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Default Encoders? Karl?

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i
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Default Encoders? Karl?


Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i


Those are pretty low resolution, only up to 720 CPR. The encoders on the
CNC machines I used to work on were all around 2,000 CPR. You have to do
the math with your ballscrew pitch to figure out what the final movement
resolution is, but I expect it will be far too low. Also since this is a
servo setup, the minimum servo error with such a low resolution encoder
could equate to an unacceptable position error at the cutter.
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Default Encoders? Karl?

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i


Those are pretty low resolution, only up to 720 CPR. The encoders on the
CNC machines I used to work on were all around 2,000 CPR. You have to do
the math with your ballscrew pitch to figure out what the final movement
resolution is, but I expect it will be far too low. Also since this is a
servo setup, the minimum servo error with such a low resolution encoder
could equate to an unacceptable position error at the cutter.


Well, on the back of the envlope calculation is: ball screw pitch 1/8"
(0.125"). Pulley ratio 1:2. So, 720 CPR results in

0.125"/2/720 = .0000868 inch per cycle
OR
0.002mm

That would be acceptable to me.

i
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Default Encoders? Karl?


Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i


Those are pretty low resolution, only up to 720 CPR. The encoders on the
CNC machines I used to work on were all around 2,000 CPR. You have to do
the math with your ballscrew pitch to figure out what the final movement
resolution is, but I expect it will be far too low. Also since this is a
servo setup, the minimum servo error with such a low resolution encoder
could equate to an unacceptable position error at the cutter.


Well, on the back of the envlope calculation is: ball screw pitch 1/8"
(0.125"). Pulley ratio 1:2. So, 720 CPR results in

0.125"/2/720 = .0000868 inch per cycle
OR
0.002mm

That would be acceptable to me.

i


..0000868" per count * servo error tolerance of +/- 128 counts (256 count
window) = 0.0222208" error which is very significant and would not be
acceptable to me.
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Default Encoders? Karl?

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i

Those are pretty low resolution, only up to 720 CPR. The encoders on the
CNC machines I used to work on were all around 2,000 CPR. You have to do
the math with your ballscrew pitch to figure out what the final movement
resolution is, but I expect it will be far too low. Also since this is a
servo setup, the minimum servo error with such a low resolution encoder
could equate to an unacceptable position error at the cutter.


Well, on the back of the envlope calculation is: ball screw pitch 1/8"
(0.125"). Pulley ratio 1:2. So, 720 CPR results in

0.125"/2/720 = .0000868 inch per cycle
OR
0.002mm

That would be acceptable to me.

i


.0000868" per count * servo error tolerance of +/- 128 counts (256 count
window) = 0.0222208" error which is very significant and would not be
acceptable to me.


that sounds bad!

i


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Default Encoders? Karl?


Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i

Those are pretty low resolution, only up to 720 CPR. The encoders on the
CNC machines I used to work on were all around 2,000 CPR. You have to do
the math with your ballscrew pitch to figure out what the final movement
resolution is, but I expect it will be far too low. Also since this is a
servo setup, the minimum servo error with such a low resolution encoder
could equate to an unacceptable position error at the cutter.

Well, on the back of the envlope calculation is: ball screw pitch 1/8"
(0.125"). Pulley ratio 1:2. So, 720 CPR results in

0.125"/2/720 = .0000868 inch per cycle
OR
0.002mm

That would be acceptable to me.

i


.0000868" per count * servo error tolerance of +/- 128 counts (256 count
window) = 0.0222208" error which is very significant and would not be
acceptable to me.


that sounds bad!

i


Bingo! That would be why the "big boys" use 2,000 CPR or better
encoders.

What is the CPR on your current encoders?
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Default Encoders? Karl?

I've had good luck with these:
http://usdigital.com/products/encode...rotary/kit/e6/

I'd get a higher count per inch encoder. get one that gives you a very even
count per inch movement number, like 20,000 or 50,000 or even 100,000.
remember you get four counts per encoder pulse.

get the differential option. use their wire and connectors.

