Kirk Wallace wrote: > On Fri, 2008-04-18 at 12:17 -0500, Jon Elson wrote: > >>Well, as for stair-step motion, there is no simple cure. You >>can gear down the stepper with a belt and pulleys, but then you >>lose top speed, rather severely with steppers. > > ... snip > I think I am not getting my point across, fortunately it is only an > academic pursuit. What I am concerned about, is axis travel due only to > position oscillation as the motor settles into its new step position. > With a properly tuned servo, the position is always arrived at from the > starting point. If P is too high, the axis will over shoot and try to > correct. If you are cutting material at this time, you will have taken > off too much, even if you finally arrive to the desired position. I > imagine that steppers do this on every step, but will never ring or over > shoot more than a half step making the effect negligible. > 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!
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.) >>... > > 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 ------------------------------------------------------------------------- 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
