Invert 4~20mA scaling

shammi79

Member
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Join Date
Sep 2014
Location
Australia
Posts
8
Hi everybody,
I’m trying to find the best way to do invert a 4-20 mA signal in the device, PLC and citect.

I have a Rotork actuator which open and closes a weir gate. 0% is fully closed and 100% is fully open. I just want to invert this process so it’s easy to understand for the operators.
0% = Opened and 100% Closed.

I have found the invert option in the instrument panel for the actuator, whats the best way to tackle this in the processor and Citect.
Any help will be much appreciated

Thank you
 
Ov = [(Osh – Osl) * (Iv - Isl) / (Ish – Isl)] + Osl

Ov = scaled output value
Iv = analog #Input value
Osh = high limit of the scale for the scaled Output value
Osl = low limit of the scale for the scaled Output value
Ish = high limit of the scale for the Input value
Isl = low limit of the scale for the Input value

If you decide to invert the signal in the PLC you are better using the full scale formula rather than the short version, because it assumes you have the lower value at zero. The formula above will scale anything to anything and still works if you go outside the maximums and minimums.
 
Use BryanG's formula to calculate the 4 to 20 value. Then subtract that value from 24.

24-20=4
24-16=8
24-12=12
24-8=16
24-4=20
 
You don't even have to do the extra maths, just make the output high limit 4 and the low limit 20. It will do the inversion for you. Have no experience with Kingfisher RTU.
 
I find that what 0%=close, 100%=open is what operators expect. 100%=closed 0% open is opposite of most controls and confuses the operators and electricians.
 
BryanG said:
You don't even have to do the extra maths, just make the output high limit 4 and the low limit 20. It will do the inversion for you. Have no experience with Kingfisher RTU.
Click to expand...

But how do i make the change in citect? i don't think i can have 100% low limit and 0% high limit.
 
proof said:
I find that what 0%=close, 100%=open is what operators expect. 100%=closed 0% open is opposite of most controls and confuses the operators and electricians.
Click to expand...

I agree. I also suggest documenting it somewhere that would be found if they were trying to troubleshoot a problem.


Will.
 
l suppose if you are more interested in the valve being closed, 100% would be a good percentage, but has been suggested 100% is normal talked about in open.
But in thinking about what you suggest, 100% closed would be a better figure as that is the safest position for a lot of processors.
Turned off, is normally the safest position.
But when it gets hooked up to a HMI and showing graphics, 100% on a valve would show full throttle (open) when looking, not unlike 100% on the throttle when in a car spinning the wheels, not 0% on the throttle spinning the wheels.
 
Last edited:

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