On 5/13/24 16:47, Todd Zuercher via Emc-users wrote:
Anyone have any brilliant ideas to stiffen a woefully inadequate cross beam on a gantry router
without adding too much mass? What is there now is a 4" x 8" rectangular 3/8"
walled extrusion that is 145" long.
That has got to be north of 100 lbs of flying weight.
Under normal jogging commands the two servos control the ends of this gantry
reasonably well, but while the axis is homing the thing shakes and wobbles
terribly bad. Also If I put a dial indicator in the center of the bridge and
hit the bridge forward or backward it will flex and wobble enough to displace
the dial indicator +/-0.03 and it takes nearly a dozen wobbles to dampen it.
But on the ends the servo's only have a few thousandths of give.
I'm less concerned about the actual stiffness and more worried about dampening
the wobble.
This might be a place to use a technique the 3d printers are using.
Called input_shaping. In the printers case the testing wiggle is 30 to
150 hz but in something this massive you might want to start the
frequency of the scan at 1hz. The amplitude of the motion is then
recorded by an accelerometer chip, usually an adxl345. The data
collected is then used to program a digital filter, which does not
effect the speed of the machine as it works to reduce the amplitude of
the drive at those frequencies where the system is resonating. Such
fancy math has been responsible for a 4 to 8x increase in the machines
actual speed as it controls the ringing.
Another different technique I have found helpful by accident is
stepper/servo's. I am engraving some text of the sides of the printed
nuts for the vise screws I'm building. Using normal steppers that bounce
back and forth magnetically, that ringing restricts the speed of the
printer to about 30mm a second if the text is not to be destroyed by the
ringing. Switching to stepper/servo's has allowed me to drive the
printer 10x faster and the text remains readable. The stepper/servo is
actually dampening that magnetic bounce in real time. And they can do
that on less power than a normal stepper, aided by the switch to higher
voltage power supplies, up to 110 volts vs the 40 or so normal steppers
limiting the bandwidth of the lower voltage normal steppers. The reduced
power is because they are now using the detected error to control the
motor current, motor working easy=low current and negligible heating, a
difference you can see in your shops power bill. I'm now using 5 of them
in the garage and have 3 more to put on my GO704. Capable of stopping
linuxcnc in it tracks if they hit an immovable object, well tested, just
one problem. It has yet to happen running a job!
Check out Hanpose for the higher sized stuff you might need to replace
the two servo's you are using now.
<https://www.alibaba.com/product-detail/Lichuan-stepper-motor-nema-42-close_60697466670.html?spm=a2700.wholesale.you_may_like.3.4c101343U7AJp5>
Drives a closed loop nema 42 motor rated 12NM from a 220 volt line. With
a matching motor and the right set of belt pulleys it can turn your
house around. For probably < $350 an axis.
Todd Zuercher
P. Graham Dunn Inc.<http://www.pgrahamdunn.com/index.php>
630 Henry Street
Dalton, Ohio 44618
Phone: (330)828-2105ext. 2031
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Cheers, Gene Heskett, CET.
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