Re: Going home / more stepper speed!

Alan,

I respectfully disagree with Jon. You cannot see ringing with either
a voltage probe or a current probe when the motor windings are driven
with a full bridge at low speed. This is because any back-EMF signal
is completely washed out by the motor's inductive current and the low
impedence voltage source that constitutes the bridge.

I have tried very hard to come up with differential canceling circuit
schemes to accomplish this because a real breakthrough would be the
result.

If it were possible to reliably retrieve this signal then it would be
very simple to use it to use it to actively damp the ringing and also
have direct missed step feedback.

The signal is there alright, at higher speeds (>100 full steps per
sec), it is easy to find. Unfortunately as speed approaches zero, so
does the back EMF.

Take another look at the high speed phase current with the current
probe, but this time with the motor stalled. What you will see is a
perfect triangle waveform. This is the motor's inductive current.

If you were to subtract that waveform from the one you see when the
motor is running (at the same speed), you would have a sinusoid that
is pure back EMF. Most interesting, the phase relationship between it
and the inductive current (full step drive freq) bears a direct
relationship between pecentage of available torque (stall) and phase
angle.

From this phase angle you can derive a feedback signal that can warn
the PC of immenent stall in time to take corrective action.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Alan Marconett KM6VV <KM6VV@a...> wrote:
> Jon, Mariss, Ballendo and the list,
>
> I did some experimenting last night, got a current probe (2ma/mv
> Tektronix) and looked at a motor winding's current. I can see the
> triangular current waveform of the +- 2A current responds to the
step
> pulses. At quite slow speeds, A pair is seen (full step). Half
step
> shows one current pulse with a lower amplitude. faster speeds, and
the
> previously separate pulses merge together, forming a continuous
change
> from one pulse to the next. Sometimes the "sides" of the pulses are
> nice and "flowing" (a little wavy), other times the motor gets a
little
> noisy, and I see "jags" on the sides of the pulses. No "ringing"
> observed. <snip>