Thanks Kirk, ... > > 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. >
Well, you are right but if you have 6 motors, and you intend to generate the PWM, and a parallel kinematics with the direct and inverse model, etc,... Then it can be possible that maximum loop frequency in EMC can go too low. Having an embebded proccesor or driving unit for each motor can eventually lower the load. Anyway, everything is possible, and best solution can be very dependent on the setup. > > 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. You are right, in fact I have pretty low following error measured in halscope. So torque ripples are dealt with by the controller, nevertheless, even if the vibration does not affect to the following error, and it is low amplitude, the vibration is still there. I presume that the lower the torque ripple the better, whatever the PID. I've seen -in a commercial demonstration- the same BLDC driver acting same BLDC motor, but the one of them with a patented -don't ask- design that lowered the torque ripple. The control was in velocity, and the noise levels lowered to less than half. Nevertheless, my encoders pulses are generated after the sin cos signals of the resolver, and I'm pretty sure that this is introducing a error, within a revolution of the shaft. This implies a inertia force in the at th frequency of the rotors and their integer multiple. This is not only a source of imprecision but also a source of noise. This is why I'm in the process of changing my "virtual encoders" to real linear ones in the linear guide trying to isolate to the lack of the stiffness in the linear guide shafts and couplings. Nevertheless the noise I'm hearing, is not in the range of frequencies of rpm, but higher as I'm using direct coupling only cogging or torque ripple (may be friction although I feel it is not) can be the sources. It would be nice to be able to remove them using a feed forward approach. > > 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 agree. Using a single pulley what is the maximum reduction ratio advisable?. Also I've a question, using toothed pulleys can be advisable, but should I consider to use a special type, to avoid additional torque ripple coming from the expectable variable stiffness of the toothed belt (this effect can introduce a parametric resonance). > > I've been thinking about increasing the bandwidth of the PID (I'm > > controlling intensity), hoping that the feedback can be able to ... Thank you again, 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
