PM demodulation

Jerry Avins escreveu:
> Bob Cain wrote:
> 
> 
>>
>>Tim Wescott wrote:
>>
>>
>>>Jerry Avins wrote:
>>>
>>>
>>>>It's actually cheaper to make a
>>>>high-resolution, two-phase optical encoder that has better resolution
>>>>and accuracy than a bulkier, higher-inertia Selsyn. 
>>>
>>>
>>>
>>>I've found that for some very high accuracy measurements, resolvers (a
>>>close cousin of the Selsyn), are superior to optical encoders.  This
>>>has happened when we need a "pancake" sensor that will go in the
>>>circumference of a fairly large hole, but I'll have resolvers on the
>>>list from now on.
>>
>>
>>Unless optical has come a long way, it couldn't compare to what you
>>could get from a resolver back when I was doing his stuff and the Selsyn
>>having so many poles was a super resolver used for the very highest
>>resolution.  That was necessasary for the rotary axes because there was
>>no lead screw multiplying the resolver resolution as there is on a
>>linear axis.
>>
>>I can't believe I still remember this stuff!  No wonder new ideas don't
>>come very often any more, there is way too much old clutter in the way
>>of their getting out.  :-)>
> 
> 
> BEI has encoders -- not cheap -- of 18,000 lines that can quantize a
> turn into 72,000 parts. Centration errors arise from the disk's being
> slightly off center, so that there is "wow" when the disk rotates at
> constant speed. It can be resolved into a power series in which (as
> usual), the first term predominates when the error is small.
> 
> A clever arrangement can double the effective number of lines, allow the
> use of only one track, and eliminate odd terms from the power series
> (including the first). An imaging system that consists od right-angle
> prisms (one of them with a roof) and a focusing lens projects an image
> of the track onto a part of the disk diametrically opposite. Rotating
> one of the prisms very slightly has the moire a quarter line out of
> phase from outside to inside. The image and wheel moving in opposite
> directions doubles the frequency, and if eccentricity makes the rim move
> faster, the image projected onto it moves more slowly, stabilizing the
> effective line spacing.
> 
> The accuracy of an encoder is limited to how well the lines van be
> placed by a ruling engine guided by a laser interferometer. Selsyns,
> resolvers, and control transformers are limited by how well silicon
> steel can be fabricated. Errors are encountered twice: once in the
> sender, and once in the receiver. (The resolver output can go to ADCs,
> but the encoder's output just needs to be counted.)
> 
> Jerry

It's possible to get some encoders that do output it's sin-cos waveform. 
with that, an ADC and some software you can get a pretty high resolution 
from an encoder with almost no inertia added to the system and a pretty 
fast response :D

Ricardo

Ricardo wrote:


> It's possible to get some encoders that do output it's sin-cos waveform. 
> with that, an ADC and some software you can get a pretty high resolution 
> from an encoder with almost no inertia added to the system and a pretty 
> fast response :D

That's a darned good idea.  If the accuracy of the sin and 
cos is great enough you can get away from the need for an 
ADC by using a digital phase locked loop.  I built such a 
beast long ago but it suffered from too much cyclic error 
due to the resolver.  The ADC method ameliorated that but 
darned if I can remember why.


Bob
-- 

"Things should be described as simply as possible, but no 
simpler."

                                              A. Einstein

Bob Cain wrote:

> 
> 
> Ricardo wrote:
> 
> 
>> It's possible to get some encoders that do output it's sin-cos
>> waveform. with that, an ADC and some software you can get a pretty
>> high resolution from an encoder with almost no inertia added to the
>> system and a pretty fast response :D
> 
> 
> That's a darned good idea.  If the accuracy of the sin and cos is great
> enough you can get away from the need for an ADC by using a digital
> phase locked loop.  I built such a beast long ago but it suffered from
> too much cyclic error due to the resolver.  The ADC method ameliorated
> that but darned if I can remember why.

How can you determine the position of a stationary resolver with a PLL?

jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Jerry Avins wrote:

> How can you determine the position of a stationary resolver with a PLL?

D'oh.  Never mind.  I was only half conscious when I wrote 
that.  You need the quadrature mixing of a resolver, of 
course, for the PLL and I doubt there are linear enough 
optical sensors to effect that with a modulated optical encoder.


Bob
-- 

"Things should be described as simply as possible, but no 
simpler."

                                              A. Einstein