Slow Response times on Compactlogix Combo Ai/AO card

[Stalak]

Hello,

I'm having an issue with response times with analog inputs from a 1769-IF4XOF2 Combo card with a 1769-L30ER PLC. The analog inputs are level sensors(3 submersible pressure transmitters, 1 radar level sensor) coming from a wetwell nearby.

The time to see a change in raw input value is anywhere between 5-30 seconds, which is excessive. The radar sensor which is a VEGA C21, has a bluetooth functionality where i can see the level being reported through some of their software, and the value seems to be changing faster than 5-30 seconds as well.

The RPI of the card is set to 80ms and the program runs on continuous task at around 1ms.

Oddly enough, when we tried disconnecting the sensor wiring from the card and used a multimeter to produce varying 4-20ma signals into the combo card, the change in raw input value was instantaneous.

Does anyone have any idea on what is happening here?

Thanks.

 

[Bob O]

Is there dampening/averaging going on in the sensors?

 

[drbitboy]

Oddly enough, when we tried disconnecting the sensor wiring from the card and used a multimeter to produce varying 4-20ma signals into the combo card, the change in raw input value was instantaneous.

Have you tried the complementary test: disconnect the sensor wiring from the card and using a multimeter to measure the current being produced by the sensor without the card?

Also, since it is a current loop, you should be able to connect the sensor to the card with the current going through the multimeter in the circuit.

What are the results of those tests?

 

[Stalak]

Is there dampening/averaging going on in the sensors?

Not 100% on the radar sensor, but im fairly confident that the other 3 do not have dampening/averaging

Have you tried the complementary test: disconnect the sensor wiring from the card and using a multimeter to measure the current being produced by the sensor without the card?

Also, since it is a current loop, you should be able to connect the sensor to the card with the current going through the multimeter in the circuit.

What are the results of those tests?

Haven't tried that, but Ill give this a shot next time im out to the site and post what i find.

 

[Stalak]

So i performed a few tests and here is what i found.

I connected my multi-meter to the current loop and found that the level sensors are indeed changing faster than the PLC is recording it. I noticed that the raw input of the signal in the PLC changes until the mA signal increases/decreases by a certain amount, about 0.08mA.

I went ahead and tried the first test again by connecting my multimeter to the combo card directly, but this time i made smaller increments to my signal by going up only about 0.001 mA at a time. The analog raw input in the plc will not change until i get to about 0.08mA increase/decrease, and oddly enough the plc only increases/decreases the raw value by 128 at a time.

To my knowledge, there arent many configuration options for this card so im a bit confused on this matter.

Any help would be appreciated.

 

[Stalak]

Hm maybe ive found out my issue, It looks like the max resolution for this card is 8 bits so that what may be causing my issue here.

 

[kamaroman68]

I believe that is the lowest level resolution card you are using.... only 8 bit. I have the same card in one of my projects for temperature and experience the same thing your describing, only temperature instead. Wondering if you need to swap to a higher resolution card to solve your issue

 

[OkiePC]

Hm maybe ive found out my issue, It looks like the max resolution for this card is 8 bits so that what may be causing my issue here.

Yes, that would explain it! Welcome back to 1979! (the last analog module I worked with that had 8 bit resolution became obsolete that year).