Thanks Peter, > > 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 dont think so (unless you are considering high speed spindle motors). For > servo motors its not really a problem. For example with a 4 pole servo motor > (so 120 Hz for a reasonable 3600 RPM max speed) a 4 KHz servo loop will keep > the maximum electrical angle error < 10.8 degrees at 3600 RPM which will keep > torque ripple contributed by angular error issues less than 1.8%, and much > better at slower speeds, decreasing to ~0.2 percent at 1200 RPM > I think I'm following you until "angle error < 10.8" but I don't understand the rest. Wouldn't it be sin(10.8*pi/180)= 0.187 approx 18%.
In any case I think you are right. It looks feasible even with a 18%. I'll try to think in an implementation within emc. > My guess is that magnetic irregularities and lack of good sine wave drive are > more likely culprits for your cogging issues. I agree with you. > > > > 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. > > That is strange, all the (larger) PMSMs we have used have very near 0 cogging > when no power is applied. > > I made a check in a BLDC supposed to be cogging free, by a special design, and i was certainly much lower than in mines, but I could feel it. > > > > 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?. > > > > 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. > > > If your system has a high enough bandwidth, running the servo thread faster > will help. I've noticed that with our test bed dynamomter and 3 KW PMSM, a 1 > KHz servo loop rate is too slow for good performance, 2 or 4 KHz being > noticebly better, allowing much larger P and D terms and coorespondingly lower > following errors. > This is a very interesting observation. I'll give it a try. Thank you very much, Javier ------------------------------------------------------------------------------ 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
