Thanks Gene, this has been a very enriching discussion. No plans still for the Hubles's mirror :).
Cheers, Javier On Wed, Apr 10, 2013 at 7:37 PM, Gene Heskett <ghesk...@wdtv.com> wrote: > On Wednesday 10 April 2013 13:01:29 Javier Ros did opine: > > > This system > > > > http://www.automationtechnologiesinc > > .com/products-page/nema23-closed-loop-stepper-motor-system-hybrid-servo- > > kit/hybrid-servo-drive-kl-5080h > > > > Is apparently a stepper motor that is controlled as a brushless. > > Essentially a stepper is a brushless. This needs a encoder, probably one > > with a index pulse correctly positioned, > > so that the electronics can compute the switching accurately. > > > > This means, even if the control of the drive looks like a STEP DIR > > control, internally there are position and current loops, theoretically > > such a drive could offer, velocity and > > current control (I've not checked for the above reference). This means > > essentially that, the motor runs cooler, because only the required > > intensity is flowing in the motor, and not > > the maximum required intensity (the one that is controlled with the > > typical potentiometer in typical stepper drives). This means as well > > that the motor runs smoother, this must > > be most noticeable at low velocities, and finally that the positioning > > can be as precise as the encoder is. > > > > I regard of precision, note that a stepper, is not as precise as > > 3600/steps_per_revolution/microsteps, because microsteps don need to be > > equally spaced, even steps are not precise due to manufacturing > > (magnetic field) > > do not have to be equally spaced. In addition to this forces make that > > the motor is not centered at the center of the microstep. > > Let me fine tune this by pointing out that the stepper motor maker can, > with access to the maps the controller uses to adjust its currents when > microstepping, could be fine tuned such that at light loading, the > microsteps can be pretty accurate. This of course means the motor and the > controller must be calibrated to each other. That will be the makers job > since few if any of us have the tools to do that, and it sure wouldn't be > feasible economically for everyone to own their own stuff to do that. > > More just plain old comment: > > The noise would likely go down a bit, but since we aren't also throttling > the current in many drivers (mine does after about a second of no motion, > so mine only heat about 15F when idle), the motor is still going to run > hot. The ideal situation would be by adjusting the overall currents to keep > the motor within say 20%/microstep of the ideal microstepped position, but > again this would require a high precision encoder, or some sort of > magnetically detected feedback to detect the error in real time & only use > enough current to achieve that. But at that point, you may as well spend > the money on a servo system, which may well be what this outfit is doing. > Net cost will be similar. My current stepper setup, using 425oz motors on > the lathe, was just under $100/axis. This is only a 252oz motor and costs > USD 210/axis. I can't seem to justify the extra sheckel's for me. > > > In comparison > > a brushless type encoder based drive for steeper can be as precise as > > the encoder, you know the actual position with the encoder position, > > although the position can be different of the commanded position, but > > you know the difference. > > > > The only limitations seem to be related to control at hih rpms, > > performance degrades in comparison with brushless. I would say that > > this is related to the higher pole count of the steeper,and the > > inherent dificulty to stablish intensity at high pole conmutation > > frecuency due to impedance, something that con be alleviated increaing > > voltage as much as possible.. > > > > In regard to this the error position, it can be even smaller in this > > brushless system because, as it runs cooler, you can allow for small > > duration current higher than the nominal. For a steeper > > you can not surpass the nominal value, not for the motor not for the > > electronics. > > Theoretically true. The motor can be banged with considerable overcurrent > when it is lightly loaded and essentially exactly in position, but if half > s step off or more due to heavy loading, then the rotor's magnetism could > be effected, permanently damaging the motor. > > > I've never run a system of this type, but I would love to use one of the > > MESA cards and brushless firmwares to test a such a setup (I'm > > interested in current control) if somebody with more experience > > thinks/knowns this is possible and not too difficult. Just using a > > double shaft stepper and a cheap encoder. I would love, to identify > > stepper cogging, and to software compensate for it. > > A moot point IMO when the gearing is such that 1 microstep is a fraction of > a micron without resorting to a doubling of cost per axis. > > I haven't actually checked, as my step accuracy is the same on both axis's > of the lathe, the x is a 2.5mm/turn screw, the z is 5, but the z is also > geared down 2/1. On my .0001 dial indicator, I can't see the individual > steps. > > > This said, if the proposed system works as theoretically expected, it > > looks to me it has a pretty reasonable price. > > Debatable, unless you are carving a new mirror for the Hubble. :) > > > Regard, > > > > Javier > > > > On Wed, Apr 10, 2013 at 6:45 AM, Viesturs Lؤپcis > <viesturs.la...@gmail.com>wrote: > > > 2013/4/10 Tomaz T. <tomaz_...@hotmail.com> > > > > > > > The whole point is that I don't have any feedback from steppers at > > > > this stage, and as I said, the "cheapest" solution would be to > > > > simply change > > > > > > the > > > > > > > existing one with the one with closed loop "future" (and also > > > > drivers). > > > > > > So > > > > > > > the basic idea might be to use In-Position signal (output) from > > > > stepper driver and when this goes fault, it triggers "following > > > > error" in > > > > > > linuxcnc. > > > > > > > Could this work? > > > > > > So why don't You put encoders on stepper motors and link encoder > > > position to axis.n.motor-pos-fb pin and let LinuxCNC track actual > > > motor position and it definitely will trigger following error, once > > > it has been reached. This way there are no fancy stepper drives and > > > motors are required. > > > > > > > Anyone using this stepper system from kelinginc? > > > > > > http://www.automationtechnologiesinc.com/products-page/nema23-closed-l > > > oop-stepper-motor-system-hybrid-servo-kit/hybrid-servo-drive-kl-5080h > > > > > > > > > No tuning of feedback loop? Well, then I do not see, how does this > > > system achieve its goal and correct for motor's position error. > > > > > > -- > > > Viesturs > > > > > > If you can't fix it, you don't own it. > > > http://www.ifixit.com/Manifesto > > Cheers, Gene > -- > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author) > My web page: <http://coyoteden.dyndns-free.com:85/gene> is up! > My views > <http://www.armchairpatriot.com/What%20Has%20America%20Become.shtml> > Only kings, presidents, editors, and people with tapeworms have the right > to use the editorial "we". > -- Mark Twain > A pen in the hand of this president is far more > dangerous than a gun in the hands of 200 million > law-abiding citizens. > > > ------------------------------------------------------------------------------ > Precog is a next-generation analytics platform capable of advanced > analytics on semi-structured data. The platform includes APIs for building > apps and a phenomenal toolset for data science. Developers can use > our toolset for easy data analysis & visualization. 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