Schneider SoMachine Analog Scaling

Hossam Azzaz

Member
Join Date
Oct 2019
Location
Cairo, Egypt
Posts
9
Dear All,
I need to know if I have a pressure analog signal input(4-20mA) and need to get an output analog signal (0-10VDC) to a pump controlled by a speed drive.
I need to know how with "Schneider SoMachine Basic V1.4 EL" could I program the analog scaling with ladder.
Details will be very appreciative.
Thanks
 
This is one way of scaling, however in this case I converted integers to real to do the calc & back again, if input & output are reals you can delete the int to real & real to int bits
So there is one input (the raw values from analog say for this purpose 0-32767), the zero & span if you like of the input for this purpose it would be 0-32767, the other two inputs are out 0 and out span in this case it could be 0-150.
so you set the input zero & span to match the raw value in and set the output zero & span to the required i.e. 0-150
 
Inverse Scaling

Thanks Perky for your great help.
Now I'd made the scaling and it works correctly now, I mean when pressure switch giving me signals from 4-20mA, the PLC analog output is 0-10V.
Now I'd like to get this to be analog input to a ABB speed drive, but:
when PLC output changes from o-10VDC
the speed drive speed changes from max to min(i.e. from 50Hz to 0Hz)
In other words I need to get a speed from the drive inversely proportional to the pressure.
can you help me with that?
 
If I understand you correctly you have a pressure transmitter that gives a value depending on pressure i.e. 0-10 bar and you want to control a pump so that if the pressure rises you reduce the speed of the drive you could just invert the value i.e. sub the actual pressure from the max pressure for example if the pressure value goes from 0 at zero bar and 100 at 10 bar then sub the actual from 100 so if the signal was 0 then 100 - 0 =100, or signal 75 then 100 - signal of 75 = 25 .
formula is output = max output - input.
It really sounds to me that you need a PID algorithm this is usually how you control a speed of a pump versus feedback from the flow/pressure or temperature. Most modern PLC's have function blocks for PID, to use direct inverse control is very unstable. I recently posted a PI block code on here, however I cannot re-post it as I'm on a different pc as Microsoft strike again switched my usual pc on at 9:35 this morning and after over 2 hours it is still configuring updates. I will update this thread when I find it.
 
Here is an example of temperature control so same principle
The PDF page 1 shows the use of the PI block
The second is the variable structures in the function
and the third shows the code of the PI function block, note: I did not include derivative function in this as it normally is not required for simple control.
 

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