analog input m/m error

[gibson]

Hey guys, I am using ML1200 with Analog module IF4. When i apply voltage (0-10V) across Terminals of Analog module i am getting wrong value on SCP instruction.


Consider, voltage across terminal is 3.38V but i am getting following values on PLC.
>> I:1/2 = 2245
SCP settings:
>> Input Min = 0
>> Input Max = 31206
>> Scaled Max = 100
>> Scaled Min = 0
>> Output = 7

Input switches are in Voltage position

Thanks

 

[Lancie1]

The SCP calculation looks correct to me. Is your Input Max correct for this PLC?

slope = ( scaled max - scaled min) / (input max - input min)
offset = scaled min - (input min X slope)
Scaled Output = (input value X slope) + offset

Slope = (100 - 0)/(31206 - 0) = 0.0032045
Offset = (0 - (0 x 0.0032045) = 0
Scaled Output = I:1.2 * 0.0032045 + 0 = 2245 * 0.0032045 = 7.194

The 7.194 would be 7.194 out of 100 (which is the same ratio as 2245 to 31206). What number did you expect? If you wanted to convert the input to voltage, then substitue Scaled Max = 10 volts (which calculates to 0.7194 volts).

On the other hand, if your 1762-IF4 module is configured for "Scaled for PID" instead of "Raw/Proportional", then your results will be different.

 

[dmargineau]

Wrong value?!...Really?!...
Let's see...

(2245 X 100) / 31206 = 7.1941293344102416202012433506377

Send your SCP Output to a Float Data Table address instead of an integer and you will get the decimals also.

 

[Mickey]

Can you tell us what two terminals you are measuring across?

Can you post your program, its .RSS file (zip it first)? Or a snap shot of the cards configuration.

Double check the switches.

 

[gibson]

The SCP calculation looks correct to me. Is your Input Max correct for this PLC?

slope = ( scaled max - scaled min) / (input max - input min)
offset = scaled min - (input min X slope)
Scaled Output = (input value X slope) + offset

Slope = (100 - 0)/(31206 - 0) = 0.0032045
Offset = (0 - (0 x 0.0032045) = 0
Scaled Output = I:1.2 * 0.0032045 + 0 = 2245 * 0.0032045 = 7.194

The 7.194 would be 7.194 out of 100 (which is the same ratio as 2245 to 31206). What number did you expect? If you wanted to convert the input to voltage, then substitue Scaled Max = 10 volts (which calculates to 0.7194 volts).

On the other hand, if your 1762-IF4 module is configured for "Scaled for PID" instead of "Raw/Proportional", then your results will be different.

Yes Input max is correct 31206 for 10V but for +3.38V the value on Input I:1/2 should be around 10000 right ?

Module is configured as Raw/proportional

 

[dmargineau]

Your OP states that the SCP instruction returns erroneous results...This is not the case...
The issue is the module's hardware configuration/scaling.
Your 1762-IF4 module set for voltage has a range of -10 VDC to 10 VDC hence your SCP instruction should have an Input Min =-32767 (-31206) and not 0.

 

[Mickey]

Your OP states that the SCP instruction returns erroneous results...This is not the case...
The issue is the module's hardware configuration/scaling.
Your 1762-IF4 module set for voltage has a range of -10 VDC to 10 VDC hence your SCP instruction should have an Input Min =-32767 (-31206) and not 0.

I disagree, the OP has the SCP right. I suspect the modules configuration or where the voltage measurement is taken. If done as you stated the scaled min would have to be -100.
The OP should post the program.

See data format info PDF

 

[dmargineau]

Mickey,

It is a 1762-IF4 module; according to Page 8 of http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1762-in012_-en-p.pdf
the Voltage Mode range is -10 VDC to +10 VDC; only the 1762 Analog Combo IF2OF2 allows a 0-10 VDC range for the Input channels when set to Voltage Mode.

 

[bernie_carlton]

Though Mickey posted data for a output module the format is still correct. I agree that the SCP is fine given that he is not measuring negative voltages. The SCP doesn't have to have the full range of the module to be correct, just a correct mapping of two seperated points. Zero minimum input to zero scaled is fine.

 

[Mickey]

Though Mickey posted data for a output module the format is still correct.

I corrected my mistake, thanks.

 

[dmargineau]

I also agree that the Scaled Min. parameter of the SCP instruction should be -100.

 

[bernie_carlton]

I'm curious about the address the OP provided.

I:1/2 = 2245

Assuming the card is in slot 1 then the appropriate addresses would be I:1.0 through I:1.3 for the data.

Ensure that you are reading the appropriate data.

 

[gibson]

Yes it is I:1.2

 

[gibson]

Here is code

 

[dmargineau]

If you are still "there", have you tried Input Min = -31206 and Scaled Min = -100?
This is quite interesting...
The 1761-IF4 configuration is done strictly via RSLogix 500:
either 4-20 mA or -10 VDC to +10 VDC for Input Ranges
and
either Raw Proportional or Scaled for PID Data Format.