RE: Stepper Motor Resonance ?

["Tim Goldstein" <timg-at-ktmarketing.com>]

500 in/oz motors on a Bridgeport sized machine seems pretty wimpy if you
are looking for good rapids. That is about the size that is normally
used on a mill drill machine. Also EMC is known as having a less than
perfectly paced pulse stream. This will tend to start resonance. You did
not mention if you are using steppermod.o or freqmod.o? freqmod.o is
better in regards to the pulse pacing than is steppermod.o. You may want
to consider one of Jon Elson's expernal pulse boards. That is reputed to
give a very smooth pulse stream which will definitely be a help to you.
The only other thing I can suggest is to get some proper 1000+ in/oz
motors on that machine.

Tim
[Denver, CO]
www.KTMakreting.com/CNC.html


> -----Original Message-----
> Has anyone out there had any problems with resonance when 
> driving stepper motors?
> 
> I have a bridgeport mill onto which I have adapted steppers 
> (Superior Electric M111 ,torque is 500ish oz/in) and am 
> running BDI 2.16. The X axis I have has been limited to a top 
> speed of about 600mm per min (240rpm & 800Hz). I have 
> occasionally lost some steps (due to lack of torque), so made 
> up a couple of reduction gearboxes (1:2.4) to over come this.
> 
> The torque is OK now, but my rapid traverse speed is now 
> limited to approx 250mm per min (240rpm & 800Hz). This is 
> quite a pain....
> 
> In an effort to figure out what was going on, I used a 
> function generator to directly pulse the stepper drivers 
> (Superior Electric STM103's) with the motors disconnected 
> from the lead screws.
> 
> I found the motors were stalling at around the same 
> frequency, but if I accelerated them quickly past this 
> frequency they would run happily up to over 3000Hz (or 
> 900rpm) which would give me 930 mm per min. While I don't 
> need to travel this fast it would be good be able to get back 
> to around 600mm/min again.
> 
> Is there a simple fix for this that I could try? or is there 
> some thing I could do to move the resonance frequency higher?
> 
> 
> Thanks
> 
> Tim Smith