Don't be a cheap skate and use the PC 5 volt supply. get one just for the
encoders. I usually get a multiple voltage out DC power supply. Seems like I
always need 24, 10, 5 I don't know if EMC does analog (10V) or opto
isolated digital I/O (24V)

Karl



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Default Encoders? Karl?

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i

Those are pretty low resolution, only up to 720 CPR. The encoders on the
CNC machines I used to work on were all around 2,000 CPR. You have to do
the math with your ballscrew pitch to figure out what the final movement
resolution is, but I expect it will be far too low. Also since this is a
servo setup, the minimum servo error with such a low resolution encoder
could equate to an unacceptable position error at the cutter.

Well, on the back of the envlope calculation is: ball screw pitch 1/8"
(0.125"). Pulley ratio 1:2. So, 720 CPR results in

0.125"/2/720 = .0000868 inch per cycle
OR
0.002mm

That would be acceptable to me.

i

.0000868" per count * servo error tolerance of +/- 128 counts (256 count
window) = 0.0222208" error which is very significant and would not be
acceptable to me.


that sounds bad!

i


Bingo! That would be why the "big boys" use 2,000 CPR or better
encoders.


I am confused.

Your accuracy number for my example was 0.02".

If you increase the count by 3 times (as in your mention of 2,000 CPR)
then the accuracy only improves to 0.006", also an unacceptable
number.

Something is not right.

What is the CPR on your current encoders?


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Default Encoders? Karl?


"Pete C." wrote:

Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i

Those are pretty low resolution, only up to 720 CPR. The encoders on the
CNC machines I used to work on were all around 2,000 CPR. You have to do
the math with your ballscrew pitch to figure out what the final movement
resolution is, but I expect it will be far too low. Also since this is a
servo setup, the minimum servo error with such a low resolution encoder
could equate to an unacceptable position error at the cutter.

Well, on the back of the envlope calculation is: ball screw pitch 1/8"
(0.125"). Pulley ratio 1:2. So, 720 CPR results in

0.125"/2/720 = .0000868 inch per cycle
OR
0.002mm

That would be acceptable to me.

i

.0000868" per count * servo error tolerance of +/- 128 counts (256 count
window) = 0.0222208" error which is very significant and would not be
acceptable to me.


that sounds bad!

i


Bingo! That would be why the "big boys" use 2,000 CPR or better
encoders.

What is the CPR on your current encoders?


For a home machine, I would want accuracy at least to 0.001", so the
normal servo error window would need to be ~0.0005" or so to be
acceptable. The big commercial machines hold much tighter accuracies, to
the extent of using liquid cooled temperature stabilized ballscrews,
double servos with glass scales on the machine, etc.
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Default Encoders? Karl?


Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

On 2010-06-04, Pete C. wrote:

Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i

Those are pretty low resolution, only up to 720 CPR. The encoders on the
CNC machines I used to work on were all around 2,000 CPR. You have to do
the math with your ballscrew pitch to figure out what the final movement
resolution is, but I expect it will be far too low. Also since this is a
servo setup, the minimum servo error with such a low resolution encoder
could equate to an unacceptable position error at the cutter.

Well, on the back of the envlope calculation is: ball screw pitch 1/8"
(0.125"). Pulley ratio 1:2. So, 720 CPR results in

0.125"/2/720 = .0000868 inch per cycle
OR
0.002mm

That would be acceptable to me.

i

.0000868" per count * servo error tolerance of +/- 128 counts (256 count
window) = 0.0222208" error which is very significant and would not be
acceptable to me.

that sounds bad!

i


Bingo! That would be why the "big boys" use 2,000 CPR or better
encoders.


I am confused.

Your accuracy number for my example was 0.02".

If you increase the count by 3 times (as in your mention of 2,000 CPR)
then the accuracy only improves to 0.006", also an unacceptable
number.

Something is not right.

What is the CPR on your current encoders?


I think the issue is encoder "lines" vs. "counts". The encoders on the
machines I worked on were 2,000 line encoders, which as Karl noted could
provide 8,000 counts per revolution. The US digital site uses counts, so
presumably the 720 CPR encoder is 180 line?


