Closed loop with linear encoders but still backlash?

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6 years 10 months ago #153171 by Gnevko
Closed loop with linear encoders but still backlash? was created by Gnevko
Hi :)

I am very close to finish the HAL configuration for my CNC Mill (converted from Wabeco F1210). I use two linear encoders (Resolution 0.001mm) for X and Y axis and JMC closed loop stepper motors for all axis. So, today I tried to make some final measurements and ... it seams I have still small backlash ca. 0.03 mm. But how it possible? May be I configured something wrong in my HAL file?

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Thank you in advance!

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6 years 10 months ago #153173 by Todd Zuercher
Replied by Todd Zuercher on topic Closed loop with linear encoders but still backlash?
The clues to what may be wrong (if anything) are most likely in your PID settings in your ini file.

That said within 2 or 3 encoder counts of your feedback device may be the realistic limit of accuracy.

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6 years 10 months ago #153174 by Gnevko
Replied by Gnevko on topic Closed loop with linear encoders but still backlash?
hm ... my INI file looks very simple or better to say usual:

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Code:
[AXIS_X] MAX_VELOCITY = 25.0 MAX_ACCELERATION = 250.0 MIN_LIMIT = -250.0 MAX_LIMIT = 250.0 [JOINT_0] TYPE = LINEAR HOME = 0.0 HOME_OFFSET = 250.0 HOME_SEARCH_VEL = 5.0 HOME_LATCH_VEL = 1.0 HOME_FINAL_VEL = 10.0 HOME_SEQUENCE = 2 HOME_IGNORE_LIMITS = YES FERROR = 10 MIN_FERROR = 1 MIN_LIMIT = -250.0 MAX_LIMIT = 250.0 MAX_VELOCITY = 25.0 MAX_ACCELERATION = 150 STEPGEN_MAXVEL = 30 STEPGEN_MAXACCEL = 300 ENCODER_SCALE = -1000 P = 90 I = 0 D = 0 FF0 = 0 FF1 = 1 FF2 = 0.0005 BIAS = 0 DEADBAND = 0.0005 MAX_OUTPUT = 0 # these are in nanoseconds DIRSETUP = 8000 DIRHOLD = 8000 STEPLEN = 3000 STEPSPACE = 3000 STEP_SCALE = 1200

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6 years 10 months ago #153176 by Leon82
Replied by Leon82 on topic Closed loop with linear encoders but still backlash?
Backlash could be mechanical and not related to your servos

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6 years 10 months ago #153177 by PCW
Replied by PCW on topic Closed loop with linear encoders but still backlash?
What is the following error when you read the 0.03 mm position error?
(that is, what is joint.0.f-error?)

A 0.03 mm error that HAL / LinuxCNC sees should result in a motion of
2.7 mm/s to correct the position error with your current tuning parameters
( 0.03 * 90 = 2.7 mm/s )

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6 years 10 months ago #153178 by Gnevko
Replied by Gnevko on topic Closed loop with linear encoders but still backlash?
Of course mechanical, but a closed loop system with linear encoders should solve exactly this problem or not?

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6 years 10 months ago #153180 by Leon82
Replied by Leon82 on topic Closed loop with linear encoders but still backlash?

Gnevko wrote: Of course mechanical, but a closed loop system with linear encoders should solve exactly this problem or not?


I would think adding BACKLASH = .03 in the ini would remove it electronically.unles there is a tuning problem

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6 years 10 months ago #153182 by Henk
Replied by Henk on topic Closed loop with linear encoders but still backlash?
I don't think so.
The backlash setting in the ini file will make the motor "catch up" a certain amount of steps or angle on each reversal. Since your feedback is a true position you don't want that.

It will work when your feedback comes from a motor encoder.

If you have mechanical backlash you will get a small overshoot because the motor is commanded to reverse but the joint will experience a small delay

Best way to fix this is to eliminate the backlash. Easier said than done, I know.

Henk

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6 years 10 months ago #153188 by Gnevko
Replied by Gnevko on topic Closed loop with linear encoders but still backlash?

PCW wrote: What is the following error when you read the 0.03 mm position error?
(that is, what is joint.0.f-error?)

A 0.03 mm error that HAL / LinuxCNC sees should result in a motion of
2.7 mm/s to correct the position error with your current tuning parameters
( 0.03 * 90 = 2.7 mm/s )


That is the exactly the strange sings - in linuxcnc it seams to be all ok: the following error is something about 0.0003 and no position error at all. But if I check the result with a dial gauge (digital with resolution 0.001 or manual with resolution 0.01) or with HAIMER 3D finder (resolution 0.01) - I see the backlash.

Steps to reproduce are simple: move the axis in "+" direction, set zero in linuxcnc and dial gauge for the axis, then move the axis again in + direction on +1mm and -1 mm. After that in Linuxcnc I read 0.0000 mm (as expected) but on dial gauges ca +0.03mm.

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6 years 10 months ago #153190 by Henk
Replied by Henk on topic Closed loop with linear encoders but still backlash?
Try making the Deadband=0 and add some I gain. That should force it back to 0.

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