Re: chopper drive - resonance problems - need ideas

Stepper motors with no load will stall due to resonance. Put a flywheel of
some sort or a load on the motor. What you are seeing is normal and will
even happen with an R/L drive. It has nothing to do with the chopper.
Darrell

----- Original Message -----
From: Joel Jacobs <jj@...>
To: <CAD_CAM_EDM_DRO@onelist.com>
Sent: Friday, February 25, 2000 10:08 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
>
>
>
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