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Default Encoders? Karl?


I think the issue is encoder "lines" vs. "counts". The encoders on the
machines I worked on were 2,000 line encoders, which as Karl noted could
provide 8,000 counts per revolution. The US digital site uses counts, so
presumably the 720 CPR encoder is 180 line?


The terminology on this is terrible. Multiply the USdigital number by four
to get counts that your control will see.

karl


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Default Encoders? Karl?

On 2010-06-04, Karl Townsend wrote:
I've had good luck with these:
http://usdigital.com/products/encode...rotary/kit/e6/

I'd get a higher count per inch encoder. get one that gives you a very even
count per inch movement number, like 20,000 or 50,000 or even 100,000.
remember you get four counts per encoder pulse.


OK, great. Just what I need. I printed out the datasheet and will
measure mounting dimensions to order them.

get the differential option. use their wire and connectors.


OK

Don't be a cheap skate and use the PC 5 volt supply. get one just for the
encoders. I usually get a multiple voltage out DC power supply. Seems like I
always need 24, 10, 5 I don't know if EMC does analog (10V) or opto
isolated digital I/O (24V)


I will check what Jon's PPMC does, but I think that he takes 5v
differential signals.

Makes perfect sense to me. Thanks Karl.

i
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Default Encoders? Karl?


Karl Townsend wrote:

I think the issue is encoder "lines" vs. "counts". The encoders on the
machines I worked on were 2,000 line encoders, which as Karl noted could
provide 8,000 counts per revolution. The US digital site uses counts, so
presumably the 720 CPR encoder is 180 line?


The terminology on this is terrible. Multiply the USdigital number by four
to get counts that your control will see.

karl


Lines = Number of physical lines on the encoder disk

Counts = Lines * 4 (leading and trailing edges of the two quadrature
channel signals)
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Default Encoders? Karl?


"Ignoramus21167" wrote in message
...
On 2010-06-04, Karl Townsend wrote:
I've had good luck with these:
http://usdigital.com/products/encode...rotary/kit/e6/

I'd get a higher count per inch encoder. get one that gives you a very
even
count per inch movement number, like 20,000 or 50,000 or even 100,000.
remember you get four counts per encoder pulse.


OK, great. Just what I need. I printed out the datasheet and will
measure mounting dimensions to order them.

get the differential option. use their wire and connectors.


OK

Don't be a cheap skate and use the PC 5 volt supply. get one just for the
encoders. I usually get a multiple voltage out DC power supply. Seems
like I
always need 24, 10, 5 I don't know if EMC does analog (10V) or opto
isolated digital I/O (24V)


I will check what Jon's PPMC does, but I think that he takes 5v
differential signals.

Makes perfect sense to me. Thanks Karl.

i


Did you notice in the reading to put a resistor and capacitor on the lines?
be sure to do this. Unless you like weird noise issues.

Karl


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Default Encoders? Karl?

On 2010-06-04, Karl Townsend wrote:

"Ignoramus21167" wrote in message
...
On 2010-06-04, Karl Townsend wrote:
I've had good luck with these:
http://usdigital.com/products/encode...rotary/kit/e6/

I'd get a higher count per inch encoder. get one that gives you a very
even
count per inch movement number, like 20,000 or 50,000 or even 100,000.
remember you get four counts per encoder pulse.


OK, great. Just what I need. I printed out the datasheet and will
measure mounting dimensions to order them.

get the differential option. use their wire and connectors.


OK

Don't be a cheap skate and use the PC 5 volt supply. get one just for the
encoders. I usually get a multiple voltage out DC power supply. Seems
like I
always need 24, 10, 5 I don't know if EMC does analog (10V) or opto
isolated digital I/O (24V)


I will check what Jon's PPMC does, but I think that he takes 5v
differential signals.

Makes perfect sense to me. Thanks Karl.

i


Did you notice in the reading to put a resistor and capacitor on the lines?
be sure to do this. Unless you like weird noise issues.


