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Author Topic: Backlash problem about step motors  (Read 1116 times)
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Hi All,

I have a tool that have some step motors in it (a 3D printer to be precise), and I am using it quite frequently for about a year now.  It works fine.  However, a few days ago, I noticed one of the step motor is moving backward a by tiny bit whenever it stop, which is not noticeable before.

Is it a backlash problem?  If so, why do it suddenly become noticeable after a year?  Does that mean the step motor is damaged?

Any information would be greatly appreciated.

Thank.
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Little more information required.
a) is there a gearbox involved or are you looking directly at stepper shaft
b) how must does it move by (part of 1 step or 1 complete step)
c) do you keep the coils energised when stopped
d) if answer to (c) is "yes" are the coils really energised
e) if the answer to (c) is "no" is there a possibility of at least one of the coils being energised
f) how many coils are used
g) and finally the model number of the stepper motor
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> a) is there a gearbox involved or are you looking directly at stepper shaft

I notice the change by examining the output of the 3Dprinter, so yes, there is a gearbox involved.
However, its just a simple one.  One drive gear mounted on a stepper motor inside a drive block to grab and push filaments.

http://www.geek-workshop.com/thread-769-1-1.html

(Note that, it is designed to move backward a tiny bit when it stop, where the amount can be adjusted.   I am sure it is configured to just stop and hold.)

b) how must does it move by (part of 1 step or 1 complete step)

Probably part of 1 step , but I'm not sure about it.

c) do you keep the coils energised when stopped

Yes, the coils are energized, burning hot.

f) how many coils are used

I'm not sure about it.

g) and finally the model number of the stepper motor

Nema 17 Stepper Motor, http://store.makerbot.com/nema-17-stepper-motor.html



By the way, what is the different between an ordinary stepper motor and a Hybrid Stepper Motor? 
« Last Edit: January 11, 2013, 09:53:25 pm by ma_hty » Logged

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Normally stepper motors are pretty bomb-proof - no brushes so only the bearings to wear out.

Is the back-pedalling just the driven gear (backlash in the gear) or is the motor shaft also involved?

If the behaviour is deteriorating then either there is more mechanical load (exceeding the hold-torque
of the motor?) or the drive electronics is failing (many stepper motor drive chips have thermal cut-out
and will drop out for brief periods if over-driven).

[edit:  actually its worth checking for slop in the motor bearings or driven-gear bearing too ]
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Hi MarkT,

I see... The problem probably came from something else...

When you say "over-driven", are you saying the stepper motor is being over-driven?
The stepper motor is burning hot, way too hot to be touched.  If you are referring to
the stepper motor, it probably be the case.  In this case, can it be fixed?
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It is more that likely that the backlash is coming from the integrated gears.
If the motor is that hot then it is likely you are driving it too hard and need to reduce the current. This might cut down on the top speed but having it run cooler will reduce the mechanical ware.
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Hi MarkT,

I see... The problem probably came from something else...

When you say "over-driven", are you saying the stepper motor is being over-driven?
The stepper motor is burning hot, way too hot to be touched.  If you are referring to
the stepper motor, it probably be the case.  In this case, can it be fixed?

No, the motor driver chip may be over-driven.
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Getting hot is 'normal' (you are locking it in position by energizing the coils). is it (much) hotter than the other motor?

If you grab the output shaft and twist, does it move/rock, if so, is the other motor just the same? (could just be low torque, so comparative force to move the shaft)

If the motors DO behave differently (but the same motor type), is it possible to reverse the motor electrical drives - you may need to disconnect the motor output to prevent damage.

If the problem moves, you have an electrical/drive/software problem.

If not, then you have a mechanical problem. Disassembly of big motors is quite easy, you may find that the field windings have broken loose and are able to rotate, although that may be accompanied by a lot of noise while in motion. It may also be a single coil that is defective. This may not be noticed if positioning is via some form of encoder rather than absolute stepping, only when holding would a problem manifest itself.
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This motor is also part of the plastic extruder for a 3D printer. It might run a little warm because of proximity to the heater for the extrusion nozzle. If there is any reverse rotoation at the end of the cycle it maight be part of the process for the extruder. By reversing the motor slightly the pressure is taken off the plastic going into the extruder and then the nozzle doesn't drip.
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If not, then you have a mechanical problem. Disassembly of big motors is quite easy, you may find that the field windings have broken loose and are able to rotate, although that may be accompanied by a lot of noise while in motion. It may also be a single coil that is defective. This may not be noticed if positioning is via some form of encoder rather than absolute stepping, only when holding would a problem manifest itself.

_Don't_ disassemble a stepper motor - some are permanently demagnetised by doing this and lose torque.
(For the same reason you shouldn't over-current them).  Also the bearing alignment is vital to stepping accuracy
so reassembling without calibration may reduce accuracy.

You can check if the coils are intact and whether they are shorting to the case using a multimeter.
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