Re: [CAD_CAM_EDM_DRO] RE: Stepper questions

Alan Marconett wrote:

>HI Dave,
>
>That's an interesting point! I don't recall reading anything like that, I
>could be wrong. Basically the windings do not interact as far as I know.
>
>

The two halves of a stepper phase coil (on 6- and 8-wire motors) DO
interact.
They are usually wound on the same coil bobbin, and act as two windings of
a transformer. If you have twice the turns, then the inductance is squared.
Because you only need half the current in full winding mode, it only
takes twice
the voltage to turn the current around in the same time. But, at the
same voltage,
it takes 4 times longer to turn the current around. That's why the
half-winding
scheme is often used on a 6-wire motor with a bipolar drive.

>Actually, one would think that "turning around" a winding, as in a bipolar
>drive would take longer. The field would have to collapse before it could
>build up in the opposite direction. Therefore wouldn't a unipolar drive,
>with its four windings have the advantage?
>

No, because the current in the winding that is shut off has to decay,
and that takes
time, or infinite voltage standoff of the transistors. Second, the
decaying of the
"old" winding interferes with the filling up of the "new" winding, so it
is the SAME
problem as the bipolar faces, really.

> And I'd think that the unipolar
>stepper would have the further advantage of having two windings available
>for microstepping. Half-stepping of a unipolar stepper is relatively easy!
>Just alternate one winding on and two windings on! Ever see a 5 phase
>stepper? I still haven't figured out the stepping sequence for that, but
>they do 400 steps!
>
>
>

You can do this with a bipolar drive, too. Just turn one winding
completely off inbetween
+ and - polarities. Of course, you lose 33% of your torque that way.

>I'd like to see the "dual" specs for such a stepper. I'm guessing that they
>rely on a difference in current, and possibly some dissipation differences
>between the two modes. Same current * turns = same torque!
>
>
>

For holding torque, this is absolutely correct. For running torque,
EVERYTHING is
up in the air. It depends entirely on how the drive manages the
collapse of the
field in the winding that is being turned off. Providing an effective
scheme to
dump the inductive energy quickly can greatly improve the running
performance.
But, that adds a lot of parts. The most common technique is to place
diodes to
return the current to the power supply, but this basically leaves the
coil short-
circuited. Putting some resistance in series with those diodes will
increase the
voltage on the transistors, but will make the field collapse faster.
But, then the
resistors will get hot. Especially in a chopper-type drive, they will
be carrying the
circulating current nearly all the time while the motor is holding
position, and get
REAL hot. So, you need more transistors to shunt the resistors except
when moving
above some speed. Eventually, it becomes obvious why a bipolar drive is the
easiest way to get a simple, high performance drive. And, as for
microstepping,
if it doesn't do microstepping, it isn't worth having! (My opinion, of
course! But,
if you've worked with a Gecko G201 drive, not much else even comes
close! And,
a full-step, or even half-step unipolar, L/R drive is so FAR from a
Gecko that it is just
a really bad joke.)

Jon