On Tue, 15 Apr 2008, Jon Elson wrote:
> Date: Tue, 15 Apr 2008 11:40:59 -0500
> From: Jon Elson <[EMAIL PROTECTED]>
> Reply-To: "Enhanced Machine Controller (EMC)"
> <[email protected]>
> To: "Enhanced Machine Controller (EMC)" <[email protected]>
> Subject: [Emc-users] brushless motors
>
> Hello, gang,
>
> I got a Giddings and Lewis brushless motor off eBay a while ago,
> and finally got around to trying it with my brushless drive.
> It is a SSM430 size 42 motor, rated at 30 Lb-In continuous
> stall. I had to fool around with the commutation encoder
> phasing to get it to work right, and then found out the
> quadrature encoder was trashed, some of the marks on the encoder
> disc were rubbed off. I patched an HP HEDS9000 series encoder
> on it, and it works quite well. I may want to go back and tweak
> the commutation phasing again. I did have some significant
> vibration of the motor at low speeds, somewhere around 25 RPM
> and a frequency of about 40 Hz. I guess if the commutation
> pulsing from 6-step drive matches the motor's natural frequency,
> you can only supply so much damping electronically. Something
> I've been troubled by for some time with the EMC(1, 2 version
> doesn't matter) PID scheme is that in many cases the damping
> "valley" between too little D and too much D is very shallow and
> narrow. With too little damping, various mechanical resonances
> can just run away. With too much D, you get large-signal
> oscillations at a lower frequency, maybe 5-10 Hz, and they get
> very violent. This is not the 1/2 sampling frequency stuff I
> was discussing a couple months ago from encoder quantization
> jitter, but real mechanical oscillations.
>
> Any suggestions? I've been doing ad-hoc servo tuning for a long
> time, and don't know if I'm off the mark. (I tried the
> auto-tune at last CNC Workshop with no sign it actually worked,
> is there a tutorial on how to use it?)
If you are just driving a bare motor, try raising your sampling rate...
Peter Wallace
Mesa Electronics
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