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?