On Fri, 2008-04-18 at 22:13 -0500, Jon Elson wrote: ... snip > No, I think I got your point exactly, and I don't think steppers > have a really good answer for this. You can't split a step, or > tell it how long to take to get from one step to the next. When > the step command is given, it goes "clunk", and depending on the > dynamics of the motor, machine, etc. it will either fall a > little short of the magnetic 'detent' or go past it. Given a > good strong stepper moving at dead-slow speed, the magnetic > detend is at maximum strength, and friction should be moderate, > so it will almost certainly go past and then pull back. That is > the nature of the beast!
I read this at Wikipedia: "A new development in stepper control is to incorporate a rotor position feedback (eg. an encoder or resolver), so that the commutation can be made optimal for torque generation according to actual rotor position. This turns the stepper motor into a high pole count brushless servo motor, with exceptional low speed torque and position resolution. An advance on this technique is to normally run the motor in open loop mode, and only enter closed loop mode if the rotor position error becomes too large -- this will allow the system to avoid hunting or oscillating, a common servo problem." > Microstepping can smooth this out to some extent, by splitting > steps, and portioning them out at some rate so as to interpolate > the movement. But, it still can't make the magnetic detents > inherent in the motor a whole lot finer, so you can maybe get > reliable motion down to quarter steps or so, but soon friction > takes over, and any attempt to move in smaller increments breaks > up into stick-slip friction. In other words, as I understand > it, in full steps, you get X torque. With half-steps, you get > X/2 torque. With quarter steps, you get X/4 pulling the rotor > into the "detent". and so on, because the size of the detent > you are aiming at is getting smaller and smaller. (I'm not > explaining this well, I'm not trying to say that the overall > torque of the motor is going down, it certainly is not. What > I'm trying to say is that as the movement increments get > smaller, the torque developed to make those smaller angular > movements gets smaller. Eventually, the motor doesn't move at > all for one microstep, then it moves more than that on the next > microstep.) > >>... 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 (?). I would think micro-stepping is just like this, except you get to try to set where the compromise is. I don't know what quarter stepping is, but if it adds a third winding in the mix, I would think the sweet spot would get wider, so even though the maximum torque is high, the stiffness at the ideal position is probably not so good. > > I am thinking that micro-stepping is like high servo encoder resolution > > and is for dynamic control, not positional control. I would think you > > would want full or half steps to set your positional accuracy and > > micro-stepping allows a means to work all the nasties while moving > > between the steps. This is pure speculation on my part. > > > There is a difference, however. With microstepping, there is no > sensing of position, so you don't know if the motor is out of > position. With a servo, the gain of the control system forces > it to respond even to VERY small errors. So, you can take a > pretty ordinary DC motor, put a 10,000 cycle/rev encoder on it, > (which gives 40,000 quadrature counts/rev) and actually MAKE it > move one 40,000th of a turn! You could never do that with a > microstepping drive. > > Jon But, I don't think you can tune a real servo system down to the last few encoder counts, or am I wrong? -- Kirk Wallace (California, USA http://www.wallacecompany.com/machine_shop/ Hardinge HNC/EMC CNC lathe, Bridgeport mill conversion, doing XY now, Zubal lathe conversion pending Craftsman AA 109 restoration Shizuoka ST-N/EMC CNC) ------------------------------------------------------------------------- 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