Yes, I did see that, and I will do so. Should be easy with Jon's breakout.

I hope that my servo motors' mounting holes match some available
options options for that encoder, that way it will be all neat and
easy.

i


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Default Encoders? Karl?


I hope that my servo motors' mounting holes match some available
options options for that encoder, that way it will be all neat and
easy.


Na, neat and easy, and refitting old machines are mutually exclusive.
Any algebra expert should know this.

Karl


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Default Encoders? Karl?

On 2010-06-04, Karl Townsend wrote:

I hope that my servo motors' mounting holes match some available
options options for that encoder, that way it will be all neat and
easy.


Na, neat and easy, and refitting old machines are mutually exclusive.
Any algebra expert should know this.


I will take a caliper to it and I will know, hopefully tomorrow or
so.

i
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Default Encoders? Karl?

On 06/04/2010 10:04 AM, Pete C. wrote:

Ignoramus21167 wrote:

On 2010-06-04, Pete wrote:

Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i

Those are pretty low resolution, only up to 720 CPR. The encoders on the
CNC machines I used to work on were all around 2,000 CPR. You have to do
the math with your ballscrew pitch to figure out what the final movement
resolution is, but I expect it will be far too low. Also since this is a
servo setup, the minimum servo error with such a low resolution encoder
could equate to an unacceptable position error at the cutter.


Well, on the back of the envlope calculation is: ball screw pitch 1/8"
(0.125"). Pulley ratio 1:2. So, 720 CPR results in

0.125"/2/720 = .0000868 inch per cycle
OR
0.002mm

That would be acceptable to me.

i


.0000868" per count * servo error tolerance of +/- 128 counts (256 count
window) = 0.0222208" error which is very significant and would not be
acceptable to me.


The normal rule of thumb in servo design is that the encoder error
should be no more than 1/10th the desired total system error.

Your 128 count rule would mean that your 1/8" pitch ball screw would
give an error of 128 * 0.125" / 2 / 2000 = .004". Is that really what
you meant? Four mils error?

'course, by my "ten times" rule, Iggy's .087 mils suddenly become .87 of
designed-for error, which is a lot more than _I'd_ be willing to
tolerate -- 2000 counts would give a designed-for error of 0.3 mil,
which isn't lovely, but is probably getting limited more by the machine
than the encoders (which is what you aim for).

Those sinusoidal encoders will give you essentially infinite resolution,
so you'll never have to wonder if you're losing accuracy to quantization
noise. You can still lose accuracy in lots and lots of other ways, but
not to quantization...

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
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Default Encoders? Karl?

Ignoramus21167 fired this volley in
:


Your accuracy number for my example was 0.02".

If you increase the count by 3 times (as in your mention of 2,000 CPR)
then the accuracy only improves to 0.006", also an unacceptable
number.

Something is not right.


Thanks for doing the math, Iggy.

Bingo! You win the big teddy bear.

If a servo misses re-positioning 128 (or 256!) times in a full stroke of
the machine, it's undersized, or being driven too fast.

I have a couple of machines that would upchuck if they detected TWO
missed steps in 38".

LLoyd
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"Pete C." fired this volley in news:4c093eef$0
:

I think the issue is encoder "lines" vs. "counts". The encoders on the
machines I worked on were 2,000 line encoders, which as Karl noted

could
provide 8,000 counts per revolution. The US digital site uses counts,

so
presumably the 720 CPR encoder is 180 line?


If, in fact, that's the case, then it's a huge difference.

But still, that only gets you to roughly 11:1 better than the .02
error... and a thou in CNC work is NOT good enough!

The thing about a servo missing up to 256 counts in a full-excursion run
is nuts. My OLD R2E4 doesn't do that sort of thing. Newer, more capable
servo drivers and faster encoders couldn't possibly miss that bad.

