Piezoelectric disks for Z contact detect and bed levelling

The piezo is actuated by the head of a bolt in the top of the carriage. I'm using smoothieware and I have a carriage rebound of 6mm set so that at rest the carriage is 6mm from the piezo. I don't think there will be a problem even if the bolt is in contact with the piezo, the system is so sensitive that the first microstep or two should trigger the endstops. I'll check it out though.

Idris

They look good. Much more straightforward to implement than the z-probe. Which is great.

Edited 1 time(s). Last edit at 02/25/2017 04:26PM by DjDemonD.

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Simon Khoury

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

Hi Idris,
Firstly, I owe you an apology for being so dismissive when you thought of using piezo elements for end stops, I clearly didn't think it through. My assumption was that if a piezo end stop was in the fully homed position when a call to home that axis was made, that the piezo would give no response. That would be a reasonable interpretation if the sensor was based on the pressure relief methods that I use, but with either a direct pressure method or a bending method, movement in the homing direction will give a signal even if the carriage is already against the end stop.
I will follow your developments with interest.

While waiting for DjDemonD's wiki page to go live, I was looking through the wiki pages for Z probe [reprap.org] and FSR sensors [reprap.org] I must admit that I was unable to supress an unplesant and snide guffaw when I read that less than 300 grams is too sensitive for an FSR based system - the piezos on my delta are set to trigger at about 10 grams. No wonder people are worried about gouging their build stages.

Mike

Piezo for endstops is intriguing, but I'm not sure I fully understand the benefits. Is it just the expectation of higher accuracy/repeat-ability? Given the extra complexity and cost compared to a simple opto or mechanical endstop, I'm not sure that I need any higher accuracy for XY endstops - it's a non-critical measurement.

Edit - Ah, but not for deltas, where all endstops are critical. Silly me, I think in cartesian.

Edited 1 time(s). Last edit at 02/27/2017 06:48AM by JamesK.

Hi Mike,

Don't worry, I didn't take offence and it is something worth investigating. It turns out that if you have zero homing rebound it is possible to use up all the compliance in the piezo and potentially cause a problem. When I set zero rebound I could home the machine six or seven times before the tone of the steppers changed (a highly accurate method of measuring exerted force...) so I think that in practice this would not be a problem, alternatively a rebound of 0.1mm prevents this completely.

I realise that in my excitement to show off the piezo endstops working I didn't actually show how they were working and how they were set up, I'll try to rectify that now.

Very simply I have a 10.5mm (also very thin, 0.2mm) piezo element centred below an 8mm hole in the mount, the head of the bolt in the carriage touches dead centre in the bottom of the piezo.


One of these at the top of each axis connects to the three channel endstop board. The sensitivity and threshold of each channel is linked so they all trigger at the same force.



I think I've reached the limits of rigidity of my printer as I'm not seeing any improvement in z-leveling. I think that DJdemonD's printer is rather more robust so hopefully when he gets round to playing with the board I sent him we'll get better data on accuracy and repeatability.

I've got six more of these endstop boards on their way from OSHpark and hopefully they'll be here within two weeks.

I'm still working on the best way to organise piezos as endstops and their signal conditioning requirements, I could use some feedback here.

One option I'm looking at is an integrated board that has a piezo soldered on as well as the signal conditioning circuit, you'd then need one of these per axis. The issue with this is that you wouldn't be able to tune them, or if you could you couldn't guarantee that they were all tuned the same, I don't know how much of a problem this would be. This is likely to be the simplest, mount it, plug it in and forget about it. the difficulty is I'd have to find tuning values that work reliably for everyone.

Option two is the same as one but the piezo mount part of the board can be snapped off and mounted separately, this gives more flexibility without having the have multiple board types.

Option three is what I have at the moment, one board handling three separate piezos, the issue here is that long wires may pick up interference and in the app note we're warned against long wires due to the capacitance affecting the strength of the signal. How long does the cable have to be before this is a problem, no idea, may need further investigation.

Opinions most welcome, or if you've got other ideas I'd love to hear them.

Idris

If I get chance I will try some Piezo endstops but it's not very high on my list. I don't think they'll beat the opto-endstops I have now for reproducibility or accuracy. Be an interesting result if they did.

