RE: [CAD_CAM_EDM_DRO] Re: Microstepping thru the Tulips

Hi all,

Text descriptions can be difficult to follow if you haven't played with
chopping stepper drives.

I tried placing this picture in the Yahoo group but can't find a button that
lets me do that so instead here's a link:

http://www.autoartisans.com/images/stepcurrent.jpg

The motor driver is an LMD18245 micro-stepping driver. The top scope trace
shows the current sensed by the driver. The bottom trace shows the voltage
across the winding which makes the current flow through the winding.

The step rate is about 460 Hz and the motor is turning so it creates both
back emf as the armature works through the windings and also as the magnetic
field from the previous step collapses.

Hopefully the labels on the photo show everything clearly but I'll add less
than a thousand words anyway. Forgive me if I'm not totally clear on this
as I did this back in Nov. 2005 and didn't keep any notes on it.

1. When the signal to push current through a coil is turn on, the voltage to
the winding is applied and current is supposed to flow.

2. But because it's an inductor current doesn't start right away. It is
also prevented because initially there is an opposite voltage bucking the
applied voltage (back emf). Until this back emf decays no current will flow
through the winding.

3. Finally, the back emf is lower than the applied voltage (and still
falling) and we start to see current flow through the winding.

4. When the maximum current is reached the applied voltage is removed and
the current starts to decay. When the current is below the set point, the
applied voltage is there again and the current climbs.

5. This happens forever if the motor is stopped; the current averages out
to the setpoint value.

6. Finally since the motor is turning the whole things starts again with a
different winding and the current builds up again.

So where does micro-stepping enter into this you might ask? If you look at
the scope photo the yellow trace stops rising at about 2V. This may be
equivalent to 3 amperes of current through the winding. If the reference is
changed so that the yellow trace stops at 1V then there would be only 1.5
amperes of current flowing through the winding.

Where 3 amperes in one winding and 0 amperes in the other pulls the armature
around to a magnetic pole 1.5 amperes in one and 1.5 in the other holds the
armature between the poles. Various ratios between the two hold the
armature at other places. The high quality micro-steppers simulate a two
sine waves offset by 90 degrees and the armature smoothly moves between the
detenting magnetic poles. (There's more to this but this is the simple
explanation).


Another interesting phenomenon that the scope photo shows clearly is why the
torque drops off as the motor turns faster. If the step pulses were
happening twice as fast, the photo would show the current rising to about
half of the ramp before the direction of the current was changed. Less
current, less torque. In fact it wouldn't even start chopping.

If the step rate increases to a point where the current hasn't a chance to
begin to flow, the armature just locks up and doesn't move. No current, no
torque.

Finally, if the applied voltage was changed to a higher value then the slope
of the ramp would be steeper and maximum current would be reached faster.
The back emf value doesn't change because the back emf is based on the
current flowing through the windings when it's interrupted at the motor
speed; not the applied voltage.

And, if I used a low inductance motor here instead of a surplus high
inductance motor, the slope of the current curve would also be dramatically
steeper. That also results in more torque at medium RPM and a higher top
speed.

Hope that helps a bit.

John Dammeyer