Re: Stepper Motor Resonance ?

[Jon Elson <elson-at-pico-systems.com>]

Tim & Heather Smith wrote:

>Has anyone out there had any problems with resonance when driving stepper
>motors?
>  
>
Of course, this is the CLASSIC difficulty with steppers!

>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.
>  
>
GEARboxes?  With real gears in them?  You should use toothed belts. 
 They don't have
backlash, and the slight stretching of the rubber can absorb some of the 
vibrational energy.

>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?
>  
>
If your gearboxes have backlash equal to even 1/2 of a step, they will 
severely exacerbate any
resonance problems.  I suspect your backlash is less than 4 steps, or 
your system would go completely
crazy even at low speeds.  Anyway, if you want to keep the gearboxes, 
you will need to add
dampers.  A really simple damper is a disc with a hub attached to the 
shaft.  You then make 2 more
heavy discs with holes to clear the first disc's hub, and clamp them to 
that disc with rubber sheet
between them. The clamp bolts can be just outside the OD of the middle 
disc, this will keep the
outer discs centered, but allow them to slip freely over the middle 
disc.  By adjusting the tightness
of the clamp screws, you can adjust the amount of damping that it delivers.

A more complex design, but similar to commercial units, is a fluid 
damper.  You make a hollow
disc, and put a heavy washer inside.  Maybe 100 mm diameter, and 5 mm 
thick would be good.
You put some viscous fluid inside, at least like way oil, or maybe gear 
case lube.  When such a
device is working well, it can completely damp out these resonances.

A quick check can be made with a rag wrapped around the shaft.  You 
would likely even be able to
feel the resonance, and I'm sure you have had no problem finding the 
resonant frequency by ear.

Jon