PID loop advice

We have a PID loop at the plant that measures our drum outlet temp. It adjusts a damper to control the temp within the set point. We have a temperature sensor at the inlet of the drum. It isn't connected to any loop it's just there for indication. When we add feed to the drum "sawdust" the material is drawn through the drum and dried. The operators usually do a moisture sample on the material after it has went through the drum to determine what temp the SP on the PID should be at. We have a very hard time tuning this PID for the different moisture of material that is going into the drum. We receive our sawdust from many different operations and the moisture could vary as much as 30%.
We recently switched the PID to control the temp from the Inlet temp not the outlet temp. It seems to control the loop more consistent but the problem now is it takes too long for us to hit our proper moisture we need to bring the material into the plant.
The inlet temp is usually around 398F and the outlet around 72F with material going through. The closer we can keep the outlet to 72F the more consistent our moisture is.
I was wondering if a feed forward or feed back PID system would be better since we have sensors at the inlet and outlet.
I was wondering if I used two PID's one for the outlet set at 72F that when it deviates away from it's set point it tells the inlet PID to increase. So the outlet temp PID would have a stationary setpoint and the inlet PID would have be constantly changing setpoint that would control the damper.

Any suggestions is greatly appreciated.

Thanks,

Not that good at this compared to others, but maybe adaptive gains depending on the moisture test?

Uhhh

What setting are you controlling? Inlet temperature?
What is the block diagram of the installation (what follows what? what are the delays? etc.)
How is moisture related to temperature?
Why is the moisture sample taken at the outlet?
Why do you need 2 controllers?
What do you ultimately want to achieve?

feed forward PID - it is a pearl

Is this a continuous or a batch process? Is the drum rotating?

If continuous, how is the flow rate of sawdust controlled?

If continuous, then the difference between inlet and outlet temperature times the heat capacity of the gas/vapor stream times the gas/vapor stream flow rate is presumably roughly proportional to the rate of moisture removed (evaporated) from the sawdust times the latent heat of vaporization, assuming some constant approach to equilibrium.

What is the RH (or dewpoint) of the inlet and outlet gas/vapor stream?

Presumably temperature setpoint is some sort of proxy for outlet moisture level in the sawdust, since the operators take moisture samples to set the temperature setpoint. So there must be some sort of model implied in that relationship. Do you know what that model is?

Also I would expect the outlet air temperature is very close to the outlet temperature of the sawdust, and that the outlet sawdust temperature is not much above, if at all, the inlet sawdust temperature. Is that the case, or is there no way to know the temperature of sawdust at inlet and outlet?

Do you have any trends of the outlet temperature, inlet temperature, and PID output (damper position)? If yes, can you post them here?

Has anyone tried any empirical tuning methods (Ziegler-Nichols, Cohen-Coon, etc.)? Are there any trends of those experiments that could be posted here?

Is it like this (with a bit more safety)?

Is the inlet air temperature constant (controlled), or is it allowed to vary as the damper changes the (flue?) gas/vapor rate?

What controls the heat input to the system (e.g. fuel flow rate to a furnace or burners)?

More popcorn is needed. Notice all the questions.
drbitboy and MaxK are pulling teeth, yet again.

Do you have any trends of the outlet temperature, inlet temperature, and PID output (damper position)? If yes, can you post them here?

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No trends, no help because then we would be guessing.

Has anyone tried any empirical tuning methods (Ziegler-Nichols, Cohen-Coon, etc.)? Are there any trends of those experiments that could be posted here?

Click to expand...

NOOOOOOOOOOOOOOOO! Don't mention those words.

Perhaps the main regulation loop should control the humidity RH at the outlet.

Anyway it seems that the drum is too small or short for the expected production.

Peter Nachtwey said:

Has anyone tried any empirical tuning methods (*******-*******, *****-****, etc.)? Are there any trends of those experiments that could be posted here?

Click to expand...

NOOOOOOOOOOOOOOOO! Don't mention those words.

Click to expand...

I'm looking for the experiments, and hopefully trends, from any of those methods that may have been attempted, not the approach results.