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TMC2130 sensorless homing; forces?

Posted by HugoW 
TMC2130 sensorless homing; forces?
November 14, 2018 02:21PM
Hi,

I am converting my DIY CoreXY printer back to RAMPS (1.4, 24V) and I bought TMC2130 drivers. I would like to use the sensorless homing but I worry about the forces. On the X and Y axis I think the frame and belts will cope with the head running into the frame to home, but my Z-axis is driven by two 8x2 spindels together on one NEMA 23 motor. I now have an aluminum tooling plate bed but I would like to convert to a mirror, soon. Do we have any way of knowing in advance what forces are when the nozzle hits the bed? Does anybody use the sensorless homing on the z-axis at all?

Cheers,

Hugo
Re: TMC2130 sensorless homing; forces?
November 14, 2018 04:06PM
There are a few Duet users who use Z motor stall detection either to home Z or in place of a Z probe. See [forum.duet3d.com] for an example.

With two Z motors it would probably be best to drive them from separate drivers and probe once near each leadscrew, because you may get a more definite stall that way. With some firmwares this setup also lets the firmware adjust the Z motors independently to level the bed from side to side, which corrects for the tendency of the motors to get out of sync when you turn the printer off and on again.

Bear in mind that stall detection is at best accurate to +/- one full step. So other types of endstop and Z probe can be more accurate.

Edited 1 time(s). Last edit at 11/14/2018 04:07PM by dc42.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: TMC2130 sensorless homing; forces?
November 18, 2018 10:44PM
A more accurate way to use stall detection for Z axis homing is to lower the Z axis in small increments and then move the nozzle back & forth in the X (or Y) direction with a very low current setting and detect the X (or Y) motor stall as the nozzle drags on the bed. This is what the MOD-t printer did, although it used servo motors and encoders rather than open-loop stepper drivers with stall detection.
Re: TMC2130 sensorless homing; forces?
November 19, 2018 11:15AM
Quote
LoboCNC
A more accurate way to use stall detection for Z axis homing is to lower the Z axis in small increments and then move the nozzle back & forth in the X (or Y) direction with a very low current setting and detect the X (or Y) motor stall as the nozzle drags on the bed. This is what the MOD-t printer did, although it used servo motors and encoders rather than open-loop stepper drivers with stall detection.

Gee, all this in lieu of a simple microswitch ? The company went belly up BTW.
The OP is using steppers. See DC42 answer.

Edited 1 time(s). Last edit at 11/19/2018 11:17AM by MKSA.


"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)
Re: TMC2130 sensorless homing; forces?
November 19, 2018 12:39PM
Quote
MKSA
Quote
LoboCNC
A more accurate way to use stall detection for Z axis homing is to lower the Z axis in small increments and then move the nozzle back & forth in the X (or Y) direction with a very low current setting and detect the X (or Y) motor stall as the nozzle drags on the bed. This is what the MOD-t printer did, although it used servo motors and encoders rather than open-loop stepper drivers with stall detection.

Gee, all this in lieu of a simple microswitch ? The company went belly up BTW.
The OP is using steppers. See DC42 answer.

With stepper motor stall detection, you can use the same basic technique as you can with servo motors. And yes, all to replace a microswitch (and its wiring and its offset adjustment). This technique actually has some advantages over a microswitch in that it is independent of bed thickness changes (switching cover glass, etc.) and it can also be used over different regions of the print bed to provide bed leveling information. (And I suspect New Matter didn't go out of business because of their Z-axis homing.)
Re: TMC2130 sensorless homing; forces?
November 20, 2018 02:13AM
Quote
LoboCNC
With stepper motor stall detection, you can use the same basic technique as you can with servo motors. And yes, all to replace a microswitch (and its wiring and its offset adjustment). This technique actually has some advantages over a microswitch in that it is independent of bed thickness changes (switching cover glass, etc.) and it can also be used over different regions of the print bed to provide bed leveling information. (And I suspect New Matter didn't go out of business because of their Z-axis homing.)

Do you use it ?

For my Prusa clone, I stick with my X and Y microswitches and my Z sensor detecting the nozzle to bed contact, force required about 1 N (ONE Z motor !). Easy, reliable, simple, fast.
Note that Prusa main advantage of the stall detection feature is to realign their two Z motors plus leadscrews.
Of course for a Delta or similar, stall detection makes sense, yet for Z home better use a nozzle to bed sensor. Oh and my security cameras does use stall detection when it zeroes ! Just the noise is a bit disturbing at first smiling smiley. The idea is not new.


"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)
Re: TMC2130 sensorless homing; forces?
November 20, 2018 06:51AM
What Z-sensor do you use, that measures / reacts to the force?

Thanks,

Hugo
Re: TMC2130 sensorless homing; forces?
November 20, 2018 07:38AM
Quote
HugoW
What Z-sensor do you use, that measures / reacts to the force?

Thanks,

Hugo

I made my own. When the nozzle moves downward and touches the bed, it triggers at about 1 N to avoid false trigger, be enough to squeeze the molten filament as I zero at working T° and low enough to damage the bed and of course not bend, push the bed, take up any play in the lead screws (no need for anti backlash nuts), bend the guides etc... all sources of errors. It is integrated in the carriage that with the guides and my own extruder/hot end are unique.
I advise you to look for of the shelf Z sensors like the piezo or IR from DC42. These are valid and fits standard carriage,hot end. Or try to design your own, piezo, strain gauge ... Stay away from gadgets like BL Touch or bulky heavy industrial proximity sensors.


"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)
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