PID and PID Auto-Tune Issue

[russher]

The short answer is that with a simple, constant, ingredient auger speed the mill amps vary minimally (1 to 2 amps).
The longer answer is that there are other variables involved in the system, that, in an attempt to not over-complicate things, I have not described. Other variables include: 1) more than one type of grain is being milled: Barley grinds harder (more amps) than wheat for a given auger speed; 2) the control system automates a gap setting for the disc mill grinding plates. The larger the gap between plates the lower the amps on the mill motor for a given product - and, conversely, the smaller the gap the higher the amps for a given product; 3) the control system also automates the speed of the mill motor for optimum grinding. The mill motor amps, for a given product, will be slightly less at 3400 rpm than at 2200 rpm. The goal - beyond accurate and efficient grinding - is a constant, stable, amperage on the mill motor. Given all these variables, and the single common denominator being the amperage on the mill motor, I was thinking PID would be the way to go for controlling the speed of the ingredient auger relative to the mill motor amps.

[Controls Guy]

Remember, AutoTune is just a convenience.  If it gets to the point that AutoTune does a better job tuning than you do manually, it means you're not doing enough of them manually and you're out of practice.

Doing your own job yourself should always be the default and AutoTune is there simply to be a convenience and give you a semi-reasonable starting point, if you bother to use it at all.  A loop that you need help to tune well is one thing, but being baffled that AutoTune can't tune it is pointless, IOW.

[russher]

You make a good point. I could use some brushing up. Can you point me towards good information for manually tuning PID's?

[Controls Guy]

Not offhand.  There's a wealth of information online which would give you a head start, but honestly, once you've done a quick review of it, I think experience is the best teacher.  Just play with loops to brush up your intuitive feel for when you need more proportional, less integral, etc., and when you can trade them off against each other.  Here's one tip:  many many loops don't need differential.  The traditional approach to tuning is to disable I&D, and tune P till you find what it takes to make the loop oscillate, back off a little and then turn on and tune your I.

Your loop however, will defy that simple works-most-of-the-time approach.  When you get into a weird situation, you just have to stop and think through the loop and figure out what's causing your problem, and what steps to take to deal with it.  Sometimes I limit the bias (accumulated integral), or keep the integral turned off till within a window around the setpoint.  That's the cool thing about PLC PID, is that you have a lot of flexibility.  I was controlling pipeline pressure with a VFD on a pump one time where it took 20 minutes to fill the two miles of pipe!  Even in "PID mode" from the operator's point of view, I had to pump for 20 minutes at a fixed speed, before actually turning over control of pump speed to the loop.

In your case, I'd consider doing a couple things.  First, it's the transfer auger that directly impacts mill load, so that's where the (a) VFD needs to be.  You can control the grain hopper augers based on the fill level at the transfer auger inlet hopper.  The other problem is the different behavior of the mill with different grains, but you know what the mix of hard and easy-milling grains is at any given time at the transfer auger (do you even mix them?).  Therefore you can tune separately for each grain (if you even need to once you're controlling the transfer auger instead of the feed augers), and switch or slide between gain sets depending on the grain being milled at any given time.  I assume the disc separation is another independent variable that must be set according to the flour particle size desired and you aren't free to manipulate it to control motor amps.

[Garyhlucas]

Now that the OP has given more information it looks to me like the issue is as simple as a speed control on the transfer conveyor that is PID controlled by the grinder motor current.  Then for each conveyor feeding into the transfer conveyor use a scaling factor such that it keeps pace with the transfer conveyor depending on how that particular grain feeds.  So if the transfer conveyor is running at half speed the conveyor feeding it runs at half of its scaled speed.  The effect will be that the transfer conveyor can respond almost instantly to the grinder motor current, and the material in the transfer conveyor will always be nearly constant along the conveyor, smoothing everything out.

[ATU]

What about the grain cleaner?  Any buffer before the grain enters?