Re: Microstepping

PJH,

A "full-step" drive is the most elementary step motor drive. The coil
exciting sequence is +-, ++, -+ and -- where "+" is current passing
thru the coils left-to-right and "-" is current passing right-to-
left. The sequence then repeats.

Anytime a fixed sequence repeats, you can think of it as being
a "cycle", and for purposes of what follows, you can think of a cycle
as being 360 degrees. Thus, a single full step occupies 90 degrees of
that cycle.

A quick examination of the above sequence bears a suspiciously close
resemblance to the trignometric identities of sine and cosine if you
squint at it just so; only the "sign" for both is preserved.

That being the case, you wonder, "what would happen if I drive the
motor windings with a current that has a higher resolution than a
single bit?".

To do that, you set up a look-up table that contains the values for
sine and cosine; if you want to "microstep" a motor at say 10
microsteps, the entries in the table would be 9 degrees apart.

You now output currents weighed on these values to the motor
windings. What does the motor do?

Here, it's helpful to remember the story of the donkey that was
exactly between two equal-sized bales of hay, who starved because he
couldn't decide to go right or left.

Forget the starving part; it's not imortant here. Rather, the donkey
is the motor rotor and the bales of hay are the motor windings.

The "donkey" would be drawn to the left "bale of hay" at 0 degrees
because there would be no right "bale of hay" if the left one is
cosine while right one is sine (sine 0 = 0, cosine 0 = 1).

As the angle advances from 0 to 90 degrees, the left "bale" (cosine)
gets smaller while the right "bale" (sine) gets larger. The "donkey"
positions itself between the two at the point where the attraction to
each is identical. His position gradually shifts from the left "bale"
to the right "bale".

That's how it works.

Mariss

--- In CAD_CAM_EDM_DRO@y..., "P. J. Hicks" <hickspj467@a...> wrote:
> Hi,
> Another newbie question.
>
> I see 'full step', 'half step', 'quarter step', and 'eighth step'
mentioned at various times. If the stepper motor moves for example,
1.8 deg/pulse (step) for each pulse (step) put out from the PC
parallel port how does the motor move 1/2 or 1/4 or 1/8 steps? Or
does it move 2, 4, or 8 steps? Or is the controller able to
manipulate the motor windings to get multiple steps OR partial steps
for each step output by the PC? Then how is the above effected by
parallel/series windings connection?
>
> Computer says move one step; does motor see 1, 2, 4, or 8 steps OR
sees 1, 1/2, 1/4, 1/8 step? How?
> PJH
>
> [Non-text portions of this message have been removed]