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. Get a free account! http://www2.precog.com/precogplatform/slashdotnewsletter _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
