how to compensate for backlash?

Bearings/Motors/Belts/Gears/Etc.

how to compensate for backlash?

Postby shatterjack » Thu Oct 13, 2011 4:25 am

Hi guys,

Can anyone tell me more about backlash? I've been having some trouble with the laser not returning to the exact same spot when it's doing a looped shape. There are settings in the firmware to compensate for backlash but I wanted to know a bit more about what people do first. Thanks

tony
shatterjack
 
Posts: 5
Joined: Wed Sep 28, 2011 4:01 am

Re: how to compensate for backlash?

Postby macona » Thu Oct 13, 2011 6:00 am

How far off? With a belt system there should be little to no backlash. If this is a case it sounds like you are loosing steps. Check you acceleration and deceleration settings and also try lowering your top speed. Also depending on the drives you are using your step pulse width may be short.
macona
 
Posts: 363
Joined: Tue Aug 17, 2010 10:30 pm

Re: how to compensate for backlash?

Postby bdring » Thu Oct 13, 2011 12:21 pm

What you are talking about could be due to flex in the system. Lasers are more prone to this than other CNC systems. Try repeating the same pattern several times. If the shape is sloppy but stays in roughly the same area it is a backlash/flex problem. If the pattern is walking off, it is more likely lost steps.

Post a picture of you laser.
Bart
"If you didn't build it, you will never own it."
bdring
Site Admin
 
Posts: 2966
Joined: Sun Nov 22, 2009 7:33 pm
Location: Chicago, IL, USA

Re: how to compensate for backlash?

Postby lasersafe1 » Thu Oct 20, 2011 1:15 am

There can be backlash due to belt stretch or different spring tensions seen on the forward vs reverse swing. You can't possibly tighten your belt so tight as to remove all backlash. It is usually more noticable at higher sweep speeds. I have a pretty high quality ULS-25 machine and can see the difference in engrave quality between runs of X-swing and X-unipolar. I never lose steps, so I know this is not the reason. The X-unipolar will always give a superior result. I know that the LO controller software has some values that can be entered to compensate for this, but I have never bothered to research it further since this is only an occasional hobby for me.
lasersafe1
 
Posts: 599
Joined: Fri Nov 27, 2009 8:23 pm

Re: how to compensate for backlash?

Postby r691175002 » Thu Oct 20, 2011 1:21 am

I'd suggest tightening belts and making sure the carriage rolls freely first.
r691175002
 
Posts: 242
Joined: Mon Dec 13, 2010 5:05 pm

Re: how to compensate for backlash?

Postby lasersafe1 » Thu Oct 27, 2011 1:15 pm

So the answer is now Crystal Clear to me after seeing results of DC tube response on the following link:
http://www.buildlog.net/forum/viewtopic.php?f=6&t=864&p=6549#p6543

It wouldn't matter if your belt had zero slop and your machine is built like a tank. If a typical DC tube can have an on-off delay of up to 2ms, then at 30ips this can result in a position error on the engraving of up to .06" between the sweep right and the sweep left. THAT is why X-unipolar gives a better result than X-Sweep.

So compensation can be done in software if the controller allows, but since the delayed response depends on tube age, temperature, power supply, etc., This value could change over time. I think I'll stick with X-unipolar engraving since the delay errors are always in the same direction and therefore invisible.

30ips=30mil per ms.

I apologize for bringing it up here, because "This" phenomon is not the cause of the problem that was stated at the start of the thread. I agree that there must be a lost step or something that causes the non-circle home position error.
lasersafe1
 
Posts: 599
Joined: Fri Nov 27, 2009 8:23 pm

Re: how to compensate for backlash?

Postby macona » Sat Oct 29, 2011 5:14 am

The belts are pretty much backlash free if you have new belts and pulleys. I have had the wear enough to get some backlash but its totally load dependant.
macona
 
Posts: 363
Joined: Tue Aug 17, 2010 10:30 pm


Return to Mechatronics

Who is online

Users browsing this forum: No registered users and 11 guests

cron