LLoyd


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"Pete C." fired this volley in news:4c094cf5$0$1092
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Lines = Number of physical lines on the encoder disk

Counts = Lines * 4 (leading and trailing edges of the two quadrature
channel signals)


yep... and I have some cheap motors from old TI printers that have 720 LINE
encoders. These are from the mid-80's, and they were not high-end
expensive equipment like CNC mills or lathes.

LLoyd
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"Lloyd E. Sponenburgh" wrote:

"Pete C." fired this volley in news:4c094cf5$0$1092
:

Lines = Number of physical lines on the encoder disk

Counts = Lines * 4 (leading and trailing edges of the two quadrature
channel signals)


yep... and I have some cheap motors from old TI printers that have 720 LINE
encoders. These are from the mid-80's, and they were not high-end
expensive equipment like CNC mills or lathes.

LLoyd


The resolution (within reason) isn't the expensive part, the rugedizing
to survive a machine tool environment is.
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"Lloyd E. Sponenburgh" wrote:

"Pete C." fired this volley in news:4c093eef$0
:

I think the issue is encoder "lines" vs. "counts". The encoders on the
machines I worked on were 2,000 line encoders, which as Karl noted

could
provide 8,000 counts per revolution. The US digital site uses counts,

so
presumably the 720 CPR encoder is 180 line?


If, in fact, that's the case, then it's a huge difference.

But still, that only gets you to roughly 11:1 better than the .02
error... and a thou in CNC work is NOT good enough!

The thing about a servo missing up to 256 counts in a full-excursion run
is nuts. My OLD R2E4 doesn't do that sort of thing. Newer, more capable
servo drivers and faster encoders couldn't possibly miss that bad.

LLoyd


+/- 128 counts isn't the same as missing by 256 counts.

Some servo drives will have much tighter tolerance, and that is the
limit under load, not normal. I used +/- 128 in the example since that
is the max error of the popular Gecko servo drives, more than 128 off
and it will generate an alarm to stop the control.
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"Lloyd E. Sponenburgh" wrote:

Ignoramus21167 fired this volley in
:


Your accuracy number for my example was 0.02".

If you increase the count by 3 times (as in your mention of 2,000 CPR)
then the accuracy only improves to 0.006", also an unacceptable
number.

Something is not right.


Thanks for doing the math, Iggy.

Bingo! You win the big teddy bear.

If a servo misses re-positioning 128 (or 256!) times in a full stroke of
the machine, it's undersized, or being driven too fast.

I have a couple of machines that would upchuck if they detected TWO
missed steps in 38".

LLoyd


You're confusing cumulative error in a stepper system with the
non-cumulative following error in a servo system.

A servo system needs fine encoder resolution so that the servo loop can
function properly and hold position against loads. The max following
error is the point that the servo will alarm and shutdown. If the system
is designed correctly, 128 counts should translate into something like
1/10 of the nominal position resolution of the system, like 0.0001" if
you expect to utilize 0.001" positioning.
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Karl Townsend wrote:

I've had good luck with these:
http://usdigital.com/products/encode...rotary/kit/e6/

I'd get a higher count per inch encoder. get one that gives you a very even
count per inch movement number, like 20,000 or 50,000 or even 100,000.
remember you get four counts per encoder pulse.

get the differential option. use their wire and connectors.

Don't be a cheap skate and use the PC 5 volt supply. get one just for the
encoders. I usually get a multiple voltage out DC power supply. Seems like I
always need 24, 10, 5 I don't know if EMC does analog (10V) or opto
isolated digital I/O (24V)

Karl



Would you have any use for some of these?

Integrated power designs 65 Watt switching power supply: Model #
SRW-65-4006 4" X 6" X 1.25" +5V @ 5A / +24V @ 1A / +15V @ 2A / -15V @
2A

http://www.ipdpower.com/home/download.asp?id=379

The website is screwed up. Right click to save and add .pdf, then
select all file types before saving.

I still have 25 NOS units on hand for $25 each.


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.


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Tim Wescott wrote:

On 06/04/2010 10:04 AM, Pete C. wrote:

Ignoramus21167 wrote:

On 2010-06-04, Pete wrote:

Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.

