On Monday 10 March 2008, Kirk Wallace wrote:
>On Mon, 2008-03-10 at 10:49 -0400, [EMAIL PROTECTED] wrote:
>> Please, I did not in any way mean to be unfair to EMC. I did not mean it
>> in that way at all.
>>
>> EMC is a fantastic program.
>>
>> What I mean is that by doing the electronic gearing there is no way to
>> avoid a condition where a stepper motor is operating at a step rate where
>> resonance is present. It will happen and there is not anything EMC can do
>> to prevent it. Its just a condition of steppers that cannot be avoided.
>>
>> It's hard to read E-mail and not have the writing be taken in the wrong
>> way. Don't take my comments to be bashing EMC in any way. It has been
>> everything I have been looking for in CNC control. I have no issues with
>> EMC at all. It's a fantastic package and there has been a ton of work put
>> into it.
>>
>> Jim C
>
>I am brainstorming here.
>
>I think you stated the crux of the problem with "there is no way to
>avoid a condition where a stepper motor is operating at a step rate
>where resonance is present."
>
>The mechanical damping can move where resonance happens.
No, it can serve to absorb it, quite well from my experiences so far. Common
sense says the weight of the damper will lower that frequency somewhat also.
In the dampers I have, I can magic marker all those fender washers, run that
azis to its limits each way once, and no washer will be where I marked it.
They work by slipping against the rubber, maybe as little as .001 degrees per
step per washer, but that slippage energy is subtracked from the stored
energy, encouraging the motor to move smoother because the slippage can be
and is, both ways on a given step's motion. The inner shaft is 2 or 3 thou
smaller than a 5/8" hole fender washer, but the rubber washers have only
about a 1/2" hole, and roughly carved at that, so that the rubber washers are
reasonably well locked to the inner shaft, and bulge out at the center when
forced over the inner shaft to lightly grip the fender washers near their
centers. Leave the retainer collar loose enough that you can turn any of the
fender washers by hand fairly easily.
>I also noticed
>from other threads, that the voltage the stepper is driven with can
>change the resonance frequency.
Yes, the higher the voltage, within limits the stiffer it will be in that rpm
range where the inductance of the motor is costing it torque. Below that
range it should be a wash cuz the driver is chopper regulating the current
and therefore the strength of the magnetic spring action.
>I think the ideal would be to move the
>resonance above the maximum operating speed, so you never see it in
>normal operation.
That would take a new concept in current profile control, and even then may
not work well as the spikes of high current that would imply might be high
enough to demagnetize the rotor, and effectively reduce its power forever.
>Isn't micro stepping the most effective tool against
>resonance?
Yes, the finer the better although going beyond 8 steps does seem to be an
area of diminishing returns.
>Or maybe an RC filter on the driver outputs to help shape the
>voltage.
Absolutely not. The inrush currents in any capacitance would quickly overheat
and destroy the H bridge components in the output stage, and any resistance
there is pure power loss. We are cautioned to not even hook a scope probe to
a motor lead because of its added capacitance.
>Of course EMC is flexible enough to be able to set up some sort
>of feedback to mechanically or electrically change the stepping
>characteristics.
There are others here who can describe the sharpness of the stones in that
path far better than I. I'll only say its a rocky road. :)
>Brushless DC motors are just low pole count steppers aren't they? I
>wonder if a stepper driver could be used to drive a BLDC? Though I
>suppose a BLDC driver might be better.
And that is another area I'll invite others to comment on.
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
QOTD:
"Even the Statue of Liberty shaves her pits."
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