Hi,
I ran across an article by one of the servo drive firms that said the
using two sensors/axis improved motion. I think they used a resolver on
the drive and an encoder on the ball screw. While this improves motion
that assumes you have very good ball screws. I think that a better
approach would be to use glass scales and then another sensor on the
ball screw or drive. Right now I have a mill with glass scales on X and
Y with Z being considered. Planned is implementation of stepper-servos
like Gene's setup. First iteration will be stepper-servos plus a DRO
simply to tell me how bad things are.
BTW -- on X using the hand wheel vis a vis an accu-rite 5 um glass scale
was dead on moving X in the positive direction I won't admit how bad
it is on reversal. ;-) Y is damaged but still pretty good going positive
but really bad in Y neg. No scale on Z so no estimate but not perfect.
The X does have a 40 K count/inch in quadrature encoder on it.
Note: Chinese glass scales are now mounted on X and Y since the
Accu-rite scales do not play well with the Chinese DRO.
Cheers,
Dave
On 1/3/22 6:58 PM, Sam Sokolik wrote:
This has been done at a bigger scale... (2 feedback loops)
https://web.archive.org/web/20160222165548/http://jmkasunich.com/cgi-bin/blosxom/shoptask/wichita-trip-02-20-08.html
http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Combining_Two_Feedback_Devices_On_One_Axis
On Mon, Jan 3, 2022, 8:42 PM Chris Albertson <albertson.ch...@gmail.com>
wrote:
CNC is about precision motion control. Here is a new idea where this
builder gets 0.05mm accuracy but uses hobby level R/C servo Not only that,
but he connects three of these in series, one to the next to the next so
all the backlashes and poor tolerances add together. Then he uses this to
do precise motion. He loads a mechanical pencil with this chain of cheap
parts.
What does this means for LCNC? It means that someone has found a software
solution to backlash. What he does is place a quadrature encoder on both
the motor and the output shaft. The difference in encoder reading is an
exact measure of mechanical backlash and effective gear ratio. He can
measure the backlash under different conditions and store the
measurements. Then he places a cascaded PID controller and Kalman filter
over this hardware.
Technically the problem with backlash control via software is the delay
from input to output pays poorly with the PID algorithm. He applies a
predictive model.
Checkup this video. It is unimpressive if you have a $100,000 CNC milling
machine, but he is using a linked chain of hobby servos. The novel idea is
his software. It is on github, you can read it.
https://youtu.be/gq-P39rfRqU
He explains it here. Notice in the video he shows the backlash. The
gear-slop is at the 1/4 inch level but using his software backlash
correction you can see the results in the dial indicator is about 0.05mm
(or about 0.002 in American units) Not bad given the truly horrible
mechanics.
https://youtu.be/SioCwvR_PYY
Why do I care? I have a robot-dog leg here on my workbench using hobby
servos, let's say performance could be improved. But the anti-lash
technique might be used on a real milling machine. Maybe one of the
experts here could look and see if it could be applied? I will use parts
of his idea on my dog-bot.
--
Chris Albertson
Redondo Beach, California
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