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Author Topic: backlash comp creates follow error  (Read 2180 times)

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backlash comp creates follow error
« on: October 31, 2013, 02:21:38 AM »
OK guys I can't figure this one out myself and am hoping that someone can shine some light on it for me. I have a bridge port mill converted with DC servos.  Spent money and got good ballscrews. The backlash I have in the system is just the flexing of the related components. About .001.  When I enable the backlash comp I get a follow error at the drive at the direction change.  If I keep the bl comp low enough it doesn't error out right away.  This is because it doesn't go outside the follow error setting of the gecko drive.  What I can see is that no matter how slow I have the percent of feed option set it still snaps the comp move at a speed that the servo can't do which is where my error is coming from. So if I put in even just .001in of comp on direction change I get an instant error. Even with the feed percent at 1.

Do I have a setting wrong or am I missing something? Thanks

Offline Hood

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Re: backlash comp creates follow error
« Reply #1 on: October 31, 2013, 04:07:20 AM »
It sounds as if you are over compensating for the backlash.
Backlash comp is designed to take up free movement and thus it will  move quickly. As you are faulting on a following error then it would seem that the motor is seeing resistance in the reversal. If it was truly free movement, to take up the slack, then there should be no significant external forces acting on  the motor and it should easily manage to do the reversal.
 As a test you could uncouple the motor from the screw and assuming you do not run into tuning issues you should be able to see if the following error still occurs.
Hood
Re: backlash comp creates follow error
« Reply #2 on: October 31, 2013, 02:56:30 PM »
Thanks.  But why does the % of feed speed setting in the backlash comp dialog have no effect on the speed of the slack take up.  That makes no sense to me.  This value is used to slow it down but seems to do nothing.  I am only looking to take up about .0005" so no matter what the move it is not like it is trying to move the table, but rather just take the flex out of the components.

This may sound picky but I do a lot of hole interpolation that if this was working would give me really good results.

Offline Hood

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Re: backlash comp creates follow error
« Reply #3 on: October 31, 2013, 04:35:03 PM »
I am not even sure if that is used any more. I have never used backlash comp. I think the shuttle setting may have more of an effect on things.

Hood

Offline RICH

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Re: backlash comp creates follow error
« Reply #4 on: November 01, 2013, 08:53:18 AM »
I have not used backlash in long time because I got rid of it on all my machines.
But that silly .001" can be a real pain to get rid as there are so many things that can add up to create it.

The  50-50-50 rule which you may want to consider is as follows:

50% - The max velocity at 50% ( works for steppers but your using servos so will need to play around with the value )
50% - Set the shuttle Wheel setting in configuration to .0050  to .050 ( .5 too slow )
50% - Backlash speed

The Shuttle Wheel Accel is in seconds. How exactly is the" time" applied to the backlash "cycle"? Well I don't really know.
 It's internal to Mach. By observing the difference in how the motor reacts to different settings you will see that it affects the overall time
 it takes for Mach to take up the backlash and at say a value of 5 you will find that over five seconds the handwheel / shaft will turn
 rather slowly. At 0.005 sec its almost instantaneous. In the real slow setting, and say you jog a distance of .1"  you will find there
seems to be almost 3 distinct motor responses in the cycle,....... a small rotation then deceleration...... then backlash taken up
over a time...and then  accel and move to finish the distance.

So to get of flavor of it you need to start slow, observe, listen to see what is happening over an operating range.
Work your way up in settings.

Hope this helps,

RICH