Re: chopper drive - resonance problems - need ideas

Joel,

Resonance is a well recognized phenomena in steppers. One of the normal
first cures is to use 1/2 steps instead of whole steps. This often
eliminates the problem. It also seems you need to be concerned about it in
the actual application, not with the motors free standing as the additional
drag, inertia, etc. of the application will change the resonance as you have
seen. Another way to reduce or eliminate it is to use micro steps instead of
1/2 steps. Finally, the software you use has a large bearing upon this
issue. I was using DeskNC and while it is inexpensive, easy to setup and use
I had a big resonance problem with it. I switched to EMC and the resonance
was much reduced. I played with the acceleration and it got better. I
switched to the new frequency based EMC stepper module and everything is
running great.

The point of all this is don't just look at the controller, resonance is a
much more inclusive issue to deal with.

Tim
[Denver, CO]

----- Original Message -----
From: Joel Jacobs <jj@...>
To: <CAD_CAM_EDM_DRO@onelist.com>
Sent: Friday, February 25, 2000 11:08 AM
Subject: [CAD_CAM_EDM_DRO] chopper drive - resonance problems - need ideas


> From: "Joel Jacobs" <jj@...>
>
> Could use some ideas here. I'm having some resonance problems with my
> chopper drive. Maybe someone else has been down this road. First a
> topology review...
>
> Looking at 1/2 the circuit - a centertap and two windings L1 and L2, it's
> kinda like a unipolar exited bipolar drive - the chopper is applied to the
> center tap with a high side switch and when it's on the current ramps up
in
> only the one winding whose low side switch is on but when it's off (90% of
> the time) the current freewheels back through the other winding so It
> actually flows in both windings during that time. The two low side
switches
> share a current sense resistor as they are never both on at the same time.
>
> The high side switch is turned on every 62us (16khz) and remains on until
> current limit is reached. I'm running a 40volt supply and when it steps,
> the current ramps up to 4 amps in ~600us and then begins chopping. The
> steps are very 'forceful - when it's stepping slowly step - step - step -
> step, the motor goes TwANG - TwANG - TwANG - TwANG.
>
> There are certain speeds where the resonance of the armature wreaks havoc
so
> severely that it looses steps with no load on the motor. Particularly bad
> at about 4000 spm.
>
> The motor is not mounted so the case and the armature resonate - if I hold
> the case down tightly to the table it improves.
>
> If I apply slight drag by holding the pulley it stops resonating and runs
> good.
>
> If I half step the motor it's much better but can still have the problem.
>
> I tried adding about 10us dead time during the ramp up each cycle and that
> improved it some but it then took almost 2ms to reach I_limit.
>
> I can program the current limit so maybe I could try making the step at
1/2
> current then go to full current.
>
> Am I going to have to 'tune' the resonance out with smoke and mirrors
> software?
> Maybe some kind of harmonic balancer attached to the motor shaft?
>
> Clueless...
>
> Joel