On Saturday 22 December 2012 18:14:35 Steve Stallings did opine: > It is evident that discussing servo tuning is going to > generate lots of interest and comments. Stephen and > Peter both have lots of academic and theoretical > background as well as practical experience to go > with it. Many of the rest of us only have casual > exposure to the concept of the response and tuning > of servo loops. In order to help keep this discussion > on track and meaningful for those of us in the casual > category I would like to suggest that the systems be > described completely when making statements about how > they behave. > > As I understand things, Peter is referring to control > of brushless DC permanent magnet servo motors with > encoders that can provide only shaft position feedback. > Velocity, if used, is computed from position. I do not > know if the control circuits in Peter's drivers are > current mode, voltage mode, or some hybrid of the two. > I suspect that they are current mode with the current > loop controlled in the drives. > > Stephen's mill I think has classic +/- 10 volt DC > controlled PWM type servo drivers running brush type > permanent magnet servo motors with both encoders for > position and tachometers for velocity. Stephen > stated that they are presently configured as voltage > mode so the +/- 10 volts DC signal adjusts the PWM > duty cycle and thus the apparent voltage seen by the > servo motor. The motor current is not being controlled > other than a protective limit on maximum current. This > classic type of driver also closes the velocity loop in > hardware, so if the motor voltage derived from just the > +/- 10 VDC input does not result in the specified speed, > then the tachometer feedback will alter the voltage > (PWM duty cycle) in an attempt to get the specified > speed. This is a servo loop in hardware and the driver > should have electronic control adjustments independent > of the PID in the LinuxCNC software. I am going to > guess that these controls address P (control voltage > gain), a gain adjustment for the Tachometer feedback > that, and an adjustment related to the time variant > response to changes in the control voltage and the > tachometer feedback voltage. This last one may be > similar in effect to a D term, but is likely not > a true D term. As best I know the driver does not > have an I term. These types of drivers usually also > have an offset or nulling term that may be similar > to FF0. All of this is happening in the servo driver, > not in the PID software in LinuxCNC, so it is an > "inside" loop. > > So now, how do we talk about apples to apples comparison > of these two control systems? In both cases the PID in > LinuxCNC receives only a position feedback and is the > outermost loop of the control system that is used to > control position. Perturbations to the PID control can > come from both changes in the requested position and > from changes in the mechanical response to the control > system. As I understand it there is a 90 degree phase > shift in the response of the position loop between the > types of loop control, current-torque, or voltage-velocity > and this alone keeps me confused about where the response > poles are and how to adjust for them. > > While it is natural to describe servo systems mathematically, > please try to include intuitive descriptions for those of > us who are mathematically challenged. > > Thanks, > Steve Stallings
What he said, +1. Count me in the 'mathematically challenged' column. :) > > > -----Original Message----- > > From: Peter C. Wallace [mailto:p...@mesanet.com] > > Sent: Saturday, December 22, 2012 4:55 PM > > To: Enhanced Machine Controller (EMC) > > Subject: Re: [Emc-users] Pid saturated, was: Servo error > > > > On Sat, 22 Dec 2012, Stephen Dubovsky wrote: > > > Date: Sat, 22 Dec 2012 16:29:39 -0500 > > > From: Stephen Dubovsky <smdubov...@gmail.com> > > > Reply-To: "Enhanced Machine Controller (EMC)" > > > > > > <emc-users@lists.sourceforge.net> > > > > > > To: "Enhanced Machine Controller (EMC)" > > > > <emc-users@lists.sourceforge.net> > > > > > Subject: Re: [Emc-users] Pid saturated, was: Servo error > > > > > > Peter, > > > I *FULLY* understand. My 8th paragraph states if the drive > > > > is poorly tuned > > > > > (voltage mode = no tuning) and the poles are low in freq > > > > (bingo) the D is a > > > > > bandaid. Voltage mode control doesn't hide/move the plant > > > > poles to a > > > > > higher freq like current mode can. Its simpler but lower > > > > performance. > > > > > Only so much bandwidth a single loop can get if it gets > > > > handed all the > > > > > multi-order system resonances in one black box. Much > > > > better to handle each > > > > > system order at a time (torque,speed,position). The D adds > > > > zero which if > > > > > you put ontop of the next systems pole you can help things > > > > out. There are > > > > > systems that it works in. Buts its a bandaid. > > > > Um no, you dont. I was giving advice on how to tune a voltage > > mode loop, Your > > advice does not apply in this context. Its very clear that > > you have not > > actually tuned a voltage mode loop. D is absolutely needed > > for stability, A > > voltage mode drive with velocity feed forward approximates a > > torque mode loop > > (a second order system). Without a D term the solution to this system > > function is a sine wave. > > > > > > Peter Wallace > > Mesa Electronics > > > > (\__/) > > (='.'=) This is Bunny. Copy and paste bunny into your > > (")_(") signature to help him gain world domination. > > > > > > -------------------------------------------------------------- > > ---------------- > > 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 > > ------------------------------------------------------------------------ > ------ 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 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! Equal bytes for women. 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
