Piezoelectric disks for Z contact detect and bed levelling

@Moriquendi, Just a brief note that if the bed on your Kossel mini is heated you may find that the piezo disks need some time to settle. This is because piezo disks act as a pretty good detector for rate of warming. On my Rostock. the sub frame with the piezos on it warms up from 20°C to 50°C as the heated bed warms up to 100°C. This takes about 12 minutes and the sensors show contact until about 5 minutes after the bed temperature stops going up. The explanation is that the piezos give an output of about 50mV for every 0.03°C per second - enough to keep the comparator tripped.
There are several things that can be done to work around this. The best is obviously to keep the heat from the bed reaching the piezo disks - better insulation. Also, keeping the low frequency roll off high enough to ignore the fairly slow changing thermal measurement both by having the piezo discharge resistor fairly low and by having a lower capacitance at the disks. As you seem to have both these accounted for you should not have any problems but just in case you do see this effect.

@elenhinan, I kludged together a charge amplifier but am getting odd results at the moment. The most prominent thing is that it seems to be sensitive to induced noise from the heater - strange as the voltage amplifier is bullet proof in that respect. As the present circuit is a bit of a patched mess I am putting together another PCB in the hopes that it will be usable.

Mike

Here we go, feedback is welcome.

Complete details for Piezo Z-probe, including tuning instructions and BOM

Files on Dropbox



This is experimental and offered without warranties, there may be errors.

Moriquendi

Any chance I could buy a populated pcb from you? I'd like to try this piezo sensor idea but my smd soldering is appalling.

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Simon Khoury

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

Sorry, I completely missed your post.

I've got twelve PCBs from OSHpark all made up, eleven are working and I want to keep four for myself (going to play with Piezo endstops).

So, I have seven PCBs made up. I don't want to make any money from them I just wanted the soldering practice, however I'd like to do some good with them. I'm a trustee of a rescue charity for Great Danes, I was thinking of offering one board in exchange for a £5 donation to that charity. I'll post them anywhere in the UK for that, anyone overseas would need to pay postage.

Let me know your thoughts, the charity is registered if you want to check.

Moriquendi

Edited 2 time(s). Last edit at 10/17/2016 05:35PM by Moriquendi.

Five left

Moriquendi

Hi guys I haven't yet used the piezo boards but I am about to. Decided to just throw together a little rig to see if you could mount one of the piezo transducers above a hot end and you can. I drilled one so that it has a 4mm hole through it for a bowden tube.
I've only got a pocket scope but these are fairly light taps on the heatsink giving fairly decent signals. Only 1.5v but this should be enough to get a trigger?




If it works, I figure this is cheaper and easier than having them under the bed, eliminates the issues with heat and the effect of two sensors cancelling each other out etc... Plus the heatsink in this rough version is quite securely mounted.

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Simon Khoury

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

That looks to be a good way of mounting the piezo disk - if it works well I will try it myself. Things to look out for are long mechanical settling times from X or Y movements and the rate of change of temperature at the piezo element which can give a false reading.

Mike

Neat idea!

Thanks. It makes sense as long it can be built so as the nozzle is firm. The issue I had with FSR hot ends is that they are less sensitive and more compressible than piezo discs meaning a vague nozzle in z more likely.

I need slightly bigger piezo's as the solder pad for the positive lead is being squashed in the clamp and I am sure this is reducing the sensitivity. And I need to neaten and miniaturise the whole assembly.

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Simon Khoury

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

Let me know if you have any issues with the boards. Adjusting them can be a bit fiddly, in hindsight I should probably have used bigger trimmers but they're designed to be adjusted and then left alone.

Moriquendi

Thanks, but as it turns out initial testing seems to suggest no fiddliness at all.




This is how it looks now, just trimmed it down a bit. Definitely found squeezing them, especially after drilling the holes, wasn't sufficient, so I redesigned the clamp so that there is a ring on the lower part 16mm wide and then the upper part now only contacts the outside of the piezo disc, made a few tiny holes for the solder pad on the disc itself in the lower ring, and the wire.



Attached it to the Moriquendi board, no resistor, no adjustments at all and it triggers really nicely.

[www.youtube.com]

Now to stick it on the delta and try it.

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Simon Khoury

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

So it works. I am testing it on my Kossel XL with Duetwifi. The sensor board works on 3.3v, which is good. It also provides an analogue output which the Duetwifi can use, and the trigger value can be adjusted. It needs to be configured so the probe output falls as it is triggered. My auto-calibration run:

[www.youtube.com]

It now achieves a deviation across the bed of 0.013mm whereas with the DC42 IR sensor I was achieving around 0.05. So a factor of 5 times more accurate calibration using this sensor.
Extremely pleased doesn't even cover it , now to rebuild the effector with this sensor and print some stuff.

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Simon Khoury

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

And here it is printing.

[www.youtube.com]

Gives a z offset of 0 after calibration (my kossel mini with FSR's used to be around 0.2mm - both with printbite) and my bed height map is now totally flat:



Which I would expect it to be given that its tooling plate and the printer is very rigid. However when I was using IR sensor I got a much less even height map, so the effector tilt + differing reflectivity of some areas of the bed resulted in some error.

