John Marston
Member
Is there a way to create proportional logic for a control valve without using a PID function? I want to create a simple P controller without using the Rslogix PID function block.
P Controller Logic without using a PID block
- Thread starter John Marston
- Start date
spaderkung
Member
One common model for controllers uses
Output = K*(setpoint-actual),
adjust K so it works and put it in a fixed scan cycle for best result.
Just copy what you need from wikipedia or internet since you can paste most of it directly into ST code.
Output = K*(setpoint-actual),
adjust K so it works and put it in a fixed scan cycle for best result.
Just copy what you need from wikipedia or internet since you can paste most of it directly into ST code.
John Marston
Member
OP
Right now I'm subtracting the valve position from the Setpoint to give me an error. I then take the error and drop it into my analog output going to the hydraulic servo valve. Being that the error reduces as the valve position gets closer to the setpoint, the servo closes and the control valve slows way down. This would be okay but I would like for it to be a little more weighted.
cardosocea
Member
What do you mean by weighed?
The problem you usually end up running into is that the valve and system will have a deadband below which your command won't make a difference and as such you won't get to the value you really want to be at.
The problem you usually end up running into is that the valve and system will have a deadband below which your command won't make a difference and as such you won't get to the value you really want to be at.
Peter Nachtwey
Member
Yes, 3 SCP instructions. One to convert counts to engineering units. One to compute the control output 0-100 using a proportional band and the third one to convert the control output to counts.
That is the way proportional only control works with servo valves.
Servo valves shouldn't have a deadband. If they have one it better be very small.
John Marston
Member
OP
I've never used an SCP instruction but I'm going to give it a try this afternoon. I'll report back my findings.
Peter, I believe I bought some converters from your company recently and they worked very well.
Peter, I believe I bought some converters from your company recently and they worked very well.
I posted this some time ago although it's a PI block but should do what you want & you can tailor it to do what you need, although it's in GX works it would not be hard to convert, just set the I component to a very low value & don't forget to only sample the block one scan every x milliseconds or even seconds.
John Marston
Member
OP
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