Jan,
Thanks. The pinion gear is held upward against the rack by
a hefty spring. I've tried the jogging with light to medium to heavy
spring settings, which while increasing the oomph required to move
the gantry, didn't seem to make much of a difference in the
inaccuracies in the jogging. I'll play some more with the
acceleration settings and see what i get out of that. Considering
what Steve was saying though, I'm thinking I may have to change sizes
on my motor pulley though to get more basic steps per rev to increase
the resolution.
mark
At 11:16 AM 3/25/2010, you wrote:
>Mark,
>I just looked through this thread and perhaps the discussion is getting a
>bit off the rails (in my humble opinion)
>Many many moons ago I also worked on stepper systems and they drove me nuts,
>exactly because of this:
>
> > I tried it at .001, .05, .01 and .1 on the jog movement. At
> >.001, sometimes I'd get no movement (I'd here stepper motor noise but
> >no movement on the dial indicator), other times I'd get a half-thou,
> >and other times it would jump .002" - .004".
>
>My take on it is that there is resonance in the stepper, and exactly when
>EMC tells it to step forward, the rotor has resonated so far to the next
>step backwards that the step impulse in the stator windings make it jump the
>wrong way i.e. backwards.
>What happens from there on is anybody's guess sometimes you just lose that
>step, sometimes the motor stops and hums sometimes it might even move some
>more backwards.
>
>The cause of the resonance needs to be addressed, it can be one of:
>1. acceleration to big, the rotor moves still syncronous but lags on the
>edge of the rotating field. Any disturbance kicks it out of syncronism.
>2. acceleration change too sudden. vaguely the same as above and the sudden
>change in acceleration is the disturbing mechanism.
>3. not enough mechanical damping. The rotor resonates all the time as it
>moves, the stepper frequency powers the resonance. Microstepping should
>solve this.
>4. sloppy or loose rack and pinion movement. I.e. there is not enough
>damping in the region of the play. See 3.
>5. Too much power. See 3
>6. Too little power. See 1. the rotor lags too much.
>8. And last but not least noise from the steppermotor power cables getting
>into the signal cable to the drive.
>One step out of time and synchronism is lost.
>9. and lastlast: lousy stepper electronics / pc interface card. With not
>enough capacitance in the right place to feed the electronic circuits and so
>cause spikes.
>
>So the first line of attack to me would be to add a bit of friction to the
>motor and see if it gets better. I say this since it is a rack and pinion.
>Else check for noise in the signal cable. You might also want to see that
>there are no irregularities in the rack. Wind the thing by hand with the
>motor shaft!
>
>So well, good luck!
>
>Cheers,
>
>Jan de Kruyf.
>
>
>On Thu, Mar 25, 2010 at 11:59 AM, Mark Wendt (Contractor) <
>[email protected]> wrote:
>
> > Mornin' Gents,
> >
> > Had a little time to play yesterday evening to play around with my
> > machine, so I decided to stick a dial indicator on both my X and Z
> > axes (I only have those two, it's a gantry machine that cuts tapered
> > triangular bamboo strips for making bamboo fly rods). The Z axis was
> > dead nuts on, but I was get varied responses to jogs on the X
> > axis. I tried it at .001, .05, .01 and .1 on the jog movement. At
> > .001, sometimes I'd get no movement (I'd here stepper motor noise but
> > no movement on the dial indicator), other times I'd get a half-thou,
> > and other times it would jump .002" - .004". At .01, the first
> > couple of jogs would show about .001" - .002" short of the full .01"
> > movement, then occasionally move the full .01", and then sometimes
> > slightly more than .01". About the same for the .05" movement. At
> > .1", the movement for the first few times was .003" - .004" short,
> > but then would move almost dead nuts on to .1" each jog. I don't
> > think backlash would come into play since all the moves were in the
> > same direction.
> >
> > I disconnected the pinion gears from the racks, moved the gantry
> > back and forth the length of the X axis a few times by hand, and
> > didn't really notice any binding. It seemed pretty smooth, and
> > relatively easy to move a 165 lb gantry on the rails.
> >
> > I thought maybe it was a software setting, so I futzed around with
> > the MAX_VELOCITY, MAX_ACCELERATION, and the STEPGEN_MAXACCEL. It is
> > pretty cool to see a heavy gantry zipping back and forth on the
> > machine at 180 inches a minute... ;-)
> >
> > Anything else I should look at? Or should I keep concentrating on
> > tuning with the three variables above?
> >
> > Thanks,
> > Mark
> >
> >
> >
> >
> >
> >
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