The inner loops output would go to the drive. The feedback for the inner loops would be the velocity feedback from the encoder, connect that to the same pin on the inner loop's PID component as you do the position feedback to the outer loop. The input command of the inner loop gets fed the velocity command from the outer loop (aka the output.). Tune the inner loop by feeding it a small square wave command that is long enough to get up to near the commanded velocity and a command velocity of maybe a 10th of the machine's max. (Not so much that a good jerk won't hurt it.) Tune a much P and D as it can handle and be stable, then add I to try to minimize following error. You want to have the fastest rise to a smooth constant velocity as you can get without overshoot and no wobble.
Tune the outer loop just like All the online velocity mode tuning tutorials. Good latency is important for tuning a torque mode servo. Running a faster thread can also be beneficial, so a good PC is a must. On Thu, Jan 9, 2025, 4:11 PM Viesturs Lācis <viesturs.la...@gmail.com> wrote: > ceturtd., 2025. g. 9. janv., plkst. 21:30 — lietotājs Todd Zuercher > (<tzuercher1...@gmail.com>) rakstīja: > > > > Ona normal velocity PID loop, the command input and feedback are in > > position units, only the output command is in velocity units. For a > torque > > command servo, if using a double nested loops the outer one would be a > > regular position/velocity loop, but the inner one would be velocity > command > > input to torque command output. A single PID loop can be used with > torque > > command servos, but tuning them is a lot harder and they are a bit > unstable > > when loads change and at high or low speed. For such a loop tune with > lots > > of I term with an i-error limit set so the I term can never saturate the > > output command. This allows a fast I reaction without windup induced > > overshoot causing instability. > > > > This makes sense. Servodrives are Mesa 8i20, input value to the drive > is current, which means "torque". I kind of have them running with > single PID loop, nothing particular to complain but am not very happy > as either the ferror is more than 0.06 mm or motors become noisy and > are vibrating. This explains why I could not get ferror down to > 0.01-0.02 mm... > Nested /cascaded PID loops sound good, I found a forum thread > mentioning that as well, but I have no idea how to start with that. I > would appreciate any advice on how to actually tackle this. I suspect > that I should start with inner loop first. I even found that P, I and > FF1 parameters should be used for inner loop, P and FF1 parameters for > outer loop. It is just that I am not sure what should be used for > inner loop's command when outer loop is not there yet. Connect inner > loop's pid.m.command to joint.n.vel-cmd? And should the > "encoder.nn.velocity" linked to pid.m.feedback? > And keeping in mind that I want some jerk limitation as well, what > could possibly go wrong if I put limit3 component between both pid > loops? It is acceptable that overall ferror will be around 0.05 mm > with max accel of 800m/s^2, I just need to minimize those kicks when > actual acceleration value instantly changes. > > Viesturs > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
