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Assisted Bed Levelling. Proposal

Posted by Nexus 
Assisted Bed Levelling. Proposal
March 25, 2017 03:05PM
I would like to know how convenient would be to have Marlin firmware assisting on the bed levelling operation.

The idea is to combine the traditional method (piece of paper or feeler gauge) with the levelling sensor, e.g. 3DTouch.

The procedure would be something like:
1) Setup the desired distance between the bed and the nozzle using the traditional method (paper or feeler gauge)
2) Go to the LCD and choose “Bed Levelling Assistant”. The firmware would activate the levelling sensor, move Z up&down and note the Z height on which the sensor gets triggered. (I will refer to this Z height as Zorig)
3) The firmware would move the nozzle to a different corner of the bed
4) The firmware activates the sensor, move Z up&down and note the Z height at which the sensor gets triggered. The comparison between this Z height and the Zorig would indicate the direction (up/down) the bed needs manual adjustment.
5) repeat step #4 until desire precision or “go to next bed corner”
6) Go to next bed corner and repeat from step #4.

The move to next corner could be done automatically (predefined positions) or manually by moving the axis from the LCD menu.

So instead of having Marlin collecting the diverse Z heights distributed along the surface of the bed, Marlin would give the user the possibility of manually adjusting the height of the bed.

The reason for using this “assisted bed levelling” process is because the Automatic Bed Levelling is based on adjusting the height of the Z axis while printing a layer. The method herein described is to keep the Z axis immobile while printing a layer. In other words, it prevents the Z axis from moving up and down while printing a layer; the Z axis moves when changing from one layer to the next only. This method would also give a levelling parametric approach rather than going by the "feeling" when using the paper method.

Edited 2 time(s). Last edit at 03/26/2017 09:21AM by Nexus.
Re: Assisted Bed Levelling. Proposal
July 18, 2017 06:11AM
Some updates:

Probe: BLTouch

I am now performing Assisted Bed Levelling thru Gcode as follows:
1) position the BLTouch in one of the corners of the bed
2) send Gcode
G1 Z0
3) Calibrate the height of the bed, e.g. using the paper/visit card method (actually I am using a 0.1mm gap measurement blade)
4) send Gcode
G30 S-1
5) Read the echo from printer, e.g. Bed X: 50.00 Y: 29.00 Z: 0.10, and pay attention to the Z value, in this example 0.10
6) Move the BLTouch (printer head) to the next corner of the bed.
7) Execute Gcode shown on step #4
8) Pay attention to the new echo from the printer, e.g Bed X: 100.00 Y: 99.00 Z: -0.02. If the new Z value is below the one returned on step #5 then the corner of the bed is too low on the contrary, if the new Z value is above the returned value of step #5 then the bed is too high.
9) Adjust the height of the bed (on that corner) and repeat from step #7 until you get the Z value echoed on step #5.
10) Repeat again from step #6 until three corners have been levelled.


Notes:
1) I installed the BLTouch according to the recommendation of having a distance of 8.3mm(+/-0.2) between the tip of the nozzle and the base of the BLTouch probe.
2) The new Marlin 1.1.4 provides manual bed levelling for those printer without levelling probe. It moves the printer head (nozzle) around the corners of the bed to manually level the bed using the paper/visit card method. This feature of the Marlin code could be used to assist you half way but you still need to manually send the Gcode shown on step #4.
3) The accuracy I get is Avg=0.107mm with StDev=0.01 (ten samples) which is fairly good. I guess the StDev can be improved by featuring Z probe slow speed (Z_PROBE_SPEED_SLOW) when executing the G30 command as Marlin does when homing Z.

Edited 1 time(s). Last edit at 07/18/2017 09:38AM by Nexus.
Re: Assisted Bed Levelling. Proposal
July 18, 2017 11:48AM
A similar mechanism is provided by the latest beta release of RepRapFirmware, but it works differently. You set up a bed.g file to probe the bed at several points, with at least one point per bed support (normally 3) but more if you like. The firmware probes the bed, then calculates and displays the optimum adjustment of the levelling screws that minimises the squares of the height errors found at the probe points.

Edited 1 time(s). Last edit at 07/18/2017 01:41PM by dc42.

Delta printer calibration calculator, mini IR Z probe, and colour touch screen control panel: [escher3d.com]

Large delta printer, and other 3D printer blog postings: [miscsolutions.wordpress.com]

Full disclosure: I have a financial interest in sales of the Panel Due, Mini IR height sensor, and Duet WiFi/Duet Ethernet.
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