cover calibration, testing, startup and

by. In our experience, calibration of analog

troubleshooting.

equipment should occur every year, but it may be more frequent depending on how

How do I test the PLC?

accurate the signal is required to be. As for

1. PLCs normally do not need calibration

the procedure to troubleshoot the equip-

after the initial programming and testing.

ment, it may closely resemble the calibra-

My experience is that they are pass/fail.

tion procedure, or at a minimum include

By that, I mean the input is either good or

components of the calibration process. If

it is bad. The PLC should have the capa-

possible, consider side-by-side testing for

bility to differentiate between the two

equipment calibration or better yet, NIST

conditions. The same goes for outputs.

traceable paths. Using NIST traceable tests may help to confirm the display readings

How do we know that the instruments are

and that the equipment is accurately cali-

accurate?

brated. Realistically, there should be a linear


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relationship between the required signal

If the emissivity and temperature of the

accuracy and how often the equipment is

measurement target are unknown, use the

maintained, or calibrated.

optional black body tape to take simplified

— Jeff Thornton, product manager—automation,

measurements.

Red Lion Controls, www.redlion.net

1. Stick the black body tape to the measurement target, heat it to a suitable tem-

BETTER TO PREDICT AND PREVENT

perature (+20 °C (+68 °F) or more above

I cannot offer much on best practices,

room temperature), and maintain that

troubleshooting and calibration procedures

temperature as best as possible.

since these are all post-mortem steps after an incident occurs. I am suggesting having the foresight to see these events occur

2. Set the emissivity to 0.95 (emissivity of black body tape). 3. Measure the temperature of the portion

before a downtime or quality event causes

of the target covered in black body tape

action to be taken. Predictive analytics and

and take note of the measured value.

applying machine-learning agents detect

4. Remove the black body tape and measure

early onset anomalies that can prevent

the temperature of the target on an area

such events. The need for developing good

that is close to the area measured in 3.

troubleshooting and calibration procedures

5. Choose “set by temperature” and enter

will always be standard practice, but the

the temperature measured in 3 to set the

value to the client to predict and apply pro-

emissivity.

active procedures to see a problem develop surpasses any traditional methods. — Aldo Ferrante, president and CEO, ITG Technologies, www.itgtec.com, CSIA (www.controlsys.org) member, Jacksonville, Florida

6. The emissivity of the measurement target is set. In terms of frequency of monitoring, the more often the better, but monthly is typically adequate, depending on how dirty the

BLACK BODY TAPE

process is. These checks can be done in a

This is directly from our instruction manual

manual or an automated fashion, but I’ve

for our temperature sensor lineup. The

seen HMI buttons for calibration that would

black body tape will have a fixed emissivity,

require setting master parts in the fixtures.

so, as long as you can adjust the emissivity

Doing it manually will take time, but hav-

of your sensor, this will work fine. If it isn’t

ing a constant-temperature-rated item with

adjustable, likely the sensor will be set at a

the black body tape applied can be a quick

.95 emissivity. At this point, take a target

check any operator can perform during

with a known temperature and apply the

changeover. Based on the complexity of the

black body tape to it and follow the se-

PLC programming, making sure it is aware

quence below.

of a calibration will be important if the cali-


8


brated black body item is not going to be in

instrument may be in error, or your device

the exact temperature range.

(PT, TT, FT, or valve) may not be receiving

— Michael Gorsky, sales specialist,

voltage (check analog output point, check

Keyence, www.keyence.com

fuses, check wiring).

TROUBLESHOOT, CALIBRATE,

Calibration instructions: Calibration of these

TEST AND PROGRAM

devices will determine if they are accurately

I manage the instrumentation service group

reading the process variable (flow, pressure

for Interstates. I have techs based in the

or temperature), as well as verify that they

Midwest that troubleshoot and calibrate

are accurately outputting the mA represent-

instrumentation on a daily basis. I ran across

ing the measured variable. Special equip-

your questions and wanted to offer my ad-

ment is needed for each type of device: a

vice regarding your request. I will touch on

hand pump and pressure gauge for the PT;

each of the following areas below regarding

a temperature calibrator (hot block) for the

your medical device cleaning skid, including

TT; and a flow calibration skid for the FT.

pressure, temperature and flow devices, as well as a control valve and a PLC.

