> -----Original Message----- > From: Peter C. Wallace [mailto:[email protected]] > Sent: Saturday, December 22, 2012 6:06 PM > To: Enhanced Machine Controller (EMC) > Subject: Re: [Emc-users] Pid saturated, was: Servo error > > On Sat, 22 Dec 2012, Steve Stallings wrote: > > > Date: Sat, 22 Dec 2012 17:52:19 -0500 > > From: Steve Stallings <[email protected]> > > Reply-To: "Enhanced Machine Controller (EMC)" > > <[email protected]> > > To: "'Enhanced Machine Controller (EMC)'" > <[email protected]> > > Subject: Re: [Emc-users] Pid saturated, was: Servo error > > > > 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 > > > Thats basically right. I think we are arguing about nothing really. > > Stevens tuning method is appropriate for first order systems > (velocity mode > servos or voltage to current loops in motor controls or > spindle speed loops) > > Tuning second order systems (and the bare hBridge systems > like the 7I39 > approximate a second order system) are tuned differently and > depend on the D > term for stability. > > > > Peter Wallace > Mesa Electronics >
OK, now I still need more education. Why is a bare H-bridge driven with PWM not equivalent to voltage mode? I thought the duty cycle of a PWM drive was effectively integrated by the inductance of the motor such that the percent duty cycle was equivalent to percentage of bus voltage on the H-bridge as far as the motor was concerned. Can you give an intuitive description or examples of a second order system as compared to a first order system? Thanks, Steve Stallings ------------------------------------------------------------------------------ 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 [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
