Re: Microstepping thru the Tulips

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
> >
> >
> >
> >
> > Addresses:
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