Another one is to reuse a converter board from the existing Heidenhain
controller. Same issue as above. (and I do have documentation, it just
does not say what is what).

I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:

http://usdigital.com/products/encode...otary/kit/e7p/

They seem like they will fit, however, I am slightly surprised at the
price.

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.

i

Those are pretty low resolution, only up to 720 CPR. The encoders on the
CNC machines I used to work on were all around 2,000 CPR. You have to do
the math with your ballscrew pitch to figure out what the final movement
resolution is, but I expect it will be far too low. Also since this is a
servo setup, the minimum servo error with such a low resolution encoder
could equate to an unacceptable position error at the cutter.

Well, on the back of the envlope calculation is: ball screw pitch 1/8"
(0.125"). Pulley ratio 1:2. So, 720 CPR results in

0.125"/2/720 = .0000868 inch per cycle
OR
0.002mm

That would be acceptable to me.

i


.0000868" per count * servo error tolerance of +/- 128 counts (256 count
window) = 0.0222208" error which is very significant and would not be
acceptable to me.


The normal rule of thumb in servo design is that the encoder error
should be no more than 1/10th the desired total system error.

Your 128 count rule would mean that your 1/8" pitch ball screw would
give an error of 128 * 0.125" / 2 / 2000 = .004". Is that really what
you meant? Four mils error?


The line / count confusion is there, so 2000 line = 8000 count, so more
like .001" error to shutdown. The +/- 128 count is based on the alarm
threshold in the popular Gecko servo drives, not the higher end stuff
and also isn't the normal following error.


'course, by my "ten times" rule, Iggy's .087 mils suddenly become .87 of
designed-for error, which is a lot more than _I'd_ be willing to
tolerate -- 2000 counts would give a designed-for error of 0.3 mil,
which isn't lovely, but is probably getting limited more by the machine
than the encoders (which is what you aim for).

Those sinusoidal encoders will give you essentially infinite resolution,
so you'll never have to wonder if you're losing accuracy to quantization
noise. You can still lose accuracy in lots and lots of other ways, but
not to quantization...

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

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Would you have any use for some of these?

Integrated power designs 65 Watt switching power supply: Model #
SRW-65-4006 4" X 6" X 1.25" +5V @ 5A / +24V @ 1A / +15V @ 2A / -15V @
2A

http://www.ipdpower.com/home/download.asp?id=379

The website is screwed up. Right click to save and add .pdf, then
select all file types before saving.

I still have 25 NOS units on hand for $25 each.


Great prcie but i need more amps @24V

Karl


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Karl Townsend wrote:

Would you have any use for some of these?

Integrated power designs 65 Watt switching power supply: Model #
SRW-65-4006 4" X 6" X 1.25" +5V @ 5A / +24V @ 1A / +15V @ 2A / -15V @
2A

http://www.ipdpower.com/home/download.asp?id=379

The website is screwed up. Right click to save and add .pdf, then
select all file types before saving.

I still have 25 NOS units on hand for $25 each.


Great price but I need more amps @24V



No problem. I just thought I owuld ask. They are low noise, and were
being installed in some sensitive audio equipment.

I may still have some good 8" floppy drive power supplies. They have
more current at 24 volts. They usually have +5 & and adjustable +12 to
+15 output that will go down to +10.


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.
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In article ,
Ignoramus21167 wrote:

The servos on my mill have sinusoidal encoders which are not
compatible with most CNC control stuff.

I am looking at various options.

One is to make a converter to convert sinusoidal to quadrature. It is
a pain in the butt to do due to lack of documentation on what wire
does what.


What do you mean by "sinusoidal encoders"? Synchro transformers running at 60
Hz or 400 Hz single phase? What is the make and model of the encoders?


If it's a synchro, there will be five wires (perhaps plus a safety ground). Two
wires will go to a single coil on the rotor. Three wires will go to a Y or
Delta sator coil.

http://www.eugeneleeslover.com/USNAVY/CHAPTER-10-B.html

http://en.wikipedia.org/wiki/Synchro

Synchros are still used because they are simple and extremely rugged.