Finally, a mA meter will be needed for measuring the output. It is recommended that

Troubleshooting: Each of your devices

all of this special equipment is certified (for

should be sending a 4-20 mA single to the

example, NIST traceable) to ensure it is ac-

PLC, and the PLC should be sending a 4-20

curate enough to serve as the reference for

mA to your control valve. I recommend the

the calibration.

following if issues occur: If, after the calibration testing, it is determined First start by checking the signal to/from

that any of the devices are outside of the pre-

each device. A good, non-intrusive method

determined accuracy specs, adjustment can

of checking for this signal would be to use a

be done to the device to bring it into toler-

clamp-on dc Amp meter, such as the Fluke

ance. This is typically carried out by operating

773. This device allows for measuring the

the user interface on the device (buttons and

current, without having to lift any wires.

display, if equipped), or by communicating to the device via HART communication, such as

If your loop current is above 20 mA, you

HART 475 handheld device.

likely have an error in the device or a short in the wiring.

PLC testing and troubleshooting: The instructions in the above section serve as a

If your loop current is below 4 mA , your

good method for periodically testing the de-


9


vice. Typically, we perform the above checks

does, the technology exists for commu-

for our clients’ instruments annually at the

nicating, configuring and troubleshooting

very least. If any of the devices are critical to

each of these devices without being on-

the process or to the safety of their opera-

site. This is done by using HART-enabled

tors, we typically test them once per quarter.

analog input/output cards, as well as connecting the skid to the client’s network

Testing of the PLC: This is most easily done

via Ethernet. To access the instruments

by simulating a 4-20 mA signal into the

remotely, all that would be needed is a

inputs or the PLC (directly at the card) or

VPN connection to the client’s network, as

measure the output of the PLC. If the PLC/

well as some special software to commu-

HMI is properly displaying the simulated

nicate with the devices. Interstates com-

valve and outputting the measured value, it

monly uses this technology to offer clients

is typically fair to assume that your PLC is

troubleshooting advice, or even fix instru-

properly functioning.

ment issues without being on-site. — Adam Dittbenner, PE, instrumentation service man-

Add programming to PLC? Some additional programming to the PLC that could aid in troubleshooting and/or calibration would

ager, Interstates Construction Services, http://www.interstates.com/ CSIA, www.controlsys.org certified systems integrator

be the following:

TROUBLESHOOT THE LOOPS • Interlock bypassing: If there are any

The temperature, pressure and flow sen-

control loops on the skid, it would be

sors on the skid ar e most likely connected

beneficial to add a bypass button to the

to 4-20 mA process transmitters. These

HMI that could be used during testing. For

transmitters will accept the temperature

example, if the control valve is modulated

sensor, flow sensor or pressure sensor

off of the signal received by one of the

as input and in turn produce a 4-20 mA

devices (pressure, temperature or flow), it

output signal, which is connected to the

would swing open or closed if the output

PLC analog input channel. At some point,

of any of the devices is being simulated.

a linear scaling would need to have been

A bypass button would allow for stopping

implemented in the PLC software for each

the valve from moving, while simulating

4-20 mA loop in order to get the process

the output of any of the devices.

readings in engineering units (°F or °C for

• Remote access to skid for monitoring and

temperature, gpm for flow, psi for pres-

troubleshooting: Each of the instruments

sure). For example, if the pressure sensor

and valves likely has HART communica-

measures 0-30 psi and the transmitter

tion if it is an analog device. Assuming it

produces 4-20 mA output, then a signal


10


of 4 mA is equal to 0 psi, and a signal

CALIBRATION

of 20 mA is equal to 30 psi with a linear

Periodic calibration is a vital procedure to

relationship (y = mx + b) in between these

maintain seamless operation and optimize

points. Once this type of relationship is

uptime for nearly all machines. But at what

programmed into the PLC for each chan-

interval should you perform these peri-

nel, then assuming that everything else in

odic maintenance operations? The answer

the loops is working properly the readings

depends entirely on your individual opera-

should be correct. If the process readings

tion, how the environment changes and

(temperature, pressure, flow) appear to

how susceptible the instrumentation is to

be incorrect, then there could be several

the change in environment. Periodic calibra-

reasons why. There could be an issue

tion ensures that measurements are within

with the 4-20 mA loop wiring (broken or

prescribed tolerance limits.

