On Thu, 5 Apr 2012, lloyd wilson wrote: > Date: Thu, 05 Apr 2012 20:32:27 -0400 > From: lloyd wilson <llwilso...@rochester.rr.com> > Reply-To: "Enhanced Machine Controller (EMC)" > <emc-users@lists.sourceforge.net> > To: Emc mail list <emc-users@lists.sourceforge.net> > Subject: [Emc-users] torque-velocity questions > > We are investigating using my favorite cnc controller for an application > which will require as much stiffness as we can arrange; websearching > suggests that torque mode rather than velocity mode is preferrred in > such applications. We are using a Pico PPMC interface set to mediate > between the computer and a Parker Aries amp/motor combination (+/- 10v > control input, 2000 line encoder, vendor rep loaded any requisite motor > parameters into the amp). We finally have the system so it doesn't > immediately fault on a following error, but only in velocity mode; we've > been totally unable to generate a set of PID parameters that are viable > when the amp is set to torque mode. Ergo, it's guru time- > > 1 - is there some inherent incompatibility between Linux CNC and torque > mode servos?
No, but a torque mode PID loop is also responsible for velocity control. This is handled by a separate high speed loop in your drive when your drive is in velocity mode. A Torque mode PID loop often needs to to run at a higher sample rate than a PID loop controlling a velocity mode drive. That is to say LinuxCNCs 1 KHz default servo thread rate may be marginal for a torque mode drive (not enough phase margin for the D term). Also because a torque mode PID control loop depends so much on the D term, its helpful if the velocity feedback is a good as possible. I'm pretty sure the PPMC has velocity estimation (as opposed to plain old crunchy dp/dt) in its encoder counter/driver, but you need to connect the velocity estimate to the feedback-deriv pin on the PID component to make use of this better velocity feedback. > 2- is there some deeply buried config parameter that needs to be changed? Not that I know of (other that using the velocity estimate instead of the PID comps default dp/dt) but torque mode PID tuning is different and somewhat tougher to do that velocity mode tuning (IMHO). > 3- can somebody point us to a strategy specifically oriented to tuning > torque mode servos? Well I'm sure there are some good tuning tutorials... But from memory this is what I have done with torque mode drives: first I would do step response plotting and then alternately bump up P till it overshoots on a step and then bump up D till you regain a little less than critical damping and then repeat bumping both up to the point you can no longer make it stable with more D. Then back P and D off a little till its stable for the next step. Next I would adjust FF2 by doing a controlled move at say 1/2 the accel capability of the motor/drive/load and adjust FF2 to null the error during accell/decell. Similarly I would adjust FF1 by doing a long slew at perhaps 1/2 of the machines maximum speed and nulling and error during constant motion. At the end I would add I term until the drive becomes unstable (jog while adjusting to make sure you notice when it becomes unstable) and then reduce the amount of I term to about 1/2 the value that caused instability. > 4- any other ideas? > > as always, thanks in advance - this community never stops astounding me > with its knowledge and its willingness to share > > > lloyd > > ------------------------------------------------------------------------------ > For Developers, A Lot Can Happen In A Second. > Boundary is the first to Know...and Tell You. > Monitor Your Applications in Ultra-Fine Resolution. Try it FREE! > http://p.sf.net/sfu/Boundary-d2dvs2 > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > Peter Wallace Mesa Electronics (\__/) (='.'=) This is Bunny. Copy and paste bunny into your (")_(") signature to help him gain world domination. ------------------------------------------------------------------------------ For Developers, A Lot Can Happen In A Second. Boundary is the first to Know...and Tell You. Monitor Your Applications in Ultra-Fine Resolution. Try it FREE! http://p.sf.net/sfu/Boundary-d2dvs2 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
