Backlash Compensation Not Working

I'm moving on to dialing in my machine a little tighter and am working on the backlash comp. I'm running 2.4.6 on a lathe - I setup the system with PncConf and some custom coding for the RPM meter off hte wiki, but nothing else fancy. When I setup originally I did not include backlash comp. I re-ran PncConf to add it, straight backlash comp, not variable. Now I have the backlash line in the .INI file, under the appropriate axis.

When I run EMC, system works fine, but it does not appear to be taking up the backlash, per the dial indicator when I move back and forth set increments. I put in 0.004 in the Z axis, so it should be a significant change.

I checked back to the HAL file, and can't find any reference to the backlash from the INI file. Where would I check to be sure all the necessary coding is there, and if I am missing some, what is it?

If that's not the case, what else can I check to be sure it is reading the backlash before I go jumping the numbers?

Thanks!

Nick

Hello,

Just edit your .ini file.

If they are not there already add
BACKLASH = x.xxx
(where .xxx ist the amount of BACKLASH) in the .ini files under each axis that you want to add backlash to.

From the manual...

BACKLASH = 0.000
Backlash in machine units. Backlash compensation value can be used to make up for small deficiencies in the hardware used to drive an axis. If backlash is added to an axis and you are using steppers the STEPGEN_MAXACCEL must be increased to 1.5 to 2 times the MAX_ACCELERATION for the axis.

For testing you can use a somewhat large number to make sure it is working and when ever the axis changes direction you should see the jump.

Rick G

That's what I am saying - the INI file has the BACKLASH=0.004 in it, but I am not seeing the jump when I switch motion on the axis. It did appear to improve by 0.001, but that's a factor of x4, so not sure why it would be so different.

I was not sure if there was any coding in HAL which was needed as well, if that is not the case, then I will have to play around with the backlash number, but wanted to be sure there was not a programming missing somewhere.

It's just funny becuase I don't see any reference to backlash in any of the files except the INI, but maybe the program reads it directly from the INI file?

Thanks!

Nick

That is all you should need to do, however have you adjusted your STEPGEN_MAXACCEL?

If backlash is added to an axis and you are using steppers the STEPGEN_MAXACCEL must be increased to 1.5 to 2 times the MAX_ACCELERATION for the axis.

Also increase the backlash to say .016 and see what happens.

You may want to also decrease your axis max velocity and or acceleration for testing (as the backlash move is done at high speed) to see if you may be losing steps on a rapid reverse of the axis.

Double check your machine for the amount of backlash it needs at different speeds and locations, is the backlash always .004 ?

Rick G

Sorry, I should have specified - I am using closed loop servos - I did see that note in the ref's but could not apply it.

If all I need is the backlash defined in the INI file, I will up the amount and see if I get any difference. I am checking it at very slow speeds F=0.2 moving back and forth using dial indicator to check it out.

Is there any other settings in PID tuning which need to be a certain way?

Thanks!
Nick

I found that each time you run Pncconf you have to re-select the backlash, it keeps dropping it out as a selection for some reason. As long as it shows up in the INI file, seems to be working!

Glad you have it sorted out.

Rick G

This bug is fixed in 2.5 now. Thanks.