Re: Microstepping thru the Tulips

I have thought a little about the relationships you mentioned over
the years.:-) I'll just cover the step frequency vs. PWM frequency
thing:

I think you misunderstand PWM works. Microstepping frequency and PWM
frequency are completely unrelated. This thought experiment may help
to straighten it out:

I think you agree that 400 Hz step rate on a full-step drive results
in a 100Hz current waveform frequency. The full-step sequence repeats
after 4 full steps after all. This modulates OK on a 20kHz PWM for a
motor speed of 120RPM.

A 250-microstep drive needs a 100kHz step pulse frequency to get a
speed of 120RPM. The current waveform frequency is still the same and
it still modulates OK.

100kHz is way above the PWM frequency but the PWM never sees it. It
only sees the resulting 100Hz reference, the same as for the full-
step drive. This also modulates OK on a 20kHz PWM.

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Phil Mattison"
<mattison20@...> wrote:

>
> I've seen a lot of comments about microstepping on this board as if

it is

> the best thing since sliced bread. I wonder how many people have

thought

> about the relationships between step rate, PWM frequency and motor
> inductance. Since microstepping supposedly uses the PWM chopper to

regulate

> current into something like a sine wave to improve motor

performance, it

> stands to reason that if the step rate gets anywhere near the PWM

frequency

> that theory goes out the window. Also, the motor coil inductance

puts an

> upper bound on the step rate at which the PWM does anything at all.

Exactly

> what those limits are depends on the combination of motor

inductance, power

> supply voltage, chopper switching speeds and probably several other

factors

> with less impact. So to say that more microstepping is better than

less is

> like saying that blue is better than orange.
> --
> Phil Mattison
> http://www.ohmikron.com/
>