I'm actually having a odd thing with my current hot end piezo sensor, it will trigger everywhere except the left side of the bed. Loose wire or something....

Interestingly what you said Idris about long wires I have a long wire on my kossel Xl as I didn't know how long I'd need when I setup the first one up ages ago. Might shorten that down a good bit.

Edited 1 time(s). Last edit at 02/27/2017 08:31AM by DjDemonD.

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Simon Khoury

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

Shortened the wire nothing changed, but loose connection on my Piezo sensor at one extreme position. So that's all sorted. Will see about getting the wiki page up later. Feel free to review/edit/add to it once it's up.

As Mike said the general z probe sensors page on reprap wiki is rather out of date and not really very accurate in terms of the pros/cons of different sensing modalities.

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Simon Khoury

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

Idris - the endstop board has only one pot on it what's the basic difference in layman's terms?

Edited 1 time(s). Last edit at 02/27/2017 02:16PM by DjDemonD.

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Simon Khoury

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

You only get to tune the threshold, not the sensitivity. I don't think this makes a great deal of difference to how you tune it. Turn the pot one way or the other (I forget which) until all the led's are out then a fraction further, tap your piezo with an insulated object, you should get a flash from the relevant channel. It's crucial that you use an insulated,hard, object. If you use something conductive and you're holding one end your body will change the capacitance of the piezo and will cause a trigger. If you get spurious triggers during normal printer motion then turn the pot slightly further, if you have to hit the piezo too hard to get a trigger then go back the other way.

The reason there is only one pot is that I wanted to ensure that all the channels were equally sensitive and tuned in unison, it would have been impossible to achieve this while still having a pot for sensitivity without using a three gang pot which would be large, difficult to find and very expensive. The threshold is easy to share between channels because you're adjusting the output of a voltage follower which can then be shared between three comparators.

Idris

Wiki page [reprap.org]

Feel free to edit if you have the access or let me know if you want something changed.

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Simon Khoury

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

Looks good, sorry I haven't been able to contribute yet but I've been busy with other things.

Idris

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DjDemonD
Wiki page [reprap.org]

Feel free to edit if you have the access or let me know if you want something changed.

A very good page in the wiki, I will try to fill in some of the details re. underbed sensors in the next few days.

I see that you have put a pointer to the Piezo-Electric Sensors page in the Z Probes page. I may try to edit the whole end section of that page to separate out the nozzle contact probes from the proximity sensor probes - and also remove the somewhat disparging "uncommon" comment.

Mike

Thanks. Yes the z-probes section needs an overhaul and since were currently pioneering in the z-probes department that sounds like a plan Mike.

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Simon Khoury

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

The wiki section Underbed_Piezo-electric_sensor [reprap.org] is now clickable. Please edit or point out any errors or improvements.
Idris, I have taken a bit of a liberty and done a drawing of my interpretation of your underbed sensors. Please check if this is correct.

Mike

Edited 1 time(s). Last edit at 03/01/2017 12:39PM by leadinglights.

Comes up as an empty page for me....

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dart16
Comes up as an empty page for me....

O.K., fixed (I think)

Mike

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leadinglights

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dart16
Comes up as an empty page for me....

O.K., fixed (I think)

Mike

Drawing looks good, I can add a photo of one of my bed supports if you think that'd help. I've added a little to the text too in that area.

Idris

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Moriquendi
Drawing looks good, I can add a photo of one of my bed supports if you think that'd help. I've added a little to the text too in that area.

Idris

By all means add a photo. I like your text but now I may have to clarify what I mean by "semi-radial" and "responds to bending in one direction only".
Shakespear and Tolkien had it easy

It looks great. Superb work. Quite flattered being invited to have a bio.

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Simon Khoury

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

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leadinglights

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DjDemonD
Those are interesting and very professionally produced results Mike. Were any of the discs drilled in the test? So potentially promising even if a bit of soldering or discarding less suitable discs applies if they function with holes.

Only the one drilled and tested so far but I will do a couple more shortly so that I know that they are consistent.

Mike

I just got some 20mm no name piezos. Using my pocket scope, which is rather limited in its capabilities I could get around 0.5v giving it a sudden bend. I think these might work fine as endstops - will try this later on.

