The attached workbook is an approach to suggestion #1 in the previous post. The optimization might work better with the fitted data, and without compromising results. If a negative CV is a problem, it can also be offset (i.e., CV Mod+ column).
Two suggestions: (1) apply smoothing functions to the plant data, and add a CV offset to create a smooth bottom to its oscillation instead of the discontinuity at zero, and (2) force a first order w/ deadtime system so there are only three optimization parameter (if I understand correctly), and run a series of evenly-spaced grid "searches," and then plot the objective on a series of 3D plots to visually interpret the "surfaces" that simplex is operating on. This may help guide the starting values to find the minimum objective of interest.
I can change that CV output to not bottom out at zero, this will also have the effect increasing the deadtime as it will take longer [because of reset wind up?] for the valve to open enough to send cooling. However I think that having it not bottom out is a good step forward.
For the PV being sent to the PID I am running it through a 10 second filter and then truncating the output to 2 decimal places. (00.00). I do this because the filter output is a 7 decimal place REAL that moves frequently in either direction and produces undesirable effects on the PID CV.
For the PV being sent to the PID I am running it through a 10 second filter and then truncating the output to 2 decimal places. (00.00). I do this because the filter output is a 7 decimal place REAL that moves frequently in either direction and produces undesirable effects on the PID CV.
I can change that CV output to not bottom out at zero, this will also have the effect increasing the deadtime as it will take longer for the valve to open enough to send cooling. However I think that having it not bottom out is a good step forward.
Regarding the TankFit document you posted, I see that the graph has averaged in the datapoints but what am I really looking at here?
I have the valve go to complete closed position 4ma when at 0%CV to ensure the valve is closed and move it to 7ma at 1%CV to just barely crack it open and proportion it up from there in small increments as it is a ball valve.
Ooop, sorry about that. I really hate when I ask a question and I don't get an answer.Hey Peter,
Update: Deadtime printed out is 0.006 (what are the units?)
@drbitboy
I am surprised the program didn't do better but the data isn't very good.
4ma to 7ma is nearly 20% of a 4-20ma signal, and that is for the 1% change in PIDE CV. Now, we do not know the actual valve characteristic, nor do we know anything about non-linearities in the cooling system characteristic, but does that seem like a bit of a big jump to anyone else? At a minimum the PIDE assumes at near-linear (or near-equal-percentage) response between CV and PV.
So the first 0-to-1% in CV from the PIDE goes from 0 to 20% of the valve position, and the next 9% CV or so gets to 29-30%; could this system not be re-scaled to simply have linear CV position from 0-30%?
Thats right at about 7ma the ball valve just barely opens up, enough to see a crack of light through it. I bench tested it before I installed it.
I want to steer clear of a 1-20% range where nothing happens as that will cause an excessive amount of wind up.
Thats right at about 7ma the ball valve just barely opens up, enough to see a crack of light through it. I bench tested it before I installed it.
I want to steer clear of a 1-20% range where nothing happens as that will cause an excessive amount of wind up.
So there is no flow below a signal of around 7ma, say 19-20%?
During the bench test, did you characterize the flow vs. linear percentage? If yes, can you post that? Or can you point us to the flow characteristic of the valve? Thanks.
How did the test, with Ti = 120min, work out?
I missed this part about valve response problems over a significant part of the CV range. If you have a flow measurement, it is not uncommon to linearize the valve response by manipulating the PID CV before sending the signal to the analog output. Similar to what you are already doing, but with a characteristic function that is the inverse of the uncompensated flow vs. CV.
You still might benefit from tuning adjustments in the short term, though it sounds like that valve configuration is a real impediment especially if the process operating window moves.