Re: [CAD_CAM_EDM_DRO] Re: Microstepping thru the Tulips

My random thoughts about stepper speed, in case you're interested.

Thinking of the problem from the perspective of requirements, what is the
maximum speed you can get out of a stepper? Consider it as a two phase AC
servo, driven with sine wave inputs. As the frequency of the input goes up,
the current lags and falls in amplitude. Eventually the reactance of the
motor can't be compensated to maintain current limited by the voltage rating
of the windings.

Clearly low inductance, high current steppers can be operated faster. Used
to be that a tradeoff existed in power amps preferring lower current and
higher voltages. MOSFETs get better every year and now there's no real
reason to design in the low-inductance steppers from the beginning.

In any case, you can calculate your maximum frequency from your power supply
voltage and current drive. Just an outside bound. Ideally, you'd like a PWM
at least 2x with at least 8 bits resolution. In any case cheap FPGAs can run
500MHz and can be had with built-in multipliers.

So what's to keep an FPGA connected to a power amp from being a universal
motor controller? (Other than someone has to write the code.)

On 8/14/06, Steve Stallings <stevesng@...> wrote:
>
> violent agreement indeed, except for your assumption
> of what most folks consider high speed.
>
> If we assume Gecko drives microstepping at 10 micro-
> steps per full step and the known appx. 20KHz PWM
> rate, then we get a possible 10 rev. per second or
> 600 RPM with microstepping still nominally functioning.
>
> Apply this to a Bridgeport with 5 TPI screws and
> 2:1 gearing (typical of Boss series) and you get
> 60 inches per minute. Not blindingly fast, but a
> long ways from slow.
>
> Cheers,
> Steve Stallings
> www.PMDX.com
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Phil Mattison"
> <mattison20@...> wrote:
> >
> > 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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> sister site to the CCED group, as many of the same members are there, for OT
> subjects, that are not allowed on the CCED list.
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> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING
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