loose wire), loss of power supply (most 4-20 mA loops require 24 Vdc power) or

Typically, calibrations are performed on

a defective transmitter. To tr oubleshoot

an annual basis, but that could change

the loops, make a visual inspection of the

based on the system requirement. Measur-

wiring and confirm that 24 Vdc power is

ing environmental factors, such as shock,

in fact being provided by the loop power

vibration and temperature, as well as opera-

supply by testing with a digital voltmeter.

tional figures within the chosen tolerance,

If the wiring and power supply are veri-

will help to build an historical basis upon

fied, then the transmitter can be checked

which necessary calibration intervals can be

by disconnecting the 4-20 mA output

determined. A robust accounting of system

signal from the PLC analog input and sub-

health and operating figures will enable

stituting a 4-20 mA input from a process

maintenance personnel to address possible

loop calibrator. With this type of calibra-

issues before they escalate into inefficient

tor, a current value anywhere between

operation or worse, downtime.

4 mA and 20 mA can be simulated to verify that the PLC analog input chan-

EtherCAT diagnostics are a standard feature

nel is reading correctly. If the PLC analog

set in the EtherCAT fieldbus. This establishes

input channel reads correctly with the

built-in topology recognition down to the in-

calibrator input but not when the actual

dividual I/O terminals. EtherCAT system veri-

process transmitter output is connected,

fication cannot only take place during start-

then most likely the transmitter needs to

up; automatic read-in of the network is also

be replaced.

possible (configuration upload). Bit faults

— Anthony Corvini, product manager,

during data transfer are reliably detected

Omega Engineering, www.omega.com

through evaluation of the CRC checksum in


11


each EtherCAT device. Apart from breaking-

Contact the manufacturer of whichever PLC

point detection and localization, the pro-

that is being used and ask that question.

tocol, transfer physics and topology of the

They will most likely say that no calibration is

EtherCAT system enable quality monitoring

needed. They also publish analog input accu-

of each individual transmission segment.

racy and it may even vary with temperature,

The automatic evaluation of the associated

but again there are is nothing to adjust.

error counters enables precise localization of

— Jim Rosner, lead industrial application engineer, con-

critical network sections. Gradual or chang-

trols and power conversion division, Eaton, www.eaton.com/electrical

ing sources of error such as EMC influences, defective connectors or cable damage are

TWO-POINT SHIFT

detected and located.

Verifying analog sensor readings are accurate can improve product quality and

This diagnostic functionality provides an

increase machine uptime. The use of this

excellent means to monitor system health,

maintenance and troubleshooting proce-

particularly by leveraging the speed of the

dure for a system containing a transmit-

EtherCAT industrial Ethernet system and

ter with a 4-20 mA analog signal, receiver

the data management and analytics capa-

(PLC) and possibly a loop power supply can

bilities of IoT solutions. System and pro-

show you how to optimize your machine.

duction data can be accessed in real-time, anywhere in the world, enabling analysis by

Measure transmitter signal: The technician

decision-makers and/or engineers who may

can measure the transmitter by removing

or may not be on-site.

the wires to the PLC and checking the wires

— Andy Garrido and Sree Potluri, I/O specialists, Beck-

from the transducer with a current meter

hoff Automation, www.beckhoff.com

to see if the device is producing 4-20 mA. A signal present means the PLC input is

NO CALIBRATION NECESSARY

bad and should be replaced. If the signal is

PLC analog inputs should not need calibra-

not present, check the transmitter and loop

tion. As a matter of fact, how would you do

power if present.

it when the analog modules have no adjustments and are all solid-state components/

Transmitter loop power: Some transmit-

microprocessor chips? Calibrating the

ters produce their own internal loop power,

transducers of the devices connected to the

while other transmitters require an external

analog inputs should probably be done ev-

loop power supply (Figure 1).

ery six months to a year, but there’s nothing to calibrate/adjust on a PLC analog module

If the system uses an external loop power

to my knowledge.

supply, a digital multimeter (DMM) can be


12


EXTERNAL LOOP POWER

INTERNAL LOOP POWER

XMITER

XMITER

+

+ –

PWR

–

PWR

+

4 to 20mA

+

+ –

–

– PLC

PLC

+

+

– –

LOOP POWER Figure 1: Some transmitters produce their own internal loop power, while other transmitters require an external loop power supply.

used to measure the power supply to see if

Signal accuracy: In some cases, the accura-

it is producing at the correct voltage. If this

cy of the process signal needs to be tested.

