Analog Output Value in Logix Designer

[RockyNivas]

Apologies if this is a basic thing to ask here.

I am controlling an Analog control valve. I need to open it for 100%. At this Juncture PID is not needed since the valve opening and closing is not depends on the flow.

I am using SCL block in FBD. I'd like to confirm if I scale the value (0 to 32767) to (0 to 100%) and let the user to enter the values through HMI whether they want it to be open fully or 20% or 50% and move that Percentage value to Analog Output Module. Does it work to open the valve for the desirable position ?

PLC : ROCKWELL
Analog Output Module Current Value : 4-20mA

Thanks in Advance for the suggestion.

 

[parky]

Normally, if not using any kind of feedback i.e. temperature or flow then just use the 0-100 the operator puts in on the HMI then scale this to 0-32767 (or what ever is the raw output to the analogue card. if not needing any zero/span (trim factor) then you can use a factor & multiply it I suggest if using decimal places i.e 0.0-100.0 then do the simple maths & then convert it to an integer.
so 32767 means for every % it is 327.67 so just multiply the 327.67 by the percentage i.e. 50.0 X 326.67 = 16383.5 (converted to integer would be 16384 as round up occurs on boundary of .5).
Normally analogue control valves have their own feedback so if told to go to half i.e. 12ma it will stop at that position i.e. 20ma - 4ma = 16ma /2 = 8ma + offset of 4ma = 12ma.

 

[RockyNivas]

Thank you so much for your response Parky. I will do that accordingly.

Could you please let me know should I need to use 0-16383 or 0-32767 for the analog Output scaling parameters. When I checked the manual it says Scaled for PID values is 0-16383 for 4000mA to 20000mA. Should I use that for math calculations and move the value directly to the analog output module for the valve open and close ?

Thanks in Advance

 

[joseph_e2]

Depending on the analog output module, you may be able to just do the scaling in the module properties. What model PLC and analog output module are you using?

 

[parky]

As Joseph stated, it does depend on the module, I'm not too sure with rockwell's latest hardware as it has been a few years since I used them, but if it is the OFE2 then it appears it is 0-4095 so it really does depend on the card. But the same applies just on a calculator divide the 20ma value what ever it is by 100 that will give you the actual count for each 1 % I do suggest you use floating point maths then convert the result to an integer as it gives a better accuracy but if that is not too important then just do it with integers it will not be far out.

 

[joseph_e2]

As an example, I set up an analog output module in a CompactLogix and it gives some simple options for scaling for each channel. One of the options is percentage, which is what I would probably try first for what you're doing. In a ControlLogix, you have more options, where you can set high and low signal and high and low engineering for each channel. That makes it really easy to see what's happening.

 

[parky]

Perhaps if you post the part number of the analog card we can help better, I'm sure there will be many here who have experience with the card you are using & will guide you through