On Wed, Feb 2, 2011 at 1:57 PM, Kirk Wallace <kwall...@wallacecompany.com>wrote:
> On Wed, 2011-02-02 at 12:13 -0700, kil...@bobodyne.com wrote: > ... snip > > Are you talking about a constant friction or somethingmore like > > sticktion where the initia zero-velocity friction is much higher than the > > constant velocity friction? > ... snip > > > That's a good point. With any kind of friction, when EMC2 calls for > motion the axis motor may move, but flex in the system will delay the > feed back, unless the feed back (encoder) is on an unloaded part of the > axis motor. No, this is regular friction, constant force that is opposed to the direction of movement. There is no sticktion. I checked it with the servo motor off, driving the worm screw with pliers. Everything is well greased, just tight. > > If the encoder is on the unloaded part of the motor, the > indicated error would be small, but the actual joint position could be > off by a bit. If the encoder is connected more towards the joint output, > the actual joint position feed back will be better, but will convey the > motor motion less accurately. The more the motor motion and the joint > motion differ, the harder the system will be to tune, because the motor > may start moving but EMC2 won't see it until the encoder starts moving. > With stiction, not only will the feed back be delayed, but the > break-free nature of the motion will cause the encoder to try to catch > up all at once. This is just like backlash in the feedback loop. To > improve the situation one needs to: > > * Reduce the friction and backlash > * Increase the feedback stiffness > * Increase the feedback loop frequency (decrease servo period as much as > possible, the loop needs to be as fast as the aberrant motion, but this > only works if the feedback is keeping up) > * Have an encoder on the unloaded motor end and another on the joint > output. > > I think one might need a motor current and error trace in HALscope to > see the stiction and backlash. > I will check some more. I am very seriously snowed in and spend appx. 4 hours digging out, with more to do. i > -- > 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 > ------------------------------------------------------------------------------ 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