Anyway, one can buy chips that will convert synchro to angle, and vice versa,
and the wiring isn't that hard to figure out, so making a converter isn't that
hard.

Joe Gwinn
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"Tim Wescott" wrote in message
...
On 06/04/2010 10:04 AM, Pete C. wrote:

snip
.0000868" per count * servo error tolerance of +/- 128 counts (256 count
window) = 0.0222208" error which is very significant and would not be
acceptable to me.


The normal rule of thumb in servo design is that the encoder error should
be no more than 1/10th the desired total system error.


The 1/10th rule of thumb is what I always used, I hardley ever see an error
of more than 2 counts at any kind of low speeds. I don't recommend trying
..0001 tolerance at rapid speeds. With 1 count error, my system slowly
corrects, with 2 counts it corrects quite rapidly, you have to be on the
ball to see it. I don't think I've ever seen 3 count error unless I use
controls to see it.


Your 128 count rule would mean that your 1/8" pitch ball screw would give
an error of 128 * 0.125" / 2 / 2000 = .004". Is that really what you
meant? Four mils error?


If I got a 128 count error then I would tune or replace the control.

'course, by my "ten times" rule, Iggy's .087 mils suddenly become .87 of
designed-for error, which is a lot more than _I'd_ be willing to
tolerate -- 2000 counts would give a designed-for error of 0.3 mil, which
isn't lovely, but is probably getting limited more by the machine than the
encoders (which is what you aim for).

Those sinusoidal encoders will give you essentially infinite resolution,
so you'll never have to wonder if you're losing accuracy to quantization
noise. You can still lose accuracy in lots and lots of other ways, but
not to quantization...

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com


Sounds reasonable to me, I don't recommend feeding 250 ipm if you have .0001
tolerance unless you have a machine specifically designed to do this. I may
get 10 count error in rapids but not on any speed I would cut at. I haven't
even optimized the PID gains on my CNC lathe but it gets within 0.0001
within a second or so.

RogerN




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Pete C. wrote:
Ignoramus21167 wrote:


I have opened up one of my servos to see the shaft. It is a 10mm
shaft.

For that size, US Digital has suspiciously cheap encoders E7P:


Well, on the back of the envlope calculation is: ball screw pitch 1/8"
(0.125"). Pulley ratio 1:2. So, 720 CPR results in

0.125"/2/720 = .0000868 inch per cycle
OR
0.002mm

That would be acceptable to me.

i


.0000868" per count * servo error tolerance of +/- 128 counts (256 count
window) = 0.0222208" error which is very significant and would not be
acceptable to me.

Where do you get this "servo error tolerance +/- 128?" That sounds like
you are assuming the Gecko 320 series of drives,
which Iggy will not be using.

But, anyway, the US Digital encoders have caused all sorts of problems
in a variety of applications, mostly due to a total lack of decoupling
capacitors in the encoder. I have worked with the CUI AMT102 encoders,
and they seem to work well. But, they only go up to 8 mm ID on the
wheel adaptor. They have a DIP switch inside to set the resolution,
and also have the index pulse. There is a cable adaptor that converts
the output to differential. They are available from Digi-Key, and quite
affordable, just over $30 with either the plain or differential cable.

Jon
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Ignoramus21167 wrote:


I will check what Jon's PPMC does, but I think that he takes 5v
differential signals.

The PPMC has jumpers to set each encoder for differential or
single-ended, and provides up to 200 mA at 5 V to each encoder with a
self-resetting thermal "fuse" (PolySwitch). It also has a termination
resistor (120 Ohms) when in differential mode to absorb reflections on
the lines.

Jon
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Joseph Gwinn wrote:
What do you mean by "sinusoidal encoders"? Synchro transformers running at 60
Hz or 400 Hz single phase? What is the make and model of the encoders?


If it's a synchro, there will be five wires (perhaps plus a safety ground). Two
wires will go to a single coil on the rotor. Three wires will go to a Y or
Delta sator coil.