I then drilled one with a 5mm hole, and basically knackered it, its still responding but its down in the 50mV range now.

Edited 1 time(s). Last edit at 03/02/2017 07:21AM by DjDemonD.

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Simon Khoury

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

The piezo disks I'm using as endstops are These and they work very well.

I've said before, though it might have been in a PM to someone, that measuring the output of a piezo sensor is not as straight forward as it first appears. I would suggest hooking one up to the endstop board and seeing whether it triggers, that might tell you more about the usability of the disk than measuring the output voltage.

Idris

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Moriquendi
I've said before, though it might have been in a PM to someone, that measuring the output of a piezo sensor is not as straight forward as it first appears.

Could you elaborate a little on that? While the sensor is obviously going to interact with the input impedance of the measuring circuit, I would have thought that looking at the raw output on a scope was a pretty good first stab at how the sensor is behaving. You get a good sense of signal to noise and a quick estimate of area under the curve, peak voltage and decay rate.

The important thing in measuring the output of a piezo is to have a high input resistance on your scope or multimeter - moving coil meters are very poor but just give a tiny kick to the needle. Anything over 1MΩ should do and 10MΩ will hold the voltage for several seconds. To check what the input resistance is: Connect a 1MΩ resistor in series with one of the leads of your scope/meter and then use it to measure a voltage. If your are measuring a 5V supply and get 2.5V then your meter resistance is 1MΩ, if you get about 4.5V then your resistance is 10MΩ. Most of the plots that I have shown are with a 10MΩ probe.

Mike

Sure,

Put simply the output pulse can be so brief that it is lost by a digital measuring device, a digital multimeter is very unlikely to give an accurate reading as they only update a few times a second. A pocket scope should be better but I don't know how much better.

There is another potential issue with reading the output of the piezo directly that's even harder to quantify:

Actuation of the piezo sensor, whether by bending or pressure, causes a charge to build on opposite terminals. This charge is small. As this charge flows from one side of the disk to the other (a current) through a resistance (VR1 on my boards) it generates a voltage. So far pretty straight forward, but (and this goes back to an earlier discussion regarding long piezo wires) if the parasitic capacitance of the circuit is too high it slows the rate at which the charge flows, reducing the current and therefore lowering the peak voltage. An analogy might be the difference between pouring a bucket of water into a full gutter and pouring a bucket of water into a full lake, eventually a bucketfull of water will flow out the other end but it will come out of the gutter much quicker than the lake. If the input capacitance of the measuring device is significant that will affect the result too.

I could be completely wrong about this but I can't quite match up what I saw on my scope with DjDemonDs report of 0.5v signal.

Idris

If the range of your scope goes down low enough you should be able to put a high value resistor in series, something like 10MΩ. Using a X10 scope probe would also work as those tend to be 10MΩ or greater.

Mike

Here's what I see on my Bitscope with a light tap on a 20mm piezo...


2v per div and 500us timebase. Note that the output voltage is the difference between the two traces as I don't have a differential probe. Edit, also there appears to be clipping at the peaks so the actual peak voltage may be rather higher.

Idris

Edited 1 time(s). Last edit at 03/02/2017 10:49AM by Moriquendi.

I tried again using the 10x probe and got a bigger spike even from the drilled one, it's quite hard to read this pocketscope. So yes I'd agree 20mm seems feasible, would make for a smaller unit especially for i3 style machines. Be nice to make an e3d titan with Piezo probe unit.

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Simon Khoury

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

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DjDemonD
I tried again using the 10x probe and got a bigger spike even from the drilled one, it's quite hard to read this pocketscope. So yes I'd agree 20mm seems feasible, would make for a smaller unit especially for i3 style machines. Be nice to make an e3d titan with Piezo probe unit.

I'm mixing a few files to get one for the titan, which will use the 27mm piezo. Will report back in a few days when bits turn up, to see if I've got it right....

I have added a drawing to the Underbed Piezo Sensor Wiki page [reprap.org] to show the modes that piezo disks can be used. Also cleaned up the Z Probes page [reprap.org] adding a few other types such as the puck probe and accelerometer probe.

Mike