is faulty, the power supply needs to be re-

It is recommended to use a high-quality

placed. If the power supply checks out, then

process meter that is calibrated to calibrate

most likely the transducer is bad.

the system. With most transducers, the manufacture will supply the scaling for the

If the device has an internal loop power, a

device in the data sheet. The information

DMM set in current mode can be used to

provides the scale the device will produce

measure the signal at the transmitter. If the

based on the 4-20 mA. For example, a tem-

signal is not present and the power sup-

perature device may produce a -50 °C to

ply powering the device is good, then the

200 °C based on the 4-20 mA signal. Know-

transducer is bad. If the signal is present,

ing this ratio allows the technician to supply

the same test can be done at the PLC input

a 4-20 mA current into the system to see if

with the system’s wiring removed from

the readout is correct.

the PLC input and the meter attached to the transmitter wiring. If the signal is not

Other times the system needs to be cali-

present at the PLC, the wiring needs to be

brated to the application. This requires an

checked for loose or broken connections.

external device to measure the amount of


13


Temperature indicated by Controller After offset Upper limit of set range, YH (e.g., 260 C)

Upper-limit temperature Input Before offset

Indicated temp. after offset, X2 (e.g., 110 C) Indicated temp. before offset, Y2 (e.g., 105 C)

Indicated temp. before offset, Y1 (e.g., 40 C) Indicated temp. after offset, X1 (e.g., 25 C) Lower-limit temperature Input Lower limit of set range, YL (e.g., 0 C)

X1, Room temp. (e.g., 25 C)

X2, Near set point (e.g., 110 C)

Temperature Indicated by thermometer

TWO-POINT INPUT SHIFT Figure 2: This example uses temperature, but this can be used for any linear application. The input value is shifted at two points: near room temperature and near the set point. Check the temperature of the sensing object with a calibrated device and the temperature indicated on the controller/display at both near room temperature and near the set point.

heat or flow. Then compare the reading from the external device to the actual sensor. In this case, an offset may need to be entered into the raw signal the measuring device is producing. If this is the case, one way to do this is to take two separate readings. The further away the two readings are, the more accurate the calibration will be. How accurate the system needs to be determines if a single offset will be sufficient or if a two-point shift would need to be implemented. With a single shift the slope remains the same and a positive or negative offset would be applied. Providing a two-point shift allows the ability to change the slope of the overall readout.

The formula is a good way to determine what values are needed when doing a two-point shift (Figure 2). The example is using temperature, but this can be used for any linear application.


14


1. The input value is shifted at two points: near room temperature and near the set point. Check the temperature of the sensing object with a calibrated device and the temperature indicated on the controller/display at both near room temperature and near the set point. 2. Use the following formulas to calculate the upper-limit temperature value (offset) and lower-limit temperature value (offset) settings based on the values checked above.

Lower-limit temperature input offset:

Example:

Upper-limit temperature input offset:

Example:

Set both the upper-limit and lower-limit temperature input offsets and then check the temperature of the sensing object and the temperature indicated on the controller/display both near room temperature and near the set point for accuracy.

Although two points were used, near room temperature and near the set point in the example, accuracy can be increased further by using two points as far away from each other as possible.

Entering the equations into a PLC can simplify the procedure by applying the offset automatically to display values and establish a process.

Testing/validating of the analog values should be done on a scheduled basis. Typically the manufacture of the device provides some guidance, but it is up to the design engineer to ultimately decide when this should be done. Documentation should be provided to note the time interval and values to be used for the calibrations.


15


Cleaning the devices should also be done

direction. At the same time the act of

on a scheduled time basis. Since there are

tightening connections has the potential

so many different types of devices that

to solve issues and give you confidence

produce an analog output, check with the

in the wiring as you move forward with

manufacture of the device to determine the

further tests.

best way to clean the device.

