DC42 IR Probe on PEI, what's most critical?

For Mr. Crocker and others with experience using his probe on PEI, what is important to control when putting together the PEI, stove paint, adhesive assembly?

Something is screwed up with my setup, I just don't know what it is.
My setup is PEI painted with Rustoleum Black High Heat Primer, baked at 250F for awhile, stuck to a glass bed with 3M 468MP adhesive

This particular piece of PEI has been around the block. It had been used for awhile with a prox sensor, pulled off and the back painted with WHITE high heat primer. That worked like crap, so I pulled it off again (most of the white came off with the 3M adhesive) and painted it black, stuck it back on the glass bed.
That works better, but...

I still have trigger height consistency issues from point to point. Reading consistency at any given point is good (well, most of them). I can read, home the head, return to the point and get virtually the same reading.
But I can move one millimeter over and get up to .2 mm difference in the trigger height.
And some points give me an inconsistent reading (#$?!#?!?).
Point 1:
SENDING:G30
Z:3.1000
SENDING:G30
Z:3.0938
SENDING:G30
Z:3.1031

Point 2, 1 millimeter away:
SENDING:G30
Z:3.1594
SENDING:G30
Z:3.1531
SENDING:G30
Z:3.1562
SENDING:G30
Z:3.1531


It looks like the probe is triggering about 4.0 mm from the PEI exposed surface (bottom of probe to print surface). The PEI is .030 in.

I figure either I am still getting reflection off of the adhesive/small bubbles or the bottom of my PEI is too rough. I've got a new piece of PEI but I am hesitant to go at it until I know how I screwed up the last one.

So, is the paint thickness critical? If it is visibly opaque, is that good enough? Should I go thick?
Do sanding scratches in the PEI surface before painting cause problems with probe triggering consistency?
Is there something else I am overlooking?


Thanks!

Visible opacity is good enough. However, PEI is transparent to IR, so scratches, coatings etc, on the top surface in particular can still affect the trigger height. As you have a new piece of PEI on order, you could try sanding the top of the old piece with fine grade sandpaper to see if that helps.

We have an effector incorporating a nozzle-contact sensor under development, and I have to admit that it gives better results that I can get from the IR sensor with a PEI bed. See [www.duet3d.com] for a preview.

Edited 2 time(s). Last edit at 04/14/2017 07:43AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

i love the carriage adaptator
do you know where it is available?

The carriage adapter is just a piece of epoxy glass PCB with the holes drilled at a precise spacing to match the effector. They will be available along with the effector from the Duet3D web shop a few weeks from now, and probably from Duet resellers too. The black part in front of the carriage adaptor is a metal carriage, available from Robotdigg.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
dc42
Visible opacity is good enough. However, PEI is transparent to IR, so scratches, coatings etc, on the top surface in particular can still affect the trigger height. As you have a new piece of PEI on order, you could try sanding the top of the old piece with fine grade sandpaper to see if that helps.

....

(my emphasis)

ermagerd.

Scratches make a HUGE difference.

I went over the center area of my PEI with 600 grit (as I have a few times but not lately) and the readings in that area got much more consistent.

Then I sanded the endsstop adjustment landing zone in front of my X tower. The trigger height lowered consistently by 0.12 mm
The same at the Z tower made the reading lower by 0.2 mm

Here I was thinking that my carriage was tilting as it moved around (and trying to fix it) . Now I find out that at least part of it (we'll see how much) was due to the difference in surface finish.

Why do you think the trigger gets lower? My PEI upper surface is the mat finish side, so the areas in front of the towers were by no means shiny.

Edited 1 time(s). Last edit at 04/14/2017 02:56PM by swoozle.

Directionality of the sanding marks (parallel to IR sensor board vs perpendicular) on the surface makes the trigger height move by more than 0.1 mm.
Still 600 grit.

This also explains why my z offset was changing from print to print if the print footprint happened to be on the sensor probe spot.
So much is explained. I am enlightened.

I like your probe, but this seems to be, if not a drawback, certainly something to be aware of if you print on PEI.

I'm using this on a X5 mini. Is it less sensitive on a Duet due to the proportional sensing you do?

Edited 1 time(s). Last edit at 04/14/2017 02:55PM by swoozle.

And now I'm realizing that the sanding I did on the lower surface prior to painting it is also contributing. I certainly didn't pay attention to sanding direction and used 400 grit paper.

