Welcome! Log In Create A New Profile

Advanced

Piezoelectric disks for Z contact detect and bed levelling

Posted by leadinglights 
Re: Piezoelectric disks for Z contact detect and bed levelling
March 17, 2017 06:10AM
Quote
DjDemonD
Yes I have been commenting about polarity a lot as it seems to now make sense based on what you've said to be quite random. I presumed it was because I was using the inverted logic instead, one way around they seem to trigger but the voltage rise (maybe Mike you've seen this on your scope?) is not fast enough to trigger quickly enough and you get a crash, the other way around they are amazingly precise.

So something to be wary of with our experimental beta units, if its performing poorly, reverse the piezo connector. For pre-built units I will make sure they are the right way round, by testing them on the rig, before they go out of the door.

That is almost something I have seen before, but not quite. The first batch of no-name 27mm piezos were all checked and recorded and they were all within a 10% spread and all had the same polarity. A batch of Murata 27mm piezos were purchased but they were without leads so had to have leads soldered on. I again classified all of these although I lost some to hamfisted soldering; they all had the opposite polarity to the no-name ones and a much worse spread of peak voltages - within a 25% spread or so. A second batch of no-name discs was purchased from a new source on ebay and some of them were checked and were identical to the first no-name batch. No results were recorded on this batch as I had gone into "same old, same old" mode. One 27mm disc was drilled and installed in one of my rigs so that it could be checked as if it were in Simon's hotend. Output was good in that mode, however when it was installed as if it was an undrilled one, under a pressure pad, it gave a worrying double hump in th voltage output. I think this was because the drilling had given a local buckle to the brass.
A single drilled 20mm disc was checked along with the 27mm disc and gave a useful output. 10 further 20mm discs were modified by having a hole milled in them, this was done to eliminate the possible buckling from the force of drilling. On testing this last batch, a much wider spread of peak outputs was obtained although voltage rise for the first few ms appears to be in the Goldilocks range (livable)
I have seen one mention before of a reversed piezo, I think it may have been on a maze runner robot forum.
I am not sure how the PZT is polarized in these discs as it is normally done by putting a high voltage across the ceramic, possibly while it cools through the Curie temperature. Since thse discs are radially polled, I imagine that the ceramic is in naked form while being polarized and is then mounted to the disc and the top contact plated on.
I have a further batch of 20mm disc and will record them before and after drilling - this may take a while though as I have managed to sprain my wrist*

Mike

* Nothing to do with printers, I was turning over a couple of tons of compost at the allotment.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 17, 2017 06:26AM
Is there an easy, no oscilloscope needed method of determining the correct polarity before installation as it doesn't seem obvious just watching the led on the piezo board?



I am developing the Piezo hotend z-probe [www.thingiverse.com]
DemonDeltaMicro - Micro Delta Printer [www.thingiverse.com]
Things I've made/remixed.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 17, 2017 06:43AM
Here's what I was referring to by a "vertical" mount:

[imgur.com]

The piezo in the photo is 27mm, so there's plenty of room in there.

g.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 17, 2017 08:05AM
Okay so the idea is one part of the disc is attached to the hotend the other to the mount?



I am developing the Piezo hotend z-probe [www.thingiverse.com]
DemonDeltaMicro - Micro Delta Printer [www.thingiverse.com]
Things I've made/remixed.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 17, 2017 08:43AM
Challenge accepted. Stay tuned for my version of an effector. Started it today.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 17, 2017 08:56AM
Quote
DjDemonD
Okay so the idea is one part of the disc is attached to the hotend the other to the mount?

Not really. The idea is to suspend the piezo between the two spacers that hold the hot end assembly to the effector platform. If the piezo by itself isn't sensitive enough to record the hit, a #6-32 nut could be glued to the ceramic face in order to add mass to that side of the device.

g.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 17, 2017 09:06AM
So you're hoping to get a trigger just with the piezo stuck on the side of it? I'd be dubious if it's that sensitive but I'd be pleased if it is. Keen to see what happens.

Clearlynotstefan looking forward to it.

Edited 1 time(s). Last edit at 03/17/2017 09:07AM by DjDemonD.


