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. 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. -- 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
