Re: chopper drive - resonance problems - need ideas

That's like fighting a windmill. It sounds like stepper motor resonace
except that you generally see resonance at a much lower speed. I's say 500
SPS is the range. But if you put a load on it and it goes thru the range
then stop fighting the windmill. Hook it up to your machine. It should be
fine.
The real test for resonace comes with it connected to your machine and you
run a G02 command say G02 X1y1J1 F15
If it goes thru the transitions from one axis to the other smoothly without
losing steps you should be fine. I have found testing a motor on the work
bench with no load is the worse case basis.
Reducing the current and also going to half step should also help
Dan

-----Original Message-----
From: Joel Jacobs <jj@...>
To: CAD_CAM_EDM_DRO@onelist.com <CAD_CAM_EDM_DRO@onelist.com>
Date: Friday, February 25, 2000 9:57 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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