I am developing the Piezo hotend z-probe [www.thingiverse.com]
DemonDeltaMicro - Micro Delta Printer [www.thingiverse.com]
Things I've made/remixed.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 17, 2017 09:13AM
Yes, that's the idea. Based on a tap test I did on Wednesday, it should work. I just need to get one of the boards soldered up so I can test it "for real". smiling smiley

I won't be offended if someone beats me to it. grinning smiley

g.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 17, 2017 10:53AM
Much thinking just done, see edit below:
Quote
DjDemonD
Is there an easy, no oscilloscope needed method of determining the correct polarity before installation as it doesn't seem obvious just watching the led on the piezo board?

A simple way is to make or print a little plastic ring that will support the edge of the piezo and connect a digital multimeter (generally an analogue one won't do) Red lead to red wire and black to black. Push in the middle with the pen and then release suddenly. Pushing should give a signal one way and releasing the other. If the multimeter is a good one you should even get an idea of how much of a signal but sampling interval may make this difficult


On this latest batch I did get two which were reversed but his is the first time I have classified by radial sensitivity and it is possible for a disk to be positive on radial but negative on axial. Needs thinking about.

Edit:Ahhh!! I believe I understand. On the rig probe goes down and releases pressure on disc; half a second later probe goes up and pressure is re-established. Poor potty cnc training means that going up is G0 Z1 and going down is G1 F60 Z-0.2, G0 and G1 are at different speeds but acceleration is the same. Oscilloscope is only triggered on the positive going pulse whether it is going down or going up, speed once it gets in its stride depends on down or up but initial speed is always the same because of acceleration so initial voltage good but peak may suck big time.

It seems likely that, for whatever reason, the 27mm discs are all one way but the 20mm discs are all over the place.

Mike

Edited 1 time(s). Last edit at 03/17/2017 11:44AM by leadinglights.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 19, 2017 05:21PM
Quote
JamesK
For my intended use case of three point under bed mounts I wanted to be able to cope with the bed moving in opposite directions at the sensors for some probing points, so my circuit rectifies the signal before passing it to the op-amp, and uses three separate channels with the output combined digitally at the comparator output. It's not particularly expensive using quad op-amp, quad comparators.

Could you share your schematic with us please. In particular how do you rectify the signal? Thanks

Edited 1 time(s). Last edit at 03/19/2017 05:37PM by Quazzer.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 19, 2017 05:41PM
Quote
Quazzer

Could you share your schematic withus please. In particular how do you rectify the signal? Thanks

Sorry, I don't have a schematic done up. Rectification was just the typical diamond arrangement using four discrete diodes. One side of the resulting output is tied to ground, the other goes to the op-amp. I'm using the LM324 which copes with running on a single supply and is cheap on ebay. The output is fed to a LM339 quad comparator which tests the output against an adjustable reference voltage (generated with a cermet and buffered by the spare 4th op-amp on the LM324).

I should 'fess up that I only tested a single channel and not on a printer, so the benefits (or even the feasibility) of the circuit are unproven.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 20, 2017 04:06AM
Quote
JamesK
Sorry, I don't have a schematic done up. Rectification was just the typical diamond arrangement using four discrete diodes. One side of the resulting output is tied to ground, the other goes to the op-amp. I'm using the LM324 which copes with running on a single supply and is cheap on ebay. The output is fed to a LM339 quad comparator which tests the output against an adjustable reference voltage (generated with a cermet and buffered by the spare 4th op-amp on the LM324).

I should 'fess up that I only tested a single channel and not on a printer, so the benefits (or even the feasibility) of the circuit are unproven.

Thanks JamesK. I guess you mean a bridge rectifier as used in this circuit.

Edited 1 time(s). Last edit at 03/20/2017 04:06AM by Quazzer.
Re: Piezoelectric disks for Z contact detect and bed levelling
March 20, 2017 04:12AM
Quote
Quazzer
Thanks JamesK. I guess you mean a bridge rectifier as used in this circuit.

Oh, good find. Yes, very much like that, but with a comparator in place of the second op-amp. I also used a zener to clamp the input to the op-amp within safe limits as the piezo's are potentially capable of generating voltages above the supply voltage.
Sorry, only registered users may post in this forum.

Click here to login