2. Check your sensor or analog output device: The goal of this step is to ensure

With these steps completed on a regular

your sensor or other analog output de-

maintenance interval, your system can run

vice is sending a reference signal. For this

more efficiently and produce better out-

step, you will need a multimeter that can

comes.

read either 0-20 mA or 0-10 Vdc, de-

— Keith Hill, product engineer, control components & re-

pending on your reference signal.

lays, Omron Automation and Safety, www.omron.com

Whenever you test an analog reference, test

TROUBLESHOOT AND CALIBRATE

it at the furthest point from the source. This

Troubleshooting analog circuits: There are

will ensure you don’t just test the sensor’s

a number of different theories and tech-

or analog output device’s ability to gener-

niques for troubleshooting analog circuits.

ate the reference signal; you also test all the

As with any troubleshooting process, your

wiring up to that point.

first goal is to narrow down the location of the problems. From there you can get into

For example, you have a 4-20 mA refer-

more specifics. I have put together a few of

ence signal from a pressure mMeter being

the general steps that I typically take when

fed into a PLC. You would want to confirm

confronting a potential analog issue.

you are able to read 4-20 mA at the analog input terminals of the PLC. Once again, this

1. Housekeeping: Let’s face it, we live in

will ensure that not only are you generating

the real world. Lighting does strike, fork-

a signal, but you are able to transmit it in

lifts do knock into enclosures, people

your system, as well.

will crimp the insulation and vibration does cause wires to work their way free.

Utilizing your meter in its correct setting:

A quick inspection and tightening of

• Measure the 0-20 mA current signal by

connection points can work wonders

placing the probes in a series connection

to fix an issue. While rarely is there just

with the circuit (break the circuit).

a single issue causing the problem, a quick inspection can provide an initial

• Or measure the 0-10 Vdc signal by placing the probes in parallel with the circuit.


16


One of my go-to tools for troubleshooting

reference signal into the device, you can use

current analog references is my mA pro-

this signal for your test. If you do not have

cess meter. This was different from your

that reference you will need to find some

classic Amp meter because it could read

way to create one to perform this test.

down to the 0-20 mA level. The benefit is that you can read the current value without

In the past I would keep a 9 Vdc battery

breaking the circuit. This is not only a time-

and potentiometer in my bag for these

saver, but it eliminates the balancing act of

types of situations. By wiring the two

trying to hold a meter, two probes and the

together I could create a quick adjustable

signal wires at the same time. Additionally,

current or voltage source to feed my analog

some of the more advanced versions can

input device. By turning the potentiometer

generate a 0-24 mA or 0-10 Vdc signal for

I could also confirm that the values seen in

testing analog input devices (this will be

the PLC, HMI, ac drive or SCADA system

discussed in more detail in Step 3).

were able to update in real time.

If you do not read an analog signal, per-

Once you can confirm you are feeding your

form another test at the sensor or analog

analog input device, you need to confirm

output device. If you are able to read a

that it is properly reading that value. This

reference signal, you have a wiring issue. If

step is dependent on the device in question.

you are not able to read a reference signal,

If the device is a PLC, you will need to either

you have a sensor issue.

log onto the software or create an object on your touchscreen (HMI) to display this

If you are reading a signal, move on to the

value. Otherwise the device will most likely

next step, checking the analog input device.

have a sub menu, or at the very least status lights that you can reference.

3. Check your analog input device: The goal of this step is to ensure your analog input

4. Check for noise issues: These are some of

device is able to read the analog reference

the more difficult issues to identify and fix.

that is being fed into it. To do this, you need

The only real way to determine if you have

the ability to feed an active analog refer-

a noise issue is to scope the analog input

ence into your device and the ability to read

and see if you are experiencing a clean or

or confirm that the analog input device is

“choppy” reference. Many times a system

reading said reference.

that is experiencing noise-related issues doesn’t have an issue all the time. In many

If you have performed Step 2 and confirmed

cases it will only experience it when a large

via meter that you are actively feeding a

load is turned on, or during certain times of


17


the day when other machines in your facility are running at the same time.

1. Expose your sensor to a known process value. 2. Determine what value your analog input

If you determine you have a noise issue, the

device interprets that known process

next course of action is to mitigate it.

value to be.

• Ensure electrical connections are properly

3. Once you obtain those two points of data

terminated (connections are tight and

above, you can subtract to determine if

wires are not crimped on the insulation).

you have any error in your system.

• Ensure that signal wires are properly shielded and grounded. • Ensure the signal wires do not run parallel, or close to, higher-voltage lines.

4. If this error is determined to be outside the acceptable range, you will then need to adjust your PLC settings/calculations to offset the difference.

• When possible, apply an input filter to the analog reference to smooth out any short

Before we run through an example, it’s

unwanted behavior.

worth mentioning system accuracy. In this case, accuracy is the difference between

For example, if in the course of your investi-

the actual measured value and the value the

gation you determine that the analog refer-

PLC interoperates it as in order to perform

ence experiences a large spike when Pump

its calculations. The user needs to under-

Motor 3 starts up, first steps may include:

stand this value and know at what point

• Ensure the signal wires are well clear of

the process is affected by an error between

any of the power wires feeding Pump

actual and interpreted values.

