On Tue, 2011-01-25 at 14:44 +0100, jros wrote: ... snip > I can see you've be seriously dealing with the internals of PMSM, and I > would like to discuss a little bit about the subject,as I'm facing some > problems related to torque rippling issue. > > I agree with your comments, but it looks like complicated, as some sort > of > identification/calibration should be needed. Plus a resolver, I mean you > need to track the absolute position of the shaft during a single > revolution (may be, there are suitable encoders for that work). > > Plus, I think that the logic should be implemented directly in the > electronics driving directly the motor, as the compensation strategy > should be fairly quick. Driving a variable intensity command from EMC > will certainly limit the strategy to low rpm.
I tend to not agree (or disagree either). It is typical for the EMC2 processor to run at 1 to 2GHz as opposed to 5 to 40mHz for an embedded processor. Granted the EMC2 processor has a lot more to do, but it seems there is evidence to question which processor would be faster at motor control. > I've seen some guys trying to do such compensation dealing with this > when using linear motors, that I presume are very similar to PMSM. > > I suppose that this problem important to them probably because the > aforementioned lack of twisting, but also because it is a Direct-Drive > application. That is, no reduction, and then low velocity. > > What do you think about this?: > > I've seen referred this torque ripple as "cogging", I can feel it > in my setup (www.imac.unavarra.es/~cnc) rotating the shaft by hand my > motors switched off. I'm pretty sure that this is the main responsable > of the vibrations I feel in the robot at low velocities. I'm using an > almost direct driving. I'm not sure uneven torque is as big of an issue as one might think. If there is enough torque, the position feedback should keep the system on the planned motion schedule. The uneven torque is evaluated with a constant input, feedback can compensate for it, but it will add to the loop load. If you are getting motion oscillation, I suspect it is due to the feedback system and not the motor. Halscoping the command and feedback signals should help determine the nature of the oscillation. > Do you think that this is an indication that my stator is not twisted?. > > So I'm thinking about using pulleys to introduce some sort of reduction, > but I'm not sure if this is going to lower the problem. I mean, for a > frictionless reduction strategy, I would get a increase in rotor > velocity, but also a lower level of torque. I see a benefit as the > rippling frequency will be increased. But wouldn't worsen the ripple to > torque ratio, and then introduce more problems?. In my opinion, the pulley ratio should match the maximum motor RPM with the maximum joint speed. This will maximize the effective torque to inertia ratio, allowing for faster position correction. > I've been thinking about increasing the bandwidth of the PID (I'm > controlling intensity), hoping that the feedback can be able to > compensate the cogging, but may be then other sources of noise can > become more important. > > Then may be an identification + feedforward compensation can be the best > solution. I think it can be implemented in EMC, up to some bandwidth. > Identification can be the hard part, but if a good homing strategy is > available, is looks like possible. > > I'm thinking about making such a implementation. Do you think that it > can improve things?. > > Also what is your position about the best motor/driver type for > direct-drive position control applications. > > > I hope this discussion helps some body else to enlighten thing. It > certainly helps me. > > Cheers, > > Javier I think looking at the command and error signals with Halscope should give you the biggest bang for the buck for now. -- Kirk Wallace http://www.wallacecompany.com/machine_shop/ http://www.wallacecompany.com/E45/index.html California, USA ------------------------------------------------------------------------------ Special Offer-- Download ArcSight Logger for FREE (a $49 USD value)! Finally, a world-class log management solution at an even better price-free! Download using promo code Free_Logger_4_Dev2Dev. Offer expires February 28th, so secure your free ArcSight Logger TODAY! http://p.sf.net/sfu/arcsight-sfd2d _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
