On Saturday 22 December 2012 18:21:46 Stephen Dubovsky did opine: > Sorry Peter, I have respect for you (great products!) but I *COMPLETELY* > disagree w/ this. I have my masters degree in power electronics and > control and have lots of experience in on and off road traction drives > (aka electric vehicles.) > > P first > I next > then increase them as a PAIR. The P term forms a zero that cancels the > pole the integrator put at the origin. You often place the zero ontop > of the pole from the next outer loop (due to torque or velocity mode > drive and the mass of the system) but thats getting ahead of ourselves. > > They should be able to get the whole system up and running and large > input stable w/ *JUST* P&I. When tuned properly together they can both > get very large. MUCH higher than if you do one than the other. > > The ratio determines the frequency of the zero you are adding. Look up > bode plots and gain/phase margin in the internet to get a better > understanding of this stuff. > > Once tuning it almost finished, look at the output of the integrator > during a steady state move.
Look at the output of the 'integrator'. Since my exposure to this is almost 100% from linuxcnc use, what pin or signal do I look at with the halscope to see this? IOW, please put all this in linuxcnc speak if possible. :) > Use that to compute your FF1 term (I > output/input = FF1 gain needed.) Now during moves, the integrator will > be almost zero during steady state. But its STILL just as stable and > ready to correct for any errors (workpiece mass added to the table or > cutting forces as examples.) > > Then tune FF2 to reduce accel/decel output from the loop. The PI will > have almost zero output now. But they are ready to spring into action > when conditions change and your FF1/FF2 aren't perfect (again workpiece > mass, cutting forces, etc.) > > If you don't have backlash problems and have high enough servo loop rate > and encoder resolution, D is completely unnecessary and its actually > destabilizing. Its adding another zero at even higher frequency which > is driving loop gain and phase BOTH in the wrong direction. This > causes WORSE phase and gain margins. > > This all works very well if the drive (which is part of the inner loop > you are controlling) is tuned correctly. You have to use bandaids like > D if the inner drive loop is poorly tuned (its poles are low in > frequency) or you got your zero in the wrong place to begin with. This > is my first time reading details about the mesa ac driver card and it > sounds like its running in voltage control mode. That doesn't decouple > the poles from the motors inductance and system mass. A current loop > control does. Though, you'll likely get lucky in your system is the > moving mass and inductance are so far apart the poles dont interact > much but it still won't move the system poles higher like a > current/torque loop would. > And this to me is another confusing point. The controller card in a mini- lathe is a pretty stiff controller in its own right, with the revs sagging maybe 10% when its working hard enough to clear the fuse when driven by the original speed potentiometer. To get rid of a huge non-linearity in the control characteristic, I installed a 10k resistor between the C41's output, and the input to that mini-lathe card, essentially converting the voltage the C41 generated into a small current that feeds the lathes card. Very small currant, the drop across the 10k when its wide open is perhaps .25 volts. In my view I have made a voltage mode control out of a current mode control input, so it should be considered a voltage controlled velocity servo. Correct me please if that is wrong. > System tuning and stability is very misunderstood on the internet. Tell me about it, I've downloaded and printed half the 'PID' tuning stuff google can find, and I am not markedly smarter than I was 2 weeks ago. Too many of these 'papers' invent their own names for what linuxcnc calls a parameter or a pin. Sometimes I can make a SWAG, but more often once I tried to see if it works that way here, I am back to square one. Not your fault of course, its the generalized lack of a common nomenclature from my view point. [...] Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> is up! Exercise caution in your daily affairs. I was taught to respect my elders, but its getting harder and harder to find any... ------------------------------------------------------------------------------ LogMeIn Rescue: Anywhere, Anytime Remote support for IT. Free Trial Remotely access PCs and mobile devices and provide instant support Improve your efficiency, and focus on delivering more value-add services Discover what IT Professionals Know. Rescue delivers http://p.sf.net/sfu/logmein_12329d2d _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