Motor 3. • If possible, apply a ramp up to that motor with either a soft start or ac drive. • If possible, apply a smoothing filter across that PLC analog input, which will slow down the response time of the system; however, it will ensure you are getting a more reliable signal.

Error is created and compounded by several factors. • Sensor accuracy to read the process value • Sensor output resolution of the analog signal • Any noise or interference that is generated in the transmission of that analog signal • Your PLC input card’s resolution, filter and

Calibrating: This really depends on the ap-

update time

plication and what aspect of your process that you are trying to measure. However,

Let’s give an example for your temperature

the basic process is still the same.

sensor. For calibration to take place, you


18


will need a method to expose the tempera-

ISOLATOR RECOMMENDATION

ture sensor to a known/calibrated tempera-

Accuracy and consistency are crucial ele-

ture and a way to monitor what the PLC is

ments in any machine or manufacturing

interpreting that value to be.

process in the medical industry; we are literally dealing with someone’s quality of

Every application is different, but when

life and not just a machine production at the

possible I recommend removing the tem-

end of it.

perature sensor and performing this test independent of the machine. A popular

Typically best practices and aids to trouble-

method for testing temperature probes is

shoot are implemented not just from the

a calibrated bath; however methods need

plant maintenance personnel, but should

to be evaluated on an application-by-ap-

be a whole design philosophy from OEM

plication basis.

engineering down. The tug of war between production downtime for calibration and

Change the known/calibrated temperature

verifications vs. loss of sales during that

the sensor is exposed to. Begin recording

period can be lessened when a machine is

several data points relating known tem-

designed to be not only accurate, but have

perature vs. PLC interoperated tempera-

minimal downtime.

ture. The difference between these two values is your error.

A key design feature is to keep the cable runs as short as possible. The longer the

If you determine the error above is not

cable run, the more prone it is to errors or

acceptable in your application, you then

fluctuations based on noise and surround-

will need to adjust your PLC settings and/

ing interference. If a cable has to be in

or code to compensate for this error. If

place, we normally recommend an isolator

you’re lucky, the error is linear. These are

for a couple of reasons.

the simplest to compensate for. Nonlinear errors are not the end of the world; it will

An isolator will prevent any voltage fluctua-

just require a bit more math to determine

tion at the sensor itself since it controls/

the corrective equation to apply to your

isolates the power and signal both to the

raw value.

sensor and to the PLC. By decreasing any potential voltage or current spikes, the lifes-

— Robert Ruber, product manager, Schneider Electric, www.schneider-electric.us

pan of the instruments is extended and the calibration remains tighter.


19


An isolator with a split output will provide

ging features and direct Modbus TCP con-

feedback or reassurance at the PLC and

version. This conversion to a bus protocol

locally. The benefit of using a splitter is to

now enables users to monitor the system

maintain the integrity of the signal from the

from practically anywhere when combined

sensor without introducing excess imped-

with cloud service. The set points and limits

ance into the line. Signal isolators and split-

can be set via the HMI/PLC or via the user’s

ters enable the user to make measurements

phone through a simple webpage.

both close to the sensor and down the line

— Andrew Barco, business development and

at the PLC.

marketing manager, Weidmuller North America, www.weidmuller.com

When choosing isolators for 4-20mA sig-

LOOK TO THE LOOP

nals, the important factor is to verify if it

The most common problem in the simplest

can be configured to handle loop powered

and most reliable method of transmitting

(sinking) or powered (sourcing) loops. One

a sensor signal is the current loop. It has

unit to do it all greatly helps to reduce the

two critical components: the signal and

training of the maintenance staff, and it can

the power for the signal, all in just a pair

easily be replaced with one from stock no

of wires. Its limitation for transmission is

matter what the input source is. Current

the EMF of the transmitter, usually 24 Vdc.

measurement with Hall-effect sensors allow

However, what if the sensor fails or the load

for simple wiring and the 4-20 mA signal

opens or the operator doesn’t pay attention

can be fed into the PLC with limits and set

or is absent?

point preconfigured on the isolators to stop and cut off power if the currents get

The same problem occurs in many critical

too high or low. To maintain the accuracy

applications, including medical and nuclear.

of a temperature sensor, converting it to a

Address those three important variables.

