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

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.
�����������������������������������������������������������������������

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

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

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

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.  :-)>


Bob
-- 

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

                                              A. Einstein

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.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Sam wrote:

> Hello all !
> 
> I would like to know if there is a quite simple (but efficient) way to 
> demodulate a PM signal who is digitalized.
> 
> I have seen some for FM demod, especially with CORDIC. Does something 
> exist for PM as well ?
> 
> Many thanks !
> 
> Sam

Using FM demodulation and taking the derivative of the result has been 
mentioned, but I haven't seen a post about using resonant filters to do 
the FM demodulation.  Since you've mentioned that you're not doing this 
with complex signals, you can approach this the way it's commonly done 
in analog:  limit the signal, then it through a resonant high- or 
low-pass filter tuned to the carrier frequency, and mix the filter 
output with the signal.  This will only work if you're sampling above 
the carrier frequency.

             Limiter              Lowpass
           .---------.      .-----------------.
           |    .--- |      |                 |
           |    |    |      |      N(z)       |
  -------->|    |    |--o-->| --------------- |----->X----->
           |    |    |  |   |  z^2 + a z + b  |      ^
           | ---'    |  |   |                 |      |
           '---------'  |   '-----------------'      |
                        |                            |
                        '----------------------------'
created by Andy�s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

The incoming signal (which you have to bandlimit) is squared up, then 
it's run through a highly resonant filter -- you want a filter that has 
a peak that's 2-3 times as wide as your expected frequency swing, and 
has a 90 degrees phase shift at the carrier frequency.  The output of 
the filter is simply multiplied by the limited signal (the 'X' in the 
diagram), and the output of _that_ is low-pass filtered (filter not shown).

Of course you'll have to make sure that your signal is bandlimited 
before you run it through the limiter, and that you lowpass the result. 
  You'll also have to pay careful attention to the lowpass filter, to 
make sure that it doesn't overflow or have numeric resolution issues.

An alternative to that is the PLL already mentioned.  Assuming that you 
have a phase detector that's linear over the expected phase swing of the 
signal you can just phase lock and take your PM output directly from the 
phase detector.  Your signal will determine which approach is best.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Ok Rick, thank you for your complete answer !

Well, I believe that, in this case, it is quite the same to delay a few 
samples by a quarter of a period of the center frequency, isn't it ?

This would be simpler to implement !

Sam

Bob Cain wrote:

> 
> 
> Jerry Avins wrote:
> 
>> The transducers were selsyns.
> 
> 
> Selsyns were still in use as late as 1985 when I was doing machine tool
> controller design.  Aren't they still?
> 
> 
> Bob

Maybe, but they are bidirectional, and so can so apply a torque on the
systems they are intended to measure. 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. One advantage that
Selsyns have is that they work simply by being connected together and
powered. Another is that they are (were?) cheaply available as military
surplus, rated 400 Hz 120 V. That makes them cheap, but being current
operated, they are just right at 18 to 20 V. A $2.50 pair makes a dandy
weather-vane indicator*, and the low voltage makes any wiring legal.

Jerry
________________________________________
* When there's not much wind, you can drive the neighbor's kids crazy by
twirling the indicator pointer.
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������