BOOM

That's my head exploding

That lines up with how the IR sensor has been behaving for me. A bare PEI sheet on matt black painted glass will not work well as the adhesive layer throws off the IR probe On a brand new PEI sheet that's well painted the IR sensor works decently, but as soon as the PEI starts to get scuffed and needs reworking results become less consistent. I really like the IR sensor because it's well designed and has a small footprint and low weight that makes it easy to mount on any effector or carriage, but I find it too sensitive to build surface finish to be the perfect Z probe.

On my cartesian printers I've relegated them to Zmin endstop duty and use manual mesh levelling to get the best results, but even then I have to use babystepping to get the Z height just right when starting a print as soon as the PEI surface quality starts to wear down or the IR sensor position changes (I have it mounted on the E3D heatsink fan assembly so unfortunately it can rotate along the heatsink axis). On the Delta's I'm using David's Delta wizard at escher3d.com with feeler gauges to calibrate them, until there is a better solution for a Delta Z probe.

I think there is a better z-probe the piezo probe. DC's sensor has been a ray of light when the alternatives were inductive/capacitative probes, and microswitches on servos, but things have moved on. The overwhelming majority of comments from people who have started using piezo (make one for yourself its fully open source) have been "wow these things are amazing". I have in all fairness had one comment that it was difficult to take apart if the hotend jammed, but the latest module we're using (and that I will be selling fully assembled versions of - but this isn't a sales pitch - make your own its not that tricky, I'm only going to sell them as it might be easier for some to buy one than make it) allows you to take the hotend out without dismantling the module.

Any printing surface, no offsets (except maybe 0.1-0.2mm in z to account for the compliance in the unit, which generates the trigger), probe anywhere on the bed that you can print, its reliable, and with the pre-tested, pre-assembled unit its play and play. There's a bit of tuning if you make your own but its not rocket science.

These things are used for high precision measurement industrially, so why we haven't been using them on 3d printers is beyond me, guess that's going to change.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

Yes that was going to be my next attempt. I've been following your design on and off (and I must say, thanks, it's really great work). Some time ago I ordered a set of Murata piezo discs off eBay - haven't gotten around to trying them out yet though (and the project seemed to be in flux still). The thread you linked to I assume has the final design? Looks like I have some reading to catch up to as it's grown to a 14 page thread , along with the thread on the duet3d forums. I'm not really clear on the electronics part and the design I saw (Moriquendi's, I believe) was using SMD components so I'd have to look for through-hole alternatives (I can whip up a PCB but don't have an SMD station). It's on my (ever growing) to-do list...

Anyhow should make a point of it as the postal deities of Spain have finally deemed the time was right to deliver my Haydn-built magnetic arms, so I'm due an effector/carriage upgrade.

edit: TI says there's a DIP version of that opamp so all good

Edited 1 time(s). Last edit at 04/16/2017 07:33PM by siddharta.

Moriquendi can sell you a board for about £10. The design using the 20mm discs and groovemount (though can be adapted for screw mount too) is pretty much final. The great thing with this system is its quite versatile and since the components are fairly cheap its easy enough to build it into your setup.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

For 10 GBP I probably won't go through the hassle of building one, thanks for pointing that out. The design using the 20mm discs you're referring to is the groovemount design on Thingiverse here? Unfortunately the piezo's I ordered are 27mm Murata's, so I'd have to rework that design or get some 20mm discs.

edit: PM'd Moriquendi , thanks again!

Edited 1 time(s). Last edit at 04/16/2017 08:04PM by siddharta.

10x20mm discs are about £3 on ebay. Or you can use a 27mm design or adapt it. We've realised that you don't need much movement in the disc, in the 20mm groovemount version you're almost listening for the tap.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

@DjDemonD - ordered some 20mm discs off eBay.

@swoozle - really really sorry for hijacking your thread got carried away with DjDemonD's design suggestion, back to topic! Though DJDemonD's probably right in that the piezo disc design is a better Z probe solution and seeing that dc42's also going that direction with his PCB effector and carriage design it would seem that's the way to go forward.

Edited 2 time(s). Last edit at 04/17/2017 05:34AM by siddharta.

Carry on, on the piezo thread if you prefer.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

No worries, this is great info and I am going to pursue the piezo solution as well.