PID Loop Issue

[Mike Nash]

I found out last night that setting the PID.Bias before enabling the PID did nothing for me. Setting the PID.Output  with a one shot a few rungs before the PID was enabled sets both output and bias and it starts from there. This was only an issue if the bias wasn't at 50% when the PID was disabled previously.

Good luck.

[Evilbeard]

Well, I set the Output to 52.5, and I had set the Bias to 0 somehow, causing it to be low. We'll see what happens here in a few minutes. Here we go!

[Evilbeard]

Turns out I was having a slight communication issue with my secondary drive. The acceleration wasn't quite acting linear because of the issue in communicating the speed ref. I made it analog and my secondary nip now accelerates smoothly. Once I got that lined out, it was just no time before I had my loop running great. In other words, I'm a doofus.

[Controls Guy]

Welcome to the club, my brother!   

[Evilbeard]

Turns out, I spoke too soon. We ran a light gauge film on the line and ran it at 500FPM start to finish with no problems. The operator came over, put on production film, and now when starting the machine out, it ramps to speed (dancer looks great), and then when it reaches speed, everything stabilizes. Then, the dancer seems to creep out of range, and everything goes to hell.  The top pane of the picture is the Dancer (white)/Setpoint (red)/PIDOutput (aqua)/Roll Diameter(yellow).  The bottom pane is the Secondary Speed (as a percentage of 0-max in RED)/Drive Base Speed Reference (before being trimmed in Yellow)/ Speed Reference (Aqua.

You can see the acceleration of the line, everything looks good. It reaches speed, and then the drive starts decelerating to allow for the roll building diameter (I verified diameter with a roll tape. I measured 9.93" on my calculated 9.95"). Then, the dancer starts going toward the limit. The secondary nip drive is a steady output frequency (it varies 0.1 Hz now that it's analog). I just don't know why it's doing this, and how I can correct it. I've tried more I to smooth things out, and that didn't help. I tried more I and more gain, it didn't seem to help. I tried less I and less gain, and it didn't seem to help. If I had any hair left, I'd pull it out.

[franji1]

FYI, Designer's PID View and Trend View have a Trend Archive file format, where you can hit the Record button or Export button in the view's toolbar, and then save it as a Trend Archive file, which you can email to your friends or post here.  Anyone can then do File->Import->Trend Archive into New Project to look at it in a static/historical mode, complete with zooming and clicking on data points, etc.

Great for late night PID view discussions (like that ever happens)  .

I've attached a sample in a .ZIP file.  You will need Designer 1.4 to open it.  After importing, please change the time scale on your Trend View to something more reasonable .

This forum has a limit of 512K per post.  Mine was for just a couple minutes, and zipped it was 64K.

[Controls Guy]

Evilbeard:  what's your I time and P gain?

BobO:  Is I dependent or independent type?

[BobO]

BobO:  Is I dependent or independent type?

Sadly, I'm going to have to admit that I don't know what that means.

[Controls Guy]

If you double your P-gain and it automagically doubles the absolute rate of change of the I term (in percent CV, say) with no change in Ki, then it's dependent.   If not, it's independent.

[BobO]

There is a single gain term that is applied to each of P, I, and D...pretty sure that means dependent.

[BobO]

I hate to sound so seriously ignorant, but we started with the equations Koyo used for DL and fixed quirks as they were pointed out. Once everything was working as expected and blessed by folks who know lots more than me, I pretty much purged it.

[Controls Guy]

I hate to sound so seriously ignorant, but we started with the equations Koyo used for DL and fixed quirks as they were pointed out. Once everything was working as expected and blessed by folks who know lots more than me, I pretty much purged it.

I think it's probably documented in the 205 manual, I'll check.  Either way is fine, but my response to Evilbeard's trends will vary a little based on the answer.

[Controls Guy]

Yup, dependent.  If you change Kc, it scales all the terms.

[Mike Nash]

Evilbeard, I can't tell what your traces are supposed to represent in that shot as there are no legends and no y scales visible.

My first thought is you maybe have something wrong in your diameter calculation - not the math, but the update speed. If your web is thin it runs fine, but if thicker it gets behind. Are you counting spindle revs and multiplying by film thickness or footage by thickness to get area, adding core area and then getting the diameter or...? If you don't update the spindle ref from diameter build fast enough you will get behind. Thicker film can be a much faster build rate. It looks like you are incrementing about once a second unless that is just your trend sample rate (I don't recall if you can set that and it is late, late after a very long day. Any chance your diameter checks after stopping but you didn't really get the info to the drive fast enough while running? Build at core should be faster, but ramping to speed can reduce that early build rate?

[Controls Guy]

Evilbeard -- your dancer position instability seems to cause, rather than result from, the CV oscillation.  If so, that would imply there's still some issues in the process.

Also, in the middle part of the trend, the PID output is dead flat, when it should be trying to reset.  Since DM PID is dependent, increasing either P or I should make it more aggressive.

I'd still be interested to see what P-only would do, but you're going to need a stable process for that.  I will tolerate transient instability better.