Re: [CAD_CAM_EDM_DRO] Re: Microstepping thru the Tulips

This diagram illustrates my point nicely. We may be in violent agreement, as
we used to say at Intel. The ramp time at the beginning of each step depends
on motor inductance and power supply voltage. It cannot be reduced without
changing one of those variables. The reciprocal of that time is the
frequency at which the chopper (PWM, fixed off-time or whatever) ceases to
have any effect. On this diagram it is a little more than half the period of
the step frequency, so let's say for the sake of argument the
chopper-irrelevant frequency would be 2*460, or about 920 steps per second.
That's well below the top speed of most motors I know of. One thing is
indisputable: there is no microstepping unless the chopper can modulate the
current.

So the question is, at what step rate can the chopper modulate the current
in a meaningful way? If you look at a 16x microstepping drive, the
literature claims to provide 8 different current levels during a single
motor step (8 ramping up, 8 ramping down.) Best case, the chopper would have
to operate for at least 16 cycles during the motor step. That means the
chopper is relevant only at full-step rates at or below 1/16th of the
chopper frequency, which commonly seems to be 20 or 30 KHz. In other words,
maybe 1000 to 2000 steps per second, depending on the motor. As we have
seen, beyond that it doesn't matter anyway because the chopper does nothing.

I would suggest that what microstepping really buys you is the ability to
operate the motor in the resonance frequency range with less chance of
stalling, and that contrary to many comments I have seen, it has little or
no effect on high-speed performance. I suspect that drivers offering the
best high-speed performance simply have better control of their switching
times.
--
Phil Mattison
http://www.ohmikron.com/

----- Original Message -----
From: John Dammeyer <johnd@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, August 13, 2006 11:54 PM
Subject: 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
>
>
>
>
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