Re: Rotary table DRO

babinda01 wrote:

>Hello,
>
> I was wondering if someone may be able to help me with a problem I
>have at the moment.
>I am trying to add a DRO to my 'little' rotary table. It is 6 meter
>diameter. I am looking to get .05mm accuracy if at all possible. I
>was thinking of puting a rotary encoder onto the main spindle which
>is coupled directly to the table, but to get the resolution I am
>looking for I need approx 400 000 pulses per rev (0.00095 deg). All
>the encoders I have looked at seem to be about 8000 pulses per rev in
>quad. Is it possible to gear an encoder up? If so how (so as to get
>minimal backlash)?
>
>Thanks for your help
>Andrew

There are several sources of high resolution rotary encoders. Sub-arc
second encoders can be had, 1 arc at 3 m is .014 mm, so that should be good
enough. The RON 800 series has ±1 arc second accuracy, 36000 count/rev,
interpolation in the electronics to get the arc second accuracy.

The electronics in theodolites measure angles to that sort of precision too.
My Wild T3A (non-electronic) reads to 0.2 arc second. Accuracy is probably
on the order of 1 arcsecond. Would probably be too slow for you though, it
takes me about 1 minute to make a reading.

Canon also makes (made?) high resolution encoders. I have a Canon K-1
encoders, 81000 pulses/rev. 36 mm diameter, 50 mm long. The test record
shows a maximum error of about 4 arc seconds. This uses interpolation too.
Interpolation is based on quadrature sine/cosine outputs.

These things aren't cheap.

Could you wrap one of the mylar scales 360 lpi around your table and put and
HEDS readout head on it? I can't remember the source for these. Someone
on this list will- they are frequently recommended. At 360 x 4 = 1440 /inch
resolution that should be more than good enough. A problem is that the
scales are limited in length, you'd have to splice them together.


regards-
Elliot B.




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