Edited 1 time(s). Last edit at 01/13/2017 01:24PM by DjDemonD.

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Simon Khoury

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

Well that looks excellent

That looks really good!

I'd expect better than 0.05 with your build, so I'm glad you're now getting the results you deserve. 0.013mm is fantastic.

The only thing I'd add is to try to get that hotend up a lot higher, it will reduce the effect that effector tilt has on the X/Y accuracy of your parts. I've got a new effector design in the final stages of testing that has really helped me with this issue. I'm getting +- 0.1% accuracy on X/Y now with the new effector. I'll make a post about it in the next week.

Thanks guys. Yes having the hot end much higher would make a lot of sense, maybe I can rework this as an effector with built in Piezo mount.

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Simon Khoury

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

[www.thingiverse.com]

If anyone wants to try it. Moriquendi, you might get a few more orders for these controllers

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Simon Khoury

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

Works for me, I've got plenty left and components to make more if there's the demand.

Moriquendi

Hope to try this on a coreXY machine.

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appjaws - Core XYUV Duet Ethernet Duex5
firmware 3.1.1 Web Interface 3.1.1
Ormerod 1-converted to laser engraver, Duet wifi
OpenSCAD version 2020.07
slic3r-1.3.0, Simplify3D 4.1.2, Cura-4.4.1

This discussion got going on duetwifi forum [www.duet3d.com]

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Simon Khoury

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

Okay so MkII of my design is now on thingiverse [www.thingiverse.com] (and on my large kossel) working well, although after assembling it my bowden clip broke in the v6, so now I will have to dismantle it again. I might do a series of photos to show how to assemble it, and a full BOM of parts needed.



Its much smaller now the whole unit is just 40mm diameter and it holds the nozzle fairly firmly. Only two bolts to hold the upper and lower sections to one another as they now slide over one another quite tightly with 4 flanges around the outside. Two m3 bolts to hold it to your carriage/effector. If printing it use plenty of support. I brushed the domed section above the hot end with acetone afterwards as it looked a little thin, but it sits nicely on top of the hotend, and insulates the piezo from the hot end.

Next step design an effector with the upper piece built-in.

I found a 5mm spur point wood drill bit cuts a much neater hole through a piezo transducer than a twist drill or diamond core drill. Use a wood board for support, low rpm and stop if it gets hot, went through it neatly like a hot knife through ABS.

Edited 1 time(s). Last edit at 01/16/2017 08:09AM by DjDemonD.

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Simon Khoury

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

Nice work!
I think I will have to try it as well. It certainly looks like a lovely idea.
Combine that mount with a new effector and a Nimble sitting on top, perfect setup!

I will download your Thingiverse files and see how well that would fit. Unless you have a step file for me?

Lykle

Hi Lykle,

I am a rank amateur and use tinkercad but I can share the design on tinkercad if you need to modify it.

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Simon Khoury

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

HI,
Nah it's ok, I will grab the Thingi files and re-create. Not too difficult.
Thanks

Lykle

Edited 1 time(s). Last edit at 01/16/2017 08:13AM by Lykle.

Great stuff. Now I'm caught between adding piezo's to the bed as I'd originally planned, or trying to figure out if I could put one in each mount of a dual extruder. I have a dovetail slide made up - if I added a small stepper to drive the dovetail I could use piezos in each hot-end to automatically set the level of the two nozzles Not sure if it's worth the weight penalty, but it sure would be a nice feature.

As I want to try this myself I will download DjDemonDs latest files and translate to STEP (my hotends are very non standard)

On a slightly different tack, I have long suspected that piezoelectric disks will work without amplification but did not have the courage or time to try it. I have found two recent entries on the German speaking part of this forum where windiesel seems to be doing just that. My understanding of German is non-existant and not much better for Google German to English translation, but I thint the gist of it is that it does work but could be better. The threads are :-

[forums.reprap.org]
[forums.reprap.org]

Mike

With the low cost of modern op-amps, and the ease of including filtering in the amp stage as well as threshold and gain settings I'm not sure it's worth trying to ditch the amplifier, even if it becomes unity gain. Maybe I'm just a control freak

Mike, Lykle is reworking my design (using proper cad skills and software) he says he'll send me some files over later so unless you fancy your own take on it, might save you some time.

Edit sorry missed the part about non standard hotends.

Edited 1 time(s). Last edit at 01/16/2017 10:01AM by DjDemonD.

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Simon Khoury

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

I don't think it's amplification that's needed. I think the most important thing is protecting the controller from the output of the piezo in the case of a head crash and providing a reliable, adjustable threshold for triggering.

Moriquendi

Ps, Boards have been posted out today, you should get them in a day or two. The charity secretary has sent thank you emails as confirmation that the charity has received your donations. We rehome around 50 dogs each year and as a small charity every penny we raise in donations is vital so thank you to everyone.

Made a few slight mods to the design, added a small lip above the lower sections to avoid the bowden coupler being squeezed (and disconnecting) when assembling the unit. This should also decrease the contact area onto the piezo for a stronger signal. Made a slight cutout in the upper section to allow the piezo to bend slightly more. On thingiverse. I figure most people here are adapting it their own setup so probably not too important.

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Simon Khoury

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