Probing issue.

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Probing issue.

Postby kig23 » Mon Jun 18, 2018 9:47 am

Hello. I'm getting a strange issue with probing. There is a difference between the Var #5063 value and the Z axis dro value. Here is simple code that i used for testing.
Code: Select all
exec.Code ("G31 Z-50");                         
while(exec.IsMoving()){}

double zvar = exec.Getvar (5063);

MessageBox.Show ("Var #5063 ---> " + zvar);


and a photo with the result.

dro_var.png
dro_var.png (29.28 KiB) Viewed 7063 times


Am i missing somthing? Thanks for any suggestions.
kig23
 
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Re: Probing issue.

Postby cncdrive » Mon Jun 18, 2018 10:20 am

This is not an issue, it is normal.
What happens is that when you probe and the probe input triggers then the motion controller saves the probed value and sends it to the software, but after that your axis has to stop, it has to deccelerate.
So, for example you probe and your probe sensor hits at -4.57, then the motion controller sends that value to the software and the axis deccelerates to stop and so the stop point will be e.g. -4.58
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Re: Probing issue.

Postby kig23 » Mon Jun 18, 2018 12:02 pm

So if i'm not wrong. The difference between var#5063 and z axis dro is related to the feedrate. High feedrate needs more time to decelerate. If i use feedrate, lets say F80 this will produce less difference between var#5063 and z axis dro instead of F600.
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Re: Probing issue.

Postby ger21 » Mon Jun 18, 2018 12:34 pm

Acceleration setting would probably have a larger effect than feedrate.
Gerry
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Re: Probing issue.

Postby kig23 » Mon Jun 18, 2018 2:14 pm

Thanks for your replies.
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Re: Probing issue.

Postby cncdrive » Mon Jun 18, 2018 3:00 pm

Yes, ofcourse. From higher feedrate you deccelerate down with the same acceleration the longer the deccleration path will be.
And the lower the acceleration setting is the decceleration path from the same feedrate to 0 speed is longer.
So, both parameters effect the length of the decceleration path.

Because stepper motors can loose steps when they are stopped without decceleration if the rotational speed is above the so called "start/stop frequency" of the motor therefor the probing just like any other motions can only work properly with acceleration and decceleration.
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Re: Probing issue.

Postby kig23 » Mon Jun 18, 2018 3:45 pm

Thanks Balazs for the explanation.
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