These have incandescent LIGHT BULBS in them, so there is NO DOUBT,
whatsoever, they are optical, and also OLD. LEDs came out in the late
1960s, and due to the safety aspects of encoder failures, this is one of
the first applications they went into. So, these must be from the late
1960's or very early 1970s.

Heidenhain made analog output encoders, that either produced a current
or voltage output, but was not converted to digital signals in the
encoder. The eclipsing of the light path by the disc caused the signals
from the encoder to vary in an approximately sinusoidal pattern. It was
possible to interpolate these signals to increase encoder resolution,
but some systems used the basic resolution without interpolation.

There are a LOT of these Heidenhain analog or sinusoidal encoders out there!

Synchro transformers are fairly rare in shaft angle measurement in the
machine tool business. A Scott-Tee transformer is needed to convert to
quadrature-related signals. Simply winding the stator with two coils at
90 degrees provides the quadrature relationship directly without the
Scott-Tee, and so is much more commonly used in this industry. That is
called a resolver. They put a rotary transformer in there, too, to get
rid of the brushes, thus making a "brushless resolver". These typically
have 6 wires, but can be made with 5.

Jon
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Ignoramus21167 writes:

I Wanted to know if anyone has any suggestions, as I do not want to go
a wrong way.


I have been using HP (Agilent) HEDS 5645-H06 encoders for years with
Geckodrives. Here is the datasheet:

http://www.truetex.com/heds.pdf

These are 400 CPR quadrature encoders, so you get a resolution of 1600
per revolution, which with a 2:1 pulley translates to 3200 per rev, which
on a Bridgeport table with 0.2" pitch screws (ballscrews or leadscrews)
gives you about 0.0004" resolution. Given that backlash and rigidity
slop, even with ballscrews and belt drive, is much more than that, this
is plenty of resolution and an excellent match to the Geckodrive methods.
The 1/10 rule-of-thumb is arbitrarily costly, less reliable, and of
little value, in my engineering opinion. There should be an "impedance
match" between the digital control resolution and the mechanical
tolerances, for optimal economy and performance.

I have a surplus quantity of the new HP items for sale at $50/each.

These are made for standard 1/4" encoder shaft mounts, but if you are
equipped for metalworking you can make your own adapters for other size
shafts and mounts. I've even inserted 1/4" dowel pins into non-servo DC
motor shafts to convert them to inexpensive high-power servos: This page
shows how I did this with the HP HEDS item:

http://www.truetex.com/servomod.htm
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On 2010-06-05, Jon Elson wrote:
Ignoramus21167 wrote:


I will check what Jon's PPMC does, but I think that he takes 5v
differential signals.

The PPMC has jumpers to set each encoder for differential or
single-ended, and provides up to 200 mA at 5 V to each encoder with a
self-resetting thermal "fuse" (PolySwitch). It also has a termination
resistor (120 Ohms) when in differential mode to absorb reflections on
the lines.


It looks like you have provided everything, and that with differential
encoders, no additional noise suppression is needed. Neat. Thanks

i


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On 06/04/2010 06:48 PM, RogerN wrote:
"Tim wrote in message
...
On 06/04/2010 10:04 AM, Pete C. wrote:

snip

snip again
Sounds reasonable to me, I don't recommend feeding 250 ipm if you have .0001
tolerance unless you have a machine specifically designed to do this.


Now _that_ would be a fun machine to do the control design on -- or a
nightmare, depending on whether I came in at the "blank sheet of paper"
stage or if I came in at the "we blundered through this without knowing
what we were doing and it's all screwed up, how little can you do and
have it all meet spec?"

Granted, the latter case is often more lucrative, but no one comes out
of it just thrilled with life.

I may
get 10 count error in rapids but not on any speed I would cut at. I haven't
even optimized the PID gains on my CNC lathe but it gets within 0.0001
within a second or so.


A second seems like a long time -- but then 0.0001 is a small error.
You do realize that 0.0001 error in the measurement may be much bigger
by the time you get out to the cutting tool?

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
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