PLC analog input & output module required calibration?

Hello everyone,

Just for general knowledge, do you think PLC analog input & output module/point needs to calibrate using external calibrator?

no, but during startup you need a calibrated injector and multimeter. I never heard of analogue I/O cards sent to be calibrated.

Singapore_Mats said:

no, but during startup you need a calibrated injector and multimeter. I never heard of analogue I/O cards sent to be calibrated.

Click to expand...

Let's say after a certain period (6 mths) running, we recalibrate the sensor & the PLC AI point together (like what we did in starup), do you think it's a good idea?

Yes, if your companys quality-program etc. or your process requieres very accurate readings, so why not every 6mth or 12mth test sensors and I/O-card with a calibrated source.

My experience with analogue signals is that the problem is more likely in the sensor, cable, the connection, junction box etc. and very seldom in the I/O-card itself.

Hi there
I dont think that AI/AO cards needs calibration.

Calibration is the requirement for transducers which converts one form of energy to another.
We need calibration of trancducers because there capablity to accurately transform the energy
differs with time, because of N numbers of factor.

Since AI card is nothing but all Microprocessors, they do not need calibration.

manmeetvirdi said:

Hi there
I dont think that AI/AO cards needs calibration.

Calibration is the requirement for transducers which converts one form of energy to another.
We need calibration of trancducers because there capablity to accurately transform the energy
differs with time, because of N numbers of factor.

Since AI card is nothing but all Microprocessors, they do not need calibration.

Click to expand...

You got the point, but some client insist us to do the calibration for the AI/AO modules, too. So, based on your explaination there is no harm doing it at the analog module. right?

Absolutely no Harm

But how you do that? I mean Calibration is comparing the measured value with that of set of standard values, and then tweak the device if there is mismatch.

So how do you calibrate AI card? Do you have standard values to compare with, or you send
these cards to OEM for this?? But what happens to production when you take your PLC down for calibration??

Also the values measured by AI is ultimately processed by CPU, so you you need to calibrate it also.
Just a suggestion ;-)

My Twido-card I happend to have on my table have a accuracy of 0.2 degrees when measuring thermocoupler.

That is VERY accurate considering it measures 0-1300 degrees!

How would you calibrate in analog module?? take an SLC analog card for example 1747-NI4 if you look on the board there is only one trim pot the rest of the circuitry is chipsets and fixed resistors with the usual line drivers and line buffers.

Certain older types of cards I have encountered (in my limited experience, admittedly) have adjustment pots on them. These are generally just for fine-tuning however.

In the case of AB SLC's, adjustment wouldn't be possible, but systems like the Yokogawa MicroXCel, ACCOS and other similar cards, pots are supplied for fine-tuning each channel.

If according to audit requirements, you are required to at least test the accuracy of the card to be within tolerances, connecting a calibrated source and logging the results or performing an adjustment should pose no problems.

Ah OK I never came across one for a PLC directly that had em, Seen em on other boards such as ones for X-ray machines or usually standalone boards.

Most programmers convert the raw value of the analog through a function so that it can be scaled into a meaningful value i.e. 0-150 Deg c, this function will contain zero & span values that can be adjusted either directly with a pc or an hmi so that any errors can be eliminated.
this is particuly true with PT100 direct inputs where no form of zero/span exists on the probe.
I tend to make all sensor inputs 4-20ma, use scaling blocks of code but as most 4-20 ma sensors/converters have trim pots etc. this means that no changes are required in the plc (good for reducing backups of code & data registers).
Although I always take regular dumps of data areas preferably when the plant is not functioning.

flyers said:

Just for general knowledge, do you think PLC analog input & output module/point needs to calibrate using external calibrator?

Click to expand...

Not necessarily. If calibration is needed, then the calibration has to be done for the measuring system as a whole. If you can't calibrate the sensor, then you have to calibrate the analog input card.

manmeetvirdi said:

I mean Calibration is comparing the measured value with that of set of standard values, and then tweak the device if there is mismatch.

Click to expand...

Not necessarily. Calibration can also consist of simply testing if the measurement is still within the set limits. This is also known as a good/bad calibration. If the calibration results in a 'bad' then you have to take actions, such as changing the input card or the sensor. And re-calibrate of course.

Most companies I work or worked for do however double-calibrate: they calibrate the sensor and they calibrate the measuring system. Sometimes the measuring system is a PLC analog input card.

I know of a company that does regular calibration on all analog input cards used for measurements which influence the quality of the job done. Therefore they take readings on the PLC while sending in signals of 0, 4, 8, 12, 16 and 20ma and reading the value on the analog input. If one or more readings are out of tolerance, the card is replaced. They also calibrate the sensors attached. They also calibrate the PLC analog outputs in the same way: set these to the values corresponding to 0, 4, 8, 12, 16 and 20ma and measure if these give in fact the desired currents on a calibrated meter.

Kind regards,

Hi there Jvdcande
Thats a new thing for me to know about.
But how often while calibrating AI/AO cards does they actually replace them,after calibration??Is it like frequent or one in a blue moon type?

Also if AI/Ao is reading bad means that some thing is wrong with electronics,right? And this should be displayed as fault in a card.