Kirk Wallace wrote: > Please correct me if I am wrong, because I am making some of this up. > With single stepping one stator winding acts on the rotor. The rotor and > stator poles match well so there is a tight sweet spot. With half > stepping, some of the steps use one winding an others use two, so their > torque is nearly twice as much except the rotor poles are trying to > compromise between two stator poles so there is a wider sweet spot (?). The rotor and stator are not the two phase windings on a stepper. It could be thought of as two separate single-phase motors with a 90 degree offset between them. The magnets are in the rotor. See http://www.ece.osu.edu/ems/ee647/Lab_Manuals/Lab5.pdf for a picture and more info.
But, you are right about the torque variation with the simple half-step scheme. But, you double the excitation frequency from full-step mode, and that apparently is more important. > > But, I don't think you can tune a real servo system down to the last few > encoder counts, or am I wrong? > Depends on the system, to a large extent. But, yes, a well-tuned servo can indeed get down to a couple encoder counts even during hard acceleration and discontrinuities in the velocity/accel curve. I hold position within 15 encoder counts on my minimill throughout a 60+ IPM move. I'm sure I could do better, but that is already down to 100 uinch, so why should I care. (That machine has an encoder resolution of 128,000 counts/inch, due to belt reduction and fine-thread screws.) My Bridgeport holds to +/- 3 encoder counts, at 20,000 counts/inch, pretty much throughout the speed range. Those are velocity servos, though, and have additional DC tachometer feedback to the servo amps. Jon ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
