WAGO AI problem

Sillver · Oct 24, 2011

Hi,
i have a 4-20mA pressure transmitter which is giving out 3.985mA when there is no pressure (indicating that the sensor is good), but it is displayed as -65psi on my HMI - it should show something close to zero.

The 4-20mA is connected to an analogue input from WAGO (750-485) and the error seems to be generated by the WAGO module. When simulating 4mA - the WAGO card gives out 6464 and HMI show -50psi. According to the data sheet the wago should give out 6552.

Have anyone seen a similar problem before or have a good idea about how to correct it?

Buzzen · Oct 24, 2011

What kind of processor are you using? Sound like you are haveing a scaling problem. 4-20= 3277-16384

Lancie1 · Oct 24, 2011

... it is displayed as -65psi on my HMI - it should show something close to zero.

Your HMI probably has some type of scaling equation, either internal, or in the PLC program. You can make the HMI read any number you want (from -largest number displayed to +largest number displayed) by changing the scaling equation.

justin.garski · Oct 24, 2011

Looking at the data sheet to that module, it looks like the data range goes down to 3.2ma. (That is before you get any sort of fault indication)
So you're scaling formula is possibly correct (seems likely) and it is just giving you numbers for the below 4mA value.

If you need more info I can have my counterpart in your neck of the woods follow up. I'm the Wago apps engineer for the north central part of the country. But we have guys scattered all over.

Random question, what's your max PSI in your equation?

bkottaras · Oct 24, 2011

justin.garski said:
Looking at the data sheet to that module, it looks like the data range goes down to 3.2ma. (That is before you get any sort of fault indication)
So you're scaling formula is possibly correct (seems likely) and it is just giving you numbers for the below 4mA value.

If you need more info I can have my counterpart in your neck of the woods follow up. I'm the Wago apps engineer for the north central part of the country. But we have guys scattered all over.

Random question, what's your max PSI in your equation?

I would add some code to clamp the scaled values and not allow anything less than 4mA or more than 20mA to display ont he HMI.
Manually move 0 when 4mA or below or MaxValue when 20mA or above.
This should take care the HMI readings.
Dealing with actual readings and taking action is a different story, up to you on how to go about tackling it.

PS: Justin, do you cover the Chicagoland area or know who does? I'm about to pick up a 750 system and may need a bit of help from your app engineering group.

justin.garski · Oct 24, 2011

Clamping by using a move when at or below 4mA would be easy enough, good point. This will "mask" the real value, but it may not matter for you.

I do not cover the Chi area, but I am good friends with the guy who is. Email me at [email protected] and I'll send you his info. Not sure if its good for putting it out there on a forum.

Sillver · Oct 25, 2011

There is no scaling equation involved here. 6464 is the number coming directly from the WAGO into my PLC over Profibus. If I understand the manual correctly it should give ut 6552 at 4mA. http://www.wago.com/wagoweb/documentation/750/eng_manu/modules/m07500485_00000000_0en.pdf

Fixing the HMI readings is easy enough, but i would prefer to fix it on the WAGO side. Perhaps a firmware upgrade could improve matters?

justin.garski · Oct 25, 2011

I pulled up the manual earlier. There must be a scaling formula somewhere, because the Wago does not read PSI. And the firmware does not control this. You are getting a value below 6552 because your current is below 4mA. The only way to get a true 6552 is to get 4mA dead on.

Sillver · Oct 25, 2011

I simulated 4mA with my multimeter - it should be pretty accurate. When i send 4mA the WAGO is giving out 6464. The range of this signal is 0-15000psi

justin.garski · Oct 25, 2011

I am going to assume there's something just a tad low on it. I'd make 0 be the 6464.
I also recommend you give your regional guy a call, he may have more info.

shooter · Nov 1, 2011

minimum reading is 6552 when running 4 mA, max is 32760.
you may scale this (it is not complete correct but it works.
-65/15000 is only 0.4 % within limits of the system. so you will have to calibrate a little. just put the sensor on 0 and read the HMI. set this as minimum. then apply 10000 PSI to it and calculate what you will need for reading.
You can use your own scaler, but have a look at www.oscat.de for some nice software for doing this.
if you send the numbers i will make calcs no problem.

WAGO AI problem

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