Re: Stepper current in partial-step positions

Hi,

There are two distinct "half-step" sequences.

The older one is for drives that feature "all or nothing" current
schemes such as R/L drives. In this type drive, a resistor in series
with the motor winding (or the winding's resistance itself) is used
to control or limit phase current. This sequence is the "both phases
on, single phase on" type. Because a physical resistance sets the
phase current, this drive type can only function fully "on" or
fully "off".

Phase A: +++0---0
Phase B: -0+++0--

It is characterised by having alternate "strong" and "weak" steps.

The other sequence applies to drives which can modulate current, such
as switching types that sense and servo a current to a reference
value. Now current can be any value between "0" and some limit.

With this type drive, half-stepping is best looked at as a 2
microstep per step sequence and treated as such. The general solution
here for discrete microstepping phase currents for any microstep
resolution is:

I PhaseA = k sin (90/2n + 90/x)
I PhaseB = k cos (90/2n + 90/x)

where:

k = peak current
n = # of microsteps/step
x = between and including 0 and less than n

For a two microstep solution the currents are:

I PhaseA = k sin 22.5 and k sin 67.5
I PhaseB = k cos 22.5 and k cos 67.5

To simplify for a 1A/phase motor (k = 1), the correct phase A
currents would be 383 mA (sin 22.5) and 924 mA (sin 67.5). Phase B
currents would be 924 mA and 383 mA respectively.

Finally you could normalize these currents to 1A and .4142 A.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Alan Marconett KM6VV <KM6VV@a...> wrote:
> Hi Mariss,
>
> Nice "aside" calcs! I plan on trying them. And great half/step
> comments!
>
> I tried running MaxNC's 4-phase (normally runs 1/2 step) at FULL
step,
> and boy did that make a difference! :>(
>
> I've heard of two sequences for half step, how many other sequences
are
> there, Assuming simple on/off control for the phases?
>
> I'd also like to find data on the 5-phase sequences, probably some
good
> motors out there! :>)
>
> Thanks again for your comments.
>
> Alan KM6VV