I think it is just easy to have 2 pid's and sum them.. (one on the motor encoder with P,D and feed forward - the other with scales and the I term) Then you are not re-writing the pid component for some odd corner case.
sam On Mon, Jan 3, 2022 at 10:36 PM gene heskett <[email protected]> wrote: > On Monday, January 3, 2022 11:00:54 PM EST Sam Sokolik wrote: > > I think this just would work with linuxcnc and scales + motor encoders. > > WIth the scales using only I in the 2 pid loops - it would be constantly > > correcting backlash to keep the position.. (that is what the G&L did) > > > > sam > > Or, better yet, using the new 3 phase servo steppers where the motor > encoder > only feeds the driver, use a PID to drive the stepgen, and feed only the > scale back to the PID with only the FF0=1, and Igain active? > > > > > On Mon, Jan 3, 2022 at 9:50 PM Chris Albertson < > [email protected]> > > wrote: > > > I think his method would work for your mill. Yes the tool pushed the > > > table and the table moves. What he does is not just a double loop. > He > > > models the movement of the table and predicts how it will move. To > do > > > this on a mill you would need encoders on the table and then run a > > > calibration cycle that measure table movement as a function of force. > > > > > > Then as the mill cuts he can update the model parameter in real time by > > > seeing if the predicted table motion matches the actual table motion. > > > > > > In this video he loads a pencil. He says (in comments some place) the > > > reason for the initial slow movement is that the system is measuring > > > flex > > > vs. force applied as it puts the lead in the pencil. after a second > or > > > so the data is good enough the system can move faster. You can see the > > > rate of lead insertion gets faster. It is because he has better data. > > > https://youtu.be/GCHXNcpq3OA > > > > > > > > > On Mon, Jan 3, 2022 at 7:02 PM John Dammeyer <[email protected]> > > > > > > wrote: > > > > Interesting concept. I notice that although he showed his finger > > > > pushing > > > > on the jaws (and text said compensation turned off) we never saw him > > > > disturb the jaws with compensation on. That he can position > > > > accurately > > > > > > is > > > > > > > pretty cool. But backlash on my X axis results in the milling cutter > > > > pulling or pushing the table under heavy cuts. > > > > > > > > The only real solution is ball screws or double nut ACME. > > > > > > > > Now for a robot arm that does pick and place of work "on to"/"off of" > > > > the > > > > mill or a conveyor belt maybe it's not an issue? > > > > John > > > > > > > > > -----Original Message----- > > > > > From: Chris Albertson [mailto:[email protected]] > > > > > Sent: January-03-22 6:40 PM > > > > > To: Enhanced Machine Controller (EMC) > > > > > Subject: [Emc-users] Very good software backlash control demo. > > > > > > > > > > CNC is about precision motion control. Here is a new idea where > > > > > this > > > > > builder gets 0.05mm accuracy but uses hobby level R/C servo Not > > > > > only > > > > > > > > that, > > > > > > > > > but he connects three of these in series, one to the next to the > > > > > next > > > > > > so > > > > > > > > all the backlashes and poor tolerances add together. Then he uses > > > > > this > > > > > > > > to > > > > > > > > > do precise motion. He loads a mechanical pencil with this chain of > > > > > > cheap > > > > > > > > parts. > > > > > > > > > > What does this means for LCNC? It means that someone has found a > > > > > > > > software > > > > > > > > > solution to backlash. What he does is place a quadrature encoder > > > > > on > > > > > > > > both > > > > > > > > > the motor and the output shaft. The difference in encoder > reading > > > > > is > > > > > > > > an > > > > > > > > > exact measure of mechanical backlash and effective gear ratio. > He > > > > > > can > > > > > > > > measure the backlash under different conditions and store the > > > > > measurements. Then he places a cascaded PID controller and Kalman > > > > > > filter > > > > > > > > over this hardware. > > > > > > > > > > Technically the problem with backlash control via software is the > > > > > delay > > > > > from input to output pays poorly with the PID algorithm. He > applies > > > > > a > > > > > predictive model. > > > > > > > > > > Checkup this video. It is unimpressive if you have a $100,000 CNC > > > > > > > > milling > > > > > > > > > machine, but he is using a linked chain of hobby servos. The novel > > > > > > idea > > > > > > > is > > > > > > > > > his software. It is on github, you can read it. > > > > > https://youtu.be/gq-P39rfRqU > > > > > > > > > > He explains it here. Notice in the video he shows the backlash. > > > > > The > > > > > gear-slop is at the 1/4 inch level but using his software backlash > > > > > correction you can see the results in the dial indicator is about > > > > > > 0.05mm > > > > > > > > (or about 0.002 in American units) Not bad given the truly > horrible > > > > > mechanics. > > > > > https://youtu.be/SioCwvR_PYY > > > > > > > > > > Why do I care? I have a robot-dog leg here on my workbench using > > > > > > hobby > > > > > > > > servos, let's say performance could be improved. But the > anti-lash > > > > > technique might be used on a real milling machine. Maybe one of > > > > > the > > > > > experts here could look and see if it could be applied? I will > > > > > use > > > > > > > > parts > > > > > > > > > of his idea on my dog-bot. > > > > > > > > > > -- > > > > > > > > > > Chris Albertson > > > > > Redondo Beach, California > > > > > > > > > > _______________________________________________ > > > > > Emc-users mailing list > > > > > [email protected] > > > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > > > _______________________________________________ > > > > Emc-users mailing list > > > > [email protected] > > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > > > -- > > > > > > Chris Albertson > > > Redondo Beach, California > > > > > > _______________________________________________ > > > Emc-users mailing list > > > [email protected] > > > https://lists.sourceforge.net/lists/listinfo/emc-users > > > > _______________________________________________ > > Emc-users mailing list > > [email protected] > > https://lists.sourceforge.net/lists/listinfo/emc-users > > . > > > Cheers, Gene Heskett. > -- > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author, 1940) > If we desire respect for the law, we must first make the law respectable. > - Louis D. Brandeis > Genes Web page <http://geneslinuxbox.net:6309/gene> > > > > > > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