4-20 mA signal first maintains the accuracy

Look for a solution that is powered by the

of the signal by eliminating any potential

loop and detects and alarms if the signal

losses that might occur on long runs for

fails, even post-mortem; that includes auto-

RTD or T/C wires. The 4-20 mA isolators

tricolor bargraph with set points to warn

should provide the choice of cold junction

the operator and PLC via serial I/O of the

compensation inside the isolator or via ex-

alarm status; and that can monitor its 4-20

ternal devices.

mA output to ensure load integrity via a second loop-powered channel that moni-

As devices move toward Industry 4.0, we

tors the output, as well.

increasingly see devices with onboard log-

— Dr. Otto Fest, president, Otek, www.otekcorp.com


20


4 QUESTIONS

the risk analysis and risk mitigation strat-

There are four questions to address here.

egy for your control process, these device

1. Is the instrument working?

calibrations may be completed on-site or

2. Is the instrument performing within ac-

may require the devices to be sent back to

ceptable tolerances?

a calibration provider.

3. Can periodic tests be performed in the PLC to indicate an out-of-tolerance per-

Using a calibration provider that is certified

formance?

ISO17025 compliant means they are accred-

4. Can a control-system function automate any of these tasks?

ited by a third party, thus ensuring that all calibrations are completed by competent personnel. A calibration provider will have

Is the instrument working? In typical control

procedures where all instruments are cali-

systems where the measured instrument

brated to their best capability.

provides a 4-20 mA signal to the control system indicating the process variable, the

These procedures are necessary to ensure

control system is blind to what this signal

metrological traceability when following

actually represents and if it is actually work-

the ISO17025 standard. The uncertainties

ing. Useful instrument diagnostic informa-

of measurement devices are reduced when

tion can only be communicated via digital

sending a measurement device back to a

network protocols like HART, Profibus, Eth-

laboratory and—depending on the required

erNet/IP or Foundation Fieldbus. Without

accuracy of the transducer—may determine

digital instrument data the only indication

how it is calibrated.

a control system might get that I/O signals are not working properly would be from

The fastest way to complete calibrations

I/O card diagnostics, if available. Examples

will be on-site. However, on-site calibrations

would be short circuit detection and over/

may increase the transducer inaccuracy as

under voltage and current. Today, most

compared to the factory-delivered speci-

4-20 mA instruments have HART function-

fication. If this increased inaccuracy is not

ality, so if your control system I/O cards

acceptable for your risk-mitigation strategy,

have HART modems on them, instrument

it will require replacing the device with a

diagnostic data can be obtained.

newly calibrated transducer kept in stock or simply sending it to a calibration provider

Is the instrument performing within accept-

to meet factory accuracy.

able tolerances? In order to know whether instrument measurements are in tolerance

After ensuring the measurement device has

requires regular calibration. Depending on

been properly ranged and put back in the


21


process, a loop calibration is then neces-

procedures for calibration and verification

sary to make sure that all of the remaining

stated in the prior question have been fol-

control items are within specifications. This

lowed, then confidence can be high that the

will require using a traceable multimeter to

process is not running properly, and appro-

simulate the process variable input of the

priate action can be taken.

control loop. Can a control system automate any of Having completed an instrument/loop cali-

these tasks? The only function that cannot

bration, a customer can then be confident

be automated via the control system is the

that the process is now in control and the

calibration process. This involves techni-

measurement device is functioning within

cians performing the work. There are instru-

factory specifications from the manufacturer.

ments coming into the marketplace that will perform onboard verifications and indicate

Can periodic tests be performed in the

back to the control system if a device has

PLC to indicate an out-of-tolerance perfor-

failed a verification. This technology will

mance? Yes, it can, if normal/typical run-

begin to address automating evaluation of

ning process conditions can be determined.

device performance, but it is not a true cali-

Programmed upper and lower control limit

bration and thus users still have to deter-

tests can be applied to any process vari-

mine if a verification can adequately reduce

able indicating when the measurement has

the out-of-tolerance risk in any particular

drifted outside a predetermined accept-

process application.

able tolerance. The PLC can be set to alarm

— Michael Robinson, national marketing manager,

users through the operator interface. If the

Endress+Hauser, us.endress.com


22

CD1609-Troubleshooting-420-mA.pdf

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