Re: [CAD_CAM_EDM_DRO] Encoder/Servo question

Ok, I think I've got all the responces figured out, but what about non-linear
applications, say for a rotary table or dividing head? Would that much flexion,
friction, and backlash, etc,. still be present? I can see some, but wouldn't
there be significantly less of it?

Bill

On Fri Jul 16 09:02:27 2004 Jon Elson wrote:

>>-----Original Message-----
>>From: Bill Vance [mailto:ccq@...]
>>Sent: Friday, July 16, 2004 4:35 AM
>>To: 2cc@...
>>Subject: [CAD_CAM_EDM_DRO] Encoder/Servo question
>>
>>Howdy;
>>
>>I've noted various references to 1000 and 2000 line encoders, but are there
>>any servos that will actually turn one or two thousandths of a turn per
>>step/line?
>>
>>
>>
>Absolutely! The closed-loop system DEMANDS that the encoder be turned
>that small fraaction of a thousandth of a turn. I'm using 1000 line
>encoders
>on my system, which give 4000 quadrature counts per revolution. I can
>generally hold the encoder within +/- 2 counts of the null point. So, the
>ENCODER is being held within a very tight bound of about 1/1000 of
>a turn. But, that has no bearing on table position! It is up to the
>leadscrew
>to force the table to be within the equivalent position, and that may be
>beyond the capabilities of the leadscrew, it's bearings, and the nut.
>I see about .0015" backlash in my 3 axes, which is 30 times the encoder
>resolution. This is NOT a failure of the servo loop to maintain encoder
>position, but a failure of the leadscrew to constrain the table to an
>equivalent precision. Much of it is due to flex in the mechanical
>mounting, compression of the leadscrew itself (it is .895" dia),
>backlash in the ballnuts and rocking of the table. If the ways are
>perfectly flat, this rocking won't happen, but my 1938 Bridgeport
>is quite worn. The table and saddle are all worn in odd convex and
>concave shapes.
>
>Jon


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